The role of sensory pathways in Pavlovian conditioning in rabbit.

In an earlier experiment we showed that selective attention plays a critical role in rabbit eye blink conditioning (Steele-Russell et al. in Exp Brain Res 173:587-602, 2006). The present experiments are concerned to examine the extent to which visual recognition processes are a separate component from the motor learning that is also involved in conditioning. This was achieved by midline section of the optic chiasma which disconnected the direct retinal projections via the brainstem to the cerebellar oculomotor control system. By comparing both normal and chiasma-sectioned rabbits it was possible to determine the dependence or independence of conditioning on the motor expression of the eye blink response during training. Both normal and chiasma-sectioned animals were tested using a multiple test battery to determine the effect of this redirection of the visual input pathways on conditioning. All animals were first tested for any impairment in visual capability following section of the optic chiasma. Despite the loss of 90% of retinal ganglion cell fibres, no visual impairment for either intensity or pattern vision was seen in the chiasma animals. Also no difference was seen in nictitating membrane (NM) conditioning to an auditory signal between normal and chiasma animals. Testing for motor learning to a visual signal, the chiasma rabbits showed a complete lack of any NM conditioning. However the sensory tests of visual conditioning showed that chiasma-sectioned animals had completely normal sensory recognition learning. These results show that NM Pavlovian conditioning involves anatomically separate and independent sensory recognition and motor output components of the learning.

Defensive conditioning-related functional heterogeneity among nuclei of the rat amygdala revealed by c-Fos mapping.

The amygdala is a complex forebrain structure proposed to play a pivotal role in fear conditioning circuitry. In this study, c-Fos immunomapping was applied to investigate the functional activation of particular amygdalar nuclei following a 50-trial training session of two-way active avoidance reaction. To dissect distinctive responses displayed by the animals and to cluster them into groups of correlated behaviors, factor analysis was employed. The training procedure resulted in an increase of c-Fos expression within the cortical, medial, lateral and basolateral, but not central, nuclei. The expression in the cortical nucleus correlated negatively with grooming behavior, whereas c-Fos immunolabeling of the other three subdivisions of the amygdala could be associated with the number of intertrial responses. No correlation was observed between c-Fos expression and avoidance reactions performed or the amount of shock received by the animal. The results obtained with c-Fos mapping of various regions of rat amygdala, combined with a fine dissection of behavioral repertoire, imply that there are specific functional links between particular parts of the structure and distinctive behaviors that reflect various emotional states of the animal.

Effect of nonaversive and aversive stimulations in infancy on the acoustic startle response in adult rats.

Two groups, each consisting of 8 three-week-old rat pups, were exposed to different behavioral treatments with the aim to determine how the experimental manipulation influenced their adult emotional reactivity. Every day for two weeks the pups from the first group received 15 min of handling whereas the animals from the second group were exposed to various aversive stimuli, differing each day. Following these manipulations, after a 5-day break the acoustic startle response (ASR) was measured in all animals and the testing was repeated after another four weeks. Statistical analysis of the data revealed significant differences between groups in the ASR parameters. Surprisingly, in the test which directly followed the treatment the mean ASR amplitudes were similar in both groups. Highly significant differences, however, were observed in the ASR amplitude four weeks later. The rats from the handling group responded with greater amplitudes. The latency of the ASR was significantly shorter in the nonaversive group compared with the second group exposed to aversive stimuli. The results suggest that early exposure to aversive stimulation significantly decreases rats emotional reactivity whereas nonaversive and impoverished stimulation clearly elevates arousal levels when the animal is placed in a novel situation.

CS modality transfer of two-way avoidance in rats with central and basolateral amygdala lesions.

