INCREASES IN VOCALIZATION AND MOTOR REFLEX THRESHOLDS GENERATED BY THE INTRATHECAL ADMINISTRATION OF SEROTONIN OR NOREPINEPHRINE

The capacity of serotonin and norepinephrine to elevate the thresholds of spinal motor reflexes (SMRs), vocalizations during shock (VDSs), a nd vocalization afterdischarges (VADs) when administered into the spin al subarachnoid space was evaluated. Both monoamines generated dose-de pendent increases in the thresholds of all 3 responses. The minimum ef fective doses of serotonin and norepinephrine that elevated all 3 resp onse thresholds were 40 mu g and 1 mu g, respectively. Monoamine-induc ed increases in response thresholds were reversed by the intrathecal a dministration of their corresponding receptor antagonists (phentolamin e or methysergide). Threshold increases generated by serotonin were al so partially reduced by phentolamine. These results indicate that dors al horn neurons that underlie flexion reflex generation (SMR) and the rostral transmission of pain information (VDS and VAD) have similar th resholds of inhibition to spinopetal monoaminergic projections.