Operant control of the pattern of licking in rats

Summary In an attempt to slow down the highly regular rate of licking by instrumental conditioning, 10 rats were trained to obtain their daily ration of water in an apparatus equipped with a retractable drinking spout. Termination of a photoelectrically monitored lick started a computer controlled delay during wich the spout was made inaccesible. The subsequent return of the spout was either permanent or limited to a time window (D or W conditions). The cycle was reset by each lick. With stationary spout, the interlick intervals (ILIs) were around 210±16 ms (median and interquartile range). The spout return was gradually delayed during 22 sessions from 140 to 260 ms and limited to a 260–340 ms window during 11 sessions. A session consisted of 512 licks with stationary spout followed by 7×512 licks under D or W conditions. Two rats were able to smoothly adjust to the limited spout availability by increasing median ILI to 290 ms. Four rats generated bimodal ILI distributions indicating gradual reduction of tongue protrusion to an undetected lick followed by abrupt increase of lick amplitude. Three rats increased the median ILI to 260 ms and the interquartile range to 80–150 ms. The above changes of lick pattern allowed the rats to attain tongue-spout contact in 41 to 89% licks. It is concluded that the rats can substantially slow down their lick rate provided that they receive feedback information about the failure or success of each lick.Visiting scientist from ICB Victoria de Girón, University of Havana, CubaVisiting scientist from the Institute of Physiology, Azerbaidhanian Academy of Sciences, Baku, USSR     

Volumetric measure and lick count of liquid food intake of rats

A simple and relatively inexpensive lick counter-volumeter is described which automatically records liquid intake and drinking behavior of rats. It counts lick contacts while accurately measuring the rate of volume intake in constant unit volumes. The unit volumes are adjustable over a wide range. For a unit volume on the order of 60 μl, the precision was 1.2% over a range of withdrawal rates from 1 to 10 ml/min. The average lick volume of rats consuming a liquid food was systematically lower by 18% during the second meal following a 15 hr fast when compared to that of the first. This change in average lick volume appears to reflect a change in the rat's motivation to drink the liquid food.     

Effects of unilateral striatal dopamine depletion on tongue force and rhythm during licking in rats.

To quantitatively assess the orolingual dysfunctions produced by unilateral striatal dopamine depletions, rats first received 6-hydroxydopamine injections into the nigrostriatal bundle and were then trained to lap water from a force-sensing disk in 2-min sessions. Compared with controls and rats with moderate (<75%) dopamine depletions, subjects with substantial (>75%) dopamine depletions showed decreases in number of licks, lick rhythm, and lick peak force. Rats with substantial lesions were also impaired in making initial, within-session adjustments in lick peak force but not in lick rhythm. The results confirm the presence of Parkinson-like deficits in tongue dynamics during consummatory licking behavior in rats. The methods used here should prove useful in providing quantitative measures of the efficacy of experimental therapies in this rodent model of Parkinson's disease.     

Genetic and environmental variability in lick rates of mice

The hypothesis of lick rate invariance was tested by comparing rates of licking among two inbred strains of mice and their F₁ hybrids across two different fluid regimens. DBA mice licked significantly faster than C57 mice, while the lick rate of both reciprocal F₁ groups was intermediate between that of the two parental strains. This ordered relationship held when either water or a morphine solution was the available fluid. C57 mice, but not others, also displayed transient changes in lick rate with changes in fluid regimens. Thus, rates of licking among mice are neither genetically nor environmentally invariant.     

Salt intake by normotensive and spontaneously hypertensive rats: two-bottle and lick rate analyses

Spontaneously hypertensive rats (SHR) overconsume NaCl compared to the normotensive Wistar Kyoto rat (WKY) rat. In the present experiment, two-bottle preference for NaCl (0.01, 0.03, 0.1, 0.3, 0.5, 1.0, 3.0 M) and lick rate analyses were used to identify the possible mechanisms that underlie the intake of NaCl by male SHR. Two-bottle preference and absolute NaCl intake by SHR were greater than that of WKY rats. When NaCl intake was calculated on the basis of body weight, SHR consumed more NaCl per 100 g body weight than did WKY. Also, during the one-bottle test, SHR consumed more 0.1 and 0.3 M NaCl per 100 g body weight than did WKY. The increased intake of NaCl by SHR was most evident for 0.3 M NaCl. Intake is determined by the initial rate of licking and the decline in lick rate over time. Nonlinear regression analysis of lick rate showed that the initial lick rates (licks/min) were similar for male WKY and SHR. Lick rate declined more rapidly when WKY rats drank 0.3 M than when they drank 0.1 M NaCl, a result consistent with the role of negative feedback in controlling the decay in lick rate. This concentration-dependent change in lick rate was not seen in SHR. Also, SHR lick rate for 0.1 and 0.3 M NaCl decelerated more slowly than that of WKY rats. The increased intake of hypertonic NaCl by SHR was due to a decrease in the decline in lick rate, suggesting that SHR are less responsive to ingestion-contingent negative feedback.     

