2015 Distinguished career award: Reflections on a career in science.

I was very pleased to receive the 2015 Distinguished Career Award from SSIB. This brief manuscript contains reminisces that might stir up pleasant memories in the older members of SSIB and also some general thoughts that I hope will be of value to the younger investigators who are closer to the beginning of their scientific careers. Although the organization has chosen to honor me with this special award, my own career was shaped by a great many people who have influenced my scientific career and I want to acknowledge them. They include Neal Miller, my doctoral mentor at Yale; Joe Holmes and Alan Epstein, my postdoctoral mentors; George Wolf and Reed Hainsworth, graduate student colleagues; John Brobeck, Paul Rozin, and Phil Teitelbaum, Michael Zigmond, Joe Verbalis, Jim Smith, and Alan Sved, faculty colleagues; Derek Denton, Paul McHugh, and James Fitzsimons, scientific role models; John Bruno, Steve Fluharty, and Linda Rinaman, post-doctoral trainees at Pitt; and Lori Flanagan, Kath Curtis, Michael Bushey, Mike Bykowski, Reza Manesh, Carrie Smith, Jennifer Vaughan, and Myriam Stricker, student trainees at Pitt. I thank them all and also my colleagues in SSIB not only for the honor of this award but for providing an abundant supply of insights and discoveries that have stimulated me throughout my adult life, in addition to being an attentive community in supporting my own work.

Variable effects of parabrachial nucleus lesions on salt appetite in rats depending upon experimental paradigm and saline concentration.

Previous studies have demonstrated that bilateral lesions of the gustatory (medial) zone of the parabrachial nucleus (PBN) in the pons eliminate the salt (sodium chloride; NaCl) appetite induced in rats by treatment with the diuretic drug, furosemide. The present studies reexamined NaCl intake of rats with PBN lesions induced by ibotenic acid, using multiple models of salt appetite. The impairment of a conditioned taste aversion, an established consequence of PBN damage, was used as an initial screen with which to assess the effectiveness of the lesions. Rats with PBN lesions did not drink either 0.3 of a molar (M) solution of NaCl or 0.5 M NaCl in response to daily treatment with desoxycorticosterone acetate. These findings suggest that the excitatory stimulus of salt appetite mediated by mineralocorticoids is abolished by PBN lesions. In contrast, rats with PBN lesions drank some 0.5 M NaCl and more 0.3 M NaCl, in addition to water, in response to hypovolemia induced by subcutaneous injection of 30% polyethylene glycol solution. Those findings suggest that an excitatory stimulus of salt appetite, presumably mediated by Angiotensin II, is not abolished by PBN lesions. These and other observations indicate that lesions of the gustatory PBN in rats may or may not eliminate salt appetite, depending on which model is used and which concentration of NaCl solution is available.

Neurochemical and behavioral analyses of the lateral hypothalamic syndrome: a look back.

Philip Teitelbaum is one of the great physiological psychologists of his generation. His early research clarified key issues regarding the effects of electrolytic lesions of the ventromedial or ventrolateral hypothalamus on food intake in rats, a subject of paramount interest during the 1950s and 1960s. Perhaps best known were his extensive studies of the lateral hypothalamic syndrome in rats, which focused on the complex and changing array of symptoms after experimental brain damage. It soon became clear from later work that his research interests were not in the brain's control of food intake but in the effects of lesions to fragment behavior and thereby allow investigators to view its components. He was the foremost proponent of the use of exquisite behavioral analysis to reveal details in movement that allowed insights into brain function, and that approach - old fashioned physiological psychology made modern and at its finest - has infiltrated the entire field of experimental psychology, including studies of ingestive behavior, even while the new field of behavioral neuroscience emerged. He extended his analytic approach to neurological issues such as autism in humans, a promising arena that fully occupied his attention during the later phases of his career. But his influence on his scientific colleagues went well beyond his careful and powerful thinking; his articles and books have been models of clarity and concision. I write in behalf of a grateful field to salute his many great contributions.

Pre-systemic controls of fluid intake and vasopressin secretion.

In dehydrated rats, an early postingestive signal associated with the concentration of ingested fluid provides a signal that can inhibit neurohypophyseal vasopressin secretion. That inhibitory signal is not generated by the act of swallowing, as it is in dogs and many other animals, but by visceral osmoreceptors that presumably send a vagal signal to the nucleus of the solitary tract and the area postrema in the brain stem. In further contrast to dogs, the act of water ingestion does not provide an early signal inhibiting thirst in rats, but gastric emptying is so rapid that thirst can be satiated relatively quickly. When saline is consumed instead of water, thirst is inhibited by a different signal that results from the volume consumed and apparently is associated with gastrointestinal distension. These and other results emphasize the need to include gastric emptying of ingested fluid in considerations of water and Na(+) balance in rats.

