Calcitonin gene-related peptide (CGRP) is regarded as a key mediator in the generation of primary
headaches.
CGRP receptor antagonists reduce
migraine pain in clinical trials and spinal trigeminal activity in animal experiments. The site of
CGRP receptor inhibition causing these effects is debated. Activation and inhibition of
CGRP receptors in the trigeminal ganglion may influence the activity of trigeminal afferents and hence of spinal trigeminal neurons. In anesthetized rats extracellular activity was recorded from neurons with meningeal afferent input in the spinal trigeminal nucleus caudalis. Mechanical stimuli were applied at regular intervals to receptive fields located in the exposed cranial dura mater. α-CGRP (10(-5) M), the
CGRP receptor antagonist olcegepant (10(-3) M), or vehicle was injected through the infraorbital canal into the trigeminal ganglion. The injection of volumes caused transient discharges, but vehicle, CGRP, or
olcegepant injection was not followed by significant changes in ongoing or mechanically evoked activity. In animals pretreated intravenously with the
nitric oxide donor glyceryl trinitrate (GTN, 250 μg/kg) the mechanically evoked activity decreased after injection of CGRP and increased after injection of
olcegepant. In conclusion, the activity of spinal trigeminal neurons with meningeal afferent input is normally not controlled by
CGRP receptor activation or inhibition in the trigeminal ganglion.
CGRP receptors in the trigeminal ganglion may influence neuronal activity evoked by mechanical stimulation of meningeal afferents only after pretreatment with GTN. Since it has previously been shown that
olcegepant applied to the cranial dura mater is ineffective, trigeminal activity driven by meningeal afferent input is more likely to be controlled by
CGRP receptors located centrally to the trigeminal ganglion.