Introduction

Circulating hormones have profound effects on the development and functioning of the central nervous system (CNS) in both vertebrates (Arnold and Gorski, 1984) and invertebrates (Truman, 1988). They act to cause adaptive changes in CNS function, thereby adjusting behavior to meet changing physiological or developmental needs. Classically, hormonal effects have been classified as organizational and activational The former are developmental actions that are usually permanent, although the behavioral results of these actions may not be manifest until weeks or months later, long after the hormone has disappeared. The classic example of an organizational effect is the perinatal action of androgens in mammals resulting in the masculinization of the brain so the individual subsequently shows “male” behaviors as an adult. Activational effects, by contrast, represent “immediate” responses to hormones that are in the physiological realm and typically subside once the hormone is withdrawn. For example, estrogen has activational actions in evoking receptive behavior in female rats at the appropriate phase of the estrus cycle.

Study of hormonal effects on behavior has been difficult because of the cellular complexity of the CNS and the nature of some of the cellular responses. Some hormone-evoked responses are biochemical, such as changes in levels of specific neurotransmitters. These can be studied directly at the level of expression of specific genes, typically the enzymes involved in transmitter synthesis or processing. Besides biochemical changes, neurons also respond to hormones by altering their shape, resulting in altered patterns of synaptic connections due to new axon or dendrite growth and synapse formation. These morphological responses are not necessarily measurable in terms of products of single genes and, indeed, it is not clear which are the critical genes to examine.