EFFECTS OF LEVODROPROPIZINE ON VAGAL AFFERENT C-FIBERS IN THE CAT

1 Levodropropizine (LVDP) is an effective antitussive drug. Its effect s on single-unit discharge of vagal afferent C-fibres were tested in a naesthetized cats to assess whether an inhibition of va al C-fibres is involved in its antitussive properties. Vagal C-fibres, identified by their response to phenylbiguanide (PEG), were recorded via suction el ectrodes from the distal part of the cut vagus. Based on their respons e to lung inflation, C-fibres were classified as pulmonary (19 fibres) or non-pulmonary (6 fibres). 2 PEG increased the discharge rate of bo th C-fibre types and activated a respiratory reflex causing apnoea. Th is reflex was abolished when the second vagus nerve was cut as well, w hile PEG-mediated stimulation of the C-fibres was not affected by vago tomy. 3 LVDP was administered intravenously and the C-fibre response t o PEG was compared with that before administration of the drug. LVDP r educed both the duration of apnoea and the response of the C-fibre to PEG. 4 Comparison of the C-fibre responses to PEG and to a mixture of PEG and LVDP revealed that the period of apnoea was shortened and the discharge rate of the C-fibre reduced when LVDP was present. 5 The LVD P-induced inhibition of the C-fibre response to PEG was on average 50% in pulmonary and 25% in non-pulmonary fibres. 6 These results suggest that LVDP significantly reduces the response of vagal C-fibres to che mical stimuli. It is, thus, likely that the antitussive effect of LVDP is mediated through its inhibitory action on C-fibres.