A role of the polysynaptic system of substantia nigra in the cholinergic-dopaminergic equilibrium in the central nervous system

Comparison of the behavioural effects of infusion of carbachol and acetylcholinesterase into the rat substantia nigra

It has been postulated for many years that acetylcholinesterase (AChE) may play a nonclassical role in the substantia nigra, unrelated to its ability to hydrolyse acetylcholine. In this study the behavioural effects of unilateral infusion of AChE and a cholinergic agonist, carbachol, were compared. Carbachol induced ipsiversive circling over a very short time scale (minutes), whereas AChE induced contraversive circling, but over a longer time course-10 days. Both agents showed selectivity of response within the substantia nigra: acetylcholinesterase was only effective when infused into the most rostral region of the substantia nigra and its effects were limited to the pars compacta. In contrast, carbachol had effects in both the pars compacta and reticulata, with a graded sensitivity to carbachol in the rostral/caudal plane; infusions into rostral regions induced high rates of circling compared to more caudal areas, suggesting that the cholinergic input to the substantia nigra is not homogenous, but greater in rostral regions. This disparity between the effects of carbachol and AChE would, therefore, suggest that AChE is not exerting its long-term behavioural actions via a cholinergic mechanism, both in terms of time course of the response and the areas within the substantia nigra sensitive to these agents.

Cholinergic stimulation of rostral and caudal substantia nigra pars compacta produces opposite effects on circling behavior and striatal dopamine release measured by brain microdialysis

Turning in circles is among the behaviors elicited by unilateral cholinergic stimulation of the substantia nigra. Recent studies have shown that microinjection of cholinergic agonists into the substantia nigra pars compacta increases dopamine release and turnover in the striatum of anesthetized rats [Hernández-López et al. (1992) Brain. Res. 598, 114-120; Blaha and Winn (1993) J. Neurosci, 13, 1035-1044]. In this study, the relationship between circling behavior and striatal dopamine release following cholinergic stimulation of the substantia nigra pars compacta neurons was assessed by brain microdialysis in awake rats. The results indicate that cholinergic stimulation of the substantia nigra pars compacta with the mixed nicotinic-muscarinic cholinergic agonist carbachol modulates striatal dopamine release, and this effect is accompanied by circling behavior and stereotypies. Microinjection of carbachol (109 nmol) in the caudal portions of the substantia nigra pars compacta induced contralateral circling associated with an increase of dopamine release in neostriatum. On the contrary, ipsilateral circling and reduction of striatal dopamine release was elicited when the same dose of the drug was applied in the rostral portions of the substantia nigra pars compacta. The above findings are in accordance with recent electrophysiological studies suggesting the existence of sub-populations of nigrostriatal dopaminergic neurons, and indicate that the substantia nigra pars compacta is functionally compartmentalized. We conclude that the cholinergic input to the substantia nigra pars compacta could modulate the motor behavior through regulating the firing rate of nigrostriatal dopaminergic neurons and dopamine release in the neostriatum.

Comparison of changes in locomotor activity with striatal homovanillic acid and 3,4-dihydroxyphenylacetic acid concentrations following the bilateral intranigral injection of dopamine agonist drugs in rats

Bilateral injection of apomorphine (2.5 micrograms) into the substantia nigra zona reticulata of rats reduced both locomotor activity and striatal HVA and DOPAC concentrations. Bilateral injection of dopamine (10 micrograms) did not affect locomotor activity whereas a higher dose of dopamine (50 micrograms) enhanced locomotor activity. Striatal HVA and DOPAC concentrations were unchanged following injection of dopamine. Bilateral injection of (+/-)-3PPP (0.1 or 2.5 micrograms) into the zona reticulata of the substantia nigra did not alter locomotor activity while a higher dose (10 micrograms) enhanced locomotion. Injection of (+/-)-3PPP (0.1-10 micrograms) into the zona reticulata was without effect on striatal HVA or DOPAC concentrations. The bilateral manipulation of nigral dopaminergic neurotransmission alters motor activity and nigrostriatal dopamine turnover in conscious rats. However, the changes in motor activity are not necessarily related to altered nigrostriatal activity, suggesting the involvement of dopamine receptors located at non-dopaminergic sites within the substantia nigra.

