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), voca lizations elicited during stimulation (VDSs), and vocalization afterdi scharges (VADs) was assessed. Responses were elicited by applying grad ed electric current to the tail. Performance (latency and amplitude) o f all three responses was monitored to determine whether elevations in threshold were confounded by performance decrements. Air three drugs were found to elevate VAD thresholds more readily than VDS and SMR thr esholds. 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 respon ses without disrupting the performance of any response, the order of s usceptibility to threshold increases remained VAD, VDS, and SMR. Diaze pam also elevated VAD thresholds more readily than VDS thresholds acro ss a dose range that failed to disrupt performance of either response. SMR thresholds were only elevated by diazepam when administered in do ses 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.