2-Deoxy-D-glucose-induced decrements in operant and reflex pain thresholds

Neurons in the caudal ventrolateral medulla mediate descending pain control

Supraspinal brain regions modify nociceptive signals in response to various stressors including stimuli that elevate pain thresholds. The medulla oblongata has previously been implicated in this type of pain control, but the neurons and molecular circuits involved have remained elusive. Here we identify catecholaminergic neurons in the caudal ventrolateral medulla that are activated by noxious stimuli in mice. Upon activation, these neurons produce bilateral feed-forward inhibition that attenuates nociceptive responses through a pathway involving the locus coeruleus and norepinephrine in the spinal cord. This pathway is sufficient to attenuate injury-induced heat allodynia and is required for counter-stimulus induced analgesia to noxious heat. Our findings define a component of the pain modulatory system that regulates nociceptive responses.

Eating is a protected behavior even in the face of persistent pain in male rats

Feeding is critical for survival. Yet, patients with chronic pain often lose their appetite and eat less. We previously showed that ad libitum fed male rats continue to feed rather than withdraw from a brief noxious stimulus. This study examined the effects of a sustained noxious stimulus on feeding by testing ad libitum fed male rats for five eating behaviors--latency to eat, time taken to eat each chip, pauses and scanning during eating, and the number of chocolate chips eaten--during the hour following a sham injection or an injection of a low (0.5%) or moderate (1.5%) dose of formalin into the hind paw. Sham-injected rats showed no pain-related behaviors, rats injected with 0.5% formalin showed very few pain-related behaviors, and rats injected with 1.5% formalin showed favoring, lifting and licking of the injured paw with a characteristic biphasic time course. Besides taking less time to commence eating during the first phase of formalin pain, rats injected with either dose of formalin did not differ from sham-injected rats on any of the other eating measures. Rats injected with 0.5% formalin showed no pain behaviors during eating, whereas those given 1.5% formalin typically ate while not exhibiting any pain behaviors but occasionally ate while favoring the paw, rarely while lifting the paw, and never while licking the paw. These results show that eating is a protected activity even in the presence of persistent pain in male rats.

Animal models of pain: progress and challenges

Many are frustrated with the lack of translational progress in the pain field, in which huge gains in basic science knowledge obtained using animal models have not led to the development of many new clinically effective compounds. A careful re-examination of animal models of pain is therefore warranted. Pain researchers now have at their disposal a much wider range of mutant animals to study, assays that more closely resemble clinical pain states, and dependent measures beyond simple reflexive withdrawal. However, the complexity of the phenomenon of pain has made it difficult to assess the true value of these advances. In addition, pain studies are importantly affected by a wide range of modulatory factors, including sex, genotype and social communication, all of which must be taken into account when using an animal model.

Differential effects of stress on escape and reflex responses to nociceptive thermal stimuli in the rat

Acute stress has been shown to increase latencies of nociceptive reflexes, and this effect is considered evidence for stress-induced analgesia. However, tests for nociception that rely on motivated operant escape assess cerebral processing of pain and could be modulated independent of reflex responses. We therefore compared the effects of an acute stressor (restraint) on escape responses and lick/guard reflexes to stimulation of the paws by a thermally regulated floor. Testing sessions included a pre-test exposure to 36 degrees C, followed by a test trial in which either escape from 44 or 36 degrees C or reflex responses to 44 degrees C were observed. Behavioral responses to stress were assessed during a three day period, with baseline testing on day 1, post-stress or control testing on day 2, and evaluation of long-term stress effects on day 3. On day 2, half the animals received 15 min of restraint stress, followed by 15-min pre-test and test trials. Licking and guarding responses to thermal stimulation during 44 degrees C test trials were significantly reduced by restraint stress, confirming previously reported stress effects on nociceptive reflexes. In contrast, learned escape responses to the same thermal stimulus were significantly enhanced after stress. The increase in operant sensitivity suggests that acute restraint, a form of psychological stress, produces hyperalgesia for a level of thermal stimulation that preferentially activates C nociceptors. These results are discussed in relation to studies involving physical or psychological forms of stress, different nociceptive stimuli, and assessment strategies used to evaluate thermal pain sensitivity.