Differential effects of striatal injections of dopaminergic, cholinergic and GABAergic drugs upon swimming behavior of rats

The present study provides a detailed report about similarities and dissimilarities between the effects of neostriatally applied dopaminergic (apomorphine, 250-300 ng; haloperidol, 250-500 ng), cholinergic (carbachol, 50-100 ng; scopolamine, 200-500 ng), and GABAergic (muscimol, 1-2 ng; bicuculline, 5-35 ng) drugs upon swimming of rats. The used swimming test consisted of 4 parts: (a) open-field test for analyzing drug-induced changes in normal behavior; (b) 'swimming without escape' test for analyzing drug-induced changes in the ability to switch from one type of behavior to another; (c) 'swimming with escape' test for analyzing drug-induced changes in the ability to switch from ongoing swimming behavior to climbing behavior by allowing the rats to escape via a rope; and (d) 'rope' test for analyzing drug-induced changes in the kind of contact behaviors needed to switch to the latter climbing behavior. In the open-field test the drugs produced neither abnormal behavior nor motor disturbances, which prevented the display of normal behavior in the remaining tests. Both apomorphine and carbachol produced identical effects in all tests. Muscimol produced overall effects which were not only opposite to those of apomorphine and carbachol, but also comparable to those of scopolamine. All effects elicited by apomorphine, carbachol and muscimol were antagonized by their corresponding antagonists: haloperidol, scopolamine and bicuculline respectively, whereas the effects of the latter were suppressed by their corresponding agonists. These data globally show that dopamine and acetylcholine act in the same direction but opposite to that of GABA as far as it concerns the regions investigated. The finding that haloperidol injected into the GABA target area produced effects which were not only similar to those of haloperidol injected into the dopamine target area, but also dissimilar to those of muscimol and bicuculline injected into the GABA target area, shows that the effects were drug-specific rather than region-specific. Though 3 distinct cholinergic regions were investigated, cholinergic-specific effects could only be elicited from one region, suggesting that the neostriatum is heterogeneous in this respect. Finally, well-delineated dissimilarities between haloperidol-, scopolamine-, and muscimol-treated rats were found in the rope test. These data show that behavior-relevant information transmitted by GABAergic drugs surmounted that transmitted by cholinergic drugs which, in turn, surmounted behavior-relevant information transmitted by dopaminergic drugs.

Biochemical correlates of motor changes caused by the manipulation of dopamine function in the substantia nigra of the mouse

2-Di-n-propylamino-5,6-dihydroxytetralin, injected bilaterally into the substantia nigra of the mouse, caused dose-dependent motor inhibition which was associated with decreased levels of DOPAC and increased levels of dopamine in the striatum. (-)Sulpiride, injected into the substantia nigra, antagonised the locomotor depression although the partial antagonism of the elevation in the level of dopamine in the striatum and of the reduction in levels of DOPAC did not achieve significance. The specificity of the action of tetralin on dopamine receptors was shown by the failure of prazosin and yohimbine to antagonise the locomotor depression induced by tetralin and the reduction in levels of DOPAC. The selectivity of the action of tetralin for the dopamine system was shown by its failure to affect levels of noradrenaline, serotonin and 5-hydroxyindoleacetic acid in the striatum. The injection of tetralin into the substantia nigra also caused biochemical changes in limbic areas (nucleus accumbens and tuberculum olfactorium), where the levels of dopamine and DOPAC were elevated, and in the frontal cortex where the levels of DOPAC were reduced. These changes were antagonised by a concomitant injection of (-)sulpiride into the substantia nigra. It is concluded that the action of dopamine agonists in the midbrain can decrease the functional activity in the ascending dopaminergic pathways.

Lack of effect of intranigral administration of a dopamine analogue, (+/-)-3-(3-hydroxyphenyl)-N,n-propylpiperidine [(+/-)-3-PPP], on nigrostriatal dopamine neurones

Bilateral injection of apomorphine into the zona reticulata of substantia nigra in rats reduced locomotor activity and striatal dopamine (DA) turnover. In contrast, bilateral injection of (+/-)-3-(3-hydroxyphenyl)-N,n-propylpiperidine [(+/-)-3-PPP] into the zona reticulata (ZR) did not reduce locomotor activity and did not affect striatal DA turnover. A high dose of (+/-)-3-PPP increased locomotor activity. Unilateral injection of (+/-)-3-PPP, but not of apomorphine, into the ZR induced a brief period of contraversive rotation. (+/-)-3-PPP may not alter nigral DA autoreceptor function but may interact with DA receptors located on non-dopaminergic nigral neurones.

