Identification of a central CGRP circuit for trigeminal V1-mediated migraine-like pain in mice

Migraine attacks can be provoked in people with underlying primary headache disorders by multiple peripheral mechanisms suggesting the possible convergence of trigeminal nociceptive inputs onto a common, unknown central circuit. As calcitonin gene-related peptide (CGRP) expressing cells in the parabrachial nucleus (PBN) have been implicated in nociplastic pains, we hypothesized that (a) PBN CGRP-expressing neurons and their projections to the central amygdala (CeA) may be an essential central relay of multiple peripheral inputs promoting injury-free migraine headache and (b) this pathway may prioritize threat from activation of cranial afferents over other nociceptive inputs. Periorbital cutaneous allodynia (CA) was determined following supradural application of a cocktail of inflammatory mediators (IM), CGRP or pituitary adenylate cyclase-activating polypeptide (PACAP), or systemic administration of nitroglycerin (NTG) in CalcaCre or wild-type mice. Decreased rearing and photophobia were respectively evaluated as measures of ongoing headache and light-evoked migraine-like pain. Phosphorylation of extracellular signal-regulated kinases (pERK) was evaluated to assess regional neuronal activation. Compared with vehicle, supradural IM increased pERK in the trigeminal nucleus caudalis and PBN, but not in the paraventricular thalamus. Chemogenetic activation (Gq-DREADD) of PBN CGRP neurons elicited CA and photophobia while decreasing rearing. Lesioning of PBN neurons with ibotenic acid suppressed CA produced by supradural IM. Chemogenetic silencing (Gi-DREADD) or expression of tetanus toxin (TeNT) in PBN Calca neurons to prevent neurotransmitter release also inhibited CA elicited by supradural IM, CGRP, PACAP or systemic NTG. CRISPR/Cas9 editing of CeA receptor-associated membrane protein-1 (Ramp1) prevented IM-induced migraine-like symptoms revealing CGRP/CGRP-receptor activation from PBN→CeA projections. In contrast, PBN lesions, chemogenetic manipulation, and TeNT expression in PBN Calca cells and CeA Ramp1 CRISPR/Cas9 editing did not affect acute pain responses induced by hind paw IM injection, intraperitoneal acetic acid, or acute nociceptive behaviors from IM stimulation of the maxillary (V2) and mandibular (V3) branches of the trigeminal nerve. These data reveal a central pathway composed of PBN Calca neurons and CeA CGRP receptor signaling that is both sufficient to elicit migraine-like pain in naive mice and necessary to promote migraine-like pain following activation of the ophthalmic (V1) branch of the trigeminal nerve. Importantly, this pathway is essential for acute pain arising from V1 trigeminal, but dispensable for acute stimuli from other nociceptive afferents. The prioritization of V1 acute nociception suggests evolutionary importance for detecting threats to the head to promote survival through advantageous behaviors such as defensive coping observed in migraine.