It is well known that nervous system plasticity (the mutable quality of brains) underlies animals’ ability to change their behaviors. However, it has been widely thought that such plasticity consists mostly of changes at synapses, the sites of communication between neurons, or in the intrinsic excitability of individual neurons – how likely a cell is to increase or decrease its voltage or to fire action potentials. New research challenges this view by showing that another type of alteration can be at work: single neurons can change the type of neurotransmitter they release in response to stimulation1.
Specifically, this work addresses tadpole camouflage. Tadpoles can rapidly change (increase or decrease) the amount of pigmentation in their skin in order to blend in with their surroundings. The research I here refer to demonstrates that when frog tadpoles (Xenopus laevis) are stimulated with 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.
When exposed to just hours of direct light, dark-reared tadpoles responded by doubling the number off dopaminergic neurons within a part of their central nervous system called the suprachiasmatic nucleus (SCN). Furthermore, these new dopaminergic neurons seem to integrate into the existing pathways for changing pigmentation. The authors were able to selectively destroy existing SCN dopaminergic neurons, eliminating the tadpoles ability to change pigment, and then rescue this ability with light exposure.
It remains unclear if this phenomenon is purely a developmental one, operating only prior to adulthood, an important caveat. None the less, the ability to change cell type is an as-yet unexplored form of neural plasticity which will have widespread implications for neuroscience research. Furthermore, this work may have even more direct application to human experience.
In human beings, the malfunction of certain dopamine-mediated signalling cascades (cell-level combinations of molecular interactions) have been implicated in seasonal affective disorder (winter depression). It is also well known that dopamine is intrinsically involved in the rewarding feelings delivered by food, sex and drugs. It may thus simply be the case that a lack of light stimulation leads to a lack of dopaminergic neurons, and thus to fewer episodes of rewarding experience during the winter months.
1. Dulcis D, Spitzer NC. Illumination controls differentiation of dopamine neurons regulating behaviour. Nature 456: 195-201, 2008.