Cholinergic modulation of stimulant-induced behavior

Stimulant-induced stereotypy, presumably mediated by nigrostriatal dopaminergic neurons, can be altered by the administration of cholinergic agonists or antagonists. Cholinergic, striatal interneurons have been postulated as the site of these effects, although the specific site of interaction between cholinergic and dopaminergic systems is unknown. The study reported here examined the effects of the cholinesterase inhibitor physostigmine, and several other cholinergic and anticholinergic drugs, on stimulant-induced behavior. Stereotypy was inhibited by physostigmine to the same degree whether induced by direct (apomorphine) or by indirect acting (amphetamine and methylphenidate) stimulants. The results are interpreted as indicating that the site of cholinergic modulation of stimulant-induced stereotypy is postsynaptic to the dopaminergic neurons which mediate stereotypy.

Do cholinomimetics specifically antagonize rotational behavior?

Asymmetries of body posture and movement (rotational behavior) following intracerebral application of dopamine (DA, 200 micrograms/2 microliters) or carbachol (30 and 60 micrograms/2 microliters) were diminished by systemically applied oxotremorine (0.5-1.0 mg/kg IP). The combined intracerebral injections of DA and carbachol show localization-specific effects in caudate nucleus, substantia nigra and globus pallidus. The experiments point to specific dopaminergic-cholinergic interaction in different brain regions. Systemically applied cholinomimetics exert a generally suppressive action on motoric effects.

Drug-induced stereotypes and asymmetric behaviour after substantia nigra pars posterior (SNPP) lesions in cats

Apomorphine- (Apo) and amphetamine- (Amph) induced behavioural phenomena were studied in cats which received either 6-hydroxydopamine (6-OHDA) or electrothermic lesions of the posterior part of the substantia nigra (SNPP). In this species, as in rats, both drugs evoked sterotypies (sniffing and head nodding) and an enhancement of locomotor activity. However, distinct differences between the reactions of cats and rats to both drugs were found, the most evident effect being the lack of sterotyped gnawing and licking. By correlating the data on behaviour, the histological examination showing the size and location of the lesions, and the dopamine (DA) content of the corresponding caudate nuclei, it is concluded that the fewer DA-specific neurons lesioned in the SNPP, the more pronounced was the ipsilateral asymmetric behaviour. We suggest, therefore, that the ipsilateral asymmetric behaviour in cats following APO- and AMPH-treatment is due to the destruction of a non-catecholaminergic output.

GABAergic and glycinergic mechanisms within the substantia nigra: pharmacological specificity of dopamine-independent contralateral turning behavior and interactions with other neurotransmitters

The pharmacological specificity of the GABA agonist muscimol-induced contralateral turning behavior after unilateral injection into substantia nigra pars reticulata (SNR) has been studied. Muscimol-induced turning was antagonized by intranigral bicuculline methochloride (BMC) and picrotoxin, whereas antagonists of glycine, morphine, dopamine, noradrenaline, and serotonin were ineffective. Glycine induced a qualitatively similar turning behavior which was strychnine-sensitive but relatively BMC and picrotoxin-insensitive. Other drugs, including substance P, kainic acid, clonidine, oxymetazoline, serotonin, and carbachol, induced turning that could be dissociated from the effect of muscimol. Muscimol-induced turning was dopamine-independent, indicated by resistance to haloperidol (1 mg/kg), to pretreatment with reserpine (7.5 mg/kg) plus alpha-methyl-p-tyrosine (200 mg/kg), to haloperidol injections into the SNR, striatum and nucleus accumbens, and finally to kainic acid lesions of the striatum. 6-Hydroxydopamine lesions increased the efficacy of intranigral muscimol, while kainic acid lesions of the SNR antagonized muscimol. Muscimol-induced turning was inhibited by oxotremorine (0.25 mg/kg), by intranigral carbachol, and by apomorphine (0.1--0.5 mg/kg), but only moderately by intranigrally injected apomorphine. These data suggest specificity of GABA-agonist-induced contralateral turning and indicate an interaction between nigral GABA and other neurotransmitters, particularly dopamine and acetylcholine.

The effects of intranigral injections of picrotoxin and carbachol in cats with a lesioned nigrostriatal pathway

Picrotoxin, carbachol and atropine were injected intranigrally through chronically implanted crannulae in cats. The drug-induced behavioural syndromes, mainly asymmetric behaviour, were analyzed. The changes in these syndromes after 6-hydroxydopamine (6-OHDA) lesion of the substantia nigra (SN) indicate that, in addition to the dopaminergic (DA), a non-catecholaminergic (non-CA) nigral output exists, both being responsible for asymmetric behaviour. It is further suggested that gamma-aminobutyric acid (GABA) receptors are present on both dopaminergic and non-CA nigral output neurons, while acetylcholine (ACh) receptors, responsible for asymmetric behaviour, are almost exclusively located on non-CA nigral output neurons.