Appetitive conditioning in neonatal rats: Conditioned orientation to a novel odor

These experiments document a form of early appetitive learning in rats obtained using classical conditioning procedures. Some of the special determinants of this conditioning are described, as well as ontogenetic changes in the effectiveness of training procedures. Learning was apparent when deprived 3- and 6-day-old rats oriented to and maintained contact with a novel and normally aversive odor after this odor had been paired with oral infusions of milk (Experiment I). The effectiveness of the conditioning procedures depended on the temperature at which pups were trained (Experiment IB). Moreover, the reinforcing properties of milk infusions depended on deprivation (Experiment IC). This conditioned change in responsiveness to odor was specific to the odor that had been paired with milk (Experiment II) and was retained for at least 24 hr (Experiment III).     

Behavioral conditioned responses to contextual and odor stimuli paired with LiCl administration.

The present experiments examined whether behavioral conditioned responses (CRs) develop to LiCl-paired contextual and odor stimuli, and whether these CRs are similar to the behaviors observed following the administration of the drug. During conditioning, a novel context (Experiment 1) or a novel odor (Experiment 2) was paired with LiCl injections, and the CRs which developed to these stimuli were assessed during testing. Suppression of general activity occurred after LiCl exposure and after exposure to the lithium-paired context and odor. However, the actual behaviors which emerged as conditioned responses were distinctly different from those which were elicited by the drug itself. Lying on belly was the predominant unconditioned response to LiCl. Freezing was the predominant CR to the context, whereas gaping was the predominant CR to the odor. These outcomes are discussed and compared to previous results using a taste stimulus.     

Dissociation of conditioned locomotion and Fos induction in response to stimuli formerly paired with cocaine

A discrete stimulus (flashing light) was paired with cocaine (20 mg/kg) to induce conditioned locomotion. To identify brain regions activated during this response, Fos was measured with immunohistochemistry. Although paired subjects displayed robust conditioned locomotion, Fos was not increased in any limbic brain regions analyzed. In contrast, pairing of cocaine with generalized contextual cues (whole room) produced conditioned locomotion and Fos activation in the prelimbic portion of prefrontal cortex and the nucleus accumbens core. These results suggest that the pattern of neuronal activation during cocaine-conditioned activity differs depending on whether a discrete or contextual stimulus is used as a conditioned stimulus. The possibility that expectancy and frustrative nonreward contribute to Fos expression in rats conditioned to contextual cues is discussed.     

Effects of conditioned stimulus pre-exposure on human electrodermal conditioning to fear-relevant and fear-irrelevant stimuli

The effects of conditioned stimulus (CS) pre-exposure and fear-relevance of the CS on human Pavlovian electrodermal conditioning were investigated. A differential delayed conditioning paradigm was used with a CS-unconditioned stimulus (US; shock) interval of 8 s. In Experiment 1, 64 subjects were randomized into four groups, two of which received fear-relevant stimuli and the other two fear-irrelevant stimuli. Half of the subjects were pre-exposed to the to-be-CSs and the other half to two not-to-be-CSs, with 15 exposure of each stimulus. During acquisition, subjects received 8 reinforced and 8 nonreinforced CS+ and CS- trials, and during the extinction phase 15 nonreinforced trials of each CS. Pre-exposure to the to-be-CSs retarded conditioning for the first and second interval anticipatory responses (FIRs and SIRs); that is, a latent inhibition effect was demonstrated, although the results for the FIR were inconclusive. The expected effects of fear-relevance were not revealed. Experiment 2 addressed the question whether the long pre-exposure period interfered with the frequently observed "preparedness effect" of higher resistance to extinction to fear-relevant stimuli. The design was similar to that of Experiment 1, but for half of the subjects the acquisition phase was initiated immediately after a short rest period, and for the other half acquisition started after an extended rest period, equal to the duration of the pre-exposure phase in Experiment 1. Twenty extinction trials of each CS were presented. A reliable difference in arousal in terms of spontaneous fluctuations was produced by the rest periods, but although differential conditioning was observed, no effect of fear-relevance was seen during extinction.     

Vasopressin secretion: suppression after light and tone stimuli previously paired with footshocks in rats

