An intriguing study by
Dulcis et al. (2013) describes a switch in neurotransmitter phenotype that may mediate the effects of changes in photoperiod on mammalian behaviors. The variations in photoperiod that occur seasonally at high latitudes can elicit physiological and behavioral changes in various organisms and influence mood in humans.
Dulcis et al. (2013) found that the number of dopaminergic neurons in hypothalamic nuclei receiving retinal input by way of the suprachiasmatic nucleus decreased in rats maintained for a week on long-day cycles (19 hours of light; 5 hours of darkness), whereas the number of somatostatin neurons increased. Conversely, in rats maintained on short-day cycles (5 hours of light; 19 hours of darkness), the number of dopaminergic neurons increased, whereas the number of somatostatin neurons decreased. These changes did not depend on neurogenesis or apoptosis; rather, they resulted from a switch in neurotransmitter expression and were accompanied by homeostatic changes in D2 dopamine receptor expression on postsynaptic corticotrophin-releasing factor (CRF) neurons. Long-day cycles (leading to decreased D2 receptor abundance) were associated with increased CRF in the cerebrospinal fluid, increased plasma corticosterone, and an increase in stress behaviors (rat models of anxiety and depression) in these nocturnal animals. Focal ablation of dopaminergic neurons (or exposure to dopamine receptor antagonists) also elicited stress behaviors; remarkably, the behavioral effects of focal ablation were partially rescued by subsequent exposure to short-day cycles. Thus, neurons in the adult brain appear to switch transmitter phenotype in response to changes in photoperiod, providing a possible mechanism linking photoperiod to mood and behavior (see
Birren and Marder, 2013).