2-Deoxy-D-glucose antinociception and serotonin receptor subtype antagonists: test-specific effects in rats

The antinociceptive actions of 2-deoxy-D-glucose (2-DG) are mediated in part by endogenous opioid, dopaminergic, cholinergic, histaminergic, and neurohormonal influences. Although 2-DG antinociception was not affected by tryptophan hydroxylase inhibition, a possible serotonergic role in 2-DG antinociception was investigated because of the existence of serotonin [5-hydroxytryptamine (5-HT)] receptor subtypes. The present study examined the effects of general (methysergide: 5 and 10 mg/kg), 5-HT2 (ritanserin: 2.5 mg/kg), and 5-HT3 (ICS-205,930: 0.25-5 mg/kg) receptor subtype antagonists upon 2-DG antinociception on the tail-flick and jump tests in rats. On the tail-flick test, 2-DG (450 mg/kg) antinociception was significantly reduced by all ICS-205,930 doses (48-58%) but unaffected by either methysergide (22-29% reduction) or ritanserin (6% reduction). In contrast, 2-DG antinociception on the jump test was significantly potentiated across the 120-min time course and across the 2-DG dose-response curve (100-650 mg/kg) by methysergide, ritanserin, and ICS-205,930 pretreatment. Each of the three antagonists produced significant leftward shifts in the peak and total 2-DG dose-response curve for the jump test. These data suggest different sites of action for 2-DG antinociception as a function of the pain test employed and a differential modulation by serotonin receptor subtypes at those sites.

Potentiation of 2-deoxy-D-glucose antinociception, but not hyperphagia by zolantidine, a Histamine (H2) receptor antagonist

Antagonism of the histamine (H2) receptor reduces antinociception induced by naloxone-resistant foot-shock, naloxone-sensitive foot-shock, and morphine with a rank-order potency similar to their H2 antagonism. The antimetabolic glucose analog 2-deoxy-D-glucose (2DG) produces antinociceptive and hyperphagic responses that dissociate from each other and are in part mediated by opioid systems. The present study determined the effects of the brain-penetrating H2 receptor antagonist zolantidine (ZOL) on 2DG antinociception on the tail-flick and jump tests, as well as on 2DG hyperphagia, in rats. ZOL (0.01-1 mg/kg) potentiated the antinociceptive responses induced by a moderate (450 mg/kg) dose of 2DG, but had lesser effects upon antinociception induced by a lower (100 mg/kg) 2DG dose. ZOL itself slightly increased jump thresholds, but not tail-flick latencies. Combinations of ZOL and 2DG produced supraadditive antinociception, even though ZOL failed to significantly shift the 2DG dose-response curve to the left. In contrast, ZOL failed to alter basal intake or 2DG hyperphagia, supporting previous evidence implicating the H1 but not the H2 receptor in these effects. These results further dissociate the antinociceptive and hyperphagic effects of 2DG, and also support previous results indicating both pro- and antinociceptive roles for H2 receptors.

Swim-stress-induced antinociception in young rats

1. Opioid and non-opioid mechanisms have been implicated in the phenomenon of stress-induced antinociception in adult rodents. We have studied stress-induced antinociception in developing rats and characterized differences in the neurochemical basis of this effect in pre- and post-weanling animals. 2. Twenty and 25 day old rats were stressed using warm water (20 degrees C) swimming for 3 or 10 min periods and antinociception was assessed by the tail immersion test (50 degrees C). 3. A 3 min swim in 20 and 25 day old rats produced marked antinociception which was blocked by naloxone, Mr 1452, 16-methyl cyprenorphine and levallorphan but not Mr 1453 or N-methyl levallorphan. The delta-opioid receptor antagonist ICI 174,864 attenuated stress-induced antinociception in 25 day old rats but was without effect in 20 day old animals. 4. A 10 min swim in 25 day old rats produced antinociception which was non-opioid in nature. In contrast, antinociception was not observed in 20 day old rats after a 10 min swim-stress. 5. Pretreatment of animals with dexamethasone blocked 3 min swim-stress antinociception in 20 and 25 day old animals but had no effect on antinociception induced by a 10 min swim. 6. Swim-stress-induced antinociception can be observed in young rats and dissociated into opioid and non-opioid types dependent on the duration of swimming stress. The non-opioid type appears to develop more slowly and cannot be observed in preweanling rats. The opioid type is a predominantly mu-receptor phenomenon in preweanling animals but delta-receptor components are observable in postweanling rats.

