Pulmonary and upper airway afferent influences on the motor pattern of vocalization evoked by excitation of the midbrain periaqueductal gray of the cat.

It has been established previously that natural-sounding vocalization is evoked by excitation of the midbrain periaqueductal gray (PAG). It is not known, however, whether the motor output for such vocalization is invariant, or whether it is influenced by afferent discharge. We have investigated, in the unanesthetized precollicular decerebrate cat, the patterns of upper airway and respiratory EMG activity evoked by microinjection into the PAG of the excitatory amino acid D,L-homocysteic acid (6 to 40 nmol in 30-200 nl), under conditions of varied pulmonary and upper airway afferent input. The PAG-evoked pattern in the control state consisted of a 1- to 2-min sequence of episodic vocalizations (hiss or howl-like) with expiratory (vocalization) durations of generally less than 2 s, separated by one or more inspirations. During such PAG-evoked vocalization, a significant prolongation of the expiratory duration and an extended discharge in laryngeal, intercostal and abdominal muscles was evoked by the application of warm, humidified static air pressures (5-30 cm H2O) either: (i) to the lungs and tracheo-bronchial tree via a tracheostomy, or (ii) as an airflow through the upper airways in an expiratory direction. In contrast, if the development of the usual vocalization-related air pressures (5-45 cm H2O) in the lungs and airways was prevented by opening a tracheal cannula to the atmosphere, shortened respiratory durations (inspiratory and expiratory) occurred. The effects evoked by static pressure inflations of the lungs could be blocked by bilateral vagotomy or reversibly blocked by unilateral vagotomy combined with vagal cooling sufficient to abolish Hering-Breuer reflexes. These results indicate that the motor pattern for vocalization evoked by excitation of PAG is exquisitely sensitive to pulmonary and upper airway afferent input.