Increase of striatal methionin enkephalin content following lesion of the nigrostriatal dopaminergic pathway in adult rats and reversal following the implantation of embryonic dopaminergic neurons: a quantitative immunohistochemical analysis.

The aim of the present study was to test whether intrastriatal implants of embryonic dopaminergic neurons are able to normalize the lesion-induced dysfunction of striatal enkephalinergic neurons, one of the major output systems of the striatum. The ascending dopaminergic pathway of adult rats was unilaterally lesioned. Three weeks later a cell suspension obtained from the mesencephali of ED14 rat embryos was implanted into the denervated striatum and striatal methionin enkephalin immunostaining was quantified six months later by the use of an image analyser. Methionin enkephalin immunostaining was unevenly distributed in the striatum of control animals. Besides the classical patch/matrix pattern, a mediolateral gradient was also present and, moreover, immunostaining decreased towards caudal levels. Seven months after the lesion of the nigrostriatal dopaminergic pathway, methionin enkephalin immunostaining was found to be increased in the denervated striatum by about 50%. However, relative increases were more sustained in the areas where basal methionin enkephalin immunostaining were lowest, i.e. the lateral striatum and posterior striatal areas. This resulted in an attenuation of the global gradients seen in the normal striatum. Increased immunostaining was also found in the ipsilateral globus pallidus. The implantation, into the denervated striatum, of embryonic dopaminergic neurons led to a reversal of the lesion-induced increase of striatal and pallidal methionin enkephalin immunostaining six months later. Moreover, this reversal resulted in an overshoot, as the level of immunostaining in the graft-bearing striatum was found to be lower than the levels found in the normal striatum. It is concluded that grafts of embryonic dopaminergic neurons can normalize the function of one of the major output systems of the striatum and, through it, influence more distant targets of this structure. This suggests a physiological basis for the behavioral effects observed previously with such grafts.