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Keywords:

  • Adrenocorticotropin-(1–24);
  • α-Melanocyte–stimulating hormone;
  • Melanocyte-stimulating hormone receptor;
  • Adenylate cyclase activity;
  • Forskolin;
  • Rats.

Abstract: The regulation of adenylate cyclase activity by adrenocorticotropin/α-melanocyte–stimulating hormone (ACTH/MSH)-like peptides was investigated in rat brain slices using a superfusion method. Adenylate cyclase activity was concentration-dependently increased by ACTH-(1–24), α-MSH (EC50 values 16 and 6 nM, respectively), and [Nle4,D-Phe7]α-MSH (EC50 value 1.6 nM), in the presence of forskolin (1 μM, optimal concentration). 1-9-Dideoxy-forskolin did not augment the response of adenylate cyclase to ACTH-(1–24). Various peptide fragments were tested for their ability to enhance [3H]cyclic AMP production. [Nle4,D-Phe7]α-MSH increased [3H]cyclic AMP formation with a maximal effect of 30% and was more potent than ACTH-(1–24), ACTH-(1–16)-NH2, α-MSH, ACTH-(1–13)-NH2, [MetO4]α-MSH, [MetO24,D-Lys8,Phe9]ACTH-(4–9), ACTH-(7–16)-NH2, ACTH-(1–10), and ACTH-(11–24), in order of potency. This structure–activity relationship resembles that found for the previously described peptide-induced display of excessive grooming. ACTH-(1–24) stimulated adenylate cyclase activity in both striatal (maximal effect, ˜20%) and septal slices (maximal effect, ˜40%), but not in hippocampal or cortical slices. Lesioning of the dopaminergic projections to the striatum did not result in a diminished effect of [Nle4,D-Phe7]α-MSH on [3H]cyclic AMP accumulation, which indicates that the ACTH/MSH receptor–stimulated adenylate cyclase is not located on striatal dopaminergic terminals. ACTH-(1–24) did not affect the dopamine D1 or D2 receptor–mediated modulation of adenylate cyclase activity. Based on the present data, we suggest that the binding of endogenous ACTH or α-MSH to a putative ACTH/MSH receptor in certain brain regions leads to the activation of a signal transduction pathway using cyclic AMP as a second messenger.