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), and vocalization afterdischarges (VADs) when administered into the spinal subarachnoid space was evaluated. Both monoamines generated dose-dependent increases in the thresholds of all 3 responses. The minimum effective doses of serotonin and norepinephrine that elevated all 3 response thresholds were 40 micrograms and 1 microgram respectively. Monoamine-induced increases in response thresholds were reversed by the intrathecal administration of their corresponding receptor antagonists (phentolamine or methysergide). Threshold increases generated by serotonin were also partially reduced by phentolamine. These results indicate that dorsal horn neurons that underlie flexion reflex generation (SMR) and the rostral transmission of pain information (VDS and VAD) have similar thresholds of inhibition to spinopetal monoaminergic projections.