Verapamil injections in the peripeduncular nucleus suppress multiunit evoked activity and sexual receptivity in female rats

Abstract

Using a combination of injection cannula and recording electrode, the effect of 1 μl of 3 mM verapamil upon the multiunit responses evoked in the peripeduncular nucleus (PPN) by electric stimuli applied to the pudendal nerves in urethane anesthetized proestrous female rats was studied in 9 experiments. It was observed that in 6 cases responses were suppressed after the injection and in 2 other cases there was a marked decrease, whereas no change was observed in 1 case. Similar injections of vehicle (saline) produced no change (7 rats) or small and short lasting depression of evoked activity (4 rats). Similar injections of verapamil applied to a region of the mesencephalic reticular formation caudal to the PPN, presumably containing fibers that carry evoked activity towards the PPN, had no effect on evoked multiunit activity in the PPN. Injection of 2% Xylocaine in the same site in the same animal eliminated the evoked responses. These experiments support our hypothesis that verapamil suppressed evoked responses in the PPN blocking Ca ++-dependent release of neurotransmitter at local synapsis, without interfering with fiber conduction of neural activity. Based on this assumption we used identical injections to test the hypothesis that activation of neurons in the PPN is necessary for the performance of lordotic behavior in ovariectomized rats primed with estradiol benzoate. Guide cannulae aimed at the PPN were implanted in castrated females which were then primed with enough estradiol benzoate to induce sexual receptivity. These animals were then injected with 1 μl of saline and after a test for behavior receptivity, they were injected with 1 μl of verapamil 3 mM and tested again for lordosis immediately after and at 20 and 60 min later. Sexual receptivity was not affected by saline injections, but was significantly decreased after verapamil administration. It is concluded that transynaptic activation of PPN neurons is necessary for the performance of sexual behavior in the female rat.