Summary In an attempt to find whether vasopressin (VP) secretion is suppressed by learned emotional stress, we have given rats under a hypertonic condition simultaneously applied light and tone that had been paired previously with footshocks and have quantified immunoreactive VP (ir-VP) in the plasma. In a training session light (60 watt) and tone (2 kz) of 3-s duration which were paired with electric footshocks (50 Hz, 1-s duration) were given to rats 11 times at an interval of 30 s. Various lengths of time after the training, the rats were tested with light and tone, which were unpaired with footshocks and repeatedly applied every 15 s for 3 min in the box used for training. Hypertonic NaCl (0.5 M, 2 ml/ 100 g b. w.) was injected s. c. 30 min before testing to increase the basal level of plasma VP. After testing, plasma ir-VP was significantly less in the experimental group than in the 0-mA control group of rats that were trained without FS. The values for the experimental group were also significantly less than those of untested control rats that had been trained with FS but were not tested. Plasma osmolality and blood haemoglobin concentration were not significantly different between control and experimental groups. Plasma immunoreactive adrenocorticotrophic hormone (ir-ACTH) level was higher and motor activity as expressed by cumulative time period of body movement during testing was lower in the experimental group than in either of the control groups. The difference in plasma ir-VP between experimental and control groups was statistically significant two days but not seven days after training, whereas ACTH and motor activity in experimental groups were still significantly different from those in control groups seven days after training. The suppressive VP and augmentative ACTH responses to testing disappeared in the rats that had received light and tone repeatedly during the intervening period between training and testing. These data support the hypothesis that emotional stimuli suppress VP secretion.Supported by grants from the Ministry of Education, Science and Culture, Japan     

Reinforcers in infancy: classical conditioning using stroking or intra-oral infusions of milk as UCS

Six-day-old rats received 20 forward pairings of an odor-conditioned stimulus (CS) with one of two unconditioned stimuli (UCS); 1) intra-oral milk infusions or 2) stroking with a sable-hair brush. These UCS's produce a common general response of increased behavioral activity, but different specific behaviors. For each UCS, additional pups received backward pairings of the CS and UCS, random pairings of the CS and UCS, CS only, UCS only, or no stimuli. Four hours later, pups received a two-odor choice test to assess the development of an odor preference and a CS-only test to assess the acquisition of conditioned responding (CR). The results of the two-odor choice test indicated that for both UCS's only forward pairings of the CS and UCS resulted in an odor preference. Similarly, the CS-only test showed that only forward pairings of the CS and either UCS were effective in producing CR's; pups that received forward pairings exhibited increased behavioral activity during presentations of the CS, which is an unconditioned response (UCR) to both UCS's. Only the forward paired CS-milk UCS group exhibited increased mouthing and probing during the CS only test; these are UCR's that occur to milk infusions but infrequently to the stroking UCS. These results demonstrate the development of similar conditioned odor preferences using behaviorally activating UCS's, but CR's which are specific to the form of the UCR.     

Use of oral antibiotics in studies of ingestive behavior in rats.

The occurrence of chronic respiratory disease (CRD) in animal colonies is a problem encountered in many laboratories. Sixteen female albino rats were used to investigate the effects on food and water intake of the oral antibiotics most often used to treat CRD. Four concentrations (0.01, 0.05, 0.25, and 0.50%) of Tetracycline hydrochloride (TH) and Sodium sulfamethazine (SS) were presented at different times mixed with food or water. The animals were permitted to choose between each drugged and normal food or water combination for 4 days. Both drugs seriously affected fluid intake when placed in solution. When mixed with food, only SS produced a preference-aversion function and increased total daily food and water intake regardless of concentration. The presence of TH in the food had no effect on food choice or on total daily food intake at any concentration.     

Modification of unit responses to gustatory stimuli by conditioned taste aversion in rats

Strong conditioned taste aversion (CTA) was established in rats by associating 0.1% saccharin drinking with subsequent LiCl (0.15 M, 2% body weight) poisoning. Unit reactions (n = 135) to mouth perfusion with saccharin (10 sec) and water (40 sec) were recorded in curarized animals and analysed with a spike detecting computer program. Saccharin induced reactions in the gustatory thalamus of naive and CTA trained rats were equally frequent. The percentage of saccharin induced reactions in lateral and ventromedial hypothalamus (LH and VMH) was affected by CTA training in a reciprocal manner: it decreased from 64 to 38% in LH neurons and increased from 40 to 70% in VMH neurons. It is argued that the modification of hypothalamic reactivity to saccharin reflects the changed signal significance of the sweet taste, which activates the feeding center and inhibits the satiety center in naive animals, but elicits opposite effects after CTA acquisition.     

Suppression of vasopressin secretion by classically conditioned stimuli in rats

The possibility that emotional stress suppresses footshock-induced secretion of antidiuretic hormone (VP) was tested in rats by a classical conditioning paradigm. As a training trial, rats received a flash and a brief sound (2 kHz, 0.5 s) as conditioned stimuli (CS) followed by footshocks (FS, 5 ms pulses of 3 mA intensity, 50 Hz) for 1 s period as unconditioned stimuli. Rats were trained by 100 trials repeated at an interval of 6 s. A various length of time after the training, rats were tested by CS repeated at an interval of 15 s for the period of 120 s and FS of 60 s period, which started 60 s after the CS onset. Testing CS further augmented FS-induced increase in plasma adrenocorticotrophic hormone (ACTH) but suppressed FS-induced increase in plasma VP in a time-dependent manner. The CS also increased the degree of an inhibition of motor behavior known as "freezing" behavior. Augmentation of ACTH response and suppression of VP response to testing FS were dependent on the training shock intensity. These data are consistent with the hypothesis that VP secretion is potentiated by physical but suppressed by emotional stress.     