Dissociation of licking and volume intake controls in rats ingesting glucose and maltodextrin

The volume of fluid that rats acquire with each lick was systematically varied across short-term tests with 12.5% glucose (Experiment 1) or 12.5% maltodextrin (Experiment 2). For glucose, rats increased the number of licks emitted as lick volume was reduced such that meal size remained remarkably stable across all (8, 4, and 2 microl) but the smallest (1 microl) lick volume conditions tested. Rats similarly compensated for lick volume reduction (8 to 4 microl) with maltodextrin by approximately doubling the number of licks emitted. Meal duration and a number of lick-microstructural parameters (initial ingestion rate, mean burst duration, terminal lick and ingestion rates, and burst duration) were not correlated with the intake outcome insofar as they varied significantly across conditions over which intake remained stable. Thus, in response to lick volume manipulation, rats demonstrated an impressive degree of behavioral flexibility in what may be regarded as a defense of meal size.     

Licking during forced spout alternation in rats: resetting the pacemaker or disconnecting the motor output?

Summary Rats trained to alternate between two adjacent drinking spouts (each spout is retracted after a single lick and presented again after the animal has licked at the other spout) emit 2.5 licks instead of the optimum 1 lick per spout presentation. The question whether the pacemaker of licking is reset or continues to run during transition between spouts was addressed in three experiments performed in 10 highly overtrained rats. Videorecording analysis of oro-facial activities showed that the rat's mouth was firmly closed during the 500 ms transition between spouts. EEG was recorded from the region of hypoglossal nucleus with implanted bipolar electrodes during continued licking at one spout and during spout alternation. Averaging (n = 256, 1024 ms) centered around the onset of the first lick at the newly contacted spout showed 2–3 lick related potentials in the post-transitional interval whereas only one such wave occasionally appeared at the end of the pre-transition period. Computer plotted distributions of post-transition licks (timed with respect to the last pre-transition lick) were examined while the horizontal or vertical distance between spouts was increased. Growing spout separation changed the height but not the timing of the modes of the post-transition lick distribution. This phase-locked synchronization of pre- and post-transition licks indicates that the central timing network of the lick generator is not reset but continues to run during transition between spouts and that the cessation of tongue movements is due to inhibition of the intracycle pattern generator and motor output. It is concluded that more sensitive recording techniques are required to detect the activity of the central timing network in absence of overt licking.     

Tolerance to amphetamine hypophagia: a microstructural analysis of licking behavior in the rat

The development of tolerance to amphetamine-induced hypophagia was assessed by recording changes in lick parameters in rats given chronic administration of the drug (2 mg/kg) and access to sweetened milk. Although licking and milk intake gradually recovered, the volume of milk ingested per lick remained suppressed. Amphetamine had no effect on the interlick interval or the force per lick. In contrast, the drug caused a sustained increase in the number of lick bursts (defined by pause criteria of 0.5-2.0 s) and a decrease in the number of licks per burst (but only at pause criteria of 0.5 and 1.0 s). These results suggest that tolerant rats frequently interrupt licking, resulting in less efficient capture of milk.     

Evaluation of lateral spinal hemisection as a preclinical model of spinal cord injury pain

Operant escape from nociceptive thermal stimulation of 13 Long-Evans rats was compared before and after lateral spinal hemisection, to determine whether this lesion configuration provides an appropriate preclinical model of the hyperalgesia that can be associated with human spinal cord injury. Escape from 44 °C and from 47 °C stimulation was not affected following sham spinal surgery but was significantly reduced over 20 weeks of postoperative testing following lateral spinal hemisection. This result is opposite to previous reports of enhanced reflex withdrawal in response to thermal stimulation of rats following lateral spinal hemisection. In addition, the latency of reflexive lick/guard responses to 44 °C was increased and the duration of lick/guard responding was decreased in the present study (hyporeflexia). Thus, previous assessments of simple withdrawal reflexes have described a hyperreflexia following lateral spinal hemisection that was not replicated by lick/guard testing, and postoperative escape responding revealed hypoalgesia rather than the increased pain sensitivity expected in a model of chronic pain.     

