Substantia nigra as a station that not only transmits, but also transforms, incoming signals for its behavioural expression: Striatal dopamine and GABA-mediated responses of pars reticulata neurons

Abstract

In order to investigate whether striatal dopaminergic mechanisms are involved in the behavioural expression of the GABAergic mechanisms within the pars reticulata of the substantia nigra, apomorphine or haloperidol were bilaterally administered into the caudate nucleus of cats pretreated with a unilateral injection of picrotoxin or muscimol into the nigral pars reticulata. Although the doses selected for the intracaudate injections have been shown to be maximally effective in affecting the behavioural expression of the caudate function, the pharmacological treatment of the caudate nucleus did not produce any significant change in the behaviour elicited from the nigra; neither the picrotoxin-induced asymmetric posturing, asymmetric circling, freezing and hind leg disorders nor the muscimol-induced asymmetric posturing, asymmetric spinning and stereotyped licking were significantly affected. The latter behaviour was absent in animals with a partial or total destruction of the nigral pars reticulata. As the behavioural expression of the nigral pars reticulata differed completely from the asymmetric head twisting known to be characteristic for the caudate nucleus, it is suggested that the behavioural expression of the caudate nucleus requires a main output station elsewhere in the brain. Furthermore, the present results demonstrate that the nigral pars reticulata does not form part and parcel of a feedback system that simply transmits incoming signals from the caudate nucleus towards the pars compacta, i.e. the origin of the dopaminergic, nigrostriatal system. Finally, the present study demonstrates that the dopaminergic activity within the caudate nucleus may only modify, but certainly not determine, the behavioural expression of the nigral pars reticulata. It is concluded that the nigral pars reticulata not only transmits, but also transforms its incoming signals.