Angiotensin II may mediate excitatory neurotransmission from the subfornical organ to the hypothalamic supraoptic nucleus: an anatomical and electrophysiological study in the rat.

In the rat, it has been proposed that angiotensin II (AII) neurons in the subfornical organ, a midline circumventricular structure, participate in the activation of hypothalamic neurosecretory neurons and promote a rise in plasma vasopressin and oxytocin. In this study, we observed AII-immunoreactive fibers coursing throughout the supraoptic nucleus as well as in other magnocellular cell groups of the hypothalamus. Moreover, following retrograde transport of Fast blue deposited within the supraoptic nucleus, cell counts in our best case revealed that 40% of AII-immunoreactive neurons in subfornical organ contained Fast blue, and 46% of the retrogradely labeled subfornical organ cells contained AII. In separate electrophysiological studies, post-stimulus histograms from 18 of 28 supraoptic neurons displayed a 30-55% reversible reduction in the excitation evoked by an electrical stimulus in the subfornical organ during local pressure applications of 100 microM to 1 mM saralasin. In 2 of 14 other cells, tubocurare (100 microM) produced only a 10% reduction in subfornical organ excitation. These observations indicate that AII may mediate an excitatory input to supraoptic neurons from the subfornical organ.

Angiotensin and the lamina terminalis: illustrations of a complex unity.

The subfornical organ, the median preoptic nucleus, and the OVLT are arranged in a dorsal to ventral column to form the lamina terminalis ("end wall") of the third cerebral ventricle. In the decade of the 1970s, several labs showed that angiotensin II produces many of its effects in the brain by interacting with the structures of the lamina terminalis. This review concentrates on drinking behavior and on two structures: The subfornical organ as an important sensor of blood-borne angiotensin, and the median preoptic nucleus as a sensitive zone for central angiotensin. In the 1980s, histochemistry and tract-tracing have begun to clarify the structural framework for a complex of interactions. All three structures of the lamina terminalis are filled with angiotensin immunoreactive neural processes and angiotensin binding sites. The evidence suggests that angiotensinergic axons have intrinsic and extrinsic origins. Neurosecretion of angiotensin is apparent in some inbred strains of rats. It is concluded that the lamina terminalis is best considered a single functional entity tuned to the detection of fluid imbalance as signaled by angiotensin.

Central serotonergic influences on renal electrolyte and water excretion.

These studies examined the effects of altered activity or levels of serotonin (5-HT) in the central nervous system (CNS) on renal water and excretion of electrolytes and on arterial blood pressure. Rats were implanted with intracerebroventricular cannulae and then continuously hydrated with a hypotonic solution in order to induce a diuresis. In two separate experimental series, samples of urine were collected before and after intraventricular (i.v.t.) administrations of drug, and the effects on the excretion of sodium were determined. In the first experimental series, 5-HT in the CNS was manipulated by intraventricular administration of p-chloroamphetamine (PCA), an agent known to increase synaptic concentrations of 5-HT. Significant increases in urinary excretion of sodium (UNa V) and the Na/K ratio were observed after the administration of p-chloroamphetamine (200-600 micrograms i.v.t.). p-Chloroamphetamine in large doses also increased blood pressure and antidiuresis. The natriuresis, but not the pressor or antidiuretic responses, were attenuated by pretreatment with either p-chlorophenylalanine, an inhibitor of tryptophan hydroxylase, or fluoxetine, a drug which inhibits the release of 5-HT following the administration of p-chloroamphetamine. Thus, the natriuretic response appeared to be due to a p-chloroamphetamine produced increase of synaptic 5-HT. A further test of the role of 5-HT in the central control of sodium excretion was made in a second experimental series where hydrated rats received intraventricular injections of 5-HT. After direct application of 5-HT to the CNS, significant increases in UNa V and in the Na/K ratio were observed, concomitant with depressor effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Bi-directional, chemically specified neural connections between the subfornical organ and the midbrain raphe system.

Reports of a serotonin-immunoreactive fiber plexus in the subfornical organ (SFO) and evidence that this input is relevant to the control of thirst and blood pressure by the SFO prompted an investigation of neural connections between the SFO and the serotonergic neurons in the raphe system. Serotonin-immunoreactive fibers were found to enter the SFO by a number of different routes and to form a plexus in the central part, which contains a bed of fenestrated capillaries. Injections of the anterogradely transported lectin, phaseolis vulgaris leucoagglutinin (PHA-L), into the dorsal raphe nucleus produced a pattern of labeled fibers in the SFO comparable to that revealed with the anti-serotonin staining. Injections into the SFO of the retrogradely transported dye, Fast Blue, labeled cell bodies in the dorsal and median raphe nuclei of the midbrain, and at least 60% of these cells also demonstrated serotonin immunoreactivity. Injections of PHA-L into the SFO identified a return projection to this part of the raphe system, where an angiotensin-immunoreactive plexus of fibers and varicosities is found. Injections of True Blue into various parts of the midbrain raphe nuclei reliably labeled neurons in the SFO, and some 56% of them could also be stained with anti-angiotensin. It is suggested that this circuitry is involved in the control of fluid balance by the SFO and that the serotonergic-raphe projection to the SFO may participate in the relay of visceral sensory information, perhaps related to blood pressure, from the nucleus of the solitary tract and the lateral parabrachial nucleus.

