C. Reavill

A comparison of in vitro and in vivo dopamine receptor antagonism produced by substituted benzamide drugs

Cerebral dopamine receptor antagonism in vivo is associated with the ability to interact with two in vitro models of these receptors. Thus the ability to inhibit dopamine stimulation of the adenylate cyclase from rat striatum has been suggested as a model of post-synaptic dopamine receptor activity and of neuroleptic potential (Clement-Cormier, Kebabian & others, 1974; Miller, Horn & Iversen, 1974). Similarly, the ability to displace radioactive ligands from their binding sites in rat striatal preparations is also used as an index of an interaction with post-synaptic dopamine receptors and of neuroleptic activity (Seeman, Chau-Wong & others, 1975; Creese, Burt & Snyder, 1976). This latter model is believed to provide a better correlation between in vivo clinical neuroleptic activity and in vitro activity than the adenylate cyclase system. We have previously reported that some substituted benzamide drugs, such as metoclopramide and sulpiride, exhibit behavioural and biochemical properties associated with a blockade of cerebral dopamine receptors (Dolphin, Jenner & others, 1975; Peringer, Jenner & Marsden, 1975; Peringer, Jenner & others, 1976; Elliott, Jenner & others, 1977). Such compounds to a variable degree block apomorphine-induced locomotor activity, inhibit apomorphine-induced circling in rodents with unilateral nigrostriatal lesions, and elevate striatal and mesolimbic concentrations of the dopamine metabolites homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC). We have, however, been unable to demonstrate a significant effect of these compounds on the dopamine stimulation of the adenylate cyclase from rat striatum. This, and other data, has led us to conclude that these compounds act on the cerebral dopamine pathways in a manner which differs from that of classical neuroleptics. The present study is a comparison of the activity of five substituted benzamide drugs in the in vitro models of dopamine receptor activity with their ability to induce catalepsy and to inhibit apomorphine-induced stereotyped behaviour, these behavioural tests being chosen since they are often used to screen neuroleptic activity. These data are then compared with the

Anticonvulsant action of 2-amino-7-phosphonoheptanoic acid in the substantia nigra

Focal injection of 2-amino-7-phosphonoheptanoic acid in the substantia nigra, pars reticulata or pars compacta, in rats produces a dose-dependent suppression of the tonic extensor seizure component in the electroshock test. Haloperidol pretreatment prevents stereotyped behaviour induced by 2-amino-7-phosphonoheptanoic acid, but does not change the effect on the electroshock test. This anticonvulsant effect of 2-amino-7-phosphonoheptanoic acid probably results from an antagonist action at a receptor site sensitive to N-methyl-D-aspartate leading to a decrease in activity in nigral efferent systems.

Repeated administration of N-methyl-4-phenyl 1,2,5,6-tetrahydropyridine to rats is not toxic to striatal dopamine neurones

N-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine ( MPTP ) (10 mg/kg/day i.p.) was administered to rats for 16 days, which were then observed for a further 9-11 days. MPTP administration did not alter spontaneous locomotor activity or amphetamine (2.5 mg/kg ip)-induced locomotion. Apomorphine (0.25 mg/kg sc) did not alter locomotion in control rats but increased activity in MPTP treated animals. The stereotyped response to apomorphine (0.25 mg/kg sc) and amphetamine (2.5 and 5.0 mg/kg ip) was unaltered by MPTP administration. The striatal content of dopamine, HVA and DOPAC was unaltered by MPTP intake. The uptake of [3H]dopamine and [3H] 5HT in striatal synpatosomes was not changed by MPTP . The results suggest that MPTP , in the dose used, is not toxic to nigro-striatal dopamine neurones in the rat. This contrasts with its neurotoxic actions in monkeys and man.

Kainic acid lesions of striatum and decortication reduce specific [3H]sulpiride binding in rats, so D‐2 receptors exist post‐synaptically on corticostriate afferents and striatal neurons

Unilateral kainic acid lesions of rat striatum reduced specific striatal [3H]spiperone and [3H]sulpiride binding sites (Bmax) by 52 and 67% respectively compared with the intact side. The dissociation constant (KD) for [3H]spiperone binding was unchanged but that for [3H]]sulpiride binding was reduced. Specific striatal [3H]spiperone and [3H]sulpiride binding was reduced by 22 and 37% respectively in unilateral decorticate animals, but there was no change in KD. Unilateral 6‐hydroxydopamine lesions of the medial forebrain bundle caused no change in striatal [3H]spiperone binding sites or KD value, but produced a 27% increase in [3H]sulpiride binding sites with no change in KD. These data support the hypothesis of D‐2 receptors located on cortico‐striate glutamate fibres, but also indicate the presence of both D‐1 and D‐2 receptors on the cell bodies of striatal neurons.

Turning behaviour induced by injection of muscimol or picrotoxin into the substantia nigra demonstrates dual GABA components.

Injection of the GABA agonist muscimol into rat caudal substantia nigra caused contralateral turning, whereas injection into the rostral substantia nigra caused ipsilateral turning. The GABA antagonist picrotoxin had the opposite effect. These findings support the hypothesis that GABA has dual actions in the substantia nigra. Ipsilateral turning induced by injection of muscimol into rostral nigra was abolished by haloperidol pretreatment, indicating the involvement of dopaminergic mechanisms. Haloperidol pre-treatment did not prevent turning induced by muscimol injected into the caudal nigra, supporting the existence of a non-dopaminergic nigral output system.