Gastric emptying and intestinal absorption of ingested water and saline by hypovolemic rats.

Recent experiments showed that in a one-bottle test conducted 16h after sc injection of polyethylene glycol (PEG) solution, hypovolemic rats consumed water or 0.30 M NaCl in an initial drinking episode but did not empty the ingested fluid from the stomach or absorb it from the small intestine very rapidly, certainly not as rapidly as when 0.15M NaCl was consumed (Smith et al., Am J Physiol 292: R2089-R2099, 2007). The present experiments examined the patterns of water and 0.30 M NaCl ingestion and the movement of consumed fluid through the gastrointestinal tract when PEG-treated rats were given a two-bottle delayed-access test. We found that both fluids always were consumed in the first drinking episode, that the fluid mixture ingested was equivalent to 0.10-0.15M NaCl, and that gastric emptying rate and net fluid absorption from the small intestine usually were much faster than when PEG-treated rats drank either water or hypertonic saline alone. Thus, ingestion of water and 0.30 M NaCl by hypovolemic rats in the same episode adaptively facilitated the movement into the circulation of a near-isotonic fluid that is ideal for restoring plasma volume deficits.

Regulation of NaCl solution intake and gastric emptying in adrenalectomized rats.

Adrenalectomized (adrex) rats adaptively increase NaCl intake to compensate for the uncontrolled loss of Na(+) in urine due to the absence of aldosterone. After a period of NaCl deprivation, they ingest saline avidly but stop drinking before hyponatremia is repaired. The present experiments determined whether pre-systemic signals inhibit further NaCl intake, and whether gastric emptying of Na(+) is modulated according to the concentration of ingested NaCl solution. After overnight deprivation, adrex rats consumed 0.05 M and 0.15 M NaCl at a maximally fast rate ( approximately 1.7 ml/min) and emptied ingested fluid from the stomach at a slower but maximally fast rate ( approximately 1.1 ml/min). When 0.30 M NaCl was consumed instead, fluid intake still was maximally fast but gastric emptying slowed in proportion to concentration so that the emptying of Na(+) was comparable to that observed when 0.15 M NaCl was ingested ( approximately 0.13 meq/min). When 0.50 M NaCl was consumed, intake slowed proportionately so that Na(+) consumption was comparable to that observed when 0.30 M NaCl was ingested ( approximately 0.5 meq/min). NaCl intake appeared to be inhibited both by the concentration of saline emptied from the stomach and by the volume of ingested fluid in the stomach and small intestine. Gastric emptying also slowed proportionately when 0.50 M NaCl was consumed, as if the rats were regulating the delivery of Na(+) to the small intestine. These results suggest that adrex rats can detect the volume and concentration of ingested NaCl solution presystematically and integrate these two variables, and thereby modulate the rates of Na(+) intake and gastric emptying.

Presystemic signals in the control of thirst, salt appetite, and vasopressin secretion.

Presystemic signals play an important role in the control of ingestive behavior by allowing animals to anticipate imminent physiological changes. The significance of such signals in the control of food intake has been amply demonstrated and is widely appreciated. Our recent experiments have revealed that presystemic signals also provide important early feedback when rats drink water or NaCl solution, before the ingested fluids are absorbed and influence cerebral osmoreceptors or cardiovascular baroreceptors. These early signals clearly affect vasopressin (VP) secretion and thirst. They relate either to the distension of the stomach and proximal small intestine (presumably mediated by local stretch receptors) or to the concentration of fluid that empties from the stomach into the small intestine (presumably mediated by visceral osmo- or Na(+)-receptors). Dehydrated dogs use functionally comparable signals from the oropharynx while drinking in order to inhibit both VP secretion and thirst. However, that system differs in several respects from the system in rats aside from the fact that the presystemic signals in rats are not oropharyngeal: in rodents, (a) separate early signals influence VP secretion and thirst, (b) early signals can provide both stimulation and inhibition of VP secretion and thirst, and (c) the early signals are associated with both the volume and concentration of ingested fluid. These presystemic signals also inhibit the intake of NaCl solution by rats with salt appetite.

Presystemic influences on thirst, salt appetite, and vasopressin secretion in the hypovolemic rat.

