|
15. Secretion of corticotrophin-releasing factor (CRF), adrenocorticotrophic hormone (ACTH) and adrenal corticosteroids as motor pattern of submission and defense; suppressed by prolactin. During defense and submission the hypothalamus apparently produces the corticotropin-releasing factor (CRF) that stimulates the release of andrenocorticotrophic hormone (ACTH) from the anterior pituitary that stimulates, in tun, the release of corticosteroid hormones from the adrenal cortex. As one would predict from the model proposed in this review, the adrenocortical activation occurs no matter how the defense or submission was activated by motivating stimuli. The following motivating stimuli produce defense accompanied by adrenocortical activity: defeat by another conspecific that involves pain, dorsal tactile stimu1ation and various defense pheromones (Bronson and Eleftheriou, 1964; touch and Higginbotham, 1967); exposure to a strange environment, i.e. neophobia (Heybach et al, 1918); pain of electric shock (Conner et al, 1971); loud noise (Henkin and Knigge, 1963); or exposure to an animal who had previously defeated the test animal, i.e. conditioned stimuli to pain and defeat (Bronson and Eleftheriou, 1964). Pituitary-adrenal activation also accompanies all of the various motor patterns of defense, including upright posture and boxing (Conner et al, 1971); freezing in response to a novel open-field environment (Chapman et al, 1969), inescapable electric shock (Madden et al, 1971) and escape or avoidance behavior (Bassett et aI, 1973). The secretion of ACTH and corticosteroids during defense is complicated by the relationship between these hormones. The ACTH that is secreted by the anterior pituita~ facilitates the secretion of corticosteroids by the adrenal cortex. The corticosteroids, in turn, then inhibit further secretion of ACTH by a negative feedback either to the anterior pituitary or to hypothalamic centers that control the anterior pituitary (Brush and Froelich, 1975). An illustration is provided in the data from Conner et a1 (1971). ACTH levels in the blood rose from resting level to a level six times higher within five minutes after administration of footshock, but then began to decline despite continuing shocks. In the meantime, corticosteroid levels began to rise five minutes after administration of shock and then continued to rise throughout the remainder of the test period of fifteen minutes. Presumably the corticosteroid secretion began in response to elevated ACTH levels and caused a feedback inhibition of ACTH secretion. Prolonged defense leads not only to increased secretion of corticosteroids by the adrenal cortex, but also to an increase in size of the gland itself, and subsequently increased capacity to secrete corticosteroids under future defense conditions. Although the increase in size of the adrenal gland is not always directly proportional to an increase in the corticosteroid secretion, under most circumstances it does indicate increased corticosteroid secretion (Christian, 1975). There is a complex relationship between the pituitary-adrenal axis and lactation. On the one hand, both suckling and exteroceptive stimuli associated with suckling facilitate the release of ACTH and corticosteroids during lactation in rats (Voogt et a1, 1969), and, as a result, circulating levels are higher than in virgin rats (Thatcher and Tucker, 1970). On the other hand, lactating female rats show a smaller response of corticosteroid secretion during shock-induced aggression (Thoman et al, 1970), in response to ether or surgical stress (Thoman et aI, 1970; Schlein et al, 1974), and in response to a novel open-field situation (Stern et aI, 1973). In order to explain the relationship of the pituitary-adrenal axis to lactation, one may suppose that CRF secretion is a motor pattern of the parental motivational mechanism, but that the secretion is suppressed by prolactin. Since prolactin secretion may also be a motor pattern of the parental motivational mechanism, the resultant activity may usually represent a compromise between these two contradictory motor patterns.
|