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Equilibrium models are derived and applied to in vivo binding of spiperone in the rat brain. The models express the concentration of the ligand in the striatum and frontal cortex as a function of the accumulation in the cerebellum. The models differ with respect to the description of specific binding. Nonlinear regression analysis shows that the in vivo specific binding of 3H-labeled spiperone in the frontal cortex (mainly serotonergic) can be described by a noninteracting sites model, whereas the specific binding in the striatum (mainly dopaminergic) can best be described by models that lead to sigmoid saturation curves. These results were tested and partly confirmed by determining the region-of-interest/cerebellar radioactivity ratio of 11C-labeled N-methylspiperone, with and without pretreatment with haloperidol. The estimated Bmax was 32 fmol/mg wet tissue in the frontal cortex and approximately 90 fmol/mg wet tissue in the striatum. The free plus nonspecific binding of spiperone was similar in the frontal cortex but lower in the striatum than in the cerebellum. The occurrence of sigmoidicity can be best explained by the existence of high-affinity/low-capacity sites in the cerebellum rather than mutual interactions of striatal sites. The consequence of the present analysis for positron emission tomography is that the striatal/cerebellar activity ratio is not an accurate parameter of specific binding features at tracer doses of spiperone or N-methylspiperone.

The potential of in vivo spiperone binding as a tool for the detection and quantitative analysis of striatal dopamine (DA) receptor alterations was studied in rat brain lesioned in several ways. Two weeks after kainate (KA) injection a significantly higher radioactivity accumulation was observed in the lesioned striatum than in the contralateral structure after a tracer dose of [3H]spiperone. The difference was maximal 2 days after surgery and it was present for at least 4 weeks while it was reversed 11 weeks after KA injection. The radioactivity uptake (tracer dose of [3H]spiperone) measured 2 weeks after surgery could be specifically prevented in both KA-lesioned and contralateral striatum by haloperidol and N-n-propylnorapomorphine while non-dopaminergic drugs were almost without effect. More than 80% of the radioactivity accumulation was saturable in both contralateral (unlesioned) and KA-lesioned striatum, leaving a slightly higher non-saturable radioactivity level in the latter. One week after unilateral ablation of the cerebral cortex overlying the striatum only minor bilateral differences in striatal radioactivity content were found after a tracer dose of [3H]spiperone. No differences were present after 6-OHDA lesion of the nigrostriatal pathway. Striatal DA receptor densities (Bmax) were determined from the dose-dependency of total striatal spiperone accumulation. This relationship was assessed using cerebellar spiperone accumulation instead of dose. Thus a Bmax of about 75 fmol X mg-1 tissue was found in the striatum of control (unoperated) rats and contralateral to the striatal KA lesion while 2 weeks after surgery it was approximately 33 fmol X mg-1 in the KA-lesioned striatum. One week after unilateral decortication Bmax values of about 50 and 65 fmol X mg-1 were found ipsi- and contralaterally to the lesion respectively.

The histamine H3 receptors were initially identified as presynaptic autoreceptors in the brain. However, recent research described here by Jan van der Werf and Hendrik Timmerman demonstrates that H3 receptors are associated with multiple functions, and their location is not confined to the CNS.

Specific tracer and maximal specific binding (Bmax) were determined in rat brain regions from radioactivity accumulation after intravenous administration of 3H N-n-propylnorapomorphine (NPA) or 3H spiperone at various specific activities. With NPA the highest Bmax-values (expressed in pmol.g-1 tissue) were found in the striatum (26 pmol.g-1) nucleus accumbens (about 27 pmol.g-1) and the olfactory tubercle (11 pmol.g-1). Saturable NPA binding was also found in the amygdaloid complex, medulla oblongata and inferior colliculi, but not in the frontal cortex. Bmax values for spiperone were high in the striatum (73 pmol.g-1), the nucleus accumbens (48 pmol.g-1), the olfactory tubercle (34 pmol.g-1) and the frontal cortex (18 pmol.g-1). A similar order was found for the tracer contents in these regions. There was no linear relationship between these contents and Bmax values. The possible implications of these findings and usefulness of NPA for brain imaging are discussed.

The accumulation and retention of radioactivity in rat brain were studied after intravenous injection of the dopamine (DA) agonist [3H]N-n-propylnorapomorphine ( [3H]NPA). Dose-dependent saturable accumulation of label was found in the striatum, nucleus accumbens and olfactory tubercle. DA agonists (apomorphine, N,N-dipropyl-5,6-ADTN) and antagonists (haloperidol, cis-flupenthixol) prevented this accumulation. Enhanced accumulation of radioactivity in the striatum was found after 6-OHDA lesions and short- and long-term treatment with reserpine. These results are an indication of specific NPA binding to presumably postsynaptically situated DA receptors. One hour after administration of the drug, the effect of NPA on striatal DA metabolism was not correlated with receptor saturation. Maximal numbers of in vivo NPA binding sites (about 30 and 22 pmol . g-1) in striatal tissue were calculated from independent measurements at 15 and 60 min after NPA injection. Regional distribution of radioactivity after a tracer dose of [3H]NPA was assessed in 35 brain areas and parts of the spinal cord. In addition to the already mentioned DA-rich areas receptor-specific NPA binding was also found in several other brain parts.

