General Aspects and History

Pituitary adenylate cyclase activating polypeptide (PACAP) is a hypothalamic peptide which activates the adenylate cyclase of the pituitary gland. PACAP was first discovered in the hypothalamus in 1989. Arimura and Shioda (1989) demonstrated that, in the pituitary gland, there are two substances which exhibit adenylate cyclase-activating properties. Both substances were isolated, purified and sequenced; and it turned out that one protein consisted of 38 amino acids (PACAP 38), while the second proved to be a 27-amino-acid (PACAP 27) fragment of the former.

PACAP shows high homology (68%) with VIP (vasoactive intestinal polypep-tide) and thus belongs to a superfamily of structurally and functionally closely related proteins, named the vasoactive intestinal polypeptide (VIP)-glucagon-growth hormone releasing factor-secretin superfamily (Fig. 4.45). Additional members of this family include corticotropin-releasing hormone (CRH), growth hormone-releasing hormone (GHRH), secretin and glucagon.

The structure of PACAP has been highly conserved through evolution among tunicata, amphibians and mammals, suggesting that PACAP is involved in the regulation of important biological functions.

PACAP is abundantly expressed in the nervous and endocrine systems where it has diverse functions. In the nervous system, PACAP peptides not only serve roles in neuronal communication and signaling but, because of their neuro-trophic properties, they also promote neuronal survival, mitosis, proliferation and differentiation. PACAP can therefore be regarded as a hypothalamic hormone with neurotransmitter, neuromodulator and neurotrophic abilities.

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