The benzodiazepine midazolam does not impair Pavlovian fear conditioning but regulates when and where fear is expressed

Rats were injected with a benzodiazepine (midazolam) and shocked after presentation of an auditory conditioned stimulus (CS). They were then tested for fear reactions (freezing) to the CS in either the original context or a 2nd context after either a short (1-day) or long (21-day) retention interval. Rats tested in the original context froze less after 1 day than rats tested after that interval in the 2nd context or rats tested after 21 days. Moreover, rats tested after the long interval in the original context froze less than rats tested after that interval in the 2nd context. Therefore, midazolam does not impair the acquisition of conditioned fear but regulates when and where that fear is expressed. These effects of midazolam were interpreted as a contextually controlled deficit in the expression of conditioned fear that is similar to that associated with latent inhibition and extinction (M. E. Bouton, 1993).

Effects of systemic, intracerebral, or intrathecal administration of an N-methyl-D-aspartate receptor antagonist on associative morphine analgesic tolerance and hyperalgesia in rats

A flavor paired with morphine shifted to the right the function relating morphine dose to tail-flick latencies and provoked hyperalgesic responses when rats were tested in the absence of morphine. These learned increases in nociceptive sensitivity were not mediated by alterations in tail-skin temperature. Microinjection of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5) into the lateral ventricle reversed the hyperalgesic responses but spared the tolerance to morphine analgesia. By contrast, systemic administration of the noncompetitive NMDA receptor antagonist MK-801 or intrathecal infusion of AP-5 reversed the hyperalgesic responses as well as the tolerance to morphine analgesia. The results demonstrate that associatively mediated tolerance to morphine analgesia can co-occur with hyperalgesic responses and are discussed relative to learned activation of endogenous pronociceptive mechanisms.

Bacterial lipopolysaccharide induces a conduction block in the sciatic nerves of rats

A single injection of Escherichia coli lipopolysaccharide (LPS; intraperitoneally [i.p.] and intravenously [i.v.]) reliably induces peripheral nerve disturbances in the hindlimbs of inbred Australian albino Wistar (AaW) rats. In the series of experiments presented here, we aimed to characterize this syndrome by examining electrophysiologic, immunologic, and immunochemical features. The LPS-induced neurologic sequelae in AaW rats were transient, at least partly reversible by drug treatment, and were not associated with any detectable neuropathologic findings by light microscopy. Neurologic sequelae were prevented by administration of dexamethasone and by pretreatment with the macrophage inhibitor gadolinium chloride, suggesting that they were caused by LPS-induced activation of peripheral macrophages. Sequelae were associated with early decreases in compound muscle-action potential amplitudes, indicating impaired functioning of either proximal sciatic nerve axons and/or neuromuscular synapses. Spinal somatosensory-evoked potential latencies also were increased, indicating impaired somatosensory function at the sciatic nerve, dorsal roots, spinal cord, and/or postsynaptic interneurons, although the precise location of impairment could not be delineated. Similarities between this syndrome and immune-mediated polyneuropathies in humans are discussed.

Test type influences the expression of lithium chloride-induced hyperalgesia

The hyperalgesic properties of the emetic drug lithium chloride (LiCl) were examined in eight experiments. At a dose of 63.6 mg/kg, LiCl produced hyperalgesia in the radiant-heat (Experiment la) and immersion (Experiment 1b) tail-flick tests. At doses of 15.9, 31.8, 63.6, and 127.2 mg/kg, LiCl failed to produce hyperalgesia during the delayed behavioral response in the formalin test (Experiments 2a and 2b), but 63.6 mg/kg LiCl did produce hyperalgesia during the normally quiescent, interphase period of formalin responding (Experiment 2c). At the dose of 63.6 mg/kg, LiCl did not produce hyperalgesia in the hotplate test (Experiments 3a and 3b) and did not exert significant motoric effects in a step-down passive-avoidance task (Experiment 4). The results were discussed with reference to the behavioral effects of LiCl and their implications for demonstrations of associatively mediated morphine analgesic tolerance.

