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Daily intake of palatable sucrose-containing, but not lipid-containing solutions or fat-enriched diets, is enhanced in oxytocin gene deletion mice
Abstracts / Frontiers in Neuroendocrinology 27 (2006) 120–128
121
Gonadotropin-releasing hormone (GnRH-1) neurons constitute the final output pathway of the neuronal network that controls the preovulatory luteinizing hormone (LH) surge and ovulation. A number of experiments suggest that oxytocin (OT) regulates events related to the LH surge. In female rats, OT induces GnRH-1 release from hypothalamic explants taken on the afternoon of proestrus and recent studies have shown that a subpopulation of GnRH-1 neurons in female rats express oxytocin receptors (OTR). However, a function for OT directly on GnRH-1 neurons has not to date been addressed. Primary GnRH-1 neurons maintained in nasal explants provide an alternative model for physiological, pharmacological, and molecular studies designed to evaluate inter and intracellular mechanisms regulating GnRH-1 neurons. In this study, we evaluated the expression of OTR in GnRH-1 neurons in mouse. The expression of OTR was examined by polymerase chain reaction (PCR) on cDNAs generated from individual GnRH-1 neurons obtained from nasal explants at 7 days in vitro (div). PCR results indicate that a subpopulation of GnRH-1 neurons express OTR mRNA, indicating that OT could directly influence GnRH-1 neuronal activity. To address whether OT modulates GnRH-1 cells prenatally, the presence of OT in nasal explants was evaluated. OT mRNA was examined by PCR in explants at 1, 3, 7, and 14 div and double label immuno-cytochemistry performed for OT and GnRH-1 in nasal explants at 7, 14, and 21 div. Neither OT mRNA nor protein was detected in the explants at the stages examined. These results suggest that endogenous OT is not present during prenatal migration of the GnRH-1 neurons, but further experiments in vivo are necessary to fully address this issue. To determine the function of OT, if any, on GnRH-1 neurons, exogenous OT will be applied to GnRH-1 neurons in nasal explants and neuronal activity monitored using calcium imaging.
signal associated with drinking, as is true in dogs and human subjects, because no such inhibition of VP secretion was observed either when dehydrated rats drank 0.15 M NaCl instead of water or when dehydrated rats drank water but the ingested fluid drained from an open gastric fistula. Thus, it appears that the presystemic signal that inhibited VP secretion was postgastric and dependent on the concentration of the ingested fluid that emptied into the small intestine. Other recent work suggested that gastrointestinal fill provides a signal inhibiting fluid consumption when dehydrated rats drank water or NaCl solution [Hoffmann et al., AJP (2006) in press]. In the present experiments, we investigated the effects of water and saline consumption by hypovolemic rats on plasma osmolality, plasma volume, and neurohypophyseal VP secretion, while additionally studying the fate of ingested fluid in the gastrointestinal tract. Rats were injected with a 30% solution of polyethylene glycol (PEG; 5 ml, sc) and deprived of food and water overnight (16 h). They were then allowed to drink either water, 0.15 M NaCl, or 0.30 M NaCl solution, after which they were killed by decapitation and their stomachs, small intestines, and blood were collected for analysis. The PEG treatment, which produced 30–40% plasma volume deficits, elicited comparable intakes (1 ml/min for 5–9 min) regardless of which of the three fluids was consumed. A close relation between fluid intake and gastrointestinal fill was observed in each of the three groups, which resembled that seen when rats drank water or saline in various models of thirst or salt appetite. Those results are consistent with the hypothesis that fluid ingestion by rats is constrained by a rapid inhibitory signal associated with distention of the stomach and small intestine. On the other hand, there was no evidence of a presystemic signal that affected VP secretion. Specifically, plasma levels of VP had the same exponential relation to plasma volume deficit regardless of which fluid was consumed. Therefore, water ingestion by hypovolemic rats did not inhibit secretion of VP before providing a detectable decrease in the osmolality of systemic blood, nor did ingestion of hypertonic saline provide an early stimulation of VP secretion.
