Long-lasting effects of neonatal stimulation on the behavior of rats.

The present study aimed to analyze the effects of neonatal stimulation on species-specific behaviors (defensive reactions to a predator and social interactions) in adult male and female rats. Handling and an unpredictable sequence of aversive stimuli were applied to male and female pups from the 1st to the 10th day after delivery; behavioral inhibition, aggression, and sexual behavior were evaluated in adulthood. Results showed that either neonatal handling or aversive stimulation decreased behavioral inhibition in a novel and potentially harmful situation (open field with a predator) in both male and female rats and increased maternal aggressive behavior. Sexual behavior in both males and females decreased, which could affect reproductive capability. The results could cast doubts on the generalization of beneficial effects of neonatal stimulation on the behavior of adult rats.

Hypothalamic paraventricular nucleus modulates maternal aggression in rats: effects of ibotenic acid lesion and oxytocin antisense.

Central oxytocin (OT) appears to be crucial for maternal behavior. OT, through the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN), can exert its physiological and behavioral effects by acting on OT receptors in nonpituitary projections of the PVN. The purpose of the present study was to analyze the role of the PVN and OT on maternal aggressive behavior in two different periods after delivery: on the fifth day (period of high aggressiveness) and on the eighteenth day postpartum (period of low aggressiveness). In the first experiment, ibotenic acid was injected into the PVN in order to lesion the parvocellular neurons. A second experiment was designed to study more specifically the effects of OT using the antisense technique. On the fifth day postpartum, both the PVN lesion by the ibotenic acid and a possible acute reduction of OT synthesis by the antisense administration in that nucleus increased maternal aggressive behavior, while on the eighteenth day postpartum no effect was recorded. We may conclude that central projections of the PVN modulate maternal aggression during a restricted period after delivery, only when lactating females show naturally high levels of aggressive behaviors.

Aversive stimulation during the stress-hyporesponsive period does not affect the number of corticotroph cells in neonatal male rats

Immunohistochemistry was used to evaluate the effects of neonatal handling and aversive stimulation during the first 10 days of life on the number of corticotrophs in the anterior lobe of the pituitary of 11-day-old male Wistar rats. Since adult rats handled during infancy respond with reduced corticosterone secretion in response to stressors and with less behavior inhibition in novel environments, we assumed that neonatal stimulation could affect pituitary morphology during this critical period of cell differentiation. Three groups of animals were studied: intact (no manipulation, N=5), handled (N=5) and stimulated (submitted to 3 different aversive stimuli, N=5). The percentage of ACTH-immunoreactive cells in the anterior lobe of the pituitary (number of ACTH-stained cells divided by total number of cells) was determined by examining three slices per pituitary in which a minimum of 200 cells were counted by two independent researchers. Although animals during the neonatal period are less reactive to stress-like stimulation in terms of ACTH and corticosterone secretion, results showed that the relative number of ACTH-stained cells of neonatal handled (0.25 + 0.01) and aversive stimulated (0.29 + 0.03) rats was not significantly different from intact (0.30 + 0.03) animals. Neonatal stimulation may have a differential effect on the various subpopulations of corticotroph cells in the anterior pituitary.

Hypothalamic paraventricular nucleus, oxytocin, and maternal aggression in rats.

Both the lesion of the parvocellular region of the PVN (FIG. 1) and the acute reduction of OT synthesis in that nucleus (FIG. 2) increase maternal aggression in rats. Previous work showed that ibotenic acid as well as the OT antisense in the PVN reduced the level of OT in the brainstem, but not in the pituitary. Therefore, the oxytocinergic parvocellular neurons of the PVN appear to exert an inhibitory effect on the aggressive behavior of the lactating female rat against an adult intruder. In a relationship of a different nature, mother-infant, a facilitatory effect of OT has been shown. Previous work showed a significant decrease of OT mRNA levels in the PVN of female rats during the first 10 days after delivery compared to late pregnancy, which is the inverse ratio of the natural temporal evolution of maternal aggressive behavior. Furthermore, in the present work, a functional decrease of OT mRNA was probably the effect of the antisense in the PVN. In conclusion, OT cells in the PVN appear to play different roles on maternal care and maternal aggression.

Aversive stimulation during the stress-hyporesponsive period does not affect the number of corticotroph cells in neonatal male rats.

Immunohistochemistry was used to evaluate the effects of neonatal handling and aversive stimulation during the first 10 days of life on the number of corticotrophs in the anterior lobe of the pituitary of 11-day-old male Wistar rats. Since adult rats handled during infancy respond with reduced corticosterone secretion in response to stressors and with less behavior inhibition in novel environments, we assumed that neonatal stimulation could affect pituitary morphology during this critical period of cell differentiation. Three groups of animals were studied: intact (no manipulation, N = 5), handled (N = 5) and stimulated (submitted to 3 different aversive stimuli, N = 5). The percentage of ACTH-immunoreactive cells in the anterior lobe of the pituitary (number of ACTH-stained cells divided by total number of cells) was determined by examining three slices per pituitary in which a minimum of 200 cells were counted by two independent researchers. Although animals during the neonatal period are less reactive to stress-like stimulation in terms of ACTH and corticosterone secretion, results showed that the relative number of ACTH-stained cells of neonatal handled (0.25 +/- 0.01) and aversive stimulated (0.29 +/- 0.03) rats was not significantly different from intact (0.30 +/- 0.03) animals. Neonatal stimulation may have a differential effect on the various subpopulations of corticotroph cells in the anterior pituitary.

The effect of testosterone and DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) on male sexual behavior of rats.

The effects of a 5-HT2 receptor agonist, DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane; 0.5 mg/kg), on the behavior of male rats at different ages when given alone or with different levels of testosterone, in the presence of sexually receptive and non-receptive females are presented. DOI increased mounting and/or mount plus thrusting behavior in adult males with receptive females. In pre-pubertal males, DOI increased the frequency of pursuit and genital sniffing in the presence of receptive females, but not of non-receptive ones, when no mounts or thrustings were recorded. In castrated rats treated with testosterone and tested with receptive females, DOI increased the frequency of thrusting behavior, but in castrated rats without testosterone treatment, DOI produced no change. DOI did not induce mounting in pre-pubertal or castrated rats without testosterone substitution therapy. These results suggest that DOI influences male sexual behavior through a neural system that is modulated by testosterone.