In vivo binding of N-n-propylnorapomorphine in the rat striatum: Quantification after lesions produced by kainate, 6-hydroxydopamine and decortication

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 (B_max) were estimated from the relationships between total striatal and cerebellar NPA accumulation. A B_max of 26.9 ± 1.6 fmol · 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 · mg^?1 saturable NPA binding sites. After unilateral decortication the receptor densities were altered in both striata resulting in ipsi- and contralateral B_max values of 23 and 36 fmol · mg^?1 respectively. With a tracer dose of [³H]NPA less radioactivity accumulated in the KA-lesioned striatum, while after unilateral destruction of the dompaminergic 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.     

Quantification of in vivo spiperone binding in the rat striatum after lesions produced by kainate or decortication

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.     

Dopamine receptor binding sites in the rat superior colliculus

Following intravenous administration of [3H]spiperone or [3H]N,n-propylnorapomorphine (NPA) to rats, radioactivity derived from the ligands accumulated in the striatum and superior colliculus when compared with cerebellar levels. The accumulation of [3H]spiperone in both areas was prevented by intraperitoneal administration of (+)-butaclamol, haloperidol and sulpiride but not by (-)-butaclamol, cinanserin, propranolol or prazosin. The accumulation of [3H]NPA was prevented by administration of (+)-butaclamol, haloperidol and apomorphine but not by (-)-butaclamol. In in-vitro experiments, membrane preparations from the superior colliculus showed a small number of specific binding sites for both [3H]spiperone and [3H]NPA. The dissociation constant (KD) for [3H]NPA was not different from that for striatal preparations but that for [3H]spiperone was 10-fold higher. We conclude that dopamine receptors may be present within the superior colliculus.     

Differential labelling of dopamine receptors in rat brain in vivo: Comparison of [3H]piribedil, [3H]S 3608 and [3H]N,n-propylnorapomorphine

The in vivo distribution of radioactivity in brain and labelling of cerebral dopamine receptors in rats derived from the administration of the atypical dopamine agonists [³H]piribedil and [³H]S 3608 has been compared to that of the classical dopamine agonist [³H]N,n-propylnorapomorphine (NPA). Radioactivity derived from [³H]piribedil accumulated in the substantia nigra, nucleus accumbens and cervical spinal cord, and this accumulation was prevented by administration of (+)-butaclamol and apomorphine only in substantia nigra and the nucleus accumbens. Radioactivity derived from [³H]S 3608 accumulated in the same areas and, additionally, in the frontal cortex and tuberculum olfactorium; this accumulation was prevented by administration of (+)-butaclamol and apomorphine only in substantia nigra, nucleus accumbens and tuberculum olfactorium. Neither ligand caused accumulation of radioactivity in the striatum. In contrast, radioactivity derived from [³H]NPA accumulated in the substantia nigra, nucleus accumbens, striatum, and tuberculum olfactorium. Radioactivity derived from [³H]NPA was prevented from accumulating in all these areas by (+)-butaclamol and by apomorphine, but piribedil only prevented accumulation in the substantia nigra and nucleus accumbens and S 3608 only prevented accumulation in substantia nigra, tuberculum olfactorium and nucleus accumbens. Neither piribedil nor S 3608 prevented accumulation of radioactivity derived from [³H]NPA in the striatum. Piribedil and S 3608 showed equal capacity in vitro to displace [³H]spiperone and [³H]NPA from striatal or nucleus accumbens tissue preparations. These results suggest that, in vivo piribedil and S 3608 selectively interact with dopamine receptors in the substantia nigra and nucleus accumbens, but not with those in striatum, perhaps due to differential distribution within brain.     

Regional in vivo binding of [3H]N-propylnorapomorphine in the mouse brain. Evidence for labelling of central dopamine receptors

Tain vein injections of [³H]N-propylnorapomorphine ([³H]NPA) in male mice resulted in a dose-related accumulation of radioactivity in the following brain regions: striatum (max), olfactory tubercle and cerebellum (min). The specific binding was saturable with increasing concentrations of the drug and stereospecifically displaced by (+)butaclamol. Dopamine agonists (apomorphine, NPA and bromocriptine) and antagonists (spiperone, haloperidol, (+)butaclamol and l-sulpiride) all caused dose-dependent prevention of [³H]NPA binding. Mianserin, phenoxybenzamine and propranolol did not prevent the in vivo [³H]NPA binding suggesting that [³H]NPA binds specifically to dopamine receptors in the striatum and the olfactory tubercle of the mouse.     