Neural projections subserving the initiation of a specific motivated behavior in the rat: new projections from the subfornical organ.

The activation of neurons in the subfornical organ (SFO) by angiotensin II in the blood is known to stimulate thirst. The projections of these neurons were examined with anterograde and retrograde axonal transport methods, and were found to end in specific parts of the prefrontal cortex, septal region, substantia innominata, medial preoptic area, zona incerta and lateral hypothalamic area. These regions in turn are thought to be involved in arousal, somatomotor control and cognitive functions, essential components of the initiation and procurement phases of motivated behavior.

The distribution of angiotensin II-immunoreactive cells and fibers in the paraventriculo-hypophysial system of the rat.

Angiotensin II (AII)-immunoreactive cell bodies were found in all parts of the paraventricular nucleus of the hypothalamus (PVH) in the normal, colchicine-treated rat. The greatest concentration of cells was found in the posterior part of the magnocellular division of the nucleus, while scattered cells were found in all 5 parts of the parvocellular division. In comparison, the Brattleboro rat showed similar cell staining in parvocellular parts of the PVH, although a substantial decrease in the number of AII-stained cells was found in the magnocellular division. In the normal animal, fiber staining was evident in both laminae of the median eminence. This immunostaining was selectively enhanced in the internal lamina following water deprivation, and was selectively enhanced in the external lamina following adrenalectomy. The Brattleboro rat was similar to the normal animal with regard to staining of the external lamina, but, consistent with the diminished number of immunoreactive magnocellular neurons, little immunostaining in the internal lamina was detected. Unilateral lesions of the PVH selectively diminished staining of fibers and varicosities in the ipsilateral external lamina, while bilateral lesions virtually eliminated staining on both sides. The findings in the Brattleboro rat indicate that specific subpopulations of both parvocellular and magnocellular neurons in the PVH contain an antigen that is immunologically similar to synthetic AII and unrelated to vasopressin or its prohormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Anatomical evidence that neural circuits related to the subfornical organ contain angiotensin II.

Bidirectional connections between the subfornical organ and the hypothalamus are reviewed, and emphasis is placed on recent evidence for the presence of angiotensin II in some of these pathways. Additionally, evidence is presented suggesting that this peptide may serve as a neurotransmitter or neuroendocrine factor in the efferent projections of cell groups receiving neural inputs from the subfornical organ. It appears that angiotensin II may serve as one of the chemical messengers at each link in multi-synaptic pathways related to the subfornical organ.

Organization of angiotensin II immunoreactive cells and fibers in the rat central nervous system. An immunohistochemical study.

The distribution of angiotensin II (AII)-immunoreactive cells and fibers was examined in adult male Sprague-Dawley rats with and without colchicine pretreatment. As seems to be the case for a number of other neuropeptides, AII is preferentially found in brain stem, hypothalamic, and limbic structures involved in the control of homeostatic functions. AII-stained cell bodies were most prominent in magnocellular parts of the paraventricular and supraoptic nuclei, and cells were also found in parvocellular parts of the former. Other hypothalamic nuclei containing cell bodies include the suprachiasmatic nucleus, the medial preoptic area, and perifornical parts of the lateral hypothalamic area. Of considerable interest was robust staining in several of the circumventricular organs, in particular the subfornical organ, where both cells and fibers were found. The results of water deprivation and nephrectomy suggest that this staining does not represent uptake of circulating peptide, but instead, represents AII-containing neural connections. In the thalamus, AII-stained cells were found in the paraventricular nucleus, the central medial nucleus, the nucleus reuniens, and rostral parts of the zona incerta. Two cell groups in the basal telencephalon, in the dorsal part of the bed nucleus of the stria terminalis and in the medial nucleus of the amygdala, lay at either end of an AII-stained pathway coursing through the stria terminalis. In the midbrain, immunoreactive cells were found in the interpeduncular and peripeduncular nuclei, and one pontine cell group was detected in the most lateral part of the lateral parabrachial nucleus. The only AII-stained cells in the medulla were in the nucleus of the solitary tract, near the margin of the area postrema. Fibers were found at all levels of the central nervous system, from the olfactory bulbs to the spinal cord, where terminal fields were observed in the substantia gelatinosa and in the intermediolateral cell column. Longitudinally oriented fibers were present throughout the periventricular fiber system and in the medial forebrain bundle, including its caudal extension in ventrolateral parts of the brain stem. It is suggested that, at many different levels, AII serves as both a hormone and neurotransmitter for fluid balance.

