Central connections of the olfactory bulb in the goldfish, Carassius auratus

The central connections of the goldfish olfactory bulb were studied with the use of horseradish peroxidase methods. The olfactory bulb projects bilaterally to ventral and dorsolateral areas of the telencephalon; further targets include the nucleus praeopticus periventricularis and a caudal olfactory nucleus near the nucleus posterior tuberis in the diencephalon, bilaterally. The contralateral bulb and the anterior commissure also receive an input from the olfactory bulb. Contralateral projections cross in rostral and caudal portions of the anterior commissure and in the habenular commissure. Retrogradely labeled neurons are found in the contralateral bulb and in three nuclei in the telencephalon bilaterally; the neurons projecting to the olfactory bulb are far more numerous on the ipsilateral side than in the contralateral hemisphere. Afferents to the olfactory bulb are found to run almost entirely through the lateral part of the medial olfactory tract, while the bulb efferents are mediated by the medial part of the medial olfactory tract and the lateral olfactory tract. Selective tracing of olfactory sub-tracts reveals different pathways and targets of the three major tract components. Reciprocal connections between olfactory bulb and posterior terminal field suggest a laminated structure in the dorsolateral telencephalon.