Evidence that GABA transmission mediates context-specific extinction of learned fear

Six experiments used rats to study the effects of the beta-carboline FG 7142 on extinction of fear responses (freezing) to an auditory cue that had signalled footshock. Subcutaneous injection of FG 7142 interfered with the development of extinction without having any detectable effect on the rats' levels of fear prior to extinction. Injection of FG 7142 also reversed extinction, partially reinstating fear responses that had been extinguished previously. A similar reinstatement of extinguished fear was seen when rats were tested for fear of the cue in a different chamber. The reinstatement produced by FG 7142 and that caused by context shift were not additive: FG 7142 did not increase extinguished fear if rats were tested in the different chamber. Finally, FG 7142 had no detectable effect on the latent inhibition of fear produced by repeatedly presenting the cue alone before conditioning with shock, even though this inhibition, like extinction, was affected by a shift in context. The present findings indicate that GABA transmission at GABA(A) receptors is involved in the inhibition of extinguished fear, and that this effect of GABA is regulated by those cues that constitute the extinction context.

Effects of systemic, intracerebral, or intrathecal administration of an N-methyl-D-aspartate receptor antagonist on associative morphine analgesic tolerance and hyperalgesia in rats

A flavor paired with morphine shifted to the right the function relating morphine dose to tail-flick latencies and provoked hyperalgesic responses when rats were tested in the absence of morphine. These learned increases in nociceptive sensitivity were not mediated by alterations in tail-skin temperature. Microinjection of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5) into the lateral ventricle reversed the hyperalgesic responses but spared the tolerance to morphine analgesia. By contrast, systemic administration of the noncompetitive NMDA receptor antagonist MK-801 or intrathecal infusion of AP-5 reversed the hyperalgesic responses as well as the tolerance to morphine analgesia. The results demonstrate that associatively mediated tolerance to morphine analgesia can co-occur with hyperalgesic responses and are discussed relative to learned activation of endogenous pronociceptive mechanisms.

Benzodiazepine-induced amnesia in rats: reinstatement of conditioned performance by noxious stimulation on test

A benzodiazepine (midazolam), injected either systemically or directly into the basolateral amygdala (BLA), differentially affected the acquisition of fear responses to a shocked context: Administration of the drug before conditioning impaired subsequent freezing to the context but spared analgesic responses in rats tested there for sensitivity to formalin pain. Moreover, the pain test not only revealed evidence for analgesic responses but also served to reinstate conditioned freezing that was otherwise absent in rats conditioned under midazolam. The results were interpreted as showing that the presence of noxious stimulation on test serves either (a) to assist in retrieval of the context-shock association whose storage had been modified by midazolam's action in the BLA, or (b) to enable performance of the context-shock association whose affective properties had been blocked by midazolam's action in the BLA.

Benzodiazepine-induced amnesia in rats: reinstatement of conditioned performance by noxious stimulation on test

A benzodiazepine (midazolam), injected either systemically or directly into the basolateral amygdala (BLA), differentially affected the acquisition of fear responses to a shocked context: Administration of the drug before conditioning impaired subsequent freezing to the context but spared analgesic responses in rats tested there for sensitivity to formalin pain. Moreover, the pain test not only revealed evidence for analgesic responses but also served to reinstate conditioned freezing that was otherwise absent in rats conditioned under midazolam. The results were interpreted as showing that the presence of noxious stimulation on test serves either (a) to assist in retrieval of the context-shock association whose storage had been modified by midazolam's action in the BLA, or (b) to enable performance of the context-shock association whose affective properties had been blocked by midazolam's action in the BLA.