Regional in vivo binding of the substituted benzamide [3H]eticlopride in the rat brain: evidence for selective labelling of dopamine receptors

The novel substituted benzamide eticlopride, (S)-(-)-5-chloro-3-ethyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-6-methoxy salicylamide hydrochloride (A38503; FLB 131), was radiolabelled to high specific activity and used for in vivo receptor binding studies in the rat brain. Intravenous injections of [3H]eticlopride resulted in a rapid accumulation of radioactivity in several brain regions: striatum greater than olfactory tubercle greater than septum greater than substantia nigra greater than frontal cortex greater than cerebellum. Approximately 95% of the radioactivity recovered from the striatum was in the form of authentic eticlopride, as determined by thin-layer chromatography. Two hours after injection, the ratio between the amount of radioactivity present in the striatum and in the cerebellum was approximately 10:1. The in vivo binding of [3H]eticlopride was saturable in all dopamine-rich areas, with a very low proportion of non-specific binding. The specific in vivo binding of [3H]eticlopride was blocked by several dopamine antagonists, including haloperidol, (+)-butaclamol, spiperone, d,l-sulpiride and remoxipride. The dopamine agonist N-n-propylnorapomorphine, but not apomorphine, was found to be a potent blocker of in vivo [3H]eticlopride binding. Serotonin and noradrenaline receptor antagonists did not prevent the in vivo binding of [3H]eticlopride. Autoradiographic analysis of the in vivo [3H]eticlopride binding showed a high density of binding sites in the striatum, nucleus accumbens and the olfactory tubercle. Moderate binding was found in the hippocampal formation and in the entorhinal area, but little or no binding was detected in other cortical regions. [3H]Eticlopride binding in all these areas was blocked by pretreatment with (+)-butaclamol. Taken together, these findings show that the substituted benzamide compound [3H]eticlopride passes readily into the brain and binds with high specificity to dopamine or neuroleptic receptors in dopamine-rich brain areas. Thus, eticlopride may be a useful tool in studies of dopamine D-2 receptors in vivo.     

Neurochemical and behavioral effects of acute and chronic treatment with apomorphine in rats

In three experiments, rats were injected once daily with 5.0 mg/kg apomorphine or vehicle and tested for locomotor activity for 10-14 days. In each experiment, apomorphine produced behavioral sensitization, characterized by a progressively greater increase in locomotor activity with each succeeding injection. On day 11 of testing, in an experiment designed to assess the synthesis of dopamine (DA), rats were injected with 5.0 mg/kg apomorphine or vehicle, followed by 100 mg/kg NSD-1015, an inhibitor of the enzyme l-aromatic amino acid decarboxylase. After administration of NSD-1015, concentrations of dihydroxyphenylalanine (DOPA) were determined in striatal and mesolimbic tissues by high performance liquid chromatography (HPLC) with electrochemical detection. The results revealed a significant decrease in accumulation of DOPA in both striatal and mesolimbic tissue after acute treatment with apomorphine. More important, chronic treatment with apomorphine produced a significant increase in accumulation of DOPA in both areas. In subsequent experiments, rats on day 14 of testing were sacrificed for determination of levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC) or specific binding of [3H]spiperone in the striatum and mesolimbic region. Although levels of DOPAC were significantly reduced in the regions of the brain after an acute injection of apomorphine, chronic treatment with apomorphine did not significantly affect levels of DA, DOPAC or specific binding of [3H]spiperone. These findings suggest that the development of behavioral sensitization to apomorphine may be related to an alteration in the synthesis of DA.     

