Activation of 5-HT5A receptor in the ventrolateral orbital cortex produces antinociceptive effects in rat models of neuropathic and inflammatory pain

The ventrolateral orbital cortex (VLO) is identified as an integral component of the endogenous analgesic system comprising a spinal cord - thalamic nucleus submedius - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study investigates the effects of 5-HT5A receptor activation in the VLO on allodynia induced by spared nerve injury and formalin-evoked flinching behavior and spinal c-Fos expression in male SD rats, and further examines whether GABAergic modulation is involved in the effects evoked by VLO 5-HT5A receptor activation. We found an upregulation of 5-HT5A receptor expression in the VLO during neuropathic and inflammatory pain states. Microinjection of the non-selective 5-HT5A receptor agonist 5-CT into the VLO dose dependently alleviated allodynia, and flinching behavior and spinal c-Fos expression, which were blocked by the selective 5-HT5A receptor antagonist SB-699551. Moreover, application of the GABAA receptor antagonist bicuculline in the VLO augmented the analgesic effects induced by 5-CT in neuropathic and inflammatory pain states, whereas the GABAA receptor agonist muscimol attenuated these analgesic effects. Additionally, the 5-HT5A receptors were found to be colocalized with GABAergic neurons in the VLO. These results provide new evidence for the involvement of central 5-HT5A receptors in the VLO in modulation of neuropathic and inflammatory pain and support the hypothesis that activation of 5-HT5A receptors may inhibit the inhibitory effect of GABAergic interneurons on output neurons projecting to the PAG (GABAergic disinhibitory mechanisms), consequently activating the brainstem descending inhibitory system that depresses nociceptive transmission at the spinal cord level.

The α1 adrenoceptor antagonist prazosin potentiates morphine induced conditioned place preference in rats

The norepinephrine (NE) system is involved in pathways that regulate morphine addiction. Here, we investigated the role of α1 adrenoceptor in the ventrolateral orbital cortex (VLO) of rats with repeated morphine treatment and underlying molecular mechanisms. The rewarding properties of morphine were assessed by the conditioned place preference (CPP) paradigm. Prazosin, an α1 adrenoceptor antagonist, was microinjected into the VLO. The expression of α1 adrenoceptor, p-CaMKII/CaMKII, CRTC1, BDNF and PSD95 in the VLO were determined by immunohistochemistry or western blotting. Neurotransmitter NE in the VLO and inflammatory factors in serum were detected separately through high-performance liquid chromatography and enzyme-linked immunosorbent assay. Our experimental results showed that repeated morphine administration induced stable CPP and prazosin promoted the morphine-induced CPP. Microinjection of prazosin in the VLO not only blocked the activity of α1 adrenoceptor, decreased CaMKII phosphorylation and CRTC1, which eventually resulted in a regression of synaptic plasticity-related proteins, but also was accompanied by significantly decreasing of NE in the VLO and increasing of inflammatory cytokines in peripheral blood. These findings suggested that prazosin potentiates the addictive effects of morphine. The effect of increased CPP through reducing α1 adrenoceptor and NE was associated with the CaMKII-CRTC1 pathway and synaptic plasticity-related proteins in the VLO and inflammatory cytokines in the peripheral blood. The NE system may therefore be an underlying therapeutic target in morphine addiction. Additionally, we believe that the clinical use of prazosin in hypertensive patients with morphine abuse may be a potential risk because of its reinforcing effect on addiction.

