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We found that both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate autoreceptors were present on the glutamate-releasing terminals of cerebellar parallel/climbing fibers and that they functioned as facilitatory autoreceptors. Extracellular cGMP inhibited the neurotransmitter release evoked by presynaptic kainate receptor activation; the inhibitory effect of extracellular cGMP was selective for the kainate autoreceptor-mediated response and did not affect the AMPA autoreceptor-mediated response. Endogenously synthesized cGMP might be the physiological source for the extracellular cGMP modulating the response to kainate autoreceptor activation.

We studied the pharmacological characterization of the 5-hydroxytryptamine2(5-HT2) heteroreceptor located on glutamatergic cerebellar mossy fiber nerve terminals. Depolarization-evoked overflow of endogenous glutamate from rat cerebellar “giant” mossy fiber synaptosomes was inhibited by 5-HT or (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI], exhibiting pD2(= −log EC50) values of 7.37 and 7.29, respectively. Trazodone inhibited the depolarization-evoked glutamate overflow, exhibiting lower potency (pD2= 6.42) and lower efficacy with respect to 5-HT or (±)-DOI (maximal inhibition, 54%, compared with 70% for either 5-HT or (±)-DOI). Ketanserin, a 5-HT2A/5-HT2Creceptor antagonist, counteracted the inhibitory effect of (±)-DOI or trazodone. Inhibition of glutamate overflow by 5-HT, (±)-DOI, or trazodone was prevented by the selective 5-HT2Areceptor antagonistR-(+)-α-(2,3-dimethyoxyphenyl)-1-(2-(4-fluorophenyl)ethyl)-4-piperidine-methanol (MDL 100907), while the potent and selective 5-HT2Creceptor antagonist 6-chloro-5-methyl-1-[6-(methylpyridin-3-yloxy)pyridin-3yl-carbamoyl] indoline (SB 242084) was ineffective. In cerebellar slices, MDL 100907 increased on its own the K+-evoked release of glutamate. It is concluded that the evoked release of glutamate from cerebellar mossy fibers can be controlled by inhibitory presynaptic 5-HT2Aheteroreceptors, the receptors can be activated by endogenously released 5-HT, and trazodone behaves as a partial agonist at these receptors.

The NMDA receptor/nitric oxide (NO)/cyclic GMP pathway and its modulation by 5‐hydroxytryptamine (5‐HT) was studied in slices of neocortical samples obtained from patients undergoing neurosurgery.The cyclic GMP elevation produced by 100 μMNMDA was blocked by 100 μMof the NO synthase inhibitor NG‐nitro‐L‐arginine (L‐NOARG) or by 10 μMof the soluble guanylate cyclase inhibitor 1H‐[1,2,4]oxadiazolo[4,3,‐α] quinoxaline‐1‐one (ODQ).The NMDA effect was prevented by 5‐HT or by the 5‐HT2agonist (±)‐1‐(2,5‐dimethoxy‐4‐iodophenyl)‐2‐aminopropane ((±)‐DOI; EC50=22 nM). The (±)‐DOI inhibition was insensitive to the 5‐HT2Areceptor antagonist MDL 100907 or the 5‐HT2Bantagonist rauwolscine; it was largely prevented by 1 μMof the non‐selective 5‐HT2Cantagonists mesulergine (5‐HT2A,B,C), ketanserin (5‐HT2A,C) or SB 200646A (5‐HT2B,C); it was completely abolished by 0.1 μMof the selective 5‐HT2Creceptor antagonist SB 242084.The NMDA‐induced cyclic GMP elevation also was potently inhibited by the selective 5‐HT2Cagonist RO 60‐0175 and by the antidepressant trazodone, both added at 1 μM, in an SB 242084‐sensitive manner.Finally, the 5‐HT1Aagonist 8‐hydroxy‐2‐(di‐n‐propylamino) tetralin (8‐OH‐DPAT; 1 μM) inhibited the NMDA‐evoked cyclic GMP response, an effect blocked by the selective 5‐HT1Areceptor antagonist WAY 100635.In conclusion, the NMDA receptor/NO/cyclic GMP pathway in human neocortex slices can be potently inhibited by activation of 5‐HT2Cor 5‐HT1Areceptors.

The pharmacological profile of the γ-aminobutyric acid (GABA)Breceptor regulating cholinergic twitch contraction in the guinea pig ileum myenteric plexus-longitudinal muscle preparation was investigated. GABA and (−)-baclofen inhibited the contraction, exhibiting quite close potencies (pD2for GABA = 5.70; pD2for (−)-baclofen = 5.33). The compound CGP 47656 also reduced the cholinergic twitch concentration (pD2= 5.42), but its efficacy was significantly lower than that of (−)-baclofen or GABA. Added at varying concentrations, CGP 47656 modified the concentration-response curve of (−)-baclofen as expected for a partial agonist. Phaclofen, CGP 36742, CGP 35348, and CGP 52432 behaved as competitive antagonists of (−)-baclofen, exhibiting the following pA2values: 3.90, 4.88, 5.02, and 7.82, respectively. The compound CGP 56999 behaved as a potent noncompetitive GABABreceptor antagonist. In comparing the pharmacological profile of the ileal receptor with those of the previously characterized pharmacological subtypes of the GABABreceptor present in the central nervous system, it can be seen that the GABABreceptor inhibiting cholinergic twitch contraction in guinea pig ileum myenteric plexus-longitudinal muscle mostly resembles the receptor located on somatostatin human neocortex nerve terminals.

