Role of gap junctional communication in pial arteriole dilation during somatosensory stimulation

Abstract

Pial arterioles supplying the hindlimb somatosensory cortex dilate in response to electrical stimulation of the contralateral sciatic nerve. The mechanism of this response is not well understood. One possibility involves the cell-to-cell conduction of dilator signals via gap junctions from parenchymal to pial vessels. To test the hypothesis that sciatic nerve stimulation (SNS)-induced dilation of pial arterioles involves gap junctional communication, we determined if pharmacological gap junction blockers may prevent the dilation response. We performed a craniotomy over the somatosensory cortex of adult chloralose-anesthetized rat to create an open cranial window. A surface electrode was placed in the same area for somatosensory evoked potential (SEP) recording. Through the craniotomy, a segment of a pial arteriole overlying the hindlimb somatosensory cortex was chosen for study (Figure 1). Microscopic streams of the gap junction blockers octanol (4 mM) and carbenoxolone (75 microM) were applied continuously onto a segment of the chosen pial arteriole by pressure-ejection using micropipettes, while vessel diameter was monitored at sites upstream and downstream from the site of application relative to the direction of blood flow (Fig. 1). Warmed mock cerebrospinal fluid was continuously superfused to disperse drugs applied by micropipette. At the upstream site, octanol (n=6) and carbenoxolone (n=6) reversibly attenuated SNS-induced dilation to 35% and 30% of the baseline response, respectively. The gap junction blockers did not alter the dilation response at the downstream site. Moreover, the blockade was not due to nonspecific impairment of the response by octanol and carbenoxolone, because 1) dilation to hypercapnia remained intact at both upsteam and downstream sites, and 2) SEP did not change. In addition, we evaluated the effect of connexin mimetic peptides homologous to extracellular loop motifs of vascular connexins, on SNS-induced pial dilation. The gap junction peptides, Gap27(37,43) [SRPTEKTIFII, 250 microM, n=5] and Gap26(37,40) [VCYDQAFPISHIR, 250microM, n=5], were superfused over the pial surface for 120 minutes while the dilation response to SNS was observed at 15-minute intervals. Both peptides significantly reduced SNS-induced dilation (by 70% and 50%, respectively), without affecting SEP. These results suggest that the dilation response of pial arterioles to somatosensory stimulation involves the conduction of dilator signals via gap junctions along the vascular wall.