Central neural pathway mediating splanchnic osmosensation

The neural basis of homeostatic and anticipatory thirst

Water intake is one of the most basic physiological responses and is essential to sustain life. The perception of thirst has a critical role in controlling body fluid homeostasis and if neglected or dysregulated can lead to life-threatening pathologies. Clear evidence suggests that the perception of thirst occurs in higher-order centres, such as the anterior cingulate cortex (ACC) and insular cortex (IC), which receive information from midline thalamic relay nuclei. Multiple brain regions, notably circumventricular organs such as the organum vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), monitor changes in blood osmolality, solute load and hormone circulation and are thought to orchestrate appropriate responses to maintain extracellular fluid near ideal set points by engaging the medial thalamic-ACC/IC network. Thirst has long been thought of as a negative homeostatic feedback response to increases in blood solute concentration or decreases in blood volume. However, emerging evidence suggests a clear role for thirst as a feedforward adaptive anticipatory response that precedes physiological challenges. These anticipatory responses are promoted by rises in core body temperature, food intake (prandial) and signals from the circadian clock. Feedforward signals are also important mediators of satiety, inhibiting thirst well before the physiological state is restored by fluid ingestion. In this Review, we discuss the importance of thirst for body fluid balance and outline our current understanding of the neural mechanisms that underlie the various types of homeostatic and anticipatory thirst.

Role of Vasopressin in Rat Models of Salt-Dependent Hypertension

PURPOSE OF REVIEW

Dietary salt intake increases both plasma sodium and osmolality and therefore increases vasopressin (VP) release from the neurohypophysis. Although this effect could increase blood pressure by inducing fluid reabsorption and vasoconstriction, acute activation of arterial baroreceptors inhibits VP neurons via GABA_A receptors to oppose high blood pressure. Here we review recent findings demonstrating that this protective mechanism fails during chronic high salt intake in rats.

RECENT FINDINGS

Two recent studies showed that chronic high sodium intake causes an increase in intracellular chloride concentration in VP neurons. This effect causes GABA_A receptors to become excitatory and leads to the emergence of VP-dependent hypertension. One study showed that the increase in intracellular chloride was provoked by a decrease in the expression of the chloride exporter KCC2 mediated by local secretion of brain-derived neurotrophic factor and activation of TrkB receptors. Prolonged high dietary salt intake can cause pathological plasticity in a central homeostatic circuit that controls VP secretion and thereby contribute to peripheral vasoconstriction and hypertension.

Effects of vagotomy, splanchnic nerve lesion, and fluorocitrate on the transmission of acute hyperosmotic stress signals to the supraoptic nucleus

The response to hyperosmotic stresses in the abdominal cavity is regulated, in part, by vasopressin (VP)-secreting neurons in the supraoptic nucleus (SON). How osmotic stress signals are transmitted to the brain is incompletely understood, and whether the transmission routes for osmotic stress signals differ between acute and chronic stresses is unknown. Here we investigated the role of the vagus, splanchnic nerves, and astrocytes in the SON in transducing acute hyperosmotic-stress signals from the abdominal cavity. We found that acute administration of hyperosmotic saline triggered the activation of neurons as well as astrocytes in the SON and the adjoining ventral glia limitans (SON-VGL). Severing the subdiaphragmatic vagal nerve (SDV) prevented the normal response of cells in the SON to HS treatment and attenuated the release of VP into the bloodstream. Lesioning the splanchnic nerves (SNL) diminished HS-induced release of VP, but to a much lesser extent than SDV. Furthermore, SNL did not significantly affect the up-regulation of Fos in SON neurons or the up-regulation of Fos and GFAP in SON and SON-VGL astrocytes that normally occurred in response to HS and did not affect HS-induced expansion of the SON-VGL. Inhibiting astrocytes with fluorocitrate (FCA) prevented the response of the SON to HS and attenuated the release of VP, similarly to SDV surgery. These results suggest that the vagus is the principle route for the transmission of hyperosmotic signals to the brain and that astrocytes in the SON region are necessary for the activation of SON neurons and the release of VP into the bloodstream.

