Association of habituation with endogenous alpha-melanocyte-stimulating hormone in the brain of the toad, Bufo cognatus
Several lines of evidence indicate that, in mammalian and nonmammalian vertebrates, alpha-melanocyte- stimulating hormone (a-MSH) acts as a neurotransmitter/neuromodulator within the brain to enhance learning and memory. Although a great deal is known about the pharmacological effects of a-MSH peptides on learning, virtually nothing is known about the physiological role of endogenous a-MSH neurons in learning behavior. I used a simple behavioral model of learning, habituation of prey-catching behavior in the toad Bufo cognatus, to examine whether learning behavior is associated with alterations in the activity of melanotropin cells in the brain and pituitary gland. Changes in the tissue concentration of a-MSH in specific brain areas were used to gauge changes in the synthetic/secretory actions of brain a-MSH cells. The distribution of a-MSH and adrenocorticotropin (ACTH) in the brain of the toad was examined by immunocytochemistry using the avidin biotinylated-peroxidase technique. Pro-opiomelanocortin (POMC) perikarya were detected in the ventral infundibular nucleus and the lateral hypothalamic nucleus. The cells projected their fibers toward brain areas that included the ventral telencephalon, the ventral hypothalamus, the tectum and pretectal area, the brainstem, and the spinal cord. Intense immunoreactive staining was observed in the olfactory nucleus and the nucleus accumbens, the medial anterior thalamus, the preoptic nuclei, and the infundibulum. The immunocytochemical localization of a-MSH-immunoreactivity in the brain of B. cognatus was confirmed by biochemical studies. First, heterologous radioimmunoassay of a-MSH in the toad was validated by the demonstration of parallelism between serially diluted brain extracts and plasma and synthetic a-MSH. a-MSH concentration was highest in the caudal thalamus and retrochiasmatic hypothalamus, followed by the caudal telencephalon and preoptic area, the rostral telencephalon, the brainstem and the optic tectum. With one exception, there were no significant differences at any time point between control and habituated animals. However, significant changes in brain a-MSH content were observed within each group over time. In the brains of both habituated and control animals, a-MSH content was increased in the hypothalamic and the preoptic areas and decreased in the telencephalic area and the brainstem. My immunocytochemical and biochemical findings suggest that, aside fi'om its hormonal activity in the control of melanosome dispersion, a- MSH may play a role as a neurotransmitter and/or neuromodulator in the toad brain. Furthermore, it seems that the habituation of prey-catching behavior is not associated with changes in the activity of a-MSH levels in the brain and the plasma of toads. Prolonged confinement might cause alterations in the activity of neuronal and pituitary POMC cells.