Conditioned and unconditioned stress-induced analgesia: Stimulus preexposure and stimulus change

Analysis of memory modulation by conditioned stimuli

Conditioned stimuli (CS) have multiple psychological functions that can potentially contribute to their effect on memory formation. It is generally believed that CS-induced memory modulation is primarily due to conditioned emotional responses, however, well-learned CSs not only generate the appropriate behavioral and physiological reactions required to best respond to an upcoming unconditioned stimulus (US), but they also serve as signals that the US is about to occur. Therefore, it is possible that CSs can impact memory consolidation even when their ability to elicit conditioned emotional arousal is significantly reduced. To test this, male Sprague–Dawley rats trained on a signaled active avoidance task were divided into “Avoider” and “Non-Avoider” subgroups on the basis of percentage avoidance after 6 d of training. Subgroup differences in responding to the CS complex were maintained during a test carried out in the absence of the US. Moreover, the subgroups displayed significant differences in stress-induced analgesia (hot-plate test) immediately after this test, suggesting significant subgroup differences in conditioned emotionality. Importantly, using the spontaneous object recognition task, it was found that immediate post-sample exposure to the avoidance CS complex had a similar enhancing effect on object memory in the two subgroups. Therefore, to our knowledge, this is the first study to demonstrate that a significant conditioned emotional response is not necessary for the action of a predictive CS on modulation of memory consolidation.

Classical conditioning and pain: conditioned analgesia and hyperalgesia

This article reviews situations in which stimuli produce an increase or a decrease in nociceptive responses through basic associative processes and provides an associative account of such changes. Specifically, the literature suggests that cues associated with stress can produce conditioned analgesia or conditioned hyperalgesia, depending on the properties of the conditioned stimulus (e.g., contextual cues and audiovisual cues vs. gustatory and olfactory cues, respectively) and the proprieties of the unconditioned stimulus (e.g., appetitive, aversive, or analgesic, respectively). When such cues are associated with reducers of exogenous pain (e.g., opiates), they typically increase sensitivity to pain. Overall, the evidence concerning conditioned stress-induced analgesia, conditioned hyperalagesia, conditioned tolerance to morphine, and conditioned reduction of morphine analgesia suggests that selective associations between stimuli underlie changes in pain sensitivity.

Pavlovian Conditioning: A Functional Perspective

From a functional perspective, Pavlovian conditioning involves learning about conditioned stimuli (CSs) that have a pre-existing relation to an unconditioned stimulus (US) rather than learning about arbitrary or neutral CSs. In addition, the most important product of learning involves changes in how the organism responds to the US, not in how it responds to the CS, because the US is the more biologically relevant stimulus. These concepts are illustrated using examples from a variety of behavioral and physiological situations including caloric intake and digestion, breast feeding, poison-avoidance learning, eyeblink conditioning, sexual conditioning, fear conditioning, aggression, and drug tolerance and sensitization.

Effects of naltrexone and cross-tolerance to morphine in a learned helplessness paradigm

Opiates have been implicated in learned helplessness (LH), a phenomenon known to be related to opiate stress-induced analgesia (SIA). In the present study, we investigated the role of opiates in the induction of LH and SIA under different conditions. Adult female Wistar rats were trained either by receiving 60 inescapable 1-mA footshocks (IS group, N = 114) or by confinement in the shock box (control or NS group, N = 92). The pain threshold of some of the animals was immediately evaluated in a tail-flick test while the rest were used 24 h later in a shuttle box experiment to examine their escape performance. The opiate antagonist naltrexone (0 or 8 mg/kg, ip) and the previous induction of cross-tolerance to morphine by the chronic administration of morphine (0 or 10 mg/kg, sc, for 13 days) were used to identify opiate involvement. Analysis of variance revealed that only animals in the IS group demonstrated antinociception and an escape deficit, both of which were resistant to the procedures applied before the training session. However, the escape deficit could be reversed if the treatments were given before the test session. We conclude that, under our conditions, induction of the LH deficit in escape performance is not opiate-mediated although its expression is opiate-modulated.

