The connections between the olfactory bulb and the brain in the goldfish

Abstract

We have investigated olfactory bulb connections in the goldfish by using both horseradish peroxidase applied to olfactory tract lesions and wheat germ agglutinin-conjugated peroxidase administered to the olfactory bulb. The projections from the bulb pass to the brain in two major bundles: the lateral and the medial olfactory tracts. The lateral tract innervates the posterior terminal field in the area dorsalis. The medial tract divides into two rami. The dorsolateral ramus is the most substantial olfactory bundle in the brain and innervates several targets in both the area dorsalis and the area ventralis. The ventromedial ramus appears to innervate targets in the area ventralis exclusively. In addition, fibers of the medial olfactory tract (both rami) innervate the preoptic nucleus as well as targets in the diencephalon and, possibly, in the mesencephalon as well. Olfactory fibers from the dorsolateral and ventro-medial rami cross the midline in the dorsal and ventral olfactory decussations, respectively. Between these two decussations is a dense olfactory plexus which has not previously been reported, and which may serve as a nexus allowing interchange of fibers between the two olfactory rami. The terminal nerve in the goldfish has two parts. The major part of the nerve projects to the ventral nucleus (Vv) and is far more extensive than has previously been reported. A much less substantial component of the terminal nerve projects to the two retinae. There are four large groups of cells in the telencephalon which project to the olfactory bulb. Of these two, the dorsal and the anterolateral groups have not been described in previous studies of the goldfish. We also report weakly labeled bulbopetal cells in the nucleus preglomerulosus and in the locus ceruleus.