2-deoxy-D-glucose modulation of T-lymphocyte reactivity: differential effects on lymphoid compartments

This study was designed to evaluate the effect of glucoprivation, as induced by 2-deoxy-D-glucose (2-DG) administration, on lymphocyte mitogen reactivity in Sprague-Dawley rats. The results showed that a single injection of 2-DG decreased reactivity in both whole-blood and spleen lymphocytes, as determined by mitogenic stimulation to concanavalin A (Con A) and phytohemagglutinin (PHA). However, the suppressed reactivity for the spleen lymphocytes attenuated with repeated injections, but the whole-blood lymphocytes did not show attenuation. Mitogen assessments of lymphocytes obtained from the thymus indicated that a single injection did not induce suppressed reactivity, but repeated injections induced a pronounced suppression of responsiveness. Furthermore, mitogen assessments of mesentery lymph nodes did not show any effect of 2-DG injections. These results corroborate other findings using electric shock as the stressor, namely that different compartments of the immune system are differentially affected by a stressor.

Intracerebroventricular alloxan reduces 2-deoxy-D-glucose analgesia

2-Deoxy-D-glucose (2DG) analgesia, mediated in part by endogenous opiate and hypothalamo-hypophysial systems is presumably activated by its stress-related properties. Recently 2DG hyperphagia, but not 2DG hyperglycemia was reduced by central pretreatment with the pancreatic beta-cell toxin, alloxan; this deficit was eliminated by co-administration of 3M D-glucose. The present experiment examined whether intracerebroventricular pretreatment with alloxan (40 or 200 micrograms) altered 2DG analgesia (400 or 700 mg/kg, IP) on the tail-flick and jump tests, and whether 3M D-glucose co-administration ameliorated any deficits. Both alloxan doses significantly reduced 2DG analgesia (400 mg/kg) on both tests. 2DG analgesia (700 mg/kg) was significantly reduced by both alloxan doses on the jump test, but only by the higher alloxan pretreatment on the tail-flick test. 3M D-glucose co-administration ameliorated alloxan-induced analgesic deficits more effectively at the lower 2DG dose. Neither alloxan nor alloxan/3M D-glucose treatments altered basal thresholds. These data pertain both to alloxan's effects upon coding of 2DG effects as stressful, and to the role of diabetes and/or central glucoreceptors in analgesic processes.

Effects of chronic amitriptyline and desipramine on food intake and body weight in rats

Long-term treatment with tricyclic antidepressant drugs (TCAs) can induce excessive body weight gain in a significant proportion of patients. Such weight gains, which appear to be largely independent of clinical improvement, are in many cases severe enough to interfere with continuation of treatment. In efforts to model this effect in experimental animals, seven experiments were performed in which two commonly used TCAs, amitriptyline and desipramine, were administered chronically to rats. Despite manipulations of drug dosages (2.5 mg-17 mg/kg), route of administration (intraperitoneal, subcutaneous, oral; daily injections vs. continuous release from osmotic pumps), diet composition and palatability (regular Purina Chow pellets or powder with or without added high fat and high carbohydrate sources; high vs. low protein diets) and animal sex and housing conditions (single vs. group housing), chronic TCA treatment was never observed to increase daily food intake or rates of body weight gain. Desipramine treatment invariably caused decreased food intake and weight loss. Amitriptyline treatment either caused no change in food intake and body weight or slightly reduced levels in comparison to vehicle-treated controls. However, both amitriptyline- and desipramine-treated rats showed a potentiation of acute caloric intake after a single systemic injection of the glucoprivic agent 2-deoxy-D-glucose. These results are considered against the background of human clinical observations. Possible reasons for the differences between human and animal data are discussed.

Effects of muscarinic receptor antagonism upon two forms of stress-induced analgesia

The present study assessed in rats the effects of muscarinic receptor antagonism upon analgesia induced by cold-water swims (CWS: 2 degrees C for 3.5 min) and 2-deoxy-D-glucose (2DG: 600 mg/kg). First, CWS analgesia was significantly reduced 30 min after the swim by scopolamine (0.01 and 0.1 mg/kg) and methylscopolamine (10 mg/kg) pretreatment, and was eliminated 60 min after the swim by scopolamine (0.01-10 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment. In contrast, scopolamine potentiated CWS hypothermia. Second, while scopolamine (1 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment prolonged 2DG analgesia, both antagonists dose-dependently reduced 2DG hyperphagia. Third, the changes in analgesic and hypothermic stress responses were not due to baseline shifts in jump thresholds or body temperatures. However the dose-dependent reductions by scopolamine and methylscopolamine in baseline food intake and 2DG hyperphagia were significantly correlated. Fourth, the dose-dependent reduction by scopolamine and methylscopolamine of pilocarpine analgesia differed in pattern from the other analgesic effects, suggesting heterogeneity in muscarinic receptor modulation of different analgesic responses.

