An immunocytochemical study of the olfactory projections in the three-spined stickleback, Gasterosteus aculeatus, L.

The distribution of olfactory fibers in the brain of the three-spined stickleback was visualized by means of immunohistochemistry. The labeling of the olfactory fibers was produced by serum containing antibodies against somatostatin-14. Olfactory fibers were observed entering the olfactory bulbs, where they terminated in the glomerular layer or collected into fascicles that coursed through the bulbs into the telencephalon without participating in the formation of the glomerules. In the telencephalon the fascicles, which belonged to the medial olfactory tract, formed two fiber systems: ventral descending fibers and dorsal descending fibers. The ventral descending fibers could be followed through the ventral telencephalon to the vicinity of the lateral tuberal nucleus. The dorsal descending fibers coursed via the anterior commissure to the posterior part of the telencephalon. Part of the postcommissural fibers of the dorsal descending system coursed to the posterior zone of the area dorsalis telencephali while others left the telencephalon via the medial forebrain bundle and could be followed to the periventricular hypothalamus. Some axons formed synaptic contacts with unlabeled cell bodies in the nucleus of the terminal nerve which, in this species, is situated immediately behind the bulbs. In addition, an extensive terminal field associated with the dorsal descending fibers was found in the ventromedial aspects of the telencephalon. It is unlikely that the labeling represents immunoreactive somatostatin-14 because: 1) the labeling persisted after the absorption of the antiserum with synthetic somatostatin-14; 2) antiserum against somatostatin-14 from another manufacturer did not have this labeling property; and 3) the production of the absorbable labeling depended on the choice of fixative whereas the production of the unabsorbable labeling did not.