Post-lesion acquisition of two-way avoidance and subsequent transfer to two warning signals (conditioned stimulus, CS) of different modality were investigated in 60 rats. In Experiment I the animals were originally trained with less salient (darkness) CS, then transferred to more salient compound (darkness and white noise), and finally to white noise CS. The opposite arrangement of the conditioned stimuli (CSi) during the subsequent stages was employed in Experiment II. In control animals, avoidance acquisition was faster and the intertrial responding (ITR) rate lower with the auditory than with the visual CS. Lesioned rats learned avoidance responses more slowly, independently of CS modality. The transfer to other CSi revealed dramatic between-group difference in the level and consistency of avoidance response, shuttle-box latencies and ITR rate. In control animals, transfer to more salient CSi enhanced avoidance performance, whereas change to less salient CS decreased it. Rather small changes in shuttle-box performance and consistency of avoidance response due to CS modality were seen in rats with the basolateral lesions. In contrast, central nucleus injury caused a strong deterioration in the avoidance transfer, especially when the visual CS followed the acoustic one. The results indicate differential involvement of the basolateral and central amygdala nuclei in stimulus-processing mechanisms of instrumental defensive behavior.

The effects of the medial and cortical amygdala lesions on post-stress analgesia in rats.

The role of the medial, and cortical nuclei of amygdala was studied in 54 Möll-Wistar rats under two modes of foot-shock analgesia. In all but control animals bilateral electrolytic lesions were performed. Pre- and post-stress pain reactivity were measured in the hot-plate and the tail-flick tests. The damage of the medial nucleus decreases animals' primordial pain reactivity. Four minutes of continuous foot-shock produced post-stress analgesia in all control and lesioned rats, but 20 min of regularly intermittent foot-shock failed to evoke analgesia in the lesioned rats, especially in subjects with the dorsal part of the medial nucleus injuries. The results indicate that the medial and cortical nuclei are important in regulation of the post-stress antinociceptive processes evoked only by prolonged intermittent shock action. It has been previously shown that the behaviour evoked by this stressor is related to opioid mechanisms, and modulated by the hypothalamic-pituitary-adrenocortical system. Present finding is in agreement with our concept of the dorsomedial amygdala involvement in painful and stressful stimuli processing.

Post-stress analgesia in rats with partial amygdala lesions.

The role of the dorsal basal and the central nuclei of the amygdala under two modes of foot-shock analgesia was studied in 45 Möll-Wistar rats. Four minutes of continuous foot-shock produced post-stress analgesia in control and in all lesioned rats, but 20 min of regularly intermittent foot-shock did not evoke analgesia in rats with central nucleus lesion. This result indicates different involvement of dorsal basal and central nuclei in processing of stressful stimuli.

Effects of partial lesion of dorsal hippocampal afferent and GM1 ganglioside treatment on conditioned emotional response and hippocampal afferent markers in rats.

Acquisition of the conditioned emotional response (CER) in 32 male hooded rats previously learned to press a bar for food and divided into four groups was studied. Two groups received electrolytic lesions of the dorsal hippocampal afferent and were thereafter injected either with GM1 ganglioside (30 mg/kg daily) or with buffer. Two remaining groups were sham operated and similarly injected. The partial hippocampal deafferentation evoked immediate enhancement of bar presses rate which persisted during the 2-week period of testing. CER training undertaken 2 days after surgical procedures appeared unsuccessful, whereas similar training with a cue of different modality initiated a week later resulted in acquisition of conditioned suppression of bar presses in all groups. Toward the end of training the conditioned suppression was more pronounced in lesioned than in control rats. The GM1 injections attenuated the conditioned suppression in control rats, presumably due to an antinociceptive role of ganglioside treatment. Behavioural training did not change the normal distribution pattern in cholinergic and serotonergic hippocampal afferent markers showing dorso-ventral gradient along longitudinal axis. The lesion-induced decrease pattern was also not affected. However, in contrast to previous findings in non-trained animals, the GM1 treatment was not effective in protecting against degenerative changes in the hippocampus of trained rats.

Latency of the two-way avoidance response in rats: inhibition of delay.

Avoidance and escape latencies were examined in 24 rats trained in two-way avoidance response to auditory and visual conditioned stimuli (CS). In rats trained with darkness CS the escape latencies decreased within the first 50-trial session but later were stable over the course of nine sessions. Avoidance latencies shortened within sessions as the response-eliciting properties of the conditioned stimuli increased. On the contrary, median daily avoidance latencies lengthened during training. Training with the more salient auditory stimulus resulted in a decrease in the probability of avoidance responses early in the CS-US interval, reflecting the development of inhibition of delay. Acquired inhibition of delay was eliminated by an increase in the fear-inducing properties of situational cues. When presented in a compound, the less salient darkness stimulus was fully overshadowed by the noise stimulus.

