Licking for taste solutions by potassium-deprived rats: specificity and mechanisms.

There has been little work on the specificity and mechanisms underlying the appetite of potassium (K(+)) deprived rats, and there are conflicting results. To investigate the contribution of oral factors to changes in intake induced by K(+) deficiency, we conducted two experiments using 20-s "brief access" tests. In Experiment 1, K(+)-deprived rats licked less for water than did replete rats. After adjusting for this difference, K(+)-deprived rats exhibited increased licking for 100 mM CaCl(2), 100 mM MgCl(2), and 100 mM FeCl(2) compared with K(+)-replete rats. In Experiment 2, which used larger rats, the K(+)-deprived and replete groups licked equally for water, 500 mM Na.Gluconate, 350 mM KCl, 500 mM KHCO(3), and 1 mM quinine.HCl, but the K(+)-deprived rats licked more for 500 mM KCl, 500 mM CsCl, and 500 mM NaCl than did the replete rats. Licking was unaffected by addition to NaCl of 200 muM amiloride, an epithelial Na(+) channel (ENaC) blocker, or 100 muM ruthenium red, a vanilloid receptor 1 (VR-1) antagonist, or by addition to KCl of 50 muM 4-aminopyridine, a K(+) channel blocker. These findings suggest that K(+)-deprivation produces a non-specific appetite that is guided by oral factors. We found no evidence that this response was mediated by ENaC, VR-1, or K(+) channels in taste receptor cells.

Influence of oral and gastric NaCl preloads on NaCl intake and gastric emptying of sodium-deficient rats.

Evidence is mixed as to whether oral metering contributes to the satiation of NaCl intake. To examine this in detail, we measured NaCl intake of sodium-deficient rats given preloads of NaCl that were sham ingested, normally ingested, or intubated into the stomach. Intake of 500 mM NaCl was reduced by prior ingestion, but not by sham ingestion, of an NaCl preload. NaCl intubation reduced NaCl intake if the test began 15 min, but not 60 min, after the preload. Gastric emptying of NaCl was initially more rapid after intubated than after ingested NaCl. Plasma aldosterone concentrations dropped more rapidly after ingested than after intubated NaCl and also dropped after sham ingestion of NaCl, raising the possibility of a cephalic-phase influence on aldosterone levels. These findings suggest that oral factors do not directly control the amount of NaCl consumed by sodium-deprived rats. Differences between the physiological effects of voluntary ingestion and intubation may be responsible for the results of several early studies purported as evidence for oral metering of sodium consumption.

Whole nerve chorda tympani responses to sweeteners in C57BL/6ByJ and 129P3/J mice.

The C57BL/6ByJ (B6) strain of mice exhibits higher preferences than does the 129P3/J (129) strain for a variety of sweet tasting compounds. We measured gustatory afferent responses of the whole chorda tympani nerve in these two strains using a broad array of sweeteners and other taste stimuli. Neural responses were greater in B6 than in 129 mice to the sugars sucrose and maltose, the polyol D-sorbitol and the non-caloric sweeteners Na saccharin, acesulfame-K, SC-45647 and sucralose. Lower neural response thresholds were also observed in the B6 strain for most of these stimuli. The strains did not differ in their neural responses to amino acids that are thought to taste sweet to mice, with the exception of L-proline, which evoked larger responses in the B6 strain. Aspartame and thaumatin, which taste sweet to humans but are not strongly preferred by B6 or 129 mice, did not evoke neural responses that exceeded threshold in either strain. The strains generally did not differ in their neural responses to NaCl, quinine and HCl. Thus, variation between the B6 and 129 strains in the peripheral gustatory system may contribute to differences in their consumption of many sweeteners.

Calcium deprivation alters gustatory-evoked activity in the rat nucleus of the solitary tract.

