The neuronal organization of the rat subfornical organ in vitro and a test of the osmo‐ and morphine‐receptor hypotheses.

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Abstract
Extracellular action potentials (units) were recorded from rat subfornical organ explants in vitro in response to addition of angiotensin II (AII) or carbamylcholine (carbachol) or serotonin (5-HT) to the superfusion solution. The frequency recorded was dose dependent over a wide range (AII, 0.05-5 nM; carbachol, 2.7-2700 nM; 5-HT, 1-100 nM). Appropriate antagonists, Sar1-Ala2 angiotensin (saralasin) or AII, atropine sulphate for carbachol and methysergide maleate for 5-HT, blocked these excitations. The effects were reversible except for that of atropine. Two populations of AII-excited units were found. A superficial population lying between 15 and 45 .mu.m from the ependymal surface was blocked only by saralasin and another population lying more than 55 .mu.m below the ependymal surface could be blocked by atropine and saralasin. Carbachol-evoked units generally lay below 45 .mu.m, and 5-HT-evoked units were scattered evenly over the subfornical organ. Superficial AII-excited neurons may have a cholinergic excitatory synapse with the deeper carbachol-excited neurons. Neurons of the subfornical organ are apparently not excited by morphine or by changes in extracellular osmotic pressure. All types of drug-excited unit, both superficial (15-55 .mu.m) and deep (below 55 .mu.m), could be driven polysynaptically from the body or columns of the fornix. Units driven antidromically or antidromically and synaptically were almost all more than 55 .mu.m from the surface. 5-HT-evoked units were driven antidromically only by stimulation of the columns of the fornix. AII- and carbachol-evoked units could be driven antidromically or antidromically and synaptically by stimulation of the body or the columns of the fornix. AII units driven antidromically are actually carbachol-sensitive neurons driven by the more superficial AII-sensitive cells. A model of the neuronal organization of the subfornical organ is suggested in which AII-sensitive neurons lying superficially are excited by substances borne by blood or CSF and synapse with deeper carbachol-sensitive neurons. The axons of the deep neurons pass out of the subfornical organ 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.