The role of brain and spinal cord norepinephrine in autoanalgesia

Classical conditioning: The new hegemony

Converging data from different disciplines are showing the role of classical conditioning processes in the elaboration of human and animal behavior to be larger than previously supposed. Restricted views of classically conditioned responses as merely secretory, reflexive, or emotional are giving way to a broader conception that includes problem-solving, and other rule-governed behavior thought to be the exclusive province of either operant conditiońing or cognitive psychology. These new views have been accompanied by changes in the way conditioning is conducted and evaluated. Data from a number of seemingly unrelated phenomena such as relapse to drug abuse by postaddicts, the placebo effect, and the immune response appear to involve classical conditioning processes. Classical conditioning, moreover, has been found to occur in simpler and simpler organisms and recently even demonstrated in brain slices and in utero. This target article will integrate the several research areas that have used the classical conditioning process as an explanatory model; it will challenge teleological interpretations of the classically conditioned CR and offer some basic principles for testing conditioning in diverse areas.

Suckling-induced activation of neural c-fos expression at lower thoracic rat spinal cord segments

Suckling stimulation is essential for neuroendocrine and sympathetic reflex activation during lactation. In the present study, the induction of c-fos gene expression was used to identify neuronal populations in the spinal cord activated by acute 5 min suckling or by electrical stimulation of the central stump of the first abdominal mammary nerve in lactating rats previously separated from their litters for 6 or 18 h. In addition, to investigate whether spinal sympathetic preganglionic neurons are activated by suckling, dual immunostaining (Fos and choline acetyltransferase) was performed. Fos was expressed at low levels in continuously suckled and 6 h nonsuckled mothers, but no expression was found after 18 h of nonsuckling. On the other hand, in 6 h nonsuckled rats, significant increments in Fos expression occurred in several regions after acute suckling and after electrical stimulation. Also, the pattern of Fos expression in each spinal laminae was different for the two stimuli, i.e. more intense effects of suckling in deep laminae V-X and more intense effects in laminae I-IV with electrical stimulation. Double-labeling after suckling was found only in sympathetic preganglionic neurons from the intermedio-medial cell column, whereas after electrical stimulation, double label was observed only in neurons from the intermedio-lateral cell column. On the other hand, no effect upon Fos protein expression was observed after suckling and only a minor effect after electrical stimulation of mammary nerve in 18 h nonsuckled rats. These results are consistent with previous findings on the sympathetic reflex regulation of the mammary gland, as well as on the importance of the nonsuckling interval for optimal functioning of lactation.

Effects of neurotropin on hyperalgesia and allodynia in mononeuropathic rats

Neurotropin is commonly used in Japan for the treatment of chronic pain. Using a rat model, we evaluated the effect of neurotropin on a unilateral peripheral mononeuropathy produced by placing loose ligatures around the sciatic nerve. The effect of neurotropin upon the resultant hyperalgesia and mechanical allodynia was assessed using the Ugo Basile Plantar test and von Frey hairs test, respectively. Neurotropin reduced thermal hyperalgesia and produced an early recovery from hyperalgesia in a dose-dependent manner. No significant reduction in mechanical allodynia, however, was noted under the tested condition. A possibility of differential drug sensitivity for thermal hyperalgesia and mechanical allodynia was indicated in this model, although the reason still remain elusive.

Identification of a sex-specific quantitative trait locus mediating nonopioid stress-induced analgesia in female mice