doi:10.1016/j.yfrne.2006.03.316
doi:10.1016/j.yfrne.2006.03.317
Water ingestion by hypovolemic rats does not provide an early signal for the inhibition of vasopressin secretion Carrie A. Smith, Edward M. Stricker Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA
Daily intake of palatable sucrose-containing, but not lipidcontaining solutions or fat-enriched diets, is enhanced in oxytocin gene deletion mice J.A. Miedlar, H.M. Cai, R.R. Vollmer, L. Rinaman, J.A. Amico Departments of Pharmaceutical Sciences, Neuroscience, and Medicine, University of Pittsburgh, Pittsburgh, PA 15261, USA
Presence of oxytocin receptor mRNA in GnRH-1 cells C.S. Caligioni, S. Wray Cellular and Developmental Neurobiology Section, NINDS, NIH, USA
Recent studies have shown that water ingestion by dehydrated rats inhibits vasopressin (VP) secretion before a significant decrease in systemic osmolality is observed (Stricker, Hoffmann, AJP 289 (2005) R1238–R1243). That inhibitory effect cannot be attributed to an oropharyngeal
The absence of oxytocin (OT) signaling pathways in OT knockout (OzTKO) mice is associated with markedly
122
Abstracts / Frontiers in Neuroendocrinology 27 (2006) 120–128
enhanced initial and sustained intake of sweet palatable nutritive and non-nutritive solutions compared to intake by WT mice of the same C57BL/6 background strain (Amico et al., 2005; Billings et al., 2006). The present study sought to determine whether OTKO mice also demonstrate enhanced initial and sustained intake of a palatable lipidcontaining solutions. For this purpose, OTKO and WT mice were given a two-bottle choice between 10% Intralipid solution and water available ad libitum for 3–4 days. Mice displayed a preference for Intralipid solution over water that did not differ between genotypes, except for significantly greater Intralipid intake by OTKO mice on the first day of exposure. Preference for Intralipid was present over a range of concentrations (2.5, 5, and 10%) and was similar between genotypes. When cumulative intake of 10% Intralipid or 10% sucrose was measured hourly from 17:00 to 23:00 h, OTKO mice consumed greater amounts of 10% sucrose, but not 10% Intralipid, compared to WT mice. Ingestion of solid fat-enriched diets also was not different between genotypes. Thus, the enhanced initial and sustained intake of palatable sweet liquid by OTKO mice does not extend to enhanced intake of a palatable lipid-containing solution or fat-enriched solid diet. Our cumulative results suggest a special role for OT signaling pathways in controlling intake of sweet-tasting, but not lipid-containing, ingesta. doi:10.1016/j.yfrne.2006.03.318
Adiponectin modulates the excitability of type I neurons in the paraventricular nucleus of the hypothalamus Ted D. Hoyda, Mark W. Fry, Alastair V. Ferguson Department of Physiology, Queen’s University, Kingston, Ont., Canada K7L 3N6 Adiponectin (ADP) is a 30 kDa adipokine secreted exclusively from adipocytes. ADP plays a number of roles in the mammalian body notably reversing insulin resistance, increasing cellular triglyceride clearance and acting as a potent angiogenic anti-atherosclerotic peptide. Recently, evidence suggests that intracerebroventricular ADP increases energy expenditure and thus decreases body weight. Here, we show that both ADP receptor 1 and 2 (AdipoR1/2) mRNA is expressed in the cells of the paraventricular nucleus of the hypothalamus (PVN), an autonomic control center which has been implicated in energy homeostasis. We have also investigated the functional consequences of activation of these receptors using current clamp recordings in rat hypothalamic slice preparations. The PVN consists of two major cell types. Type 1 or magnocellular (MNC) neurons synthesize and secrete either vasopressin or oxytocin into the circulation. ADP (10 nM) depolarized 10/26 MNC neurons (6.44 ± 0.86 mV), while the remainder of neurons tested hyperpolarized ( 6.78 ± 0.95 mV, n = 5) or
had no discernable response (n = 11). In addition, single cell RT-PCR analysis shows that 3/3 identified vasopressin neurons depolarized to ADP while 2/2 identified oxytocin expressing neurons hyperpolarized to ADP. These data are the first to show direct effects of ADP on neuronal populations in the hypothalamus and suggest important roles for this peptide in controlling the excitability of PVN neurons potentially involved in the maintenance of energy homeostasis. doi:10.1016/j.yfrne.2006.03.319