Calcium-deprived rats develop a compensatory appetite for substances that contain calcium. To investigate the role of gustatory factors in calcium appetite, we recorded the extracellular activity of single neurons in the nucleus of the solitary tract of calcium-deprived and replete rats. The activity evoked by a broad array of taste stimuli was examined in 51 neurons from replete rats and 47 neurons from calcium-deprived rats. There were no differences between the groups in the responses of all neurons combined. However, neurons with sugar-oriented response profiles gave significantly larger responses to 3, 10, and 100 mM CaCl(2) in the calcium-deprived group than did corresponding cells in the replete group. This difference in taste-evoked responding may underlie an increase in the palatability of CaCl(2) and, in turn, contribute to the expression of calcium appetite.

Soa genotype selectively affects mouse gustatory neural responses to sucrose octaacetate.

In mice, behavioral acceptance of the bitter compound sucrose octaacetate (SOA) depends on allelic variation of a single gene, Soa. The SW.B6-Soa(b)congenic mouse strain has the genetic background of an "SOA taster" SWR/J strain and an Soa-containing donor chromosome fragment from an "SOA nontaster" C57BL/6J strain. Using microsatellite markers polymorphic between the two parental strains, we determined that the donor fragment spans 5-10 cM of distal chromosome 6. The SWR/J mice avoided SOA in two-bottle tests with water and had strong responses to SOA in two gustatory nerves, the chorda tympani (CT) and glossopharyngeal (GL). In contrast, the SW.B6-Soa(b) mice were indifferent to SOA in two-bottle tests and had very weak responses to SOA in both of these nerves. The SWR/J and SW.B6-Soa(b) mice did not differ in responses of either nerve to sucrose, NaCl, HCl, or the bitter-tasting stimuli quinine, denatonium, strychnine, 6-n-propylthiouracil, phenylthiocarbamide, and MgSO(4). Thus the effect of the Soa genotype on SOA avoidance is mediated by peripheral taste responsiveness to SOA, involving taste receptor cells innervated by both the CT and GL nerves.

Rapid induction of sodium appetite modifies taste-evoked activity in the rat nucleus of the solitary tract.

Sodium-deprived rats develop a salt appetite and show changes in gustatory responses to NaCl in the periphery and brain stem; salt-sensitive neurons respond less to hypertonic NaCl than do corresponding cells in replete controls. By administering DOCA and renin, we generated a need-free sodium appetite quickly enough to permit us to monitor the activity of individual neurons in the nucleus of the solitary tract before and after its creation, permitting a more powerful within-subjects design. Subjects received DOCA pretreatment followed by an intracerebroventricular infusion of renin. In animals that were tested behaviorally, this resulted in elevated intake of 0.5 M NaCl. In neural recordings, renin caused decreased responding to hypertonic NaCl across all neurons and in the salt-sensitive neurons that were most responsive to NaCl before infusion. Most sugar-sensitive cells, in contrast, gave increased phasic responses to NaCl. These results confirm that sodium appetite is accompanied by decreased responding to NaCl in salt-sensitive neurons, complemented by increased activity in sugar-sensitive cells, even when created rapidly and independently of need.

Calcium-deprived rats sham-drink CaCl2 and NaCl.

Calcium-deprived rats are often thought to increase their calcium intake as a result of learning, but recent studies indicate that there is also an unlearned component to the appetite. They also ingest large amounts of some non-calcium minerals, including sodium. We examined the contribution of post-ingestive feedback to drinking using calcium-deprived and replete rats that could sham-drink CaCl2 and NaCl. Rats fitted with gastric cannulae in order to allow ingested fluids to drain freely drank 0.3 M NaCl in six 1-h sessions with their cannulae open (sham), followed by two sessions with their cannulae closed. Their intake of 0.03 M CaCl2 was then measured in a similar series of tests (six with cannula open followed by two with it closed). Ingestion of both NaCl and CaCl2 was significantly greater in calcium-deprived than in replete subjects under both open and closed conditions. These differences reached significance within 15 min after the onset of drinking during the first test with NaCl, and within 5 min in subsequent tests. The differences in CaCl2 intake generally reached significance within 5 min, including during the first test. Because there was minimal opportunity for post-ingestive NaCl or CaCl2 to mediate learning, the results provide additional support that the appetite for CaCl2 and NaCl in calcium-deprived rats can be driven solely by orosensory factors.

