Involvement of spinal 5-HT1A receptors in isolation rearing-induced hypoalgesia in mice

Neonatal nociceptive stimulation results in pain sensitization, reduction of hippocampal 5-HT1A receptor, and p-CREB expression in adult female rats

Painful invasive procedures are often performed on newborns admitted to intensive care units (ICU). The acute and long-term effects caused by these stimuli can be investigated in animal models, such as newborn rats. Previous studies have shown that animals subjected to nociceptive stimuli in the neonatal period show sex-specific behavioral changes such as signs of anxiety or depression. Under the same conditions, neonatal stimuli also provoke an increase in the rate of neurogenesis and cell activation in the hippocampal dentate gyrus. So, this study aims to identify the possible roles of central monoamines, receptor expression (5-HT1A), and signaling factors (p-CREB) underlying the long-term effects of neonatal nociceptive stimulation. For this, noxious stimulation was induced by intra-plantar injection of Complete Freund´s adjuvant (CFA) on the postnatal day 1 (P1) or 8 (P8). Control animals were not stimulated. On P75 the behavioral tests were conducted (hotplate and elevated plus maze), followed by sacrifice and molecular studies. Our results showed that neonatal nociceptive stimulation alters pain sensitization specially in females, while stimulation on P1 increases pain threshold, P8-stimulated animals respond with reduced pain threshold (P < 0.001). Hippocampal expression of 5-HT1A receptor and p-CREB were reduced in P8 F group (P < 0.001) in opposition to the increased utilization rate of dopamine and serotonin in this group (P < 0.05). This study shows sex- and age-specific responses of signaling pathways within the hippocampus accompanied by altered behavioral repertoire, at long-term after neonatal painful stimulation.

Synergistic effect of cannabidiol and transcutaneous electrical nerve stimulation on neuropathic and inflammatory pain in mice

OBJECTIVES

Pain is the most common cause of seeking healthcare and the leading cause of disability worldwide. Although cannabidiol and transcutaneous electrical nerve stimulation (TENS) are effective and safe strategies for treating chronic pain, the combined effect of these interventions remains overlooked. To compare the isolated and combined effect of cannabidiol and TENS in the treatment of experimental neuropathic and inflammatory pain.

METHODS

Swiss mice were subjected to chronic constriction injury (CCI)-induced neuropathic or carrageenan-induced inflammatory pain models. Cannabidiol or TENS alone and the combination of these therapies were administered once. The nociceptive threshold was measured by the von Frey test. IL-1β, TNF-α and IL-10 cytokine levels were measured by ELISA from spinal cord samples.

RESULTS

Combined, cannabidiol and TENS potentiate antinociception only in neuropathic pain. IL-1β and TNF-α levels were similarly reduced when TENS or cannabidiol were administered alone or in combination. However, only cannabidiol and TENS combined increased IL-10 levels.

CONCLUSIONS

Our findings indicated TENS and cannabidiol combined were effective in potentiating antinociception in a neuropathic pain model, an effect potentially associated with spinal IL-10 upregulation.

Effect of Acute Brief Social Isolation on Visceral Pain

Objective

The sensitivity of somatic pain could be affected by social isolation; however, few studies have examined the impact of social isolation on visceral pain. In the present study, the effect of acute brief social isolation on visceral pain response was investigated.

Methods

Adult male rats were either reared individually or grouped for 2 hours or 24 hours. Colorectal distention (CRD)-induced abdominal withdrawal reflex (AWR) score and pain threshold were used to assess visceral pain sensitivity. The amount of fecal bolus was used to determine the stress level.

Results

Acute brief isolation rearing for 2 hours significantly increased AWR score and reduced visceral pain threshold in rats when compared to group rearing. Similarly, acute isolation for 24 hours resulted in visceral hypersensitivity, as indicated by an increase in the AWR score and a decrease in the visceral pain threshold. Furthermore, the amount of fecal bolus in acute isolation rearing (2 or 24 hours) rats was considerably higher than in the control group rearing rats.

Conclusion

Acute short-term social isolation enhances visceral pain sensitivity, which could be related to an increase in stress levels.

