Ectopic expression of TrKA in the adult rat basal ganglia induces both nerve growth factor-dependent and -independent neuronal responses.

Ectopic expression of tropomyosin-related kinase A (TrkA), the high-affinity receptor of nerve growth factor (NGF), has been widely used in cell culture systems to uncover its role in cell survival or death events. In contrast, little is known about the consequences of its expression in vivo. To address this question, adeno-associated virus (AAV) vectors were used to express TrkA in the substantia nigra (SN) and striatum of adult rats. Nine weeks after transfer, tyrosine hydroxylase (TH) and dopamine transporter (DAT) mRNAs were slightly decreased in the ipsilateral SN. This decrease was no longer significant when NGF was delivered into the striatum. There was no change of DAT binding sites or D1 or D2 receptor mRNAs and binding sites in the striatum, suggesting that ectopic TrkA exerts a limited effect on the pool of TH and DAT transcripts, without affecting overall dopamine signaling. When transferred into the striatum, TrkA transgene had no effect on the size of the cholinergic interneurons, but it exerted typical neurotrophic effects, as shown by an enlargement of the projection neurons and nitric oxide synthase (nNOS)-expressing interneurons. This trophic action was amplified by a delivery of NGF. No toxic effect of the transgene was noted. These data indicate that ectopic expression of TrkA may result in the promotion of neurotrophic effects or can influence neuronal plasticity in the absence of exogenous NGF in neuronal populations that naturally fail to respond to this factor.