Ultrastructure of Met5-enkephalin terminals in the caudate-putamen nuclei of adult rats receiving neonatal intranigral 6-hydroxydopamine.

Destruction of dopamine neurons of the nigrostriatal pathway in the early postnatal rat enhances the levels of Met5-enkephalin in the adult dorsal striatum (caudate-putamen nuclei) and may contribute to the abnormal self-injurious behavior seen in humans with Lesch-Nyhan disease. We examined whether there were ultrastructural changes in Met5-enkephalin immunoreactive terminals in the rat model that might reflect cellular sites for enhanced activity of these opioid neurons. At 3 days postnatal, 10-20 nl injections of a 1% solution of the dopamine neurotoxin, 6-hydroxydopamine (6-OHDA), or vehicle were placed unilaterally in the region of the substantia nigra of 25 litters of male rat pups. In adulthood (72-80 days postnatal), the brains of these animals were fixed by vascular perfusion with an aldehyde solution. Met5-enkephalin immunolabeling was examined in coronal sections at three rostrocaudal levels through the caudate-putamen nuclei of control (ipsilateral and contralateral to vehicle and contralateral to 6-OHDA) and experimental (ipsilateral to 6-OHDA) groups. In selectively lesioned animals, there was a significant increase in the relative optical density of immunoautoradiographic labeling for enkephalin throughout the rostrocaudal striatum ipsilateral to 6-OHDA as compared to control groups. Electron microscopy revealed immunoperoxidase labeling for enkephalin in axon terminals and more rarely in soma and dendrites irrespective of drug treatment. In both experimental and control striatal tissues, the enkephalin immunoreactive terminals formed primarily symmetric synapses with unlabeled dendrites or spines. However, ipsilateral to 6-OHDA injections there was a small (5.4%), but significant increase in the proportion of enkephalin immunoreactive terminals contacting dendritic spines, the known targets of dopamine terminals. Appositions were commonly detected between enkephalin immunoreactive terminals and other morphologically heterogeneous axons in the striatum ipsilateral to 6-OHDA and in control tissues. Met5-enkephalin immunoreactive terminals in adult striatum ipsilateral to 6-OHDA injections showed a 214% increase in volume as compared to vehicle-injected controls. Concurrently, there was a small (13%), but significant increase in the numerical density (number/volume) of enkephalin-labeled terminals both contralateral and ipsilateral to 6-OHDA injections. These results suggest that a change in bouton size is the major mechanism by which striatal enkephalin neurons alter their synaptic efficacy and target associations to compensate for damage to the nigrostriatal dopamine neurons.