Effects of naloxone on penile erection in cats

Male cats were anesthetized with pentobarbital. A Foley catheter was placed in the urinary bladder and physiologic saline, under a head of pressure, was allowed to flow at a constant rate through the bladder. Naloxone, 0.2 mg, caused penile erection in 5 of 11 experiments. The onset of action was 0.5 to 4 minutes, and the duration of the erection was 5 to 36 minutes. In two of three experiments a second injection of naloxone caused a second erection. The erection caused by naloxone was not changed by pre- or posttreatment with morphine or by posttreatment with propranolol. It was suggested that the erection could be due either to altered levels of hormones released from the central nervous system or to removal of reflex inhibitory tone in the spinal cord or sacral parasympathetic ganglia.