Maristela J. Padoin

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.

Effects of neonatal handling on the behavior and prolactin stress response in male and female rats at various ages and estrous cycle phases of females

Neonatal handling induces behavioral and hormonal changes, characterized by reduced fear in novel environments, and lesser elevation and faster return to basal levels of plasma corticosterone, prolactin and adrenaline, in response to stressors in adulthood. The present study aimed to analyze the effects of neonatal handling from Days 1 to 10 postnatal on prolactin response to ether stress in male and female rats at three life periods: neonatal, peripubertal and adulthood. Moreover, adult females were tested in two different phases of the estrous cycle, i.e., diestrus and estrus. In another set of experiments, the behavior of peripubertal and adult males and females in estrus and diestrus was analyzed in the elevated plus maze test. Pups were either handled for 1 min (handled group) or left undisturbed (nonhandled group) during the first 10 days after delivery. In adults, in the handled females in diestrus, stress induced a lesser increase in plasma prolactin compared with nonhandled ones, as in males. However, in estrus, handled females showed no difference in the prolactin response to stress. In the elevated plus maze, handled females in diestrus, but not in estrus, showed higher locomotor activity compared with nonhandled ones. Peripubertal male and female rats handled during the neonatal period showed no difference in behavior in the elevated plus maze compared with nonhandled animals. Early-life stimulation can induce long-lasting behavioral and stress-related hormonal changes, but they are not stable throughout life and phases of the estrous cycle.

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.

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.