Disappearance of supra-ependymal 5-HT axons in the rat forebrain after electrolytic and 5,6-DHT-induced lesions of the medial forebrain bundle.

The present study was intended to demonstrate the origin of supra-ependymal 5-hydroxytryptamine axons in the rat forebrain. Electrolytic lesions and injections of 5,6-dihydroxytryptamine (10 mug in 4 mul) were carried out unilaterally in and close to the medial forebrain bundle in the posterior hypothalamus of rats. Ten to 14 days later, terminal axons and formaldehyde-induced indolealkylamine fluorescence had virtually disappeared supra-ependymally in the lateral ventricles and interventricular foramina ipsilateral to the lesion if the indolealkylamine axons passing through the medial part of the medial forebrain bundle had been destroyed. No changes were observed, electron microscopically or fluorescence histochemically, in ventricles contralateral to the lesion. It is concluded that the supra-ependymal serotonergic nerve terminals in the lateral ventricles and interventricular foramina originate, uncrossed, from non-terminal axons passing through the medial forebrain bundle in the posterior hypothalamus.