Antagonism of CaC12 injected into periaqueductal gray on the inhibitory action of morphine on pain-evoked discharges of parafasicular nucleus in rats

Electrical stimulation of the peroneal nerve in Sprague-Dawley rats induced pain-evoked discharge (PEvD) of the parafasicular nucleus (Pf). Morphine (3mg/kg, ip) alone or plus saline 0.5 UI into the periaqueductal gray area (PAG) markedly inhibited the PEvD of unit discharges of pain-excitatory neurons in Pf. Injection of CaC12 (15 nmol/L, 0.5 UI) into PAG 5 min before ip morphine attenuated or abolished the inhibitory action of morphine on PEvD. The fact that no antagonistic effect of CaC12 injected into the site lateral to PAG was seen suggests the specificity of the site of action. CaCl2, EGTA or verapamil injected into PAG without ip morphine did not influence the PEvD. Injection of Ca2+ into PAG antagonized the inhibitory action of morphine on the PEvD, suggests that the action of morphine takes place via PAG instead of directly in the Pf and can not be attributed entirely to the lack of Ca2+ in the PAG. It is concluded that morphine may activate certain structures in the PAG first and then inhibit PEvD of Pf and that Ca2+ is involved in this process.