Defensive and Emotional Behavior Modulation by Serotonin in the Periaqueductal Gray

Serotonin 5-hydroxytryptamine (5-HT) is a key neurotransmitter for the modulation and/or regulation of numerous physiological processes and psychiatric disorders (e.g., behaviors related to anxiety, pain, aggressiveness, etc.). The periaqueductal gray matter (PAG) is considered an integrating center for active and passive defensive behaviors, and electrical stimulation of this area has been shown to evoke behavioral responses of panic, fight-flight, freezing, among others. The serotonergic activity in PAG is influenced by the activation of other brain areas such as the medial hypothalamus, paraventricular nucleus of the hypothalamus, amygdala, dorsal raphe nucleus, and ventrolateral orbital cortex. In addition, activation of other receptors within PAG (i.e., CB1, Oxytocin, µ-opioid receptor (MOR), and γ-aminobutyric acid (GABA_A)) promotes serotonin release. Therefore, this review aims to document evidence suggesting that the PAG-evoked behavioral responses of anxiety, panic, fear, analgesia, and aggression are influenced by the activation of 5-HT_1A and 5-HT_2A/C receptors and their participation in the treatment of various mental disorders.

Differential rearing alters taste reactivity to ethanol, sucrose, and quinine

RATIONALE

Early-life environment influences reinforcer and drug motivation in adulthood; however, the impact on specific components of motivation, including hedonic value ("liking"), remains unknown.

OBJECTIVES

The current study determined whether differential rearing alters liking and aversive responding to ethanol, sucrose, and quinine in an ethanol-naïve rat model.

METHODS

Male and female rats were reared for 30 days starting at postnatal day 21 in either an enriched (EC), isolated (IC), or standard condition (SC). Thereafter, all rats had indwelling intraoral fistulae implanted and their taste reactivity to water, ethanol (5, 10, 20, 30, 40% v/v), sucrose (0.1, 0.25, 0.5 M), and quinine (0.1, 0.5 mM) was recorded and analyzed.

RESULTS

EC rats had higher amounts of liking responses to ethanol, sucrose, and quinine and higher amounts of aversive responses to ethanol and quinine compared to IC rats. While EC and IC rats' responses were different from each other, they both tended to be similar to SCs, who fell in between the EC and IC groups.

CONCLUSIONS

These results suggest that environmental enrichment may enhance sensitivity to a variety of tastants, thereby enhancing liking, while isolation may dull sensitivity, thereby dulling liking. Altogether, the evidence suggests that isolated rats have a shift in the allostatic set-point which may, in part, drive increased responding for a variety of rewards including ethanol and sucrose. Enriched rats have enhanced liking of both sucrose and ethanol suggesting that enrichment may offer a unique phenotype with divergent preferences for incentive motivation.

The analgesic effect of trans-resveratrol is regulated by calcium channels in the hippocampus of mice

Resveratrol has been widely studied in terms of it's potential to slow the progression of many diseases. But little is known about the mechanism of action in neuropathic pain. Neuropathic pain is the main type of chronic pain associated with tissue injury. Calcium channels and calcium/caffeine-sensitive pools are associated with analgesic pathway involving neuropathic pain. Our previous study suggested that the antinociceptive effect of resveratrol was involved in Ca²⁺/calmodulin-dependent signaling in the spinal cord of mice. The aim of this study was to explore the involvement of Ca²⁺ in analgesic effects of trans-resveratrol in neuropathic pain and signal pathway in hippocampus. Hot plate test was used to assess antinociceptive response when mice were treated with trans-resveratrol alone or in combination with Mk 801, nimodipine, CaCl₂, ryanodine or EGTA. The effects of trans-resveratrol and the combination on Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and BDNF (brain-derived neurotrophic factor) expression in hippocampus were also investigated. The results showed that trans-resveratrol increased paw withdraw latency in the hot plate test. The effect of resveratrol was enhanced by Mk 801 and nimodipine. Central administration of Ca²⁺, however, abolished the antinociceptive effects of resveratrol. In contrast, centrally administered EGTA or ryanodine improved trans-resveratrol induced antinociception. There was a significant increase in p-CaMKII and BDNF expression in the hippocampus when resveratrol were combined with Mk 801, nimodipine, ryanodine and EGTA. Administration of CaCl₂ blocked changes in p-CaMKII and BDNF levels in the hippocampus. These findings suggest that trans-resveratrol exerts the effects of antinociception through regulation of calcium channels and calcium/caffeine-sensitive pools.