It is increasingly appreciated that the sexes differ in their perception of noxious stimuli and in their responsivity to exogenous and endogenous analgesic manipulations. We previously reported the existence of qualitative sex differences in the neurochemical mediation of nonopioid (i.e., naloxone-insensitive) stress-induced analgesia (SIA) produced by forced swims and suggested that female mice possess a sex-specific SIA mechanism. This female-specific system is now known to be estrogen-dependent, to be ontogenetically organized, and to vary with reproductive status; however, its neurochemical identity remains obscure. In an attempt to identify candidate genes underlying SIA in both sexes, we performed a two-phase quantitative trait locus (QTL) mapping experiment using the BXD/Ty recombinant inbred (RI) set derived from DBA/2J (D2) and C57BL/6J (B6) inbred mouse strains and (B6xD2)F2 hybrid mice derived from these same progenitors. All mice were subjected to 3 min forced swims in 15 degrees C water; nociceptive sensitivity on the 54 degrees C hot-plate assay was assessed immediately before and 2 min after cessation of the swim. We report the localization of a QTL statistically associated with SIA magnitude [p = 0.00000012; logarithm of the odds (LOD) = 6.1] in female mice only. This female-specific QTL, which we name Fsia1, is located on chromosome 8 at 52-84 cM from the centromere and accounts for 17-26% of the overall trait variance in this sex. The present data provide further evidence of the existence of a female-specific SIA mechanism and highlight the important role of both genetic background and gender in the inhibition of pain.

Hypnosis treatment of clinical pain: understanding why hypnosis is useful

Clinical and experimental research literature indicates hypnosis is very useful for severe and persistent pain, yet reviews suggest hypnosis is not widely used. To encourage more widespread clinical application, the author reviews recent controlled clinical studies in which hypnosis compares favorably with other interventions; links advances in understanding endogenous pain modulation to a neurophysiologic view of hypnosis and hypnoanalgesia; relates the neurophysiology of hypnoanalgesia to management of chronic pain; challenges the view that hypnotic pain control is only for the highly hypnotizable patient; and raises issues about how people learn to control pain with hypnosis. Training in hypnotic analgesia may usefully enhance nervous system inhibitory processes that attenuate pain.

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.

Transcranial electrostimulation effects on rat opioid and neurotransmitter levels

A specific form of Transcranial Electrostimulation Treatment (TCET) has been shown to induce analgesia, alleviate symptoms of opiate withdrawal and alter nociceptive responses in neurons in the midbrain and hypothalamus of rats. TCET consists of a 10Hz, charge balanced, 10 mu A current passed for 30 minutes between electrodes placed in the ears. Both serotonin (5HT) and endogenous opioids have been strongly implicated in TCET responses. This study directly measured brain levels of several neurotransmitters and their metabolites in anesthetized rats stimulated with either 10 mu A TCET or 0 mu A (Sham). Neurotransmitters measured in selected homogenized brain areas by high performance liquid chromatography were 5HT and its metabolite, 5-hydroxyindolacetic acid (5HIAA); norepinephrine (NE) and its metabolite, 3-methoxy-4-hydroxyphenethyleneglycol (MHPG); and dopamine (DA). Levels of NE and DA were significantly higher in the hypothalamic region of TCET rats than of control rats. The midbrains of TCET rats contained significantly elevated levels of DA, MHPG, 5HT and 5HIAA. In the hindbrain no significant differences were observed. Thus, TCET appears to cause an increase in the synthesis or release of 5HT, DA and NE in the midbrain and DA and 5HT in the hypothalamus. In a separate experiment, beta-endorphin-like immunoreactivity was measured in blood plasma taken from rats at intervals before, during and after a 30 minute TCET treatment, but no demonstrable TCET effect was observed. The lack of change in serum endorphin levels suggests that TCET-induced opioid activity may be confined to the central nervous system, a reasonable theory because the current passes only through the head.

Effects of acute selective 5-HT1, 5-HT2, 5-HT3 receptor and alpha 2 adrenoceptor blockade on naloxone-induced antinociception

