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Smoking during adolescence may increase the likelihood to develop nicotine dependence and to abuse other drugs such as cocaine. Despite great efforts to understand underlying neurobiological mechanisms of this progression, less attention has been paid to the role of genetic factors. Here, we investigated the influence of both genetic background and age at first nicotine exposure in the long-lasting effects on mesolimbic dopamine transmission including the increased cocaine-rewarding effect. Mid-adolescent and adult rats of inbred strains Lewis (addiction prone) and Fischer 344 (addiction resistant) were administered nicotine (0.4 mg/kg) or vehicle once daily for 5 days. Changes in dopamine transmission were investigated by in vivo microdialysis and electrophysiology after 30 days of withdrawal, whereas changes in cocaine-rewarding effect were assessed via conditioned place preference paradigm. Nicotine pre-exposure differentially changed mesolimbic dopamine transmission depending on strain and age of pre-exposure. A potentiation of dopamine response to nicotine was observed in nucleus accumbens (NAc) core of both strains and age groups, whereas dopamine response in NAc shell was enhanced exclusively in Lewis rats exposed to nicotine during adolescence. A similar response was observed following cocaine challenge at adulthood. Changes in VTA dopamine cell population and activity were observed only in adolescent nicotine-pretreated Lewis rats, which also showed an increased cocaine-rewarding effect at adulthood. These results highlight the influence of genetic background in the long-lasting effects of nicotine exposure and suggest that exposure during adolescence might increase nicotine and cocaine-rewarding properties in genetically vulnerable individuals, thereby facilitating progression toward dependence.

Adolescent Cannabis exposure has been hypothesized to act as a gateway to opiate abuse. In order to investigate the role of genetic background in cannabinoid-opiate interactions, we studied the effect of Δ(9) -tetrahydrocannabinol (THC) exposure of adolescent Lewis and Fischer 344 rats on the responsiveness of accumbens shell and core dopamine (DA), as monitored by microdialysis, to THC and heroin at adulthood. Heroin reward and reinstatement by heroin priming were studied by conditioned place preference (CPP) and cognitive and emotional functions by object recognition, Y maze and elevated plus maze paradigms. THC stimulated shell DA in Lewis but not in Fischer 344 rats. Adolescent THC exposure potentiated DA stimulant effects of heroin in the shell and core of Lewis and only in the core of Fischer 344 rats. Control Lewis rats developed stronger CPP to heroin and resistance to extinction compared with Fischer 344 strain. In Lewis rats, THC exposure did not affect heroin CPP but potentiated the effect of heroin priming. In Fischer 344 rats, THC exposure increased heroin CPP and made it resistant to extinction. Lewis rats showed seeking reactions during extinction and hedonic reactions in response to heroin priming. Moreover, adolescent THC exposure affected emotional function only in Lewis rats. These observations suggest that long-term effects of Cannabis exposure on heroin addictive liability and emotionality are dependent on individual genetic background.

Conditioned saccharin avoidance (CSA) can be produced when either lithium chloride (LiCl) or a reinforcing drug, such as morphine, is administered following exposure to the taste of saccharin. In this study we investigated the involvement of dopamine (DA) transmission in the acquisition of morphine and LiCl-CSA. CSA was evaluated in a two-bottle choice paradigm with two conditioning pairings between saccharin and morphine or LiCl as unconditioned stimulus (US). Morphine hydrochloride (7.5 mg/kg s.c.) or LiCl (40 mg/kg i.p.), administered 45 and 120′ respectively after saccharin-drinking session, induced strong CSA. The DA D 1 receptor antagonist, SCH 39166 (0.1 mg/kg s.c.), impaired morphine-CSA if administered 15′ and, to a lesser extent, 30′ but not 45′ before the drug (i.e immediately after saccharin drinking). In contrast SCH 39166 reduced LiCl-CSA when administered 45′ before the drug and even more so when administered 105′ before LiCl i.e. immediately after saccharin drinking. Therefore SCH 39166 impaired morphine-CSA when given shortly before the drug, while it impaired LiCl-CSA when given shortly after saccharin. Raclopride, a specific antagonist of D 2 receptors, failed to affect LiCl- and morphine-CSA. These results are consistent with the idea that DA, acting on D 1 receptors, plays a differential role in morphine- and LiCl-CSA. In LiCl-CSA DA is necessary for the processing (consolidation) of the short-term memory trace of the saccharin taste to be associated with the lithium-induced aversive state, while in morphine CSA contributes to mediate the appetitive properties of the drug.