Adolescent social rejection alters pain processing in a CB1 receptor dependent manner

Experiences of social rejection represent a major source of distress and in particular peer rejection during adolescence has been implicated in various psychiatric disorders. Moreover, experimentally induced acute social rejection alters pain perception in humans, implicating overlapping neurocircuits for social and physical pains. We recently demonstrated that rearing of adolescent Wistar rats with inadequate, less playful play partners (Fischer 344) persistently decreases pain sensitivity, although the detailed mechanisms mediating the aversiveness during the social encounter remained unsettled. With the present study we examined the behavioral performance during acute interaction of female adolescent Wistar rats with either age-matched same-strain partners or rats from the Fischer 344 strain. We here identify the low responsiveness upon playful attacks, which appears to be characteristic for social play in the Fischer 344 strain, as one of the main aversive components for adolescent Wistar animals during cross-strain encounters, which subsequently diminishes thermal pain reactivity. A detailed behavioral analysis further revealed increased ultrasonic vocalization at 50kHz and an increased frequency of playful attacks for adolescent Wistar animals paired with a Fischer 344 rat compared to same-strain control pairs. Finally, an acute injection of a subthreshold dose of the cannabinoid type 1 receptor inverse agonist/antagonist SR141716 before the social encounter abolished enhanced play-soliciting behavior in Wistar/Fischer 344 pairs as well as the behavioral consequences of the rejection experience in adolescent Wistar rats, further emphasizing an important modulatory role of the endocannabinoid system in mediating the effects of social behavior and social pain.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Repeated forced swim stress enhances CFA-evoked thermal hyperalgesia and affects the expressions of pCREB and c-Fos in the insular cortex

Stress affects brain activity and promotes long-term changes in multiple neural systems. Exposure to stressors causes substantial effects on the perception and response to pain. In several animal models, chronic stress produces lasting hyperalgesia. The insular (IC) and anterior cingulate cortices (ACC) are the regions exhibiting most reliable pain-related activity. And the IC and ACC play an important role in pain modulation via the descending pain modulatory system. In the present study we examined the expression of phospho-cAMP response element-binding protein (pCREB) and c-Fos in the IC and ACC after forced swim stress (FS) and complete Freund's adjuvant (CFA) injection to clarify changes in the cerebral cortices that affect the activity of the descending pain modulatory system in the rats with stress-induced hyperalgesia. FS (day 1, 10min; days 2-3, 20min) induced an increase in the expression of pCREB and c-Fos in the anterior IC (AIC). CFA injection into the hindpaw after the FS shows significantly enhanced thermal hyperalgesia and induced a decrease in the expression of c-Fos in the AIC and the posterior IC (PIC). Quantitative image analysis showed that the numbers of c-Fos-immunoreactive neurons in the left AIC and PIC were significantly lower in the FS+CFA group (L AIC, 95.9±6.8; L PIC, 181.9±23.1) than those in the naive group (L AIC, 151.1±19.3, p<0.05; L PIC, 274.2±37.3, p<0.05). These findings suggest a neuroplastic change in the IC after FS, which may be involved in the enhancement of CFA-induced thermal hyperalgesia through dysfunction of the descending pain modulatory system.

Isolation rearing reduces mechanical allodynia in a mouse model of chronic inflammatory pain

Social isolation rearing in mice after weaning reduces pain sensitivity to acute pain, and this hypoalgesia is mediated by the descending serotonergic pain inhibitory system in which the spinal serotonin (5-HT)1A receptor is involved. However, it is not known whether isolation rearing affects pain sensitivity to neuropathic or inflammatory chronic pain. In this study, we examined the effects of isolation rearing on chronic pain induced by Freund's complete adjuvant (FCA) and partial sciatic nerve ligation using the von Frey test (to assess mechanical allodynia) and the plantar test (to assess thermal hyperalgesia). In the FCA model, isolation rearing reduced mechanical allodynia, but not thermal hyperalgesia. In contrast, isolation rearing had no effect on allodynia or hyperalgesia in the sciatic nerve ligation model. The isolation rearing-induced inhibition of allodynia was alleviated by intrathecal injection of WAY100635, a selective 5-HT1A receptor antagonist. FCA increased 5-HT turnover and decreased 5-HT1A receptor expression in the spinal cord of group-reared mice, while it did not have these effects in isolation-reared mice. These results suggest that FCA suppresses the serotonergic pain inhibitory system selectively in group-reared mice. Moreover, systemic administration of osemozotan, a selective 5-HT1A receptor agonist, inhibited FCA-induced mechanical allodynia in group-reared mice, and this effect of the drug was suppressed by intrathecal injection of WAY100635. Collectively, these findings suggest that isolation rearing selectively reduces FCA-induced mechanical allodynia in mice and that this effect is mediated by the activation of spinal 5-HT1A receptors.