The 5-HT7 receptors in the VLO contribute to the development of morphine-induced behavioral sensitization in rats

The 5-hydroxytryptamine 7 receptor (5-HT₇R) is one of the most recently cloned serotonin receptors which have been implicated in many physiological and pathological processes including drug addiction. Behavioral sensitization is the progressive process during which re-exposure to drugs intensified the behavioral and neurochemical responses to drugs. Our previous study has demonstrated that the ventrolateral orbital cortex (VLO) is critical for morphine-induced reinforcing effect. The aim of the present study was to investigate the effect of 5-HT₇Rs in the VLO on morphine-induced behavioral sensitization and their underlying molecular mechanisms. Our results showed that a single injection of morphine, followed by a low challenge dose could induce behavioral sensitization. Microinjection of the selective 5-HT₇R agonist AS-19 into the VLO during the development phase significantly increased morphine-induced hyperactivity. Microinjection of the 5-HT₇R antagonist SB-269970 suppressed acute morphine-induced hyperactivity and the induction of behavioral sensitization, but had no effect on the expression of behavioral sensitization. In addition, the phosphorylation of AKT (Ser 473) was increased during the expression phase of morphine-induced behavioral sensitization. Suppression of the induction phase could also block the increase of p-AKT (Ser 473). In conclusion, we demonstrated that 5-HT₇Rs and p-AKT in the VLO at least partially contribute to morphine-induced behavioral sensitization.

Electrical lesion of bilateral ventrolateral orbital cortex impairs fear- and space-related learning and affects subsequent choice behavior

OBJECTIVES

Although many studies have indicated that orbitofrontal cortex plays an important role in the learning and retrieval of memory and subsequent decision-making, the role of ventrolateral orbital cortex (VLO) still remains unclear, especially related to fear and space.

METHODS

Four separate cohorts of rats were used in this study. After sham surgery and electrical lesion of bilateral VLO, four cohorts received active avoidance test, passive avoidance test, Morris water maze and T maze separately.

RESULTS

Firstly, data shown that electrolytic lesions of bilateral VLO of Sprague-Dawley rats shortened the latency of rats to escape to darkroom in passive avoidance test. Besides, the damage of VLO also resulted in decrease of the number of active avoidance of rats from the third day during 5 consecutive days' training in active avoidance test. What's more, the impairment of VLO significantly shortened the exploring time in the target quadrant of rats in Morris water maze. Furthermore, VLO-lesions group shown lower correct alternation percentage than sham group in T maze.

CONCLUSIONS

These results indicated that not only in the learning and retrieval of fear-related memory, VLO also plays an important role in the learning and retrieval of spatial-related memory guided by visual cues.

Electrolytic lesions of the bilateral ventrolateral orbital cortex not only directly reduce depression-like behavior but also decreased desperate behavior induced by chronic unpredicted mild stress in rats

BACKGROUND

Previous studies have revealed that ventrolateral orbital cortex (VLO) may play an important role in the regulation of emotional behavior. However, it is not known what effect VLO damage will have on emotion regulation.

RESULTS

Data showed that damage of VLO increased the anxiety-like behavior in open field test and elevated plus maze, and decreased the depressive behavior in forced swimming test and learned helplessness test. Besides, the impulsive aggressive behaviors were also increased while the attack latency decreased after VLO lesion. What's more, damage of VLO decreased depressive behaviors induced by chronic unpredicted mild stress in rats.

CONCLUSIONS

These results suggest that the integrity of VLO plays an important role in emotional regulation, and the damage of VLO may inhibit the development of depression-like behavior.

Sirtuin 1 inhibitor EX527 suppresses morphine-induced behavioral sensitization

Morphine addiction is categorized as a chronic recurrent brain disease which always results in mental disturbance, concomitant diseases and early death. Recent evidence suggested that Sirtuin 1 (SIRT1) played a crucial role in learning, memory and reward, nevertheless, its role in morphine addiction is still unclear. We explored whether SIRT1 in the ventrolateral orbital cortex (VLO) is associated with morphine addiction and its potential mechanism. We applied the morphine-induced behavioral sensitization paradigm to investigate whether microinjection of EX527, a SIRT1 inhibitor, into the VLO could affect the rat behaviors. Furthermore, we focused on the expression of extracellular signal-regulated protein kinases (ERK) and brain-derived neurotrophic factor (BDNF), potential downstream targets of SIRT1. Microinjecting EX527 into the VLO significantly suppressed morphine-induced behavioral sensitization. We found that the expression of SIRT1, phosphorylated ERK (p-ERK) and BDNF in the VLO were markedly up-regulated by morphine administrations in expression phase. These positive changes were significantly inhibited by microinjecting EX527 into the VLO. These results suggest that SIRT1 in the VLO may mediate morphine-induced behavioral sensitization and the overexpression of SIRT1, p-ERK and BDNF could be the potential mechanism. Taken together, the results of our research provide evidence to support that SIRT1 play an important role in morphine vulnerability and microinjecting EX527 into the VLO could significantly suppress morphine addiction in rats.

