Embryonic cortical neurons differentiate into various types of interneurons when heterotopically transplanted into the adult rat brain.

The adult cortex represents a heterogeneous mixture of different classes of pyramidal neurons and non-pyramidal interneurons. After grafting embryonic cortical anlage into the adult striatum, the present study investigated whether the development of different populations of interneurons in heterotopic cortical grafts is similar to the adult cortex. The presence of specific subpopulations of interneurons in grafts was assessed by immunocytochemistry using various antibodies against marker molecules for interneurons such as neuropeptides and calcium-binding proteins. These molecules are expressed to a different extent in specific subpopulations of cortical interneurons. Cortical primordia obtained on embryonic day 14 (E14) were stereotactically grafted into the center of the left striatum of adult recipient rats. After an 8-week differentiation period, host rats were perfusion fixed and immunocytochemistry was performed using antibodies against neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), somatostatin, parvalbumin and calbindin D-28k. Within the grafts, the number of immunopositive interneurons as well as the intensity of immunostaining for different marker molecules corresponded well with those of the adult cortex. In contrast, the expression pattern in the graft demonstrated clear differences when compared with the surrounding host striatum. The present study demonstrates, that at E14 at least some cells of the cortical anlage are primed to develop into different classes of interneurons independent of their normal environment and their regular synaptic connections. Thus, different interneuron progenitor cells survive transplantation and develop cell-specific morphological and cytochemical characteristics. Differentiation into various subpopulations of neurons may be a prerequisite for potential therapeutic approaches in humans.