The present studies investigated the influence of presystemic signals on the control of thirst, salt appetite, and vasopressin (VP) secretion in rats during nonhypotensive hypovolemia. Rats were injected with 30% polyethylene glycol (PEG) solution, deprived of food and water overnight, and then allowed to drink water, 0.15 M NaCl, or 0.30 M NaCl. The PEG treatment, which produced 30-40% plasma volume deficits, elicited rapid intakes in an initial bout of drinking, but rats consumed much more 0.15 M NaCl than water or 0.30 M NaCl. In considering why drinking stopped sooner when water or concentrated saline was ingested, it seemed relevant that little or no change in systemic plasma Na(+) concentration was observed during the initial bouts and that the partial repair of hypovolemia was comparable, regardless of which fluid was consumed. In rats that drank 0.15 M NaCl, gastric emptying was fastest and the combined volume of ingested fluid in the stomach and small intestine was largest. These and other observations are consistent with the hypothesis that fluid ingestion by hypovolemic rats is inhibited by distension of the stomach and proximal small intestine and that movement of dilute or concentrated fluid into the small intestine provides another presystemic signal that inhibits thirst or salt appetite, respectively. On the other hand, an early effect of water or saline consumption on VP secretion in PEG-treated rats was not observed, in contrast to recent findings in dehydrated rats. Thus the controls of fluid ingestion and VP secretion are similar but not identical during hypovolemia.

Inhibition of NaCl appetite when DOCA-treated rats drink saline.

Marked increases in the consumption of concentrated NaCl solution were elicited in rats by daily injection of the synthetic mineralocorticoid, deoxycorticosterone acetate (DOCA). DOCA-treated rats drank different volumes of NaCl solution depending on its concentration (between 0.15 M and 0.50 M), with less consumed (in milliliters) the more concentrated the fluid was. In consequence, total Na(+) intake (in milliequivalents) was roughly similar in all groups. Gastric emptying of Na(+) also diminished as the concentration of the ingested NaCl solution increased, and the delivery of Na(+) to the small intestine was remarkably similar in all groups. Cumulative volume of ingested fluid in the stomach and small intestine was very closely related to intake (in milliliters) of the concentrated NaCl solutions. Systemic plasma Na(+) levels did not increase until after rats stopped consuming concentrated NaCl solution, although they were elevated at the onset of water ingestion. The situation appeared to be different when 0.15 M NaCl was consumed. This isotonic solution emptied and was absorbed relatively rapidly, and DOCA-treated rats drank larger amounts of it throughout a 1-h test period than when they drank concentrated NaCl solutions. Collectively, these findings suggest that saline consumption by DOCA-treated rats may be inhibited by two presystemic factors, one related to the volume of ingested fluid (i.e., distension of the stomach and small intestine) and one related to its concentration (i.e., elevated osmolality of fluid in the small intestine and/or in adjacent visceral tissue).

Control of thirst and salt appetite in rats: early inhibition of water and NaCl ingestion.

Most previous studies on the controls of thirst and salt appetite in rats have focused on systemic factors. Our recent experiments suggest that presystemic factors also are likely to play an important role. For example, dehydrated rats were found to consume similar volumes in an initial drinking bout when given access either to water or 0.05, 0.10, 0.15, or 0.20 M NaCl solution. Thus, cessation of the bouts evidently was not related to the concentration of fluid consumed but to its volume. It occurred despite the continued presence of known systemic stimuli for thirst (i.e. either increased plasma osmolality or decreased plasma volume), and therefore it resulted from inhibition rather than satiation. This inhibition appeared to derive from signals related to the cumulative volume of ingested fluid in the stomach and small intestine. Similar findings were obtained in studies of NaCl solution intake by NaCl-deprived adrenalectomized rats. These and other observations suggest that gastrointestinal fill generates stimuli that inhibit drinking in rats regardless of whether thirst or salt appetite motivates fluid consumption and regardless of whether rats consume water or NaCl solution.

Water ingestion by rats fed a high-salt diet may be mediated, in part, by visceral osmoreceptors.

After surgical removal of all salivary secretions ("desalivation"), rats increase their consumption of water while eating dry laboratory chow. In the present experiments, desalivated rats drank even more water while they ate "powdered" high-salt food (i.e., <15-mg food particles). The Na+ concentration of systemic plasma in these animals was not elevated during or immediately after the meal, which suggests that cerebral osmoreceptors were not involved in mediating the increased water intake. A presystemic osmoregulatory signal likely stimulated thirst because the Na+ and water contents of the gastric chyme computed to a solution approximately 150 mM NaCl. In contrast, desalivated rats drank much smaller volumes of water while eating "pulverized" high-salt food (i.e., 60-140-mg food particles), and the fluid mixture in the gastric chyme computed to approximately 280 mM NaCl solution. These and other findings suggest that the NaCl ingested in the powdered high-salt diet was dissolved in the gastric fluid and that duodenal osmoreceptors (or Na+-receptors) detected when the concentration of fluid leaving the stomach was elevated after each feeding bout, and promptly stimulated thirst, whereupon rats drank water until the gastric fluid was diluted back to isotonicity. However, when rats ate the pulverized high-salt diet, much of the NaCl ingested may have been embedded in the gastric chyme and therefore was not accessible to visceral osmoreceptors once it emptied from the stomach. Consistent with that hypothesis, fluid intakes were increased considerably when desalivated rats drank 0.10 M NaCl instead of water while eating either powdered or pulverized high-salt food.