Presystemic stimulation of neurohypophyseal vasopressin secretion while rats eat high-NaCl diet Myriam L. Hoffmann, Edward M. Stricker Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA, USA Recent work has shown that dehydrated rats use presystemic, postgastric signals to inhibit thirst and neurohypophyseal vasopressin (VP) secretion. The inhibition of thirst appears to arise from distension signals related to the volume of ingested fluid in the stomach and small intestine [Hoffmann et al., Am. J. Physiol. 2006 (in press)]. In contrast, the presystemic inhibition of VP secretion appears to be related to the concentration of ingested fluids, not the volume, because consumption of water causes an early inhibition of VP secretion whereas consumption of equal amounts of 0.15 M NaCl solution does not. Furthermore, the ingested water evidently must pass through the stomach and empty into the small intestine for this effect to occur because VP secretion is not inhibited when ingested water drains from an open gastric fistula [Stricker, Hoffmann, Am. J. Physiol. 289 (2005) R1238]. These findings are consistent with the idea of an early modulation of VP secretion that is mediated by visceral osmo- (or Na+) receptors. The current study focused on the hypothesis that presystemic signals can stimulate VP secretion in rats. Previous investigations of relevance used hyperosmotic NaCl loads administered by gavage to stimulate VP secretion. Instead, in our experiment rats self-administered a salt load by eating high-salt diet (8% NaCl) after a period of overnight food deprivation. Because solid foods generally form a dense chyme in the animals’ stomachs, which empties very slowly, little of the ingested NaCl actually empties quickly. Consequently, we increased gastric emptying of the NaCl load by presenting the food in a wet mash formed by combining powdered high-salt diet in a 1:1 ratio with water. The results indicated that plasma VP (pVP) was elevated whether food-deprived rats ate high-salt food pellets or wet mash. When rats consumed dry food, the increase in pVP occurred while plasma sodium (pNa) was rising; thus, it is unclear whether to attribute the stimulation of VP secretion to a signal from visceral osmoreceptors, from cerebral osmoreceptors, or from both. However, when ani-
121
Gonadotropin-releasing hormone (GnRH-1) neurons constitute the final output pathway of the neuronal network that controls the preovulatory luteinizing hormone (LH) surge and ovulation. A number of experiments suggest that oxytocin (OT) regulates events related to the LH surge. In female rats, OT induces GnRH-1 release from hypothalamic explants taken on the afternoon of proestrus and recent studies have shown that a subpopulation of GnRH-1 neurons in female rats express oxytocin receptors (OTR). However, a function for OT directly on GnRH-1 neurons has not to date been addressed. Primary GnRH-1 neurons maintained in nasal explants provide an alternative model for physiological, pharmacological, and molecular studies designed to evaluate inter and intracellular mechanisms regulating GnRH-1 neurons. In this study, we evaluated the expression of OTR in GnRH-1 neurons in mouse. The expression of OTR was examined by polymerase chain reaction (PCR) on cDNAs generated from individual GnRH-1 neurons obtained from nasal explants at 7 days in vitro (div). PCR results indicate that a subpopulation of GnRH-1 neurons express OTR mRNA, indicating that OT could directly influence GnRH-1 neuronal activity. To address whether OT modulates GnRH-1 cells prenatally, the presence of OT in nasal explants was evaluated. OT mRNA was examined by PCR in explants at 1, 3, 7, and 14 div and double label immuno-cytochemistry performed for OT and GnRH-1 in nasal explants at 7, 14, and 21 div. Neither OT mRNA nor protein was detected in the explants at the stages examined. These results suggest that endogenous OT is not present during prenatal migration of the GnRH-1 neurons, but further experiments in vivo are necessary to fully address this issue. To determine the function of OT, if any, on GnRH-1 neurons, exogenous OT will be applied to GnRH-1 neurons in nasal explants and neuronal activity monitored using calcium imaging.