Activation of CB1 receptors on GABAergic interneurons in the ventrolateral orbital cortex induces analgesia

The prefrontal ventrolateral orbital cortex (VLO) is involved in antinociception. It has been found that dopamine receptors, adrenoceptors, serotonin receptors and μ-opioid receptors are involved in this effect through direct/indirect activation of the VLO output neurons. However, the effect of CB1 receptors on the VLO modulation of pain has not been studied. In this study, we investigated whether activation of CB1 receptors in the VLO modulates nociception. A common peroneal nerve (CPN) ligation model was used to induce neuropathic pain in male mice. On day 13 after CPN ligation, spontaneous firing of the VLO pyramidal neurons was recorded and CB1 receptor level in the VLO was detected. Mechanical allodynia was measured after HU210 was microinjected into the VLO. Relative contribution of CB1 receptors on GABAergic neurons and glutamatergic neurons was determined by CB1 receptor knockdown using a viral strategy. Our data indicated that on day 13 after nerve injury, spontaneous firing of the VLO pyramidal neurons reduced significantly but was enhanced by intraperitoneal injection of HU210 (20 μg/kg), a potent CB1 receptor agonist. Expression of CB1 receptor in the VLO was up-regulated. Microinjection of HU210 into the VLO attenuated allodynia, and this effect was blocked by pre-microinjection of specific CB1 receptor antagonist AM281. Deletion of CB1 receptors on GABAergic neurons in the VLO can completely block HU210-induced analgesia. Thus, it can be concluded that activation of CB1 receptors on GABAergic interneurons in the VLO may be involved in analgesia effect of cannabinoids.

Microinjection of valproic acid into the ventrolateral orbital cortex exerts an antinociceptive effect in a rat of neuropathic pain

RATIONALE

Ventrolateral orbital cortex (VLO) has been found to play an important role in the regulation of neuropathic pain (NPP). As a traditional mood stabilizer, valproic acid (VPA) is currently employed in the treatment of NPP. However, whether VPA plays an analgesic role in VLO is still unknown.

OBJECTIVES

To elucidate the underlying analgesic mechanism of microinjection of VPA into the VLO on spared nerve injury (SNI), an animal model of NPP.

METHODS

We firstly examined the role of VPA by intraperitoneal and intral-VLO injection. Then, we accessed its role as a histone deacetylase inhibitor by intral-VLO microinjection of sodium butyrate. Finally, the GABAergic mechanism was measured through the intra-VLO microinjection of several agonists and antagonists of various GABAergic receptor subtypes.

RESULTS

Both intraperitoneal and intral-VLO injection of VPA attenuated SNI-induced mechanical allodynia. Microinjection of sodium butyrate, one of the histone deacetylase inhibitors, into the VLO attenuated the mechanical allodynia. Besides, microinjection of valpromide, a derivative of VPA which is a GABAergic agonist, into the VLO also attenuated allodynia. Furthermore, microinjection of picrotoxin, a GABAA receptor antagonist, into the VLO attenuated mechanical allodynia; microinjection of picrotoxin before VPA into the VLO increased VPA-induced anti-allodynia. Besides, microinjection of CGP 35348, a GABAB receptor antagonist, into the VLO attenuated allodynia; microinjection of CGP 35348 before VPA into the VLO also increased VPA-induced anti-allodynia. What is more, microinjection of imidazole-4-acetic acid (I4AA), a GABAC receptor antagonist, into the VLO enhanced allodynia; microinjection of I4AA before VPA into the VLO decreased VPA-induced anti-allodynia.

