Modulation of the cough reflex by antitussive agents within the caudal aspect of the nucleus tractus solitarii in the rabbit

We have previously shown that ionotropic glutamate receptors in the caudal portion of the nucleus tractus solitarii ( NTS), especially in the commissural NTS, play a prominent role in the mediation of tracheobronchial cough and that substance P potentiates this reflex. This NTS region could be a site of action of some centrally acting antitussive agents and a component of a drug-sensitive gating mechanism of cough. To address these issues, we investigated changes in baseline respiratory activity and cough responses to tracheobronchial mechanical stimulation following microinjections (30-50 nl) of centrally acting antitussive drugs into the caudal NTS of pentobarbitone-anesthetized, spontaneously breathing rabbits. [D-Ala(2), N-Me- Phe(4), Gly(5)-ol]-enkephalin ( DAMGO) and baclofen decreased baseline respiratory frequency because of increases in the inspiratory time only at the higher concentration employed ( 5 mM and 1 mM, respectively). DAMGO ( 0.5 mM) and baclofen ( 0.1 mM) significantly decreased cough number, peak abdominal activity, peak tracheal pressure, and increased cough-related total cycle duration. At the higher concentrations, these agents suppressed the cough reflex. The effects of these two drugs were counteracted by specific antagonists ( 10 mM naloxone and 25 mM CGP-35348, respectively). The neurokinin-1 (NK1) receptor antagonist CP-99,994 ( 10 mM) abolished cough responses, whereas the NK2 receptor antagonist MEN 10376 ( 5 mM) had no effect. The results indicate that the caudal NTS is a site of action of some centrally acting drugs and a likely component of a neural system involved in cough regulation. A crucial role of substance P release in the mediation of reflex cough is also suggested.