Several studies have demonstrated a paradoxical form of antinociception induced by the repeated administration of opioid antagonists accompanied by exposure to a painful stimulus. The underlying mechanism of this naloxone-induced antinociception (NIA) is still unknown, but the results of several studies suggest that it is a non-opioid response. This study was designed to investigate serotonergic and noradrenergic involvement in NIA. Rats were treated daily with systemic injections of 5 mg/kg naloxone, followed by a 45-s hot plate test of nociception (temperature = 51.5 +/- 0.5 degree C). After rats reached plateau levels of NIA, they received a test trial in which they were treated with various doses of different selective 5-HT or alpha 2 adrenoceptor antagonists in addition to naloxone before the hot plate test. Rats treated with 0.16, 0.32 and 0.63 mg/kg pirenperone or 2.5 mg/kg ritanserin showed significant reductions in paw lick latency with respect to rats treated with vehicle. In addition, high doses of yohimbine (7.5-10 mg/kg) also effectively reversed NIA. In contrast, NIA was not affected by acute blockade of 5-HT1 or 5-HT3 receptors by methiothepin or MDL 72222, respectively, or by the alpha 2 adrenoceptor blocker idazoxan. None of the 5-HT or alpha 2 adrenoceptor antagonists had any effect on the paw lick latencies of saline-treated rats. A possible role of 5-HT2 receptors in the antinociception induced by opioid receptor blockade is discussed.

Effect of naloxone on the habituation of novelty-induced hypoalgesia: the collateral inhibition hypothesis revisited

Repeated daily administration of the opiate receptor antagonist naloxone prior to hotplate tests provokes longer paw-lick latencies by attenuating the habituation of novelty-induced hypoalgesia. This hypoalgesia has been found to persist when pain tests are subsequently conducted following saline administration. The present experiments were conducted to determine whether the substrates mediating the hypoalgesia observed during naloxone and saline tests are similar or distinct. Neither the hypoalgesia observed during naloxone nor saline tests were affected by the induction of tolerance to the hypoalgesic effect of morphine, suggesting that both effects are mediated by nonopioid antinociceptive mechanisms. Previous work from our laboratory demonstrated that the hypoalgesia observed during naloxone tests is inhibited by clonidine, enhanced by yohimbine, and unaffected by prazosin and phentolamine. In the present article, we report a similar pattern of results for the hypoalgesia observed during saline tests. It is concluded that the substrates mediating both effects are similar. The results are discussed in relation to the possibility that an opioid substrate involved in habituative learning may be inhibitory on a nonopioid antinociceptive substrate.

Spinal cord alpha-2 noradrenergic receptors mediate conditioned analgesia

The present experiment investigated the effects of direct spinal administration of the monoaminergic receptor blockers yohimbine, phentolamine and methysergide on the expression of conditioned analgesia. Animals in the Paired group received classical conditioning trials in which one context was paired with footshock administration (1 mA shock for 15 s). Animals in the Unpaired control group were administered shock in a second, different, context. On the test day animals within each condition were administered saline (20 microliters), yohimbine (30 micrograms), phentolamine (30 micrograms), or methysergide (30 micrograms) prior to receiving a hot plate test (50 degrees C) in the context previously used to shock the Paired group. These ligands were administered into the spinal fluid through a chronic, indwelling spinal catheter. Animals in the Paired group which received saline displayed longer paw lick latencies than saline-treated animals in the Unpaired group. Yohimbine, but not phentolamine or methysergide, attenuated this conditioned analgesia. These results suggest that spinal cord noradrenergic substrates mediate conditioned analgesia, and that this mediation occurs specifically through the alpha-2 noradrenergic receptor.

Clonidine and yohimbine modulate the effects of naloxone on novelty-induced hypoalgesia

Previous research has shown that repeated daily pretreatment with the opiate receptor blocker naloxone retards the development of habituation to novelty-induced hypoalgesia. The present experiments were conducted in order to determine whether noradrenergic substrates mediates this effect. Animals in the NAL condition were administered 10 mg/kg naloxone prior to assessment of pain sensitivity on a 48.5 degrees C hot plate. Control animals (SAL condition) were administered saline prior to pain assessment, and naloxone 2-4 h later. Paw lick latencies declined over repeated tests in SAL animals, suggesting the habituation of novelty hypoalgesia. Naloxone pretreatment attenuated this decline. The longer paw lick latencies observed in NAL condition animals were reduced by administration of 2 microgram/kg clonidine, a specific noradrenergic alpha-2 receptor agonist, and enhanced in a dose dependent (0.5-4.0 mg/kg) fashion by the alpha-2 antagonist yohimbine. Clonidine and yohimbine either failed to alter pain reactivity in control animals, or produced less marked effects than those observed in naloxone-exposed animals. These results suggest that noradrenergic substrates mediate naloxone's effects on novelty hypoalgesia.

