GABA AGONISTS AND OMEGA CONOTOXIN GVIA MODULATE RESPONSES TO NERVE ACTIVATION OF THE PERFUSED RAT MESENTERY

The modulatory actions of gamma-aminobutyric acid (GABA) receptor agonists and omega-conotoxin GVIA (CTX) on sympathetic and sensory nerves were examined on contractile responses of the perfused rat mesentery to transmural nerve stimulation (TNS). GABA and baclofen, a selective GABA(B) receptor agonist, significantly inhibited vasoconstrictor responses to TNS, while muscimol, a selective GABA(A) receptor agonist, had no effect. In the guanethidine treated and methoxamine-contracted mesentery, TNS caused a vasodilator response which was unaffected by GABA. CTX (10(-8) M) markedly suppressed the vasoconstrictor response to TNS, but did not affect vasodilator responses. These findings suggest that in the rat mesentery: (1) GABA receptors modulate the activity of sympathetic nerves via prejunctional GABA(B) receptors, but do not influence sensory nerves, and (2) calcium channels which participate in sympathetic nerve activation have different properties than calcium channels in capsaicin-sensitive sensory nerves.