Arginine vasotocin (AVT) is a neurotransmitter in the amphibian central nervous system and is released from the neurohypophysis in the regulation of hydromineral balance and other homeostatic functions. Many amphibians experience drastic changes in habitat with respect to water availability during their transformation from aquatic larvae to terrestrial adults. To examine whether metamorphosis is accompanied by a reorganization of central vasotocinergic neurons, the developmental organization of vasotocin neurons and nerve fibers was studied with immunocytochemistry in the brains of bullfrogs (Rana catesbeiana) and woodfrogs (R. sylvatica). In bullfrogs, early limb-bud-stage tadpoles had AVT-immunoreactive neurons and nerve fibers in the lateral septal nucleus, amygdala, preoptic hypothalamus, suprachiasmatic nucleus, and posterodorsal tegmentum. Woodfrog larvae showed similar patterns of hypothalamic AVT immunoreactivity, although neuronal staining in the amygdala did not appear until metamorphic climax, and never appeared in septal neurons or in the posterodorsal tegmentum. Whereas the highly terrestrial R. sylvatica adults must adapt to an adult habitat with prolonged periods of dehydration, R. catesbeiana adults remain semiaquatic and, as such, need not develop extreme mechanisms for water retention. Nonetheless, vasotocinergic pathways showed developmental similarities in the two species. The early appearance of AVT innervation in both Rana suggests that AVT has neuroregulatory functions well before metamorphosis.

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Amphibians rely exclusively on behavioral thermoregulation to maintain body temperature within species- and developmental stage-specific critical limits. Several members of the bombesin family of peptides and histamine are included in a class of neurochemicals that have potent thermoregulatory effects in ectothermic and endothermic vertebrate species and may be involved in behavioral thermoregulation in amphibians. Because amphibians respond to environmental temperature cues differently in larval versus adult animals, we used immuno-cytochemistry to study developmental changes in bombesin-like (BN) and histamine-like (HA) innervation in the bullfrog brain and spinal cord. Neurons and fibers that were BN-immunoreactive and HA-immunoreactive were present in the earliest stage tadpoles examined (Gossner stage 29); BN-immunoreactive perikarya were found only in the preoptic area, posterior thalamic nucleus and in the rostroventral tegmentum of the mesencephalon. In the preoptic area, dramatic changes were observed in the number and staining intensity of BN-ir so-mata; neuronal labelling was greatest in tadpoles undergoing tail resorption (i.e. metamorphic climax) and was nearly absent in adults. Neurons immunoreactive to BN in the ventral mesencephalon also were developmental stage-dependent; limb-bud growth stage tadpoles had the largest numbers of labelled neurons, whereas in the adults, labelled cells were rarely visible in this area. The highest density of fibers was in the medial septum, lateral amygdala, and the optic tectum. Fewer fibers were observed within the dorsal and ventral hypothalamus, the pineal gland, and all the thalamic nuclei. Perikarya immunoreactive to HA were localized in the dorsal infundibular nucleus of the hypothalamus. Immunoreactivity was present in all developmental stages examined, and the numbers of labelled cells increased throughout metamorphosis to a maximum in adult brains. Fibers were found in the medial septum, medial amygdala, preoptic area, thalamus, pineal gland, hypothalamus and optic tectum. These results show that BN- and HA-immunoreactivities are established early in larval development, but their phenotypes are differentially expressed during larval and adult growth stages. This pattern suggests that reorganization of BN-like and HA-like neural circuitry may occur during metamorphosis and may be involved in the reported developmental changes in amphibian thermoregulation. In addition, BN-like peptides and HA may modulate other related mechanisms of amphibian thermoregulation and behaviour, such as thermal acclimation, circadian shifts in temperature selection and feeding. To what extent they are involved in amphibian thermoregulation remains to be investigated.

Bombesin is a member of a class of neuroactive chemicals that have potent thermoregulatory effects in ectothermic and endothermic vertebrate species. Bombesin-like peptides are found in the brains of ectothermic and endothermic vertebrates and have been implicated in the central nervous system modulation of behavioral thermoregulation. Amphibians rely on behavioral thermoregulation to maintain their body temperature within developmental stage-dependant critical limits. To investigate the influence of bombesin on behavioral thermoregulation, we examined the effects of central injections of bombesin on thermal habitat selection at different stages of bullfrog development. Tadpoles and adult male and female frogs were allowed to select a preferred temperature, within an aquatic thermal gradient, before and after receiving an intracerebroventricular injection of bombesin. In larval and adult female bullfrogs, bombesin administration caused a decrease in preferred temperature values. This effect was clearly dose-dependent in tadpoles. Bombesin effects were variable in adult males, probably due to an overriding stress response to handling exhibited by males. The bombesin-induced hypothermia was blocked by [D-Phe⁶, Des-Met¹⁴]-bombesin (6–14), ethyl amide, a bombesin/gastrin-releasing peptide receptor antagonist. These data suggest that bombesin/gastrin-releasing peptide receptors are functional in the central nervous system of larval and adult amphibians and that receptor binding can modulate thermoregulation. They raise the question: under what natural conditions is endogenous bombesin/gastrin-releasing peptide released in the brain to activate thermoregulatory behavior?

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