Roles of gender and gonadectomy in pilocarpine and clonidine analgesia in rats

Central and systemic morphine analgesia as well as both opioid and nonopioid forms of swim analgesia display gender differences with male rats showing greater magnitudes of analgesia than female rats. Since nonopioid swim analgesia is dependent upon muscarinic cholinergic and alpha 2-noradrenergic mechanisms, the present study evaluated in rats whether gender, adult gonadectomy or estrous phase altered analgesia induced by either the muscarinic cholinergic receptor agonist, pilocarpine or the alpha 2-noradrenergic receptor agonist, clonidine. Pilocarpine (1-10 mg/kg) analgesia was significantly greater in male rats. Female rats displayed 7-fold and 3-fold rightward shifts in peak analgesia on the tail-flick and jump tests respectively. Clonidine (100-500 micrograms/kg) analgesia was significantly greater on both nociceptive tests in males, but only produced a 2-fold rightward shift in peak analgesia in females on the jump test. Whereas castration failed to shift either dose-response curve, ovariectomy mitigated the gender differences in pilocarpine and clonidine analgesia. Both pilocarpine and clonidine analgesia were not altered by estrous phase changes. These data indicate that gender differences in analgesia are not specific to opioid systems.

Mechanism of hyperalgesia in SART stressed (repeated cold stress) mice: antinociceptive effect of neurotropin

Exposing mice to 24 and 4 degrees C in alternate 1 hr periods in the day time and maintaining 4 degrees C at night for several days decreases the tail clamp pressure required to evoke pain behavior. This model is referred to as SART (specific alternation rhythm of temperature) stress. An extract from inflamed skin of rabbits inoculated with vaccinia virus (neurotropin) clearly normalized the hyperalgesia in this SART stress model. To clarify the mechanism of the hyperalgesia in SART mice and the mode of the antinociceptive action of neurotropin in this model, the influence of systemically administered neurotransmitter related drugs was studied. 1) Neurotropin, 5-hydroxytryptophan and L-dihydroxyphenylalanine significantly normalized the decrease in nociceptive threshold, and muscimol tended to inhibit it in nociceptive threshold in SART stressed mice. 2) Haloperidol, phenoxybenzamine, reserpine, bicuculline, scopolamine, physostigmine and naloxone alone did not influence the nociceptive threshold in SART stressed mice. 3) The antinociceptive effect of neurotropin was significantly attenuated by p-chlorophenylalanine, haloperidol and phenoxybenzamine; and it was completely inhibited by reserpine. 4) Naloxone, bicuculline, scopolamine and physostigmine had no influence on the antinociceptive effect of neurotropin. These results suggest that hypofunction mainly of the monoaminergic systems contributes to hyperalgesia in SART stressed mice and that neurotropin produces the antinociceptive effect by restoring these neural functions.

Naloxone-induced hypoalgesia: effects of noradrenergic antagonists and agonist

The present experiments confirmed that rats injected with naloxone and exposed to a heated floor acquired a hypoalgesic response, as indexed by the latencies to lick their paws. The expression of these latencies were unaffected by yohimbine, clonidine, propranolol, or by relatively moderate doses of prazosin, suggesting that the conditioned hypoalgesic response induced by pairings of naloxone and a heated floor is not mediated by the release and turnover of noradrenaline.

