Behavioral taste responses of developmentally NaCl-restricted rats to various concentrations of NaCl.

The behavioral taste responses of developmentally NaCl-restricted rats were examined with a brief-exposure taste test. Neurophysiological and morphological alterations have been reported in rats whose dietary sodium is restricted during pre- and postnatal development, yet there exists little data discerning their behavioral response to tastants. Control and developmentally NaCl-restricted rats were maintained on a low-NaCl diet and trained to lick from individually presented sipper tubes. Each subject received 4 days of testing on various NaCl concentrations. Results indicate that developmentally NaCl-restricted rats have dramatically increased lick rates to NaCl solutions. These responses are likely due to some combination of factors including (a) numbers and type of active chorda tympani fibers, (b) compensatory responses to NaCl-solutions from other nerves of the oral cavity, and (c) increased sensitivity of central taste systems to NaCl.

c-fos-like immunoreactivity in the subfornical organ and nucleus of the solitary tract following salt intake by sodium-depleted rats.

Acute sodium depletion by furosemide induces a robust salt appetite in the rat which is satiated rapidly by ingestion of sodium chloride (salt) solutions. To identify neuronal populations activated by sodium depletion and by salt intake, we quantified c-fos-like immunoreactivity (c-FLI) in the subfornical organ (SFO) and nucleus of the solitary tract (NTS) after sodium depletion and at time intervals from 30 min to 12 h after 1 h of access to 0.3 M NaCl. Rats drank 10+/-1.6 mL over 1 h, with most of the intake occurring by 30 min. Increased numbers of c-FLI-positive cells were observed in the SFO 24 h after sodium depletion; c-FLI remained elevated for 90 min after 0.3 M NaCl intake and then declined until the number of c-FLI-positive cells at 12 h was not significantly different from mock-depleted levels. Sodium depletion alone did not significantly elevate c-FLI in the NTS, but the number of c-FLI-positive nuclei in the NTS was significantly increased after 0.3 M NaCl intake. The cellular location and temporal pattern of c-FLI expression are consistent with activation of neural circuitry sensitive to humoral, gustatory, and postingestive stimuli accompanying sodium depletion and 0.3 M NaCl ingestion. c-FLI in the SFO and NTS may serve as quantifiable markers in the central nervous system of the state of sodium depletion and of ingestive (orosensory and gastrointestinal) sensory stimulation, respectively.

TongueTwister: an integrated program for analyzing lickometer data.

The analysis of lickometer data is often rendered prohibitively tedious by the large volume of data generated by the typical experiment. TongueTwister is an integrated program for the rapid and automatic analysis, presentation, and summary of long- and medium-access data collected by lickometers or of brief-access data collected by multi-bottle lickometers such as the DiLog Instruments MS80. The program was written in C+2 for Macintosh computers, and analyzes data collected by MS-DOS PCs. It takes advantage of the Macintosh user interface to provide quick and convenient output from all the files of a single experimental session, and to export the data to third-party statistical software or other documents. It can batch-process data files by automatically opening and analyzing all the files in a directory; thus, the user can employ directories as a simple database for organizing experimental groups. When a lickometer data file is opened, a textual summary, a raster plot of the lick pattern, the cumulative licks, the lick rate, a histogram of inter-lick intervals, and a breakdown of the session by fractions are automatically calculated and displayed. When an MS80 brief-access file is opened, the lick pattern for each tube presentation and a textual summary of the mean values derived for each tube are automatically displayed. If a directory of files is opened, the mean values derived across all the individual files are calculated and graphed. Analysis parameters can be tailored to the investigator's liking. Tables or graphs can be saved to disk, or copied and pasted into other Macintosh programs for additional analysis. The program may also be used for general-purpose analysis of periodic event records.

Sodium appetite in the sham-drinking rat after chorda tympani nerve transection.

