| To analysis the effect of Cobratoxin on pain-evoked discharge of neurons in thalamic parafascicular nucleus (Pf) and its mechanism, the pain-evoked discharge was recorded extracellularly with glass microelectrode from Pf. 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. (2)There were three types of responses after noxious stimulation: pain excitatory type, pain inhibitory type and no pain response type. (3)The frequency, the duration of pain-evoked discharge of Pf neurons were inhibited after i.c.v. injection of high, middle and low doses of Cobratoxin. The effect was dose-dependent. (4) The inhibitory effect of Cobratoxin was stronger and lasted longer than morphine. (5) The inhibitory effect of Cobratoxin on pain-evoked discharge of Pf neurons was reversed by i.c.v. injection of atropine, but not by naloxone. (6)The inhibitory effect of Cobratoxin on pain-evoked discharge of Pf neurons was partially reversed by pCPA pre-treatment.The results suggested that: (1) Cobratoxin has an inhibitory effect on pain-evoked discharge of Pf neurons which is dose-dependent. (2) The inhibitory effect of Cobratoxin has a long duration, and stronger than morphine. (3)Central opioidergic system may not participate in the inhibitory effect of Cobratoxin. (4)Central cholinergic system may participate in the inhibitory effect of Cobratoxin. (5)Central serotonergic system may be partially involved, in the inhibitory effect of Cobratoxin on pain-evoked discharege of Pf neurons.
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