The biochemical identity of putative neurotransmitters in autonomic neurons of different vertebrate groups is discussed. Cholinergic and serotonergic neurons contain the same transmitters in all vertebrate groups: acetylcholine and 5-hydroxytryptamine, respectively. The occurrence of adrenaline and noradrenaline in different proportions in adrenergic neurons is pointed out, and the possibility of purinergic neurons releasing purine derivatives considered.

Bombesin, gastrin/CCK/caerulein, neuropeptide Y (NPY), neurotensin, opioids, somatostatin, substance P (tachykinins) and vasoactive intestinal polypeptide (VIP) are all ‘members of different families of peptides’ that occur in neurons and/or endocrine or paracrine cells. Sequence variations of these peptides between vertebrate groups, and the occurrence of several related peptides in one animal are discussed. Available information on the biosynthesis of a neuropeptide in non-mammalian vertebrates has been included.