Effect of drugs active at adenosine receptors upon chronic stress-induced hyperalgesia in rats

Effects of gestational and breastfeeding caffeine exposure in adenosine A1 agonist-induced antinociception of infant rats

OBJECTIVES

Caffeine is extensively consumed as a psychostimulant drug, acting on A₁ and A_2A adenosine receptors blockade. Chronic exposure to caffeine during gestation and breast-feeding may be involved in infant rat's behavioral and biochemical alterations. Our goal was to evaluate the effect of chronic caffeine exposure during gestation and breast-feeding in the functionality of adenosine A₁ receptors in infant rats at P14. NTPDase and 5'-nucleotidase activities were also evaluated.

METHODS

Mating of adult female Wistar rats was confirmed by presence of sperm in vaginal smears. Rats were divided into three groups on the first day of pregnancy: (1) control: tap water, (2) caffeine: 0.3 g/L until P14, and (3) washout caffeine: caffeine was changed to tap water at P7. Evaluation of nociceptive response was performed at P14 using hot plate (HP) and tail-flick latency (TFL) tests. A₁ receptor involvement was assessed using caffeine agonist (CPA) and antagonist (DPCPX). Enzymatic activities assays were conducted in the spinal cord.

RESULTS

Gestational and breastfeeding exposure to caffeine (caffeine and washout groups) did not induce significant alterations in thermal nociceptive thresholds (HP and TF tests). Both caffeine groups did not show analgesic response induced by CPA when compared to the control group at P14, indicating chronic exposure to caffeine in the aforementioned periods inhibits the antinociceptive effects of the systemic A₁ receptor agonist administration. No effect was observed upon ectonucleotidase activities.

CONCLUSIONS

Our results demonstrate that chronic caffeine exposure in gestational and breastfeeding alters A1-mediated analgesic response in rats.

Pain in neuropsychiatry: Insights from animal models

Pain is the most common symptom reported in clinical practice, meaning that it is associated with many pathologies as either the origin or a consequence of other illnesses. Furthermore, pain is a complex emotional and sensorial experience, as the correspondence between pain and body damage varies considerably. While these issues are widely acknowledged in clinical pain research, until recently they have not been extensively considered when exploring animal models, important tools for understanding pain pathophysiology. Interestingly, chronic pain is currently considered a risk factor to suffer psychiatric disorders, mainly stress-related disorders like anxiety and depression. Conversely, pain appears to be altered in many psychiatric disorders, such as depression, anxiety and schizophrenia. Thus, pain and psychiatric disorders have been linked in epidemiological and clinical terms, although the neurobiological mechanisms involved in this pathological bidirectional relationship remain unclear. Here we review the evidence obtained from animal models about the co-morbidity of pain and psychiatric disorders, placing special emphasis on the different dimensions of pain.

Diazepam as attenuator of pain induced by dentin hypersensitivity in rats exposed to stress

The pain in dentin hypersensitivity (DH) has distinct sensory and emotional origins, with variations that occur in different intensities for each individual. The aim of this study is to evaluate the effects of diazepam in the attenuation of the pain induced by DH.

DESIGN

Fifty male Wistar rats were divided into five groups: control group received water ad libitum (C); stress group received water ad libitum plus stress (S); DH induced by erosion challenge with isotonic solution ad libitum (G); DH and stress (GS); and DH, stress and diazepam (GSD) groups. Animals of the GS group were exposed to the New York Subway Stress Model. Animals treated with diazepam (GSD group) received 1 mg/kg every 3 days, from the 15th day of treatment until the end of the stress-inducing period. The body weights of rats were weekly registered. After 30 days, all groups were submitted to the DH test, which was assessed using cold water stimuli, and were graded 0, 0.5, 1, 2, or 3. Dental elements were evaluated using scanning electron microscopy (SEM).

RESULTS

1) Groups G and GS presented the highest DH scores, which confirms that stress increased pain response; 2) GSD group had significantly reduced DH scores compared to G and GS groups; 3) SEM of dental elements showed exposed dentin tubules in G, GS, and GSD groups, as expected.

CONCLUSIONS

diazepam attenuated pain induced by dentin hypersensitivity in rats exposed to stress.

