Depression by nicotine of pain-related nociceptive activity in the rat thalamus and spinal cord.
To assess the possible role of nicotinergic control in nociception and pain, experiments were carried out on rats under urethane anesthesia in which nociceptive activity was elicited by electrical stimulation of afferent C fibers in the sural nerve and recorded from single neurones in the thalamus and from ascending axons in the spinal cord. Intravenous administration of nicotine (0.01-0.5 mg/kg) depressed the nociceptive activity evoked in the thalamus and the spinal cord in a dose-dependent way. The maximum depression in thalamus and spinal cord was 40% of control activity and obtained at a dose of 0.025 mg/kg. Likewise, local administration of nicotine to the spinal cord by intrathecal injection (5, 10, and 30 micrograms) reduced the nociceptive activity evoked in neurones of the thalamus and in ascending axons of the spinal cord, the maximum of the depression being 40% of control activity. The depressant effect of nicotine (0.05 mg/kg) was reduced by mecamylamine (1 mg/kg) but not by atropine (0.5 mg/kg). It is concluded that the antinociceptive effect of nicotine is due to a specific action of the alcaloid at the spinal level.