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 The rats were trained in an apparatus where the contacted spout was withdrawn after completion of a lick and the other spout was presented.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.     

Inefficient licking during forced spout alternation in rats: violation of the law of effect?

The efficiency of the neural control of licking was assessed in rats (n = 6) allowed 30-min daily access to water in a box equipped with two retractable drinking spouts. After completion of a photoelectrically monitored lick, the contacted spout was withdrawn by an electromechanical device which simultaneously made the other spout available at an adjacent wall opening. Although economy of behavior required the animals to lick at each spout only once, computer monitored licking showed that the rats emitted 2.9 +/- 0.2 and 2.5 +/- 0.1 licks per spout presentation (LPSP) on the first and fifth days of training, respectively. The transition time between spouts and the interlick interval were not significantly changed. Separation of the spouts by a vertical partition extending 24 mm into the box prolonged the transition time and increased LPSP to 3.9 and 3.3 on Days 1 and 4 of training, respectively. These values were not changed when the length of the partition was increased to 40 mm. The failure of rats to maximize reward by emitting single licks probably reflects limited control of the generator of licking and/or the tendency to avoid its frequent switching on and off.     

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.     

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.     

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     

Multidimensional aspects of drinking in the rat.

In 60-min test sessions, thirsty rats licked an empty metal spout for 0.2-milliliter shots of water delivered audibly to a neighboring spout that remained accessible after the first delivery. As the instrumental empty-lick requirement increased they drank less water, in accordance with the demand law, but made the same number of licks at the water delivery spout. In terms of water licks/milliliter they, therefore, licked less efficiently at higher behavioral prices, opposite to the relation obtained when rats respond for licks at a spout full of water. Given a continuous supply of free water they licked more efficiently, and licks varied in strict proportion to the amount drunk. Yoked controls with no instrumental requirement drank the same amount and made the same number of water licks as their price-paying experimental partners. Thus, intermittent delivery of a small amount of water induced rats to lick the delivery spout at a relatively high average rate that was unaffected by the behavioral price of the delivery or the interdelivery interval. Opposite efficiency effects suggest that external constraint on any dimension of drinking will affect that dimension more than any other left unconstrained.     

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.     

The development of water-intake regulation during schedule-induced polydipsia

Water intake by rats exhibiting asymptotic levels of schedule-induced polydipsia during a fixed-interval (FI) 1-min food schedule was decreased by reducing the diameter of the drinking spout orifice from 2.5 mm (LT-condition) to 1.0 mm (ST-condition). During initial exposure to the ST condition, rate of licking was immediately enhanced but session intake was not regulated. Lick rate continued to increase across subsequent ST sessions and session intake similar to that during LT conditions was achieved after several ST sessions. However, volume regulation failed to occur during the first session following each transition from LT to ST conditions. Restrictions in the rate of consumption increased the duration but not the frequency of post-pellet licking bouts, and resulted in decreased rates of bar-pressing with enhanced pause time. Reinforcement frequency across conditions was unaffected. Increased licking rates during ST sessions also resulted in enhanced licking rates during sugsequent LT sessions. These results suggest that volume regulation during schedule-induced polydipsia is achieved through reductions in competing responses, and changes in both the discriminative control of drinking and topography of licking.     

Aged Fischer 344 rats exhibit altered orolingual motor function: relationships with nigrostriatal neurochemical measures

The present study utilized a novel behavioral preparation to measure differences in orolingual motor function between young (6 months) and aged (24 months) Fischer 344 (F344) rats. Rats were trained to lick an isometric force-sensing operandum for water reinforcement so that the number of licks per session, licking rhythm and lick force could be compared between the two groups. The aged rats exhibited a greater number of licks per session, but a slowed licking rhythm, compared to the young rats. Lick force did not differ significantly between the groups. The dopamine (DA) uptake inhibitor nomifensine decreased all three measures in both groups. Analyses of whole brain tissue content of DA, 3,4 dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in the substantia nigra and dorsal striatum revealed no significant differences between the two age groups. Differences were observed between the two groups with respect to relationships between behavioral and neurochemical tissue measures. Striatal DA content and the number of licks per session were positively correlated for the young rats but not for the aged rats. In the aged rats, but not the young rats, positive correlations were also observed between licking rhythm and the DOPAC+HVA/DA ratio in the substantia nigra. These findings suggest that age-related alterations in orolingual motor function may relate in part to functional changes in DA neuronal circuits.     

Age-related changes in orolingual motor function in F344 vs F344/BN rats

Normal aging is associated with both locomotor and orolingual motor deficits. Preclinical studies of motor function in normal aging, however, have focused primarily on locomotor activity. The purpose of this study was to measure age-related changes in orolingual motor function and compare these changes between two rat strains commonly used in aging studies: Fischer 344 (F344) and Fischer 344/Brown Norway hybrid (F344/BN) rats. Rats (6-, 12-, 18- and 24-months of age) were trained to lick water from an isometric force-sensing operandum so that the number of licks per session, licking rhythm (licks/second) and lick force could be measured. In both strains, the number of licks per session was greatest in the oldest group, while this measure was greater for F344/BN rats at all ages. Peak tongue force increased with age in F344/BN rats, did not change with age in the F344 rats, and was greater for the F344/BN rats at all ages. Both strains exhibited an age-related slowing of licking rhythm beginning with the 18-month-old group. These findings suggest that despite lifespan differences between these two rat strains, diminished tongue motility emerges at the same age.     