Alpha 2-adrenoceptors inhibit a nociceptive response in neonatal rat spinal cord

Alpha 2-Adrenoceptors mediate analgesia in vivo. The present study explored the actions of the alpha 2-adrenoceptor agonists dexmedetomidine and clonidine on a nociceptive response in isolated neonatal rat spinal cord. Stimulation of a dorsal root generates a slow ventral root potential (slow VRP) at the corresponding ipsilateral ventral root. The slow VRP meets several criteria for a nociceptive response. Dexmedetomidine (10 nM) and clonidine (200 nM) depressed the slow VRP by approximately 80%. Dexmedetomidine's action was approximately linear over the concentration range 0.5-500 nM, whereas clonidine (20 nM-5 microM) exerted biphasic effects. The profile of agonist and antagonist effectiveness characterized the receptor(s) as alpha 2-adrenoceptors; the subtype could not be identified as either alpha 2A or alpha 2B. Naloxone pretreatment partially blocked dexmedetomidine's effect, suggesting a possible endogenous opiate involvement. Dexmedetomidine (0.5-2.0 nM) also depressed the VRP evoked by application of substance P to the cord, implicating postsynaptic as well as possible presynaptic actions. At high concentrations, dexmedetomidine (50-500 nM) depressed the monosynaptic reflex, probably through non-alpha 2-receptor(s). Results from the neonatal spinal cord correlate well with those from in vivo analgesia studies. They suggest an important direct spinal contribution to alpha 2-adrenoceptor-mediated analgesia.

Alpha-adrenoceptor involvement in swim stress-induced antinociception in the mouse

Three different intensities of swim stress produced stress-induced antinociception (SIA) in mice which was assessed either by the reduction in the number of abdominal constrictions produced by acetic acid or by an increase in reaction time on a hot-plate. The involvement of alpha-adrenoceptors in the three models of SIA was investigated using selective antagonists. SIA produced by the mild stress of a 30 s warm water swim was attenuated by idazoxan (0.5-1 mg kg-1), and by yohimbine at a dose (1 mg kg-1) which reduced antinociception produced by clonidine (12.5-50 micrograms kg-1). Indoramin (1-2 mg kg-1) did not affect this model of SIA, but reversed phenylephrine induced inhibition of the constrictions. A 3 min room temperature swim increased reaction times on the hot-plate and this naloxone-sensitive SIA was reduced significantly by prazosin (1-2 mg kg-1), idazoxan (0.5-1 mg kg-1) and yohimbine (0.5-1 mg kg-1) but enhanced by clonidine (0.5 mg kg-1) and noradrenaline (NA) (10 micrograms i.c.v.). Mice treated with 6-hydroxydopamine (60 + 60 micrograms i.c.v.) were hypersensitive to the hot-plate and did not develop SIA. Levels of noradrenaline in the brain (minus the cerebellum) were decreased after the room temperature swim SIA. The most severe stress of a cold water swim produced SIA on the hot-plate which was initially naloxone-insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)

Yohimbine potentiates cold-water swim analgesia: re-evaluation of a noradrenergic role

Continuous cold-water swims (CCWS) elicit a nonopioid and neurohormonal analgesia which displays adaptation. The norepinephrine (NE) system has been implicated since parallel alterations in NE occur following acute and repeated CCWS exposure, and since CCWS analgesia is reduced by locus coeruleus lesions and is potentiated by clonidine and desipramine. The present study evaluated the effects of the alpha-2 NE receptor antagonist, yohimbine upon CCWS (2 degrees C for 3.5 min) analgesia on the jump and tail-flick tests, CCWS hypothermia, and basal nociceptive and thermoregulatory measures in rats. Yohimbine (0.1-2.0 mg/kg, IP) dose-dependently increased basal jump thresholds and potentiated CCWS analgesia: these effects appeared to be additive. Yohimbine potentiated CCWS analgesia on the tail-flick test without altering basal latencies. Yohimbine failed to alter either CCWS hypothermia or basal thermoregulation. Since yohimbine and clonidine, an alpha-2 NE receptor antagonist and agonist respectively, similarly potentiate CCWS analgesia, it appears that NE effects are orthoganol to the intrinsic system mediating CCWS.

Antagonism of stress-induced analgesia by quaternary naloxone

Both naloxone hydrochloride and the quaternary compound, naloxone methylbromide, significantly reduced antinociception elicited by 90 s of continuous footshock. Peripheral specificity of quaternary naloxone was demonstrated by its lack of antagonism of morphine analgesia. These results suggest that a significant portion of the antinociception elicited by these parameters of footshock and assessed by tail-flick procedures in rats is due to the activation of peripheral endorphins.