Experimental evidence indicates that the mesolimbic dopamine (DA) pathway innervating the ventral striatum is critically involved in the motivational effects of drug abuse. However, the role of DA transmission of the two main subdivisions of the nucleus accumbens (NAc), the shell and the core, in the motivational properties of nicotine is unknown. The aim of this study was to investigate the role of DA D1 and D2 receptors of the rat NAc shell and core in the motivational effects of nicotine using a conditioned place preference (CPP) paradigm. The effect of the intracerebral infusion of DA antagonists specific for DA D1 (SCH 39166) and D2 receptors (l-sulpiride) was studied in a single-trial place-conditioning paradigm with fixed assignment of the drug to the unpreferred compartment. Nicotine induced significant CPP at the dose of 0.4 and 0.6 mg/kg subcutaneously (s.c.). Intra-NAc shell infusion of SCH 39166 (6.25, 12.5, 25 and 50 ng bilaterally, 10 min before nicotine administration), impaired in a dose-dependent manner the acquisition of CPP by nicotine (0.4 mg/kg s.c.). SCH 39166 failed to affect nicotine CPP when infused into the NAc core. l-Sulpiride (25 and 50 ng bilaterally) had no effect on acquisition after intra-Nac shell infusion. SCH 39166 and l-sulpiride were ineffective after infusion in the NAc shell and core 10 min before the test session. The results indicate that dopamine D1 but not D2 receptors of the NAc shell are specifically involved in the acquisition of nicotine-induced CPP.

Acute administration of the acetylcholinesterase inhibitor tacrine to rats induces tremulous jaw movements which can be used as a valuable model of parkinsonian tremor. In the present study, the number of tremor episodes and jaw movements were evaluated to assess the effects of the selective A2A antagonists SCH 58261 and SCH BT2 on tremorgenesis. SCH 58261 dose-dependently, and maximally at 5 mg/kg, reduced the number of both tremor episodes (−35%) and jaw movements (−50%), induced in rats by tacrine (2.5 mg/kg ip). Since adenosine A2A receptors are largely expressed throughout the striatum, chronic cannulae were implanted in the rat dorsomedial (DMS) and ventrolateral striatum (VLS) to investigate whether A2A antagonists could act at this level. Infusion of SCH BT2 (5 μg/μl), a water-soluble analogue of SCH 58261, in VLS antagonized both tremor episodes (−68%) and jaw movements (−76%) elicited by tacrine (2.5 mg/kg ip), whereas SCH BT2 infusion in DMS was less effective in blocking jaw movements (−50%) and did not significantly affect the number of tremor episodes. Taken together, the results of this study indicate that A2A antagonists effectively reduce the magnitude of tremulous jaw movements induced in rats by acute tacrine, mainly by an action in VLS and suggest that A2A antagonists might be used as specific agents against parkinsonian tremor.

The present study investigated long-term alterations in striatal gene expression after single exposure of unilaterally 6-hydroxydopamine-lesioned rats to different dopamine agonists (priming). Rats were primed with the D 1 agonist SKF38393 (10 mg/kg), the D 2 /D 3 agonist quinpirole (0.2 mg/kg), the dopamine precursor L-DOPA (50 mg/kg) or with vehicle (drug-naive), and GAD67, dynorphin and enkephalin mRNAs were evaluated in the striatum by in situ hybridization, 3 days after priming. To evaluate GAD67 mRNA in striatonigral and striatopallidal neurons, identified as enkephalin (-) and (+) neurons, double-labelling in situ hybridization was used. Drug-naive lesioned rats showed an increase in GAD67 mRNA in enkephalin (-) and (+) neurons, an increase in enkephalin and a decrease in dynorphin mRNAs. Priming with either SKF38393 or quinpirole further increased GAD67 mRNA in enkephalin (-) and (+) neurons, however, while SKF38393 produced a high and unbalanced activation toward enkephalin (-) neurons, after quinpirole the increase was of low intensity and similar in the two pathways. Dynorphin mRNA was increased by SKF38393 but not by quinpirole, whereas enkephalin mRNA was not changed by either priming. L-DOPA produced a high and similar increase in GAD67 mRNA in enkephalin (-) and (+) neurons. Priming differentially affected peptides and GAD67 mRNA in striatopallidal and striatonigral neurons depending on the dopamine agonist used. The degree of enduring overactivity of the striatopallidal and striatonigral pathways may be related to the ability of L-DOPA and D 1 or D 2 /D 3 receptor agonists to prime motor behavioural responses and to produce dyskinetic side-effects.

