ULTRASTRUCTURAL TRACING STUDIES ON THE DIRECT AND INDIRECT FIBER CONNECTIONS BETWEEN NUCLEUS ACCUMBENS AND MIDBRAIN PERIAQUEDUCTAL GRAY IN RAT

Employing a double labeling technique by combination of anterograde tracing with retrograde tracing, we investigated the synaptic connections between the descending fibers from the nucleus accumbens (Acb)and the projection neurons from the midbrain periaqueductal gray(PAG)to the nucleus raphe magnus(NRM)or the projection neurons from the lateral habenular nucleus (LHb)to the PAG. After injecting phaseolus vulgaris leucoagglutinin(PHA L)into the Acb and horseradish peroxidase (HRP)into the NRM or PAG, respectively, PHA L anterogeradely labeled fibers and terminals and HRP retrogradely labeled neurons overlapped mainly in the caudal segment of the ventrolateral subdivision(VLS)of the PAG and dorsal raphe nucleus(DR)or medial portion of the LHb, respectively. PHA L anterogradely labeled axon terminals mainly made asymmetrical axo somatic and axodendritic synapses with HRP retrogradely labeled neuronal cell bodies and dendrites within the VLS of the PAG, DR or medial portion of the LHb. The present results showed that direct descending fibers from the Acb to the PAG mainly terminated onto the neurons in the PAG and DR which sent projection fibers to the NRM, indirect descnding fibers relayed at the medial portion of the LHb, in turn, the neurons in the medial portion of the LHb which receied termination of the Acb descending fibers projected to the PAG. These results indicated that both direct and indirect descending projections from the Acb to the PAG and DR might regulate the neuronal activities within the PAG and DR.