Increased responses in trigeminocervical nociceptive neurons to cervical input after stimulation of the dura mater

Pain referral and spread in headache patients may be attributed to a sensitization of central nociceptive neurons with an increased excitability to afferent input. We investigated if noxious dural stimulation evokes sensitization of second-order neurons that leads to an increased responsiveness to stimulation of cervical afferents. Recordings were made from 29 nociceptive neurons in the C-2 dorsal horn of the rat that received convergent synaptic input from trigeminal and cervical afferents. Trigeminal afferents of the supratentorial dura mater were activated by mustard oil (MO) and the responses of second-order neurons to stimulation of the greater occipital nerve (GON) were studied before and after dural stimulation. Projection sites to the contralateral thalamus were determined by antidromic stimulation. After dural application with MO, mechanical thresholds of the dura significantly decreased (P<0.05) and an enlargement of the trigeminal and cervical cutaneous mechanoreceptive fields was observed in 71% of neurons. The responses to noxious mechanical stimulation of deep paraspinal muscles increased after MO application (P<0.001). Similarly, an increase in the excitability to electrical stimulation of the GON was observed in C-fibre responses (P<0.001). These results suggest that stimulation of nociceptive afferent C-fibres of the dura mater leads to a sensitization of second-order neurons receiving cervical input. This mechanism might be involved in the referral of pain from trigeminal to cervical structures and might contribute to the clinical phenomena of cervical hypersensitivity in migraine and cluster headache. Understanding this interaction is likely to be pivotal in characterizing the physiology of treatment with manipulations involving cervical input, such as GON injection.