Prolactin-induced sensitization of trigeminal nociceptors promotes migraine co-morbidity in endometriosis

Abstract

Background: Women with endometriosis are more likely to have migraine. The mechanisms underlying this co-morbidity are unknown. Prolactin, a neurohormone secreted and released into circulation from the anterior pituitary, can sensitize sensory neurons from female, but not male, rodents, monkeys and human donors.

Methods: We used a syngeneic model of endometriosis to determine whether elevated prolactin levels can sensitize trigeminal ganglion neurons and increase vulnerability to migraine pain.

Results: Mice with endometriotic lesions showed increased serum prolactin levels and developed persistent abdominal, but not cephalic, allodynia. However, inhalation of a transient receptor potential ankyrin 1 agonist, umbellulone, a known environmental trigger of headache in some patients, elicited cephalic allodynia in mice with endometriosis but not sham controls, suggesting that endometriosis can promote sensitization of trigeminal neurons and migraine attacks. Endometriosis dysregulated the expression of prolactin receptor isoforms in trigeminal neurons and increased their excitability measured by in vitro patch clamp electrophysiology. Inhibition of pituitary prolactin following a 2-week treatment with a dopamine receptor agonist, cabergoline, prevented cephalic allodynia elicited by activation of trigeminal afferents with umbellulone. Cabergoline treatment also normalized the expression of prolactin receptor isoforms in trigeminal ganglia and the hyperexcitability of trigeminal neurons.

Conclusions: These data demonstrate that circulating prolactin in endometriosis promotes vulnerability to migraine through sensitization of trigeminal afferents. Clinically available dopamine receptor agonists or novel monoclonal antibodies targeting prolactin signaling may be effective for migraine prevention in women with endometriosis.

Keywords: chronic overlapping pain conditions; endometriosis; migraine; nociceptor sensitization; prolactin; prolactin receptor.

Figure 1.

Prolactin (PRL) mechanism of trigeminal nociceptor sensitization and migraine-like pain in a mouse model of endometriosis. (a) Endometriosis model and experimental design. (b) Gross morphology and histological features of endometriotic lesions. Scale bar = 90 μm. Abdominal (c) and cephalic (d) tactile frequency of response was collected before endometriosis induction (time = 0) and repeated weekly for 6 weeks after induction to assess allodynia (n = 8 per group). (e) Six weeks after endometriosis induction, baseline cephalic responses were collected, followed by inhalation of umbellulone (UMB) and cephalic measurements collected hourly for 5 hours. (f) Serum prolactin levels in sham and endometriosis mice (Sham, n = 6; Endo, n = 7). (g) Representative western blots and (h) quantification of prolactin receptor (PRLR) isoform protein expression in trigeminal ganglion (TG) from sham and endometriosis mice (Sham, n = 6; Endo, n = 5). (I–L) V1 region of the trigeminal ganglia were collected and dissociated neurons cultured for patch clamp electrophysiological assessment. (i) Average number of action potentials (APs) in response to incremental 50 pA current steps in TGV1 neurons from sham and endometriosis mice treated with either vehicle or UMB (Sham + Veh, n = 14, Sham + UMB; n = 18, Endo + Veh, n = 16; Endo + UMB, n = 18. (I, right) Representative action potential traces at 600 pA. Scale bars = 20 mV/100 ms. Comparisons of (j) the number of APs at 600 pA, (k) rheobase and (l) resting membrane potential (RMP). Results are expressed as the mean ± SEM of measurements. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 2.

Systemic cabergoline reverses umbellulone-induced cephalic allodynia and endometriosis-induced priming of trigeminal neurons. (a) Experimental timeline. (b) Cephalic tactile allodynia was assessed once a week for 6 weeks and every 3 days after starting cabergoline (CBG) or vehicle (Veh) treatment (n = 8 mice/group). (c) After baseline (BL) measurement on week 9, mice were exposed to inhalation of umbellulone (UMB), and periorbital allodynia was assessed hourly for 5 hours. (c) Serum prolactin (PRL) levels were measured after termination of the experiment in vehicle and CBG treated endometriosis mice. Quantification (e) and representative western blots (f) of prolactin receptor (PRLR) isoform expression in TGV1 tissues from sham and vehicle or CBG treated endometriosis mice (sham, n = 4; Endo + Vehicle, n = 5; Endo + CBG, n = 5). (g) Representative action potential (AP) traces at 600 pA. Scale bars = 20 mV/100 ms. (h) Average number of APs in response to 50 pA current steps in TGV1 neurons from vehicle or CBG treated endometriosis mice with in vitro UMB application. Comparisons of (i) the number of APs at 600 pA, (j) rheobase and (k) resting membrane potential. Results represent the mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3.

Proposed mechanism underlying migraine co-morbidity in endometriosis. Endometriosis increases blood levels of prolactin (PRL) (1). High circulating PRL levels lead to upregulation of the prolactin receptor short (PRLR-S) isoform on trigeminal neurons that shifts PRL signaling to the pronociceptive pathways (2), sensitizing transient receptor potential ankyrin 1 (TRPA1) channels and priming the neurons to normally innocuous stimuli (3). Exposure of primed trigeminal neurons in vivo or in vitro to a low dose of TRPA1 activator (4) increases neuronal excitability (5) and elicits migraine pain (6). PRLR-L, prolactin receptor long. Created with BioRender.com.