Receptor mechanisms of nicotine-induced locomotor hyperactivity in chronic nicotine-treated rats

Rats were pretreated with saline or nicotine (1.5 mg/kg per day) by subcutaneously implanting each animal with an Alzet osmotic mini-pump which continuously released saline or nicotine for 1, 5 and 14 days. At the end of each pretreatment period, animals were used for (i) determining their locomotor response to acutely injected nicotine (0.2 mg/kg, s.c.) and (ii) measuring the density of L-[3H]nicotine and [3H]spiperone binding sites in the striatum. We observed no changes in nicotine-induced locomotor response, striatal L-[3H]nicotine and [3H]spiperone binding in the animals pretreated with nicotine for 1 day. In rats which were pretreated with nicotine for 5 days, there was a significant increase in the nicotine-stimulated locomotor response which was associated with an increase in the number of L-[3H]nicotine binding sites and also with an elevated dopamine (DA) level in the striatum. The number of striatal [3H]spiperone binding sites was not affected. In animals pretreated with nicotine for 14 days, the nicotine-induced locomotor response remained to be potentiated. However, this response was correlated with an elevated number of striatal [3H]spiperone binding sites, whereas the number of striatal L-[3H]nicotine binding sites and the striatal DA level were normal. These results suggest that chronic nicotine-treated rats develop locomotor hyperactivity in response to nicotine initially due to increases of both the density of nicotinic receptors and DA concentration, followed by inducing DA receptor supersensitivity in the striatum.     

Irreversible blockade of striatal dopamine receptors in vivo by a derivative of α-flupenthixol

Flupenthixyl chloride (FPT-Cl), a derivative of the dopamine (DA) receptor antagonist and neuroleptic α-flupenthixol possessing a cloroethyl side chain, has been prepared and evaluated for use in vivo in affinity labeling of DA receptors. Binding of [³H]spiperone to rat striatal DA receptors was markedly altered up to 12 h after intraventricular injection of FPT-Cl as compared with controls, while Scatchard plots of [³H]spiperone binding obtained on rat striatal homogenates 24 and 48 h after injection of FPT-Cl had values of B_max significantly lower than in controls. The results suggest that the administration of FPT-Cl leads to irreversible and possibly covalent blockade of a portion of the spiperone binding sites in rat striatum. A second population of receptors appears to be blocked reversibly and presumably noncovalently by FPT-Cl, and these spiperone binding sites are partially reactivated in vivo after 48 h.     

Dissociation of the striatal D-2 dopamine receptor from adenylyl cyclase following 6-hydroxydopamine-induced denervation.

Intracellular cyclic AMP accumulation following exposure to dopamine (DA) agonists and and antagonists was measured in striatal slices from rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway and which showed contralateral circling to apomorphine. Both DA (10-320 microM) and the D-1 agonist SKF 38393 (0.1-32 microM) increased cyclic AMP accumulation in striatal slices from the lesioned and intact hemispheres. The EC50 for DA to increase cyclic AMP accumulation in slices was greater in the 6-OHDA-lesioned striata compared to the intact striatum, but the EC50 for SKF 38393 was not affected. The D-1 antagonist SCH 23390 (10 microM) completely inhibited the ability of DA and SKF 38393 to increase cyclic AMP accumulation in striatal slices from both denervated and intact sides of the brain. In slices from the intact hemisphere the increase in DA-induced cyclic AMP accumulation was enhanced by the D-2 antagonist (+/-)-sulpiride (50 microM) but (+/-)-sulpiride had no effect on the DA response in slices from the lesioned side. Similarly, the ability of SKF 38393 to enhance cyclic AMP accumulation was blocked by the D-2 agonist quinpirole (10 microM) in striatal slices from the intact hemisphere but not in tissue from the lesioned side. The density of striatal D-1 and D-2 receptors assessed by [3H]SCH 23390 and [3H]spiperone binding did not differ between the hemispheres although there was an increase in the affinity of D-1 receptors for [3H]SCH 23390 in the lesioned striatum. After striatal deafferentiation there appears to be an uncoupling of the "inhibitory" D-2 receptor from the D-1 receptor-associated adenylyl cyclase.     

Differential anatomical location of [3H]-N,n-propylnorapomorphine and [3H]-spiperone binding sites in the striatum and substantia nigra of the rat.

