Innate instincts and the brain
Understanding the neuronal circuitry that controls instincts such as thirst and hunger has been an intriguing challenge to neuroscientists. In the last decade or so, some thirst-responding neurons were identified in the circumventricular organs (CVO) of the hypothalamus. The CVO are a collection of highly vascularized structures in the brain that have a somewhat “leaky” blood-brain barrier (BBB) in comparison to other brain regions. Their incomplete BBB is thought to allow these organs to respond to peptides and hormone signals that control neurological responses to physiological stimuli. These triggered innate responses allow for homeostasis maintenance; with respect to thirst, salt and water homeostasis, specifically. In a recent Nature publication, a team led by Dr. Charles S. Zucker of Columbia University pinpoint neurons within a specific CVO - the subfornical organ (SFO) - that directly stimulate or suppress thirst (Oka et al. 2015).
Thirst control center identified in the mouse brain
To identify candidate neurons in the SFO that regulate thirst, the Zucker group looked for neurons in C57BL/6J (000664) mice that were activated after 48 hours of dehydration. Based on increased levels of cFos, CamKII, and nNOS - all surrogate markers of activated excitatory neurons - they found that one-third of the SFO neurons are strongly activated when water is withheld. The researchers hypothesized that the same neurons that are excited by dehydration likely act as the control center that drives water consumption and that stimulating these same neurons should induce the mice to drink. To activate these neurons specifically, the team used an optogenetic strategy involving sterotactic injection of an adeno-associated viral (AAV) vector expressing a Channelrhodopsin 2 (ChR2)-eYFP construct under the regulation of the CamKII promoter. When the AAV-injected mice were subsequently exposed to activating laser light to stimulate ChR2-expressing neuron, they actively sought out water and began to drink within, on average, 10-20 seconds. The light-induced drinking response was so strong that even water-satiated mice sought water and drank after ChR2 activation. These light-induced thirst control neurons are specific for water, because drinking was not stimulated following light activation when other fluids, including mineral oil, glycerol, polyethylene glycol, and honey, were offered to the mice.
Drink and do not drink impulses are controlled by distinct neuron types in the SFO
Further examination using Cre-Lox reporter systems demonstrated that three genetically distinct, non-overlapping neuronal subtypes are present in the SFO, each defined by a gene expression signature as described below:
- CamKII/nNOS and overlapping with transcription factor ETV-1 expression
- Vesicular GABA transporter (Vgat)
- Glial fibrillary acidic protein (GFAP)
Specific photostimulation of each of these SFO neuronal subtypes using similar AAV-ChR2 injected vectors demonstrated that only the type 1 neurons stimulate drinking, regardless of whether the animal was water-deprived or not. In contrast, type 2 neurons suppress thirst and water intake, even in water-deprived animals that were actively drinking. These neurons are specific for regulating water intake, because photostimulation did not affect the animals’ responses to salt or food, even when they were deprived of these substances.
The Zucker team’s results demonstrate that specific ETV-1- and Vgat-expressing neurons in the SFO regulate opposite responses to thirst: to drink or not to drink, respectively. Further detailed dissection of specific neurons in the SFO and other members of the CVO may reveal the control centers that underlie other physiological response behaviors, such as hunger. Mapping these innate control centers and their interactions with other neurons will provide unprecedented understanding of the neurobiology that underlies motivational states and responses in the mammalian brain.