The first experiment, according to Beach's' account, in behavioral endocrinology was that of Professor A. A. Berthold, who in 1849, castrated cockerels. He observed that castrated birds demonstrated atrophy of their comb and spurs, cessation of crowing, and loss of sexual interest. Progress after Berthold's observations began slowly. The only comprehensive text published during the next 50 years was that of Beach, Hormones and Behavior. 2 Then in the late 1960s and early 1970s several texts became available including Whalen's Hormones and Behavior, 3 Levine's Hormones and Behavior,4 Eleftheriou's and Sprott's Hormonal Correlates of Behavior, 5 and Leshner's An Introduction to Behavioral Endocrinology. 6 In that time period, interest had broadened beyond the effects of hormones on sexual behavior. However, as noted earlier by Whalen, "The typical textbooks in physiological psychology and in introductory endocrinology fail to deal adequately with the currently blossoming study of the relationships between hormones and behavior... The major reason for these failures probably lies in the fact that our current understanding of hormone-behavior relationships has come about through the research efforts of scientists from several different disciplines. " 3
More current interest has included the effects of hormones on learning and memory. This trend is reflected by the fairly recent publication of an edited volume, Endogenous Peptides and Learning and Memory Processes, devoted entirely to the effects of peptide hormones and their analogs on learning and memory. 7 Much of this interest stems from early observations that hypophysectomy resulted in deficits in learning in rats. 8 ·9 These deficits in learning were shown to be reversed when hypophysectomized animals were given specific hormones of pituitary origin, i.e., adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone (MSH), or vasopressin. 10• 11 These findings Jed to an examination of the effects of these hormones on learning in intact animals. The earliest experiments demonstrated that intact animals treated with ACTH displayed prolonged responding during extinction of learned behaviors coupled with no influence on the acquisition of the response. 12 · 13 Similar results were found when adrenalectomized rats were treated with ACTH, which suggested that these actions were independent of adrenocorticosteroids. 12· 14
The purpose of the present chapter is to review the effects of a specific hypophyseal peptide hormone, a-MSH or melanotropin, on learning and memory. The review will first focus upon the actions of this peptide on learning in nonhuman and human subjects. Thereafter, attention will be directed toward elaboration of the possible theoretical explanations of how the hormone may act. Finally, the discussion will briefly tum to effects that MSH may produce on organizational processes during development which are manifested in later learning and memory.