Neurophysiological characterization of mammalian osmosensitive neurones

In mammals, the osmolality of the extracellular fluid is maintained near a predetermined set-point through a negative feedback regulation of thirst, diuresis, salt appetite and natriuresis. This homeostatic control is believed to be mediated by osmosensory neurones which synaptically regulate the electrical activity of command neurones that mediate each of these osmoregulatory effector responses. Our present understanding of the molecular, cellular and network basis that underlies the central control of osmoregulation is largely derived from studies on primary osmosensory neurones in the organum vasculosum lamina terminalis (OVLT) and effector neurones in the supraoptic nucleus (SON), which release hormones that regulate diuresis and natriuresis. Primary osmosensory neurones in the OVLT exhibit changes in action potential firing rate that vary in proportion with ECF osmolality. This effect results from the intrinsic depolarizing receptor potential which these cells generate via a molecular transduction complex that may comprise various members of the transient receptor potential vanilloid (TRPV) family of cation channel proteins, notably TRPV1 and TRPV4. Osmotically evoked changes in the firing rate of OVLT neurones then regulate the electrical activity of downstream neurones in the SON through graded changes in glutamate release.

Thermoregulatory role of periventricular tissue surrounding the anteroventral third ventricle (AV3V) during acute heat stress in the rat

1. Thermoregulatory effector mechanisms are strongly influenced by hydration status. Dehydration delays the onset of evaporative heat loss and the redistribution of cardiac output in response to elevations in core temperature, yet very little is known about how and where thermal and non-thermal information is integrated. 2. The anteroventral third ventricular (AV3V) region encompasses several distinct neural structures, including the organum vasculosum of the lamina terminalis, the median preoptic nucleus, the preoptic periventricular nucleus and the medial aspects of the medial preoptic nucleus. In addition to its well-documented role in body fluid and cardiovascular homeostasis, recent anatomical and in vitro evidence has indicated the AV3V region may also be pivotal in the integration of thermal and osmotic information. 3. Electrolytic lesions of the AV3V region produce a markedly reduced thermal tolerance in rats. Elevations in mean arterial pressure, heart rate and mesenteric resistance were all attenuated in the AV3V-lesioned animals in response to a heat stress; however, hindquarter resistance was unaffected. Heat-induced salivation was also attenuated, severely reducing the ability of rats to lose heat via evaporation. 4. The AV3V region clearly has a functional role in thermoregulation, as well as cardiovascular and body fluid homeostasis. These data add further support to the hypothesis that thermal and non-thermal information may be integrated within this region.

The vagus nerve and thirst

This paper reviews the experiments, which demonstrate conclusively the involvement of the abdominal vagus nerve in normal expression of most aspects of thirst in rats, by Gerard P. Smith and his colleagues published between 1975 and 1984. The nature of that vagal contribution differs with the type of primary thirst signal. Thus, there is no clear or unitary answer concerning whether the contribution of the vagus nerve is purely sensory, or some general tonic action within the central nervous system. Subsequent studies using cFos mapping of intracellular dehydration in conjunction with vagotomy and/or hepatic manipulations are also reviewed and further illustrate the involvement of abdominal information, both in the initiation as well as the termination of drinking. Many of the questions that were raised by Smith during these pioneering studies remain unaddressed and unanswered.

Vasopressin response to osmotic and hemodynamic stress: neurotransmitter involvement

Osmotic and hemodynamic stress are the two primary regulators of vasopressin (VP) release from the posterior pituitary. The pathways providing information about plasma osmolality and blood pressure or blood volume are distinct and utilize different chemical neurotransmitters. Osmotic regulation of VP release is dependent upon afferents from the lamina terminalis region. Glutamate is an important transmitter in this system and angiotensinergic afferents from this region to the VP neurons modulate responses to osmotic challenges. Hemodynamic information is transmitted to the VP neurons via multisynaptic pathways from the brainstem with the A1 catecholamine neurons of the ventrolateral medulla providing the final link for information about decreases in blood pressure and volume. Several neurotransmitters and neuropeptides are expressed in the A1 neurons including norepinephrine (NE), ATP, neuropeptide Y, and substance P. The impact of co-release of these agents on VP release is reviewed and the potential physiological significance is discussed.