The acute motor response to caffeine was studied in rats repeatedly treated with vehicle or the dopamine D(2) agonist bromocriptine either in a novel cage or in the home cage. Rats receiving bromocriptine (5 mg/kg i.p.) in a novel cage were sensitized to the motor stimulating effects of bromocriptine itself and showed cross-sensitization to the acute administration of low (10 mg/kg s.c. ) but not high (25 mg/kg s.c.) doses of caffeine, no matter if the novel cage was identical or different from the test cage. In contrast, caffeine (10 mg/kg i.p.) administered to rats which had received bromocriptine (5 mg/kg i.p.) in the home cage and which showed no sign of a sensitized response to bromocriptine, failed to show an increased locomotor and stereotyped response as compared to vehicle pretreated rats. Similarly to caffeine, the selective adenosine A(2A) antagonist SCH 58261 (3 mg/kg i.p.) showed an increased motor response in bromocriptine sensitized rats. The sensitized response to caffeine or SCH 58261 did not appear to be due to an higher basal motor activity of bromocriptine sensitized rats since acute administration of vehicle induced a similar motor response in bromocriptine and vehicle pretreated rats. Dopamine D(2) and adenosine A(2A) receptors are colocalized in striatal efferent neurons where they control in an opposite direction motor behavior. The results of the present study showed that changes in the sensitivity of D(2) receptors influenced the sensitivity of the adenosine antagonist caffeine through an action on A(2A) receptors. D(2) and A(2A) receptors, therefore, not only acutely interact in the mediation of motor behavior but long-term modification of the D(2) receptors, such as sensitization, affected the response of adenosine A(2A) receptors.

In order to define the role of dopamine receptors in the contralateral rotational behavior induced by caffeine in unilaterally 6-hydroxydopamine-lesioned rats, we evaluated the influence of previous exposure (priming) to dopamine receptor agonists and the effect of dopamine receptor blockade on the rotational behavior induced by caffeine. 6-Hydroxydopamine-lesioned rats received single or repeated administrations of the D 1 /D 2 receptor agonist apomorphine (0.3 mg/kg s.c.) (primed) or vehicle (drug-naive). Three days later all rats received caffeine (30 mg/kg s.c.). Drug-naive and single-primed rats did not rotate in response to caffeine, whereas rats repeatedly primed with apomorphine rotate contralaterally. When apomorphine priming was paired to the environment (hemispherical bowls) where rats received caffeine, rotational behavior was significantly higher than that obtained in rats primed in an unpaired environment (cylinders). Repeated priming with the D 2 /D 3 receptor agonist quinpirole (0.2 mg/kg s.c.) induced a totally context-dependent contralateral rotation in response to caffeine, while caffeine contralateral rotation was not dependent on the context after repeated priming with the D 1 agonist SKF 38393 (3 mg/kg s.c.). Caffeine context-dependent rotational behavior was antagonized by either D 1 or D 2 receptor antagonists SCH 23390 (0.03 mg/kg s.c.) and eticlopride (0.03 mg/kg s.c.), whereas caffeine context-independent rotation was not antagonized by SCH 23390 or eticlopride. The results show that: 1) caffeine does not induce any contralateral rotation in drug-naive 6-hydroxydopamine-lesioned rats; 2) repeated priming with a dopamine agonist enables caffeine to induce contralateral rotation; this rotation is, however, dependent on the context when D 2 receptors are stimulated; 3) caffeine context-dependent contralateral rotation is counteracted by dopamine antagonists, whereas context-independent rotation is not antagonized by dopamine receptor blockade.

An involvement of globus pallidus (GP) in the antiparkinsonian effects of A 2A receptor antagonists has been proposed on the basis of the selective localization of A 2A receptors on the striatopallidal pathway. In order to investigate this possibility, the present study evaluated rotational behavior in unilaterally 6-hydroxydopamine-lesioned rats following infusion of the water-soluble A 2A receptor antagonist SCH BT2 into GP. SCH BT2 (5 μg/1 μl) altered neither motor behavior nor produced postural asymmetry by itself. However, when infused concomitantly with a parenteral subthreshold dose of l -DOPA (3 mg/kg i.p.) capable of inducing modest contralateral rotational behavior (34.7 ± 20.7/1 h), SCH BT2 significantly potentiated the number of contraversive rotations (167.4 ± 16.3/1 h). These results suggest that A 2A receptors located in the globus pallidus may be involved in the antiparkinsonian effects of A 2A antagonists.

In order to investigate the role of adenosine A2A receptor blockade on dopamine-mediated motor responses, contralateral turning behaviour and expression of the early-gene c-fos was evaluated in rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway. SCH 58261, (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1 , 5-c]pyrimidine) a potent and selective antagonist of adenosine A2A receptors (5 mg/kg i.p.), induced a 70-fold increase in the contralateral turning behaviour induced by a low dose (2 mg/kg i.p.) of the dopamine precursor L-DOPA (L-3, 4-dihydroxyphenylalanine). Expression of c-fos as measured by Fos-like immunoreactivity after SCH 58261 plus L-DOPA was also potentiated as compared with L-DOPA alone, both in striatum and globus pallidus of the 6-hydroxydopamine-lesioned side of the brain. SCH 58261 induced a less marked potentiation (7-fold) of turning behaviour induced by dopamine D2 receptor stimulation with quinpirole, while Fos-like immunoreactivity in the striatum and globus pallidus was not affected. Previous studies have shown that SCH 58261 strongly potentiated dopamine D1 receptor-mediated responses. The results of the present study therefore indicate that the positive interaction between SCH 58261 and L-DOPA, in 6-hydroxydopamine-lesioned rats, is mainly due to an interaction with dopamine D1 receptors. The data also suggest that adenosine A2A receptor antagonists might be useful for potentiating the effects of L-DOPA in Parkinson's disease.

