The effects of muscimol and picrotoxin injections into the cat substantia nigra.

The behaviour of cats after unilateral injections of muscimol, picrotoxin and bicuculline into the posterior and lateral parts of the substantia nigra was observed. The antagonism between muscimol and picrotoxin was limited to some effects of the drugs: (1) muscimol-induced sniffing, licking and enhanced locomotor activity was attenuated by picrotoxin in contrast to the muscimol-induced contralateral turning which was even enhanced by picrotoxin; (2) picrotoxin-induced balance disorders as well as characteristic motor disturbances in the hind legs remained unaffected following an additional treatment with muscimol. Furthermore, subcutaneously given apomorphine did not affect the picrotoxin-induced balance and hind leg disorders, although the apomorphine-induced stereotyped behaviour itself was enhanced and suppressed by intranigral administration of muscimol and picrotoxin respectively. The conclusion is reached that there are at least two distinct populations of GABA receptors within the substantia nigra: one group controlling or being controlled by dopaminergic neurons and one group operating fully independently of the former neurons. The similarities to experiments on rats are stressed. The mechanism underlying the muscimol-resistant picrotoxin-induced effects (balance disorders and motor disturbance of the hind legs) is discussed.