Osmotic regulation of estrogen receptor-β expression in magnocellular vasopressin neurons requires lamina terminalis

Estrogen receptor-β (ER-β) expression in rat magnocellular vasopressin (VP) neurons of the supraoptic and paraventricular nuclei (SON and PVN, respectively) becomes undetectable after 72 h of 2% NaCl consumption. To test the hypothesis that osmosensitive mechanisms that originate in the region of the organum vasculosum lamina terminalis (OVLT) control ER-β expression in the SON and PVN, animals were water deprived after electrolytic lesions were performed on the area anterior to the ventral third ventricle (AV3V). Such lesions prevent osmotic stimulation of VP release. Four weeks after surgery, male rats [lesioned ( n = 16) or sham ( n = 14)] were water deprived for 48 h or allowed water ad libitum. Water deprivation eliminated ER-β-immunoreactivity (-ir) in SON and magnocellular PVN of sham-lesioned animals. Fos-ir was evident in these neurons, and plasma osmolality (Posm) and hematocrit (Ht) were significantly elevated compared with the sham-hydrated rats (Posm, 304 ± 1 vs. 318 ± 2 mosmol/kgH2O; P < 0.001; Ht, 49.6 ± 0.6 vs. 55.0 ± 0.9%; P < 0.001). ER-β expression was comparable in sham-hydrated, AV3V-hydrated, and 6 of 8 AV3V-dehydrated rats despite significant increases in Posm in both groups (AV3V hydrated, 312 ± 2; AV3V dehydrated, 380 ± 10 mosmol/kgH2O; P < 0.001). OVLT was not ablated in the AV3V-dehydrated rats in which ER-β was depleted. Fos-ir was low or undetectable in SON in the AV3V-hydrated animals despite elevated Posm values. In AV3V-dehydrated rats, Fos-ir was significantly less than in sham-dehydrated animals but was significantly increased compared with the sham-hydrated group. This could reflect activation by nonosmotic parameters that do not inhibit ER-β expression. These data support the hypothesis that inhibition of ER-β expression in the SON by osmotic stimulation is mediated by osmoreceptive neurons in the lamina terminalis.

Functional and anatomic relationship between cholinergic neurons in the median preoptic nucleus and the supraoptic cells

The median preoptic nucleus (MePO) has been suggested to be an important area in the brain for the regulation of vasopressin (VP) release under the condition of osmotic stimulation. Fos immunoreactivity (Fos-ir), choline acetyltransferase (ChAT) immunoreactivity and retrograde labeling with fluoro-gold were used in this study to determine whether cholinergic neurons in the MePO can be activated by hypertonic NaCl, and to characterize the specific MePO cells that have anatomic projections to the supraoptic nuclei (SON). The results showed that c-fos expression specifically induced by hypertonic NaCl was found in the ChAT cells of the MePO. A retrograde tracing experiment demonstrated that the MePO neurons projecting to the SON were cholinergic. In addition, hypertonic saline-induced Fos-ir was colocalized with the MePO neurons back labeled with fluoro-gold from the SON. Together, these data provide evidence that the MePO cholinergic neurons are activated by osmotic stimulation, and suggest that cholinergic cells in the MePO are functionally important in the control of the SON neurons under the condition of hypertonic stimulation.

Central osmotic regulation of sympathetic nerve activity

AIM

In this review, we will focus on the central neural mechanisms that couple osmotic perturbations to changes in sympathetic nerve discharge, and the possible impact these actions have in cardiovascular diseases such as arterial hypertension and congestive heart failure.

RESULTS

Changes in extracellular fluid osmolality lead to specific regulatory responses in defence of body fluid and cardiovascular homeostasis. Systemic hyperosmolality is well known to stimulate thirst and the release of antidiuretic hormone. These responses are largely due to osmosensing neurones in the forebrain lamina terminalis and hypothalamus and are critical elements in a control system that operates to restore body fluid osmolality. An equally important, but less characterized, target of central osmoregulatory processes is the sympathetic nervous system.

CONCLUSION

Understanding the neurobiology of sympathetic responses to changes in osmolality has important implications for body fluid and cardiovascular physiology. By stabilizing osmolality, vascular volume is preserved and thereby relatively normal levels of cardiac output and arterial pressure are maintained.

The awareness of thirst: proposed neural correlates

The neural and endocrine bases of the generation of thirst are reviewed. Based on this review, a hierarchical system of neural structures that regulate water conservation and acquisition is proposed. The system includes primary sensory-receptive areas; secondary sensory structures (circumventricular organs), which detect levels of hormones, including angiotensin II and vasopressin, which are involved in generating thirst; preoptic and hypothalamic structures; and an area within the ventrolateral quadrant of the periaqueductal gray matter. Hodological and other data are used to determine the hierarchical organization of the system. Based on studies of the effects of lesions to various structures within the hierarchy of the system, it is proposed that the awareness of thirst in rodents is either entirely or predominantly due to neuronal activities in a subsection of the ventrolateral periaqueductal gray matter. It is also hypothesized that the awareness of thirst in primates is due to neuronal activities in both the ventrolateral periaqueductal gray and in a region within the medial prefrontal and anterior cingulate cortex.

