Comparison of motor reflex and vocalization thresholds following systemically administered morphine, fentanyl, and diazepam in the rat: assessment of sensory and performance variables.

The relative influence of systemically administered morphine, fentanyl, and diazepam on the thresholds of spinal motor reflexes (SMRs), vocalizations elicited during stimulation (VDSs), and vocalization afterdischarges (VADs) was assessed. Responses were elicited by applying graded electric current to the tail. Performance (latency and amplitude) of all three responses was monitored to determine whether elevations in threshold were confounded by performance decrements. All three drugs were found to elevate VAD thresholds more readily than VDS and SMR thresholds. VADs were also most susceptible to the deleterious effects of these drugs on motor performance. Nevertheless, across the dose range of morphine and fentanyl that elevated thresholds of all three responses without disrupting the performance of any response, the order of susceptibility to threshold increases remained VAD, VDS, and SMR. Diazepam also elevated VAD thresholds more readily than VDS thresholds across a dose range that failed to disrupt performance of either response. SMR thresholds were only elevated by diazepam when administered in doses that significantly disrupted performance. Results are discussed in terms of supporting the validity of VADs as a model of the affective-motivational dimension of pain.