Quantitative autoradiography of the serotonin transporter to assess the distribution of serotonergic projections from the dorsal raphe nucleus

Abstract

The binding of ³H‐CN‐IMI to 5‐HT uptake sites, as measured by quantitative autoradiography, was used as a marker of serotonergic neurons. Within the dorsal raphe nucleus the binding of ³H‐CN‐IMI was compared in adjacent coronal sections of rat brain to the binding of ³H‐DPAT to 5‐HT_1A receptors, which have a known somatodendritic localization. The heterogeneous pattern of binding of these two radioligands within the dorsal raphe nucleus was similar and corresponded to the distribution of serotonergic cell bodies as visualized by 5‐HT immunohistochemistry. Intracerebroventricular administration of 5,7‐dihydroxytryptamine (5,7‐DHT), which caused a dramatic loss of 5‐HT immunoreactivity and ³H‐DPAT binding to 5‐HT_1A receptors, resulted in a marked reduction of ³H‐CN‐IMI binding in this nucleus. Treatment of rats with a dose of parachloroamphetamine (PCA) which has been reported to selectively lesion serotonergic processes arising from the dorsal raphe nucleus, while sparing serotonergic cell bodies and projections from the median raphe nucleus, did not alter the binding of ³H‐DPAT or ³H‐CN‐IMI in the dorsal raphe nucleus; serotonergic cell bodies appeared morphologically unaffected. The lack of effect of PCA treatment on the binding of ³H‐DPAT and ³H‐CN‐IMI is consistent with a somatodendritic localization of the 5‐HT transporter in the dorsal raphe nucleus. PCA treatment appeared to produce a moderate loss of serotonergic innervation in serotonergic terminal field areas as visualized by serotonin immunohistochemistry. The reductions in ³H‐CN‐IMI binding observed in terminal field areas (24 to 69%) following treatment of rats with PCA did not reflect a marked differential innervation of forebrain areas by the dorsal and medial raphe nuclei as expected from previous biochemical studies, and were not entirely consistent with the findings of neuroanatomical studies using histochemical techniques. Site‐specific injection of 5,7‐DHT into the dorsal raphe nucleus produced an 80 ± 11% reduction in the binding of ³H‐CN‐IMI in this nucleus, whereas the binding of ³H‐CN‐IMI in the median raphe nucleus was not reduced. The reductions in ³H‐CN‐IMI binding measured in the caudate putamen, frontal and entorhinal cortex as a result of specific lesion of the dorsal raphe nucleus were suggestive of a heavy innervation of these areas by the dorsal raphe nucleus as indicated in neuroanatomical studies. In the hippocampus, our data were consistent with an overlapping innervation of these areas by both the dorsal and median raphe nuclei and are not reflective of predominantt innervation by the medial raphe nucleus.