QUANTITATIVE AUTORADIOGRAPHY OF THE SEROTONIN TRANSPORTER TO ASSESS THE DISTRIBUTION OF SEROTONERGIC PROJECTIONS FROM THE DORSAL RAPHE NUCLEUS

The binding of H-3-CN-IMI to 5-HT uptake sites, as measured by quantit ative autoradiography, was used as a marker of serotonergic neurons. W ithin the dorsal raphe nucleus the binding of H-3-CN-IMI was compared in adjacent coronal sections of rat brain to the binding of H-3-DPAT t o 5-HT1A receptors, which have a known somatodendritic localization. T he heterogeneous pattern of binding of these two radioligands within t he dorsal raphe nucleus was similar and corresponded to the distributi on of serotonergic cell bodies as visualized by 5-HT immunohistochemis try. Intracerebroventricular administration of 5,7-dihydroxytryptamine (5,7-DHT), which caused a dramatic loss of 5-HT immunoreactivity and H-3-DPAT binding to 5-HT1A receptors, resulted in a marked reduction o f H-3-CN-IMI binding in this nucleus. Treatment of rats with a dose of parachloroamphetamine (PCA) which has been reported to selectively le sion serotonergic processes arising from the dorsal raphe nucleus, whi le sparing serotonergic cell bodies and projections from the median ra phe nucleus, did not alter the binding of H-3-DPAT or H-3-CN-IMI in th e dorsal raphe nucleus; serotonergic cell bodies appeared morphologica lly unaffected. The lack of effect of PCA treatment on the binding of H-3-DPAT and H-3-CN-IMI is consistent with a somatodendritic localizat ion of the 5-HT transporter in the dorsal raphe nucleus. PCA treatment appeared to produce a moderate loss of serotonergic innervation in se rotonergic terminal field areas as visualized by serotonin immunohisto chemistry. The reductions in H-3-CN-IMI binding observed in terminal f ield areas (24 to 69%) following treatment of rats with PCA did not re flect a marked differential innervation of forebrain areas by the dors al and medial raphe nuclei as expected from previous biochemical studi es, and were not entirely consistent with the findings of neuroanatomi cal studies using histochemical techniques. Site-specific injection of 5,7-DHT into the dorsal raphe nucleus produced an 80 +/- 11% reductio n in the binding of H-3-CN-IMI in this nucleus, whereas the binding of H-3-CN-IMI in the median raphe nucleus was not reduced. The reduction s in H-3-CN-IMI binding measured in the caudate putamen, frontal and e ntorhinal cortex as a result of specific lesion of the dorsal raphe nu cleus were suggestive of a heavy innervation of these areas by the dor sal raphe nucleus as indicated in neuroanatomical studies. In the hipp ocampus, our data were consistent with an over-lapping innervation of these areas by both the dorsal and median raphe nuclei and are not ref lective of predominant innervation by the medial raphe nucleus. (C) 19 94 Wiley-Liss, Inc.