SUMMARY1. Extracellular action potentials (units) were recorded from rat subfornical organ explants in vitro in response to addition of angiotensin II (All) or carbamylcholine (carbachol) or serotonin (5-HT) to the superfusion solution. The frequency recorded was dose dependent over a wide range (All, 0 05-5 nM; carbachol, 2-7-2700 nM; 5-HT, 1-100 nM). Appropriate antagonists, sarcl-ala2 angiotensin (saralasin) for All, atropine sulphate for carbachol and methysergide maleate for 5-HT, blocked these excitations. The effects were reversible except for that of atropine.2. Two populations of AII-excited units were found. A superficial population lying between 15 and 45 4um from the ependymal surface was blocked only by saralasin and another population lying more than 55 ,#m below the ependymal surface could be blocked by atropine as well as saralasin. Carbachol-evoked units generally lay below 45 #um, and 5-HT-evoked units were scattered evenly over the subfornical organ. It is suggested that superficial AII-excited neurones have a cholinergic excitatory synapse with the deeper carbachol-excited neurones.3. No evidence was found for the hypothesis that neurones of the subfornical organ are excited by morphine or by changes in extracellular osmotic pressure.4. All types of drug-excited unit, both superficial (15-55 gim) and deep (below 55 gin), 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 stm from the surface. 5-HT-evoked units were driven antidromically only by stimulation of the columns of the fornix. All-and carbachol-evoked units could be driven antidromically or antidromically and synaptically by stimulation of the body or the columns of the fornix. It is suggested that All units driven antidromically are actually carbachol-sensitive neurones driven by the more superficial AII-sensitive cells. 5. A model of the neuronal organization of the subfornical organ is suggested in which AII-sensitive neurones lying superficially are excited by substances borne by blood or cerebrospinal fluid and synapse with deeper carbachol-sensitive neurones. The axons of these deep neurones pass out of the subfornical organ in the columns and body of the fornix. Afferent fibres from the body and columns of the fornix polysynaptically excite both superficial and deep neurones. A recurrent inhibitory circuit is suggested on the output path.

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2. The neurones of the subfornical organ appeared to have the characteristics of neurosecretory neurones. Their action potentials were prolonged and their antidromic spike was easily broken down into components by repetitive stimulation.3. The organization of the subfornical organ inferred from extracellular recording was confirmed by the results of intracellular recording.