Neurobiology Select

The fundamental function of the nervous system is to accurately process and respond to external stimuli. This issue's Neurobiology Select highlights recent findings exploring how neurons and their networks within the nervous system may be modulated to ensure accurate signal detection, transmission, and response. In larvae of the frog Xenopus laevis, light regulates a neural circuit in the ventral suprachiasmatic nucleus (VSC) of the brain, enabling the larvae to adapt their skin pigmentation to the environment in an elegant example of camouflage behavior. Light-induced signals trigger the activation of neurons expressing the neurotransmitter dopamine, which suppress the secretion of hormones that activate pigment cells. Light stimuli also promote adaptation of this neural circuit to ensure faster responses during subsequent exposures to light. In a new study, Dulcis and Spitzer (2008) now show that light stimuli also promote sensitization of this neural circuit to ensure faster responses during subsequent exposures to light. Contrary to the assumption that only modifications to neural circuitry are required for this adaptation, rapid changes in the number of dopamine-expressing neurons also play a role in providing the neural plasticity required for stimulus adaptation. Using a specific dopamine receptor inhibitor, the authors identify a population of VSC dopamine-expressing (dopaminergic) neurons that regulate the pigment response. Strikingly, frog larvae raised in the dark (dark-raised) and then exposed to light for two hours show a 2-fold increase in the number of VSC dopaminergic neurons, as revealed by immunostaining for the expression of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. Quantification of neuron nuclei and characterization of neuronal subtypes by immunostaining reveal no changes in the total number of VSC neurons between animals exposed to two hours of light and control animals kept in the dark. This demonstrates that the observed expansion of dopaminergic neurons is not due to cell proliferation or migration. Instead, light seems to induce the respecification of neurotransmitter expression in these VSC neurons, a process that normally occurs during neuronal differentiation. Dulcis and Spitzer propose that this expanded population of dopaminergic neurons, whose projections can be traced by immunofluorescence to melanocyte-activating hormone-releasing cells, improve the sensitivity of the light-induced pigment response. Indeed, larvae raised in light (thus possessing more VSC dopaminergic neurons) adapt to additional light stimuli more rapidly than animals with the same level of pigment but raised in the dark and harboring fewer dopaminergic neurons. These findings provide evidence for the exciting notion …