In rats with a unilateral 6-hydroxydopamine lesion of the ascending dopamine neurons, we investigated the relationship between the expression of Fos-like immunoreactivity in the caudate-putamen and contralateral turning behavior in response to dopamine agonists during the induction and expression of sensitization (priming) to D1-dependent turning behavior. Priming was induced by apomorphine (0.1 mg/kg s.c.) or by SKF 38393 (10 mg/kg s.c.) 14 days after 6-hydroxydopamine lesions and was expressed by challenge with SKF 38393 (3 mg/kg s.c.). In the induction phase of priming, administration of MK 801 (0.1 mg/kg s.c.) potentiated contralateral turning but differentially influenced stimulation of Fos expression in the caudate-putamen by apomorphine and by SKF 38393. Thus, MK 801 reduced in the expression phase of priming the stimulation of Fos expression by apomorphine in the dorsolateral caudate-putamen, but did not affect that by SKF 38393. MK 801, while preventing priming of SKF 38393-induced turning by apomorphine, failed to affect priming by SKF 38393. MK 801, given with apomorphine in the induction phase, reduced the stimulation of Fos expression in the dorsolateral caudate-putamen by SKF 38393. No such inhibitory effect of MK 801 on SKF 38393-stimulated Fos expression was observed in rats primed with SKF 38393. These results are consistent with the possibility that MK 801 disrupts sensitization of D1 transduction by reducing the activation of c-fos by the DA agonist during the induction phase of priming.

In order to study the possible contribution of the substantia nigra (SN) in the positive interaction between dopamine D1 receptor agonists and glutamate antagonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, the effect of the D1 agonist, SKF 38393, was studied in combination with intranigral infusions of glutamate antagonists of the NMDA (MK 801, CPP) or AMPA (NBQX) type of receptor. Local infusion into the SN of the 6-OHDA lesioned side of MK 801, CPP or NBQX at doses inducing no or minimal behavioral effects significantly increased the turning behavior and the expression of c-fos induced, in the lesioned caudate-putamen (CPu), by a parenteral administration of SKF 38393. The same result was obtained after intra-SN infusion of the GABA agonist, muscimol. High doses of MK 801, CPP or muscimol infused into the SN produced intense contralateral turning per se and induced a sparse c-fos expression in the lesioned CPu which was antagonized by parenteral administration of MK 801. The results indicate that a depression of SN pars reticulata efferent neurons potentiates D1-mediated responses and suggest that this area may play a role in the positive interaction between glutamate antagonists and D1 receptor agonists. © 1995 Wiley-Liss, Inc.

In rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigro-striatal pathway, striatal Dl-receptor-stimulated c-fos expression and turning behavior are positively modulated by D2 receptor stimulation and by blockade of N-methyl-D-aspartate (NMDA) or muscarinic receptors. Combined D1/D2 receptor stimulation by L-dopa activates c-fos in a manner not additive with muscarinic receptor blockade by scopolamine. On the other hand, blockade of NMDA receptors by MK 801 reduced c-fos expression induced by L-dopa while, depending on the dose of L-dopa, differentially affecting contralateral turning behavior. The results are interpreted to suggest that D2 receptor stimulation amplifies D1-receptor-mediated c-fos expression by two mechanisms differentially related to muscarinic and NMDA receptors. ©1994 Wilev-Lisa. Inc.

In rats bearing a unilateral 6-hydroxydopamine lesion of the dopaminergic nigro-striatal pathway, stimulation of adenosine A2 receptors by CGS 21680 reduced the contralateral turning behavior induced by L-3,4-dihydroxyphenylalanine (L-DOPA). Administration of CGS 21680 completely blocked the contralateral turning induced by the dopamine D1 receptor agonist, SKF 38393, and reduced the turning induced by the dopamine D2 receptor agonist, LY 171555. Quinolinic acid lesion of the striatum or 6-hydroxydopamine lesion of the dopaminergic nigro-striatal neurons demonstrated that [3H]CGS 21680 binding sites are associated to striatal intrinsic neurons. This study provides evidence for a negative postsynaptic interaction of both dopamine D1 and D2 receptors with adenosine A2 receptors.

