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.

Activation of mu-opioid receptors in the ventrolateral orbital cortex inhibits the GABAergic miniature inhibitory postsynaptic currents in rats.

Previous studies have indicated that mu-opioid receptors in the ventrolateral orbital cortex (VLO) are involved in antinociception in tail flick tests and GABAergic neurons or terminals express mu-opioid receptors in the VLO. The current study examined the effect of selective mu-opioid receptor agonist DAMGO on the GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the VLO in rats using the whole-cell patch clamp. The results demonstrated that 5 µM DAMGO application into the rat VLO slices significantly reduced the GABAergic mIPSCs frequency, without any effect on its amplitude, and this effect of DAMGO was reversed by pretreatment with selective mu-opioid receptor antagonist 1 µM CTOP. Importantly, application of CTOP alone into the VLO slices did not produce any effect on the frequency and amplitude of GABAergic mIPSCs. These results indicate a presynaptic effect of mu-opioid receptor activation on the GABAergic neurons in the VLO. The current data suggests that a presynaptic inhibition of the GABA release may contribute to the mu-opioid receptor mediated effects in the VLO and provides novel electrophysiological evidence for the underlying mechanisms of mu-opioid receptors in the VLO.

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.

Microinjection of valproic acid into the ventrolateral orbital cortex enhances stress-related memory formation.

There is collecting evidence suggesting that the process of chromatin remodeling such as changes in histone acetylation contribute to the formation of stress-related memory. Recently, the ventrolateral orbital cortex (VLO), a major subdivision of orbitofrontal cortex (OFC), was shown to be involved in antidepressant-like actions through epigenetic mechanisms. Here, we further investigated the effects of the histone deacetylase inhibitor (HDACi) valproic acid (VPA) on stress-related memory formation and the underlying molecular mechanisms by using the traditional two-day forced swimming test (FST). The results showed that VPA significantly increased the immobility time on day 2 when infused into the VLO before the initial forced swim stress on day 1. The learned immobility response to the stress was associated with increased phosphorylation of extracellular signal-regulated kinase (ERK) in VLO and hippocampus on the first day. The levels of phosphorylated ERK (phospho-ERK) in VLO and hippocampus were significantly decreased when retested 24 h later. The pretreatment with intra-VLO VPA infusion further reduced the activation of ERK on day 2 and day 7 compared with the saline controls. Moreover, the VPA infusion pretreatment also induced a significantly decreased BDNF level in the VLO on day 2, whereas no change was detected in the hippocampus. These findings suggest that VPA enhance the memories of emotionally stressful events and the ERK activity is implicated in stimulating adaptive and mnemonic processes in case the event would recur.

The role of dopamine receptors in ventrolateral orbital cortex-evoked antinociception in a rat formalin test model.

The present study examined the roles of dopamine and D(1)- and D(2)-like dopamine receptors in ventrolateral orbital cortex (VLO)-evoked antinociception in rats with persistent inflammatory pain. Following formalin injection into the rat unilateral hindpaw pad, the effects of dopamine receptor agonist and antagonist microinjections into the VLO on nociceptive behavior were observed. Results demonstrated that VLO microinjection of the non-selective dopamine receptor agonist apomorphine (R(-)-apomorphine hydrochloride, 1.0, 2.5 and 5.0µg) depressed later-phase nociceptive behavior induced by formalin injection; this effect was attenuated by the D(2)-like dopamine receptor antagonist S(-)-raclopride(+)-tartrate salt (raclopride, 3.0µg), but not by the D(1)-like dopamine receptor antagonist R(+)-SCH-23390 hydrochloride (SCH-23390, 1.0µg). Apomorphine-induced antinociception was mimicked by microinjection of the D(2)-like dopamine receptor agonist (-)-quinpirole hydrochloride (2.0 and 5.0µg) into the same VLO site, and this effect was antagonized by raclopride (3.0µg). In addition, microinjection of the D(1)-like dopamine receptor agonist R(+)-SKF-38393 hydrochloride (5.0µg) had no effect on formalin-induced nociceptive behavior during the later phase. However, the D(1)-like dopamine receptor antagonist SCH-23390 (2.5, 5.0 and 10µg) depressed nociceptive behavior in a dose-dependent manner. These results suggested that dopamine mediated VLO-induced antinociception via different mechanisms in the persistent inflammatory pain model; D(2)-like receptors mediated dopamine-induced antinociception, while D(1)-like dopamine receptors exhibited tonic facilitatory action on nociceptive behavior, thereby blocking D(1)-like dopamine receptors could induce antinociception.