Chronic restraint stress exacerbates nociception and inflammatory response induced by bee venom in rats: the role of the P2X7 receptors

OBJECTIVE

Chronic restraint stress exacerbates pain and inflammation. The present study was designed to evaluate the effect of chronic restraint stress on inflammatory pain induced by subcutaneous injection of bee venom (BV).

METHODS

First, we investigated: (1) the effect of two-week restraint stress with daily 2 or 8 h on the baseline paw withdrawal mechanical threshold (PWMT), paw withdrawal thermal latency (PWTL) and paw circumference (PC); (2) the effect of chronic stress on the spontaneous paw-flinching reflex (SPFR), decrease in PWM, PWTL and increase in PC of the injected paw induced by BV.

RESULTS

The results showed that (1) chronic restraint decreased significantly the PWMT and inhibited significantly the increase in PC, but had no effect on PWTL, compared with control group; (2) chronic restraint enhanced significantly BV-induced SPFR and inflammatory swelling of the injected paw. In a second series of experiments, the role of P2X7 receptor (P2X7R) in the enhancement of BV-induced inflammatory pain produced by chronic restraint stress was determined. Systemic pretreatment with P2X7R antagonist completely reversed the decrease in PWMT produced by chronic restraint, inhibited significantly the enhancement of BV-induced inflammatory pain produced by chronic restraint stress.

CONCLUSION

Taken together, our data indicate that chronic restraint stress-enhanced nociception and inflammation in the BV pain model, possibly involving the P2X7R.

Involvement of trigeminal astrocyte activation in masseter hyperalgesia under stress

It is commonly accepted that psychological stress contributes to the development of temporomandibular joint disorders, in which chronic orofacial pain is the main symptom. However, the central mechanism underlying the development of these disorders has remained unclear. The current study was performed to determine the involvement of the glia in the trigeminal spinal subnucleus caudalis in stress-induced increases in masseter muscle hyperalgesia in rats. After being subjected to chronic restraint stress, the animals showed decreased body weight gain, behavioral changes and marked masseter allodynia. We also found that astrocytes, but not microglia, in the trigeminal subnucleus caudalis (Vc) were dramatically activated. A further analysis was undertaken to investigate the contribution of the glia; we intrathecally injected l-α-aminoadipate (astrocyte-specific inhibitor) and/or minocycline (microglia-specific inhibitor) into the stressed rats. Our results showed that l-α-aminoadipate (LAA), but not minocycline, could significantly attenuate the mechanical masseter allodynia and behavioral changes induced by restraint stress. In addition, the expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-d-aspartic acid receptor 1 (p-NR1) in the Vc was significantly increased after chronic restraint stress, whereas LAA dramatically inhibited the overexpression of IL-1β and p-NR1. Taken together, these results suggest that activated astrocytes in the Vc may be one of the most important factors in the pathophysiology of masseter hyperalgesia induced by restraint stress and the following overexpression of IL-1β and excessive NMDAR phosphorylation may ultimately contribute to masseter hyperalgesia. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for the treatment of orofacial pain induced by stress.

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

Stress, caffeine and ethanol trigger transient neurological dysfunction through shared mechanisms in a mouse calcium channelopathy

Several episodic neurological disorders are caused by ion channel gene mutations. In patients, transient neurological dysfunction is often evoked by stress, caffeine and ethanol, but the mechanisms underlying these triggers are unclear because each has diverse and diffuse effects on the CNS. Attacks of motor dysfunction in the Ca(V)2.1 calcium channel mouse mutant tottering are also triggered by stress, caffeine and ethanol. Therefore, we used the tottering mouse attacks to explore the pathomechanisms of the triggers. Despite the diffuse physiological effects of these triggers, ryanodine receptor blockers prevented attacks induced by all of them. In contrast, compounds that potentiate ryanodine receptors triggered attacks suggesting a convergent biochemical pathway. Tottering mouse attacks were both induced and blocked within the cerebellum suggesting that the triggers act locally to instigate attacks. In fact, stress, caffeine and alcohol precipitated attacks in Ca(V)2.1 mutant mice in which genetic pathology was limited to cerebellar Purkinje cells, suggesting that the triggers initiate dysfunction within a specific brain region. The surprising biochemical and anatomical specificity of the triggers and the discovery that the triggers operate through shared mechanisms suggest that it is possible to develop targeted therapies aimed at blocking the induction of episodic neurological dysfunction, rather than treating the symptoms once provoked.

