In tadpoles, the number of neurons expressing the neurotransmitter dopamine increases on exposure to light. Such plasticity might allow animals to match their brains' response to environmental stimuli.
Nervous systems are known to adapt to environmental inputs. But such plasticity has been thought to involve modifications of neural circuits and communication between neurons via synaptic junctions — as in learning and memory — rather than alterations in the numbers of distinct classes of neuron. On page 195 of this issue, Dulcis and Spitzer1 challenge this view. They show that, when frog tadpoles (Xenopus laevis) are exposed to bright light, the number of dopamine-secreting (dopaminergic) neurons in the animals' brains increases, allowing them to adapt more rapidly to subsequent exposure to light.
Anyone who has caught wild frog tadpoles from a pond has probably been surprised to see the captive animals turn pale after a couple of hours. This rapid change in pigmentation allows tadpoles to better blend in with their surroundings, reducing their risk of becoming prey. A distinct neural circuit controls this process. Specifically, light-induced signals from the eye are relayed to a brain region called the suprachiasmatic nucleus, which contains dopaminergic neurons. From there, signals pass on to another region containing neurons that secrete melanocyte-stimulating hormone to trigger pigment cells in the skin (Fig. 1a). This circuit works in an alternating manner such that, in response to light, positive inputs from the eye to the suprachiasmatic nucleus trigger increased dopamine release. High dopamine levels then provide negative inputs to the hormone cells, resulting in reduced hormone secretion and so decreased pigmentation of the peripheral skin.
The pigmentation response is modulated by previous experience, because prolonged or repeated exposure to bright light results in tadpoles adapting more rapidly to subsequent exposures2. Changes in this response and its underlying circuitry have been studied extensively2,3,4,5, and were believed to primarily involve plasticity at the level of synaptic connections and signals passing through the circuit. Dulcis and Spitzer1 reveal that this adaptation also involves a rapid increase in the number of dopaminergic neurons within the circuitry.
The speed of the response that the authors observe is remarkable — when exposed to only two hours of light, tadpoles that had been raised in the dark exhibited a doubling of dopaminergic neurons within the suprachiasmatic nucleus. What's more, the newly emerging dopaminergic neurons seem to affect the pigmentation process on subsequent exposures to light by reducing pigmentation more rapidly (Fig. 1b). The authors traced these neurons' axonal processes and found that they project onto the hormone-releasing neurons. They next ablated the baseline dopaminergic neurons using specific drugs to show that this treatment completely abolishes light adaptation. But when animals with ablated dopaminergic neurons were exposed to light on subsequent occasions, dopaminergic neurons that had appeared after drug treatment could restore light adaptation.
Where do these 'new' dopaminergic neurons come from? Do they result from a change in the type of neurotransmitter secreted by pre-existing neurons, or are they generated de novo? Earlier work revealed that the mammalian brain (even that of adult mammals) constantly generates new neurons6,7, and that this process can be enhanced in response to environmental cues8,9,10. For instance, adult laboratory mice living in an enriched environment — large cages containing running wheels, nesting material and toys — have increased numbers of neurons in specific brain areas, particularly those crucial for spatial orientation10. Likewise, songbirds add and remove neurons to certain brain regions on a seasonal basis, a mechanism that acts to match brain anatomy to appropriate seasonal behaviour11. In the case of light adaptation in tadpoles, however, Dulcis and Spitzer find no evidence for new cells being generated within the suprachiasmatic nucleus. Given the rapid appearance of the extra dopaminergic neurons, this observation was perhaps expected: it is unlikely that additional neurons could be generated de novo within the relatively short time frame of only two hours. Instead, it seems that pre-existing neurons expressing a different neurotransmitter now co-produce dopamine.
Dulcis and Spitzer's findings advance the idea that external sensory inputs modulate a specific response by regulating the population size of specific neuronal subtypes — those that are involved in controlling the physiological response to the input — in the brain. From a broader perspective, their observation that pre-existing neurons can switch on the expression of an additional type of neurotransmitter adds to the growing list of ways in which brain plasticity can arise: weakening or strengthening of communication between neurons, formation of new connections, and the recent findings6,7,8,9,10 that additional neurons of certain types can be added to the system de novo. An issue that Dulcis and Spitzer do not address, however, is whether the particular type of brain plasticity they observe is limited to developing tadpoles, or whether it also applies to adult frogs — and mammals, for that matter.
In humans, dysfunction of signalling cascades mediated by dopamine may be an essential element of seasonal affective disorder, also known as winter depression12. Here, a way forward might be to analyse patients using positron emission tomography (PET), a technique that is routinely used to visualize dopaminergic cells13. Although PET images are of limited resolution, a dramatic increase in the number of dopamine neurons — possibly in response to seasonal changes in day length or diurnal changes in light intensity — may be detectable. If plasticity mediated by changes in the pattern of neurotransmitter production applies to humans, it is likely to open fresh avenues towards combating neurological diseases.
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Tyrosine hydroxylase-immunoreactive neurons in the brain of tadpole of the narrow mouthed frog Microhyla ornata
Journal of Chemical Neuroanatomy (2020)