An interaction between basolateral amygdala orexinergic and endocannabinoid systems in inducing anti-nociception in the rat formalin test.

The amygdala has emerged as the main brain center for the emotional affective dimension of pain and pain modulation. In the amygdala, orexin and cannabinoid receptors are expressed in relatively high concentrations. To investigate the possible interaction between the amygdala orexin and cannabinoid systems on the modulation of inflammatory pain, we conducted formalin, rotarod, and plethysmometer tests, as well as analyzing mRNA expression of orexin and cannabinoid receptors in male rats. The basolateral amygdala (BLA) was unilaterally implanted by a guide cannula. Our results showed that, compared to saline and DMSO/saline, intra-BLA microinjection of orexin-A (50 and 100 µM) decreased flinch response in the early phase, but not in the late phase of the formalin test. However, these injections had no significant effect on the mRNA expression level of BLA, orexin receptor type-1 (Orx1), and cannabinoid receptor type-1 (Cb1). Moreover, intra-BLA administration of Orx1 receptor antagonist (SB-334867; 50 nM) and Cb1 receptor antagonist (AM251; 250 and 500 nM) decreased flinch response only in the early phase of the formalin test as compared to the DMSO group. Although the intra-BLA infusion of orexin-A alone and along with SB-334867 or AM251 decreased flinch response in the early phase of the formalin test, intra-BLA co-microinjection of SB-334867/AM251/OrxA increased flinch response in both early and late phases of the formalin test when compared to the DMSO/OrxA group. Interestingly, in the SB-334867/AM251/OrxA group, the Cb1 receptor was upregulated in all groups in comparison to Orx1 receptors. Our results revealed an interaction between BLA, orexin-A, and Cb1 receptors in inducing anti-nociception in the formalin test.

Effects of social hierarchy on innate fear­induced panic responses.

Studies have previously demonstrated a relationship between social status and anxiety disorders such as panic disorder. Repeated episodes of panic attacks do not occur in combination with an actual fear stimulus or stressor. However, social ranking modulates the perception of the social signals of a threat or stressor. The hypothalamic nuclei are well­known for their role in the elaboration of fear­induced reactions. The dorsomedial hypothalamus (DMH) and the ventromedial hypothalamic (VMH) nuclei are hypothalamic subnuclei involved in the processing of threatening stimuli­evoked aversive response and innate fear development. These structures are also located in the medial amygdala­hypothalamus­brainstem circuit that modulates innate fear­induced defensive behaviors. This work aimed to investigate the relationship between social hierarchy and innate fear­induced panic­like responses in male rats. In our study, the dominance tube test was used to determine the social hierarchy. Then, DMH/VMH nuclei were unilaterally implanted with a guide cannula. After intra­DMH/VMH injection of bicuculline (GABAA receptor antagonist), both innate fear induction and differences in dominant/subordinate rats were evaluated by the open field test. Intra­DMH/VMH bicuculline increased the frequency of defensive immobility, forward escape movements, and crossing behaviors, as well as the duration of defensive immobility and forward escape movements in dominant rats. Subordinate rats showed a higher frequency of defensive attention, defensive immobility, and crossing than dominant rats. Additionally, dominant rats demonstrated a lower duration of defensive attention and defensive immobility than subordinate rats. Dominant rats seemed to adopt a form of innate­fear characterized by increased proactivity with the environment. In contrast, subordinate rats exhibited a reactive form of innate­fear characterized by passivity and freezing.

Intra­LPGi microinjection of glutamate receptors antagonists abolish 17ß­estradiol­induced analgesic effect in the ovariectomized rats.