Specific [3H]-spiperone and [3H]-N,n-propylnorapomorphine (NPA) binding was measured in striatum and substantia nigra of the rat following unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle, kainic acid lesions of the substantia nigra or striatum, and following decortication. Binding sites labelled by [3H]-spiperone in striatum were found to lie on striatal cell bodies and on the terminals of cortico-striate glutamate fibres, but not on presynaptic dopamine terminals. In contrast, binding sites labelled by [3H]-NPA were demonstrated on striatal cell bodies and on the terminals of nigro-striata dopamine fibres, but not on cortical afferents. In substantia nigra, specific [3H]-spiperone binding sites were found only on non-dopamine cell bodies. No clear evidence was found for their existence on dopamine cell bodies, the terminals of strio-nigral fibres or the terminals of cortico-nigral fibres. In contrast, specific binding sites for [3H]-NPA were found on dopamine cell bodies and the terminals of strio-nigral fibres. Localization on non-dopamine cell bodies or on cortico-nigral fibres was not demonstrated. These studies support the concept of differential localization of agonist and antagonist binding sites.     

In vivo interaction of cabergoline with rat brain dopamine receptors labelled with [3H]N-n-propylnorapomorphine

Cabergoline is a potent dopaminergic agent that interacts with agonists and antagonists of dopamine receptors in vitro. We studied the binding of [3H]N-n-propylnorapomorphine ([3H]NPA) to dopamine receptors after i.v. and oral administration of cabergoline to determine whether cabergoline crosses the blood-brain barrier; bromocriptine was used as a reference drug. Cabergoline and/or its active metabolite(s) did cross the blood-brain barrier and reach dopamine receptors. Comparative time-course analysis of the regional inhibition of [3H]NPA binding showed that cabergoline was more potent than bromocriptine in inhibiting [3H]NPA binding and that it occupied the receptor for longer. These effects were observed in all areas of the rat brain studied (striatum, olfactory tubercles, adeno- and neurohypophysis, thalamus and hypothalamus). Further studies in the striatum and adenohypophysis showed that cabergoline receptor occupancy was dose-dependent and still detectable 72 h after i.v. administration of the drug. While cabergoline was more potent in the striatum than in the adenohypophysis when administered i.v., the reverse was observed after its oral administration. Cabergoline was equally potent in the adenohypophysis after oral and i.v. administration, as determined 1 and 8 h later.     

Definition of the in-vivo accumulation of [3H]spiperone in brain using haloperidol and sulpiride to determine functional dopamine receptor occupation

The in-vivo administration of [3H]spiperone caused an accumulation of radioactivity in the substantia nigra, tuberculum olfactorium, nucleus accumbens, striatum and frontal cortex when compared with cerebellar levels. Haloperidol (0.01-1.0 mg kg-1 i.p.) dose-dependently prevented the accumulation of [3H]spiperone in the substantia nigra, tuberculum olfactorium, striatum and nucleus accumbens. Sulpiride (10-160 mg kg-1 i.p.) dose-dependently prevented the accumulation of [3H]spiperone only in the substantia nigra. The effects of sulpiride on other areas were not consistent; there was a suggestion of a reduction in the accumulation of [3H]spiperone in tuberculum olfactorium and striatum, but not in nucleus accumbens. Neither haloperidol (0.01-1.0 mg kg-1 i.p.) nor sulpiride (10-160 mg kg-1 i.p.) caused displacement of [3H]spiperone from the frontal cortex. Both haloperidol (0.01-0.5 mg kg-1) and sulpiride (10-80 mg kg-1) increased striatal and mesolimbic HVA concentrations. Haloperidol potently blocked apomorphine-induced stereotypy but sulpiride was only effective at the highest dose employed. The functional effect produced by haloperidol correlated with its ability to define [3H]spiperone binding in-vivo to dopamine receptors in the substantia nigra, striatum and tuberculum olfactorium. In contrast, there was no correlation between functional effect of sulpiride and its ability to define [3H]spiperone binding in-vivo.     