Inhibition of thirst when dehydrated rats drink water or saline.

The present experiments sought to identify the physiological signals that inhibit thirst when dehydrated rats drink water or NaCl solution. Rats were deprived of drinking fluid but not food overnight. When allowed to drink again, the dehydrated animals consumed water or saline (0.05 M, 0.10 M, 0.15 M, or 0.20 M NaCl solution) almost continuously for 5-8 min before stopping. The volumes consumed were similar regardless of which fluid they ingested, but blood analyses indicated that increased plasma osmolality and decreased plasma volume, or both, still remained when drinking terminated. These results suggest that the composition of the ingested fluid is less significant than its volume in providing an early signal that inhibits thirst and fluid consumption by dehydrated rats. Analyses of the gastrointestinal tracts revealed that the cumulative volume in the stomach and small intestine correlated highly with the amount consumed regardless of which fluid was ingested. These and other results suggest that the volume of fluid ingested by dehydrated rats is sensed by stretch receptors detecting distension of the stomach and small intestine, which provide an early inhibitory stimulus of thirst.

Inhibition of vasopressin secretion when dehydrated rats drink water.

The present study determined whether vasopressin (VP) secretion is inhibited by an oropharyngeal signal associated with swallowing fluids when dehydrated rats drink water, as it is when dehydrated dogs are used as experimental subjects (Thrasher, TN, Keil LC, and Ramsay DJ. Am J Physiol Regul Integr Comp Physiol 253: R509-R515, 1987). VP levels in systemic plasma (pVP) fell rapidly when rats drank water after overnight water deprivation. Systemic plasma Na+ concentration (pNa) also fell, but that change likely contributed little to the early inhibition of VP secretion. In contrast, consumption of water by dehydrated rats with an open gastric fistula had no effect on pVP, nor did consumption of isotonic saline by dehydrated rats; in neither case was pNa affected by fluid consumption. These findings provide no evidence that the act of drinking inhibits VP secretion in dehydrated rats. Thus some post-gastric effect of the ingested water seems to be responsible for the inhibitory signal. These results are consistent with previous suggestions that an early inhibitory stimulus for VP secretion in rats is provided by post-gastric visceral osmo- or Na+ receptors that sense the composition of the ingested fluid.

Hypovolemia stimulates intraoral intake of water and NaCl solution in intact rats but not in chronic decerebrate rats.

This experiment tested the hypothesis that afferent signals from cardiac baroreceptors to the caudal brain stem are integrated by hindbrain systems to control ingestive behavior in response to plasma volume deficits in rats. A supracollicular transection was made which should not interfere with the neural signal of volume depletion to the hindbrain. Decerebrate (n=5) and control rats (n=7) were given subcutaneous injections of 30% polyethylene glycol (PEG) to induce hypovolemia or of isotonic saline as a control. Four hours after the injection, either water or 0.1 M NaCl was administered through an intraoral cannula, and intakes were measured. Decerebrate rats did not ingest significantly more water or saline after PEG treatment than after the control treatment, whereas control rats ingested both fluids in significantly larger volumes after PEG treatment. In another test using the same animals, heart rate was monitored after intravenous injections of phenylephrine (to raise blood pressure) and nitroprusside (to lower it). Similar reflexive changes in heart rate were observed in control and decerebrate rats, showing that baroreflex function was not impaired by decerebration. These results indicate that baroafferent signals are processed at multiple levels of the neuraxis, with hindbrain systems mediating autonomic cardiovascular reflexes in response to changes in blood pressure, and midbrain or forebrain systems mediating behavioral responses associated with thirst.

Increased water intake by rats maintained on high NaCl diet: analysis of ingestive behavior.

