Preoptic LH-RH and somatostatin in the rat median eminence. An experimental light and electron microscopic immunocytochemical study.

Glass microknife lesions and immunocytochemistry were used to evaluate luteinizing hormone-releasing hormone (LH-RH)- and somatostatin (SS)-immunoreactive pathways from the preoptic region to the rat median eminence. Cuts were so placed that axons of more caudally located neurons in the periventricular hypothalamic areas were spared. Light and EM observations of LH-RH-immunostained preparations indicated that following the midline periventricular cuts the density of LH-RH labelled axons and axon terminals in the ME appeared similar to that of nonlesioned animals. Following bilateral lateral hypothalamic cuts placed between the preoptic area and the ME, LH-RH immunostaining in the ME was markedly reduced. This provides evidence that the preponderance of LH-RH axons originating from the preoptic area reach the ME by a lateral hypothalamic route. In contrast to the LH-RH findings, midline lesions made using the same coordinates caused a noticeable reduction in SS immunostaining in the accurate nucleus and ME. There was either no change or only minimal change after the lateral cut. Somatostatin axons arising from the preoptic periventricular nucleus take a periventricular route and contribute to median eminence innervation, but much less extensively than the more caudally located somatostatin neurons in the hypothalamic periventricular nucleus [19].