Selective blockade of dopamine D-1 receptors by SCH 23390 discloses striatal dopamine D-2 receptors mediating the inhibition of adenylate cyclase in rats

l-Glutamate but not methyl-D-aspartate (NMDA) or quisqualate (Quis) (10^?6 M) in vitro with or without preincubation increased significantly the K_D value of the [³H]N-propylnorapomorphine ([³H]NPA) binding sites by 21 and 36% respectively in striatal membranes of rat without influencing the striatal [³H]spiperone binding sites. The number of striatal [³H]NPA binding sites was not changed by l-glutamate (10^?6 and 10^?5 M) in vitro. There may thus exist interactions between striatal glutamate receptors — not related to excitatory amino-acid receptors of the NMDA or the QUIS type - and high affinity striatal DA receptors.     

Turnover of specific [3H]spiperone and [3H]N,n-propylnorapomorphine binding sites in rat striatum following phenoxybenzamine administration

Inclusion of phenoxybenzamine into incubates containing rat striatal preparations equipotently displaced specific striatal [3H]spiperone and [3H]NPA binding. Pre-incubation of striatal membranes with phenoxybenzamine followed by extensive washing equipotently inhibited the subsequent specific [3H]spiperone or [3H]NPA binding. In both displacement and pre-incubation experiments phenoxybenzamine caused complete inhibition of specific [3H]spiperone binding to rat striatal membranes, but only partially inhibited specific [3H]NPA binding. Following parenteral administration to rats, phenoxybenzamine caused a marked inhibition of ex vivo specific [3H]spiperone binding in striatal tissue preparations from these animals which lasted approximately 24 hr following in vivo drug administration. In contrast, administration of phenoxybenzamine caused only a transient change in ex vivo specific [3H]NPA binding. Phenoxybenzamine causes irreversible inhibition of [3H]spiperone and [3H]NPA binding in vitro. In vivo administration of phenoxybenzamine discriminates between [3H]spiperone and [3H]NPA in ex vivo studies suggesting that these binding sites have different turnover rates.     

Repeated administration of HA-966 and haloperidol to rats: similar tolerance to striatal dopamine accumulation after HA-966 challenge, but dissimilar effects on striatal [3H]spiperone binding

Repeated administration of 1-hydroxy-3-amino-pyrrolidone-2 (HA-966) to rats induces tolerance, as shown by a decreased, drug-stimulated accumulation of dopamine (DA) in the striatum. In the present study we compared the adaptive response of the striatal dopaminergic system to repeated administration of HA-966 with the adaptive response observed after repeated haloperidol. These treatments deprive dopamine (DA) receptors from their agonist and cause a blockade of DA receptors, respectively. Tolerance to HA-966 was not accompanied by a change in the specific binding of [3H]spiperone to striatal membranes. This is in contrast to the well-documented up-regulation of DA receptors that occurs with tolerance to haloperidol. Repeated haloperidol pretreatment also diminished DA accumulation following a challenge dose of HA-966, to a similar extent as that caused by repeated pretreatment with HA-966. These similar effects of pretreatment with HA-966 or haloperidol on the response to the HA-966 challenge are in line with, and strengthen, the idea that an increased sensitivity of presynaptic DA receptors is responsible for the decreasing effect of HA-966 after its repeated administration. Haloperidol and HA-966 clearly have different effects on postsynaptic DA receptors, as is shown by their differential effects on striatal [3H]spiperone binding.     

Effect of buflomedil (4-(1-pyrrolidinyl)-1-(2,4,6-trimethoxy phenyl)-1-butanone hydrochloride) on the function of striatal dopaminergic neurons

Effect of buflomedil (4-(1-pyrrolidinyl)-1-(2,4,6-trimethoxy phenyl)-1-butanone hydrochloride) on the release and uptake of dopamine (DA) and the function of DA receptors in the striatum was investigated using male Wistar rats. In vitro addition of buflomedil (10(-5)-10(-8) M) had no effect on the uptake of [3H]-DA in striatal slices. On the other hand, buflomedil (10(-5)-10(-7) M) increased the spontaneous as well as high K+ (30 mM)-evoked releases of [3H]DA from striatal slices. Buflomedil inhibited the bindings of [3H]SCH23390, [3H]spiperone and [3H]apomorphine to striatal D1, D2 and D3 receptors only at a high concentration. On the other hand, buflomedil inhibited [3H]quinuclidinyl benzilate (QNB) binding to striatal muscarinic cholinergic receptors, which was similar to the action of carbachol. Pretreatment with scopolamine (0.5 mg/kg) in vivo inhibited the facilitation of striatal DA turnover induced by oral administration of buflomedil (300 mg/kg). In contrast, continuous oral administration of buflomedil (30 mg/kg x 7 days) to rats had no significant effect on the specific bindings of [3H]SCH23390, [3H]spiperone, [3H]apomorphine and [3H]QNB to synaptic membrane preparations obtained from the striatum. These results suggest that buflomedil may enhance striatal DA release by stimulating muscarinic cholinergic receptor and that DA receptors may not be involved in the enhancing effect of buflomedil on DA release.     