Opioid and non-opioid stress analgesia from cold water swim: importance of stress severity

We have previously reported that stress severity plays an important role in determining the neurochemical basis of stress-induced analgesia from inescapable footshock. Increasing severity (duration or intensity of continuous footshock) causes a shift in mediation of the resultant analgesia from opioid to non-opioid. In this study, we find that stress severity plays a similar role in analgesia from cold water swim. More severe swims (longer duration or lower water temperature) produce stress analgesia insensitive to the opiate antagonist, naltrexone, whereas less severe swims produce analgesia significantly attenuated by this drug.

Dissociation of analgesic and hyperphagic responses following 2-deoxy-D-glucose

Injection of 2-deoxy-D-glucose (2DG) elicits both analgesic and hyperphagic responses in rats. While pituitary dysfunction, decreased dopamine availability, or neonatal monosodium glutamate treatment decreases 2DG hyperphagia, they increase 2DG analgesia. In contrast, 2-DG analgesia alone is decreased by repeated 2-DG injections, while 2-DG hyperphagia alone is decreased following naloxone pretreatment. The present four experiments examined further mechanisms subserving these two induced responses. In the first experiment, rats were deprived of food for 6 h following 2-DG (600 mg/kg). While 2-DG hyperphagia persisted in the absence of glucoprivation, 2-DG analgesia failed to occur after this delay. In the second experiment, acute exposure to inescapable foot shock (4 mA, 0.5 s/5 s for 1 h) preceded administration of 2-DG (600 mg/kg). While 2-DG hyperphagia was eliminated by this procedure, 2-DG analgesia was significantly potentiated. In the third experiment, repeated morphine (10 mg/kg) injections over 14 days eliminated 2-DG analgesia on the fifteenth day, but failed to affect 2-DG hyperphagia. In the fourth experiment, lesions placed in either the lateral hypothalamus or zona incerta decreased 2-DG hyperphagia, but failed to affect 2-DG analgesia. These results are discussed in terms of common and dissociative mechanisms mediating both responses.

Opioid modulation of appetite

The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.

Inhibition of acute feeding responses to systemic 2-deoxyglucose or insulin in rats pretreated with the GABA-transaminase blocker ethanolamine-O-sulfate (EOS)

Acute feeding responses to 2-deoxyglucose (750 mg/kg) or insulin (12 U/kg) were examined 24 hr after intracisternal injection of the GABA-transaminase inhibitor ethanolamine-O-sulfate (EOS, 400 micrograms) in female rats. EOS pretreatment completely abolished acute feeding responses to both challenges. These findings complement recent research showing that central EOS can reverse chronic overeating in several experimental preparations. The present results are consistent with previous indications that EOS treatment may induce a metabolic shift away from brain glucose utilization, thus making glucoprivation irrelevant as a metabolic challenge. An alternative possibility is that EOS-induced increases of brain GABA may offset specific neural mechanisms through which these glucoprivic agents normally induce feeding.

Neuroleptic and analgesic interactions upon pain and activity measures

Previous data in rats indicate that while dopamine receptor blockers like haloperidol (HAL) potentiate opiate analgesia, dopamine receptor stimulants like apomorphine reduce cold-water swim (CWS) and 2-deoxy-D-glucose (2-DG) analgesia. Yet recently, HAL and chlorpromazine (CBZ) have been shown to reduce heat and immobilization analgesia. To address these differences, the present study investigated whether HAL (10, 50, 100 microgram/kg) or CPZ (1, 3, 5 mg/kg) would potentiate or reduce the effects of morphine (MOR), CWS, 2-DG and chlordiazepoxide (CDP) upon analgesia and activity. While HAL increased jump thresholds in a dose-dependent manner, CPZ doses exerted erratic effects. MOR analgesia was potentiated by the two higher CPZ doses and by the highest HAL dose. 2-DG analgesia was potentiated by only the highest HAL dose while CDP analgesia was potentiated by the moderate CPZ dose. While all CPZ doses potentiated CWS-induced increases in jump thresholds, the lowest HAL dose reduced this effect. These effects are considered in terms of the analgesic manipulation and its magnitude of effect, the neuroleptic and its dose, the pain test, and possible concurrent effects upon activity.