The taste of sodium.

Sodium is crucial to physiological function. The responsibility for detecting it is assumed by the taste system, which devotes perhaps one quarter of its resources to the task. Sodium is transduced by passage into a subset of receptor cells, whose activity is relayed to the brain through a discrete gustatory channel. Responses in hindbrain, thalamus, and gustatory cortex identify the quality and concentration of sodium on the tongue. Coding of reinforcement may begin with the pons and ventral forebrain, particularly the lateral and medial hypothalamic nuclei. When body stores are sufficient, behavioral preference for sodium is mild, encompassing low concentrations and marked by weak avidity. This languid response disappears during sodium shortages. Avidity increases, and hypertonic concentrations are most preferred. This behavioral change may result from altered responsiveness in sodium-specific neurons that offer the sodium signal access to mechanisms of reinforcement. Thus, the taste system detects and recognizes sodium, and accords it a reward value commensurate with the needs of the animal.

Preference conditioning alters taste responses in the nucleus of the solitary tract of the rat.

Aversive conditioning has an impact on the neural signal for the gustatory conditioned stimulus (CS). Here, we determined whether the code is also affected by preference conditioning. We paired the taste of MgCl2 (CS+) with intragastric nutrients in some rats (MG), and citric acid (CS+) with nutrients in others (CI). A control group (Control) experienced both tastants without nutrients. Preferences (>90%) developed for each CS+. We recorded responses to 16 taste stimuli in the nucleus of the solitary tract. Responsiveness of acid-oriented neurons to MgCl2 in MG rats was lower than in Controls, and its profile was more distinct from those of acidic and bitter stimuli. Total activity to citric acid was unchanged in CI rats. However, its temporal profile showed a decreased phasic component, making citric acid temporally distinct from nonsugars. Therefore, the responses to both CS+ were modified, each in its own manner, to be more distinct from those of aversive stimuli. The effects of preference conditioning, however, were weaker than those of aversive conditioning.

Extinction of a conditioned taste aversion in rats: II. Neural effects in the nucleus of the solitary tract.

The formation of a conditioned taste aversion (CTA) in rats results in neural changes at several levels of the gustatory system. In the nucleus of the solitary tract (NTS), the outstanding feature of the response to a CS is a brief burst of activity that is absent in unconditioned animals. The burst occurs about 1 s after stimulus onset and is seen only in neurons that respond well to sugars and the CS (0.0025 M NaSaccharin). We recorded single neuron activity in response to 12 stimuli from taste cells in the NTS of 8 rats, in which a CTA to NaSaccharin had been created and fully extinguished, and in 8 unconditioned controls. The issue was if the neural effects of the CTA in NTS were reversed with extinction. We recorded the activity of 41 neurons in controls and 55 in CTA-extinguished rats. Responses measured across all neurons were not significantly different in spontaneous activity, breadth of tuning, overall response magnitude to each of the 12 stimuli, relationship among stimuli in taste spaces, or time-course. However, cells in the sugar-sensitive subgroup showed a clear vestige of the conditioning experience. They gave a well-defined burst of activity to the CS, though of reduced amplitude and slightly longer latency than in fully conditioned rats. This burst was no longer associated with the conditioned behavior-which was fully extinguished-though it may be a permanent marker for the once-salient CS that can influence subsequent reacquisition of the aversion.

Extinction of a conditioned taste aversion in rats: I. Behavioral effects.

