Phenomenon of non-associative learning in Hering-Breuer reflex simulated by electrical vagal stimulation in rabbits

The purpose of this study was to explore learning and memory in the Hering-Breuer (HB) reflex simulated by a 60-second-long electrical stimulation of vagus nerve. The responses of phrenic nerve discharge to electrical stimulation (10-100 Hz, 20-60 muA, pulse duration 0.3 ms, for 60 s) of the vagus nerve were observed in rabbits. The results showed that 60-second-long stimulation of vagus nerve produced classic HB reflex, which is composed of two components - lung inflation reflex that is the inhibition of inspiration, and lung deflation reflex that is the facilitation of inspiration. (1) High frequency stimulation (>/=40 Hz, 60 s) of the central end of vagus nerve induced shortening of the inspiratory phase and lengthening of expiratory duration. The inhibitory effect on phrenic discharge was released gradually during sustained vagal stimulation, indicating the habituation of the inhibition. At the cessation of stimulation, the phrenic discharge showed transient post-stimulus rebound. Low frequency stimulation (<40 Hz, 60 s) of the central end of vagus nerve caused an increase in respiratory frequency (f) and shortening of expiratory duration. The excitatory effect on phrenic discharge was also released gradually during the vagal stimulation. The phrenic discharge returned to control level gradually after the removal of the vagal stimulus, indicating short-term potentiation (STP). (2) The habituation of HB reflex was inversely dependent on stimulus intensity and frequency. With an increase in the stimulus frequency or intensity, the degree of the habituation decreased. On the other hand, with the decrease of stimulation intensity and frequency, the degree of the habituation increased. These data indicate a phenomenon of non-associative learning in HB reflex simulated by vagal stimulation. Neural synaptic plasticity and accommodation may exist in the reflex control of respiration in rabbits.