Potential processes involved in the initiation and maintenance of whiplash-associated disorders: discussion paper 3

STUDY DESIGN

Nonsystematic review and discussion of the etiological processes involved in whiplash-associated disorders (WAD).

OBJECTIVE

To summarize the research and identify priorities for future research.

SUMMARY OF BACKGROUND DATA

Although there is convergent evidence of a peripheral lesion in some individuals after whiplash injury, in the majority of injured people, a lesion cannot be established with current imaging technology. Therefore, it is important to consider processes that underlie the initiation and maintenance of whiplash pain as this may allow for the development and testing of interventions to target these processes and improve outcomes.

METHODS

A nonsystematic review was performed to summarize current knowledge regarding potential etiological processes involved in the initiation and maintenance of WAD and to identify future research priorities.

RESULTS

There are several etiological processes potentially involved in the initiation and maintenance of WAD. These include augmented nociceptive processing, stress system responses, and psychosocial and sociocultural factors. Recent findings also indicate that morphological changes in the neck muscles of injured people show some association with poor recovery, but the mechanisms underlying these changes are not clear. Preliminary evidence indicates associations between these processes. Future research priorities include the following: more sophisticated investigation and analysis of interactions between the various processes; whether the modification of these processes is achievable and if modification can improve health outcomes; and to clarify factors involved in the initiation of whiplash pain versus those involved in symptom maintenance.

CONCLUSION

Research to date indicates that there are several physiological and psychological etiological processes that may underlie the initiation and maintenance of whiplash-related pain and disability. Further research is required to determine relationships and interactions between these factors and to determine whether their modification is possible and will improve outcomes after injury.

The potential contribution of stress systems to the transition to chronic whiplash-associated disorders

STUDY DESIGN

A narrative description highlighting preclinical and clinical evidence that physiologic stress systems contribute to whiplash-associated disorders (WAD) pathogenesis.

OBJECTIVE

To present several lines of evidence supporting the hypothesis that physiologic stress systems contribute to WAD pathogenesis.

SUMMARY OF BACKGROUND DATA

In addition to subjecting soft tissue to biomechanical strain, a motor vehicle collision (MVC) event is also an acute stressor which activates physiologic stress systems. Increasing data from animal and human studies suggest that the activation of these stress systems may contribute to long-lasting changes in pain sensitivity after tissue injury.

METHODS

Nonsystematic review of several lines of evidence that together suggest that physiologic systems involved in the stress response may contribute to the development of WAD.

RESULTS

Stress systems which appear capable of producing hyperalgesia and allodynia include catecholaminergic systems, serotonin systems, and the hypothalamic-pituitary-adrenocortical system. Evidence for the role of these systems comes, in part, from studies examining the association between genetic variants and chronic pain outcomes. For example, in a recent study of acute neck pain after MVC, patients with certain genotypes of an enzyme involved in catecholamine metabolism were more than twice as likely to report moderate or severe neck pain in the emergency department. Such pain vulnerability because of stress system function may interact with the effects of biomechanical injury and psychobehavioral responses to influence the development of WAD.

CONCLUSION

More research examining the influence of stress systems on WAD are needed. If these systems do influence WAD outcomes, then treatments which diminish the adverse effects of stress systems may be a useful component of multimodal therapeutic interventions for individuals at risk of chronic pain development after MVC.

Selective ablation of mu-opioid receptor expressing neurons in the rostral ventromedial medulla attenuates stress-induced mechanical hypersensitivity

AIMS

Chronic stress-related conditions are often associated with stress-induced hyperalgesia. However, the neural circuitry responsible for producing stress-induced hyperalgesia is not well characterized. The aim of this study was to determine the contribution of mu-opioid expressing brainstem neurons to the expression of stress-induced hyperalgesia.

MAIN METHODS

The present study utilized a model of stress-induced mechanical hypersensitivity that involved application of repeated, light tactile whisker pad stimulation (WPS) in rats. Repeated WPS (10 applications/session, 4 sessions/h in 1 day, sessions on days 1-5 and 8-12) increased defensive-aggressive and hypervigilant behaviors, and produced hypersensitivity to tactile stimulation of the hind paw. In order to test the possible involvement of mu-opioid receptor expressing neurons in the rostral ventral medulla (RVM) to this response, rats received RVM microinjections of the toxin conjugate dermorphin-saporin or its control, saporin. Fourteen days later rats underwent either WPS or sham conditioning.