Stereoselective actions of substituted benzamide drugs on cerebral dopamine mechanisms

Apomorphine‐induced locomotor activity in reserpine‐pretreated mice was antagonized by pretreatment with (‐)‐sulpiride and (‐)‐sultopride. The (+)‐enantiomers were inactive. Apomorphine‐ and amphetamine‐induced stereotyped behaviour in rats were antagonized by (‐)‐sultopride but not by the (+)‐enantiomer. Neither enantiomer of sulpiride prevented the onset of the stereotyped response. Both (‐)‐sulpiride and (‐)‐sultopride induced increases in striatal and mesolimbic HVA and DOPAC concentrations; (+)‐sultopride elevated striatal and mesolimbic DOPAC concentrations but not HVA, while (+)‐sulpiride had no effect on HVA or DOPAC in either area. Dopamine concentrations were reduced by the enantiomers of sultopride but not by sulpiride. Low concentrations (10−9 −10−66 M) of the (‐)‐enantiomers of both drugs displaced [3 H]spiperone from its specific binding site in rat striatal preparations, but the (+)‐enantiomers were 40 and 100 times less active. However, neither enantiomer of either drug anatagonized the dopamine‐induced stimulation of adenylate cyclase in rat striatal preparations. The data suggest that the central pharmacological activity of sulpiride and sultopride resides in the (‐)‐enantiomers and that this activity occurs at cerebral dopamine receptors not dependent on adenylate cyclase for functional activity.

Dopamine-mediated circling behaviour does not involve the nigro-tectal pathway

SummaryExtensive unilateral or bilateral electrolytic ablation of the rat superior colliculus failed to reduce apomorphine- or amphetamine-induced rotation in animals with a unilateral 6-hydroxydopamine lesion of one nigro-striatal dopaminergic pathway. These findings suggest that a nigro-tectal pathway does not play a crucial role in mediating the circling response caused by striatal dopamine receptor stimulation.However, electrolytic lesions of the dorsal tegmental decussation reduced apomorphine- but not amphetamine-induced rotation in such animals, perhaps by sectioning some commissural pathway between the two nigro-striatal systems.

Effects of somatostatin on dopamine sensitive adenylate cyclase activity in the caudate-putamen of the rat.

SummaryThe effect of somatostatin-14 (SRIF) on dopamine-sensitive adenylate cyclase in caudateputamen pellets was studied in naive female rats, and in rats with chemical lesions of the nigrostriatal dopaminergic tract produced by injecion of 6-hydroxydopamine, or of the caudate-putamen itself produced by injection of kainic acid 3 week earlier. In unlesioned rats somatostatin at a concentration of 10−7 moles/1 inhibited adenylate cyclase activation by submaximal concentrations of dopamine, increasing the apparent Km but not altering Emax. In 6-hydroxydopamine lesioned rats somatostatin no longer influenced adenylate cyclase activity, whereas in kainic acid lesioned rats somatostatin still increased the apparent Km for dopamine activation. The effect of somatostatin in untreated and lesioned rats is compatible with a partial competitive antagonism to dopamine. Although the data from the lesioned rats present preliminary results, the dose response characteristics and the effects in lesioned animals suggest a more complex interaction, possibly by binding of somatostatin to an inhibitory subunit of regulatory adenylate cyclase components.

Differentiation of dopamine agonists using drug-induced rotation in rats with unilateral or bilateral 6-hydroxydopamine destruction of ascending dopamine pathways

Eighteen compounds with dopamine agonist properties were examined in two rat rotational models. In the classical Ungerstedt model, a unilateral 6-hydroxydopamine (60HDA) lesion destroyed nigrostriatal and mesolimbic dopamine pathways on one side. Indirectly acting compounds, amphetamine, amantadine, methylphenidate and S1694, produced ipsiversive rotation, which was inhibited by pretreatment with alpha-methyl-p-tyrosine (AMPT). All other compounds produced contraversive rotation, but the rotation caused by CM 29-712, bromocriptine and ET 495 was reduced by AMPT. In animals with a bilateral 60HDA lesion removing both dopaminergic inputs to nucleus accumbens and the dopaminergic input into one striatum, indirectly acting drugs caused ipsiversive posturing prevented by AMPT, but little rotation. All other compounds produced contraversive rotation, but the effects of CM 29-712, bromocriptine and ET 495 were reduced by AMPT pretreatment. Inhibition of monooxygenase drug metabolising activity utilising SKF-525A inhibited contraversive turning induced by bromocriptine and ET 495 in the unilateral lesion model, but had no effect on rotation caused by apomorphine or CM 29-712. We conclude that, in addition to indirect pre-synaptically acting agonists and direct post-synaptic receptor agonists, there are a small group of compounds which produce rotation associated with direct receptor activation, which is inhibited by disruption of pre-synaptic dopamine function. The mechanism of action of this latter group is not understood, but cannot be attributed solely to active metabolite formation.

The role of nigral projections to the thalamus in drug-induced circling behaviour in the rat

Abstract Unilateral injection of 6-hydroxydopamine (6-OHDA) into the ascending nigrostriatal pathway caused contraversive circling to apomorphine and ipsiversive circling to amphetamine respectively. An electrolesion of the ventromedial thalamic nucleus on the same side as the 6-OHDA lesion reduced apomorphine-induced circling, but not that to amphetamine. An electrolesion of the ventromedial thalamic nucleus on the side opposite to the 6-OHDA lesion reduced amphetamine circling but not that to apomorphine. Bilateral electrolesions of the ventromedial thalamic nucleus reduced neither apomorphine- nor amphetamine-induced circling. Electrolytic lesions of the parafascicular thalamic nucleus did not reduce apomorphine- or amphetamine-induced circling in animals with a unilateral 6-OHDA lesion of the nigrostriatal pathway. Knife cuts rostral and dorsal to the substantia nigra did not attenuate circling induced by injection of muscimol into the substantia nigra. Circling due to activition of nigral output pathways can be mediated by descending nigro-reticular pathways.