Subfornical Organ - Hormones and Receptors

Hormones and Receptors

Neurons in the subfornical organ have receptors for many hormones that circulate in the blood but which do not cross the blood–brain barrier, including angiotensin, atrial natriuretic peptide, endothelin and relaxin. The role of the SFO in angiotensin regulation is particularly important, as it is involved in communication with the nucleus medianus (also called the median preoptic nucleus). Some neurons in the SFO are osmoreceptors, being sensitive to the osmotic pressure of the blood. These neurons project to the supraoptic nucleus and paraventricular nucleus to regulate the activity of vasopressin-secreting neurons. These neurons also project to the nucleus medianus which is involved in controlling thirst. Thus, the subfornical organ is involved in fluid balance.

Other important hormones have been shown to excite the SFO, specifically serotonin, carbamylcholine (carbachol), and atropine. These neurotransmitters however seem to have an effect on deeper areas of the SFO than angiotensin, and antagonists of these hormones have been shown to also primarily effect the non-superficial regions of the SFO (other than atropine antagonists, which showed little effects). In this context, the superficial region is considered to be 15-55μm deep into the SFO, and the "deep" region anything below that.

From these reactions to certain hormones and other molecules, a model of the neuronal organization of the SFO is suggested in which angiotensin-sensitive neurons lying superficially are excited by substances borne by blood or cerebrospinal fluid, and synapse with deeper carbachol-sensitive neurons. The axons of these deep neurons pass out of the SFO in the columns and body of the fornix. Afferent fibers from the body and columns of the fornix polysynaptically excite both superficial and deep neurons. A recurrent inhibitory circuit is suggested on the output path.