Vagal afferent activity and body temperature in 3 to 10-day-old and adult rats

Abstract

This study examined the possible contribution of vagal stretch receptor activity to the increased power of the Hering-Breuer reflex in hyperthermia in rats during the early postnatal period. Experiments were performed on 10 anesthetized (pentobarbital 40 mg/kg, ip) 3 to 10-day-old (body weight of 16 ± 1 g; SE) and, for comparison, 18 adult Sprague-Dawley rats (body weight of 336 ± 35 g). Animals were tracheostomized and artificially ventilated with oxygen. The left vagus nerve was cut. In adult animals, single receptor fibers or a bundle of a few fibers were recorded using a bipolar stainless-steel electrode under mineral oil. In the young rats, a suction electrode filled with normal saline was used. Positive pressure of either 5 or 10 cmH2O was applied to the trachea when the respirator was turned off. The vagal activity was amplified and monitored on a storage oscilloscope for calculation of the frequency of vagal afferent activity during a given pressure application at different rectal temperatures (TR; range 28 to 42°C). In total, 30 and 31 sets of vagal activity in the young and adult rats, respectively, were analyzed. In all cases, an increase in tracheal pressure (PTR) from 5 to 10 cmH2O increased the frequency of vagal firing. The increase was greater in the adult versus the young animals; at 36°C the increase was 49 ± 11% and 16 ± 3% in the adult and young rats, respectively (P < 0.01). In all animals, vagal receptors showed temperature-sensitivity, but less so in the young than in the adult rats (P < 0.0004 and P < 0.003; for PTR of 5 and 10 cmH2O, respectively). In addition, the relationship between temperature-sensitivity and TR had significant slopes (P < 0.001 for both inflation pressures) in the adults but not in the young rats, indicating that in the latter the temperature-sensitivity of vagal receptors is independent of TR. These results imply that temperature-sensitivity of vagal receptors could have contributed to the increased power of the Hering-Breuer reflex in rats during the early postnatal period in the warmer environment.Key words: vagal receptors, temperature, Hering-Breuer reflex, young, adults, rats.