A. Campbell

Localization of dopamine receptor subtypes in corpus striatum and nucleus accumbens septi of rat brain: comparison of D1-, D2- and D4-like receptors

Changes in D1-, D2- and D4-like dopamine receptor binding in rat brain were examined by quantitative autoradiography following: (i) unilateral surgical ablation of frontal cerebral cortex to remove descending projections to corpus striatum and nucleus accumbens, (ii) unilateral injections of kainic acid into corpus striatum or nucleus accumbens to degenerate local intrinsic neurons, (iii) unilateral injections of 6-hydroxydopamine into substantia nigra to degenerate ascending dopamine projections. Rats were killed one week after lesioning, with contralateral tissue controls. Radioligands were: [3H]SCH-23390 for D1-like (D1/D5) receptors, [3H]nemonapride alone for D2-like (D2/D3/D4) receptors, and [3H]nemonapride with 300 nM S[-]-raclopride and other masking agents for D4-like receptors (identified by blockade with D4 selective L-745,870). Frontal cerebral cortex ablation did not alter D1- or D2-like receptor density, but D4-like binding decreased significantly in both corpus striatum (18%) and nucleus accumbens (23%). Kainic acid markedly reduced D1-like (75% and 84%) and D2-like binding (44% and 52%), with smaller D4-like losses (28% and 27%) in corpus striatum and nucleus accumbens, respectively. Nigral 6-hydroxydopamine lesions (verified by autoradiographic loss of dopamine transporters labelled with [3H]GBR-12935) did not significantly change D1-, D2-, or D4-like binding in the corpus striatum. These results suggest that the majority of D1-, and D2-like, and a smaller portion of D4-like receptors in corpus striatum and nucleus accumbens arise on intrinsic postsynaptic neurons, and that some D4-like, but neither D1- nor D2-like, receptors are found on presynaptic corticostriatal afferents.

GABAB receptors: altered coupling to G-proteins in rats sensitized to amphetamine

Modified dopamine and glutamate neurotransmission in discrete brain regions is implicated in stimulant-induced behavioral sensitization. Release of both neurotransmitters is influenced by GABA(B) metabotropic receptors for the principal inhibitory neurotransmitter GABA. Accordingly, GABA(B) receptors were examined in rats sensitized to amphetamine by measuring receptor density and coupling to G-proteins indicated as [(3)H]baclofen binding and baclofen-mediated [(35)S]GTP gamma S binding. Repeated treatment with (+)-amphetamine (5mg/kg per day, i.p., for five days) sensitized the rats to amphetamine challenge (1mg/kg) at 14 days, but not one day, later. GABA(B) receptor density was not altered at either time. Baclofen-mediated [(35)S]GTP gamma S binding, however, was selectively augmented in the prefrontal cortex and attenuated in the nucleus accumbens at 14 days, but not one day, after amphetamine treatment. Changes in GABA(B) receptor coupling to G-proteins in rats sensitized to amphetamine, but not in similarly treated but unsensitized rats, lead us to suggest that altered GABA(B) receptor functioning may contribute to the expression of amphetamine-induced behavioral sensitization.

Decreasing sensitivity to neuroleptic agents in developing rats; evidence for a pharmacodynamic factor

Developing rats are far more sensitive than adults to the behavioral effects of haloperidol. The present results support the hypothesis that this change may reflect age-related changes in brain responses such as alterations in drug-receptor or drug-effector mechanisms. Dose-response studies of catalepsy and ptosis were conducted in male Sprague-Dawley rats aged 30, 56, or 100 days. Resulting dose-effect curves were approximately parallel and showed rightward shifts with highly significant progressive increases of ED50. Similar developmental decreases in drug sensitivity (3–6 ×) were found following systemic (PO or IP) administration of haloperidol or the phenothiazine neuroleptic perphenazine, which differ markedly in structure, potency, distribution, and metabolism. Age-related decreases in drug sensitivity (3–4 ×) were also found using intracerebroventricular (ICV) administration of both agents in an attempt to bypass potential “pharmacokinetic” influences. Since the age-dependent decrease in sensitivity to both neuroleptics was found during the rising phase of drug action (1st hour) and ranked: PO>IP>ICV, some change in absorption and distribution of both drugs may occur in addition to the apparently important maturational decrease in target-organ sensitivity indicated by the responses to direct ICV injection and by the similarity of results obtained with dissimilar agents.

