Search

The present study examined the role of dopamine and D(1)-and D(2)-like dopamine receptors in ventrolateral orbital cortex (VLO)-evoked anti-hypersensitivity in a rat model of neuropathic pain, as well as the possible underlying mechanisms. Results showed that microinjection of apomorphine [(R(-)-apomorphine hydrochloride)], a non-selective dopamine receptor agonist, into the VLO attenuated spared nerve injury (SNI)-induced mechanical allodynia in a dose-dependent manner. This effect was completely blocked by the D(2)-like dopamine receptor antagonist S(-)-raclopride(+)-tartrate salt (1.5 microg), but was enhanced by the D(1)-like dopamine receptor antagonist SCH23390 (R(+)-SCH-23390 hydrochloride, 5.0 microg). The attenuating effect of apomorphine on mechanical allodynia was mimicked by application of the D(2)-like dopamine receptor agonist quinpirole [((-)-quinpirole hydrochloride, 0.5, 1.0, and 2.0 microg)]. In addition, microinjection of larger doses (10 and 20 microg) of SCH23390 into the VLO significantly attenuated allodynia. Furthermore, microinjections of GABA(A) receptor antagonists, bicuculline [(+)-bicuculline,(S), 9(R)] and picrotoxin (200 and 300 ng for both drugs), into the VLO attenuated mechanical allodynia. A small dose of bicuculline or picrotoxin (100 ng) resulted in increased quinpirole (0.5 microg)-induced anti-allodynia. In contrast, GABA(A) receptor agonists, muscimol hydrochloride (250 ng) or THIP [(2,5,6,7-retrahydroisoxazolo(5,4-c)pyridine-3-ol hydrochloride, 1.0 microg)], blocked quinpirole (2.0 microg)-induced attenuation. These results suggest that the dopaminergic system is involved in mediating VLO-induced anti-hypersensitivity, activation of D(2)-like dopamine receptors, and inhibition of D(1)-like receptors resulting in anti-hypersensitivity. In addition, the mechanisms of GABAergic disinhibition might be involved in D(2)-like receptor mediating effects in neuropathic pain.

Previous studies have indicated that μ-opioid receptors in the thalamic nucleus submedius (Sm) are involved in descending antinociception in behavioral tests. The present study examined the effect of μ-opioid receptor activation in the Sm upon bee venom–evoked c-Fos expression in the spinal dorsal horn associated with flinching behavior, and determined whether the ATP-sensitive potassium channel (K–ATP channel) was involved in this effect in a rat model. A dilute bee venom solution, subcutaneously injected unilaterally into a rat hind paw pad, induced significant c-Fos expression in the lumbar spinal dorsal horn, which is associated with paw flinching behavior. This effect was depressed by microinjection of the μ-opioid receptor agonist [ d -Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO) into the Sm, which was antagonized by pre-treatment with μ-receptor antagonist β-funaltrexamine at the same Sm site. Further studies found that glibenclamide, a K–ATP channel inhibitor, also blocked DAMGO-induced inhibition. These results provide functional anatomic support for the involvement of Sm and μ-opioid receptors in the modulation of persistent inflammatory nociception, and suggest that these effects were produced by opening K–ATP channel and inhibiting neuronal activity. Together with previous studies, the inhibition of the neuronal activity induced by μ-opioid receptor activation may activate descending antinociceptive pathways through a GABAergic disinhibitory mechanism and depress the nociceptive information transmission at the level of the spinal cord.

The present study examined the involvement of 5-HT in the ventrolateral orbital cortex (VLO) on descending antinociception and determined which subtypes of 5-HT receptors mediated this effect. This study focused on the effects of 5-HT microinjection in the VLO of lightly anesthetized male rats on the radiant heat-evoked tail flick (TF) reflex, as well as the influence of 5-HT(1A), 5-HT(2), 5-HT(3), and 5-HT(4) receptor subtype antagonists on the effect of 5-HT. Results showed that 5-HT microinjection (2, 5, 10 microg, in 0.5 microl) into the VLO depressed the TF reflex in a dose-dependent manner. Pretreatment with 5-HT receptor antagonists (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190), cyproheptadine hydrochloride (CPT) and 1-methyl-N-(8-methyl-8-azabicyclo[3.2.3]-oct-3-yl)-1H-indazole-3-carboxamide maleate salt (LY-278,584)), specific for 5-HT(1A), 5-HT(2) and 5-HT(3) receptors, respectively, partially reversed the 5-HT-evoked inhibition. In contrast, the 5-HT(4) receptor antagonist, 1-[2-[(methylsulfonyl)-amino]ethyl]-4-piperidinyl]methyl1-methyl-1H-indole-3-carboxylate (GR 113808), had no effect on the inhibition of 5-HT. Microinjections of NAN-190, CPT and LY-278,584 alone into the VLO had no effect on the TF reflex. These results suggest that 5-HT(1A), 5-HT(2) and 5-HT(3), but not 5-HT(4) receptors, are involved in mediating 5-HT-induced antinociception in the VLO. According to different properties and distribution patterns of the 5-HT receptor subtypes on neurons, the possible mechanism of 5-HT activation of the VLO-periaqueductal gray (PAG) descending antinociceptive pathway is discussed.

