Brain Research, 521 (1990) 1-6 1 Elsevier BRES 15614 Research Reports Grafting of midbrain neurons into the hippocampus restores serotonergic modulation of hippocampal activity in the rat Gal Richter-Levin and Menahem Segal Center for Neuroscience, The Weizmann Institute of Science, Rehovot (Israel) (Accepted 19 December 1989) Key words: Raphe; Rat; Dentate; Serotonin The ability of midbrain raphe (MR) cells grafted into serotonin-depleted brain to restore hippocampal responses to synaptically released serotonin was studied using a serotonin-releasing drug, D-fenfluramine (FFA). In normal rats, FFA enhances reactivity of the dentate gyrus (DG) to perforant-path stimulation, while suppressing spontaneous activity. These effects of the drug were dependent on the presence of normal serotonin neurotransmission in the hippocampus. Depletion of brain serotonin markedly attenuated DG reactivity to FFA. MR grafts (4-5 months after transplantation) restored the reactivity of the hippocampus to FFA. Immature MR grafts (3 weeks after transplantation) or septai control grafts could not reproduce the effects of mature grafts. These experiments demonstrate the possible functional incorporation of neural transplants in a host brain and the possible involvement of these grafts in regulation of normal host electrical activity. The combined utilization of a serotonergic graft and a releasing drug (i.e. FFA), in the serotonin-deprived brain, allows the study of serotonin functions in restricted parts of the brain. INTRODUCTION Embryonic raphe cells, grafted into adult rat hippo- campus, were shown to survive and to innervate the host with a dense network of serotonin-containing fibers 3'17. In earlier studies we found that the grafted neurons develop electrophysiological properties similar to those of in situ raphe neurons and, upon stimulation, produce inhibitory action in host hippocampal neurons 13'14. We now proceed to explore the possible modulation by the midbrain raphe (MR) graft of electrical activity of the host hippocampus. We have recently described effects of 5-HT-releasing drugs (i.e. D-fenfluramine (FFA) and p-chloroamphetamine (PCA)) on spontaneous activity and on evoked responses of dentate granule cells (DG) to perforant path (PP) stimulation 9'1°. Population spike (PS) response to PP stimulation was increased 9,w, whereas hippocampal EEG and single unit activity were suppressed 9'11 by these drugs. These effects were depen- dent on the presence of an intact serotonergic neuro- transmission system, p-Chlorophenylalanine (PCPA), a blocker of serotonin synthesis, blocked the effects of these drugs. Restoration of 5-HT levels in PCPA-treated rats, using 5-hydroxytryptophan (5-HTP), restored DG response to FFA application 9A°. In the present report we examine the ability of MR cells grafted into the hippo- campus to restore the response of serotonin-denervated hippocampi to application of FFA. MATERIALS AND METHODS Adult (200-300 g) male Wistar rats of a local breeding colony were used. Initially, rats were injected, under chloral-hydrate (3.5% solution, 10 ml/kg, i.p.) anesthesia, with the neurotoxin 5,7- dihydroxytryptamine (5,7-DHT, Sigma) 16. The injection was made into the fornix-fimbria and cingulum bundle (coordinates: 1 mm posterior to bregma, 1.2 mm lateral to midline, 15 ° off vertical. Depth, 4 mm for fornix-fimbria, 3 mm for cingulate bundle). 5/~g of 5,7-DHT were injected in a volume of 0.5/A in each site. In some experiments 5,7-DHT was injected into the lateral cerebral ventri- cles (200/~g in a volume of 10/tl in each ventricle). One week after the injection the rats were grafted with MR tissue as described previously 14'17. Briefly, the medial part of the midbrain of 13- to 14-day embryos was dissected out, minced and injected stereotaxically into the hippocampus of the 5,7-DHT-pretreated rats. Rats were returned to their home cage for 4-5 months before experiments commenced. Control grafts included raphe grafts 3 weeks after transplantation and septal neurons (16-17-day embryos) injected into fornix-transected rat brains one week after the lesion. Four to 5 months after transplantation, rats were anesthetized with chloral hydrate and prepared for recording. Stimulating electrodes (stainless steel twisted bipolar electrode, 125/~m diam- eter) were placed in the angular bundle (7.9 mm posterior to bregma, 4.2 mm lateral to midline). The vertical position (approx- imately 3 mm below dura) was adjusted to yield the largest response in the DG. Recording electrodes (monopolar nichrome, 62 /~m diameter enamel insulated wire) were positioned in the ipsilateral granule cell layer (3.5 mm posterior to bregma, 2.5 mm lateral to midline). Correspondence: M. Segal, Center for Neuroscience, The Weizmann Institute, Rehovot 76100, Israel. 0006-8993/90/$03.50 (~ 1990 Elsevier Science Publishers B.V. (Biomedical Division)