KEY FINDINGS

Repeated WPS produced defensive-aggressive behaviors directed towards the stimulus and mechanical hypersensitivity of the hind paw that persisted for up to 2 weeks after the final WPS session. Dermorphin-saporin, but not saporin, microinjections prevented the development of hind paw mechanical hypersensitivity, but did not affect the defensive-aggressive behaviors.

SIGNIFICANCE

The finding that chronic stress produces mechanical hypersensitivity through circuitry that involves the RVM provides a potential neurobiological basis for the complex interaction between chronic stress and pain.

Psychological stress induces temporary masticatory muscle mechanical sensitivity in rats

To explore the relationship between psychological stress and masticatory muscle pain, we created a communication stress animal model to determine whether psychological stress could induce increased mechanical sensitivity in masticatory muscles and to study the changes of mechanical nociceptive thresholds after stress removal. Forty-eight male Sprague-Dawley rats were divided into a control group (CON), a foot-shocked group (FS, including 3 subgroups recorded as FS-1, FS-2, and FS-3), a psychological stress group (PS), and a drug treatment group (DT). PS and DT rats were confined in a communication box for one hour a day to observe the psychological responses of neighboring FS rats.Measurements of the mechanical nociceptive thresholds of the bilateral temporal and masseter muscles showed a stimulus-response relationship between psychological stress and muscle mechanical sensitivity. The DT rats, who received a diazepam injection, showed almost the same mechanical sensitivity of the masticatory muscles to that of the control in response to psychological stress. Fourteen days after the psychological stressor was removed, the mechanical nociceptive thresholds returned to normal. These findings suggest that psychological stress is directly related to masticatory muscle pain. Removal of the stressor could be a useful method for relieving mechanical sensitivity increase induced by psychological stress.

Neonatal morphine exposure alters E-NTPDase activity and gene expression pattern in spinal cord and cerebral cortex of rats

The neonate opioid system has been frequently investigated, and studies have shown that exposure to drugs in early life can have implications for nervous system development. It has been proposed that adenosine is involved in opioid antinociception, and ATP is involved in central and peripheral mechanisms of nociception. Extracellular nucleotides can be hydrolyzed by E-NTPDases and ecto-5'nucleotidase, which present the functions of removing ATP and generating adenosine. In this study, we evaluated ATP, ADP, and AMP hydrolysis in synaptosomes from spinal cord and cerebral cortex of rats at postnatal day 16 after repeated morphine exposure in early life (postnatal day 8 to 14). Additionally, we evaluated E-NTPDase (1, 2 and 3) and ecto-5'nucleotidase gene expression by semi-quantitative RT-PCR analysis. We observed an increase in ATP hydrolysis in the cerebral cortex, and a decrease in ADP hydrolysis in spinal cord. Expression levels of E-NTPDase 1 decreased in cerebral cortex and increased in spinal cord. Our findings highlight the importance of the purinergic system in young rats submitted to repeated morphine exposure by showing that in the neonatal period such exposure is capable of affecting the control system for nucleotide levels, which can promote changes in modulation or transmission of painful stimuli.

Protective effect of adenosine reuptake inhibitors in haloperidol-induced orofacial dyskinesia and associated behavioural, biochemical and neurochemical changes

Chronic administration of typical neuroleptics is known to cause persistent oral dyskinesia in rats, an alleged animal model of tardive dyskinesia (TD). The pathophysiology of the syndrome remains unclear. Adenosine is now widely accepted as the major inhibitory neuromodulators in the central nervous system besides gamma-aminobutyric acid. Based on the hypothesis that adenosinergic receptor system may involve in the pathogenesis of TD, we investigated the effect of dipyridamole (5 and 10 mg/kg, i.p.), an adenosine reuptake inhibitor and nimodipine (10 and 20 mg/kg, i.p.) an adenosine transport inhibitor in haloperidol-induced TD by using different behavioural, biochemical and neurochemical parameters in rats. Chronic administration of haloperidol (1 mg/kg, i.p., for 21 days) significantly increased vacuous chewing movements, tongue protrusion, facial jerking which was prevented by adenosine reuptake inhibitors. Chronic administration of haloperidol also resulted in the development of dopamine sensitivity as suggested by increased locomotor activity and stereotypy and decreased % retention time on elevated plus maze paradigm. Pretreatment with adenosine reuptake/transport inhibitors, dipyridamole and nimodipine prevented all these behavioural changes. Chronic administration of haloperidol also resulted in increased oxidative damage in all brain regions which was prevented dose-dependently by both dipyridamole and nimodipine in different brain regions. Chronic administration of haloperidol resulted in decreased turnover of dopamine and norepinephrine in both cortex and subcortical regions which was dose-dependently prevented by adenosine reuptake/transport inhibitors. The major findings of the present study suggested that adenosine reuptake inhibitors dipyridamole and nimodipine could be a possible therapeutic option in neuroleptic induced TD.

