Functional expression of bradykinin B(1) and B(2) receptors in neonatal rat trigeminal ganglion neurons

Bradykinin (BK) and its receptors (B(1) and B(2) receptors) play important roles in inflammatory nociception. However, the patterns of expression and physiological/pathological functions of B(1) and B(2) receptors in trigeminal ganglion (TG) neurons remain to be fully elucidated. We investigated the functional expression of BK receptors in rat TG neurons. We observed intense immunoreactivity of B(2) receptors in TG neurons, while B(1) receptors showed weak immunoreactivity. Expression of the B(2) receptor colocalized with immunoreactivities against the pan-neuronal marker, neurofilament H, substance P, isolectin B4, and tropomyosin receptor kinase A antibodies. Both in the presence and absence of extracellular Ca(2+) ([Ca(2+)](o)), BK application increased the concentration of intracellular free Ca(2+) ([Ca(2+)](i)). The amplitudes of BK-induced [Ca(2+)](i) increase in the absence of [Ca(2+)](o) were significantly smaller than those in the presence of Ca(2+). In the absence of [Ca(2+)](o), BK-induced [Ca(2+)](i) increases were sensitive to B(2) receptor antagonists, but not to a B(1) receptor antagonist. However, B(1) receptor agonist, Lys-[Des-Arg(9)]BK, transiently increased [Ca(2+)](i) in primary cultured TG neurons, and these increases were sensitive to a B(1) receptor antagonist in the presence of [Ca(2+)](o). These results indicated that B(2) receptors were constitutively expressed and their activation induced the mobilization of [Ca(2+)](i) from intracellular stores with partial Ca(2+) influx by BK. Although constitutive B(1) receptor expression could not be clearly observed immunohistochemically in the TG cryosection, cultured TG neurons functionally expressed B(1) receptors, suggesting that both B(1) and B(2) receptors involve pathological and physiological nociceptive functions.