Alterations in Brain and Plasma Concentrations of Alpha-Melanocyte-Stimulating Hormone during Habituation of Prey-Catching Behavior in Toads

Abstract

Alpha-melanocyte-stimulating hormone (α-MSH) and adrenocorticotropin (ACTH) promote habituation of prey-catching behavior in toads. We tested the hypothesis that habituation is associated with alterations in the activity of α-MSH neurons in a toad, Bufo cognatus. We used immunocytochemistry and RIA to determine the organization and distribution of α-MSH neurons in the brain. In addition, we measured brain α-MSH and plasma concentrations in toads at different times before and during stimulus specific habituation of the turning response to a rotating prey dummy. Cell groups immunoreactive for α-MSH-/and ACTH were detected only in the infundibular hypothalamus. Projections from these cells innervated the olfactory nucleus, nucleus accumbens, septum, habenular nucleus, the preoptic area, the hypothalamus, the caudal thalamus, optic tectum and brain stem. The rank order of α-MSH concentrations in the brain of Bufo cognatus was caudal thalamus/hypothalamus (T/HT) > preoptic area (PO) ≧ rostral telencephalon (Tel) > brain stem (BS) > optic tectum (OT). The only difference observed in tissue α-MSH levels between habituated and non-habituated toads was an elevation in T/HT α-MSH levels in control toads after 20 min in the testing apparatus. In contrast, many differences were observed in both groups as a function of time spent in the test apparatus. Concentrations of α-MSH in the T/HT and PO increased while concentrations in the Tel decreased over time for both control and habituated toads. Plasma α-MSH titers rose over time in control toads but not in habituated toads. Our findings suggest that habituation per se is not associated with dramatic alterations in brain or plasma α-MSH levels. On the other hand, prolonged confinement causes pronounced alterations in the activity of brain and pituitary proopiomelanocortin cells.

Distribution of Neuronal Melanocortins in the Spadefoot Toad <i>Spea multiplicata</i> and Effects of Stress

“…See list of abbreviations. and Carr, 1997], with the exception that αMSH/ACTH-ir neurons were not observed in the preoptic area of Bufo. In S. multiplicata, neurons in the preoptic area are immunoreactive for peptides from both the mid-region (αMSH, ACTH) and C-terminus (βE) of POMC, suggesting that they are indeed authentic POMC-producing neurons.…”

“…The organization of brain POMC systems has been examined in only a handful of anuran genera: Rana [DoerrSchott, 1981;Yui, 1983;Benyamina et al, 1986;Vallarino, 1987]; Bufo [Kim and Carr, 1997], and Xenopus [Hayes and Loh, 1990;Tuinhof et al, 1998]. In the present study we describe the distribution of neuronal melanocortins in S. multiplicata, a representative of the family Pelobatidae.…”

“…For determination of αMSH content by radioimmunoassay (RIA), brains were rapidly removed, placed into near-freezing amphibian saline (0.6% NaCl), and dissected into 5 regions [Kim and Carr, 1997] [Markwell et al, 1981] of Lowry's method [Lowry et al, 1951].…”

“…Radioimmunoassay for αMSH in acid extracts of toad brain was performed using a disequilibrium assay [Khorram et al, 1984] as described by Kim and Carr [1997]. Synthetic αMSH (Bachem, Torrance, Calif., USA) was iodinated with chloramine-T and purified by gel filtration (Sephadex G-10).…”

“…Evidence that neuronal melanocortins play a role in the stress response is not restricted to mammals. Previous work in our lab indicates that exposure of toads (Bufo speciosus) to a noxious stimulus selectively alters regional brain αMSH content [Kim and Carr, 1997;Olsen et al, 1999] and reduces prey-oriented turning reactions [Carr et al, 2000].…”