Potentiation of salivary fluid secretion in ixodid ticks: a new receptor system for gamma-aminobutyric acid

gamma-Aminobutyric acid (GABA), having minimal intrinsic activity, potentiates dopamine-induced fluid secretion in salivary glands of female ixodid ticks. Because the effect of GABA was similar to that of spiperone, we tested whether these two drugs act at a common recognition site. Potentiation was not augmented when salivary glands were exposed to supramaximal concentrations of spiperone (1 microM) plus GABA (100 microM). (+/-)-Sulpiride (100 microM), a spiperone antagonist in this system, also blocked GABA-induced potentiation. Picrotoxin (100 microM) and (-)-bicuculline (100 microM), two GABA antagonists, blocked GABA-induced and spiperone-induced potentiation. Inhibition of GABA by picrotoxin and (-)-bicuculline was noncompetitive. Muscimol (an agonist at GABAA receptors) also potentiated dopamine-induced secretion. Baclofen (an agonist at GABAB receptors) did not elicit potentiation. We suggest that GABA may function as a neuromodulator for dopamine-induced fluid secretion in tick salivary glands.