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 dorsa l periaqueductal gray and overlying collicular layers. Rats that had chemit rodes in the dorsal midbrain and which stimulation produced freezing or fli ght behaviors with less than 55 mu A were selected for drug experiments. St imulation was repeated the day after the screening session 20 min following the microinjection into the dorsal periaqueductal gray of 15 nmol of eithe r verapamil, a selective L-type calcium channel antagonist, or cobalt chlor ide (CoCl2), 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 CoCl2 significantly att enuated the output of immobility, exophthalmus, running and jumping. Althou gh to a lesser degree, verapamil also attenuated defecation. Because CoCl2 had no effect on defecation, the attenuation of this response by verapamil suggests a non-specific action of this drug. Neither verapamil nor CoCl2 ch anged the output of micturition. Finally, whereas there was a complete reco very of defensive thresholds following the microinjection of verapamil, the attenuating effects of CoCl2 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 ju mping, but not defecation and micturition responses. (C) 2000 Elsevier Scie nce B.V. All rights reserved.