Increased density and synapto-protective effect of adenosine A2A receptors upon sub-chronic restraint stress

Stress initially causes adaptive changes in the brain and can lead to neurodegeneration if continuously present. Noxious brain conditions trigger the release of adenosine that can control brain function and neurodegeneration through inhibitory A(1) and facilitatory A(2A) receptors. We tested the effect of restraint stress on the density of adenosine receptors and their effect on the outcome of stress, focusing in a known affected region, the hippocampus. Sub-chronic restraint stress (6 h/day for 7 days) caused a parallel decrease of the density of A(1) receptors (15-20%) and an increase (near 250%) of A(2A) receptor density in rat hippocampal nerve terminals. This indicates that sub-chronic stress unbalances adenosine receptors, up-regulating A(2A) and down-regulating A(1) receptors. Sub-chronic stress did not cause hippocampal neurodegeneration but decreased the immunoreactivity (immunohistochemistry and Western blot) of a synaptic marker, synaptophysin. The blockade of A(2A) receptors with 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (0.05 mg/kg, daily i.p. injection) attenuated the loss of synaptophysin immunoreactivity observed in the hippocampus of rats subjected to sub-chronic restraint stress. This ability of A(2A) receptor antagonists to prevent the earliest stress-induced synaptic modifications provides a neurochemical and morphological correlate for the interest of A(2A) receptor antagonists to attenuate the burden of chronic stress.

Central Adenosine Signaling Plays a Key Role in Centrally Mediated Hypotension in Conscious Aortic Barodenervated Rats

We tested the hypothesis that clonidine-evoked hypotension is dependent on central adenosinergic pathways. Five groups of male, conscious, aortic baroreceptor-denervated (ABD) rats received clonidine (10 microg/kg i.v.) 30 min after i.v. 1) saline, 2) theophylline (10 mg/kg), or 3) 8-(p-sulfophenyl)theophylline (8-SPT) (2.5 mg/kg) or 1 h after i.p. 4) dipyridamole (5 mg/kg) or 5) an equal volume of sesame oil. Blockade of central (theophylline) but not peripheral (8-SPT) adenosine receptors abolished clonidine hypotension. In contrast, dipyridamole substantially enhanced the bradycardic response to clonidine. In additional groups, intracisternal (i.c.) dipyridamole (150 microg) and 8-SPT (10 microg) enhanced and abolished, respectively, clonidine (0.6 microg i.c.)-evoked hypotension. Because clonidine is a mixed I1/alpha2 agonist, we also investigated whether adenosine signaling is linked to the I1 or the alpha2A receptor by administering the selective I1 (rilmenidine, 25 microg) or alpha2A [alpha-methylnorepinephrine (alpha-MNE), 4 microg] agonist 30 min after central adenosine receptor blockade (8-SPT; 10 microg i.c.) or artificial cerebrospinal fluid. The hypotensive response elicited by rilmenidine or alpha-MNE was abolished in 8-SPT-pretreated rats. To delineate the role of the adenosine A2A receptor in clonidine-evoked hypotension, i.c. clonidine (0.6 microg) was administered 30 min after central adenosine receptor A2A blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-epsilon]-1,2,4-triazolo[1,5-c]-pyrimidine (SCH58261); 150 microg i.c.]. The latter virtually abolished the hypotensive and bradycardic responses elicited by clonidine. In conclusion, central adenosine A2A signaling plays a key role in clonidine-evoked hypotension in conscious aortic barodenervated rats.

