Tolerance to increases in striatal acetylcholine concentrations after repeated administration of apomorphine dipivaloyl ester

Cortical and nigral deafferentation and striatal cholinergic markers in the rat dorsal striatum: different effects on the expression of mRNAs encoding choline acetyltransferase and muscarinic m1 and m4 receptors

The regulation of the striatal m1 and m4 muscarinic receptor mRNA as well as the choline acetyltransferase (ChAT) mRNA expression by nigral dopaminergic and cortical glutamatergic afferent fibres was investigated using quantitative in situ hybridization histochemistry. The effects induced by a unilateral lesion of the medial forebrain bundle and a bilateral lesion of the sensorimotor (SM) cortex were analysed in the dorsal striatum 3 weeks after the lesions. Dopaminergic denervation of the striatum resulted in a marked decrease in the levels of m4 mRNA throughout the striatum, while the levels of muscarinic m1 mRNA and ChAT mRNA in cholinergic neurons were unaffected by the lesion. In contrast, following bilateral cortical ablation, the levels of the muscarinic m1 mRNA were significantly increased in the striatal projection area of the SM cortex, whereas the expression of m4 mRNA remained unchanged. Single cholinergic cell analysis by computer-assisted grain counting revealed a decreased labelling for ChAT mRNA per neuron following cortical ablation. However, in contrast to the topographical m1 mRNA changes, the decreased ChAT mRNA expression was evenly distributed within the striatum, suggesting an indirect cortical control upon striatal cholinergic interneurons. Altogether, these data suggest that dopaminergic nigral and glutamatergic cortical afferents modulate differentially cholinergic markers, at the pre- and post-synaptic levels. Beside the fact that nigral and cortical inputs exert an opposite control on cholinergic neurotransmission, our study further shows that this control involved different muscarinic receptor subtypes: the m4 and m1 receptors, respectively.

Evidence for, and nature of, the tonic inhibitory influence of habenulointerpeduncular pathways upon cerebral dopaminergic transmission in the rat

The potential role of the habenula in the transsynaptic regulation of the activity of ascending dopaminergic systems has been investigated in the rat by studying the effect of an acute interruption of impulse traffic in the diencephalic conduction system (stria medullaris-habenula-fasciculus retroflexus) and of pharmacological manipulation of various neurotransmitter systems in the interpeduncular nucleus on dopamine metabolism in several dopaminergic projection fields. The bilateral infusion of tetrodotoxin into the fasciculus retroflexus (which conveys the habenulointerpeduncular tract) of conscious rats markedly increased homovanillic acid levels and dopamine synthesis and utilization in the medial prefrontal cortex, nucleus accumbens, olfactory tubercle and striatum. Similar changes in dopamine metabolism were observed in these areas after bilateral infusion of tetrodotoxin into the stria medullaris (which conveys most of the afferents to the habenula). Infusion of atropine (0.4-1 micrograms) into the interpeduncular nucleus increased homovanillic acid concentrations and dopamine utilization in the medial prefrontal cortex and nucleus accumbens but not in the olfactory tubercle and striatum. Moreover, intra-interpeduncular injection of oxotremorine (17 micrograms) antagonized the increase in dopamine utilization in the nucleus accumbens (but not in the olfactory tubercle) induced by an intrafasciculus retroflexus infusion of tetrodotoxin. Local infusion of naloxone (20 micrograms) into the interpeduncular nucleus increased homovanillic acid concentrations in the nucleus accumbens and olfactory tubercle but not in the medial prefrontal cortex and striatum. In contrast, intra-interpeduncular nucleus infusion of the substance P antagonist D-Arg1, D-Pro2, D-Trp7,9, Leu11-substance P or of substance P antiserum failed to alter homovanillic acid levels in the 4 dopamine-rich areas investigated. Finally, intraraphé medianus (but not intraraphé dorsalis) infusion of muscimol (25 ng) moderately increased dopamine synthesis in the nucleus accumbens and striatum. The present findings suggest that the habenulointerpeduncular pathways exert a tonic inhibitory influence on mesocortical, mesolimbic and mesostriatal dopaminergic neurons. Cholinergic and/or opioid peptidergic neurons coursing through the fasciculus retroflexus as well as ascending serotonergic neurons originating in the raphé medianus could take part in this inhibitory control of ascending dopaminergic neurons.

