Presynaptic GABAA receptors facilitate GABAergic transmission to dopaminergic neurons in the ventral tegmental area of young rats

gamma-Aminobutyric acid A receptor (GABA(A)R)-mediated postsynaptic currents (IPSCs) were recorded from dopaminergic neurons of the ventral tegmental area of young rats in acute brain slices and from mechanically dissociated neurons. Low concentrations (0.1-0.3 mu M) of muscimol, a selective GABAAR agonist, increased the amplitude, and reduced the paired pulse ratio of evoked IPSCs. Moreover, muscimol increased the frequency but not the amplitude of spontaneous IPSCs (sIPSCs). These data point to a presynaptic locus of muscimol action. It is interesting that 1 pm muscimol caused an inhibition of sIPSCs, which was reversed to potentiation by the GABAB receptor antagonist CGP52432. Isoguvacine, a selective GABAAR agonist that belongs to a different class, mimicked the effects of muscimol on sIPSCs: it increased them at low (<= 0.5 mu M), and decreased them at a higher concentration (1 mu M). Hence, the activation of presynaptic GABA(A)Rs facilitates GABA release, which is limited by presynaptic GABA(B)Rs. Furthermore, facilitation of sIPSCs by muscimol was eliminated in a medium containing tetrodotoxin or cadmium. It is noteworthy that sIPSC frequency was greatly increased by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol(gaboxadol, or THIP), an agonist with preferential effects on extrasynaptic GABAARs containing alpha 4 beta delta subunits, or by guvacine, a GABA transport blocker, which increases ambient GABA levels. In addition, sIPSC frequency was attenuated by furosemide, a selective antagonist of alpha 6 subunits. Thus, the presynaptic GABAARs may be situated at extrasynaptic sites and may contain alpha 4/6 beta delta subunits. Given the marked sensitivity of extrasynaptic GABAARs to ambient GABA, alcohols and anaesthetics, these receptors may present a critical site for regulating synaptic function in the developing brain in both physiological and pathological situations.