Bradykinin and sympathetic neurons

Bradykinin has been detected as a a hypotensive and smooth muscle stimulating factor (Rocha e Silva et al, 1949). Nowadays, the peptide is known as a major mediator of inflammation and pain. Nevertheless, bradykinin has also been described to regulate functions of the sympathetic nervous system by causing excitation as well as inhibition. These effects of bradykinin have been described primarily in preparations of sympathetic ganglia or sympathetically innervated organs. In the present project, we will use primary cultures of rat superior cervical ganglia (SCG) to further elucidate as to how bradykinin controls signalling in sympathetic neurons. Previously, we were able to show that bradykinin elicits transmitter release from SCG cultures (Boehm and Huck, 1997). This action appeared to be related to the inhibition of M-type K + (KM) channels by this peptide (Jones et al, 1995), an effect that involves inositol phosphate synthesis and liberation of Ca2+ from intracellular stores (Cruzblanca et al, 1998). In preliminary experiments of this project, we confirmed that bradykinin (i) stimulates the formation of inositol phosphates, (ii) inhibits KM channels, and (iii) triggers noradrenaline release in SCG neurons. However, the secretagogue action appeared independent of inositolphosphate-dependent signalling cascades, but involved proteinkinase C. In the course of the project, we will determine which proteinkinase C isoform mediates the release stimulating action of bradykinin. In addition, we will use the novel anticonvulsant retigabine, which activates KM channels, to reveal to which extent these ion channels are involved in the excitation of SCG neurons by the peptide. These experiments will also deliver new information about potential modulatory effects of retigabine on neurotransmitter release. In additional preliminary experiments, we found that bradykinin also causes presynaptic inhibition of transmitter release from SCG neurons. The mechanisms underlying this inhibition were different from, and additive to, those of the presynaptic a 2 -adrenergic autoinhibition. In the further course of this project, we will characterize the receptor and the associated signalling mechanisms mediating this presynaptic inhibitory effect. Bradykinin exerts noxious as well as protective effects in sympathetically innervated tissues, such as the heart. Moreover, bradykinin is believed to mediate some of the beneficial effects of angiotensin converting enzyme inhibitors in hypertension and cardiac insufficiency (Dell`Italia and Oparil, 1999). Results obtained in this project will clarify as to how the sympathetic nervous system contributes to these therapeutically relevant effects of bradykinin.

Bradykinin exerts noxious as well as protective effects in sympathetically innervated tissues, such as the heart. Moreover, bradykinin is believed to mediate some of the beneficial effects of angiotensin converting enzyme inhibitors in hypertension and cardiac insufficiency (Dell`Italia and Oparil, 1999). Previously, these actions were suggested to be mainly mediated by the vasculature (Emanueli and Madeddu, 2001), but the present data confirm that the sympathetic nervous system is a multimodal effector for bradykinin (Boehm and Kubista, 2002) and reveal novel actions and signalling mechanisms of the peptide. Previously, it had been established that bradykinin excites sympathetic ganglia through an inhibition of M-type K + (KM) channels in postganglionic sympathetic neurons (Jones et al, 1995). Thereby, bradykinin elicits transmitter release from these neurons (Boehm and Huck, 1997). In experiments supported by this project, we confirmed these actions and described additional mechanisms: the peptide activated Ca2+-independent proteinkinase C via B2 receptors and this effect together with the inhibition of K M channels elicited action potentials which then led to transmitter release from axon terminals. A similar mechanism was described in this project for the activation of M 1 muscarinic receptors in sympathetic neurons. In addition, B2 bradykinin receptors were found to operate directly at axon terminals to restrict transmitter release. This latter effect is mediated by a depletion of membrane phosphatidylinositol-4,5-bisphosphate via phospholipase C-ß and a resulting inhibition of voltage-activated Ca2+ channels (VACCs). Although the same signalling mechanism is involved in the inhibition of K M channels via B2 receptors, the time courses of inhibition of these two channels were different: The inhibitory action on VACCs desensitized completely within a few minutes, whereas the inhibition of K M channels was sustained for more than 10 minutes. The rapid desensitization was attenuated significantly when protein kinase C (PKC) was inactivated. Likewise, the presynaptic inhibition of transmitter release by bradykinin desensitized entirely within less than 8 minutes and this was prevented by inactivation of PKC. Taken together, sympathetic neurons may mediate both, harmful and protective effects of B2 receptor activation on the cardiovascular system by enhancing or inhibiting noradrenaline release, respectively. However, the presynaptic inhibition is subject to a rapid desensitization mediated by proteinkinase C. Accordingly, the attenuation of this desensitization can be expected to enhance the protective effects of bradykinin in ischemic heart diseases (Dell`Italia and Oparil, 1999).