Muscarinic M2 receptors interact with neurokininergic NK1 and NK2 receptors in the contractions of isolated bronchi in the horse.

Acetylcholine and neurokinins, known as neurotransmitters that induce bronchoconstriction in horses and other species, are involved in the pathogenesis of obstructive respiratory diseases. However, the knowledge about the role of muscarinic receptors in the control of airway contraction in equines is not fully elucidated and that of neurokininergic receptors is limited. Bronchial smooth muscle rings of slaughtered horses were obtained from healthy lungs, or from lungs with macroscopic signs of inflammation, put into organ baths and connected to isotonic transducers. Electrical field stimulation was applied (50 Hz, 1 ms, 300 mA, 50 V every 120 s), and phasic contractions of bronchial smooth muscle were evoked. Previous experiments showed the neurogenic cholinergic nature of these contractions and a central role of muscarinic M3 receptors [1]. The effects of drugs were expressed as variation of the pre-drug contraction amplitude, assumed as 100%. Bronchial contractions were partially inhibited by selective M3 receptor antagonist (pFHHSiD) up to 10-6 M, and totally abolished at 10-5 M in normal and inflamed tissues. Methoctramine, selective M2 receptor antagonist, was able to reduce the contractions only in healthy bronchial rings, whereas selective M1 receptor antagonist, VU0255035, reduced the contractions only in pathologic ones. L-732,138 and GR159897, selective NK1- and NK2-receptor antagonist respectively, slightly reduced the contractions in normal and pathologic tissues, whereas SB218795, selective NK3-receptor antagonist, was ineffective. In presence of either NK1, NK2, but not NK3 receptor block, M2 antagonist, methoctramine, was able to reduce the contractions of the pathologic bronchi, while the response in normal tissue was unchanged. Muscarinic M1, M2, and M3 receptors are involved in the contractions of horse bronchi, M3 receptors playing a major role. Present results suggest an interference between cholinergic and neurokininergic systems involving M2, NK1 and NK2 receptor subtypes. The influence of the excitatory peptidergic system seems to be more evident in pathologic tissue, and indeed up-regulation of NK2 receptors in horses with RAO was detected [2]. A localization of inhibitory M2 receptors on neurokininergic neurons releasing substance P and/or neurokinin A could be hypothesized, as already observed in other species [3]. A better knowledge of the interactions between cholinergic and neurokininergic systems could help the development of more effective drugs for the treatment of bronchial hyperactivity in horses and, possibly, in humans. [1] Menozzi A, Pozzoli C, Poli E, Delvescovo B, Serventi P, Bertini S. J vet Pharm Ther 2014; in press [2] Venugopal CS, Holmes EP, Polikepahad S, Laborde S, Kearney M, Moore RM. Can J Vet Res 2009; 73:25- 33 [3] Bernardini N, Roza C, Sauer SK,1 Gomeza J, Wess J, Reeh PW. J Neurosci 2002; 22:RC229