In rats with unilateral lesion of the nigrostriatal dopaminergic pathway, L-DOPA induces contralateral turning through activation of denervated D-1 and D-2 receptors. Blockade of N-methyl-D-aspartate (NMDA) receptors by the non-competitive antagonist (+)MK-801, potentiated the contralateral turning induced by L-DOPA as well as that induced by the D-1 agonist SKF 38393, while D-2 mediated turning was almost completely inhibited. Administration of the D-1 antagonist SCH 23390 blocked (+)MK-801-induced potentiation of L-DOPA contralateral turning, confirming the D-1 nature of the effects observed. Immunohistochemical studies on the early gene c-fos, which is known to be activated by stimulation of supersensitive D-1 receptors, revealed sparse c-fos positive nuclei in the lesioned CPu after SKF 38393, while after combined administration of (+)MK-801 and SKF 38393 dense labelling was obtained. Blockade of NMDA receptors, differentially affects D-1 and D-2 mediated turning behavior, suggesting that different neuronal pathways are involved in the mediation of D-1 and D-2 responses.

Preclinical evidence strongly indicate that adenosine A(2A) receptor antagonists represent a promising class of drugs for the treatment of motor deficits associated to Parkinson's disease. The effects of adenosine A(2A) receptor antagonists were here assessed in a rat model of parkinsonian tremor induced by cholinomimetic drugs by evaluating the counteraction of tremulous jaw movements. Systemic administration of the A(2A) antagonist SCH 58261 dose-dependently reduced the magnitude of perioral tremor induced by the acetylcholinesterase inhibitor tacrine (2.5 mg/kg). Furthermore, intrastriatal infusion of SCH BT2 (5 microg/microl), a water-soluble analogue of SCH 58261, antagonized tacrine-induced jaw movements with a maximal effect in the ventrolateral striatum. On the other hand, SCH 58261 (5 mg/kg) was ineffective in blocking tremulous jaw movements stimulated by the direct muscarinic agonist pilocarpine (1 mg/kg). Taken together, these results indicate that A(2A) antagonists reduce parkinsonian tremor stimulated in rats by tacrine and that the striatum is deeply involved in the observed effect. Moreover, the ineffectiveness of SCH 58261 in blocking pilocarpine-stimulated perioral tremor suggests that the antitremorigenic effects of A(2A) antagonists described here are not related to a direct action on muscarinic receptor. The prospective of providing additional antitremor benefits considerably enhances the therapeutic potential of A(2A) antagonists.

Rats learn to avoid palatable saccharin solutions that predict the systemic administration of reinforcing drugs as well as malaise-inducing lithium chloride (conditioned saccharin avoidance, CSA). In the present study the involvement of dopamine (DA) transmission in the acquisition of morphine, nicotine and lithium-conditioned CSA was investigated in a two-bottle choice paradigm. Nicotine tartrate (0.2 and 0.4 mg/kg s.c.) administered 15 min after saccharin presentation induced CSA, with a maximum effect at 0.4 mg/kg. The DA D1 receptor antagonist, SCH 39166 (0.1 mg/kg s.c.) and the DA D2 receptor antagonist raclopride (0.3 mg/kg s.c.), administered immediately after saccharin, prevented CSA induced by the lower but not by the higher dose of nicotine. However, combined administration of the two antagonists prevented CSA induced by the higher dose of nicotine. SCH 39166 prevented CSA induced by all morphine doses while raclopride prevented only CSA induced by the lowest dose of morphine (1.75 mg/kg). CSA induced by different doses of lithium given by the same schedule of drug-CSA (i.e. two pairings, 15 min after saccharin) was not affected by SCH 39166. However SCH 39166 impaired the acquisition of lithium-CSA when lithium was given 60 min after saccharin. In contrast, raclopride failed to affect lithium-CSA independently from the delay between saccharin and lithium. These results suggest that DA can play different roles in drug- and in lithium-CSA and are consistent with a different mechanism of drug- as compared to lithium-CSA.

In the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease, administration of a dopaminergic agonist sensitizes rats to a subsequent administration of dopaminergic drugs given days apart (priming). In situ hybridization was used to evaluate changes on striatal gene expression of rats primed three days previously with either L-dopa, SKF38393 or quinpirole. Double labeling was used to identify the neuronal population in which such alterations occurred. GAD67 and enkephalin mRNA were increased by the lesion whereas dynorphin mRNA was decreased as compared to the intact striatum. Priming with L-dopa and SKF38393 significantly increased GAD67 mRNA in the lesioned striatum and reversed dynorphin mRNA reduction, as compared to drug-naive rats, whereas quinpirole failed to produce any effect. Enkephalin mRNA was not affected by priming. Results suggest that 6-OHDA lesion-induced adaptive changes on striatal gene expression are modified by priming. Priming brings striatal output neurons to a higher level of activity, which may explain the sensitized behavioral response observed following a dopaminergic agonist challenge. These changes are in relation to the different types of dopamine agonists utilized and suggest that modifications in gene expression induced by priming might be predictive of the dyskinetic potential of a drug.

