Cobratoxin (Cobra),the long-chain neurotoxin from Thailand cobra,Naja kaouthia,has been previously reported to have a behavioral analgesic effect in rats.The present study investigated the effect of Cobra on pain-evoked unit discharge of neurons in thalamic parafascicular nucleus (Pf) and underlying mechanisms of its inhibitory on electrophysiology.The pain-evoked unit discharge of Pf neurons was recorded extracellularly by glass microelectrode,while the right sciatic nerve was exposed for noxious stimulation.Different doses of Cobratoxin were injected intracerebroventricularly(i.c.v.).Normal saline and morphine were used as negative and positive control,respectively.To analyze the mechanism of effects of Cobratoxin on pain-evoked discharge of neurons in Pf,i.c.v.injection of atropine and naloxone,as well as intraperitoneal injection (ip) para-chlorophenylalanine (pCPA) were preformed.The experimental results indicated that:(1)The spontaneous unit discharges had two types:single impulse discharge and burst discharge.