This study was designed to investigate a possible interaction between 17ß­estradiol and glutamate receptors of the paragigantocellularis lateralis (LPGi) nucleus on pain coping behavior using the formalin test in ovariectomized (OVX) rats. The results showed that intra­LPGi injection of 17ß­estradiol declined flexing behavior in both phases of the formalin test. Still, it only diminished the late phase of licking behavior in the OVX rats. NMDA receptor antagonist, AP5, reversed the analgesic effect of 17ß­estradiol on flexing behavior in both phases of the formalin test in the OVX rats. The 17ß­estradiol­induced anti­nociceptive effect on the flexing duration was prevented by CNQX (AMPA receptor antagonist) only in the early phase of the formalin test in the OVX rats. AP5 and CNQX reduced the anti­nociceptive effect of 17ß­estradiol in the late phase, but not the early phase of licking response in the OVX rats. These results suggested: (i) The intra­LPGi injection of 17ß­estradiol is satisfactory in producing modest analgesia on the formalin­induced inflammatory pain in the OVX rats; (ii) Co­treatment of glutamate receptors (NMDA and AMPA) antagonists and 17ß­estradiol in the LPGi nucleus decrease the analgesic effect of 17ß­estradiol in the OVX rats; (iii) There is a possible association between 17ß­estradiol and glutamate receptors of the LPGi nucleus on pain coping behavior in the OVX rats.

Modulation of anxiety behavior in gonadectomized animals.

Anxiety is a complex psychological state which happens after stressful life experiences. Many factors such as daily life events, neurotransmitter systems, and different brain areas could influence anxiety behavior in humans and animals. For example, opioids and androgens decrease anxiety behavior both in humans and animals. Furthermore, removing the testes (gonadectomy) causes higher levels of anxiety­like behaviors, in which the administration of testosterone and opioid antagonist can reverse some of these behaviors. We review the effects of morphine and androgens on the modulation of anxiety behavior in gonadectomized animals. We begin by highlighting the effects of opioid drugs and androgens on the modulation of anxiety behavior that have been implicated in anxiety behavior. We then discuss the functional consequences of gonadectomy on anxiety behavior. Finally, we consider how the opioids and androgens may contribute to adaptive responses associated with anxiety.

Involvement of glutamate receptors of the paragigantocellularis lateralis nucleus in the pain modulatory effect of 17ß-estradiol in male rats.

The pain modulatory role of the paragigantocellularis lateralis nucleus (LPGi) and the 17ß-estradiol has thoroughly been probed. This study investigates the contribution of ionotropic glutamate receptors in pain modulatory effect of intra-LPGi injection of 17ß-estradiol. For this purpose, the LPGi nucleus cannulation was performed and drugs were injected into this nucleus, 15 min prior to the formalin test. The duration of formalin-induced flexing and licking behaviors was recorded for 60 min immediately after formalin injection. The results showed that the flexing behavior is significantly decreased by intra-LPGi injection of 0.8 µmol 17ß-estradiol duringboth phases of formalin test (P < 0.001). However, 17ß-estradiol attenuated the licking duration only in the second phase (P < 0.001). Interestingly, NMDA and AMPA/kainate receptor antagonists (AP5 and CNQX, respectively) significantly counteracted the analgesic effect of intra-LPGi injection of 17ß-estradiol in both phases of the formalin test (P < 0.001). Consequently, the revealing results showed that the analgesic effect of intra-LPGi injection of 17ß-estradiol on acute inflammatory pain might be mediated via the activation of ionotropic glutamate receptors.

Analgesic Effect of 17ß-Estradiol on Nucleus Paragigantocellularis Lateralis of Male Rats Mediated Via GABAA Receptors.

INTRODUCTION: Beside its autonomic functions, the nucleus paragigantocellularis lateralis (LPGi) is involved in the descending pain modulation. 17ß-Estradiol is a neuroactive steroid found in several brain areas such as LPGi. Intra-LPGi microinjection of 17ß-estradiol can elicit the analgesic responses. 17ß-Estradiol modulates nociception by binding to estrogenic receptors as well as allosteric interaction with other membrane-bound receptors like GABAA receptors. This study aimed to examine the role of GABAA receptors in the pain modulating effect of intra-LPGi injection of 17ß-estradiol. METHODS: To study the antinociceptive effects of 17ß-estradiol, cannulation into the LPGi nucleus of male Wistar rats was performed. About 500 nL of drug was administered 15 minutes prior to formalin injection (50 µL of 4%). Then, formalin-induced flexing and licking behaviors were recorded for 60 minutes. For evaluating the role of GABAA receptors in the estradiol-induced pain modulation, 17ß-estradiol was administered into the LPGi nucleus 15 minutes after the injection of 25 ng/µL bicuculline (the GABAA receptor antagonist). Then, the formalin-induced responses were recorded. RESULTS: The results of the current study showed that intra-LPGi injection of 17ß-estradiol decreased the flexing duration in both phases of formalin test (P<0.001); but it only attenuated the second phase of licking behavior (P<0.001). 17ß-estradiol attenuated the second phase of formalin test of both behaviors (P<0.001). Bicuculline prevented the antinociceptive effect of intra-LPGi 17ß-estradiol in both first and second phases of formalin-induced responses (P<0.001). CONCLUSION: According to the results of this study, the analgesic effect of intra-LPGi 17ß-estradiol on the formalin-induced inflammatory pain might be mediated via GABAA receptors.

