Primary afferent neurons innervating guinea pig dura

We made recordings from filaments of guinea pig nasociliary nerve to study response properties of afferent axons innervating the anterior superior sagittal sinus and surrounding dura mater. We analyzed 38 units in 14 experiments. Units were initially located with the use of mechanical stimuli, and were then characterized by their conduction velocity and sensitivities to mechanical, thermal, and chemical stimuli. Single-unit recordings revealed innervation of dura and superior sagittal sinus by slowly conducting axons, mostly in the unmyelinated range. The receptive fields were 1-30 mm(2), and typically had one to three punctate spots of highest sensitivity. All units tested responded to topical application of chemical agents. Ninety-seven percent of units responded to 10(-5) M capsaicin, 79% responded to a mixture of inflammatory mediators, and 37% responded to an acidic buffer (pH 5). These data underline the importance of chemical sensitivity in intracranial sensation. Heat and cold stimuli evoked responses in 56 and 41% of units tested, respectively. Although the response patterns during heating were typical of polymodal nociceptors innervating other tissues, the thresholds were lower than for other tissues (32.3-42 degrees C). Cooling led to a phasic discharge, with thresholds between 25 and 32 degrees C. Although units had different combinations of responses to mechanical, chemical, and thermal stimuli, when grouped by their sensitivities the groups did not differ regarding mechanical thresholds or presence of ongoing activity. This suggests that meningeal primary afferents are relatively homogeneous. Sensitivities of these units are in general consistent with nociceptors, although the thermal thresholds differ. These data provide the first detailed report of response properties of intracranial primary afferent units, likely to be involved in transmission of nociception and possibly mediation of intracranial pain.