Angiotensin II immunoreactivity in the neural afferents and efferents of the subfornical organ of the rat.

Angiotensin II (AII) immunoreactive cells and fibers were identified in the subfornical organ (SFO) of the rat. Cells were distributed in an annulus around the periphery of the SFO and were most visible in the Brattleboro rat treated with colchicine. Fibers were observed in a plexus, located centrally within the ring of cells, and knife-cuts suggested that they arise from parent cell bodies lying outside of the SFO. Studies combining immunohistochemistry with retrograde transport identified the perifornical zone of the lateral hypothalamus, the rostral zona incerta, and the nucleus reuniens of the thalamus as the source of AII-stained inputs to the SFO, and the region of the median preoptic nucleus as a recipient of AII-immunoreactive SFO efferents. It is suggested that these biochemically defined connections of the SFO participate in the central neural control of fluid balance.

Evidence for corticotropin releasing factor and Leu-enkephalin in the neural projection from the lateral parabrachial nucleus to the median preoptic nucleus: a retrograde transport, immunohistochemical double labeling study in the rat.

Neurons in the lateral parabrachial nucleus of the pons were retrogradely labeled by True Blue deposited into the median preoptic nucleus. Immunohistochemical staining indicated that some 7% of these cells also contained corticotropin releasing factor, and another 5% were stained for Leu-enkephalin. Labeled cells were not found throughout the lateral parabrachial nucleus, but were confined to distinct cytoarchitectonically defined subdivisions. These findings suggest the presence of two peptides that have been implicated in central cardiovascular control within a neural projection between two cell groups that have been similarly implicated. The origins of the presently described pathways in distinct parts of the lateral parabrachial nucleus suggest an underlying topographical organization of this cell group that had not previously been appreciated.

The subfornical organ and the integration of multiple factors in thirst.

Rats with lesions centered on the ventral stalk of the subfornical organ (SFO) were used to characterize the participation of this structure in the control of drinking. It is concluded that the SFO does indeed play some minor role in the mediation of drinking following intraventricular injections of angiotensin. Further, it is shown that lesions of the SFO, but not lesions of the adjacent septal-hippocampal tissue, attenuate osmotic thirst elicited by two doses of hypertonic saline. Diminished drinking responses following water deprivation, and normal feeding responses following food deprivation, underscore the importance of the SFO for drinking behavior in general, and an expanded role for the SFO in fluid regulation is suggested. However, some incidental observations suggest that the SFO is less than an equal partner with structures in the AV3V region in the overall control of water balance.

Transection of subfornical organ neural connections diminishes the pressor response to intravenously infused angiotensin II.

Knife-cut lesions were used to assess the participation of the subfornical organ (SFO) in the central pressor action of intravenously administered angiotensin. Knife-cuts of the ventral stalk of the SFO significantly attenuated pressor responses during infusion of 3 doses of angiotensin, although responses to bolus injections were unaffected. These results are consistent with previous work in implicating the SFO as an important mediator of the central pressor action of circulating angiotensin.

A further characterization of the effects of AV3V lesions on ingestive behavior.

Rats prescreened for responses to dipsogens received anteroventral third cerebral ventricular (AV3V) or sham lesions (ether anesthesia). Many known effects of AV3V lesions were observed, and new findings include a failure to drink adequately in response to polyethylene glycol (PEG) and water deprivation (regardless of responses to other dipsogens) and abnormally high levels of water intake during food deprivation. The groups did not differ in the osmotic suppression of deprivation-induced feeding. Drinking responses to angiotensin, isoproterenol, and PEG were highly intercorrelated in the experimental animals, perhaps due to destruction of an angiotensin-related substrate. Five months after surgery, rats with lesions were hyperphagic and hyperdipsic per gram of body weight, but actually ate a smaller amount of food than control rats, and absolute levels of water intake did not differ. The range of amount of daily water intake, but not food intake, was significantly greater in the experimental rats. These findings suggest that lower levels of neural organization mediate fluid balance in rats with AV3V lesions.

An HRP study of the connections of the subfornical organ of the rat.