Nociception- and anxiety-like behavior in rats submitted to different periods of restraint stress

The aim of this study was to evaluate the effect of acute, sub-chronic and chronic stress on nociception induced by formalin injection in rats' temporomandibular joint (TMJ). It was evaluated the relation between blood levels of adrenocorticotropin, corticosterone, the levels of anxiety and nociceptive responses recorded after different stress protocols. Animals were initially submitted to acute restraint stress (15; 30 min and 1 h), or exposed to sub-chronic (3 days-1 h/day) or chronic stress (40 days-1 h/day). Then, animals were (1) killed immediately to collect blood for hormonal determinations; or (2) submitted to the elevated plus-maze to evaluate anxiety; or (3) submitted to the TMJ formalin test to evaluate nociception. It was also evaluated the role of serotoninergic and opioid systems in nociceptive changes induced by stress. For this, the serotonin-selective reuptake inhibitor (fluoxetine 10 mg/kg) and the opioid agonist (morphine 1-5 mg/kg) were administered before the nociception test. All stress protocols significantly raised the levels of ACTH or corticosterone, as well as the anxiety behavior. In relation to nociception, the chronic stressed animals showed an increase in nociceptive responses (hyperalgesia). In this group, there was a reduction in the morphine analgesic effects, suggesting dysfunction in the endogenous opioid system. Fluoxetine had an analgesic effect in both stressed and control groups, although this effect was more evident in the stressed group. It was concluded that stress-induced hyperalgesia may result from changes in the serotoninergic and opioid systems, which can explain, at least in part, the important link between stress and orofacial pain.

The effects of restraint stress on nociceptive responses induced by formalin injected in rat's TMJ

It has been reported that stress can alter nociception from superficial tissues, such as skin and subcutaneous region. However, the influence of stress on an experimental deep nociception model is not understood. In this study, the temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute and chronic restraint stress on nociceptive responses in rats. Animals were initially submitted to one session of acute restraint stress (1 h) or exposed to chronic stress (40 days-1 h/day). Then, animals were killed immediately to collect blood for hormonal determinations by radioimmunoassay, or submitted to the TMJ formalin test to evaluate nociception. Rats submitted to acute restraint presented a performance similar to unstressed controls in the TMJ formalin test, whereas chronically stressed rats showed an increase in nociceptive responses. After 40 days of restraint, morphine was injected i.p. (1, 5 mg/kg or saline). The stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. These findings suggest that repeated stress can produce hyperalgesia, which is, at least in part, due to alterations in the activity of opioid systems. This model may help elucidate the underlying neural mechanisms that mediate the effects of repeated stress on orofacial pain.

Repeated sound stress enhances inflammatory pain in the rat

While it is well established that acute stress can produce antinociception, a phenomenon referred to as stress-induced analgesia, repeated exposure to stress can have the opposite effect. Since, chronic pain syndromes, such as fibromyalgia and rheumatoid arthritis, may be triggered and/or exacerbated by chronic stress, we have evaluated the effect of repeated stress on mechanical nociceptive threshold and inflammatory hyperalgesia. Using the Randall-Selitto paw pressure test to quantify nociceptive threshold in the rat, we found that repeated non-habituating sound stress enhanced the mechanical hyperalgesia induced by the potent inflammatory mediator, bradykinin, which, in normal rats, produces hyperalgesia indirectly by stimulating the release of prostaglandin E2 from sympathetic nerve terminals. Hyperalgesia induced by the direct-acting inflammatory mediator, prostaglandin E2 as well as the baseline nociceptive threshold, were not affected. Adrenal medullectomy or denervation, reversed the effect of sound stress. In sound stressed animals, bradykinin-hyperalgesia had a more rapid latency to onset and was no longer inhibited by sympathectomy, compatible with a direct effect of bradykinin on primary afferent nociceptors. In addition, implants of epinephrine restored bradykinin-hyperalgesia in sympathectomized non-stressed rats, lending further support to the suggestion that increased plasma levels of epinephrine can sensitize primary afferents to bradykinin. These results suggest that stress-induced enhancement of inflammatory hyperalgesia is associated with a change in mechanism by which bradykinin induces hyperalgesia, from being sympathetically mediated to being sympathetically independent. This sympathetic-independent enhancement of mechanical hyperalgesia is mediated by the stress-induced release of epinephrine from the adrenal medulla.

