Abstract

Previous research has shown that suppressing the production of nitric oxide (NO) by administering an inhibitor of nitric oxide synthase (NOS), L-NAME, into the third ventricle disrupts pup retrieval and maternal aggression in postpartum rats. The current experiments were carried out to investigate the sites of action of NO in the control of maternal behavior. We first identified a dose of L-NAME that was ineffective when administered into the third ventricle. Results obtained from this experiment (Experiment 1) indicated that L-NAME at the two highest doses (250 og and 200 og) but not at lower doses (0, 20 og, 80 og, and 160 og) increased the latency to retrieve the first pup as well as reduced the number of pups retrieved by day 4 postpartum rats. These two high doses of L-NAME were also effective in reducing the percentage of females aggressing against a male intruder. In experiment 2, doses of L-NAME that were shown to be ineffective when injected into the 3 rd ventricle in Experiment 1 were administered bilaterally into the MPOA of rats on Day 4 postpartum. These rats were then tested for pup-retrieval and maternal aggression. Results from this study demonstrated that L-NAME at doses of 20 og and 40og/side significantly increased the latency to retrieve the first pup and reduced the total number of pups retrieved. Interestingly, aggression towards a male intruder in the home cage was much less affected by these treatments. All rats in both experiments showed normal maternal behavior 24h after drug administration. These data suggest that NO acts within the MPOA to play a critical role in the facilitation of retrieval behavior but not maternal aggression in lactating rats.