Extinction of response to urine odor as a consequence of vomeronasal organ removal in male guinea pigs.

It has been found that removal of the vomeronasal chemoreceptor organ (VNO) in male guinea pigs disrupted investigatory responses to conspecific odors but this disruption was time-dependent; immediately following surgery, behavior appeared almost normal, whereas several months following surgery, animals became very unresponsive to conspecific odor. It was hypothesized that in the absence of a functional VNO, the main olfactory system (MOS) was capable of maintaining response to conspecific odor but that this response extinguished following repeated exposures. However, postsurgical change in the central nervous system, unrelated to exposure to the bioassay, remained a possible explanation. In order to separate these hypotheses, the VNO was surgically removed in two groups of animals (A and B), and a third group (C) experienced sham surgery. Beginning 3 weeks following surgery, males in Groups A and C were given two standard urine-response tests/week for 24 weeks. Initiation of testing of Group B animals was delayed until Week 15 following surgery. Results indicated that (a) responses of Group A declined relative to those of Group C, (b) at the first postsurgical test, Group B was as responsive to urine as Group C and much more responsive than Group A and (c) a decline in responses for Group B occurred during tests after Week 15. The data therefore strongly supported the extinction hypothesis. The MOS is capable of eliciting a high level of investigatory behavior in response to female urine odors, but in the absence of the VNO, this response wanes, perhaps due to a loss of reinforcing properties associated with VNO stimulation.