To determine the temporal relation between the ingestion of dry food containing 8% NaCl and the increased daily consumption of water that occurs when rats eat this diet, rats were placed in specially designed cages linked to microprocessors that allowed the continuous monitoring of food and water ingestion. The increase in water intake was found to result from increases both in number and size of individual drinking bouts. Approximately 75% of the water intake was consumed in drinking bouts that occurred less than 5 min after feeding. Indeed, rats rarely consumed 8% NaCl diet without also drinking water in the same ingestive episode, and the volume of water they drank was proportional to the food intake in that episode. These and other observations suggest that ingestion of the high salt diet stimulated thirst rapidly. As such, they are consistent with previous reports that visceral osmoreceptors (or Na(+)-receptors) detect osmolytes passing through the gastrointestinal tract and provide an early stimulus of thirst in rats that precedes large increases in systemic plasma osmolality.

Elevated dietary salt suppresses renin secretion but not thirst evoked by arterial hypotension in rats.

Increased dietary salt intake was used as a nonpharmacological tool to blunt hypotension-induced increases in plasma renin activity (PRA) in order to evaluate the contribution of the renin-angiotensin system (RAS) to hypotension-induced thirst. Rats were maintained on 8% NaCl (high) or 1% NaCl (standard) diet for at least 2 wk, and then arterial hypotension was produced by administration of the arteriolar vasodilator diazoxide. Despite marked reductions in PRA, rats maintained on the high-salt diet drank similar amounts of water, displayed similar latencies to drink, and had similar degrees of hypotension compared with rats maintained on the standard diet. Furthermore, blockade of ANG II production by an intravenous infusion of the angiotensin-converting enzyme inhibitor captopril attenuated the hypotension-induced water intake similarly in rats fed standard and high-salt diet. Additional experiments showed that increases in dietary salt did not alter thirst stimulated by the acetylcholine agonist carbachol administered into the lateral ventricle; however, increases in dietary salt did enhance thirst evoked by central ANG II. Collectively, the present findings suggest that hypotension-evoked thirst in rats fed a high-salt diet is dependent on the peripheral RAS despite marked reductions in PRA.

Early osmoregulatory signals in the control of water intake and neurohypophyseal hormone secretion.

Dehydrated dogs inhibit secretion of vasopressin (VP) within minutes after drinking water, before plasma osmolality (pOsm) diminishes. In recent studies, we found that water ingestion by rats similarly inhibits VP and oxytocin (OT) secretion rapidly, before pOsm is diluted. Adult male rats were infused with 1 M NaCl (2 ml/h iv) for 240 min to stimulate VP and OT secretion. After 220 min of infusion, rats were given water or isotonic saline (IS) to drink for 5 min, and blood samples were taken 5 and 15 min later. Plasma levels of VP (pVP) and OT (pOT) were much lower when rats ingested water instead of IS, even though rats drank comparable amounts of both fluids ( approximately 5.5 ml) and pOsm was not significantly affected in either case. In another study, rats were infused with 1 M NaCl (2 ml/h iv) for 120 min before receiving 4-ml gastric loads of either 0.5 M NaCl (HS) or IS; blood samples taken 25 min later showed that pVP and pOT were much higher when rats were given gastric loads of HS instead of IS, even though pOsm was not significantly altered. Comparable results were obtained when gastric loads of HS or IS were given to rats that had been deprived of drinking water overnight. Other dehydrated rats treated similarly but given access to drinking water consumed much more when they had been given gastric loads of HS instead of IS. Collectively, these and other findings suggest the importance of early signals, perhaps from hepatoportal osmoreceptors or Na(+) receptors, in the control of VP and OT secretion and water intake in rats.

Early osmoregulatory stimulation of neurohypophyseal hormone secretion and thirst after gastric NaCl loads.

Cerebral osmoreceptors mediate thirst and neurohypophyseal secretion stimulated by increases in the effective osmolality of plasma (P(osmol)). The present experiments determined whether an intragastric load of hypertonic saline (ig HS; 0.5 M NaCl, 4 ml) would potentiate these responses before induced increases in P(osmol) in the general circulation could be detected by cerebral osmoreceptors. Adult rats deprived of water overnight and then given intragastric HS consumed much more water in 15-30 min than rats given either pretreatment alone, even though systemic P(osmol) had not yet increased significantly because of the gastric load. In other rats pretreated with an intravenous infusion of 1 M NaCl (2 ml/h for 2 h), plasma levels of vasopressin and oxytocin were considerably elevated 15 and 25 min after intragastric HS treatment, whereas systemic P(osmol) was not increased further. These and other findings are consistent with previous reports that hepatic portal osmoreceptors (or Na(+) receptors) stimulate thirst and neurohypophyseal hormone secretion in euhydrated rats given gastric NaCl loads and indicate that these effects are potentiated when animals are dehydrated.