Effects of retention interval on latent inhibition and perceptual learning

Repeated, non-reinforced preexposure to a context slowed development of conditioned freezing to that context when it was subsequently paired with footshock (latent inhibition) and enhanced discriminability of that context from a similar context (perceptual learning) whether assessed by a generalization test or by explicit discrimination training. Latent inhibition was eliminated by a delay between conditioning sessions and test (Experiments 1a and 1b) and reduced by a delay between preexposure and conditioning (Experiment 2). However, perceptual learning was unaffected by either of these intervals (Experiments 1b and 2). These results are discussed in terms their impact on theories that have latent inhibition as a possible mechanism of perceptual learning, and on theories of latent inhibition that consider the retardation of conditioned responding to be the result of an acquisition failure.

Loss of latent inhibition of contextual conditioning following non-reinforced context exposure in rats

Three experiments with rats demonstrated that preexposure to an experimental environment retarded the level of conditioned freezing observed on a test in that environment after it had been paired with mild footshock. Furthermore, Experiment 1 demonstrated that this latent inhibition effect could be abolished if preexposed rats were exposed to a second experimental environment following conditioning to the preexposed environment. Experiments 2 and 3 demonstrated that this second environment had to be similar, but not identical, to the preexposed environment, and that the influence of exposure to the second environment on latent inhibition could be abolished by exposure to that environment prior to footshock conditioning. These results are considered in terms of the Dickinson-Burke (1996) theory of retrospective revaluation, and their implications for experiments demonstrating a loss of latent inhibition across a delay are considered.

Microinjection of morphine into the nucleus accumbens impairs contextual learning in rats

A unilateral microinjection of morphine into the amygdala impaired fear conditioning to both a conditioned stimulus (CS) paired with shock and the context where shock occurred, whereas a microinjection of morphine into the nucleus accumbens (NA) spared fear conditioning to the CS but impaired, in a dose-dependent and receptor-specific manner, fear conditioning to the context. Morphine in the NA also spared extinction and latent inhibition of a CS but abolished the context specificity of these effects and eliminated the increase in discriminability that results from preexposure to a to-be-shocked context. The results identify a role for the NA in the processes by which rats learn about a context and are discussed in terms of an opioid disruption of either within-context associations or of attentional processes that contribute to such associations.

Microinjection of morphine into the nucleus accumbens impairs contextual learning in rats

A unilateral microinjection of morphine into the amygdala impaired fear conditioning to both a conditioned stimulus (CS) paired with shock and the context where shock occurred, whereas a microinjection of morphine into the nucleus accumbens (NA) spared fear conditioning to the CS but impaired, in a dose-dependent and receptor-specific manner, fear conditioning to the context. Morphine in the NA also spared extinction and latent inhibition of a CS but abolished the context specificity of these effects and eliminated the increase in discriminability that results from preexposure to a to-be-shocked context. The results identify a role for the NA in the processes by which rats learn about a context and are discussed in terms of an opioid disruption of either within-context associations or of attentional processes that contribute to such associations.

Cognitive deficits in patients suffering from chronic fatigue syndrome, acute infective illness or depression

BACKGROUND

Patients with chronic fatigue syndrome (CFS) report neuro-psychological symptoms as a characteristic feature. We sought to assess cognitive performance in patients with CFS, and compare cognitive performance and subjective workload experience of these patients with that of two disease comparison groups (non-melancholic depression and acute infection) and healthy controls.

METHOD

A computerized performance battery employed to assess cognitive functioning included tests of continuous attention, response speed, performance accuracy and memory. Severity of mood disturbance and subjective fatigue were assessed by questionnaire.

RESULTS

All patient groups demonstrated increased errors and slower reaction times, and gave higher workload ratings than healthy controls. Patients with CFS and non-melancholic depression had more severe deficits than patients with acute infection. All patient groups reported more severe mood disturbance and fatigue than healthy controls, but patients with CFS and those with acute infection reported less severe mood disturbance than patients with depression.

CONCLUSIONS

As all patients demonstrated similar deficits in attention and response speed, it is possible that common pathophysiological processes are involved. The differences in severity of mood disturbance, however, suggest that the pathophysiological processes in patients with CFS and acute infection are not simply secondary to depressed mood.

