The chemical neuroanatomy of the mammalian pineal gland: neuropeptides.

The mammalian pineal gland contains multiple afferent peptidergic nerve fibres. Sympathetic nerve fibres, with their origin in the superior cervical ganglia, contain neuropeptide Y colocalized with norepinephrine. Other pinealopetal nerve fibres, probably originating in the pterygopalatine ganglion, contain vasoactive intestinal peptide and peptide histidine isoleucine. Fibres containing substance P and calcitonin gene-related peptide have also been demonstrated in pinealopetal nerve fibres. These fibres might originate in the trigeminal ganglion. The neurotransmitter content of the fibres of the central innervation, innervating the gland from the brain via the pineal stalk, has not been elucidated. However, strong indications for the presence of neuropeptide Y, substance P, somatostatin, and vasopressin in these fibres have been presented. Recent immunohistochemical studies have further shown the presence of subtypes of pinealocytes containing neuropeptides. Thus, pinealocytes containing beta-endorphin, leu-enkephalin, and somatostatin have been demonstrated in the gland. Immunohistochemistry at the electron microscopical level has shown, that in some species, leu-enkephalin containing pinealocytes make synaptic contacts with other pinealocytes indicating of paracrine regulation of the pineal gland. It must however be emphasized that large interspecies variations exist with regard to the peptidergic pineal innervation and its content of peptidergic cells.