Steroids may rapidly alter neuronal function and behavior through poorly characterized, direct actions on neuronal membranes. The membrane-bound receptors mediating these behavioral responses have not been identified. [3H]Corticosterone labels a population of specific, high-affinity recognition sites (dissociation constant = 0.51 nanomolar) in synaptic membranes from an amphibian brain. These binding sites were localized by receptor autoradiography in the neuropil, outside the regions of perikarya. The affinities of corticoids for this [3H]corticosterone binding site were linearly related to their potencies in rapidly suppressing male reproductive behavior. Thus, it appears that brain membranes contain a corticosteroid receptor that could participate in the regulation of behavior.

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It is clear that the behavioral actions of oxytocin and vasopressin in mammals are not newly acquired, but have evolutionary antecedents. Injection studies with fish, amphibians, reptiles, and birds indicate that AVT can activate certain reproductive behaviors. The strongest evidence that AVT acts centrally to control reproductive behaviors comes from research on T. granulosa. In this amphibian, injections of AVT agonists activate courtship behaviors (amplectic clasping) in males and egg-laying behaviors in females, whereas injections of AVT antagonists inhibit the behaviors. Also, in Taricha males, AVT concentrations in specific brain areas are associated with the expression of courtship behaviors. Several conclusions about steroid-peptide interactions can be drawn, based on research with this amphibian. First, gonadal steroid hormones act to maintain the behavioral actions of AVT in both males and females. In Taricha, gonadectomy abolishes and steroid implants restore AVT-induced courtship in males and egg-laying in females. Second, gonadal steroids maintain the behavioral actions of AVT, in part, by modulating AVT receptor numbers on target neurons. In Taricha males and females, gonadectomy reduces AVT receptor concentrations (but not binding affinity) in certain brain areas (amygdala pars lateralis) and not others. Third, the type of gonadal steroid determines whether AVT elicits male-like or female-like reproductive behaviors. Ovariectomized Taricha females respond to AVT injections with egg-laying behaviors when implanted with estradiol and with male-like amplectic clasping when implanted with dihydrotestosterone. Fourth, the masculinization of AVT-induced behaviors in females most likely reflects site-specific actions of androgens on AVT-synthesizing neurons. In Taricha, AVTir concentrations in the optic tectum are sexually dimorphic (higher in males than females) and reach peak levels in males during the breeding season. Fifth, AVT content in specific brain areas increase as a function of performing the behaviors. In Taricha, AVTir concentrations in DPOA, CSF, and ventral infundibulum are higher in males that exhibit courtship behaviors than in males that do not. These conclusions illustrate how steroid-peptide interactions in the control of behaviors entail multiple neuroanatomical sites and neurochemical actions.

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The nervus terminalis (TN), a component of the olfactory system, is found in most vertebrates. The TN of some fishes and mammals contains neurons immunoreactive (ir) to gonadotropin-releasing hormone (LHRH), and to several other neuropeptides and neurotransmitter systems, but there is little information on TN chemistry in other vertebrate taxa. Using immunocytochemical techniques, we found LHRH-ir neurons in amphibian TNs. In anurans, but not in a urodele, the TN was also found to contain Phe-Met-Arg-Phe-NH₂ (FMRFamide) immunoreactivity. LHRH-ir neurons of the TN and those of the septal-hypothalamic system are morphologically homogeneous and form a distinct anatomical continuum in amphibians. Based upon topographical and cytological criteria, we hypothesize that LHRH-ir systems in vertebrates might derive embryonically from the TN.