Trigeminal antinociception induced by bicuculline in the periaqueductal gray (PAG) is not affected by PAG P/Q-type calcium channel blockade in rat

We have recently shown that injection of the P/Q-type (Ca(v)2.1/alpha(1A)) calcium channel blocker, omega-agatoxin IVA, into the periaqueductal gray (PAG) facilitates meningeal dural stimulation-evoked trigeminal nociceptive processing. We injected the GABA(A) antagonist bicuculline into the PAG in addition to the agatoxin and observed bicuculline's effect on neurons responding to dural stimulation recorded in the trigeminal nucleus caudalis of rats in order to determine if P/Q channel-mediated changes acted through GABAergic mechanisms. The inhibition of trigeminal nociceptive neurons characteristic of bicuculline administered into the PAG was maintained in the presence of blocked PAG P/Q-type calcium channels. This suggests the PAG descending pain modulatory pathway is not affected by P/Q-type calcium channel blockade at the postsynaptic GABAergic inhibitory interneuron and the facilitation produced by agatoxin is mediated by another mechanism. These findings have implications for disorders involving the PAG or P/Q-type channels, such as migraine, in particular for the development of preventative treatments, suggesting GABAergic and voltage-gated calcium channels could be separately modulated. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.