In Vivo Calcium Imaging Visualizes Incision-Induced Primary Afferent Sensitization and Its Amelioration by Capsaicin Pretreatment.

Previous studies have shown that infiltration of capsaicin into the surgical site can prevent incision-induced spontaneous pain like behaviors and heat hyperalgesia. In the present study, we aimed to monitor primary sensory neuron Ca2+ activity in the intact dorsal root ganglia (DRG) using Pirt-GCaMP3 male and female mice pretreated with capsaicin or vehicle before the plantar incision. Intraplantar injection of capsaicin (0.05%) significantly attenuated spontaneous pain, mechanical, and heat hypersensitivity after plantar incision. The Ca2+ response in in vivo DRG and in in situ spinal cord was significantly enhanced in the ipsilateral side compared with contralateral side or naive control. Primary sensory nerve fiber length was significantly decreased in the incision skin area in capsaicin-pretreated animals detected by immunohistochemistry and placental alkaline phosphatase (PLAP) staining. Thus, capsaicin pretreatment attenuates incisional pain by suppressing Ca2+ response because of degeneration of primary sensory nerve fibers in the skin.SIGNIFICANCE STATEMENT Postoperative surgery pain is a major health and economic problem worldwide with ∼235 million major surgical procedures annually. Approximately 50% of these patients report uncontrolled or poorly controlled postoperative pain. However, mechanistic studies of postoperative surgery pain in primary sensory neurons have been limited to in vitro models or small numbers of neurons. Using an innovative, distinctive, and interdisciplinary in vivo populational dorsal root ganglia (DRG) imaging (>1800 neurons/DRG) approach, we revealed increased DRG neuronal Ca2+ activity from postoperative pain mouse model. This indicates widespread DRG primary sensory neuron plasticity. Increased neuronal Ca2+ activity occurs among various sizes of neurons but mostly in small-diameter and medium-diameter nociceptors. Capsaicin pretreatment as a therapeutic option significantly attenuates Ca2+ activity and postoperative pain.

Alleviation of mechanical stress-induced allodynia by improving blood flow in chronic constriction injury mice.

Reduced blood flow in the skin is observed in patients with neuropathic pain and in animal models. The aim of the present study was to elucidate the relationship between reduced skin blood flow and neuropathic pain in mice with a chronic constriction injury (CCI). Noradrenaline-induced contraction was enhanced in isolated plantar arteries ipsilateral to the CCI surgery compared to the contralateral arteries. Ten µM hydralazine, a peripheral vasodilator, at improved the enhanced contractile response in the ipsilateral arteries. The plantar blood flow in vivo was lower on the ipsilateral side of the CCI mice than on the contralateral side, and a 50% paw withdrawal threshold, as measured using the von Frey filament test, was lower on the former than on the latter side. An intraperitoneal injection (i.p.) of hydralazine (1 mg/kg) or phentolamine (5 mg/kg) improved blood flow in the skin and hyperalgesia in the ipsilateral plantar. In adrenalectomized CCI mice, plantar blood flow in the skin on the ipsilateral side was increased compared to in sham-operated mice, which was accompanied by alleviation of hyperalgesia. Moreover, the enhanced contractile response to noradrenaline was also observed in the ipsilateral plantar arteries isolated from the adrenalectomized CCI mice. Either hydralazine (1 mg/kg, i.p.) or an adrenalectomy barely affected mean arterial pressure in the CCI mice, whereas phentolamine (5 mg/kg, i.p.) lowered it. These results suggest that reduced blood flow in the skin contributes to neuropathic pain and that improving that blood flow with peripheral vasodilators, such as hydralazine, can alleviate it.

G-protein coupled estrogen receptor-mediated non-genomic facilitatory effect of estrogen on cooling-induced reduction of skin blood flow in mice.

An enhanced vasoconstrictor activity of cutaneous arteries participates in the reduction of skin blood flow induced by cooling stimulation. Raynaud's phenomenon, which is characterized by intense cooling-induced constriction of cutaneous arteries, is more common in women during the period from menarche to menopause. We thus investigated the effect of 17ß-estradiol (E2) on cooling-induced reduction of plantar skin blood flow (PSBF) in mouse in vivo. Ovariectomized female ddY mice, anaesthetized with pentobarbital, were treated with tetrodotoxin for eliminating the sympathetic nerve tone and artificially ventilated. The PSBF was measured by laser Doppler flowmetry. Cooling air temperature around the foot from 25 to 20, 15, or 10°C decreased the PSBF in a temperature-dependent manner, which was suppressed by the specific α2C-adrenoceptor antagonist MK-912. When E2 was intravenously administered as a bolus followed by a constant infusion for 10min just before the cooling stimulation, the cooling-induced reduction of PSBF was facilitated by E2 in a dose-dependent manner. The facilitatory effect of E2 was not induced after the treatment with MK-912. Similar facilitatory effect was induced by an intravenous application of G-1, an agonist of G protein-coupled estrogen receptor (GPER, also termed GPR30). Moreover, the facilitatory effect of E2 was abolished by the GPER antagonist G15. These results suggest that acute administration of E2 leads to the facilitation of cooling-induced, α2C-adrenoceptor-mediated reduction of skin blood flow via the activation of the non-genomic estrogen receptor GPER.