Bacterial lipopolysaccharides induce peripheral nerve disturbances in rats that mimic human immune-mediated polyneuropathies

A single injection of Escherichia coli lipopolysaccharide (LPS; i.v. and i.p.) reliably induced peripheral nerve disturbances in male Australian albino Wistar (AaW) rats. Signs developed 6 to 24 h after LPS inoculation and persisted only transiently. Most AaW rats had variable degrees of bilateral hind limb impairment, and rarely had forelimb, tail, or central impairment. Signs included gait abnormalities, proprioceptive loss, and to a lesser extent hind limb weakness and sensory deficits. Signs were more severe in male than female AaW rats and were induced in a number of genetically related rat strains (e.g., AaW and outbred Wistar and inbred Lewis rats, but not Sprague Dawley or inbred Fischer 344 rats). Development and severity of these signs were found not be related to animal body weight, but were dependent on LPS dose. Signs were not associated with LPS-induced alterations in pain perception, or occurrence of spontaneous pain, as indexed by tail-flick and hot-plate tests. Taken together, these data indicate that LPS induced transient peripheral nerve disturbances in rats, the severity of which was influenced by genetic, sex-related, and dose-related factors.

Context dependency of conditioned aversions to water and sweet tastes

Three experiments exposed rats (Rattus norvegicus) to a discriminative conditioning procedure whereby a specific fluid was followed by lithium in one environment but not in another. This produced context-specific aversion to water, as detected by 2-bottle tests in Experiment 1, and a context-dependent saccharin aversion, which was unaffected by context extinction, in Experiment 2. Experiment 3 found that sucrose preexposure increased contextual control over the aversion established by sucrose-lithium pairings but had no effect on the target context. By contrast, target context exposure during conditioning reduced aversion to this context but did not affect contextual control of the sucrose aversion. In conclusion, depending on the conditioning procedures, contextual control of a taste aversion can be independent of the context's Pavlovian properties.

Context dependency of conditioned aversions to water and sweet tastes

Three experiments exposed rats (Rattus norvegicus) to a discriminative conditioning procedure whereby a specific fluid was followed by lithium in one environment but not in another. This produced context-specific aversion to water, as detected by 2-bottle tests in Experiment 1, and a context-dependent saccharin aversion, which was unaffected by context extinction, in Experiment 2. Experiment 3 found that sucrose preexposure increased contextual control over the aversion established by sucrose-lithium pairings but had no effect on the target context. By contrast, target context exposure during conditioning reduced aversion to this context but did not affect contextual control of the sucrose aversion. In conclusion, depending on the conditioning procedures, contextual control of a taste aversion can be independent of the context's Pavlovian properties.

Midazolam impairs the acquisition of conditioned analgesia if rats are tested with an acute but not a chronic noxious stimulus

Three experiments investigated the effects of midazolam on acquisition of fear-mediated analgesic responses in rats conditioned on the heated floor of a hot-plate apparatus. Experiment 1 demonstrated that a moderate dose (1.25 mg/kg) of midazolam administered prior to conditioning impaired acquisition of conditioned analgesia in rats retested on the heated floor 24 h later. This effect of midazolam was reversed by the benzodiazepine receptor antagonist flumazenil. In contrast, in Experiment 2, the same or higher (2.5 mg/kg) dose of midazolam did not appear to affect the acquisition of conditioned analgesia in rats tested 24 h later with a formalin-injected paw on the non-heated floor of the hot plate apparatus. By testing rats with the opioid antagonist naloxone, Experiment 3 revealed that the higher dose of midazolam did disrupt the acquisition of conditioned analgesia in rats tested with formalin, but only by preventing acquisition of an immediate but brief analgesic response that was insensitive to naloxone. Midazolam was shown to have no effect on the acquisition of the enduring naloxone-reversible analgesia. These results are discussed in terms of benzodiazepines acting within the amygdala to produce a retrieval deficit whereby fear conditioning that takes place under the influence of a benzodiazepine can only be accessed if the animal is tested in the presence of ongoing noxious stimulation.

