Modulation of mechanosensation by endogenous dopaminergic signaling in the lateral parabrachial nucleus in mice

Abstract

Introduction: The lateral parabrachial nucleus (LPBN), a crucial hub for integrating and modulating diverse sensory information, is known to express both D1 and D2 dopamine receptors and receive dopaminergic inputs. However, the role of the LPBN's dopaminergic system in somatosensory processing remains largely unexplored. In this study, we investigated whether mechanical sensory stimulation triggers dopamine release in the LPBN and how D1- and D2-like receptor signaling in the LPBN influences mechanosensitivity in mice.

Methods: We used a G-protein-coupled receptor-based dopamine sensor to monitor dopamine release in the LPBN and a von Frey filament assay to measure the mechanical threshold for nocifensive withdrawal in mouse hind paws after unilateral microinjection of D1- or D2-like receptor antagonist into the LPBN.

Results: Noxious mechanical stimulation increased the dopamine sensor signal in the LPBN. Thresholds of nocifensive withdrawal from mechanical stimulation were decreased by the D1-like receptor antagonist SCH-23390 (0.1 µg) but increased by the D2-like receptor antagonist eticlopride (1 µg). In the intraplantar capsaicin injection model that develops mechanical hypersensitivity in the injected paw, the dopamine sensor signal in the LPBN was increased, and eticlopride (1 µg) in the LPBN significantly inhibited the capsaicin-induced mechanical hypersensitivity.

Conclusions: These results suggest that endogenous dopaminergic signaling occurs in the LPBN upon noxious mechanical stimulation, inhibiting mechanosensitivity through D1-like receptors while enhancing it through D2-like receptors. D2-like receptor signaling in the LPBN may contribute to an injury-induced increase in mechanical nociception, indicating that inhibiting the receptor within the LPBN could offer potential as a novel analgesic strategy.

Keywords: Dopamine; Dopamine receptors; Lateral parabrachial nucleus; Mechanosensitivity; Somatosensation.

Figure 1.

Noxious mechanical stimulation increases dopamine release in the lateral parabrachial nucleus (LPBN) in awake mice. (A) Dopamine biosensor (GRAB_DA2h) was expressed in the LPBN through viral vectors, and optic fibers were implanted in the LBPN. Although awake, these mice were mechanically stimulated with a tail pinch (noxious) or tail touch/stroking (innocuous) stimulus, and the dopamine levels in the LPBN were measured using fiber photometry. (B) The GRAB_DA2h fluorescent signal was robustly increased in the LPBN in response to tail pinch compared to tail touch (n = 5). (C) A D2-like receptor antagonist (eticlopride 1 mg/kg, i.p.) blunted the tail pinch-evoked signal increase (n = 2). The dotted line and shaded area in the plots indicate the stimulation initiation and duration.

Figure 2.

Dopamine is released in the lateral parabrachial nucleus (LPBN) upon mechanical stimulation of either side of the paw in anesthetized mice. Mice were under anesthesia for reliable fiber photometry during hind paw stimulation. GRAB_DA2h signals were recorded from LPBN on one side while stimulating each hind paw. (A) Pinching, but not brushing, the paw increased the dopamine sensor signals in the LPBN. Note that the signal increase was more pronounced when the contralateral paw was stimulated (n = 3). (B) Probing the paw with stiff von Frey filaments also increased the dopamine sensor signals in the LPBN (n = 3). The dotted line and shaded area in the plots indicate the stimulation initiation and duration.

Figure 3.

Dopamine in the lateral parabrachial nucleus (LPBN) decreases mechanosensitivity through D1-like receptors. Paw withdrawal thresholds (PWT) were measured using von Frey filaments before and after microinjecting SCH-23390, a D1-like receptor antagonist, into the right-side PBN. (A-C) In males (n = 5–6/dose), the antagonist at a 0.1-µg dose showed a trend toward decreased mechanical thresholds (0.1 µg). (D-F) In females (n = 6–7/dose), the antagonist at a 0.1-µg dose significantly decreased mechanical thresholds in both hind paws ipsilateral and contralateral to the antagonist injection side. In both sexes, a higher dose (0.5 µg) did not produce such an effect, yielding a V-shape dose–response curve (C and F). *P < 0.05 and **P < 0.01 vs saline (vehicle) at a given time point by post hoc Sidak test followed by linear mixed model analysis.

Figure 4.

Dopamine in the lateral parabrachial nucleus (LPBN) increases mechanosensitivity through D2-like receptors. Paw withdrawal thresholds (PWT) were measured using von Frey filaments before and after microinjecting eticlopride, a D2-like receptor antagonist, into the right-side PBN. In both males (A-C, n = 5–7/dose) and females (D-F, n = 4–6/dose), the D2-like receptor antagonist (1 µg) significantly increased mechanical thresholds in hind paws. In males, the effect was more pronounced in the ipsilateral than in the contralateral paw. *P < 0.05 and **P < 0.01 vs saline (vehicle) at a given time point by post hoc Sidak test followed by linear mixed model analysis in (A, B, D, and E); **P < 0.01 between contra and ipsi in (C).

Figure 5.

D2-like receptor antagonism in the lateral parabrachial nucleus (LPBN) alleviates capsaicin-induced mechanical hypersensitivity. (A) Capsaicin was injected at the base of the toes on the plantar side, and von Frey filaments (VFF) were applied for mechanical stimulation to the center of the capsaicin-injected paw. (B) Under anesthesia, intraplantar capsaicin (Cap) injection (red arrow) increased the dopamine biosensor signals in the ipsilateral LPBN (n = 5). (C) D2-like receptor antagonist (eticlopride 1 µg), microinjected into the LPBN ipsilateral to the capsaicin-injected paw, temporarily restored the decreased mechanical thresholds in the paw in both males (n = 4) and females (n = 4). *P < 0.05 and **P < 0.01 vs saline (vehicle) at a given time point by post hoc Sidak test followed by linear mixed model analysis.