SEROTONERGIC AND NONSEROTONERGIC PROJECTIONS FROM THE RAPHE NUCLEI TOTHE PIRIFORM CORTEX IN THE RAT - A CHOLERA-TOXIN-B SUBUNIT (CTB) AND 5-HT IMMUNOHISTOCHEMICAL STUDY

Retrograde axonal transport of the cholera toxin B subunit (CTb) was c ombined with 5-HT immunohistochemistry to determine the origin of the serotonergic innervation of the piriform cortex (PC) in the rat. After iontophoretic CTb injections in the PC, a substantial number of retro gradely labeled cells were found in the middle and medio-ventral part of the dorsal raphe nucleus (RD). A few retrogradely labeled cells wer e also observed in the median raphe nucleus (MnR) and the B9 serotoner gic cell groups. Following CTb and 5-HT immunohistochemistry on the sa me sections, double-labeled cells were observed in the RD, MnR and B9 groups. In the RD, 30% of CTb stained cells were immunoreactive to 5-H T. After colchicine or nialamide (a monoamine oxidase inhibitor) pretr eatment the percentage of these double-labeled cells reached 70%. Thes e results indicate that both 5-HT and non-5-HT neurons in the RD inner vate the PC and that the percentage of double-labeled cells is influen ced by drug pretreatment. To determine the terminal fields of the RD e fferent fibers in the PC, injections of the anterograde tracer PHA-L w ere also performed. Analysis of the fiber distribution in the PC furth er revealed some medio-lateral and antero-posterior differences.