Immunohistochemical studies on the distribution of neuropeptides and serotonin in the suprachiasmatic nucleus of the Brattleboro rat.

The distribution of vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP)/bombesin, somatostatin, vasopressin, neuropeptide Y (NPY) and serotonin was examined immunohistochemically in the suprachiasmatic nucleus (SCN) of male rats genetically deficient for vasopressin (Brattleboro strain). VIP-containing neurons and their varicose fibers were preferentially distributed in large numbers in the ventromedial part of the SCN. GRP/bombesin-containing neurons and their fibers were also gathered in the ventral part of the SCN, particularly in the ventromedial region of the nucleus. Somatostatin-containing neurons and their fibers were prominent in the rostral and middle portions of the SCN, where the highest concentration of immunoreactivity was restricted in their ventromedial part. No vasopressin-immunoreactivity was found at all throughout the SCN. Profuse NPY-containing varicose fibers were observed in the ventrolateral part of the SCN, but no immunoreactive neurons were distributed in this nuclear region. Serotonergic fibers showed a topographic arrangement in the SCN: a serotonin-immunoreactive nerve plexus was predominantly distributed in the ventrolateral part. These findings indicate that the SCN of Brattleboro rats is composed of distinct subdivisions of immunoreactive cell bodies and fibers. The distribution of the five peptides and indoleamine within the SCN in the Brattleboro strain was compared with that in normal Long-Evans rats. Furthermore, both strains of rats were exogenously administered with arginine-vasopressin, but no conspicuous difference in the regional patterns of immunoreactivity was detected. The possible role of vasopressin in the SCN is discussed.