An experimental attempt to determine the site of the neurohypophysial and osmoreceptors in the dog

The object of this investigation has been to define the site of the osmoreceptors, a term that has been applied to those hypothetical sensory elements that respond to changes in the osmotic pressure of their vascular environment, and through which the release of antidiuretic hormone from the neurohypophysis is physiologically regulated. Confirmation is given to the cephalic localization of these receptors, and an attempt has been made to discover where, within the substance of the brain, they reside. The test of their activation has been the inhibition of urine flow by intracarotid infusions of hypertonic solutions during established water-diuresis. By surgical means the cephalic field of distribution of the carotid arteries in the dog has been restricted to defined regions of the brain, and information has thereby been acquired on the osmoreceptive status of these regions. In this way it has been contrived that the evidence for the localization of the osmoreceptors should rest solely on the use of a blood-borne physiological stimulus appropriate to the sensory elements in question. The earlier sections of the paper present the results of the broader studies that this work has necessitated; first, an anatomical investigation of the arterial connexions of the circle of Willis and of the detailed vascular architecture of the diencephalon and hypophysis; and secondly, the devising of a method to trace the distribution of arterial blood and the application of this method to the demarcation of the cerebral fields of carotid and vertebral arteries. The middle sections of the paper describe the surgical and experimental techniques employed, and the manner in which the posterior lobe of the pituitary gland, the posterior brain stem and the cerebral hemisphere have been excluded from being the site of receptors. Also described are experiments in which the permanent unilateral suppression of responses following intradural ligation of an internal carotid artery has placed beyond cavil the assertion that the receptors he within the substance of the brain. The final sections present the evidence for the hypothalamic localization of the osmoreceptors and for the suggestion of a possible involvement of the thalamic paraventricular nucleus in the osmoreceptive process. , To trace the distribution of arterial blood two coloured suspensions have been employed. The suspensions are freely miscible with blood, cause no circulatory disturbance in the living animal during the period of their infusion, and tolerate histological procedures. The suspensions are infused into appropriate arteries at a terminal experiment in which the animal is killed before the suspensions have had time to recirculate. In the dog the brain is normally supplied with blood from both carotid and vertebral arteries. The telencephalic field of the carotid arteries is that part of the hemisphere supplied by the anterior and middle cerebral arteries and includes the striate body; the vertebral arteries supply the hippocampus and posterior cerebral artery field, the midbrain, cerebellum and medulla. However, this strict partition is defied by two seemingly normal occurrences: common carotid blood may pass via the occipito-vertebral anastomosis to join the basilar stream, and vertebral blood may pass forward from the posterior communicating artery to mix with the carotid cerebral supply. The thalamus is supplied almost exclusively by vertebral blood that reaches it via branches of the posterior cerebral artery and the thalamic branch of the posterior communicating artery. The hypothalamus is divided in the sources of its supply: carotid blood irrigates the anterior nuclei via arterioles arising directly from the internal carotid and the immediate vicinity of its trifurcation; vertebral blood supplies the posterior nuclei via arteriolar branches of the posterior communicating artery, but, in addition, this blood may stream forward to supplement the carotid supply to the pre-infundibular nuclei. Situated near that part of the circle where the carotid and vertebral streams meet are the main nuclear groups of the neurohypophysis, the supraoptic nuclei. Particular attention has been paid to the volume partition of these nuclei according to the origins of their blood supply, in order to gain an index of blood distribution in the anterior hypothalamus. In the normal animal between 10 and 30 % of the total supraoptic nuclear material is supplied with blood of vertebral origin. The naturally occurring asymmetry of the arterial supply to the posterior lobe of the pituitary gland has led to the elimination of this structure as a site of osmoreceptors. In each of a total of eight animals it was found that the posterior lobe was supplied with blood originating exclusively from one carotid, yet an osmotic release of antidiuretic hormone had been obtained from infusions of hypertonic solutions into the other carotid. The exclusion of carotid blood from the posterior brain stem has been achieved by ( the ligation of the two occipital arteries, and, on one side, by (b) the ligation of the occipital and posterior communicating arteries. Osmotic responses to carotid infusions were retained after these procedures and thus made secure the assignment of the receptors to the prosencephalon. Of prosencephalic structures the greater part of the telencephalon has been eliminated from being the receptor site by demonstrating the ipsilateral retention of responses after total hemispherectomy. This conclusion was supported by evidence from animals in which the carotid bloods, while evoking osmotic responses, were asymmetrically distributed within the telencephalon. The heavy degenerative cell loss in all thalamic nuclei of the hemispherectomized animal, excluding some cell groups of the midline, substantiated earlier indications that the receptors were not in the dorsal diencephalon. Alternative and equally compelling evidence for this conclusion was obtained from experiments in which carotid blood was directly excluded from the thalamus. Such exclusion was attained by tying the posterior communicating and occipital arteries of one side together with the middle cerebral or anterior cerebral artery. It was also attained in experiments in which the internal carotid of one side was ligated intradurally, this resulting in a redistribution of blood from the circle such that the thalamus of one side was deprived of any carotid flow. These same experiments afforded, too, unequivocal evidence for the exclusion of the posterior nuclear groups of the hypothalamus as a possible site for the receptors. The collation of the several evidences summarized above has led to the inference that the osmoreceptors are situated somewhere in the anterior hypothalamus or preoptic areas, that is, in the region comprised by the medial and lateral preoptic areas, the suprachiasmatic nucleus, the nucleus supraopticus diffusus, the anterior hypothalamic area, the paraventricular and supraoptic nuclei, the dorsomedial and ventromedial nuclei, and the dorsal and lateral hypothalamic areas. This region has always received carotid blood when there have been osmotic responses to intracarotid infusions. The restriction of the cephalic distribution of the carotid of one side of this receptive zone was successfully achieved by ligature of the anterior cerebral, middle cerebral and posterior communicating arteries just beyond the carotid trifurcation, together with occlusion of the ipsilateral occipital artery. The sequel, however, was disappointing in that osmotic responses were lost on that side; but the experiment provoked conjecture upon the possible neuronal organization of the osmoreceptive apparatus, since it was found that in this animal the operation had caused cystic destruction of the thalamic paraventricular nucleus together with all other anterior and medial thalamic nuclei. In all previous animals, including the hemispherectomized one, in which osmotic responses had been retained, the thalamic paraventricular nucleus and its connexions with the hypothalamus had remained intact. The conclusion is drawn that the osmoreceptors are situated in the anterior hypothalamus. There are indications that they are not of unvarying sensitivity, and that their functioning may be dependent upon the integrity of nervous connexions with the thalamic paraventricular nucleus.