Role of nucleus parabrachialis complex in inspiratory off-switch studied in isolated brainstem-spinal cord preparation from neonatal rat

The nucleus parabrachialis complex (NPB) of the pons is known as a respiratory modulating center. The NPB and the medullary respiratory center are mutually interconnected and vagus nerve afferents from pulmonary stretch receptors project to the NPB [1]. There is much evidence in in vivo preparations from adult mammals [2] that the NPB plays a crucial in the inspiratory off-switch. In the present study, we examined how the NPB participates in the inspiratory off-switch using brainstem-spinal cord preparations obtained from 0–4 days old rats. We hemi-sectioned the pons keeping one side intact.

First, we examined the effects of NPB electrical stimulation on C4 ventral nerve inspiratory activity. The electrical stimulation (0.1 ms duration, 1–5 trains, 20 ms interval) applied at various times (100–300 ms after the onset of C4 inspiratory activity) induced a transient depression or termination in C4 inspiratory activity. This depression was stronger when the stimulation was applied near the end of inspiratory activity but less conspicuous when it was applied at around the peak of inspiratory activity. Thus, activation of the NPB neuronal elements could produce termination of inspiratory activity in this in vitro preparation similar to the in vivo preparation.

Previous in vivo experiments in adult mammals suggested that an NMDA receptor- mediated off-switch mechanism is present in the inspiratory termination [3]. Then, we examined whether a similar mechanism is involved in the above-observed inspiratory off-switch in the in vitro preparation. It was shown that the inhibition of C4 inspiratory activity induced by the NPB stimulation was greatly reduced by perfusion of NMDA antagonists (MK-801, APV) and that the inhibition was blocked by perfusion of a GABA_A-antagonist (Bicuculline). To determine the location of NMDA receptors responsible for this inhibition by the NPB stimulation, an NMDA-antagonist MK-801 (1 mM) was microinjected into the NPB using a glass capillary placed in the vicinity of the stimulation electrode. The injection of MK-801 reduced the inhibition of C4 inspiratory activity by the NPB stimulation, indicating that NMDA receptor-mediated processes take place within the NPB.

Since neurons and/or neuron networks responsible for inspiratory off-switch are expected to exist in the NPB, we tried to find respiratory neurons and analyzed their distribution and firing patterns. Neurons were recorded from the cut-surface of the intact half of the pons using whole cell patch clamp method. We found several types of respiratory neurons: inspiratory, tonic inspiratory, expiratory and inspiratory-expiratory (I-E) neurons. The population of I-E neurons was the largest, being more than 50% of the recorded neurons. I-E neurons fired from middle of C4 inspiratory activity through the early expiratory phase and were inhibited at the early part of C4 inspiratory activity. Perfusion of MK-801 (10 M) decreased significantly the driving potential and burst duration of I-E neurons. In addition, this perfusion decreased also the amplitudes of EPSPs induced by vagal nerve stimulation in the I-E neurons.

In conclusion, 1) the NPB is involved in the inspiratory off-switch in in vitro preparations, 2) NMDA receptors within the NPB mediate, at least partly, the NPB-related inspiratory off-switch, and 3) a relatively large population of I-E neurons of the NPB could be responsible for the NPB-related inspiratory termination.