Naloxone Disrupts the Development of a Conditioned Ejaculatory Preference Based on a Somatosensory Cue in Male Rats

Male rats develop a conditioned ejaculatory preference (CEP) toward females bearing an odor or somatosensory cue (rodent jacket) when those stimuli are paired with the postejaculatory reward state. As with a copulatory conditioned place preference, CEP for an odor depends on endogenous opioid transmission after ejaculation. The nonselective opioid receptor antagonist naloxone (NAL) disrupts CEP for an odor cue on female rats when injected systemically to males prior to each conditioning trial. Here, we evaluated whether NAL would disrupt the development of a CEP for the somatosensory cue. Long'Evans males were assigned randomly to two groups and underwent 14 copulatory conditioning trials for 30 min each, spaced every 4 days, and consisting of sequential pairing of a jacket on a sexually receptive female and no jacket on a sexually nonreceptive female. The control group was injected with saline (SAL) in both conditions throughout training, whereas the experimental group was injected with NAL when females were receptive and wore a jacket, and with SAL when they were not receptive and did not wear a jacket. On the final test, all males were injected with SAL and placed into an open field with two sexually receptive females, one with the jacket and the other without the jacket. Control males displayed a significant CEP for females with the jacket on, whereas males injected with NAL during sexually receptive jacket conditions displayed a significant CEP for the nonjacketed female. This study confirms that opioid transmission is necessary for the establishment of a somatosensory CEP.