Introduction

The introduction of in situ hybridisation for the localisation of specific peptide mRNAs at the site of synthesis is a useful adjunct to immunocytochemistry and has been used here to provide some answers concerning the expression of peptide mRNAs in the diffuse neuroendocrine system. Complementary ribonucleotide probes were transcribed from subcloned inserts of cDNAs encoding specific peptide sequences. The peptides studied include the neuropeptides calcitonin gene-related peptide (CGRP), substance P, vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) as examples of neurally borne substances in the central and peripheral nervous system and atrial natriuretic peptide (ANP) and prolactin as examples of circulating hormones produced by endocrine cells. These examples emphasise the value of in situ hybridisation by elucidating sites of peptide mRNA production in the neuroendocrine system and their change in different physiological conditions.

It is now widely accepted that regulatory peptides are present in the typical endocrine cells of the ‘diffuse endocrine system’ described by Feyrter (1938) or the APUD (amine or amine precursor uptake and decarboxylation) system of Pearse (1969) as well as in nerves of the central and peripheral nervous system. This newly recognised ‘diffuse neuroendocrine system’ is thus characterised by the synthesis and release of a number of regulatory peptides which mediate the actions of these cells in a number of varied ways. Some act as circulating hormones, while others have a more restricted effect, acting only in the region of their release from endocrine or neuronal cells as potent modulators, neurotransmitters or long-term growth factors.