Enhancements in swim stress-induced hypothermia, but not analgesia, following amygdala lesions in rats

Lesions placed in the rat amygdala significantly reduce analgesic responses induced either by conditioning or exposure to a cat. Such lesions have alternatively reduced or failed to affect unconditioned foot shock analgesia. The present study expanded the situational determinants by examining whether lesions placed in the amygdala altered analgesia or hypothermia elicited by exposure to either continuous (CCWS) or intermittent (ICWS) cold-water swims. Lesion extent included the central, medial cortico-medial, baso-lateral, baso-medial and lateral amygdaloid nuclei. Basal jump thresholds, but not core body temperatures were significantly increased by unilateral and bilateral amygdala lesions. In contrast, the hypothermic, but not the analgesic responses following CCWS and ICWS were significantly enhanced by unilateral and bilateral amygdala lesions. These data support a hypothesis suggesting that these lesions are effective in reducing those classes of analgesic responses related to the signals of stressors than to the stressors themselves.

A permissive role of corticosterone in an opioid form of stress-induced analgesia: blockade of opiate analgesia is not due to stress-induced hormone release

The 100 inescapable tail-shock paradigm produces three sequential analgesic states as the number of shocks increases: an early opioid analgesia (after 2 shocks) that is attenuated by systemic naltrexone, a middle analgesia (after 5-40 shocks) that is unaffected by systemic naltrexone, and a late opioid analgesia (after 80-100 shocks) that is attenuated by systemic naltrexone. In order to determine whether the absence of adrenal hormones would affect any of these analgesias, we tested adrenalectomized (ADX) versus sham-operated control rats 2 weeks post-surgery. Pain threshold was assessed using the tail-flick (TF) test. ADX attenuated both the early (2 shock) and late (80-100 shock) opiate analgesias and failed to reduce the naltrexone-insensitive analgesia after 5-40 shocks. We demonstrated that a loss of adrenomedullary catecholamines does not underlie the ADX-induced attenuation of opioid analgesia since sympathetic blockade using systemic chlorisondamine (6 mg/kg) failed to reduce analgesia at any point in the shock session. It was further shown that stress levels of adrenal hormones are not critical since (a) analgesia was unaffected when animals were tested 48 h after ADX, (b) 2 shocks do not produce a surge in corticosterone (CORT) over and above levels observed in animals restrained and TF tested in preparation for shock, and (c) basal CORT replacement in drinking water fully restored analgesia in ADX rats. These experiments demonstrate that basal CORT, rather than adrenomedullary substances, is critical to the expression of analgesia. The function of CORT here is not linked to a shock-induced surge of the steroid. CORT appears to play a permissive role in the expression of analgesia. Potential effects of the absence of corticosteroids on neurotransmitter biosynthesis important in analgesia production are discussed.

Latent inhibition, overshadowing, and blocking of a conditioned antinociceptive response in spinalized rats

Prior research has shown that a conditioned antinociceptive response can be established in spinalized rats by pairing stimulation to one hind leg (the conditioned stimulus, or CS) with tailshock (the unconditioned stimulus, or US). This suggests that spinal mechanisms can support classical conditioning. It is well known that in intact subjects, classical conditioning is undermined by preexposure to the CS (latent inhibition) or the concurrent presentation of either a more salient CS (overshadowing) or one that has already been associated with the US (blocking). In the present paper we show that these manipulations have a similar impact on the acquisition of a conditioned antinociceptive response in spinalized rats. These findings imply that similar principles may govern the acquisition of a conditioned response across different levels of the nervous system.

