From sensory circumventricular organs to cerebral cortex: neural pathways controlling thirst and hunger

Abstract

Much progress has been made during the past thirty years elucidating neural and endocrine pathways by which bodily needs for water and energy are brought to conscious awareness through the generation of thirst and hunger. One way that circulating hormones influence thirst and hunger is by acting on neurons within sensory circumventricular organs (CVOs). This is possible because the subfornical organ and organum vasculosum of the lamina terminalis (OVLT), the sensory CVOs in the forebrain, and the area postrema in the hindbrain, lack a normal blood‐brain barrier so that neurons within them are exposed to blood‐borne agents. The neural signals generated by hormonal action in these sensory CVOs are relayed to several sites in the cerebral cortex to stimulate or inhibit thirst or hunger. The subfornical organ and OVLT respond to circulating angiotensin II, relaxin and hypertonicity to drive thirst‐related neural pathways; whereas circulating amylin, leptin and possibly GLP‐1, act at the area postrema to influence neural pathways inhibiting food intake.

As a result of investigations using functional brain imaging techniques, the insula and anterior cingulate cortex, as well as several other cortical sites, have been implicated in the conscious perception of thirst and hunger in humans. Viral tracing techniques show that the anterior cingulate cortex and insula receive neural inputs from thirst related neurons in the subfornical organ and OVLT, hunger‐related neurons in the area postrema have polysynaptic efferent connections to these cortical regions. For thirst, the median preoptic nucleus initially, and thereafter the thalamic paraventricular nucleus and lateral hypothalamus have been identified as likely sites of synaptic links in pathways from subfornical organ and OVLT to the cortex. The challenge remains to identify the links in the neural pathways that relay signals originating in sensory CVOs to cortical sites subserving either thirst or hunger.

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