GnRH inhibits neuronal activity in the ventral tegmental area of the estrogen-primed ovariectomized rat

In urethane-anesthetized ovariectomized rats, estrogen-sensitive descending neurons were identified in the midbrain ventral tegmental area (VTA), based on estrogen-induced changes in the excitability in antidromic responses to midbrain central gray stimulation. Estrogen increased the threshold and decreased the firing rate of the identified neurons. Responses of the identified neurons to the microiontophoresis of gonadotropin-releasing hormone (GnRH) or D-Phe2, D-Ala6-GnRH, a behaviorally active analog, but not to glutamate or gamma-aminobutyric acid (GABA), depended on estrogen. In the ovariectomized rat, GnRH excited a few neurons; the analog had no effect. GnRH suppressed spontaneous or glutamate-induced firing in almost all neurons in the estrogen-primed rat. The analog had mixed effects. The facilitation of female rat sexual behavior induced by infusion of GnRH in the VTA is due to the inhibition of VTA neurons.