Vasopressin analog delays extinction of classically conditioned bradycardia

Albino rabbits were subjected to Pavlovian (classical) conditioning and extinction of concomitant heart rate and eyeblink responses. Sixty minutes before each of three extinction sessions animals were treated with 5 or 20 μg/kg of deamino-dicarba-arginine-8-vasopressin or saline. Vasopressin treatment delayed extinction of bradycardiac conditioned responses but did not affect concomitant eyeblink conditioned responses. It was concluded that classically conditioned autonomic responses may be useful tools for studying the effects of peptides on learning.     

Distinctive stress effects on learning during puberty

Puberty is a time of significant change in preparation for adulthood. Here, we examined how stressful experience affects cognitive and related hormonal responses in male and female rats prior to, during and after puberty. Groups were exposed to an acute stressor of brief periodic tailshocks and tested 24 h later in an associative memory task of trace eyeblink conditioning. Exposure to the stressor did not alter conditioning in males or females prior to puberty but enhanced conditioning in both males and females during puberty. The enhancement occurred in pubescent females irrespective of the estrous cycle. In adulthood, sex differences in trace conditioning and the response to stress emerged: females outperformed males under unstressed conditions, but after stressor exposure, trace conditioning in females was impaired whereas that in males was enhanced. These differences were not related to changes in gross motor activity or other nonspecific measures of performance. The effects of acute stress on corticosterone, estradiol, progesterone, and testosterone were also measured. Stressor exposure increased the concentration of corticosterone in all age groups, although sex differences were only evident in adults. All reproductive hormones except estradiol increased with age in a predictable and sex dependent fashion and none were affected by stressor exposure. Estradiol decreased in male rats across age, and remained stable for female rats. Together, these data indicate that males and female respond similarly to learning opportunities and stressful experience before and during puberty; it is in adulthood that sex differences and the opposite responses to stress arise.     

Learning during motherhood: A resistance to stress

Hormonal and emotional responses to stress are diminished during pregnancy and the postpartum period. However, the effects of stress on learning during these stages of the female life span have not been examined. In previous studies, we have reported that exposure to an acute stressful event reduces classical eyeblink conditioning 24 h later in adult virgin female rats that are experiencing an ovarian cycle. Here we show that conditioning during late pregnancy was similarly reduced by stressful experience. However, conditioning in postpartum females was unaffected by stressor exposure. The resistance to stress during the postpartum period was evident as early as 2 days after parturition and persisted until the late postpartum period, just prior to weaning. Postpartum conditioning was unresponsive to numerous types of stressors, including brief inescapable tailshocks, swim stress, and exposure to a male intruder. The resistance to stress appears to be dependent on the presence of the offspring, because the impairment in conditioning returned when postpartum females were separated from their pups. Moreover, the resistance to stress occurred in virgin females that behaved maternally after being exposed to young pups for several days. Together, these data suggest that the presence of offspring and the nurturing and care-giving activities that they elicit protect females from the adverse effect of stress on processes involved in learning and memory.