Receptor affinity, neurochemistry and behavioral characteristics of the enantiomers of thioridazine: Evidence for different stereoselectivities at D1 and D2 receptors in rat brain

The binding characteristics of the enantiomers of thioridazine were assessed in the brain of the rat using competitive radioreceptor assays with tritiated ligands selective for dopamine D1 (SCH-23390), D2 (spiperone), norepinephrine alpha-1 (prazosin) and muscarinic (quinuclinidinyl benzilate) receptors. (+)-Thioridazine was shown to have 2.7 and 4.5 times higher affinity than (-)-thioridazine for D2 and alpha-1 receptors, respectively. In contrast, (-)-thioridazine had 10 times higher affinity for the D1 receptor. Both enantiomers showed similar affinities for the muscarinic receptor. In a second experiment, thioridazine, dopamine, norepinephrine, serotonin and their metabolites were assayed in the brain of the rat after acute administration of the enantiomers of thioridazine and the assessment of catalepsy. (+)-Thioridazine was 4.1 times as potent as (-)-thioridazine in elevating the turnover of dopamine in the striatum, but neither enantiomer affected the other monoamines. The concentration of thioridazine and its metabolites in the brain, for a given dose, was similar for both enantiomers. (-)-Thioridazine induced slightly more catalepsy than (+)-thioridazine and appeared to be more toxic at large doses. While racemic thioridazine had an intermediate effect between that of its two enantiomers in the binding and neurochemical assays, it appeared to induce more catalepsy than either enantiomer, suggesting a synergistic effect in this behavioral assay. It was concluded that (+)- and (-)-thioridazine act as partially selective D2 and D1 antagonists, respectively. Therefore, clinical administration of only one enantiomer of thioridazine, rather than the currently prescribed racemate, may result in an improved therapeutic profile and so be worthy of further investigation.

Effect of age on behavioral responses and tissue levels of apomorphine in the rat

Rats of several ages between 2 and 24 months were tested for stereotyped behavioral responses to R(-)apomorphine-HCl (APO), a potent and selective dopamine agonist. Between 2 and 24 months, the ED50 for apomorphine decreased 2.5-fold (0.14-0.06 mg/kg, i.p.), as assessed by a microcomputer-assisted technique. Not only were older rats more sensitive to apomorphine, but the duration of the behavioral effects increased with age and showed a greater change at doses which were greater than the ED50. When levels of apomorphine in brain were assayed by liquid chromatography and electrochemical detection, there was a progressive increase in peak levels of the agent in the tissue as well as a delay in its elimination from brain, with increasing age. Moreover, there was a highly significant correlation between increased behavioral effect and the level of apomorphine in brain with increasing age (r greater than 0.8). These results indicate that increased levels in brain or decreased elimination of apomorphine may be an important factor contributing to a marked increase in behavioral sensitivity to apomorphine with age in the rat.

Role of Dopamine in Behavioral Effects of Serotonin Microinjected into Rat Striatum

Bilateral local microinfusion of serotonin (5-hydroxytryptamine; 5-HT) into the ventrolateral striatum (VLS) of the rat forebrain induces quantifiable stereotyped orofacial behaviors. The role of presynaptic dopamine (DA) and structural requirements of indoles for expression of this behavioral effect and for inhibition of neuronal transport of [3H]DA were examined. Bilateral local injection of 6-OHDA (8 micrograms/side) into VLS depleted DA and markedly diminished the behavioral effects of 5-HT. Intracerebral pretreatment with the potent DA transport inhibitors GBR-12909 (6 micrograms/side) or nomifensine (4 micrograms/side) also markedly decreased behavioral responses to 5-HT. A series of indoles and tyramine were examined for ability to induce stereotypy following infusion into the VLS. Of compounds tested, only p-tyramine, 5-HT, tryptamine and L-5-hydroxytryptophan (5-HTP) elicited strong orofacial behaviors; indoles lacking a free amino group or containing other substituents were virtually inactive in vivo, and the effect of 5-HTP was prevented by systemic pretreatment with the decarboxylase inhibitor NSD-1015, indicating its required conversion to 5-HT. Uptake of [3H]DA (0.1 microM) into rat striatal synaptosomes was inhibited in a concentration-dependent manner in the following apparent rank-order: p-tyramine, N-methyl-5-HT, tryptamine, 5-HT, N-methyltryptamine (IC50 = 44-718 nM), other indoles (IC50 = 10-100 microM). These results support the conclusion that oral stereotypy induced by microinjection of 5-HT or other aromatic amines into rat VLS is mediated by local release of endogenous DA. These results extend previous findings indicating that this effect of 5-HT was not blocked by 5-HT receptor antagonists, and suggest mediation by a neuronal transport process involved in the uptake or storage of DA.