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.     

Lick triggered intracranial stimulation interferes with retrieval of conditioned taste aversion

The effect of brain stimulation on retrieval of conditioned taste aversion (CTA) to 0.15 M LiCl was investigated in 15 overtrained rats. The photoelecttrically detected licks at two drinking spouts delivering water or salt solutions, respectively, were used to trigger single electrical pulses, applied through implanted electrodes to basolateral amygdala or lateral hypothalamus. Current intensities not interfering with licking increased the number of licks required for recognition of the aversive fluid after spout switching from 1 or 2 to more than 5 and occasionally blocked gustatory discrimination completely. Discrimination was disrupted both by amygdalar and lateral hypothalamic stimulation of the same intensity, but amygdalar stimulation was more effective: interference was obtained from all amygdalar (n=21) but from only half of hypothalamic (n=17) stimulation points. The disruptive effects were cumulative. They were almost independent of the lick stimulus delay (1 to 100 ms) and often outlasted the period of stimulation for several min. The interfering effects of electrical stimulation are in accordance with the results of lesion studies indicating that the amygdala and the lateral hypothalamus participate in CTA retrieval, probably by mediating stimulus recognition and/or interruption of consummatory behavior.     

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.     

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.     

Rhythmometry on licking rate of the mouse

Licking rates of four male and four female mice in ad lib drinking were measured over a 24 hr period. The rates of each mouse widely ranged even within a certain period of a day, while no significant difference in the rate was found between sexes. When measured by rhythmometry, that is, by fitting a sine curve to the data on mean licking rates for every 2 hr period, the average of licking rate in 8 mice was 6.87 licks per sec., and the peak of the rate was found at 1:34. The difference between the peak and the trough of the diurnal variation in licking rate was statistically significant in six mice out of eight. These data suggest that it may be advisable to reinvestigate whether drinking rate is indeed invariant in given animals.     

A test of the generality of schedule-induced polydipsia to wild-caught Norway rats.

Four of 5 food-deprived wild-caught Norway rats failed to develop schedule-induced polydipsia when placed on a fixed-time 1-min food schedule of 45 mg pellets for 30 1-hr sessions. The one wild-caught rat that displayed schedule-induced polydipsia was retarded in the development and only reached session intake level approximately one half that of a control group of domestic laboratory-reared Norway rats. All 5 domestic rats readily acquired schedule-induced polydipsia. The wild-caught and domestic Norway rats differed in their probability of initiating a bout of drinking but not in their lick pattern, bout size or lick efficiency. These data appear to challenge the generality of schedule-induced polydipsia.     

Schedule-induced drinking and reinforcement omission

Rats were given extended training on a fixed-interval one-minute schedule in the presence of freely available water. Each food pellet delivery was accompanied by brief stimuli of blackout and tone. A total of 10 probe test sessions was conducted after stability was obtained with a maximum of 9 within-session probes during each session. On probes brief stimuli, food, or both were omitted. Omitting brief stimuli, singularly or in combination, did not reliably affect licks or bar presses. Omitting food, in the presence or absence of brief stimuli, eliminated polydipsic rates of licking but rates of bar presses were low early in the interval if brief stimuli were present and high throughout the interval if brief stimuli were absent. These effects were similar to those obtained with second-order schedules and suggest that polydipsic levels of schedule-induced drinking are under the control of the discriminative properties of food reinforcers.     

Tolerance to amphetamine hypophagia: a real-time depiction of learning to suppress stereotyped movements in the rat

To analyze how tolerance develops to amphetamine-induced hypophagia, the authors recorded real-time licking responses in rats given chronic injections of the drug and access to milk for 30 min. Initially, licking was greatly reduced and occurred only late in the session. The acquisition of tolerance was characterized by a decrease in the latency to initiate licking, a gradual increase in the number of licks, and a reorganization of the temporal licking pattern such that licks were distributed throughout the session, interspersed with pauses. On post-tolerance dose-response tests, licking was directly proportional to drug dose in some rats. The results support the view that tolerance to amphetamine hypophagia involves a behavioral adaptation to the motor effects of the drug.     

IV nicotine self-administration in rats using the consummatory operant licking response

Nicotine self-administered by tobacco smoking or chewing is very addictive in humans. Rat models have been developed in which nicotine is self-administered IV by the rats pressing a lever. However the reinforcing value of nicotine is much less in these models than the addictiveness of human tobacco use would indicate. The IV nicotine self-administration operant lever press model does not fully capture important aspects of tobacco use in humans. Conditioned oral consumption actions of smoking or chewing tobacco may play important roles in tobacco addiction. Neural circuitry underlying essential food consummatory responses may facilitate consummatory aspects of tobacco intake. To capture this motor consummatory aspect of tobacco addiction in the rat model of nicotine self-administration, we have developed a method of using a licking response instead of a lever press to self-administer IV nicotine. Sprague-Dawley rats were trained to lick one of two waterspouts for IV nicotine (0.03 mg/kg/infusion). With the licking response rats self-administered stable nicotine levels throughout 24 sessions (45 min each) of testing. The number of total licks/session significantly increased in a linear fashion over that period. The number of licks/infusion was substantial, rising steadily with training to an average of over 100 licks/infusion. Including the consummatory motor act with nicotine self-administration could help better model the compulsive aspects of tobacco addiction in humans.