The literature is divided over whether a conditioned taste aversion (CTA) can be fully extinguished. In Experiment 1, we created a powerful aversion in 54 rats by pairing the taste of 0.0025 M NaSaccharin (CS) with intraperitoneal injections of 127 mg/kg LiCl (US) on 3 occasions. We then offered 23-h deprived rats NaSaccharin for 10 min/day to observe the course of recovery. Extinction occurred in three phases: static, dynamic, and asymptotic. During the static phase (mean = 9.6 days), rats consumed the CS at < 10% of their preconditioned rate. With dynamic recovery (6.0 days), they increased acceptance to > 80% of preconditioning levels. Finally, they achieved asymptote (3.1 days) at 100% acceptance. In Experiment 2, we used 8 additional conditioned rats and 8 unconditioned controls. We followed the same 1-bottle extinction procedure and, again, obtained 100% acceptance. Then we offered both NaSaccharin and water for 8 days at 23 h/day and monitored lick patterns every 6 s to determine taste preferences. The conditioned animals consumed less NaSaccharin than controls on Day 1, and less NaSaccharin as a percentage of total fluid as late as Day 3. For the last 5 days of 2-bottle preference testing, there were no significant differences between the groups with regard to 1. volume of NaSaccharin or water consumed, 2. percentage of total fluid taken as NaSaccharin, 3. consumption of each fluid associated with a meal or taken spontaneously, 4. intake during the light or dark periods, or 5. the characteristics of ingestion, including number of drinking bouts, duration of bouts, number of licks/bout, and rate of licking. Therefore, a robust CTA is subject to complete behavioral extinction.

Activity in rat nucleus tractus solitarius after recovery from sodium deprivation.

Sodium depletion has powerful effects on ingestive behavior. Depleted rats consume NaCl avidly at first, but decrease their intake to normal levels as they restore their sodium balance. However, vestiges of the depletion experience are expressed as a more rapidly induced and robust sodium consumption when the rat is challenged with a second depletion. Thus, the salience of sodium to the rat is modified in a lasting manner by severe deprivation. Sodium depletion also causes changes in the responses of taste cells in the nucleus tractus solitarius (NTS). In the present study, we examined whether gustatory-evoked responses in rat NTS continue to reflect the condition induced by sodium deprivation after sodium balance is restored. Single-unit recordings were made in response to 13 taste stimuli in two groups of rats: an experimental group that underwent 10-16 days of sodium deprivation followed by a 2-week recovery period, and a control group that never experienced deprivation. Experimental animals were tested for daily intake of 0.5 M NaCl before and after deprivation; they demonstrated a clear salt appetite only on the first day of the recovery period. Electrophysiological recordings revealed no significant differences between the two groups in response to any single stimulus. Neurons from each group of rats were categorized into three subtypes: sugar-sensitive, salt-sensitive, and nonsugar cells. A comparison of responses in these three subtypes offered no significant differences across groups. Thus, as rats restore depleted sodium levels following deprivation, the responsiveness of cells in the NTS also returns to a predeprivation state.

Taste responses in the nucleus of the solitary tract in saccharin-preferring and saccharin-averse rats.

A minority of rats consistently reject the taste of sodium saccharin at concentrations that the majority find palatable. We chose rats that selected either water (WP), or 0.03 M NaSaccharin (SP) in two-bottle preference tests and monitored single unit responses to a range of taste qualities in the nucleus of the solitary tract. WP rats gave significantly greater responses to Na/Li salts and QHCl. Their responses to sugars were equal to those from SP rats. Total activity to NaSaccharin did not differ between the two groups, but its distribution across the three identified neuron types did. The response was skewed from one in which sugar (S) and sodium salt (N) participated nearly equally (SP) to one dominated by the activity of N cells and nearly devoid of an S cell contribution (WP rats). Accordingly, the response profile for NaSaccharin was correlated nearly as well with those of the sugars (+ 0.60) as with the Na/Li salts (+ 0.73) in SP rats, but was reshaped in WP rats to be nearly identical with those of the salts (+ 0.85) and unlike sugars (+ 0.30). In their heightened sensitivity to stimuli that humans call salty and bitter, and in their rejection of the complex taste of NaSaccharin, WP rats showed many of the characteristics of human tasters of PTC/PROP.