Activation of serotonin 1A receptors in ventrolateral orbital cortex depresses persistent nociception: a presynaptic inhibition mechanism.

The present study examined the effect of serotonin 1A (5-HT(1A)) receptor activation in the ventrolateral orbital cortex (VLO) upon formalin-evoked flinching behavior and spinal Fos expression, and further determined whether activation of 5-HT(1A) receptors affected the spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in rat VLO slice by pharmacologically separated neurons to understand the possible mechanism underlying this effect. Microinjection of the 5-HT(1A) receptors agonist 8-OH-DPAT (8-hydro-2-(di-n-propylamino) tetralin) into the VLO depressed the formalin-evoked nociceptive behavior flinching response and the Fos expression in the lumbar spinal cord dorsal, which was antagonized by pre-treatment with 5-HT(1A) receptors antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide). Furthermore, application of 8-OH-DPAT into VLO slice inhibited GABAergic mIPSC frequency in a dose-dependent manner without effects on amplitude of the GABAergic mIPSCs, this effect was blocked by NAN-190. These results provide evidence for the involvement of 5-HT(1A) receptors in VLO in the modulation of persistent inflammatory nociception, and suggest that a presynaptic inhibition of the GABA release may contribute to the 5-HT(1A) receptor-mediated descending antinociception.

The thalamic nucleus submedius and ventrolateral orbital cortex are involved in nociceptive modulation: a novel pain modulation pathway.

Recently, a series of studies have given rise to and provided evidence for the hypothesis that the nucleus submedius (Sm) in the medial thalamus is involved in modulation of nociception. The Sm, ventrolateral orbital cortex (VLO) and the periaqueductal gray (PAG) constitute a pain modulatory pathway, activation of which leads to activation of the PAG-brainstem descending inhibitory system and depression of the nociceptive inputs in the spinal cord and trigeminal nucleus. Other studies have indicated that the Sm-VLO-PAG pathway plays an important role in the analgesia induced by electroacupuncture stimulation of the acupuncture point (acupoint) for exciting small diameter fiber (A-delta and C group) afferents. Opioid peptides, serotonin, dopamine, glutamate and their related receptors are involved in Sm- and/or VLO-mediated descending antinociception, and a GABAergic disinhibitory mechanism participates in mediating the antinociception induced by activation of mu-opioid receptors, serotonin 1(A) receptors, and dopamine D(2)-like receptors. This review describes these findings, which provide important new insights into the roles of the thalamus and cerebral cortex in descending pain modulation.

D2-like but not D1-like dopamine receptors are involved in the ventrolateral orbital cortex-induced antinociception: a GABAergic modulation mechanism.

The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system consisting of an ascending pathway from the spinal cord to VLO via the thalamic nucleus submedius (Sm) and a descending pathway to the spinal cord relaying in the periaqueductal gray (PAG). This study examines whether activation of D(1)-like and D(2)-like dopamine receptors in VLO produces antinociception and whether GABAergic modulation is involved in the VLO, D(2)-like dopamine receptor activation-evoked antinociception. The radiant heat-evoked tail flick (TF) reflex was used as an index of nociceptive response in lightly anesthetized rats. Microinjection of the D(2)-like (D(2)/D(3)) dopamine receptor agonist quinpirole (0.1-2.0 microg), but not D(1)-like (D(1)/D(5)) receptor agonist SKF-38393 (1.0, 5.0 microg), into VLO produced dose-dependent antinociception which was antagonized by the D(2)-like (D(2)/D(3)) receptor antagonist raclopride (1.5 microg). We also found that VLO application of the GABA(A) receptor antagonist bicuculline or picrotoxin (100 ng) enhanced the quinpirole-induced inhibition of the TF reflex, whereas the GABA(A) receptor agonist muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the quinpirole-induced inhibition. These results suggest that D(2)-like, but not D(1)-like, dopamine receptors are involved in VLO-induced antinociception and that GABAergic disinhibitory mechanisms participate in the D(2)-like receptor mediated effect. These findings provide support for the hypothesis that D(2)-like receptor activation may inhibit the inhibitory action of the GABAergic interneurons on the output neurons projecting to PAG leading to activation of the brainstem descending inhibitory system and depression of nociceptive inputs at the spinal dorsal horn.

Synaptic connections between GABAergic elements and serotonergic terminals or projecting neurons in the ventrolateral orbital cortex.