CONCLUSIONS

These results suggest that both the histone acetylation mechanism and GABAergic system are involved in mediating VLO-induced anti-hypersensitivity.

The pronociceptive role of 5-HT6 receptors in ventrolateral orbital cortex in a rat formalin test model

Recent studies have shown the 5-HT₆ receptors are expressed in regions which are important in pain processing such as the cortex, amygdala, thalamus, PAG, spinal cord and dorsal root ganglia (DRG), suggesting a putative role of 5-HT₆ receptors in pain modulation. The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord - thalamic nucleus submedius (Sm) - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study assessed the possible role of 5-HT₆ receptors in the VLO in formalin-induced inflammatory pain model. Firstly we found that microinjection of selective 5-HT₆ receptor agonists EMD-386088 (5 μg in 0.5 μl) and WAY-208466 (8 μg in 0.5 μl) both augmented 5% formalin-induced nociceptive behavior. Microinjection of selective 5-HT₆ receptor antagonist SB-258585 (1,2 and 4 μg in 0.5 μl) significantly reduced formalin-induced flinching. Besides, the pronociceptive effects of EMD-386088 and WAY-208466 were dramatically reduced by SB-258585, implicating 5-HT₆ receptor mechanisms in mediating these responses. In addition, the pronociceptive effect of EMD-386088 was also prevented by the adenylate cyclase (AC) inhibitor SQ-22536 (2 nmol in 0.5 μl) and the protein kinase A (PKA) inhibitor H89 (10 nmol in 0.5 μl), respectively. We further confirmed the above results with quantification of spinal c-fos expression. Taken together, our results suggested that 5-HT₆ receptors play a pronociceptive role in the VLO in the rat formalin test due to its activation of AC - PKA pathway. Therefore, cerebral cortical 5-HT₆ receptors could be a new target to develop analgesic drugs.

Activation of α1 adrenoceptors in ventrolateral orbital cortex attenuates allodynia induced by spared nerve injury in rats

Recent studies have demonstrated that noradrenaline acting in the ventrolateral orbital cortex (VLO) can potentially reduce allodynia induced by spared nerve injury (SNI), and this effect is mediated by α2 adrenoceptor. The present study examined the effect of the α1 adrenoceptors in the VLO on allodynia induced by SNI in the rats. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of selective α1 adrenoceptor agonist methoxamine (20, 50, 100 μg in 0.5 μl) into the VLO, contralateral to the site of nerve injury, increased PWT in a dose-dependent manner. This effect was antagonized by pre-microinjection of the selective α1 adrenoceptor antagonist benoxathian into the same VLO site, and blocked by electrolytic lesion of the ventrolateral periaqueductal gray (PAG). Furthermore, pre-administration of non-selective glutamate receptor antagonist kynurenic acid, phospholipase C (PLC) inhibitor U73122, and protein kinase C (PKC) inhibitor chelerythrine to the VLO also blocked methoxamine-induced inhibition of allodynia. These results suggest that activation of α1 adrenoceptors in the VLO can potentially reduce allodynia induced by SNI. This effect may be direct excitation of the VLO neurons, via PLC-PKC signaling pathway, projecting to the PAG or facilitating glutamate release and then indirectly exciting the VLO output neurons projecting to the PAG, leading to activation of the PAG-brainstem descending inhibitory system which depresses the nociceptive transmission at the spinal cord level.

