Effects of 5,7-dihydroxytryptamine on serotonin₁ and serotonin₂ receptors throughout the rat central nervous system using quantitative autoradiography

The effects of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), on serotonin₁ (5-HT₁) and 5-HT₂ receptors were investigated using the high degree of resolution provided by quantitative autoradiography in an effort to determine the synaptic location of these receptors 5,7-DHT treatment resulted in a decrease in 5-HT₁ binding in the dentate gyrus and CA _{3c 4} of the anterior hippocampus and in the dorsal raphe nucleus, whereas no changes were observed in the posterior hippocampus nor in many other brain structures. 5-HT₂ receptors exhibited no changes in any brain area examined in response to 5,7-DHT treatment, despite over 90% serotonin depletion in most of the forebrain nuclei examined. The results indicate that at least some of the 5-HT₁ sites labelled by [³H]5-HT in the hippocampus and dorsal raphe nucleus are presynaptic, whereas 5-HT₂ receptors are probably postsynaptic. In addition, the distribution profiles of 5-HT₁ and 5-HT₂ binding sites were compared in the rat central nervous system at various anatomical levels. 5-HT₁ binding sites were identified using [³H]5-HT, while 5-HT₂ binding sites were labelled with [³H]ketanserin. Both receptor subtypes displayed distinctly different localization patterns, which, in most cases was the inverse of the other pattern. In the brainstem it is significant that 5-HT₂ receptors are concentrated in the facial nucleus and the motor nucleus of the trigeminal nerve, areas known to influence head and facial movement. The serotonin-mediated head-shake response occurs when 5-HT₂ receptors are activated. In contrast, 5-HT₁ receptors are distributed throughout the brainstem and in specific portions of the spinal cord. These areas are thought to control the serotonin behavioral syndrome and this behavior is 5-HT_1A-mediated. All raphe nuclei were devoid of 5-HT₂ receptors; only 5-HT₁ receptor were found in these nuclei. Correlations with serotonin terminal distribution patterns are discussed. The pattern of 5-HT₂ receptor distribution was also compared with the pattern of α₁ receptors, using [³H]prazosin in order to determine whether [³H]ketanserin significantly labels α₁ receptors. Although some similarities exist, overlap of binding did not occur in other nuclei, indicating that α₁ contamination of this system is probably negligible.