Neurotransmitter receptors in fetal tissue transplants: expression and functional significance

Numerous studies have examined receptor expression in neural transplants and their possible role in transplant-induced functional recovery from lesion-induced deficits. Herein we attempt to summarize the results of these studies, especially those from studies involving striatal transplants. Autoradiographic studies indicate that dopamine D1 and D2, muscarinic, cholinergic, 5-HT2, opiate mu, beta adrenergic and cholecystokinin (CCK) receptors are present in striatal transplants. Many of these receptors are present regardless of the transplant location and surrounding environment. This suggests that the expression of these receptors is determined by intrinsic properties of transplanted tissue, and is independent of transplant location and environment. Some transplant receptors, such as dopamine D1 and D2 and muscarinic receptors in striatal transplants, or 5-HT2 receptors in cortical transplants, display a patchy distribution which is dissimilar to that in the corresponding adult host tissue. This manuscript discusses this "abnormal" receptor distribution and possible explanations. Electrophysiological studies have indicated that some of the transplant receptors respond to physiological and pharmacological stimulation, suggesting that they are functional. However, the association of receptor expression with behavioral recovery is uncertain. The expression of neurotransmitter receptors in neural transplants may not be essential for the functional recovery associated with trophic mechanisms. However, neurotransmitter receptors may play an important role when functional recovery requires neuroanatomical integration between the host brain and the transplanted tissue.