Reduced palatability in lithium- and activity-based, but not in amphetamine-based, taste aversion learning

Conditioned taste aversions (CTA) based on lithium chloride (Experiment 1), amphetamine (Experiment 2), and wheel running (Experiment 3) were examined using the analysis of the microstructure of licking to measure the palatability of the taste serving as the conditioned stimulus (CS). Pairing saccharin with amphetamine reduced saccharin intake without reducing the size of licking clusters, initial lick rate, or the distribution of inter-lick intervals (ILIs) within a cluster. By contrast, pairing saccharin with lithium or wheel-running reduced saccharin intake as well as lick cluster size, initial lick rate, and the distribution of ILIs within a cluster. As lick cluster size, initial lick rate, and ILI distribution can be used as indices of stimulus palatability, the current results indicate that taste aversions based on either lithium or activity reduced the palatability of the CS. This suggests that aversions based on both lithium and wheel running involve conditioned nausea to the CS taste. The absence of similar changes in licking microstructure with amphetamine-based CTA is consistent with other evidence indicating this does not involve nausea.     

Liquid lost during drinking

Rats fail to transfer into their mouth all liquid removed from the drinking tube when they lick water in a licking environment which permits only the anterior portion of the tongue to contact the drinking tube. To quantitatively measure the liquid splattered during 10 sec licking trials in this high restriction licking environment, the red-dyed water observed on the surfaces of the environment was collected with 1 μl pipettes. Data were corrected for evaporation occurring before collection. Median volume lost per lick was 0.07 μl, corresponding to 1.2% of the median initial volume removed by a lick, 5.88 μl. Such loss can probably be considered negligible. Consequently, estimates of lick-volume parameters which are based upon amount of liquid removed from a drinking tube reservoir divided by number of licks, can have almost 1% accuracy.     

Low doses of haloperidol interfere with rat tongue extensions during licking: a quantitative analysis.

Orolingual dyskinetic effects of haloperidol were studied in rats trained to lick drops of water from a force-sensing disk. Haloperidol dose-dependently decreased force of tongue protrusion, decreased lick duration, increased interlick interval, and decreased number of licks. Fourier methods showed that haloperidol, in doses as low as 0.06 mg/kg, slowed the lick oscillations from an average of 5.250 Hz to 5.096 Hz (p = .014). Lick force was modulated by water drop delivery, but this effect was not influenced by haloperidol, which suggests no interference of the drug with sensory regulation of licking. The drug-induced deficits in tongue force may be manifestations of either postural alterations (pseudoparkinsonism) or orobuccolingual dyskinesias or both. Orolingual motor impairments may account for some of the neuroleptic-engendered behavioral decrements that have previously been explained in terms of reward, motivational deficits, or both.     

Cholecystokinin-A but not cholecystokinin-B receptor stimulation induces endogenous opioid-dependent antinociceptive effects in the hot plate test in mice

The effects of intracerebroventricular administration of the cholecystokinin (CCK) analogue, BDNL, and the selective CCK-B agonist, BC 264, were determined using the hot plate test in mice. BDNL (0.2 nmol and 0.5 nmol) increased the jump and the paw lick latencies. These effects were blocked by the CCK-A antagonist MK-329 (0.02 mg/kg), supporting the involvement of CCK-A receptors in CCK-induced analgesia. In contrast, the selective CCK-B agonist BC 264 produced, at one dose (2.5 nmol), a slight decrease in the lick latency that was only antagonized by the CCK-B antagonist. Naloxone, but not naltrindole, antagonized BDNL-induced analgesia. The results suggest that activation of CCK-A receptors by BDNL leads to antinociceptive responses indirectly mediated by stimulation of μ-opioid receptors by endogenous enkephalins.     

The effect of bilateral sectioning of the chorda tympani and the greater superficial petrosal nerves on the sweet taste in the rat

The effects of bilateral deafferentation of the greater superficial petrosal (GSP) and the chorda tympani (CT) nerves on the ingestion of sucrose solutions were studied in rats. The rats received five daily sequential 30 second exposures for each sucrose concentration, and the average number of licks per exposure was calculated. Sucrose concentrations of 0.01, 0.03, 0.10, 0.32, and 1.00 M were presented in ascending order across days, both before and after bilateral sectioning of both the CT and the GSP nerves, the CT alone, the GSP alone, or a sham surgery. Prior to surgery, mean lick rate increased with increasing concentrations of sucrose. Following surgery, the rats with combined GSP and CT nerve sections showed a significant decrease in mean rate of licking to the sucrose solutions. The rats with GSP sections showed a similar decrease in mean lick rate to the sucrose solutions. Animals with sections of the CT nerve and sham-operated animals showed no significant reduction in mean lick rate to the sucrose solutions. These results indicate that the GSP nerve is important to the rat in reinforcing high lick rates to sucrose.     