Synaptic concentration of dopamine in rat striatal slices in relationship to [3H]raclopride binding to the dopamine D2 receptor

The in vivo binding of dopamine (DA) radioligands to D2 receptors can be affected by competition with endogenous dopamine. In the present study, we used a brain slice preparation that provides more controlled conditions than in vivo preparations in order to examine the relationship between synaptic DA and the binding of [3H]raclopride to D2 receptors. We also estimated the synaptic DA concentration in rat striatal slices by determining the changes in [3H]raclopride binding. To correlate the changes in [3H]raclopride binding with the concentration of synaptic DA, the kinetic parameters were determined. [3H]Raclopride reached equilibrium binding conditions within two hours. The Ki value for DA in inhibiting [3H]raclopride binding was about 2.2 nM. The increase in synaptic DA evoked by electrical stimulation decreased the striatal binding of [3H]raclopride in a frequency-dependent manner. Increases in the DA concentration evoked by amphetamine (AMPH) or cocaine decreased [3H]raclopride binding by 74% or 20%, respectively, corresponding to increases in the synaptic DA concentrations of 1.6 nM or 0.6 nM, respectively. Pargyline also decreased [3H]raclopride binding by 36% corresponding at a concentration of 1.2 nM. In contrast, the depletion of synaptic DA by alpha-methyl-p-tyrosine (alpha-MpT) increased the specific binding of [3H]raclopride by 43% when the DA concentration was decreased to 0.7 nM. The changes in the DA concentration at the synapse were responsible for the changes in the striatal binding of [3H]raclopride. The values calculated in this study may therefore approximate the changes in the synaptic DA concentration in rat striatal slices following manipulation.     

In-vivo radiolabelled oxiracetam binding to rat brain

The in-vivo binding of [3H]oxiracetam has been studied in brain areas of rats examined 30 min after i.c.v. injection. Soluble radioactivity accounted for more than 90% of total radioactivity in all the structures considered and was not affected by co-injection with a 1000-fold excess of unlabelled oxiracetam. Both total and bound radioactivity showed a marked regional distribution, with highest concentrations in the septum, followed by the hippocampus; the cerebral cortex, striatum and cerebellum had the lower concentrations of radioactivity. Computer-assisted quantitative autoradiography with [14C]oxiracetam confirms these findings. Analysis of [3H]oxiracetam bound to membranes indicated that, after co-injection with a 1000-fold excess of unlabelled oxiracetam, there was a significant reduction of binding only in the septum, hippocampus and cerebral cortex. These results suggest that in those cerebral structures oxiracetam binds to saturable sites.     

Caffeine mimics dopamine receptor agonists without stimulation of dopamine receptors

The purpose of this study was to examine whether caffeine stimulates dopamine (DA) receptors. The effects of caffeine on the binding of [3H]spiperone to membranes from the striatum, accumulation of L-DOPA in the striatum in mice receiving gamma-butyrolactone, and regional levels of 3,4-dihydroxyphenylacetic acid in the brain of the rat were investigated and compared with those elicited by DA receptor agonists. Caffeine did not inhibit the binding of [3H]spiperone to membranes or the accumulation of L-DOPA in the striatum, produced by gamma-butyrolactone. Caffeine decreased the levels of 3,4-dihydroxyphenylacetic acid significantly in the striatum, olfactory tubercle and nucleus accumbens and slightly in the frontal cortex of rats, whereas it delayed utilization of DA in all those regions except the frontal cortex. Taken together these results suggest that caffeine fails to stimulate pre- or postsynaptic DA receptors. The possible mechanism by which caffeine mimics DA receptor in DA metabolism and behavior are discussed.