A method for the rapid production and purification of 11C-labelled N-alkylated derivatives of the dopamine agonist 2-amino-6,7-dihydroxytetralin (ADTN) is described. The label is introduced by N-methylation with no-carrier-added 11CH3I of the corresponding secondary amines via their lithium salts. Following systemic injection in rats a uniform distribution of radioactivity in the brain was found for both the labelled 2-(N-methyl-N-n-propylamino)- and 2-(N,N-dimethylamino)-6,7-dihydroxytetralin.

In an attempt to quantify the interactions between dopaminergic and cholinergic processes, the consequences of complete or partial activation (with N-n-propylnorapomorphine) or blockade (with spiperone) of dopamine receptors for the acetylcholine levels in the rat striatum were studied. The number of specific striatal binding sites (receptors) of spiperone was nearly three times that of N-n-propylnorapomorphine (76 and 26 pmol g-1 wet weight, respectively). The agonist produced a significant increase in the striatal levels of acetylcholine, but there was no simple relationship between receptor binding and these levels. A linear negative correlation was found between the striatal levels of acetylcholine and specific spiperone binding, showing that further receptor blockade induces a decrease in acetylcholine levels, which is independent of the receptors already occupied by the antagonist. The results of this study are evidence that one striatal dopamine receptor regulates the metabolism of at least 400 molecules of acetylcholine.

The inhibition of histamine release after depolarization of rat cerebral cortex slices by electrical stimulation and mediated by the postulated presynaptic autoreceptor (H3) depends strongly on the conditions of stimulation. Using electrically stimulated slices of the cortex a rightwards shift of the concentration-response curve of histamine (an H3 agonist) was observed on increasing the frequency of stimulation. The pA2 value of the H3 antagonist impromidine was, however, not altered at different stimulation frequencies; for a partial agonist only the maximal effect was influenced. These results indicate the existence of a receptor reserve at the H3 autoreceptor.

Slices of rat brain cortex previously loaded with [3H]histamine ([3H]HA) via de novo synthesis from [3H]histidine released tritiated histamine ([3H]HA) Ca2+ dependently in a superfused system. Both electrical field stimulation and high levels of K+ ions elicited this release. The extent of release depended on stimulation intensity. Rather strong stimuli, either by high frequency or longer stimulation, were required to elicit sufficient HA release for proper assessment of the concentration-dependence of release inhibition by drugs. The system showed marked depletion (less response per pulse) upon long-continued or successive stimulations. HA added to the superfusion medium inhibited the release evoked by stimulation at frequencies up to 10 Hz or with 30 mM K+ but not the release at higher frequencies or with 45 mM K+. The inhibition was mediated by H3 receptors, was concentration-dependent (pD2 = 7.4) and was complete at 10(-6) M. The H2 agonist impromidine antagonized the inhibition competitively (pA2 = 7.1). It is concluded that this assay in a superfusion system with electrical stimulation is suitable for the assessment of H3 receptor activity of drugs.

The neuronal localization of in vivo N-n-propylnorapomorphine (NPA) binding in the rat striatum was studied using 3 types of lesions. Striatal dopamine (DA) receptor densities (Bmax) were estimated from the relationships between total striatal and cerebellar NPA accumulation. A Bmax of 26.9 +/- 1.6 fmol X mg-1 wet weight tissue was found in the striata of non-lesioned (unoperated) rats. Similar values were obtained for striata with 6-hydroxydopamine-lesioned dopaminergic fibres. Kainate (KA)-lesioned striata contained 4.6 +/- 0.5 fmol X mg-1 saturable NPA binding sites. After unilateral decortication the receptor densities were in both striata resulting in ipsi- and contralateral Bmax values of 23 and 36 fmol X mg-1 respectively. With a tracer dose of [3H]NPA less radioactivity accumulated in the KA-lesioned striatum, while after unilateral destruction of the dopaminergic pathway more radioactivity was found in the ipsilateral striatum and no bilateral differences in striatal radioactivity concentration were found after unilateral cortical ablation. These observations show that all in vivo saturable striatal NPA binding sites are situated on striatal neurons and cortico-striatal afferents and that the effects of lesions on striatal DA receptor densities cannot be predicted from bilateral differences in the accumulation of tracer doses of [3H]NPA.