Effects of a microinjection of morphine into the amygdala on the acquisition and expression of conditioned fear and hypoalgesia in rats

A unilateral microinjection of morphine into the amygdala impaired the acquisition of fear and hypoalgesic responses in rats exposed to a heated floor in a hot-plate apparatus. This impairment was dose dependent, receptor specific, and not observed in rats microinjected with morphine into the caudal basolateral amygdala. A microinjection of morphine into the amygdala reduced the expression of fear responses and of naloxone-sensitive hypoalgesic responses, but did not reduce the expression of naloxone-insensitive hypoalgesic responses. The results document an involvement of opioidergic mechanisms in the amygdala in learned danger and of the amygdala in the control of opioid hypoalgesic responses. They also suggest that learned danger can activate antinociceptive mechanisms independently of the amygdala.

Extinction of within-event learning is contextually controlled and subject to renewal

Five experiments examined within-event learning in rats by inducing an appetite for one of the elements (salt) of a compound stimulus and assessing preference for the other element (almond). Almond preference was conditional upon (1) the almond flavour having been presented in compound with the salt, and (2) the assessment being conducted when the rats were out of sodium balance (Experiment 1). Presentations of the compound in one environment (A) and of the salt and almond elements in a second environment (B) resulted in greater almond preference when rats were tested in A than in B (Experiment 2). Almond preference was reduced when separate presentations of the compound and almond (Experiment 3) or of the compound and salt (Experiment 4) occurred in the same environments but not when these presentations occurred in different environments. Rats exposed to the compound in A and then extinguished to the elements in either A or B showed a reduced almond preference when tested in the extinction environment, but not when tested in the other environment (Experiment 5). Thus, extinction of within-event learning is context-specific and subject to renewal. The results were interpreted in terms of an associative model whereby separate presentations of the elements result in a symmetrical inhibitory link which is contextually gated (Bouton, 1993).

Effects of benzodiazepine microinjection into the amygdala or periaqueductal gray on the expression of conditioned fear and hypoalgesia in rats

Four experiments studied the effects of an intracranial microinjection of a benzodiazepine (midazolam) on the expression of conditioned fear (measured as passive avoidance) and conditioned hypoalgesia in rats. Unilateral microinjection of midazolam into the basolateral amygdala reduced both hypoalgesic and avoidance responses, whereas unilateral microinjection of midazolam into the ventrolateral region of the periaqueductal gray (vlPAG) reduced the hypoalgesic response but not the avoidance response. The results are discussed in terms of gamma-aminobutyric acid-ergic inhibition of antinociceptive mechanisms in the vlPAG and of the activation of these mechanisms by amygdala-based fear processes.

Effects of a microinjection of morphine into the amygdala on the acquisition and expression of conditioned fear and hypoalgesia in rats

A unilateral microinjection of morphine into the amygdala impaired the acquisition of fear and hypoalgesic responses in rats exposed to a heated floor in a hot-plate apparatus. This impairment was dose dependent, receptor specific, and not observed in rats microinjected with morphine into the caudal basolateral amygdala. A microinjection of morphine into the amygdala reduced the expression of fear responses and of naloxone-sensitive hypoalgesic responses, but did not reduce the expression of naloxone-insensitive hypoalgesic responses. The results document an involvement of opioidergic mechanisms in the amygdala in learned danger and of the amygdala in the control of opioid hypoalgesic responses. They also suggest that learned danger can activate antinociceptive mechanisms independently of the amygdala.

Effects of benzodiazepine microinjection into the amygdala or periaqueductal gray on the expression of conditioned fear and hypoalgesia in rats

Four experiments studied the effects of an intracranial microinjection of a benzodiazepine (midazolam) on the expression of conditioned fear (measured as passive avoidance) and conditioned hypoalgesia in rats. Unilateral microinjection of midazolam into the basolateral amygdala reduced both hypoalgesic and avoidance responses, whereas unilateral microinjection of midazolam into the ventrolateral region of the periaqueductal gray (vlPAG) reduced the hypoalgesic response but not the avoidance response. The results are discussed in terms of gamma-aminobutyric acid-ergic inhibition of antinociceptive mechanisms in the vlPAG and of the activation of these mechanisms by amygdala-based fear processes.