signal associated with drinking, as is true in dogs and human subjects, because no such inhibition of VP secretion was observed either when dehydrated rats drank 0.15 M NaCl instead of water or when dehydrated rats drank water but the ingested fluid drained from an open gastric fistula. Thus, it appears that the presystemic signal that inhibited VP secretion was postgastric and dependent on the concentration of the ingested fluid that emptied into the small intestine. Other recent work suggested that gastrointestinal fill provides a signal inhibiting fluid consumption when dehydrated rats drank water or NaCl solution [Hoffmann et al., AJP (2006) in press]. In the present experiments, we investigated the effects of water and saline consumption by hypovolemic rats on plasma osmolality, plasma volume, and neurohypophyseal VP secretion, while additionally studying the fate of ingested fluid in the gastrointestinal tract. Rats were injected with a 30% solution of polyethylene glycol (PEG; 5 ml, sc) and deprived of food and water overnight (16 h). They were then allowed to drink either water, 0.15 M NaCl, or 0.30 M NaCl solution, after which they were killed by decapitation and their stomachs, small intestines, and blood were collected for analysis. The PEG treatment, which produced 30–40% plasma volume deficits, elicited comparable intakes (1 ml/min for 5–9 min) regardless of which of the three fluids was consumed. A close relation between fluid intake and gastrointestinal fill was observed in each of the three groups, which resembled that seen when rats drank water or saline in various models of thirst or salt appetite. Those results are consistent with the hypothesis that fluid ingestion by rats is constrained by a rapid inhibitory signal associated with distention of the stomach and small intestine. On the other hand, there was no evidence of a presystemic signal that affected VP secretion. Specifically, plasma levels of VP had the same exponential relation to plasma volume deficit regardless of which fluid was consumed. Therefore, water ingestion by hypovolemic rats did not inhibit secretion of VP before providing a detectable decrease in the osmolality of systemic blood, nor did ingestion of hypertonic saline provide an early stimulation of VP secretion.
doi:10.1016/j.yfrne.2006.03.316
doi:10.1016/j.yfrne.2006.03.317
Water ingestion by hypovolemic rats does not provide an early signal for the inhibition of vasopressin secretion Carrie A. Smith, Edward M. Stricker Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA
Daily intake of palatable sucrose-containing, but not lipidcontaining solutions or fat-enriched diets, is enhanced in oxytocin gene deletion mice J.A. Miedlar, H.M. Cai, R.R. Vollmer, L. Rinaman, J.A. Amico Departments of Pharmaceutical Sciences, Neuroscience, and Medicine, University of Pittsburgh, Pittsburgh, PA 15261, USA
Presence of oxytocin receptor mRNA in GnRH-1 cells C.S. Caligioni, S. Wray Cellular and Developmental Neurobiology Section, NINDS, NIH, USA
Recent studies have shown that water ingestion by dehydrated rats inhibits vasopressin (VP) secretion before a significant decrease in systemic osmolality is observed (Stricker, Hoffmann, AJP 289 (2005) R1238–R1243). That inhibitory effect cannot be attributed to an oropharyngeal
The absence of oxytocin (OT) signaling pathways in OT knockout (OzTKO) mice is associated with markedly
122
Abstracts / Frontiers in Neuroendocrinology 27 (2006) 120–128
enhanced initial and sustained intake of sweet palatable nutritive and non-nutritive solutions compared to intake by WT mice of the same C57BL/6 background strain (Amico et al., 2005; Billings et al., 2006). The present study sought to determine whether OTKO mice also demonstrate enhanced initial and sustained intake of a palatable lipidcontaining solutions. For this purpose, OTKO and WT mice were given a two-bottle choice between 10% Intralipid solution and water available ad libitum for 3–4 days. Mice displayed a preference for Intralipid solution over water that did not differ between genotypes, except for significantly greater Intralipid intake by OTKO mice on the first day of exposure. Preference for Intralipid was present over a range of concentrations (2.5, 5, and 10%) and was similar between genotypes. When cumulative intake of 10% Intralipid or 10% sucrose was measured hourly from 17:00 to 23:00 h, OTKO mice consumed greater amounts of 10% sucrose, but not 10% Intralipid, compared to WT mice. Ingestion of solid fat-enriched diets also was not different between genotypes. Thus, the enhanced initial and sustained intake of palatable sweet liquid by OTKO mice does not extend to enhanced intake of a palatable lipid-containing solution or fat-enriched solid diet. Our cumulative results suggest a special role for OT signaling pathways in controlling intake of sweet-tasting, but not lipid-containing, ingesta. doi:10.1016/j.yfrne.2006.03.318