Intra-periaqueductal gray matter microinjection of orexin-A decreases formalin-induced nociceptive behaviors in adult male rats.

UNLABELLED: Intracerebroventricular injection of orexin-A (hypocretin-1) has been shown to elicit the analgesic responses. However, the locations of central sites that may mediate these effects have not been clearly elucidated. This study was performed using male Sprague Dawley rats to investigate the antinociceptive effects of intra-periaqueductal gray matter (PAG) administration of orexin-A, 5 minutes prior to formalin (50 µL of 2%) injection. Orexin-A had no effect on tail-flick test as thermal and acute model. In the formalin test, intra-PAG injection of orexin-A (10 nM) decreased the formalin-induced nociceptive behaviors in the interphase and phase 2, but not in phase 1, indicating an antinociceptive role of exogenous orexin-A in the PAG. While Orexin-A failed to produce a dose-dependent decrease in formalin-evoked behaviors in phase 1, it may have induced a dose-dependent decrease in formalin-evoked behaviors in early phase 2. Control injections of orexin-A into the sites near the PAG resulted in less or no reduction in pain, indicating that the analgesia observed is probably due to a site of action within the PAG rather than at surrounding neural structures. The antinociceptive effect of orexin-A was compared with intra-PAG administration of morphine (.5 µL of 20 mM, 5 minutes before the formalin injection). Morphine decreased the formalin-induced nociceptive behaviors in all phases. To investigate whether the orexin has a special action on the early part of the second phase, or its delayed effects are related to its pharmacokinetics, the orexin-A was injected into the PAG, 10 minutes before the formalin injection. No difference was observed between 5 and 10 minutes injection of orexin-A prior to formalin injection. The antinociceptive effect of orexin was blocked by intra-PAG injection of SB-334867, a putative type 1 orexin receptor antagonist, suggesting the involvement of orexin receptor type 1 in antinociception produced with intra-PAG injection of orexin-A. The results showed that the orexin-A plays an antinociceptive role in PAG in the interphase and the late phase of formalin test through type 1 orexin receptor dependent mechanism. PERSPECTIVE: Orexin is produced exclusively in the lateral hypothalamus, where it is known to modulate the pain processing through PAG. The antinociceptive effect of orexin in PAG may provide a role for this neurotransmitter in the up-down modulating pain system and further support the development of orexin-1 agonists for pain treatment.

The effect of intra-locus coeruleus injection of 17beta-estradiol on inflammatory pain modulation in male rat.

Estradiol is a neuroactive steroid found in several brain areas such as locus coeruleus (LC). It modulates nociception by binding to its receptors and also by allosteric interaction with other membrane-bound receptors like glutamate and GABA(A) receptors. LC is involved in noradrenergic descending pain modulation. In order to study the effect of 17beta-estradiol on both acute and persistent pain modulation and its mechanisms, formalin was injected into the male rat's hind paw. Formalin-induced responses including licking, flexing duration and paw jerking frequency were recorded for 60 min after injection of 50 microl of 2% formalin. The results of the current study showed that intra-locus coeruleus injection of 17beta-estradiol attenuated the second phase, but not the acute phase of formalin-induced pain (P<0.05). AMPA receptor antagonists CNQX had no effect on pain-modulatory effect of 17beta-estradiol. Estrogen and GABA(A) receptor antagonists (ICI 182,780 and bicuculline, respectively) could not reverse the antinociceptive effect of 17beta-estradiol. However, NMDA receptor antagonist APV significantly antagonized the analgesic effect of 17beta-estradiol on flexing behaviour (P<0.05). It may be concluded that the analgesic effect of 17beta-estradiol in formalin-induced inflammatory pain is mediated through interaction with membrane-bound receptors, probably the NMDA receptors.