Hyperthyroidism changes nociceptive response and ecto-nucleotidase activities in synaptosomes from spinal cord of rats in different phases of development

Changes in transport, receptors and production of extracellular adenosine have been observed after induction of hyperthyroidism. Adenosine is associated with inhibitory actions such as reduction in release of excitatory neurotransmitters and antinociception at spinal site. In contrast, ATP acts as an excitatory neurotransmitter and produces pronociceptive actions. ATP may be completely hydrolyzed to adenosine by an enzyme chain constituted by an ATP diphosphohydrolase and an ecto-5'-nucleotidase, as previously described in the spinal cord. Thus, we now investigated the effects of the hyperthyroidism on adenine nucleotide hydrolysis in the spinal cord and verified the nociceptive response in this pathology during different phases of development. Hyperthyroidism was induced in male Wistar rats, aged 5, 60 and 330 days by daily intraperitoneal injections of L-thyroxine (T4) for 14 days. Nociception was assessed with a tail-flick apparatus. Rats starting the treatment aged 5 days demonstrated a significant increase in ADP and AMP hydrolysis and increased tail-flick latency (TFL). In contrast, in the spinal cord from hyperthyroid rats aged 60 and 330 days old, the hydrolysis of ATP, ADP and AMP were significantly decreased. Accordingly, the tail-flick latency was decreased, indicating a hyperalgesic response. These results suggest the involvement of ecto-nucleotidases in the control of the hyperthyroidism-induced nociceptive response in rats at distinct developmental stages.

Induced depressive behavior impairs learning and memory in rats

While it is generally accepted that cognitive processes such as learning and memory are affected by emotion, the impact of depression on learning and memory has rarely been directly studied in experimental animals. Effects of induced depressive behavior on learning and memory were determined in rats, using an open space swim test, a novel animal model of depressive behavior that is developed recently in our laboratory. The model indexes searching activity of the animals, with the induced depressive immobility behavior showing specific sensitivity to three major prototypic classes of antidepressants and a selective serotonin reuptake inhibitor. The induced depressive behavior in rats showed a delayed response to chronic antidepressant treatment and had a lasting effect on the ability of rats to learn and recall the learned experience. It impaired the subsequent ability of rats to learn and recall both a spatial water maze task and a multi-trial passive avoidance task. These impairments were all sensitive to antidepressant therapeutics, but not to buspirone, an anxiolytic. By way of contrast, the ability of the rats to sense and move to a visible platform and to escape from an unconditioned shock stimulus was neither impaired by inducing the depressive behavior nor altered by the drug treatment, suggesting that non-specific changes in sensorimotor ability were not involved. These impairments of learning and memory indicate that the depressive behavior-induced deficits show generalizability and are not context-limited. This animal model of depressive behavior shows promising potential as a screen for novel antidepressive therapeutics and as a disease model for revealing network/cellular/molecular mechanisms in the pathophysiology of depression and depression-induced cognitive deficits.

The anti-hyperalgesic effects of carbamazepine and oxcarbazepine are attenuated by treatment with adenosine receptor antagonists

The antinociceptive effects of carbamazepine and oxcarbazepine, and the influence of caffeine, were examined in a paw pressure test in rats. Carbamazepine (10-40 mg/kg; intraperitoneal, i.p.) and oxcarbazepine (40-160 mg/kg; i.p.) caused a significant dose-dependent reduction of the paw inflammatory hyperalgesia induced by concanavalin A (Con A), intraplantarly (i.p1.). A comparable pattern of antinociceptive effect of carbamazepine and oxcarbazepine was observed; the only difference is their potency, in that carbamazepine was about three times more potent than oxcarbazepine. Caffeine (5-20mg/kg; i.p.), a non-selective adenosine receptor antagonist, significantly depressed the antinociceptive effects of carbamazepine and oxcarbazepine, in a dose- and time-dependent manner. Also, a significant depression of the antinociceptive effects of carbamazepine and oxcarbazepine was observed by pretreatment with 1,3-dipropyl-8-cyclopentylxantine (DPCPX, 0.4 and 0.8 mg/kg; i.p.), an adenosine A(1) receptor antagonist. These findings indicate that, in a paw inflammatory hyperalgesia in rats, the antinociceptive effects of both drugs are, at least partially, mediated by adenosine A(1) receptors. In conclusion, the present study suggests the potential clinical importance of carbamazepine and oxcarbazepine in the treatment of inflammatory pain. In addition, caffeine consumption could possibly depress the analgesic effects of both anticonvulsive drugs.