~ Pergamon 0306-4522(94)00371-8 Neuroscience Vol. 64, No. 1, pp. 61 70, 1995 Elsevier Science Ltd Copyright © 1994 IBRO Printed in Great Britain. All rights reserved 0306-4522/95 $9.50 + 0.00 INTRACRANIAL TRANSPLANTATION AND SURVIVAL OF TUBEROMAMMILLARY HISTAMINERGIC NEURONS H. BERGMAN,*t~ J. I. NAGY§ and A. C. GRANHOLM:~ tDepartment of Cell Biology, University of Link6ping, Link6ping, Sweden :~Department of Basic Science, University of Colorado School of Dentistry, Denver, CO 80262, U.S.A. §Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada Abstract--Investigations were undertaken to determine whether fetal histaminergic neurons in the tuberomammillary nucleus of the posterior hypothalamus survive intracranial transplantation to adult hosts. Two methods of transplantation were utilized. Grafts were placed either into the delayed cavity of a fimbria-fornix lesion or directly into the hippocampus using stereotaxic techniques. The tissue was taken from rat fetuses at embryonic days 16-17 and grafted into adult rats of either the Sprague-Dawley or the Fischer 344 strain. Routine histology and immunohistochemistry were used to evaluate the grafts. All transplants to Sprague-Dawley rats showed signs of rejection, while no signs of rejection were seen in any of the Fischer 344 rats. Transplants placed directly into the delayed fimbria-fornix cavity did not grow as well or contain as many surviving neurons as the intraparenchymal grafts. The largest number of surviving histamine-positive neurons was obtained with grafts of posterolateral blocks of hypothalamus from fetal day 17 placed directly into the CAI region of the rostral hippocampal formation of Fischer 344 hosts. Histamine-immunoreactive cell bodies with neuritic outgrowth were found in all Fischer 344 rats that received hypothalamic grafts. Cell bodies exhibited histamine immunoreactivity evenly through- out the cytoplasm and had morphological characteristics resembling histaminergic neurons in situ. Axonal outgrowth extended throughout the grafted hypothalamic tissue, and was sometimes seen in the host hippocampal tissue as well. It is concluded that fetal histaminergic neurons survive transplantation to the adult hippocampal formation, and that this allograft procedure can supplement current strategies to investigate the function of histaminergic tuberomamillary neurons in the central nervous system. The existence of magnocellular neurons in the tuberal region of the hypothalamus has been known for several decades) ° These neurons in the tubero- mammillary nucleus (TM) were subsequently found to be histaminergic, 43 and were described as consist- ing of five subgroups, located medially, laterally and ventrally. 2°'26,35'44 The target areas of histaminergic neurons in the rodent brain have been mapped using antibodies directed against conjugated histamine 27'37 or the histamine-synthesizing enzyme histidine decar- boxylase. 12'22'41'42 The ontogenesis of histaminergic neurons appears to be distinctly different from that of the catecholaminergic neurons, 21 but closely related to the development of serotonergic neurons.l'4° Their neurogenesis in the rat begins at embryonic day (E) 13, peaks on El6 and is completed by El8. 31 Hista- minergic neurons with well developed dendrites and axons extending considerable distances from cell bodies were shown to survive in hypothalamic explants from neonatal rats for up to nine weeks in vitro. 32 It has also been demonstrated that *To whom correspondence should be addressed, at Denver. Abbreviations: ADA, adenosine deaminase; BSA, bovine serum albumin; E, embryonic day; GFAP, glial fibrillary acidic protein; PB, phosphate buffer; PBS, phosphate- buffered saline; TH, tyrosine hydroxylase; TM, tubero- mammillary nucleus. 61 explanted histaminergic neurons can innervate co- explanted hippocampal tissue in vitro. 19 However, long-term studies of histaminergic plasticity and axonal growth properties have not been performed to date. Biochemical and immunohistochemical data has demonstrated that TM histaminergic neurons inner- vate all areas of the hippocampal formation with moderate densities of histamine-positive axons, and a large proportion of the histidine decarboxylase was reported to be concentrated in synaptosomal frac- tions indicating localizations in axons and terminals in this brain region. 2,29 Approximately 60% of these histaminergic projections were suggested to enter the hippocampal formation via a dorsal pathway consist- ing of the fimbria-fornix and cingulum, and the remainder via a ventral route through the amygdaloid area. 2 The same routes of hippocampal innervation have been described for other monoaminergic neur- ons) 8 Since the anatomy and physiology of the hippocampal formation are well known, and the afferent and efferent connections can be relatively easily manipulated by, for example, surgical fimbria- fornix transection,~l this brain region has often served as an innervation target for studies of neuronal plasticity. Bilateral fimbria-fornix transection results in a partial deafferentation of the hippocampal for- mation, producing specific spatial memory impair-