L-type calcium channels selectively control the defensive behaviors induced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers.

The present study reports the involvement of L-type calcium channels in the control of defensive behaviors produced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers. Rats that had chemitrodes in the dorsal midbrain and which stimulation produced freezing or flight behaviors with less than 55 microA were selected for drug experiments. Stimulation was repeated the day after the screening session 20 min following the microinjection into the dorsal periaqueductal gray of 15 nmol of either verapamil, a selective L-type calcium channel antagonist, or cobalt chloride (CoCl(2)), a calcium-specific channel modulator. Post-drug sessions were performed 48 h after. Threshold functions were obtained by logistic fitting of accumulated response frequencies. Verapamil and CoCl(2) significantly attenuated the output of immobility, exophthalmus, running and jumping. Although to a lesser degree, verapamil also attenuated defecation. Because CoCl(2) had no effect on defecation, the attenuation of this response by verapamil suggests a non-specific action of this drug. Neither verapamil nor CoCl(2) changed the output of micturition. Finally, whereas there was a complete recovery of defensive thresholds following the microinjection of verapamil, the attenuating effects of CoCl(2) were still present 48 h after. These results support an important role of L-type calcium channels in the neurogenesis of dorsal periaqueductal gray-evoked immobility, exophthalmus, running and jumping, but not defecation and micturition responses.