Hepatic denervation does not affect plasma vasopressin response to intragastric hypertonic saline in conscious rats

Peripheral osmoreceptors monitor dietary NaCl and modify central nervous system and renal sympathetic nervous system activity accordingly. Experimental evidence suggests that these responses are dependent on the hepatic nerves. Peripheral osmoreceptors also modify arginine vasopressin (AVP) secretion. However, although hepatic denervation reportedly blunts activation of both supraoptic and paraventricular hypothalamic neurons after intraportal NaCl infusion, the role of the hepatic nerves in the AVP release has not been directly examined. The present study tests the hypothesis that the hepatic nerves modify AVP release in response to intragastric NaCl infusion. Wistar-Kyoto rats (WKY) received either hepatic denervation or a sham operation. Intragastric NaCl infusion significantly elevated plasma AVP in both sham-operated WKY and hepatic-denervated WKY, and the responses were not different between these groups. Second, previous studies suggest that both AVP secretion and baroreflexes are blunted in spontaneously hypertensive rats (SHR), deficits that contribute to the observed hypertension in SHR. We hypothesized that SHR also have a blunted peripheral osmoreceptor reflex and that this contributes to NaCl-sensitive hypertension. In contrast to our prediction, in SHR intragastric NaCl infusion induced an increase in plasma AVP that was similar to that in the WKY groups. Thus, although hepatic osmoreceptors are important for chronic regulation of arterial pressure, renal sympathetic nervous system activity, and the activity of hypothalamic neurons, they do not appear to influence plasma AVP concentration in response to intragastric NaCl.

Expression of Fos immunoreactivity in rat brain during dehydration: effect of duration and timing of water deprivation

Water deprivation induces expression of the immediate early gene c-fos in specific brain regions, most likely as a result of the activation of cells that are responsive to changes in osmolality and/or blood volume. We hypothesized that the magnitude of c-fos expression would be a function of both the duration of water deprivation and the time of day at which the deprivation started. This study was designed to examine the pattern of Fos-like immunoreactivity (FLI) following water deprivation in rats under normal light/dark conditions (nLD) and reverse light/dark conditions (rLD). Rats were deprived of water but not food either for 0, 5, 16, 24 or 48 h. As expected, hematocrit ratio (HCT), osmolality (OSM), plasma renin activity (PRA) and weight loss increased as a function of duration of water deprivation. In non-deprived rats (0 h), very little FLI was observed in most brain regions. The number of cells showing FLI increased with duration of water deprivation in the supraoptic nucleus (SON), paraventricular nucleus (PVN), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO) and subfornical organ (SFO) in both nLD and rLD conditions. However, the pattern of FLI differed between nLD and rLD conditions. Compared to corresponding nLD groups after 5 or 24-h water deprivation, rLD groups had significantly more FLI in SON and PVN, and higher PRA and HCT. Also, weight loss and FLI in the MnPO were greater after 5 h, and FLI in the SFO was greater after 24 h under rLD compared to nLD conditions. Our findings indicate that the magnitude of c-fos expression, and change in weight and plasma parameters were a function of both the duration of water deprivation and the time of day at which the deprivation started. This may result from ingestion of food early in the deprivation periods during the rLD tests, thus producing greater change in osmolality and blood volume.

Effects of SFO lesion or captopril on drinking induced by intragastric hypertonic saline

This study examined the hypothesis that the subfornical organ (SFO), a circumventricular organ with both osmosensitive elements and dipsogenic receptors for circulating angiotensin (ANG) II, is important for the water drinking response that follows an intragastric (ig) load of hypertonic NaCl. A 2-ml saline load was administered ig at 300, 900, or 1200 mOsm/kg to rats with sham lesions or lesions of the SFO, and intake was measured periodically for 2 h. Hypertonic loads caused sham-lesioned rats, but not SFO-lesioned rats, to drink earlier in the test or to drink more water than did the isotonic load. Inhibition of ANG II synthesis in unoperated rats with 100 mg/kg of captopril reduced water intake only during the initial 15 min after a gavage of 1200 mOsm/kg saline. Loads of 900 and 1200 mOsm/kg both increased plasma osmolality and sodium concentration by 15 min after gavage without greatly affecting hematocrit or plasma protein concentration. Thus, the SFO is important for the osmotically-induced water drinking response after acute ig administration of hypertonic saline. With the possible exception of the first 15 min, this drinking response is independent of the peripheral synthesis of ANG II.

