The evidence for tonic GABAergic regulation of basal l-DOPA release via activation of inhibitory GABA A receptors in the nucleus tractus solitarii of anesthetized rats

We have proposed that DOPA is a neurotransmitter of the primary baroreceptor afferents terminating in the rat nucleus tractus solitarii (NTS). GABA is a putative inhibitory neuromodulator for baroreflex inputs in the NTS. Thus, GABA may inhibit DOPAergic transmission in the NTS. We tried to clarify whether basal DOPA release is inhibited by muscimol, a GABA A agonist, and facilitated by bicuculline, a GABA A antagonist, during microdialysis of the NTS in anesthetized rats. DOPA release was consistently detectable. Muscimol 10–100 μM perfused via probes gradually inhibited concentration-dependently DOPA release. Peak 30% inhibition occurred 2 h after perfusion. Muscimol (30 μM)-induced inhibition was antagonized by non-effective 10 μM bicuculline. Bicuculline (30 μM) elicited peak 30% facilitation of DOPA release 2 h after perfusion. Endogenous GABA seems to regulate tonically basal DOPA release via activation of inhibitory GABA A receptors in the rat NTS. These findings further support the above proposal.