PROJECTION OF NONCHOLINERGIC BASAL FOREBRAIN NEURONS ENSHEATHED WITH PERINEURONAL NETS TO RAT MESOCORTEX

The existence of non-cholinergic (GABAergic) components in the septo-h ippocampal system but also in basal forebrain projections terminating in the olfactory bulb and certain cortical areas has been documented b y several authors using retrograde and anterograde tracing techniques. On the other hand, the basal forebrain also contains a high number of mainly parvalbumin-positive neurons ensheathed by a lattice-like matr ix of polyanionic proteoglycans forming so-called perineuronal nets of as yet unknown function. By a combination of retrograde tracing using Fluoro-Gold injections into mesocortical areas of rats and staining o f perineuronal nets by Wisteria floribunda agglutinin (WFA) the presen t study describes the projection pattern and distribution of non-choli nergic projection neurons characterized by perineuronal nets in the an terior parts of the basal forebrain complex (medial septal nucleus, nu cleus of the diagonal band of Broca, magnocellular preoptic nucleus). After tracer injection into the cingulate cortex labelled net-associat ed neurons were distributed within the rostrocaudal extension of the b asal forebrain complex but were predominantly found in the horizontal limb of the diagonal band of Broca. Retrograde labelling of neurons wi th perineuronal nets after tracer injection into the retrosplenial cor tex was more pronounced in the medial septum. Choline acetyltransferas e-immunoreactive (ChAT-ir) projection neurons were in no case associat ed with perineuronal nets. The results demonstrate that a large portio n of the non-cholinergic projection neurons of the basal forebrain are endowed with a specialized microenvironment of proteoglycans and form a strong input system of mesocortical components of the limbic system .