Physiologic and pharmacologic characterization of a novel group of substantia nigra zona reticulata neurons involved in striatal dopamine regulation.

The physiologic and pharmacologic characteristics of a novel group of substantia nigra zona reticulata (SNR) neurons were studied using standard electrophysiologic techniques in lightly pentobarbital anesthetized rats. Microinjection of dopamine (DA) into the caudate nucleus (CN) (25-40 micrograms) sufficiently activated these SNR cells. Application of the inhibitory amino acid, glycine (7 micrograms), into the substantia nigra zona compacta (SNC) simultaneously suppressed compacta neuronal discharges and enhanced CN activities. Direct nigral injection of the glycine-antagonist, strychnine (5 micrograms), on the other hand, resulted in a parallel decrease in CN activities and an increase in SNC discharges. The glycine/strychnine effects on CN were blocked by haloperidol (100 micrograms/kg, i.p.) pretreatment. These observations suggested that this novel group of SNR cells may participate in the striatonigral feedback regulation of caudate DA contents. Furthermore, these reticulata neurons may exert a tonic inhibition on the compacta cells, possibly using glycine as the putative neurotransmitter. Pharmacologically, morphine (5 mg/kg, i.v.) may promote a suppression of spontaneous CN activities by simultaneously exciting the DA-containing SNC nigrostriatal neurons and inhibiting these novel SNR neurons, in a process that was reversed by naloxone (1 mg/kg, i.v.).