Brain vasotocin pathways and the control of sexual behaviors in the bullfrog

The neurohypophysial peptide arginine vasotocin (AVT) alters the display of several sexually dimorphic behaviors in the bullfrog (Rana catesbeiana). These behaviors include mate calling, release calling, call phonotaxis, and locomotor activity. Populations of AVT-immunoreactive cells are present in six areas of bullfrog brain and fibers are widespread. Neural areas involved in vocalization, in particular, contain AVT cells and fibers. As well, AVT concentrations in a subset of brain areas are sexually dimorphic and steroid sensitive. Effects of gonadectomy and gonadal steroid treatment vary, depending on the brain area and sex of the frog. For example, some anterior areas are sensitive to changes in both dihydrotestosterone (DHT) and estradiol. In some posterior brain areas, on the other hand, AVT levels are affected only by DHT. A similar situation exists for putative AVT receptors in bullfrogs. Receptors are widespread, occurring in many areas that have been linked to behavior. Receptor concentrations are sexually dimorphic in the amygdala pars lateralis, hypothalamus, pretrigeminal nucleus, and dorsolateral nucleus. Estradiol alters AVT receptor level in the amygdala of both sexes of bullfrog and both estradiol and DHT alter the receptor number in the pretrigeminal nucleus, but only in males. The mechanisms responsible for steroid effects on vasotocin neurons and their targets are unknown. Specific AVT cells, fiber terminal fields, and receptor populations are likely influenced by gonadal steroids for effective timing of individual behaviors displayed by bullfrogs.