Administration of somatostatin analog octreotide in the ventrolateral orbital cortex produces sex-related antinociceptive effects on acute and formalin-induced nociceptive behavior in rats

The present study was designed to examine whether somatostatin analog octreotide (OCT) was involved in antinociception in the ventrolateral orbital cortex (VLO) and determine whether this effect had a sex difference between male and female rats. The radiant heat-evoked tail flick (TF) reflex was used as an index of acute nociceptive response in lightly anesthetized rats. The number of flinches evoked by formalin injection into the hindpaw was used to evaluate inflammatory persistent pain in conscious rats. Administration of OCT (2.0, 5.0 10.0 ng in 0.5 µl) into the VLO depressed the TF reflex in a dose-dependent manner only in female rats, but not male rats. Pretreatment with a nonselective somatostatin receptor antagonist cyclo-somatostatin (c-SOM) (25.0 µg in 0.5 µl) into the VLO antagonized 10.0 ng OCT-induced inhibition of the TF reflex in female rats. Similarly, application of high dose of OCT (10.0 ng in 0.5 µl) into the VLO depressed formalin-induced flinching response in the early and late phases only in female rats, and had no any effects in male rats. Pretreatment with c-SOM (25.0 µg in 0.5 µl) into the VLO totally antagonized the 10 ng OCT-induced inhibition of the flinches in both phases in female rats. Additionally, single administration of c-SOM into the VLO failed to alter tail reflex latencies and formalin-induced nociceptive behaviors in female rats. The results provide the first valuable evidence that somatostatin and its receptors are involved in antinociception in acute heat-evoked nociception and inflammatory persistent pain only in female rats, not male rats, in the VLO.

Effect of microinjecting of 5-aza-2-deoxycytidine into ventrolateral orbital cortex on depressive-like behavior in rats

DNA methylation and histone modification are two major epigenetic mechanisms involved in the pathophysiology of major depressive disorder (MDD) and the action of antidepressants. We and others have recently shown that epigenetic regulation through histone acetylation within ventrolateral orbital cortex (VLO) contributes to the antidepressant-like effects of histone deacetylase inhibitors [HDACi, such as valproic acid (VPA) and MS-275] observed in rats. However, there is so far no investigation focused on the effect of DNA methylation in VLO on depressive-like behaviors. Here, we examined the effects of the DNA methyltransferases (DNMTs) inhibitor 5-aza-2-deoxycytidine (5-aza) on rat forced swimming test (FST) and locomotor activity when microinjected into VLO. We found that bilateral intra-VLO injections of 5-aza increased the duration of immobility in FST in a dose-dependent manner compared to vehicle-treated controls. The effects of 5-aza observed in the FST paradigms could not be attributed to non-specific decreases in activity since the inhibition of DNA methylation in VLO did not cause general impairment in locomotor activity. These results add to the evidence that DNA hypomethylation in VLO is involved in regulating depressive-like behaviors, and suggest that the effect of DNA methylation on depressive-like behaviors appear to be brain region-dependent.

The role of α₂ adrenoceptor in mediating noradrenaline action in the ventrolateral orbital cortex on allodynia following spared nerve injury

The present study examined the role of α₂ adrenoceptor in mediating noradrenaline action in the ventrolateral orbital cortex (VLO) on allodynia induced by spared nerve injury (SNI) in the rat. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of noradrenaline (1, 2, 4 μg in 0.5 μl) into the VLO, contralateral to the site of nerve injury, reduced allodynia; PWT increased in a dose-dependent manner. Similar to noradrenaline, microinjection of selective α₂ adrenoceptor agonist clonidine into the same VLO site also reduced allodynia, and was blocked by selective α₂ adrenoceptor antagonist yohimbine. Furthermore, administration of γ-aminobutyric acid A (GABAA) receptor antagonist bicuculline or picrotoxin to the VLO significantly enhanced clonidine-induced inhibition of allodynia, while GABAA receptor agonist muscimol or THIP (2,5,6,7-retrahydroisoxazolo(5,4-c)pyridine-3-ol hydrochloride) attenuated clonidine-induced inhibition. These results suggest that noradrenaline acting in the VLO can potentially reduce allodynia induced by SNI, and this effect is mediated by α₂ adrenoceptor. Moreover, GABAergic disinhibition may participate in α₂ receptor mediating effects in neuropathic pain in the central nervous system.