Efficiency and persistence of licking in rats

Thirsty rats tested in 60-min sessions licked an empty spout for access to a water spout. We raised the behavioral price of the water lick by requiring more licks at the empty spout per lick at the water spout. At relatively high prices the rats made relatively few water licks but licked more efficiently, getting more water per lick. Controls showed that the rise in efficiency with price was not attributable to two variables previously confounded with price: the total number of water licks, and the temporal pattern of access. At a higher price the rats also licked more persistently, making more extra water licks before the shutter could occlude the spout. However, yoked controls showed that the greater persistence resulted not from price, but from the relatively stringent pattern of access that accompanied the higher price. Extra water licks diminished as the session progressed, apparently from satiety, not fatigue. The results have implications for behavioral ecology, behavioral economics, motivation, and the methodology of reward.     

Pro-Leu-Gly-NH2 serves as a conditioned stimulus in the acquisition of conditioned tolerance

The effect of Pro-Leu-Gly-NH2 (MIF) on the acquisition of tolerance to morphine-induced antinociception and its efficacy as a cue predictive of morphine administration was examined. Daily administration of MIF prior to morphine injection did not attenuate the acquisition of tolerance to the antinociceptive properties of morphine, as measured by the latency to hindpaw lick in a hot-plate test of analgesia. When the animals were tested 72 hr later without MIF pretreatment, they appeared to lose tolerance, as indicated by longer latencies to paw lick. These data suggest that in some situations MIF may interfere with the acquisition of tolerance by acting as a cue that reliably predicts the antinociceptive properties of morphine.     

Redirected oral behavior in rats induced by tail-pinch and electrical stimulation of the tail

Two seemingly non-specific stimuli, tail-pinch and electrical stimulation of the tail (EST), produce different responses to food in rats: tail-pinch induces eating whereas EST elicits licking of food pellets. However, even if administered several months earlier, prior tail-pinch changes the response to pellets during EST from licking to eating. If their tail is within reach, naive rats lick it rather than food pellets. However, they rarely pursue the tail after their initial attempts to reach it are prevented; instead, they eat or lick food. In fact, if they have prior tail-pinch experience, they first eat during EST even with the tail in sight and reach. Thus the response to tail stimulation is directed specifically at the tail and is redirected towards food when the tail is not available. Different oral responses during tail-pinch and EST suggest that the two stimuli induce different sensations; however, the redirected response is modifiable by learning.     

Licking patterns for sucrose solutions by young and aged squirrel monkeys

The licking elicited by 0.1, 0.3, and 1.0 M sucrose solutions was measured in six young and six aged squirrel monkeys. All tongue-on and tongue-off times were recorded in addition to conventional measures of lick rate and consumption. Consummatory activity by both groups increased monotonically with sucrose concentration. Aged monkeys displayed greater within animal variability of tongue-on times than did young monkeys. Distributions of pause lengths between licking bursts contained two components, one varying inversely with sucrose concentration, the other varying inversely with age. Distributions of licks per burst contained a component that increased with sucrose concentration but none varying with age. Results indicated that licking deteriorated little with age and that consumption is controlled mainly by length and spacing of bursts, not by individual lick parameters.     

Reduced palatability in drug-induced taste aversion: I. Variations in the initial value of the conditioned stimulus

Like illness-inducing agents (e.g., lithium chloride), drugs of abuse also suppress intake of a taste solution. To explore the nature of this drug-induced intake reduction, in the current study three aqueous stimuli with different initial values served as the conditioned stimuli (CSs) that were paired with a standard dose of amphetamine in a voluntary intake procedure and lick patterns were analyzed. Consistent with earlier studies, amphetamine significantly reduced intake of all three CSs (quinine, sodium chloride, and orange odor). In contrast to studies that analyze orofacial responses, we found that lick cluster size was significantly lowered by amphetamine, indicating that the psychoactive drug induced a conditioned reduction in taste palatability.     

Differential control of operant and consummatory licking in rats

The failure of rats to optimize their behavior when drinking from two retractable spouts available for single licks was analyzed. The rats were trained in an apparatus where the contacted spout was withdrawn after completion of a lick and the other spout was presented. After 5 days of such forced spout alternation training the animals continued to emit 2.7 instead of the optimum 1.0 lick per spout presentation (LPSP). With water available in one spout only the average LPSP at the empty spout dropped to 1.2 and increased to 3.6 at the water containing spout. It is argued that the operant licks at the dry spout approach the ideal value of 1.0 LPSP whereas the number of LPSPs at the water spout is increased by the consummatory nature of licking triggered by the presence of water in the oral cavity.