The present study examined the roles of ventrolateral orbital cortex (VLO) 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7 receptor subtypes in mediating 5-HT-induced antiallodynic actions in the rat spared nerve injury (SNI) pain model. Changes in paw withdrawal threshold (PWT) were measured using von-Frey filaments. Microinjection of 5-HT (2, 5 and 10μg, in 0.5μl) into the VLO depressed allodynia induced by SNI, and the PWT increased in a dose-dependent manner. Microinjection of selective 5-HT1A, 5-HT2, 5-HT3, 5-HT4, 5-HT5A, 5-HT6 and 5-HT7 receptor antagonists, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190) (10μg), cyproheptadine (50ng), granisetron hydrochloride (granisetron) (10μg), 1-[2-[(methylsulfonyl)-amino]ethyl]-4-piperidinyl]methyl1-methyl-1H-indole-3-carboxylate (GR113808) (5μg), SB699551 dihydrochloride (SB699551) (10μg), SB258585 dihydrochloride (SB258585) (2μg) or SB269970 hydrochloride (SB269970) (10μg) into the VLO 5-min prior to 5-HT (10μg) injection, all antagonized the 5-HT-induced inhibition of allodynia. In addition, these antagonists applied alone to VLO did not influence allodynia. These results suggest that although 5-HT1-7 receptor subtypes in the VLO do not have a tonic modulatory action on the allodynia induced by SNI, they are involved in mediating the depression of the SNI allodynia produced by injection of 5HT into VLO.

Recent study from our laboratory has indicated that microinjection of glutamate into the nucleus tractus solitarius (NTS) facilitates the cardiac-somatic reflex induced by pericardial capsaicin. Further, N-methyl- d -aspartate (NMDA) receptors and metabotropic glutamate receptors (mGluRs) mediate this function. However, the roles of the individual receptor subtypes or subunits in modulating cardiac nociception are unknown. Among the three groups of mGluRs, group III mGluRs are the primary mGluR subtype expressed in visceral afferent neurons in the NTS. The present study examined the roles of group III mGluRs and their subtype 7 and 8 receptors (mGluR7 and mGluR8) in modulating the cardiac-somatic reflex induced by pericardial capsaicin, which was monitored by recording electromyogram (EMG) activity from the spinotrapezius muscle in anesthetized rats. Intra-NTS microinjection of a group III mGluR agonist, l -(+)-2-Amino-4-phosphonobutyric acid ( l -AP4, at 1, 10, and 20 nmol) or a selective mGluR7 agonist, N,N′-diphenylmethyl-1,2-ethanediamine dihydrochloride (AMN082, at 1, 2, and 4 nmol) both decreased the EMG response in a dose-dependent manner. This decrease was inhibited by the group III mGluR antagonist (RS)-α-Methylserine-O-phosphate (MSOP, at 20 nmol). In contrast, intra-NTS microinjection of a selective mGluR8 agonist, (S)-3, 4-dicarboxyphenylglycine (DCPG, at 6 and 8 nmol), significantly increased the EMG response above control levels. This effect was eliminated by intra-NTS MSOP and by vagal deafferentation. These data suggest that group III mGluRs and mGluR7 in the NTS display an inhibitory effect, while mGluR8 displays a facilitatory effect in modulating cardiac nociception, and this facilitatory effect is dependent on vagal afferents.

