Role of Dopaminergic Receptors Within the Ventral Tegmental Area in Antinociception Induced by Chemical Stimulation of the Lateral Hypothalamus in an Animal Model of Orofacial Pain

Abstract

Introduction: The ventral tegmental area (VTA), as one of the classical components of the brain reward circuitry, shares large neural networks with the pain processing system. We previously showed the role of VTA dopamine receptors in modulation of lateral hypothalamus (LH)-induced antinociception in acute pain conditions. However, considering the fact that the neural systems involved in the mediation of tonic pain are not the same as those that mediate phasic pain. In the present study, we aimed to examine the role of intra-VTA dopamine receptors in LH-induced antinociceptive responses during tonic orofacial pain conditions.

Methods: Male Wistar rats weighing 230-250 g were implanted with two separate cannulae into the LH and VTA on the same side. Different solutions of carbachol (62.5, 125 and 250 nM), as a non-selective cholinergic receptor agonist that activates the LH projecting neurons, were microinjected into the LH. In the other groups, D1-like dopamine receptor antagonist, SCH-23390 (0.25, 1 and 4 µg/03 µL saline) or D2-like dopamine receptor antagonist, Sulpiride (0.25, 1 and 4 µg/0.3 µL DMSO 12%) were microinjected into VTA, 5 min prior intra-LH carbachol (250 nM), then subjected to orofacial formalin test. Intra-LH carbachol microinjection dose-dependently attenuated biphasic orofacial pain.

Results: Intra-VTA administration of SCH-23390 or Sulpiride dose-dependently decreased intra-LH carbachol-induced antinociception during both phases of orofacial formalin test with further effects in the late phase.

Discussion: The findings suggest that chemical stimulation of the LH by carbachol possibly activates the orexin projecting neurons and subsequently, the VTA dopaminergic neurons involved in the orofacial pain modulation. Detecting such neural circuitry offers an alternative approach in the development of more efficient therapies for such debilitating pain conditions.

Keywords: D1-like dopamine receptor; D2-like dopamine receptor; lateral hypothalamus; orofacial formalin test; pain; ventral tegmental area.

Figure 1

Coronal brain sections show the microinjection sites in the (left panel) lateral hypothalamus (○Saline; ●Carbachol; ▲Misplacement), and (right panel) ventral tegmental area (○Saline; □DMSO; ●SCH-23390; ■Sulpiride;▲Misplacement). Abbreviations: D3V, dorsal 3^rd ventricle; LV, lateral ventricle; CPu, caudate putamen (striatum); ic, internal capsule; cc, corpus callosum; DA, dorsal hypothalamic area; CPu, caudate putamen (striatum); D3V, dorsal 3^rd ventricle; DA, dorsal hypothalamic area; mt, mammillothalamic tract; PeF, perifornical nucleus; PeFLH, perifornical part of lateral hypothalamus; MTu, medial tuberal nucleus; f, fornix; VMH, ventromedial hypothalamic nucleus; pc, posterior commissure; 3V, 3^rd ventricle; mL, medial lemniscus; Rad, radiatum layer of the hippocampus; fr, fasciculus retroflexus; SNR, substantia nigra, reticular part; str, superior thalamic radiation; VTA, ventral tegmental area; ML, medial mammillary nucleus, lateral part; PBP, parabrachial pigmented nucleus of the VTA; SuM, supramammillary nucleus; MCLH, magnocellular nucleus of the lateral hypothalamus; scale bar = 1 mm.

Figure 2

(A) The time course of face rubbing as the nociceptive responses immediately after subcutaneous injection of 1% formalin or normal saline into the orofacial region. Following formalin injection into the upper lip a significant increase in face rubbing time spent compared to respective 3-min block in normal saline group was observed. (B) The effect of microinjection of different solutions of carbachol into the LH on formalin-induced orofacial nociception. Intra-LH microinjection of carbachol (62.5, 125 and 250 nM/rat) attenuated face rubbing time spent (sec) during both phases of formalin orofacial nociception in a dose-dependent manner. Each point represents the mean ± SEM for 6–8 rats in each group. *P < 0.05, **P < 0.01 and ***P < 0.001 compared to previous 3-min block. ^†P < 0.05, ^††P < 0.01 and ^†††P < 0.001 compared to respective 3-min block in normal saline group. **P < 0.01 and ***P < 0.001 compared to vehicle (Saline) group.

Figure 3

(A) Effect of intra-VTA administration of SCH23390 (D1-like dopamine receptor antagonist) on the antinociception induced by chemical stimulation of LH using carbachol. Intra-VTA administration of SCH23390 (0.25, 1 and 4 μg/0.3 μL saline) significantly attenuated antinociception induced by intra-LH microinjection of carbachol (250 nM/rat) during both early and late phases of formalin-induced orofacial nociception. (B) A log dose–response curve of the effect of intra-VTA administration of different solutions of SCH-23390 (0.25, 1and 4 µg/0.3 μL saline) on carbachol-induced antinociception during the early compared to that of late phase of formalin-induced orofacial nociception. The effective dose (ED50) of SCH-23390 in the late phase (0.33 µg) was saliently less than that in the early phase (1.78 µg). Each point represents the mean ± SEM for 7–8 rats in each group. *P < 0.05, **P < 0.01 and ***P < 0.001 compared to saline-carbachol group. ⁺P < 0.05 and ⁺⁺P < 0.01 compared to another group. Abbreviation: ns, non-significant.

Figure 4

(A) Effect of intra-VTA injection of Sulpiride (D2-like dopamine receptor antagonist) on the LH stimulation-induced antinociception during orofacial formalin nociception. Intra-VTA administration of different doses of Sulpiride (0.25, 1 and 4 μg/0.3 μL DMSO 12%) dose-dependently attenuated antinociception produced by intra-LH microinjection of carbachol (250 nM/rat) during both early and late phases of formalin-induced orofacial nociception. (B) A log dose–response curve of the effect of intra-VTA administration of different solutions of Sulpiride (0.25, 1and 4 µg/0.3 μL DMSO 12%) on antinociception produced by carbachol during the early compared to that of late phase of formalin-induced orofacial nociception. The effective dose (ED50) of Sulpiride in the late phase (0.67 µg) was obviously less than that in the early phase (1.93 µg). Each point represents the mean ± SEM for 7–8 rats in each group. * P < 0.05, **P < 0.01 and ***P < 0.001 compared to DMSO-carbachol group ⁺P < 0.05 and ⁺⁺⁺P < 0.001 compared to another group. Abbreviation: ns, non-significant.