Stereoselective effects of opiate agonists and antagonists on ingestive behavior in rats

In male Sprague Dawley rats, the (-)-isomer of the opiate antagonist GPA 1843 (beta-9-methyl-5-phenyl-2-allyl-2'-hydroxy-6, 7-benzomorphan) produced dose-related decreases in nocturnal feeding and of hyperphagias induced by 2-deoxy-D-glucose (2-DG; 400 mg/kg) and 24 hr food deprivation (FD). The hyperphagia induced by insulin (10 U/kg) was not significantly decreased by GPA 1843. In contrast, comparable doses of the (+)-stereoisomer, GPA 1847, had no effect on nocturnal, 2-DG or FD hyperphagia. In addition, hyperphagia and hyperdipsia were observed following administration of the opiate agonist levorphanol, but not its stereoisomer, dextrorphan. Thus, the effects of these agents on consummatory behavior are mediated by a stereospecific interaction with opiate receptors, which further indicates that endogenous opiate peptides are involved in the expression of these opiate-related hyperphagias.

Analgesia following intraventricular administration of 2-deoxy-D-glucose

The glucose analogue, 2-deoxy-D-glucose (2-DG) elicits hyperphagic and analgesic responses in rats. The former response appears to be mediated by central processes since overeating is elicited following intraventricular administration of 2-DG at low (3.5 and 5.0 mg/kg) doses. The present study examined whether flinch-jump threshold would increase 30, 90 and 180 min following intraventricular injections of 2-DG at low (3.5, 5.0 and 10.0 mg/kg) doses and compared these effects with systemically-applied 2-DG doses of 350 and 500 mg/kg. Intraventricular 2-DG administration increased jump thresholds for up to 180 min across all test doses. Flinch thresholds were also increased, but in a manner dissociated from jump thresholds. Animals with cannulae located near, but not in the lateral ventricle, displayed delayed analgesic effects. The magnitude of intraventricular 2-DG analgesia was not at potent as the 100-fold higher systemic injections. It appears that central mechanisms mediated intraventricular 2-DG analgesia at the low dose range since higher, systemic 2-DG doses have previously failed to increase flinch-jump thresholds.

Reversal of stress-induced analgesia by apomorphine, but not by amphetamine

Acute exposure to severe stressors induce profound analgesia as well as depleting catecholamine levels. The present study examined whether d-amphetamine and apomorphine, agents which increase catecholamine availability, would alter the analgesic effectiveness of cold-water swims (CWS) and 2-deoxy-D-glucose (2-DG) as measured by an operant liminal escape procedure. Two groups of 10 rats each were tested to determine alterations in liminal escape threshold functions following amphetamine at doses of 0.25, 0.5, 1, 2 mg/kg and following apomorphine at doses of 0.025, 0.05, 0.1, 0.2 mg/kg. Half of the amphetamine and half of the apomorphine groups were tested across their respective dose ranges for the drug effects upon CWS analgesia. The remaining animals in each group received 2-DG (600 mg/kg IP) alone followed by 2-DG paired with each stimulant dose. No dose of amphetamine or apomorphine alone altered escape thresholds. While amphetamine produced slight potentiations of 2-DG analgesia at the two low doses, apomorphine at the 0.05 and 0.1 mg/kg doses returned CWS and 2-DG analgesia to within normal placebo values. These results provide indirect evidence for a role for brain norepinephrine and dopamine in stress-induced analgesia, and these data are discussed with respect to catecholamine involvement in pain-inhibitory processes.

The role of endorphins in animal learning and behavior

The present review examined the influence of endorphins in animal learning and behavior. It was suggested that in learning paradigms involving stress, the stressor elicits the release of endorphins. Given the evidence on endorphin-mediated, stress-induced analgesia, it was further suggested that the stress-induced release of endorphins modulates the aversiveness of the stressor, and as such, affects the learning based on this stressor. A number of learning paradigms, e.g., the conditioned emotional response, preference for signaled shock, conditioned taste aversions, and learned helplessness, were presented in support of this mediation of learning by the endorphins. A possible interaction between the endorphins and adrenocorticotropic hormone was offered as a physiological basis for this mediation.