C-fos protooncogene expression in rat brain after long-term training of two-way active avoidance reaction.

C-fos nuclear protooncogene encodes a regulatory protein (Fos), able to directly influence both expression of itself and other genes. It has been repeatedly shown that c-fos expression coincides with different forms of cell activation, probably being functionally involved in the coupling of extracellular ligands to long-term cellular responses. In this study it has been found that c-fos mRNA accumulation in rat brain, as measured by northern blotting coincides with increase of performance level of learned behavior of a two-way active avoidance task. We have previously reported (Nikolaev et al., Brain Res. Bull., in press) that a single training session of two-way active avoidance strongly induces c-fos mRNA accumulation but that after long-term training up to the asymptotic level of performance no c-fos expression was detectable. In this paper we show that c-fos still remains inducible even after long-term, asymptotic training to darkness as conditioned stimulus (CS), provided that a novel stimulus, wide band noise, which elevated performance level, was given together with darkness as compound CS.

Intertrial responses of rats in two-way avoidance learning to visual and auditory stimuli.

Learning and performance of two-way avoidance were investigated in a total of 68 rats trained with either a visual (change in illumination to darkness or to light) or an auditory (white noise of 70 or 60 dB intensity) conditioned stimulus (CS). Experiment I showed that the darkness CS produce lower avoidance performance and a much higher rate of intertrial responses than either a auditory or a compound (visual plus auditory) CS. A monotonic within-session increase of avoidance performance and a similar, but less regular increase of intertrial responses were found at the beginning of training in each group. In later sessions such trends were observed only in rats trained with a visual CS. Experiments II and III showed also rapid transfer of avoidance response and a corresponding change of intertrial response rate with the change of CS modality. When the compound CS was used, the effects of the visual element were completely overcome by the auditory one. Rats trained with a visual CS in Experiment IV showed a positive correlation between avoidance performance and the number of intertrial responses, which was more pronounced in earlier than in later training sessions. We consider the rise of intertrial behaviour as an adaptive response to the increase of task difficulty. CSi of different modalities differ not only in relative saliency, but also in the discriminability between their onset and offset. The modality of the CS influences not only avoidance performance but also the course of learning.

Post-stress analgesia after lesions to the central nucleus of the amygdala in rats.

In two experiments the role of the central nucleus of the amygdala under two modes of foot-shock analgesia was studied in 39 male Möll-Wistar rats. In Experiment I a 4 min continuous foot-shock, dependent on neural mechanism was used as a stressor. Analgesia was produced by regularly intermitted 20 min of foot-shock action in Experiment II which evoke an opioid, humorally mediated mechanism. The results suggested that the central nucleus is involved only in the humoral regulation of the opioid form of analgesia. This finding fits well with the concept of limbic control of the anterior pituitary and the pituitary-adrenocortical axis, and points to the role of the amygdaloid complex in processing of stressful stimuli.

Partial lesion of dorsal hippocampal afferents and GM1-ganglioside treatment effects on the two-way avoidance in rats.

Acquistion and retention of the two-way avoidance was studied in 32 male Moll-Wistar rats, divided into four groups. The first two received electrolytic dorsal hippocampal afferent injuries and were injected either with GM,-ganglioside or buffer. Two remaining groups were sham-operated and injected similarly with either GM, or buffer. The acquistion and retention of avoidance was better in lesioned than is sham-operated groups. Short avoidance latencies, small number of trials to the first avoidance response, and high intertrial responses frequency were also observed in those groups. Ganglioside treatment tended to elihance all behavioral effects of lesions, contrary to its opposite effects in sham-operated group. The results suggest that the GM, ganglioside administration in lesioned rats decreases pain reactivity and/or unconditioned stimulus signaling properties, since the longest initial shock duration was observed in operated, GM, injected group.

Do amygdaloid nucleus centralis and nucleus lateralis serve similar functions in defensive responding in cats?

