Deciphering migraine pain mechanisms through electrophysiological insights of trigeminal ganglion neurons

Abstract

Migraine is a complex neurological disorder that affects millions of people worldwide. Despite extensive research, the underlying mechanisms that drive migraine pain and related abnormal sensation symptoms, such as hyperalgesia, allodynia, hyperesthesia, and paresthesia, remain poorly understood. One of the proposed mechanisms is cortical spreading depression (CSD), which is believed to be involved in the regulation of trigeminovascular pathways by sensitizing the pain pathway. Another mechanism is serotonin depletion, which is implicated in many neurological disorders and has been shown to exacerbate CSD-evoked pain at the cortical level. However, the effects of CSD and serotonin depletion on trigeminal ganglion neurons, which play a critical role in pain signal transmission, have not been thoroughly studied. In this study, we aimed to investigate the association between CSD and serotonin depletion with peripheral sensitization processes in nociceptive small-to-medium (SM) and large (L) -sized trigeminal ganglion neurons at the electrophysiological level using rat models. We divided the rats into four groups: the control group, the CSD group, the serotonin depletion group, and the CSD/serotonin depletion group. We induced CSD by placing KCl on a burr hole and serotonin depletion by intraperitoneal injection of PCPA (para-chlorophenoxyacetic acid). We then isolated trigeminal ganglion neurons from all groups and classified them according to size. Using patch-clamp recording, we recorded the excitability parameters and action potential (AP) properties of the collected neurons. Our results showed that in SM-sized trigeminal ganglion neurons, the CSD-SM and CSD/serotonin depletion groups had a higher positive resting membrane potential (RMP) than the control-SM group (p = 0.001 and p = 0.002, respectively, post-hoc Tukey's test). In addition, the gap between RMP and threshold in the CSD-SM group was significantly narrower than in the control-SM group (p = 0.043, post-hoc Tukey's test). For L-sized neurons, we observed prolongation of the AP rising time, AP falling time, and AP duration in neurons affected by CSD (p < 0.05, pairwise comparison test). In conclusion, our study provides new insights into the underlying mechanisms of migraine pain and abnormal somatosensation. CSD and serotonin depletion promote the transmission of pain signals through the peripheral sensitization process of nociceptive small-to-medium-sized trigeminal ganglion neurons, as well as nociceptive and non-nociceptive large-sized trigeminal ganglion neurons.

Figure 1

Experimental Timeline and Animal Grouping. A schematic representation of the experimental timeline and the grouping of animals in the study. Rats were categorized into four groups: control, serotonin (5-HT) depletion, cortical spreading depression (CSD), and combined CSD with 5-HT depletion (CSD/5-HT depletion). Control: Received intraperitoneal saline injections and NaCl placement on the burr hole as mimics. 5-HT Depletion: Intraperitoneal PCPA (100 mg/kg, 3 days) for serotonin depletion. CSD: Dura exposure, KCl (3 mg) applied for 1 h to induce CSD. CSD/5-HT depletion: PCPA injections (3 days), KCl applied after 24 h, mimicking CSD process.

Figure 2

Excitability parameters and action potential parameters of small-to-medium (SM)-sized trigeminal ganglion neurons in the control-SM group, the 5-HT depletion-SM group, the CSD-SM group, and the CSD/5-HT depletion-SM group recorded by patch-clamp recording technique. (a) Resting membrane potential (RMP) (b) threshold potential (c) gap between RMP and threshold potential (threshold-RMP) (d) AP height (e) AP rising (f) AP falling (g) AP duration. The data shown represent mean ± SEM. Statistical differences between the control-SM group and other groups are displayed as *. The significance levels are indicated by *p < 0.05, **p < 0.01, ***p < 0.001 based on one-way ANOVA and post-hoc Tukey’s test.

Figure 3

Excitability parameters and action potential parameters of large (L)-sized trigeminal ganglion neurons in the control-L group, the 5-HT depletion-L group, the CSD-L group, and the CSD/5-HT depletion-L group recorded by patch-clamp recording technique. (a) Resting membrane potential (RMP) (b) threshold potential (c) gap between RMP and threshold potential (threshold-RMP) (d) AP height (e) AP rising (f) AP falling (g) AP duration. The data shown represent mean ± SEM. Statistical differences between the control-L group and other groups are displayed as * while those between the 5-HT depletion-L group and the CSD/5-HT depletion group are displayed as #. The significance levels are indicated by *, ^#p < 0.05, **, ^##p < 0.01, ***, ^###p < 0.001 based on Kruskal–Wallis and pairwise comparison test.