The releases of [3H]5‐hydroxytryptamine ([3H]5‐HT) and of endogenous glutamic acid and their modulation through presynaptic h5‐HT1Bautoreceptors and h5‐HT1Dheteroreceptors have been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery.The inhibition by 5‐HT of the K+(15 mM)‐evoked overflow of [3H]5‐HT was antagonized by the 5‐HT1B/5‐HT1Dreceptor ligand GR 127935, which was ineffective on its own; this drug was previously found to behave as a full agonist at the h5‐HT1Dheteroreceptor regulating glutamate release.The recently proposed selective h5‐HT1Breceptor ligand SB‐224289 also prevented the effect of 5‐HT at the autoreceptor, being inactive on its own; in contrast, SB‐224289, at 1 μM, was unable to interact with the h5‐HT1Dheteroreceptor.The inhibitory effect of 5‐HT on the K+‐evoked overflow of glutamate was antagonized by the h5‐HT1Dreceptor ligand BRL‐15572; added in the absence of 5‐HT the compound was without effect. BRL‐15572 (1 μM) was unable to modify the effect of 5‐HT at the autoreceptor regulating [3H]5‐HT release.The selective 5‐HT1Areceptor antagonist (+)‐WAY 100135, previously found to be an agonist at the h5‐HT1Dheteroreceptor regulating glutamate release, could not interact with the h5‐HT1Bautoreceptor when added at 1 μM.It is concluded that native h5‐HT1Band h5‐HT1Dreceptors exhibit a hitherto unexpected pharmacological diversity.

The effects of trazodone on the cyclic GMP elevation elicited by N-methyl-d-aspartate in rat cerebellar slices were analyzed. Trazodone inhibited in a concentration-dependent manner (EC50= 0.82 nM) the cyclic GMP response evoked by 0.1 μM N-methyl-d-aspartate. The inhibition was near complete at 10 nM trazodone. The effect of 10 nM trazodone was unaffected by 0.3 μM spiperone or rauwolscine, antagonists with selectivity for the 5-HT(serotonin)2Aor the 5-HT2Bsubtype, respectively, but it was totally prevented by 0.01 μM mesulergine, a 5-HT2A/5-HT2B/5-HT2Creceptor antagonist. Trazodone was potently counteracted (IC50= 2.7 nM) by the selective 5-HT2B/5-HT2Creceptor antagonist N-(1-methyl-5-indolyl)-N-(3-pyridil) urea HCl and, less potently (IC50= 95 nM), by ketanserin, a 5-HT2A/5-HT2Creceptor blocker. It is concluded that trazodone behaves as a potent full agonist at the 5-HT2Creceptor mediating inhibition of the cerebellar N-methyl-d-aspartate/nitric oxide/cyclic GMP system.

In a previous investigation it was suggested that the a2-adrenoceptors regulating 3H-noradrenaline (3H-NA) release and the a2-heteroreceptors regulating the release of 3H-5-hydroxytryptamine (3H-5-HT) from rat cerebrocortex synaptosomes represent different subtypes of the a2-adrenoceptor in that (-)-mianserin potently blocked the receptors sited on 5-HT terminals but was ineffective at the autoreceptors (Raiteri et al. 1983).

The interaction between clonidine, as an agonist at the a2-autoreceptors regulating noradrenaline release, and inhibitors of noradrenaline neuronal uptake was investigated in superfused synaptosomes, i.e. in conditions preventing accumulation of the released transmitter in the vicinity of presynaptic autoreceptors. Desipramine or cocaine did not decrease the inhibitory action of clonidine on the release of 3H-noradrenaline evoked by 15 mM KCl from rat cortex synaptosomes, even when the concentration ratio between uptake inhibitor and clonidine was very high. The present results do not support either the hypothesis of an interaction between imidazolines and noradrenaline uptake inhibitors at the level of a2-autoreceptors, or that of a functional coupling between presynaptic a2-autoreceptors and noradrenaline uptake mechanism.

The effects of 5-hydroxytryptamine (5-HT) and of a number of 5-HT receptor agonists and antagonists on the release of endogenous aspartate were investigated in rat cerebellum slices and synaptosomes depolarized with high K+. The release of endogenous aspartate evoked from slices by 35 mmol/l KCl and from synaptosomes by 15 mmol/1 KCl was strongly (about 90%) calcium-dependent. In slices the release of aspartate was inhibited by exogenous 5-HT (0.1–100 nmol/1) in a concentration-dependent manner. The indoleamine was very potent, producing 30% inhibition at 0.1 nmol/l. The effect of 10 nmol/1 5-HT was partly but maximally counteracted by ketanserin (300–1000 nmol/1), a 5-HT2 receptor antagonist, but fully blocked by 300 nmol/1 of the mixed 5-HT1/5-HT2 receptor antagonist methiothepin. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) inhibited the K+-evoked release of endogenous aspartate in a concentration-dependent manner. The effect of 8-OH-DPAT was antagonized by methiothepin, but not by ketanserin which fully antagonized the inhibition produced by DOI. In cerebellar synaptosomes the release of endogenous aspartate evoked by 15 mmol/l K+ was inhibited by exogenous 5-HT and by 8-OH-DPAT, but not by DOI. Methiothepin (100–300 nmol/1) antagonized the inhibitory effects of 100 nmol/l 5-HT or 8-OH-DPAT. However, 1000 nmol/l of various 5-HT receptor antagonists [ketanserin, methysergide, (-)-propranolol, spiperone or ICS 205–930] did not counteract the effect of 100 nmol/15-HT.