In rats lesioned with 6-hydroxydopamine (6-OHDA) the effect of the noncompetitive N -methyl d -aspartate (NMDA) receptor antagonist, MK-801, the dopamine (DA) D 2 receptor agonist quinpirole and the A 2A adenosine antagonist SCH 58261 was studied on acetylcholine (ACh) release in the lesioned striatum and contralateral turning behaviour stimulated by the administration of the DA D 1 receptor agonist CY 208-243. Administration of CY 208-243 (75, 100 and 200 μg/kg) to 6-OHDA-lesioned rats dose-dependently stimulated ACh release and induced contralateral turning. MK-801 (50 and 100 μg/kg) reduced basal ACh release (max 22%) and did not elicit any turning. MK-801 (50 and 100 μg/kg) potentiated the contralateral turning, but failed to modify the stimulation of ACh release elicited by 100 and 200 μg/kg of CY 208-243. MK-801 (100 μg/kg) prevented the increase in striatal ACh release evoked by the lower dose of CY 208-243 (75 μg/kg) but contralateral turning was not observed. The D 2 receptor agonist quinpirole (30 and 60 μg/kg) elicited low-intensity contralateral turning and decreased basal ACh release. Quinpirole potentiated the D 1 -mediated contralateral turning behaviour elicited by CY 208-243 (100 μg/kg), but failed to affect the increase in ACh release elicited by the D 1 agonist. The adenosine A 2A receptor antagonist SCH 58261 (1 mg/kg i.v.) failed per se to elicit contralateral turning behaviour. SCH 58261 potentiated the contraversive turning induced by CY 208-243 but failed to affect the increase of ACh release. The results of the present study indicate that blockade of NMDA receptors by MK-801, stimulation of DA D 2 receptors by quinpirole and blockade of adenosine A 2A receptors by SCH 58261 potentiate the D 1 -mediated contralateral turning behaviour in DA denervated rats without affecting the action of the D 1 agonist on ACh release. These observations do not support the hypothesis that the potentiation of D 1 -dependent contralateral turning by MK-801, quinpirole or SCH 58261 is mediated by changes in D 1 -stimulated release of ACh in the striatum.

In rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway, blockade of muscarinic receptors by scopolamine potentiates the contralateral turning induced by selective dopaminergic D1 agonists (SKF 38393, A 68930), but does not influence the contralateral turning induced by the D2 agonist LY 171555. Studies on the expression of the early gene c-fos as reflected by the immunohistochemical demonstration of the Fos protein, show that administration of scopolamine (5 mg/kg, i.p.) potentiates c-fos expression elicited by SKF 38393 (1.5 mg/kg, s.c.) in the caudate-putamen of the lesioned side. The results indicate that cholinergic transmission is differentially involved in the behavioral expression of D1 versus D2 receptor stimulation in a denervated condition and suggest that blockade of central cholinergic transmission might be useful in improving the antiparkinsonian efficacy of D1 receptor agonists.

The non competitive antagonist of N-methyl-D-aspartate (NMDA) receptors MK-801, at doses which did not produce any behavioural modification per se, increased by about 40% the contralateral turning induced by the dopaminergic (DA) D-1/D-2 agonist L-dopa in rats bearing a unilateral lesion of the DA nigro striatal neurons. Administration of MK-801 in combination with selective agonists of the D-1 (SKF 38393) or D-2 (LY 171555) receptor, induced an increase of about 280% of D-1 mediated turning and a 70% decrease of D-2 mediated turning. Analysis of the potentiation of L-dopa mediated turning by MK-801 after selective D-1 receptor blockade by SCH 23390, revealed that the increase of turning was related to the stimulation of D-1 receptors by L-dopa. Immunocytochemical visualization of the proto-oncogene c-fos in the caudate-putamen (CPu) of lesioned rats, revealed a sparse c-fos positive nuclei in the lesioned CPu of rats treated with SKF 38393 alone, while after combined administration of MK-801 and SKF 38393 dense labelling of nuclei was found. The results indicate that blockade of NMDA receptors by MK-801 acts synergistically with D-1 agonists in the induction of turning after DA denervation and shows that these changes are correlated with c-fos induction in specific areas of the CPu.

In drug-naive rats unilaterally lesioned with 6-hydroxydopamine (6-OHDA) from 17 days, stimulation of D-1 receptors by SKF 38393 (2 mg/kg s.c.) failed to induce contralateral turning. Administration, three days before, of a single dose of a dopaminergic agonist which elicited contralateral turning, made SKF 38393 very active in producing contralateral turning (priming). The effectiveness of the D-1/D-2 agonist apomorphine as a primer of SKF 38393-induced turning was critically dependent on the interval between the administration of the two agonists. Effectiveness was minimal after 3 h, increased after 6–12 h, peaked at 72 h and was reduced after 10 days. In drug-naive 6-OHDA lesioned rats, administration of the selective antagonists SCH 23390 (D-1) or raclopride (D-2) abolished apomorphine induced contralateral turning, while in primed rats both antagonists only modified the pattern of apomorphine turning but failed to abolish it. Analysis of D1 receptor binding in striata of drug-naive and primed rats, showed no change in the Bmax and Kd, while dopamine stimulated adenylate cyclase showed a decreased Km, for dopamine in the lesioned side after priming. Finally, administration of the N-Methyl-D-Aspartate (NMDA) receptor antagonist (+) MK 801 in conjunction with apomorphine, prevented the ability of apomorphine to act as a primer, indicating that the NMDA receptor exerts a permissive role on priming.