Effects of prenatal treatment of rats with haloperidol due to altered drug distribution in neonatal brain

Abstract Maternal exposure to haloperidol during gestation produced decreased behavioral responsiveness to apomorphine in rat offspring at 10 days after birth, but not at 15 or 30 days. An apparent decrease in the affinity of specific binding of [3H]piroperidol to striatal tissue of rats so treated was observed at 7 days after birth, with no difference in receptor affinity or total binding capacity at 15 or 30 days. Because these effects could result from the continued presence of haloperidol after birth, the distribution and clearance of small quantities of [3H]aloperidol were studied in neonates, older controls, and mothers, up to 10 days after drug administration. The concentration of labelled drug in the brains of the neonates was consistently more than three times higher than in adults, although serum and liver concentrations, and apparent rates of elimination from brain tissue were approximately equal. Postpartum persistence of prenatally administered neuroleptics may account for transient behavioral and biochemical alterations.

S(+)Apomorphines. Selective inhibition of excitatory effects of dopamine injected into the limbic system of the rat.

Dopamine (DA) injected unilaterally into the corpus striatum of the brain of the rat after pretreatment with a monoamine oxidase inhibitor, induced contralateral deviation of the head (ED50 = 23 micrograms). Dopamine, administered similarly into the nucleus accumbens septi, induced strong locomotor arousal effects (ED50 = 32 micrograms), as reported by others. Systemic administration of the S(+) isomers of apomorphine (APO) and its N-n-propyl analog (NPA) had no significant action on the effects on posture of DA given intrastriatally, even in large doses. In striking contrast, both agents selectively inhibited the locomotor-arousal effects of DA injected into the accumbens, but (+)NPA was seven times more potent (ED50 = 0.5 mg/kg vs 16 micrograms of DA) and its effects lasted for about 70 min. The dose-effect curves for (+)NPA against two doses of DA demonstrated parallel, lateral displacement, indicating a competitive interaction. These results support the impression that enantiomers and analogs of apomorphine, and especially S(+)N-propylnorapomorphine, have potent antidopaminergic actions that may be selective for limbic areas of the forebrain.

Effects of hippocampal kainic acid lesions on striatolimbic dopamine D1-, D2-, and D4-like receptors

Changes in D1-, D2- and D4-like dopamine receptor binding in rat caudate-putamen, nucleus accumbens and hippocampus were examined by quantitative in vitro receptor autoradiography after unilateral injections of kainic acid into hippocampus to degenerate local intrinsic neurons and remove hippocampal-striatal projections. Radioligands were: [3H]SCH-23390 for D1-like (D1/D5) receptors, [3H]nemonapride alone for D2-like (D2/D3/D4) receptors and with 300 nM S[-]-raclopride and other masking agents for D4-like receptors. Kainic acid injections produced massive local hippocampal losses of D1-like (75%), D2-like (77%) and D4-like (79%) receptor binding but did not significantly alter D1-, D2- or D4-like receptor density in caudate-putamen or nucleus accumbens. Most D1-, D2- and D4-like receptors in rat hippocampus evidently arise on intrinsic postsynaptic neurons whereas none was detected on presynaptic terminals of hippocampal-striatal afferents innervating the striatal complex.

Bromocriptine antagonizes behavioral effects of cocaine in the rat

Rats given cocaine or bromocriptine under conditions of low basal arousal showed dose-dependent increases of locomotor activity for less than or equal to 1 hours, followed by depression of activity that diminished gradually over the next 2 hours. Arousal was related biphasically to dose (maximum at ca. 5 mg/kg) but depression increased monophasically with the dose of either agent. Both behavioral arousal and depression induced by cocaine were antagonized by bromocriptine, even at doses lacking behavioral effects alone (ID50 = 1.0 and 0.36 mg/kg [1.3 and 0.5 mumol/kg], respectively). Bromocriptine blocked depression of locomotion even when given after the initial stimulation by cocaine. Bromocriptine induced very weak stereotypy, and neither increased nor blocked stereotypy induced by cocaine or apomorphine. Cocaine, at maximally effective doses, did not deplete catecholamines or serotonin in brain regions at times of maximum behavioral arousal or depression, nor did bromocriptine after metabolic turnover of dopamine. Bromocriptine antagonized arousal induced by direct injection of dopamine into the nucleus accumbens. The ability of bromocriptine to block both the behavioral arousal and depression induced by cocaine may reflect activity of bromocriptine as a mixed agonist-antagonist with limited intrinsic agonistic activity at central dopaminergic D2 receptors, perhaps with particular reference to limbic mechanisms.