Effect of lesions in the lateral nucleus of the amygdala on fear conditioning using auditory and visual conditioned stimuli in rats

The lateral nucleus of the amygdala (LA) is believed to be the site of auditory conditioned stimulus (CS) relay in classical fear conditioning. The present study attempts to determine whether the LA is specifically involved in fear conditioning using an auditory CS. Seven rats with lesions in the LA (Tone-Lateral group) and eight sham-operated rats in the control group were trained using an auditory CS (overtone based on an 800 Hz fundamental tone, 70 dB, 3.7 s) paired with foot shock (1.0 mA, 0.5 s). Five rats with lesions in the LA (Light-Lateral group) and eight unoperated rats in the control group were trained using a visual CS (25 W light, 3.7 s). The behavioral index of fear conditioning was a potentiation of the startle reflex in the presence of CS. All rats in the control group and Light-Lateral group showed this potentiation, whereas those in the Tone-Lateral group did not. These results suggest that the LA is an input site of auditory CS information into the amygdala, and that it is not a site of visual CS information input in fear conditioning. Thus, each modality of CS may have a specific subnucleus of the amygdala that mediates fear conditioning.     

Selective responding of nucleus accumbens core and shell dopamine to aversively conditioned contextual and discrete stimuli.

Dopamine transmission within the nucleus accumbens has been implicated as a neurochemical substrate of associative learning processes. It has been suggested that the acquisition of classically conditioned fear to a specific environment, or context, differs fundamentally from the development of conditioned fear to a discrete stimulus, such as a light or a tone. In this study, we assessed extracellular dopamine in the rat nucleus accumbens shell and core during the expression of a conditioned fear response. Animals were aversively conditioned to either a context or a tone and extracellular dopamine was measured in the nucleus accumbens shell and core by in vivo microdialysis over the next 2 days as animals were returned first to the conditioning chamber (day 1: context test), and subsequently as animals were again returned to the chamber and presented with the conditioned tone stimulus (day 2: tone test). Dopamine levels in the core were significantly higher in the Context-Shock group compared to the Tone-Shock group during the 30-min exposure to context while dopamine levels in the nucleus accumbens shell did not differ significantly during the context test between groups. In contrast, extracellular dopamine in the shell but not the core of Tone-Shock animals increased significantly during presentation of the tone. Dopamine in both the shell and core remained unchanged during the tone test in the Context-Shock groups.These data suggest distinct roles for shell and core dopamine transmission in the expression of a conditioned emotional response. While dopamine increased in the shell primarily during the presentation of a discrete tone conditioned stimulus, core dopamine responded more to a contextual conditioned stimulus. These results may reflect differences in either the type of information acquired or the salience of the learned associations which are formed to a context vs. a discrete tone cue.     

Catecholaminergic mechanisms underlying neurohypophysial hormone responses to unconditioned or conditioned aversive stimuli in rats

Oxytocin release from the neurohypophysis is facilitated by systemic cholecystokinin octapeptide (CCK) administration and noxious stimuli. Oxytocin release after CCK administration is mediated by A2 noradrenergic neurones while the release after noxious stimuli appears to be mediated by A1 noradrenergic neurones. On the other hand, facilitation of vasopressin release after noxious stimuli is not dependent upon noradrenergic neurones but on dopamine receptors. Environmental stimuli previously paired with noxious stimuli (conditioned fear stimuli) or novel environmental stimuli facilitate oxytocin release and suppress vasopressin release. These neuroendocrine responses to conditioned fear stimuli, but not to novel stimuli, are impaired by central noradrenaline depletion or i.c.v. adrenoceptor antagonists. These data suggest that there are at least two types of stress responses in neuroendocrine systems, one noradrenaline dependent, and one noradrenaline independent. It is also suggested that noradrenergic neurones are functionally heterogeneous in the control of oxytocin release.     

An alternative scoring method for skin conductance responding in a differential fear conditioning paradigm with a long-duration conditioned stimulus

Researchers examining skin conductance (SC) as a measure of aversive conditioning commonly separate the SC response into two components when the CS-UCS interval is sufficiently long. This convention drew from early theorists who described these components, the first- and second-interval responses, as measuring orienting and conditional responses, respectively. The present report critically examines this scoring method through a literature review and a secondary data analysis of a large-scale study of police and firefighter trainees that used a differential aversive conditioning procedure ( n =287). The task included habituation, acquisition, and extinction phases, with colored circles as the CSs and shocks as the UCS. Results do not support the convention of separating the SC response into first- and second-interval responses. It is recommended that SC response scores be derived from data obtained across the entire CS-UCS interval.     