Stress-induced analgesia: neural and hormonal determinants

Extensive evidence has indicated that distinct neural systems specifically designed to inhibit sensitivity to painful stimuli exist. Recent advances suggest that the endorphins, enkephalins and the opiate receptor interact with a descending serotonergic bulbospinal system to mediate the analgesic responses to opiates and electrical stimulation. In assessing the evolutionary and behavioral significance of this pain-inhibitory system, several laboratories discovered that acute exposure to a wide variety of stressful events results in a transient analgesia. Chronic exposure to a number of these stressors results in adaptation of the analgesic response. The purpose of this review is to identify and characterize the mechanisms by which these stressors activate pain-inhibition. The relationship of stress-induced analgesia to each of the following is reviewed: (a) the role of endorphins, enkephalins and the opiate receptor; (b) the role of the descending serotonergic bulbospinal system; (c) the role of the pituitary gland; and (d) the role of hypothalamic mechanisms. Data will be discussed in terms of "opiate" and "non-opiate" pain-inhibitory mechanisms, in which some stressors act through the former and other stressors act through the latter.

2-Deoxy-D-glucose analgesia: influences of opiate and non-opiate factors

Acute administration of 2-deoxy-D-glucose (2-DG), an antimetabolic glucose analogue induces a powerful analgesia which adapts following repeated administration. 2-DG analgesia displays significant cross-tolerance with morphine, and like morphine analgesia, is potentiated in hypophysectomized rats. The present study examined further the role of opiates in 2-DG analgesia by examining whether the opiate antagonist, naloxone, would affect 2-DG analgesia, and whether ineffective doses of 2-DG and morphine would interact in a synergistic fashion to induce analgesia. Nociceptive thresholds were measured by the flinch-jump test. Naloxone doses of 1, 5, 10 and 20 mg/kg were all ineffective in reducing significantly 2-DG (600 mg/kg) induced pain threshold elevations. Naloxone failed to attenuate 2-DG (350 mg/kg) analgesia whether administered before or after the 2-DG injection. On the other hand, simultaneous administration of sub-analgesic doses of 2-DG (200 mg/kg) and morphine (2.5 mg/kg) summated to produce significant analgesia. This, 2-DG analgesia is similar to opiates in its tolerant and summative actions, yet dissimilar in that naloxone is ineffective in reversing its effects.

Differential effects of hypophysectomy upon analgesia induced by two glucoprivic stressors and morphine

Pain threshold elevations induced in rats following acute exposure to stressful cold-water swims and to inescapable foot shocks are significantly attenuated by hypophysectomy. The present study investigated the effects of hypophysectomy upon the dose-dependent and time-dependent analgesia induced by morphine and by the glucoprivic agents, 2-deoxy-D-glucose (2-DG) and insulin. Two reflex pain tests, the tail-pinch and the flinch-jump were employed. In normal rats, insulin induced prolonged (180 min) analgesia at doses of 16 U/kg on the tail-pinch test and 256 U/kg on the flinch-jump test. However, the same agents induced small and brief pain threshold elevations in hypophysectomized animals. By contrast, though 2-DG increased both measures in both groups, its effects were more marked in hypophysectomized rats. Hypophysectomized rats also exhibited a potentiated analgesic effect on both tests following high doses of morphine. On the other hand, low doses of morphine transiently increased tail-pinch thresholds in normal, but not hypophysectomized subjects. These data provide further evidence of multiple pain-inhibitory mechanisms in which the pituitary plays a complex, but integral part.

Analgesia induced by cold-water stress: attenuation following hypophysectomy

In addition to the well-known activation of the pituitary-adrenal axis, acute exposure to severe stressors includes a temporary analgesia in rats. Thus, the present study investigates whether the pituitary was involved in the mediation of analgesia induced by severe cold-water swim (CWS) stress. Flinch-jump thresholds were measured 30 min following 3.5-min swims in water temperatures ranging from 2-35 degrees C. Compared with untreated normal rats, hypophysectomized rats, receiving corticosterone and thyroxin, displayed significantly less CWS-induced analgesia, while similarly-supplemented normal rats exhibited significantly more CWS-induced analgesia. In a second experiment, operant liminal escape pain thresholds were determined following acute and chronic CWS. Whereas normal rats exhibited profound analgesia following the initial swims, the hypophysectomized rats never displayed any CWS-induced operant escape shifts. Stress-induced alterations in general activity levels and/or thermoregulation were shown to be unrelated to the diminished effectiveness of CNS to produce analgesia in hypophysectomized rats. These data imply that the pituitary is involved in the mediation of CWS-induced analgesia.