GABA may function tonically via GABA A receptors to inhibit hypotension and bradycardia by l-DOPA microinjected into depressor sites of the nucleus tractus solitarii in anesthetized rats

We have proposed that DOPA is a transmitter 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. GABA may inhibit DOPAergic transmission. Drugs were microinjected into depressor sites of the NTS in anesthetized rats. DOPA (10–60 ng) elicited dose-dependent depressor responses. GABA (3–300 ng) elicited dose-dependent pressor responses. Nipecotic acid (100 ng) elicited pressor responses. Bicuculline (10 ng) elicited depressor responses. Responses to DOPA (30 ng) were inhibited by pretreatment with GABA and nipecotic acid, but potentiated by bicuculline, when vascular responses to pretreated drugs returned to basal levels. DOPA ME, a competitive DOPA antagonist, did not displace specific [ 3H]GABA binding. Prior DOPA ME (1 μg) inhibited by one-half pressor responses to 300 ng GABA. GABA seems to inhibit tonically via GABA A receptors depressor responses to DOPA and to elicit pressor responses partially by inhibition of tonic function of endogenous DOPA to activate depressor sites in the NTS. These findings further support the above proposal.