Previous exposure to a dopaminergic agonist (priming) strongly potentiates contralateral turning behaviour in response to D 1 and D 2 agonists in unilaterally 6-hydroxydopamine-lesioned rats. In order to study the influence of priming on the behavioural interaction of D 1 and D 2 receptors, we examined the effect of selective D 1 and D 2 receptor blockade on the contralateral turning induced by the mixed D 1 /D 2 agonist apomorphine in drug-naive and primed 6-hydroxydopamine-lesioned rats. In drug-naive rats, apomorphine induced a dose-related, apparently monophasic rotation curve. Administration of selective D 1 (SCH 23390) or D 2 (raclopride) antagonists abolished the contralateral turning induced by 0.1 mg/kg of apomorphine and partially inhibited that induced by 0.5 mg/kg. In primed rats low doses of apomorphine (0.05 mg/kg) induced an apparently monophasic contralateral turning which was reduced by D 1 receptor blockade and completely abolished by D 2 receptor blockade; a higher dose of apomorphine (0.1 mg/kg) instead elicited a biphasic (two-peak) pattern of rotation. After this dose of the agonist, blockade of D 1 or D 2 receptors abolished the second peak of rotation but, while D 1 blockade reduced the total number of turns, D 2 blockade failed to do so. Quantitative analysis of the interaction between D 1 and D 2 receptors in the overall turning effect, as well as in the time-course of turning behaviour, indicates that D 1 and D 2 receptors interact not only positively but also negatively. After higher doses of apomorphine, both negative and positive interactions take place sequentially during the time-course of apomorphine action and provide a clue for explaining the two-peak pattern of rotation observed after apomorphine in rats previously exposed to the drug.

The D-1 agonist SKF 38393 (2 mg/kg s.c.) failed to induce contralateral turning in drug-naive rats, lesioned unilaterally with 6-hydroxydopamine (6-OHDA) from 17 days, while elicited intense contralateral turning 90 days post lesion. Priming with a single administration of a dopaminergic (DA) agonist which elicited contralateral turning by itself, made SKF 38393 very active in producing contralateral turning in rats, lesioned with 6-OHDA from 17 days. The effectiveness of the D-1/D-2 agonist apomorphine as a primer of SKF 39393 induced turning was critically dependent on the interval between the administration of the two agonists. Effectiveness was minimal with an interval of 3 h, increased after 6–12 h, peaking at 72 h and was reduced after 10 days. Priming took place also when the primer and SKF 38393 were administered in different environment, indicating that priming in not dependent from behavioral conditioning. Finally, administration of the N-Methyl-D-Aspartate (NMDA) receptor antagonist (+) MK 801 in conjunction with apomorphine prevented its ability to act as a primer, indicating that the NMDA receptors exert a permissive role on priming.

SKF 38393 (2 mg/kg s.c.), a reportedly selective D-1 agonist, failed to induce contralateral turning behaviour in naive rats bearing 12 days old unilateral 6-hydroxydopamine lesions. On the other hand strong contraversive turning in response to SKF 38393 was obtained if rats had been tested 2 or 7 days before with apomorphine (0.1 mg/kg s.c.) or with LY 171555 (0.2 mg/kg s.c.), a selective D-2 receptor agonist. Contraversive turning in response to SKF 38393 was blocked by a low dose (0.05 mg/kg s.c.) of the specific D-1 antagonist SCH 23390. The results indicate that the behavioural expression of D-1 receptor supersensitivity following lesion of dopaminergic neurons depends on previous exposure to a stimulation of D-2 receptors.

Opioid agonists having different affinity for delta and mu receptors were injected bilaterally in the substantia nigra (SN) of rats. The selective agonist of mu receptors N-MePhe3,-D-Pro4 morphiceptin (PLO 17) produced a stereotyped behavior characterized by stereotyped sniffing and gnawing antagonized by the irreversible antagonist of mu receptors beta-funaltrexamine. In contrast, bilateral intranigral injection of the selective delta agonist D-Pen2,D-Pen5 enkephalin (DPDPE) elicited dose-dependent exploratory behavior and rearing but failed to produce gnawing. The behavioral syndrome induced by DPDPE was significantly reduced by the selective delta antagonist ICI 174,864. Naloxine, a non-selective opioid antagonist, antagonized the effects of both compounds. SCH 23390 and haloperidol, two antagonists of dopaminergic D1 and D2 receptors, respectively, blocked the effects of PLO 17 and DPDPE. The results indicate that stimulation of specific opioid receptor types in the SN elicits specific behavioral syndromes and suggest that the SN might be the site of origin of certain items of the behavioral syndrome evoked by systemic opiates. These items might be mediated by activation of dopaminergic neurons of the ventral mesencephalon.

