Localization of immunoreactive GABA and enkephalin and NADPH‐Diaphorase‐positive neurons in fetal striatal grafts in the quinolinic‐acid‐lesioned rat neostriatum

Abstract

Fetal striatal tissue grafts have been shown to partially reverse the biochemical and behavioral deficits induced by excitotoxic lesions. To determine if grafted striatal neurons contain neurochemical markers similar to those in neurons in the caudate nucleus and to establish the morphological characteristics and relative frequency of labeled neurons in the grafts, the localization of immunoreactive GABA and leucine-enkephalin (ENK) and of NADPH-diaphorase (NADPH-d) activity was examined in fetal striatal grafts at the light and electron microscopic levels. Striatal tissue from 17-day fetuses was grafted into the caudate nucleus of adult rats 1 week after intracaudate injections of either a low or high dose of quinolinic acid. At the light microscopic level, immunoreactive GABA and ENK and NADPH-d-positive neurons, processes, and punctate structures were present within adjacent sections of the same grafts. The frequency and morphological features of these labeled cell populations were similar in grafts placed into either minimally or extensively lesioned striata. Immunoreactive GABA and ENK neurons in the grafts constituted 28% and 13.5%, respectively, of the neuronal population of the graft and their mean diameters were 22 and 14% larger, respectively, than neostriatal neurons that contained the same chemical markers. NADPH-d-positive neurons in the grafts formed 3.5% of total grafted neurons and exhibited characteristics of neostriatal NADPH-d-containing aspiny cells, including medium-sized somata, indented nuclei, and varicose dendrites. At the electron microscopic level most GABA-positive neurons in the grafts contained indented nuclei and most immunoreactive ENK somata had unindented nuclei. Dendrites and dendritic spines with GABA or ENK immunoreactivity were present in the grafts where they were postsynaptic to unlabeled axons. Immunoreactive GABA and ENK axon terminals formed synapses with unlabeled neuronal profiles in the grafts. These findings demonstrate that fetal striatal grafts contain chemically defined neuronal populations that form synaptic connections within the graft and share some features with corresponding cell groups in the neostriatum. These results provide an anatomical basis for the graft-induced recovery from behavioral and biochemical deficits caused by instrastriatal lesions reported in other studies.