Sodium depletion in the neurologically intact rat (Intact) produces a prompt and robust intake of NaCl. After chorda tympani nerve transection (CTX), there is a longer latency and a reduced intake of NaCl. The CTX rat depends on remaining gustatory and postingestive information to direct NaCl intake. In the present study, the effect of the removal of the postingestive signals of ingested NaCl (by means of a chronic gastric fistula) on the NaCl intakes and licking patterns of Intact and CTX rats was studied. When the gastric fistula was open (Sham), ingested NaCl did not pass beyond the stomach, thus negative postingestive stimulation was absent. After overnight sodium depletion, when postingestive stimulation was present (i.e., gastric fistula closed; Real), the CTX group drank significantly less 0.3 M NaCl than the Intact group over the 2-h test [11.7 +/- 1.6 (CTX) vs. 15.3 +/- 2.8 (Intact) ml]. In contrast, when postingestive signals were absent (i.e., Sham) the Intact group ingested 52.5 +/- 4.4 ml, whereas the CTX group had ingested only 12.4 +/- 3.1 ml of 0.3 M NaCl. Lickometer data analysis revealed that even during the first minute of the test the CTX/Real group generated significantly fewer licks than any of the other groups. Thus, although the CTX group was sensitive to inhibitory postingestive signals in the early portion of the appetite test, the absence of these signals did not release the robust and sustained intake of NaCl characteristic of the Intact group. These results suggest that information provided by the chorda tympani nerve is critically important to the strong motivational properties of NaCl after sodium depletion.

Cyclic AMP tastes aversive, not sweet, to rats.

Electrophysiological and biochemical evidence suggests that cAMP mediates sweet taste transduction. Neural recordings from anesthetized rats and in vitro preparations demonstrate that membrane-per-meable cAMP analogues mimic the effects of sucrose and artificial sweetners. We presented solutions of sodium 8-(4-chlorophenylthio)-adenosine 3'-5'-cyclic monophosphate (8cpt-cAMP), a water-soluble, membrane-permeable cAMP analogue to freely behaving rats in short-term lickometer tests. Rats licked significantly less to 8cpt-cAMP than to sucrose or palatable saccharin solutions. Rats could taste 8cpt-cAMP solutions, however, because they licked less to 8cpt-cAMP in mixture with sucrose than to sucrose alone. Because 8cpt-cAMP decreased licking when mixed with sucrose, we conclude that the taste of 8cpt-cAMP is aversive, not sweet, to freely behaving rats.

Differential changes in central monoaminergic metabolism during first and multiple sodium depletions in rats.

To investigate the role of central monoamines in the behavior of sodium appetite, serotonergic and dopaminergic metabolism in regions of the forebrain and dorsal hindbrain were measured in sodium-depleted rats. Male Sprague-Dawley rats were sodium depleted by injection of the diuretic-natriuretic drug, Lasix (furosemide, 10 mg), and maintained on a sodium-deficient diet overnight. Rats were tested at a first sodium depletion or after multiple sodium depletions. There were three test conditions: 1) mock depletion consisted of vehicle injection and overnight sodium-deficient diet; 2) sodium depletion alone consisted of Lasix injection and overnight sodium-deficient diet; and 3) intake of NaCl following sodium depletion consisted of 9-min access to 0.3 M NaCl after Lasix injection and overnight sodium-deficient diet. Dopamine (DA), DOPAC, HVA, 5-HT, and 5-HIAA were measured by HPLC in the olfactory tubercle, amygdala-pyriform lobe, n. accumbens, hypothalamus, caudate nucleus, and the dorsal hindbrain. Ingestion of 0.3 M NaCl for 9 min after the first sodium depletion increased 5-HIAA/5-HT ratio significantly in the n. accumbens and amygdala, and increased DOPAC/DA ratio in the n. accumbens and olfactory tubercle compared to the mock treatment. In contrast to the results after the first sodium depletion, rats that ingested 0.3 M NaCl after four or more sodium depletions had no changes in 5-HT and DA metabolism in the forebrain, but they did have a significantly larger 5-HIAA/5-HT ratio in the dorsal hindbrain than mock-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic vagotomy does not disrupt the normal satiation of NaCl appetite.