The D-1 receptor agonist, SKF 38393 (2 mg/kg s.c), failed to elicit contralateral turning when administered to drug-naive rats 17 days after unilateral 6-hydroxydopamine (6-OHDA) lesioning of the medial forebrain bundle, while it elicited intense contralateral turning 90 days post-lesioning. On the other hand the D-1/D-2 receptor agonist, apomorphine (0.1 mg/kg s.c.), induced contralateral turning in drug-naive rats lesioned 14 days earlier and made the administration of SKF 38393 (2 mg/kg s.c.) 3 days later effective to evoke contralateral turning (‘priming’). The effectiveness of apomorphine as a primer of SKF 38393-induced turning depended critically on the interval between the administration of the two agonists. Effectiveness was minimal after 3 h and increased after 6–12 h, peaked at 72 h and was reduced after 10 days. The D-2 receptor agonist, LY 171555 (0.2 mg/kg s.c.), was also effective as a primer of SKF 38393-induced contralateral turning and this effect also was dependent upon the interval between priming and SKF 38393 administration. Moreover, priming was dependent on the dose of drug used as primer and on the dose of SKF 38393 used as a challenge. In contrast to SKF 38393, priming was unable to make effective a dose of LY 171555 that was ineffective in drug-naive rats, suggesting that LY 171555 affects D-1-dependent turning to a greater extent than D-2-dependent turning. The results indicate that the priming phenomenon is rather strictly time- and dose-dependent.

Contralateral turning can be evoked in rats with unilateral lesion of the nigro-striatal dopaminergic projection by the administra, tion of drugs which stimulate DA transmission. Two types of DA-receptors are currently distinguished (Garau et al., 1978; Kebabian and Calne, 1979); the D-1 type, stimulated preferentially by SKF 38393 and blocked by SCH 23390 and the D-2 type, stimulated, among other drugs, by LY 171555 and specifically blocked by various substituted benzamides. In unilaterally 6-hydroxydopamine (6-OHDA)-denervated rats, supersensitivity appears to develop specifically for each DA-receptor type both biochemically and behaviourally (Creese et al., 1977; Arnt and Hyttel, 1985). We have recently found that previous stimulation of D-2 receptors exerts a ‘priming’ effect on the ability of D-1 receptor stimulation to induce contraversive turning in the unilaterally DA-denervated rat (Morelli et al., 1987), indicating that the behavioural expression of D-1 receptor supersensitivity depends on previous stimulation of D-2 receptors. We now present evidence that the D-1 and D-2 receptor stimulation influence the expression of D-1 and D-2 dependent behavioural supersensitivity.

In adult rats bearing unilateral 6-hydroxydopamine (6-OHDA) lesions of the ascending dopaminergic neurons, a single administration of a dopamine (DA) receptor agonist results in strong sensitization ("priming") of contralateral turning in response to D2 and particularly D1 receptor agonists. In order to investigate the role of distinct environmental cues associated with the effect of the agonist during exposure to the primer, rats bearing 15-day-old unilateral 6-OHDA lesions were primed in their home cage with L-dopa or with saline. L-Dopa but not saline induced medium to low but steady contralateral turning. Three days later, challenge with the D1 agonist SKF 38393 in the home cage also resulted in contralateral turning in the rats previously primed with L-dopa, but not in those primed with saline. In a second experiment rats lesioned with 6-OHDA were primed in two different contexts (hemispheres versus cylinders) with a single administration of the D2/D3 agonist quinpirole (LY 171555: 0.2mg/kg s.c.) or saline. Three days later the rats were placed in hemispheres and tested for contraversive turning in response to saline or to SKF 38393. SKF 38393 elicited high rate contraversive turning independently of the environment where priming with quinpirole took place; on the other hand no conditioned contraversive turning was observed after saline. In a third experiment, the possibility of priming SKF 38393-induced turning by stimulation of nigral or striatal DA receptors was investigated. Rats lesioned unilaterally with 6-OHDA were locally infused on the lesioned side in the substantia nigra with SKF 38393 or in the striatum with quinpirole. Both these treatments elicited contralateral turning, the intranigral injection of SKF 38393 eliciting a stronger and longer lasting contraversive turning than intrastriatal quinpirole. Challenge with SKF 38393 (3mg/kg s.c.) 3 days later induced contralateral turning only in rats previously primed with intrastriatal quinpirole. The results of these studies are consistent with the idea that "priming" is an example of non-associative sensitization induced by stimulation of denervated striatal DA receptors and expressed as an increased efficiency of post-synaptic dopaminergic transduction in the striatum.