The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord-thalamic nucleus submedius-VLO periaqueductal gray (PAG)-spinal cord loop. The present study examined morphological connections of GABAergic (gamma-aminobutyric acidergic) neurons and serotonergic projection terminals from the dorsal raphe nucleus (DR), as well as the relationship between GABAergic terminals and VLO neurons projecting to the PAG, by using anterograde and retrograde tracing combined with immunofluorescence, immunohistochemistry, and electron microscopy methods. Results indicate that the majority (93%) of GABAergic neurons in the VLO also express the 5-HT(1A) (5-hydroxytryptamine 1A) receptor, and serotonergic terminals originating from the DR nucleus made symmetrical synapses with GABAergic neuronal cell bodies and dendrites within the VLO. GABAergic terminals also made symmetrical synapses with neurons expressing GABA(A) receptors and projecting to the PAG. These results suggest that a local neuronal circuit, consisting of 5-HTergic terminals, GABAergic interneurons, and projection neurons, exists in the VLO, and provides morphological evidence for the hypothesis that GABAergic modulation is involved in 5-HT(1A) receptor activation-evoked antinociception.

GABAergic modulation is involved in the ventrolateral orbital cortex 5-HT 1A receptor activation-induced antinociception in the rat.

The ventrolateral orbital cortex (VLO) is a component of an endogenous analgesic system consisting of an ascending pathway from the spinal cord to VLO via the thalamic nucleus submedius (Sm) and a descending pathway relaying in the periaqueductal gray matter (PAG). This study examines whether the activation of 5-HT 1A receptors in VLO produces antinociception and whether GABAergic modulation is involved in the VLO 5-HT 1A receptor activation-evoked antinociception. The radiant heat-evoked tail flick (TF) reflex was used as an index of nociceptive response in lightly anesthetized rats. Microinjection of the 5-HT 1A receptor agonist 8-OH-DPAT (1.0, 2.0, 5.0 microg) into VLO produced dose-dependent antinociception, which was reversed by the 5-HT 1A receptor antagonist (NAN-190, 20 mug). We also found that VLO application of the GABA A receptor antagonist bicuculline or picrotoxin (100 ng) enhanced the 8-OH-DPAT-induced inhibition of the TF reflex, whereas the GABA A receptor agonist muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the 8-OH-DPAT-induced inhibition. These results suggest that 5-HT 1A receptors are involved in VLO-induced antinociception and that GABAergic disinhibitory mechanisms participate in the 5-HT 1A receptor-mediated effect. These findings provide support for the hypothesis that 5-HT 1A receptor activation may inhibit the inhibitory action of the GABAergic interneurons on the output neurons projecting to PAG leading to activation of the brainstem descending inhibitory system and depression of nociceptive inputs at the spinal cord level.

[Roles of ventrolateral orbital cortex in pain modulation and acupuncture analgesia].

The ventrolateral orbital cortex (VLO) is a major component of orbital cortex, which has extensive connections with periaqueductal gray (PAG), thalamus and other cortical regions. Researches suggest that the VLO is involved not only in nociception, but also in pain modulation, through activation of PAG brainstem descending antinociceptive pathway to inhibit the nociceptive inputs at the spinal/trigeminal level. Furthermore, many results demonstrate that opioid, 5-HT, GABA and their receptors are involved in the VLO antinociception. VLO plays an important role in acupuncture analgesia. In this review we summarized the roles of ventrolateral orbital cortex in pain modulation and acupuncture antinociception.

Involvement of GABAergic modulation of antinociception induced by morphine microinjected into the ventrolateral orbital cortex.

Previous studies have shown that microinjection of morphine into the prefrontal ventrolateral orbital cortex (VLO) produces antinociception. The current study examined whether gamma-aminobutyric acid (GABA) containing neurons in the VLO were involved in this antinociception. Under light anesthesia, the GABA(A) receptor antagonist bicuculline and picrotoxin or agonist muscimol and THIP was microinjected into the VLO in non-morphine-treated (control) and morphine-treated (microinjection into the VLO) rats. Noxious heat-evoked tail flick (TF) latencies (TFLs) were measured in all of these groups of rats every 5 min. Bicuculline or picrotoxin (100, 200, 500 ng in 0.5 microl) depressed the TF reflex in a dose-related fashion. A smaller dose (100 ng) of bicuculline or picrotoxin microinjected into VLO significantly enhanced the VLO morphine-evoked inhibition of the TF reflex. In contrast, administration of muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the morphine-induced antinociception in the VLO morphine-treated rats. These results suggest that the GABA(A) receptor is involved in the modulation of VLO morphine-induced antinociception, and provide a behavioral support for the hypothesis that morphine may directly inhibit the GABAergic inhibitory interneurons leading to indirect activation of the descending antinociceptive pathway through a disinhibitory effect on the VLO output neurons and depression of the nociceptive inputs at the spinal cord level.