Expression and physiological regulation of BDNF receptors in the neuroendocrine melanotrope cell of Xenopus laevis

Brain-derived neurotrophic factor (BDNF) and α-melanophore-stimulating hormone (α-MSH) are co-sequestered in secretory granules in melanotrope cells of the pituitary pars intermedia of the amphibian Xenopus laevis. α-MSH is responsible for pigment dispersion in dermal melanophores during the process of black-background adaptation. BDNF-production in melanotrope cells is increased by placing animals on a black background, and BDNF acts as an autocrine stimulatory factor on the melanotrope cells. However, the repertoire of possible neurotrophin receptors of the melanotrope is unknown. In this study we have established the expression of full length TrkB (TrkB.FL), truncated TrkB (TrkB.T) and p75NTR receptors in the Xenopus neurointermediate lobe by RT-PCR. In situ hybridization reveals the presence of TrkB.FL mRNA and p75NTR mRNA in melanotrope cells. Quantitative RT-PCR shows that in animals on a black background the amounts of TrkB.T and p75NTR mRNA are about three times higher than in white background-adapted animals. We suggest that the amount of p75NTR sets the sensitivity of the melanotrope cells for the stimulatory action of BDNF during physiological adaptation to background light intensity.