The necessity of the hepatic branch of the vagus nerve for the normal satiation of NaCl intake under the condition of sodium depletion was tested. Sham- or hepatic-vagotomized male, Long-Evans rats were sodium depleted by injection with Lasix (furosemide, 10 mg) and were maintained overnight on sodium-deficient diet and water. The intake of 0.3 M NaCl during a 2-h salt appetite test was not significantly different between the sham- and hepatic-vagotomized groups. In a second group of sham- or hepatic-vagotomized rats, a load of 7.5 ml of 0.5 M NaCl was given by gavage at 15 or 90 min prior to a 1-h NaCl appetite test. The preload decreased NaCl intake equivalently in both groups. Gastric emptying of a preload of NaCl at 15 and 90 min was also the same for sham- and hepatic-vagotomized rats. Thus, the hepatic branch of the vagus nerve is not necessary for the normal short-term satiation of NaCl intake under the condition of acute sodium depletion produced by furosemide.

Transient and lasting effects of reproductive episodes on NaCl intake of the female rat.

Following recovery from experimental sodium depletion, both the need-free and the future need-induced NaCl intakes of the rat are increased. The present experiments asked whether a naturally occurring episode of sodium need, pregnancy and lactation, would also enhance NaCl intake. The daily 0.3 M NaCl intake of Long-Evans rats increased from 18.2 ml (prior to pregnancy) to 28.7 ml during pregnancy, and increased further to 36.2 ml during lactation. The daily intakes remained increased after the weaning of the second litter, at 28.5 ml/24h for females which had access to 0.3 M NaCl during the reproductive episodes (DAM + Na) and at 27.6 ml/24h for those which did not (DAM). When they were subsequently sodium depleted, the need-induced NaCl intake of the DAM, but not the DAM + Na group, was significantly increased compared to virgin female rats. The sodium depletion-induced water intakes of both groups were significantly increased. Neither the need-free nor the need-induced intakes of their offspring (as adults) were increased. In summary, pregnancy and/or lactation, in the absence of a sodium deficit, produced increased intakes of NaCl which persisted beyond the reproductive episodes.

Prior episodes of sodium depletion increase the need-free sodium intake of the rat.

Prior episodes of sodium depletion increase the daily 3% NaCl intake of rats. They ingest large volumes and continue to do so for as long as 3 months after recovery from sodium deficit while eating sodium-rich food and while plasma sodium concentration and renal function are normal. The increased daily intake of sodium is, therefore, need-free. There is a marked sex difference in the need-free intake of 3% NaCl. Female rats drink more salt than do male rats when they are sodium replete and depletion naive. Repeated depletions raise the need-free intakes of both sexes but the effect is greater in females. Plasma concentrations of angiotensin II and aldosterone, which are markedly elevated by each episode of sodium depletion, return to basal levels between and after depletions, and are not the cause of the chronically increased need-free salt intake of the multi-depleted rat. These results suggest that the persistent increase in daily 3% NaCl intake that occurs in the rat with a history of repeated sodium depletions is a permanent, nonpathological increase in avidity for the taste of salty substances that results in life-long overconsumption of salt.

The effect of cooling on the perception of carbohydrate and intensive sweeteners.

The effect of cooling on the perceived intensity of sweetness was measured for four different compounds: glucose, fructose, aspartame and saccharin. Perceived sweetness was rated in a sip and spit paradigm when the taste solution, the tongue, or both were either held at 36 degrees C or cooled to 20 degrees C. It was discovered that for glucose and fructose perceived sweetness was significantly reduced by cooling, and that cooling the tongue reduced sweetness more than did cooling the solution. The perceived sweetness of aspartame was also lessened by cooling, but in that case the temperature of the tongue and the temperature of the solution had similar effects on sweetness. In contrast to the other compounds, the sweetness of saccharin was not affected by temperature. These results (when combined with data previously reported for sucrose) raise the possibility that as a group, carbohydrate sweeteners are affected similarly by temperature, whereas thermal effects on intensive sweeteners may be less predictable. The data thereby provide indirect support for the hypothesis that the sweet taste is mediated by more than one type of gustatory receptor.

