Hypothalamic Cellular and Molecular Mechanisms Helping to Satisfy Axiomatic Requirements for Reproduction

In the absence of universal equations expressing neurobiological findings, the safest theoretical approach for the neuroendocrinologist is to start from axiomatic requirements for biologically adaptive neural mechanisms, in our case for reproduction. From this emerge two themes: the likely importance of interactions between internal (hormonal) and external signals in controlling gene expression relevant to reproductive functions; and, second, the vision of molecular interactions on DNA subserving environmental impacts on reproduction. The first theoretical notion has so far yielded data showing a role for synaptic inputs during the onset of estradiol actions for the hormone's induction of enkephalin mRNA, a finding which parallels earlier behavioral results. As well, noxious somatosensory inputs interact with estrogens and progesterone in their influence on enkephalin gene expression. The second theme led to novel investigations of thyroid influences on reproductive molecular biology and behavior, including the ability of exogenous or endogenous thyroid hormones to reduce female mating responses. Since elevated thyroid hormone levels could signal environmental cold, our experiments offer the possibility of explaining ethological facts at a molecular level. More generally, nuclear hormone receptor interactions on the surface of DNA may offer a new level of neural integration revealed first by hormone effects in neuroendocrine cells.