Fos expression in the spinal cord is suppressed in rats displaying conditioned hypoalgesia

Two experiments used c-fos expression as a marker of spinal nociceptive processing to study the neural correlates of hypoalgesic responses to conditioned stimuli (CSs) paired with an aversive event. Immunoreactive (ir) neuronal labeling of Fos, the nuclear protein encoded by the c-fos gene, was examined in the spinal cords of rats killed 2 hr after injection of dilute formalin into a hind paw. Compared with control rats either not conditioned or conditioned in one environment but tested elsewhere, there were significantly fewer Fos-ir neurons in the spinal cords of rats displaying hypoalgesic responses when tested in the presence of aversive CSs. Naloxone abolished hypoalgesic responses and reinstated spinal Fos expression, indicating that aversive CSs activated opioid-based antinociceptive mechanisms. The results confirm that aversive CSs produce hypoalgesia by inhibiting the transmission of ascending nociceptive information.

Stress-induced changes in respiratory quotient, energy expenditure and locomotor activity in rats: effects of midazolam

Changes in O2 consumption, CO2 production and locomotor activity were examined in rats exposed to (1) brief footshock, (2) an aversive conditioned stimulus (CS) predicting footshock, or (3) the anxiogenic drug FG-7142. Respiratory quotient (RQ = CO2 produced/O2 consumed) and energy expenditure [EE = O2 consumed (364 + 113RQ)] were derived to give an estimate of the energy substrate (fat, carbohydrate or protein) being utilised and total substrate oxidation respectively. In experiment 1, footshock (4 x 5 s 0.6 mA shocks over 2 min) produced an immediate increase in RQ, EE and activity. The RQ and EE effects were attenuated by the benzodiazepine midazolam (1 mg/kg). In experiment 2, an aversive CS, consisting of flashing light and buzzer that had 24 h earlier been repeatedly paired with foot-shock (20 x 5 s 0.6 mA shocks) caused a pronounced drop in RQ, an increase in EE and locomotor activity suppression. The effects of the aversive CS on RQ and EE were reversed by midazolam (1 mg/kg). In experiment 3, FG-7142 (10 mg/kg) produced a steep drop in RQ that persisted for at least 2 h and which was reversed by midazolam (1 mg/kg) and delayed by the benzodiazepine antagonist RO 15-1788 (10 mg/kg). FG-7142 also tended to inhibit EE and locomotor activity, but these effects did not reach statistical significance. Overall, these data show that stress causes profound alterations in RQ, EE and activity and that the pattern of change in these parameters differs with the nature of the stressor involved.

Effects of the interval between exposure to a novel environment and the occurrence of shock on the freezing responses of rats

Rats were exposed to a novel environment (E1) at time T1, given a footshock at time T2, and tested for freezing in E1 or in a second environment (E2). The function relating freezing to the T1-T2 interval among rats tested in E1 was an inverted U-shape. Rats exposed to short T1-T2 intervals displayed just as much freezing in E2 as in E1, whereas rats exposed to longer intervals froze less in E2 than in E1. These differences between the freezing responses in E1 and in E2 were obtained when the T1-T2 intervals were varied, but time spent in the shocked E1 was equated. Rats given two shocks in E1 differentiated between E1 and E2 when the initial shock occurred some time after exposure to E1, but not when the initial shock was presented shortly after that exposure. Rats shocked some time after exposure to E1 on Day 1 and shortly after exposure to that environment on Day 2 differentiated between E1 and E2 more than did rats exposed to the reverse sequence of T1-T2 intervals. The results were attributed to the formation of a network of connections among the E1 cues in rats exposed to moderate or long T1-T2 intervals, and to an impairment in the formation of this network as a result of the conditioning of a subset of cues in rats exposed to short T1-T2 intervals.