The nature of conditioned anti-analgesia: spinal cord opiate and anti-opiate neurochemistry

The central nervous system contains circuitry that inhibits pain sensitivity (analgesia), as well as circuitry that opposes pain inhibition (anti-analgesia). Activation of analgesia systems and anti-analgesia systems can each be brought under environmental control using classical conditioning procedures. Analgesia can be produced by cues present before and during aversive events such as electric shock, while active inhibition of analgesia comes to be produced by cues never present immediately before or during shock and therefore signal safety. We have recently reported that these analgesia and anti-analgesia systems interact at the level of the spinal cord. A series of 3 experiments were performed to examine how such interactions occur. First, potential opioid mediation of conditioned analgesia was investigated using systemic and intrathecal (i.t.) delivery of opiate antagonists. Conditioned analgesia was found to be mediated by activation of spinal mu and delta opiate receptors. Second, analgesia produced by each of these receptor subtypes was challenged by environmental signals for safety. Analgesias produced by mu and delta opiate agonists were each abolished by safety signals. Third, antagonists/antisera directed against several putative anti-opiate neurotransmitters were tested i.t. to identify which mediate conditioned anti-analgesia at the level of the spinal cord. A cholecystokinin antagonist abolished conditioned anti-analgesia. In contrast, neuropeptide FF antiserum and a kappa opiate antagonist were without effect.

Learned helplessness inducing foot shock can exacerbate morphine responsiveness

Exposure to inescapable tail shock or foot shock has been shown to produce effects on a number of learning tasks. Tail-shock exposure is also known to influence nociception and morphine reactivity. The present series of experiments investigated the effects of foot shock known to induce learned helplessness effects in our laboratory on the subsequent reactivity to morphine. A first set of experiments investigated the hypoalgesic response to a 4 mg/kg dose morphine over 4 consecutive days following exposure to foot shock. Experiment 1A did not reveal an effect of foot shock on morphine-induced hypoalgesia when testing was conducted in a novel context. In Experiment 1B, we observed an increased hypoalgesic response to morphine when testing was conducted in the shock context. The findings of Experiment 1B were replicated in Experiment 2 and extended to assess the contribution of conditioned fear hypoalgesia to these effects. The possible mechanisms responsible for these findings are discussed.

Cognitive aspects of classical conditioning

Cognitive processes have been increasingly implicated in Pavlovian conditioning. Research in the past year has focused on questions of stimulus selection and the internal representation of events and the relations between them. Recent data support negative feedback models of selection that assume conditioning-dependent changes in processing of conditioned and unconditioned stimulus events, and suggest potential neural mechanisms that may underlie these processes. New models of conditioning propose a more detailed representation of individual conditioning episodes than traditionally assumed. The results of investigations into conditional discrimination learning imply a hierarchical organization of event representations, and illustrate the importance of conditioned modulatory processes as distinct from response elicitation.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Delta opiate receptors mediate tail-shock induced antinociception at supraspinal levels

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The preceding companion paper and the present work were aimed at identifying the neuroanatomical loci at which opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 8 experiments included in the present paper examined the effect of microinjecting either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist) either into the third ventricle or over the frontal cortex. Taken together, these experiments demonstrate that the late (80-100 shock) opiate analgesia is mediated by delta opiate receptors located within subcortical structures rostral to the 4th ventricle. No evidence for supraspinal opiate involvement in the early (2 shock) opiate analgesia was found.

Kappa opiate receptors mediate tail-shock induced antinociception at spinal levels

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The present work and the following companion paper were aimed at identifying the neuroanatomical loci at which endogenous opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 3 experiments included in the present paper focus on the role of spinal opiates in tail-shock induced analgesia. The first experiment demonstrates that the tail-shock parameters used do not directly activate pain suppressive circuitry within the spinal cord, but rather activate centrifugal pain modulation circuitry originating within the brain. The last two experiments examine the effect of intrathecal microinjection of either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist). Taken together, these latter 2 experiments demonstrate that both the early (after 2 shocks) and late (after 80-100 shocks) opiate analgesias are mediated by kappa opiate receptors within the spinal cord.