Endocrinological abnormalities in angiotensinogen-gene knockout mice: studies of hormonal responses to dietary salt loading

OBJECTIVE

Physiological roles of the renin-angiotensin system in maintaining blood pressure and sodium-water balance in angiotensinogen gene-knockout mice were evaluated with special reference to endogenous pressor substances.

METHODS

Angiotensinogen-gene knockout mice and control mice were fed a 0.3 or 4% NaCl diet for 2 weeks. Systolic blood pressure and urinary excretions of electrolytes, creatinine, aldosterone, adrenaline, noradrenaline, dopamine and vasopressin were measured.

RESULTS

About 60% of our angiotensinogen-gene knockout mice did not survive until weaning. These mice presented with hypotension and polyuria. Urinary excretion of aldosterone from such mice was significantly lower (not detected) than that from control mice (2.0+/-0.3 pg/mg creatinine). In contrast, urinary excretion of vasopressin from angiotensinogen-gene knockout mice (0.7+/-0.1 ng/mg creatinine) was greater than that from control mice (0.3+/-0.1 ng/mg creatinine), and those of adrenaline and of noradrenaline were similar for knockout and control mice. After salt loading (a 4% NaCl diet), angiotensinogen-gene knockout mice exhibited a significant increase in systolic blood pressure (from 68.3+/-2.9 to 95.9+/-5.9 mmHg), significant decreases in urinary excretions of adrenaline (from 65+/-8 to 40+/-7 pg/mg creatinine) and noradrenaline (from 467+/-48 to 281+/-41 pg/mg creatinine) and no change in excretion of vasopressin compared with such mice fed a 0.3% NaCl diet

CONCLUSION

The present results with angiotensinogen-gene knockout mice confirm that the renin-angiotensin system plays fundamental roles in maintaining the blood pressure and sodium-water balance. Because the vasopressin and catecholaminergic systems may be altered by lack of angiotensin in angiotensinogen-gene knockout mice, these systems perhaps are not able to restore blood pressure and sodium-water depletion to normal levels in these mice.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.

Mediation of dehydration-induced peptidergic gene expression in the rat lateral hypothalamic area by forebrain afferent projections

We have previously shown in dehydrated rats that cellular levels of the mRNAs encoding the precursor peptides for corticotropin-releasing hormone and neurotensin/neuromedin N significantly increase in a restricted region of the lateral hypothalamic area (Watts, 1992, Brain Res. 581:208-216). The experiments reported here address the role that forebrain osmosensitive cells groups or regions associated with autonomic regulation play in developing this mRNA response. The first experiment showed that unilateral knife cuts placed between the rostral forebrain and the lateral hypothalamic area (LHA) will unilaterally attenuate the mRNA response in the LHA to dehydration. In a second experiment, small injections of the retrograde tracer Fluorogold into the region of the LHA containing these mRNAs revealed a direct input from the osmosensitive median preoptic nucleus and subfornical organ and from the fusiform nucleus of the bed nuclei of the stria terminalis, which is part of a complex of cell groups associated with autonomic regulation. We found that at least 30% of the neurons in the median preoptic nucleus and subfornical organ and 14% of the neurons in the fusiform nucleus of the bed nuclei of the stria terminalis that project to the LHA responded to a rapid increase in plasma osmolality with increased c-fos mRNA levels. In the final experiment, injections of Fluorogold into the LHA were made simultaneously with ipsilateral rostral knife cuts. Here the numbers of neurons accumulating Fluorogold in the median preoptic nucleus, subfornical organ, and the fusiform nucleus were all significantly decreased concomitantly with attenuated mRNA responses in the LHA to dehydration. We conclude that the LHA receives direct and functional projections from the median preoptic nucleus, subfornical organ, and the fusiform nucleus. These projections appear capable of mediating a substantial part of the response of peptidergic mRNAs in the LHA to dehydration.