The connections of the subfornical organ (SFO) were investigated by using the HRP technique. Injections into the SFO labeled neurons in the medial septum, but not in lateral septum nor in the diagonal band nucleus. Labeled cells were observed in the median preoptic nucleus, below the ependyma of the third ventricle, in the dorsal preoptic region near the anterior commissure, and diffusely throughout the medial preoptic and anterior hypothalamic areas. Fibers were followed from the ventral stalk of the SFO. Precommissural fibers enter the median preoptic nucleus where many of them appear to terminate. Others continue on to the medial septum, the OVLT, the supraoptic nucleus, and suprachiasmatic nucleus. HRP injections into the median preoptic nucleus labeled many neurons in the SFO. Postcommissural fibers reach the hypothalamus by descending along the walls of the ventricle in the subependymal space, by traveling in the columns of the fornix and the medial corticohypothalamic tract, or by passing through the paraventricular nucleus of the thalamus. Some postcommissural fibers turn rostrally and enter the median preoptic nucleus while others join precommissural fibers bound for the supraoptic nucleus. More caudally directed fibers appear to innervate the paraventricular nucleus of the hypothalamus and the medial preoptic and anterior hypothalamic areas. HRP injections into the paraventricular nucleus of the hypothalamus labeled neurons in the SFO. These findings corroborate and extend previous work in describing neural connections between two brain regions that are important for fluid balance.

Subfornical organ-median preoptic connections and drinking and pressor responses to angiotensin II.

The subfornical organ (SFO) and the periventricular tissue of the anteroventral third ventricle (AV3V) have been shown to be important for the central action of circulating angiotensin. Recent anatomical findings have elucidated neural connections between the SFO and several structures within the AV3V region. The present study examined the function of fibers between the SFO and the median preoptic nucleus. Horizontal knife cuts rostral to the anterior commissure severed precommissural fibers between the SFO and the median preoptic nucleus. Cuts immediately dorsal to the anterior commissure interrupted both pre- and postcommissural connections. Rats with cuts of both sets of fibers evidenced a virtual absence of drinking responses following subcutaneous injections of angiotensin, while rats with selective cuts of precommissural fibers manifested a partial, but significant, reduction in responding. The two groups of animals showed similar attenuations in drinking responses following subcutaneous injections of hypertonic saline. Rats with cuts of both pre- and postcommissural fibers evidenced an attenuation of drinking responses elicited by centrally administered angiotensin. Pressor responses following intravenous and intraventricular angiotensin injections were not reduced specifically by the experimental knife cuts. These findings are consistent with a model that postulates that angiotensin receptors, and perhaps osmoreceptors, in the SFO send excitatory neural information to the median preoptic nucleus for the mobilization of thirst.

On the separation of functions mediated by the AV3V region.

Knife-cuts were used to separate the disruptive effects on fluid balance that are produced by electrolytic lesions of the anteroventral third ventricle (AV3V) region. It was observed that vertical cuts of the dorsal stalk of the subfornical organ (SFO) produced none of these effects. Horizontal cuts between the SFO and the anterior commissure produced neither of the acute effects of AV3V lesions (adipsia and diuretic weight loss) but they did mimic AV3V lesions in disrupting drinking responses to peripherally injected angiotensin and hypertonic saline. In contrast, horizontal cuts between the anterior commissure and the organum vasculosum of the lamina terminalis (OVLT) did not reduce drinking responses to angiotensin but they did cause a large weight loss during the 24 hours following surgery. It is suggested that these ventral cuts severed neural connections between the medial septum and the ventral medial preoptic area in producing the large weight loss. Together with findings from other experiments, these findings support the hypothesis that distinct neural elements mediate the various functions that are disrupted by lesions of the AV3V region.

Localization of the anterior hypothalamic angiotensin II pressor system.

Previous studies by this laboratory have shown that an electrolytic lesion of tissue surrounding the anteroventral third cerebral ventricle (AV3V) produces pressor deficits to both intravenously (i.v.) and intracerebroventricularly (i.c.v.) administered angiotensin II (AII). These studies were designed to identify the neural substrates critical to the central AII pressor response. The AII pressor system was mapped employing a spectrum of overlapping electrolytic lesions within the medial preoptic-anterior hypothalamic area. The effect of each lesion on the pressor response to AII (i.c.v.) was tested in each animal, which was then grouped as a responder (R) or nonresponder (NR). The extent of damage produced by lesions that abolished the AII response was mapped. Bilateral destruction of tissue along the lamina terminalis (LT) either below or at the level of the anterior commissure eliminated the AII pressor response as did destruction of tissue near the margin of the preoptic and anterior hypothalamic nuclei. These data suggested that an AII pressor pathway originating in the ventral AV3V region ascends along the LT to the level of the anterior commissure and then descends through the anterior hypothalamus. The path of the descending projection through the anterior hypothalamus was ascertained by making a series of horizontal knife cuts. Transections were found that effectively eliminated the central AII pressor response without impinging upon the LT. It is concluded that the anterior hypothalamus contains an efferent pathway from the AV3V region associated with the central AII pressor response.