Transplantation of Mesencephalic Cell Suspension in Dopamine-Denervated Striatum of the Rat I. Effects on Spontaneous Activity of Striatal Neurons S. DI LORETO,* T. FLORIO,† A. CAPOZZO,† A. NAPOLITANO,* D. ADORNO,* AND E. SCARNATI† ,1 †Department of Biomedical Technology, School of Medicine University of L’Aquila, and *CNR Tissue Typing Institute, I-67100 L’Aquila, Italy These studies have examined the extent to which intrastriatal grafts of embryonic mesencephalic neu- rons induce recovery of normal discharge patterns in striatal neurons of rats after a unilateral 6-hydroxydo- pamine (6-OHDA) lesion of the nigrostriatal dopamine (DA) pathway. Lesioned rats were tested for rotational behavior induced by amphetamine and apomorphine. Animals which responded positively to these tests received two suspensions of mesencephalic embryonic neurons into the dorsal striatum (ST) ipsilateral to the denervated side. Sham-grafted rats received the sus- pension medium only. The vitality of the graft was assessed by the disappearance or reversion of rota- tional movements induced by amphetamine. Extracel- lular recordings of neurons located throughout the ST were carried out 3 months after grafting, when the animals reached the age of 6 months. The 6-OHDA- induced nigral lesion caused a net increase both in the number of striatal neurons spontaneously active and in their discharging rates. The signs of increased neuronal activity were also present in sham-grafted animals. The grafting of embryonal cells strongly re- duced the number of active neurons and decreased significantly their discharging rate. The effects of the intrastriatal graft appeared to be present within a radius of 1.5–2 mm from the core of the grafted area. The presence of tyrosine-hydroxylase-immunopositive neurons innervating the host ST confirmed the viabil- ity of the grafts at the time of electrophysiological record- ing. The results show that besides compensating motor asymmetries caused by DA denervation, intrastriatally grafted dopaminergic neurons are able to only partially restore the electrophysiological action of DA in dis- crete striatal domains. r 1996 Academic Press, Inc. INTRODUCTION It is now well established that embryonic nigral neurons transplanted in the striatum (ST) of rats bearing 6-hydroxydopamine (6-OHDA)-induced degen- eration of the substantia nigra (SN) remain physiologi- cally active, provide extensive terminal innervation within the ST, establish synaptic relationships with the host brain, and maintain their own electrophysiological proper- ties (4, 5, 13, 31, 36). The high levels of dopamine (DA) measured within the grafted areas and in the surrounding striatal tissue (30, 37) may reverse some of the behav- ioral deficits caused by the DA denervation (1, 12). To date, little is known about the effects of grafts on the electrophysiological properties of neurons within the reinnervated ST. After a solid mesencephalon graft into the dorsal ST, neurons adjacent to the implant discharge at frequencies lower than those of neurons located distally (31). When mesencephalic neurons are implanted as cell suspensions, striatal cells within the transplanted regions exhibit spontaneous frequencies similar to those commonly encountered in the intact ST (13). However, little is known of the ability of grafted DA neurons to affect, selectively, any of the different types of neurons identifiable in the rat ST on the basis of electrophysiological criteria (19, 25, 33–35). We have therefore undertaken the present study to investigate the extent to which intrastriatal implanta- tions of mesencephalic cell suspensions are effective in restoring spontaneous activity of striatal neurons in rats bearing a 6-OHDA-induced lesion of the nigrostria- tal pathway. MATERIALS AND METHODS Induction of nigral lesions. Three-month-old male Sprague–Dawley rats were used to initiate experi- ments. The rats, under chloral hydrate (400 mg/kg ip) anesthesia, received, stereotaxically, 4 μl of sterile saline containing 8 μg of 6-OHDA into the left SN as previously described (14). Coordinates used for the SN location were 5.3 mm caudal to bregma, 2.2 mm lateral to the midline, and 7.4 mm below the skull surface, as described in the atlas of Paxinos and Watson (26). Behavioral effects of nigral lesions. As illustrated in Table 1, 15 days following the 6-OHDA treatment rats 1 To whom correspondence should be addressed. Fax: 39 862 433433. e-mail: Scarnati@aquila.infn.it. EXPERIMENTAL NEUROLOGY 138, 318–326 (1996) ARTICLE NO. 0070 318 0014-4886/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.