Dopamine receptor supersensitivity: an outcome and index of neurotoxicity.

The characteristic feature of neurotoxicity is a definable lesion which can account for observed deficits, corresponding to loss of nuclei or axonal fibers normally comprising a specific pathway or tract. However, with ontogenetic lesions, the operative definition fails. In rats lesioned as neonates with 6-hydroxydopamine (6-OHDA), near-total destruction of dopamine- (DA-) containing nerves is produced, and this itself is definable. However, the most prominent feature of rats so-lesioned is the DA receptor supersensitivity (DARSS) that develops and then persists throughout the lifespan. DA D(1) receptors show overt supersensitivity to agonists producing vacuous chewing movements (VCMs), while D(1) receptors associated with locomotor activity have a latent supersensitivity that must be unmasked by repeated D(1) or D(2) agonist treatments - a 'priming' phenomenon. This D(1) DARSS is not usually associated in either a change in D(1) receptor number (B(max)) or affinity (K(d)). In contrast to D(1) DARSS, D(2) receptors are not so predictably supersensitized by a lesion of DA neurons. In reality, the permanently exaggerated response to an agonist by supersensitized receptors is per se a manifestation of neurotoxicity. Despite dramatic behavioral responses mediated by supersensitized receptors, DARSS has not been easy to correlate with enhanced production of second messengers or early response genes. Altered signaling (i.e., neuronal cross-talk) in defined pathways may represent the mechanism that produces so-called receptor supersensitization. Long-lived agonist-induced behavioral abnormality, with or without anatomic evidence of a neuronal lesion, is one of the products of DA D(1) receptor supersensitization -- itself an index of neurotoxicity.