Olfactory and nonolfactory projections in the river lamprey (Lampetra fluviatilis) telencephalon

The projections of the olfactory bulb, the primordial dorsal, piriform and hippocampal pallia, and of the dorsal thalamus were studied in the lamprey Lampetra fluviatilis using horseradish peroxidase (HRP) and HRP coupled to the wheat germ agglutinin (WGA-HRP). There was obtained an experimental morphological evidence of the presence of the direct thalamo-telencephalic projections in this vertebrate species. The anterior and posterior parts of the dorsal thalamic nucleus, the nucleus of Bellonci, the primordial geniculate bodies, the rostral part of the midbrain were identified as the sources of the telencephalic afferents. These connections may serve as a morphological substrate for transmission of nonolfactory impulses to the telencephalon of the lamprey. The projections of the nucleus of Bellonci into the primordial hippocamp were compared to the limbic thalamo-hippocampal pathways of other vertebrates. We have established, that the fibers ascending from the dorsal thalamus were distributed in the same areas, as those descending from the olfactory bulb. These are: mainly the primordial hippocamp and only a few fibers reach the dorsal and piriform pallia, as well as an area free of olfactory projections--the dorsal part of the subhippocampal lobe. We have also demonstrated that, the secondary olfactory fibers mainly projected ipsilaterally to the primordial dorsal and piriform pallia. A lesser dense bulbar projection has been observed ipsilaterally in the primordial hippocamp and in the ventral part of the subhippocampal lobe. Only few olfactory projections were found in the pallial areas and in the subhippocampal lobe contralaterally. The olfactory fiber terminals were also observed ipsilaterally in the septum, striatum, preoptic area and in the contralateral olfactory bulb. Bilateral bulbofugal projections also occur in the diencephalon, namely in the ventral thalamus and in the hypothalamus. Caudally, the secondary olfactory fibers can be traced up to the area of the posterior tuberculum. Afferents to the olfactory bulb in the river lamprey originate in the subhippocampal lobe, in all three pallial formations and probably in the dorsal thalamus. These structures are at the same time the target zones for the olfactory bulb efferent projections, thus being connected reciprocally with the olfactory bulb.