Acute and conditioned blood pressure changes in relation to social and psychosocial stimuli in rats

The naturally occurring tendency to compete with other rats for territorial space has been used to study individual behavior characteristics and blood pressure reactivity to social stimuli in adult male TMD-S3 rats. The competitive characteristics of the individual rats are consistent in two different social situations (victory and defeat). Blood pressure responses during the victory of home territory rats over intruders was more pronounced in the more competitive animals. In addition to defeat by a trained fighter rat, the experimentals were also psychosocially stimulated by the fighter while it was confined in a small wire mesh cage. The blood pressure response to this event was enhanced by the prior defeat of the test animal by the one now confined to the small cage. This response was more pronounced in competitive rats. This approach has potential as an animal model of etiological processes in socially induced hypertension.     

Comparison of nutritive and nonnutritive stimuli in intestinal and oral conditioned taste aversion paradigms

The intestinal taste aversion paradigm has previously demonstrated that animals could orally discriminate between carbohydrate and fat subsequent to pairing a gastrointestinal (GI) infusion of 1 nutrient with lithium chloride (LiCl), whereas they could not discriminate between 2 nonnutritive flavors (A. L. Tracy, R. J. Phillips, M. M. Chi, T. L. Powley, & T. L. Davidson, 2004). The present experiments assessed the relative salience of nutritive and nonnutritive stimuli when presented either intestinally or orally. Two compound stimuli, each comprising 1 nutrient and 1 nonnutritive flavor, were presented in training and were paired with LiCl or saline. Subsequent oral intake of the nutrients alone, the flavors alone, or the compounds was measured. Results showed that rats discriminated both nutrients and flavors independently after GI or oral training, whereas the compounds were discriminated only after oral training, indicating substantive differences in the processing of these stimuli. This suggests that nutrient activation of the GI tract may potentiate learning about nonnutritive flavors analogously to taste-potentiated odor conditioning. The ability to learn about the oral properties of stimuli in the GI tract suggests a new account of delayed taste aversion learning as well as learning about the positive nutritive consequences of food consumption.     

Conditioned changes in appetitive and consummatory responses to flavors paired with oral or nutrient reinforcement among adult rats

Recent studies of the behavioral organization of conditioned flavor preferences have suggested the involvement of at least two separate learning systems-an appetitive response system sensitive to the oral hedonic properties of the reinforcer, and a consummatory response system sensitive to its nutrient properties. However, these prior studies were conducted with weanling rats, that differ from adults in terms of their prior experience with food, their learning competencies, and the peculiar ontogenetic constraints on their behavior. It is, therefore, unknown whether flavor preference behaviors are similarly organized in adult rats. In this experiment, adult rats were trained to associate a specific CS flavor with either the sweet taste or the postingestive nutrient effects of sucrose. Conditioned appetitive orienting and consummatory oral responding to the CS flavors were then measured. Unlike weanling rats, adult rats exhibited both conditioned appetitive behavior and conditioned consummatory behavior in response a CS that was previously paired with either oral hedonic or nutrient reinforcement. These results suggest a set of important developmental changes in the neurobehavioral mechanisms of flavor preference learning in the postweaning period.     

Motivational control of heroin seeking by conditioned stimuli associated with withdrawal and heroin taking by rats

The authors investigated the impact of conditioned withdrawal on drug seeking by training rats to work for a heroin infusion on a seeking-taking schedule, which required responding on a seeking lever in order to gain the opportunity to self-administer the drug by a single response on a taking lever. Following the establishment of opiate dependence, a conditioned stimulus (CS) that had been previously paired with naloxone-precipitated withdrawal suppressed heroin seeking in extinction. However, when the rats had prior experience of heroin taking in the presence of the withdrawal CS, drug seeking was elevated in the presence of this stimulus. The authors conclude that the conditioned motivation of drug seeking in withdrawal depends on previous association of the CS with drug taking.     

D1 and D2 receptor contributions to ingestive and locomotor behavior are altered after dopamine depletions in neonatal rats

The effects of selective D1 and D2 antagonists on ingestive and locomotor behavior were studied in adult rats depleted of dopamine (DA) as neonates and their vehicle-treated controls. Blockade of either D1 or D2 receptors inhibited food intake and produced akinesia in control animals. Simultaneous administration of subthreshold doses of these antagonists produced similar behavioral deficits, indicating a synergism between the two receptor subtypes. In contrast, adults depleted of DA as neonates were unaffected by either D1 or D2 blockade. However, combined administration of these antagonists produced behavioral deficits comparable to those seen in controls. These data demonstrate that while activity within residual DA neurons remains critical for the expression of ingestive and locomotor behavior in rats depleted of DA as neonates, the precise nature of the interactions between the D1 and D2 receptor subtypes is altered after the depletion.