Effects of portal infusion of hypotonic- and hypertonic solutions on neuronal activity in the rat dorsal motor nucleus of the vagus

The present experiment was designed to elucidate the characteristics of the response of neurons in the dorsal motor nucleus of the vagus (DMV) to stimulation of the hepatoportal area by hypotonic as well as hypertonic solutions. Responses of 81 neurons that exhibited an antidromic response to electrical stimulation of the ventral gastric vagus were recorded in the left DMV in urethane-chloralose anesthetized rats. The effects on these 81 neurons of portal infusion of hypertonic saline (3.6% NaCl) and of pure water were examined. The discharge rates of 16 neurons increased in response both to portal infusion of hypertonic saline and to that of water. Portal infusion of 0.9% NaCl produced no changes in firing rates. Their discharge rates of seven neurons increased in response to portal infusion of hypertonic saline but not to that of water. The other 58 neurons did not respond to these stimuli. Jugular infusion of water produced no response. Therefore, the responses to portal infusion of water appear to be derived from activation of the hepatoportal receptors. These results indicate that a certain fraction of DMV neurons respond similarly to portal infusions of hypertonic and hypotonic saline. It is possible that there exist some reflex arcs that mediate a similar response to both an increase and a decrease in portal blood osmolarity (or Na+ concentration), namely, a suppression of absorption.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

Abdominal vagal afferents excite A1 area neurons antidromically activated from the region of the supraoptic nucleus in the rabbit

We made extracellular recordings from 113 spontaneously active neurons in the A1 area, after identifying the cells by antidromically activating them from the region of the supraoptic nucleus in urethane-anesthetized rabbits. We tested the response of these neurons to inputs from abdominal vagal, renal and somatic nerves. Electrical stimulation of the abdominal vagus nerve activated 64/85 neurons tested (75%), and had no effect on the remaining 25%. Latency was 195 +/- 25 ms, (conduction velocity 0.7 m/s). Stimulation of renal afferents had no effect on the discharge rate of 4 neurons tested. Stimulation of branches of the sciatic nerve inhibited 7/17 A1 area neurons tested, excited 4 and had no effect on 6 neurons. Stimulation of the central ear nerve inhibited 4/17 neurons tested, excited 6 and had no effect on 7 neurons. Gastric distension had no effect on 20/24 neurons tested. Lightly touching the animals back and legs had no effect on the discharge of 45/49 neurons tested. Similarly, painful stimuli failed to affect 44/49 neurons tested. Our results indicate that A1 area neurons, with projections to the region of the supraoptic nucleus, receive excitatory inputs from the abdominal vagus nerve. The visceral information transmitted to A1 cells by these abdominal vagal afferents is not yet determined, but acute gastric distention does not appear to be a physiological stimulus. A1 area neurons seem not to be involved in transmitting somatic information to the hypothalamus.

Ventral pontine catecholaminergic pathway mediates the vasopressin response to splanchnic osmostimulation in conscious rats

This study examines the role of catecholamines, cell bodies and fibers of passage within the subcoeruleus area (subLC) in the arginine vasopressin (AVP) response to splanchnic osmotic stimulation and hemorrhage. Bilateral chemical lesions were induced into the subLC, approximately 1 mm ventral to the locus coeruleus (LC), using 6-hydroxydopamine (6-OHDA) and ibotenic acid to selectively destroy catecholaminergic components and cell bodies, respectively. Vehicle and 5,7-dihydroxytryptamine (5,7-DHT) injections into the subLC area, 6-OHDA injections into the LC, as well as systemic desipramine pretreatment, were performed as controls for the possible non-specific effects of the lesions. Seven and 8 days later, plasma AVP level, plasma osmolality, mean arterial pressure and heart rate were measured following either gastric infusion of hypertonic (598 mOsm/kg; 2 ml/4 min) or isotonic (290 mOsm/kg) saline or a mild hemorrhage (2.5 ml/300 g) in conscious rats with indwelling tail artery catheters and naso-gastric tubes. 6-OHDA injections into subLC reduced the AVP response to the osmotic stimulation by 62.3% (P less than 0.01), as compared to vehicle-injected controls. These same rats demonstrated a normal AVP response to hemorrhage implying a specificity of the disrupted pathway. All controls confirmed that the effects of the 6-OHDA were due to specific action on noradrenergic components within the subLC area. Ibotenic acid lesions in the subLC did not significantly decrease the AVP response, demonstrating that mainly fibers and not cell bodies in this region are part of the pathway. 6-OHDA injections just anterior to the LC, where the dorsal noradrenergic bundle (DNAB) forms, reduced the AVP secretion due to hemorrhage by 77.0% (P less than 0.05), but had minor effects on the response to osmotic stimulation. These results indicate that catecholaminergic fibers travelling primarily within the subLC, in the ventral noradrenergic bundle (VNAB), carry splanchnic osmotic input to the hypothalamus, whereas the DNAB may mediate the AVP response to hemorrhage.