Salt appetite is enhanced by one prior episode of sodium depletion in the rat.

A single sodium depletion enhances the salt appetite that is expressed after a second and subsequent sodium depletions. The enhanced salt intake, as measured by a decrease in latency to drink and an increase in volume of 3% NaCl ingested, is not accounted for by an increased sodium loss. The enhanced salt intake occurs even when the interval between first and second depletion is as long as 4 months. The enhanced salt appetite does not depend on the drinking of salt after the animal's first sodium depletion and is specific for NaCl but not for KCl. Moreover, it can be produced without sodium depletion by the actions of the hormones aldosterone and angiotensin on the brain. These results suggest that angiotensin and aldosterone, which are released in response to sodium depletion, (a) increase renal sodium conservation, (b) evoke a salt appetite to restore the lost sodium, and (c) produce enduring changes in the brain that prepare it for more rapid and more vigorous expression of salt appetite in response to future sodium depletions. Thus the neural mechanisms that govern salt appetite are not only activated by the hormones of sodium conservation but appear also to be organized by them for a lifelong increase in avidity for salty substances.

Sodium appetite and cerebrospinal fluid sodium concentration during hypovolemia.

A lowered concentration of sodium in the cerebrospinal fluid (CSF) has been suggested as a critical signal for the elicitation of sodium appetite. This was evaluated in naive male Long-Evans rats made hypovolemic by subcutaneous injection of the colloid polyethylene glycol (PEG) in a hyperoncotic concentration. At 2, 5 or 8 h after PEG or vehicle injection, under conditions of availability or non-availability of water and 0.3 M NaCl for ingestion, the rats were anesthetized and samples of CSF and blood were taken. Water intakes of the PEG treated rats were elevated at 5 and 8 h, and 0.3 M NaCl intakes were elevated at 8 h in comparison to vehicle treated rats. When fluids were not available, CSF sodium concentrations were greater in PEG than in vehicle treated rats at 2, 5 and 8 h. When fluids were available, CSF sodium concentrations were greater in PEG than in vehicle treated rats at 5 and at 8 h. There was no correlation between CSF sodium concentration and saline intake. In summary, a lowered CSF sodium concentration is not a necessary condition for sodium appetite in the hypovolemic rat.

A technique for repeated sampling of CSF from the anesthetized rat.

A method for repeatedly sampling cerebrospinal fluid (CSF) from anesthetized rats is described. The technique reliably and quickly yields blood-free samples of CSF and requires supplies that are commonly available. Samples as large as 250 microliter can be collected in a few minutes. There is no apparent malaise even when CSF is withdrawn once every three days for two weeks. This technique offers an alternative to surgical cannulation for sampling of CSF from the rat.

Immunoreactive insulin levels are elevated in the cerebrospinal fluid of genetically obese Zucker rats.

Immunoreactive insulin (IRI) concentrations were measured in plasma and cerebrospinal fluid (CSF) of four-month old genetically obese Zucker rats, their heterozygote lean littermates, and age-matched normal-weight Wistar rats. Basal plasma IRI was 201 + 35 microU/ml (means +/- SEM) in the obese animals and was significantly elevated compared to both lean Zucker rats (18 +/- 2.4 microU/ml, P less than 0.001) and Wistar rats (12 +/- 2.4 microU/ml, P less than 0.001). The mean CSF IRI concentration of fasted obese Zucker rats was 1.59 +/- 0.19 microU/ml; this was significantly higher than the CSF IRI level of either fasted Zucker lean rats (0.31 +/- 0.08 microU/ml, P less than 0.001) or Wistar rats (0.34 +/- 0.12 microU/ml, P less than 0.001). Plasma and CSF IRI concentrations were increased in free-feeding as compared with fasted animals. These data provide evidence that endogenous CSF insulin is derived from circulating plasma insulin in the rat and suggest that the hyperphagia and obesity of the Zucker fatty rat are not due to an inability of circulating insulin to gain access to the CSF.