Time course of the changes in striatal acetylcholine levels induced by pergolide and haloperidol after lesion of the nigrostriatal dopaminergic pathways in the rat

Lesion of the nigrostriatal dopaminergic pathway in the rat by 6-hydroxydopamine enhances the ability of pergolide to increase striatal acetylcholine levels and prevents the haloperidol-induced decrease in acetylcholine concentrations. This supersensitive response of striatal cholinergic cells is already maximal 6 days after lesion but tends to decrease thereafter. As the time course of the development of the supersensitivity of cholinergic cells differs from that of increased dopamine binding site density, the two are probably not causally related, the former reflecting rather a change occurring beyond the dopamine recognition site.

Hypophysectomy fails to affect the supersensitivity of striatal dopamine target cells induced by prolonged haloperidol treatment

The influence of hypophysectomy on biochemical indices of striatal dopamine target cell supersensitivity induced by prolonged haloperidol treatment was investigated in the rat. Hypophysectomy itself did not modify dihydroxyphenylacetic acid (DOPAC) levels but slightly enhanced acetylcholine concentrations in the striatum. Hypophysectomy failed to affect the ability of haloperidol, apomorphine and pergolide to alter these biochemical parameters after acute administration. Prolonged administration of haloperidol (by means of osmotic minipumps delivering 2.5 micrograms/h) for 14 days caused a decrease in DOPAC and an increase in acetylcholine levels in the striatum during withdrawal; these effects were of a similar magnitude in sham-operated and hypophysectomized rats. Moreover, there was a similar degree of tolerance to the elevation of DOPAC and to the diminution of acetylcholine concentrations in striatum in response to challenge with haloperidol during withdrawal in sham-operated and hypophysectomized animals. Finally, a similar supersensitive biochemical response to pergolide (decrease in DOPAC and increase in acetylcholine levels) was observed in both hypophysectomized and sham-operated animals after prolonged haloperidol treatment. These data suggest that hypophyseal factors do not affect the development of striatal dopamine target cell supersensitivity caused by prolonged haloperidol treatment.

Effects of cyclo (Leu-Gly) on neurochemical indices of striatal dopaminergic supersensitivity induced by prolonged haloperidol treatment

The effects of a prolonged treatment with cyclo (Leu-Gly) and/or haloperidol on biochemical parameters indicative of striatal dopamine target cell supersensitivity have been investigated in the rat. When given acutely, cyclo (Leu-Gly) (2 mg/kg sc) did not affect striatal homovanillic acid, dihydroxyphenylacetic acid and acetylcholine levels both under basal conditions or after acute haloperidol (1 mg/kg ip) treatment. When given concomitantly with haloperidol (infused by means of osmotic minipumps at a rate of 2.5 micrograms/h sc) for 14 days, cyclo (Leu-Gly)(2 mg/kg sc once daily) failed to prevent the fall of striatal dopamine metabolites observed 2 days following withdrawal and the tolerance to the elevation of dopamine metabolites which occurs in response to challenge with the neuroleptic during withdrawal. Prolonged treatment with cyclo (Leu-Gly) also failed to affect the tolerance to the decrease of striatal acetylcholine levels which occurs under chronic haloperidol treatment. These data suggest that the mechanism whereby cyclo (Leu-Gly) inhibits the development of neuroleptic-induced dopaminergic supersensitivity does not involve an action of the peptide on nigro-striatal dopaminergic and striatal cholinergic neurons and is probably exerted distally to both dopaminergic and cholinergic synapses.

Influence of lithium on biochemical manifestations of striatal dopamine target cell supersensitivity induced by prolonged haloperidol treatment

The effects of prolonged treatment with dietary lithium and/or haloperidol (infused by means of osmotic minipumps) on biochemical parameters indicative of striatal dopamine target cell supersensitivity have been investigated in the rat. When given concomitantly with haloperidol, lithium failed to prevent the fall of striatal dopamine metabolites observed 2 days following withdrawal and the tolerance to the elevation of dopamine metabolites in response to challenge with the neuroleptic during withdrawal. Prolonged treatment with lithium also failed to modify the changes in nigral dihydroxyphenylacetic acid levels and in striatal acetylcholine levels which occur under chronic neuroleptic treatment. Chronic dietary lithium alone consistently elevated substance P levels in substantia nigra. The usual decrease in nigral levels of the peptide that occurs in response to chronic treatment with haloperidol was prevented in animals treated concomitantly with lithium. These data suggest that the mechanism whereby lithium stabilizes dopaminergic supersensitivity does not seem to involve an action of the compound on the neuronal mechanisms regulating the activity of the nigro-striatal dopaminergic system or on striatal cholinergic neurons but may be related to the restoration of normal striato-nigral substance P ergic transmission.