Bar pressing escape response has been trained, extinguished, retrained and then transformed into avoidance response in cats subjected to electrocoagulation of either the dorsolateral part of the central nucleus (Group CE) ox the lateral nucleus (Group L) of amygdala. In comparison with normal carts Group L was deficient in acquisition of the short latency escape responses and Group CE was handicapped in reacquisition of the escape response after the extinction procedure. Both lesioned groups were unable to acquire avoidance responding. Group CE was marked by gradual decrease of the sensitivity to shock in the course of learning, whereas Group L was less sensitive to changes in experimental procedures than the other two groups. However, most of the observed group differences were related to a decrease of sensitivity to shock and to a lowered general emotionality produced by lesions in both amygdaloid nuclei.

The role of contextual and sporadic stimuli in escape conditioning in dogs.

Instrumental escape bar-pressing responses were rapidly acquired in four male dogs and showed great resistance despite a long pause in experimental sessions. More intertrial responses were performed after short- than after long-latency escape responses. In the next stage of the experiment shock trials were discontinued and on the defensive context novel accoustical stimuli were introduced. The auditory stimuli provoked instrumental bar-press responses that were somewhat dependent on the intensity of stimuli.

Conditioned emotional response after dorsohippocampal, ventrohippocampal or entorhinal cortical lesions in rats.

Acquisition and extinction of the conditioned emotional response (CER) was studied in 24 male hooded rats which, prior to training, received small electrolytical lesions in the dorsal hippocampus, the ventral hippocampus, the entorhinal cortex, or were given sham operations. The rate of the on-going bar-pressing behavior for food pellets was highest in rats with lesions in the ventral hippocampus and the lowest after lesions in the entorhinal cortex. CER acquisition was markedly retarded and suppression was the weakest only in rats with dorsal hippocampal lesions. When compared with the level of suppression established toward the end of training, the greatest resistance to extinction of the CER was observed after entorhinal cortical lesions. The results of the experiment indicate functional heterogenity of the hippocampal formation.

Acquisition of the escape reflex in cats after the nucleus centralis of the amygdala lesions.

Three groups of 6 cats were trained in bar-press escape responses from unsignalled shock. Lesions of dorsolateral part of the amygdaloid nucleus centralis were made ,either 10 days or 35 days before training in two groups of subjects. The remaining 6 cats served as intact control ,group. The lesions resulted in a lengthening of instrumental response latencies especially in the final period of training. This was not caused by any changes in animals sensitivity and reactivity to painful stimulation since no differences between control and lesioned groups were observed at earlier stages of training. Further, no group effects were observed in threshold reactivity to shock. However, in contrast to the control cats, both lesioned groups performed escape responses to low intensity stimuli with very long latencies.

Exploration and avoidance in rats with lesions in amygdala and piriform cortex.

Lesions localized to specific areas of the amygdala and overlying cortex in rats produced differential effects in several behavioral tasks. Three different types of lesions were tested: central, basolateral, and cortex lateral to the amygdala. Lesions restricted to the central nucleus produced increased activity on all parameters studied in an open-field test, but the other two groups were not changed. In one-way active avoidance all three groups with lesions showed deficits. The most pronounced change was observed in the central group. All groups showed the same degree of retention loss, but in forced extinction of one-way active avoidance after retraining, the cortical and basolateral groups were most defective. A fear-reduction hypothesis is proposed for the central lesion. The basolateral and cortical areas may be more specifically involved in passive avoidance behavior.

Effects of lesions in the amygdaloid nucleus centralis on acquisition and retention of avoidance reflexes in cats.

Acquisition and retention of bar-press avoidance responses after lesions of the lateral part of the amygdaloid nucleus centralis were investigated in 17 adult male cats. The lesions were administered either 10 days (5 cats), or 35 days (7 cats) before training. The remaining 5 cats served initially as intact controls and then, after extensive training, were given similar lesions to test for the effects on retention of the task. The lesions resulted in both retarded avoidance acquisition and a decreased proportion of short latency responses. A short post-operative pause resulted in stronger lesion effects on the length of training and deterioration in short-latency response performance, but a smaller effect on the increase in intertrial responding than was found after a long post-operative pause. The lesioned cats showed more oscillations between runs of escape and avoidance responses than normal, control cats. The immediate effect of lesions on well-trained avoidance reflexes consisted of a decrease in avoidance performance and marked prolongation of escape latencies. The deterioration in short-latency avoidance responding was irreversible in spite of extensive post-operative retraining.