Adiponectin modulates the excitability of type I neurons in the paraventricular nucleus of the hypothalamus Ted D. Hoyda, Mark W. Fry, Alastair V. Ferguson Department of Physiology, Queen’s University, Kingston, Ont., Canada K7L 3N6 Adiponectin (ADP) is a 30 kDa adipokine secreted exclusively from adipocytes. ADP plays a number of roles in the mammalian body notably reversing insulin resistance, increasing cellular triglyceride clearance and acting as a potent angiogenic anti-atherosclerotic peptide. Recently, evidence suggests that intracerebroventricular ADP increases energy expenditure and thus decreases body weight. Here, we show that both ADP receptor 1 and 2 (AdipoR1/2) mRNA is expressed in the cells of the paraventricular nucleus of the hypothalamus (PVN), an autonomic control center which has been implicated in energy homeostasis. We have also investigated the functional consequences of activation of these receptors using current clamp recordings in rat hypothalamic slice preparations. The PVN consists of two major cell types. Type 1 or magnocellular (MNC) neurons synthesize and secrete either vasopressin or oxytocin into the circulation. ADP (10 nM) depolarized 10/26 MNC neurons (6.44 ± 0.86 mV), while the remainder of neurons tested hyperpolarized ( 6.78 ± 0.95 mV, n = 5) or
had no discernable response (n = 11). In addition, single cell RT-PCR analysis shows that 3/3 identified vasopressin neurons depolarized to ADP while 2/2 identified oxytocin expressing neurons hyperpolarized to ADP. These data are the first to show direct effects of ADP on neuronal populations in the hypothalamus and suggest important roles for this peptide in controlling the excitability of PVN neurons potentially involved in the maintenance of energy homeostasis. doi:10.1016/j.yfrne.2006.03.319
Presystemic stimulation of neurohypophyseal vasopressin secretion while rats eat high-NaCl diet Myriam L. Hoffmann, Edward M. Stricker Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA, USA Recent work has shown that dehydrated rats use presystemic, postgastric signals to inhibit thirst and neurohypophyseal vasopressin (VP) secretion. The inhibition of thirst appears to arise from distension signals related to the volume of ingested fluid in the stomach and small intestine [Hoffmann et al., Am. J. Physiol. 2006 (in press)]. In contrast, the presystemic inhibition of VP secretion appears to be related to the concentration of ingested fluids, not the volume, because consumption of water causes an early inhibition of VP secretion whereas consumption of equal amounts of 0.15 M NaCl solution does not. Furthermore, the ingested water evidently must pass through the stomach and empty into the small intestine for this effect to occur because VP secretion is not inhibited when ingested water drains from an open gastric fistula [Stricker, Hoffmann, Am. J. Physiol. 289 (2005) R1238]. These findings are consistent with the idea of an early modulation of VP secretion that is mediated by visceral osmo- (or Na+) receptors. The current study focused on the hypothesis that presystemic signals can stimulate VP secretion in rats. Previous investigations of relevance used hyperosmotic NaCl loads administered by gavage to stimulate VP secretion. Instead, in our experiment rats self-administered a salt load by eating high-salt diet (8% NaCl) after a period of overnight food deprivation. Because solid foods generally form a dense chyme in the animals’ stomachs, which empties very slowly, little of the ingested NaCl actually empties quickly. Consequently, we increased gastric emptying of the NaCl load by presenting the food in a wet mash formed by combining powdered high-salt diet in a 1:1 ratio with water. The results indicated that plasma VP (pVP) was elevated whether food-deprived rats ate high-salt food pellets or wet mash. When rats consumed dry food, the increase in pVP occurred while plasma sodium (pNa) was rising; thus, it is unclear whether to attribute the stimulation of VP secretion to a signal from visceral osmoreceptors, from cerebral osmoreceptors, or from both. However, when ani-