Characterization of the excitatory amino acid receptor-mediated release of [3H]acetylcholine from rat striatal slices

The pharmacological nature of the interaction of excitatory amino acids with striatal cholinergic neurons was investigated in vitro. Agonists of excitatory amino acid receptors evoked the release of [3H]acetylcholine from slices of rat striatum, in the presence of magnesium (1.2 mM). Removal of magnesium from the medium markedly increased the release of [3H]acetylcholine evoked by all excitatory amino acid receptor agonists tested, with the exception of kainate. In the absence but not the presence of magnesium, a clear rank order of potency was found: N-methyl-DL-aspartate = ibotenate greater than L-glutamate greater than L-aspartate greater than or equal to cysteate greater than kainate = quisqualate. The excitatory amino acid receptor mediating [3H]acetylcholine release resembles the N-methyl-D-aspartate preferring (N-type) receptor, as previously characterized electrophysiologically, according to 3 criteria: (1) rank order of potency of agonists; (2) magnesium-sensitivity; and (3) antagonism by 2-amino-5-phosphonovalerate. The release of [3H]acetylcholine evoked by N-methyl-DL-aspartate was blocked by tetrodotoxin (0.5 microM). Moreover, N-methyl-DL-aspartate failed to evoke [3H]acetylcholine release from slices of hippocampus, where cholinergic afferents, rather than interneurons, are found. These results suggest that excitatory amino acids act at receptors on the dendrites of striatal cholinergic interneurons, giving rise to action potentials and release of acetylcholine from cholinergic nerve terminals.

Cortical modulation of striatal function

The effect of bilateral section of the corticostriatal projections or of selective bilateral ablation of the frontal cortex on behavioral and biochemical parameters related to striatal function were investigated in the rat. Either lesion almost completely prevented the cataleptogenic action of haloperidol: this effect was observed as soon as 3 days and lasted for at least 3 months after surgery, paralleling a reduction in striatal glutamate uptake. Also, such lesions enhanced the apomorphine-induced stereotyped behavior (as measured 21 days after surgery). In the striatum, dopamine, dihydroxyphenylacetic acid, acetylcholine and substance P levels as well as choline acetyltransferase and glutamic acid decarboxylase activities were unaffected 10 or 21 days after either type of lesion. In the substantia nigra, substance P levels were unchanged 10 days following suction of the frontal cortex, but glutamic acid decarboxylase was reduced at 21 days postsurgery. Cortical lesions only partially prevented the reduction in striatal acetylcholine concentrations and did not affect the increase in striatal dihydroxyphenylacetic acid caused by haloperidol. Finally, lesions of the corticostriatal pathways failed to affect the apomorphine-induced increase in striatal acetylcholine levels, reduction of the potassium (20 mM) evoked [3H]acetylcholine release in striatal slices preloaded with [3H]choline and decrease of striatal dihydroxyphenylacetic acid concentrations. These findings indicate that the frontal cortex influences extrapyramidal function by a mechanism which--in behavioral terms--is antagonistic to dopamine-mediated events. As indicated by the biochemical data, this mechanism does not involve changes in striatal dopaminergic and cholinergic neuron activity. This mechanism may utilize: (1) corticostriatal glutamatergic neurons as suggested by the reduction in striatal glutamate uptake following lesions; and (2) GABAergic pathways as suggested by the reduction of nigral glutamic acid decarboxylase activity as well as by the finding that GABA receptor agonists reinstate haloperidol-induced catalepsy.

Increase in striatal acetylcholine levels by GABAergic agents: dependence on corticostriatal neurons

Surgical lesion of the corticostriatal projections almost totally reduced the ability of GABA agonist agents but not apomorphine to increase rat striatal ACh concentrations. This suggests that in intact rats, the GABAergic inhibition of striatal cholinergic neurons depends, at least in part, upon the corticostriatal (possibly glutamatergic) tract whereas the action of the dopaminergic pathway on cholinergic cells is independent of corticostriatal neurons.