Many patients suffer from secondary muscle hyperalgesia after experiencing angina pectoris. In this study, we examined the role of the nucleus tractus solitarius (NTS) and glutamate receptors in modulating cardiac-evoked muscle hyperalgesia induced by pericardial capsaicin, which was monitored by recording electromyogram (EMG) activity from the spinotrapezius muscle in the anesthetized rat. Unilateral chemical lesioning of the commissural NTS with the neurotoxin ibotenic acid significantly depressed the cardiac-somatic reflex; the EMG responses decreased to 56.4 ± 6.9% of that of the controls (5 of 5). Microinjection of the excitatory amino acid glutamate, at 10, 20, and 50 nmol, into the commissural NTS increased the EMG response, in a dose-dependent manner, to 116.9 ± 4.9%, 143.9 ± 10.2%, and 214.2 ± 15.8% (n=8), respectively, of that of the controls. In contrast, microinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a, d]-cyclohepten-5,10-imine maleate (MK-801) at 4 and 6 nmol, decreased the EMG response to 45.2 ± 10.6% and 36.8 ± 14.3%, respectively, of that of the controls (n=8 for each dose). Similarly, the metabotropic glutamate receptor (mGluR) antagonist (RS)-a-methyl-4-carboxyphenylglycine (MCPG), at 2.5 and 5 nmol, decreased the EMG response to 65.2 ± 16.3% and 57.0 ± 4.2%, respectively, of that of the controls. When a combination of MK-801 and MCPG was administrated, the EMG response further decreased to 22.5 ± 13.2% (n=6) of that of the controls. However, administration of a non-NMDA receptor antagonist 6, 7-dinitroquinoxaline-2, 3-dione (DNQX), at 2 and 5 nmol, had no effect on the EMG response. These results suggest that the NTS is involved in the facilitation of the cardiac-somatic reflex, and that the NMDA receptor and mGluRs play an important role in mediating this effect.

GABAergic projection from thalamic reticular nucleus to thalamic nucleus submedius in the medial thalamus of the rat was studied by using immunohistochemistry for GABA, retrograde labeling with Fluoro-Gold combined with immunohistochemistry for GABA, and anterograde labeling with biotinylated dextranamine. Immunohistochemistry displayed that only GABA immunoreactive terminals were observed in the thalamic nucleus submedius, while GABA immunoreactive neuronal cell bodies were located in the thalamic reticular nucleus and lateral geniculate nucleus. Injection of Fluoro-Gold into the thalamic nucleus submedius resulted in massive retrogradely labeled neuronal cell bodies in the rostroventral portion of the ipsilateral thalamic reticular nucleus and a few in the contralateral thalamic reticular nucleus, and most of these cell bodies showed GABA immunopositive staining. Many biotinylated dextranamine anterogradely labeled fibers and terminals in the thalamic nucleus submedius were observed after injection of biotinylated dextranamine into the thalamic reticular nucleus. The present results provide a morphological evidence for a hypothesis that a disinhibitory effect on output neurons elicited by opioid or 5-hydroxytryptamine inhibiting a GABAergic terminal in the thalamic nucleus submedius may lead to activation of the descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

The present study was designed to investigate the roles of different subtypes of opioid receptors in ventrolateral orbital cortex (VLO) opioid-evoked antinociception in formalin test by using an automatic detection system for recording the nociceptive behavior (agitation) and a manual method for detecting the duration of licking the injected paw in the conscious rat. Formalin (5%, 50 microl) s.c. injected into the hindpaw produced a biphasic agitation response or lengthening duration of licking. Morphine (5 microg) microinjected unilaterally into VLO significantly inhibited the agitation response and the licking time, and these effects were blocked by pre-administration of the non-selective opioid receptor antagonist naloxone (1.0 microg) into the same site. Microinjection of endomorphin-1 (5 microg), a selective micro-receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (DADLE, 10 microg), a delta-/micro-receptor agonist also inhibited the nociceptive behaviors, and both the effects were blocked by selective mu-receptor antagonist beta-funaltrexamine hydrochloride (beta-FNA; 3.75 microg), but the DADLE-evoked inhibition was not influenced by the selective delta-receptor antagonist naltrindole (5 microg). Microinjection of selective kappa-receptor agonist (+/-)-trans-U-50488 methanesulfonate salt (1.5 microg) failed to alter the nociceptive behaviors induced by formalin injection. The beta-FNA and naloxone applied into VLO and morphine into the adjacent regions ventral and dorsal to VLO had no effect on the formalin-evoked nociceptive behaviors. These results suggest that mu- but not delta- or kappa-opioid receptor is involved in the VLO opioid-evoked antinociception in formalin test rat.