Respiratory-related neuronal activity in the nucleus ambiguus-retroambigualis complex and their responses to peripheral and central stimulation in the rat

This study aimed to locate respiratory-related neurons in NA/NRA complex within the ventrolateral medulla (VLM), characterise them, and study their responses to the stimulation of midbrain periaque-ductal gray matter (PAG) and vagal afferents.

Stereotactic mapping of the medullary NA/NRA complex using stainless steel microelectrodes was undertaken in Nembutal-anaesthetised (80 mg/kg), spontaneously-breathing rats (n = 60, 300-400 g). Individual cells and cell populations showing respiratory-related firing patterns were recorded along the VLM, 2.5 mm rostral -2.1 mm caudal to the obex, 1.4-2.2 mm lateral to midline and 1.5-3.00 mm below dorsal medullary surface. The cells were classified according to their temporal relationship with the diaphragm electromyogram (dEMG). Early inspiratory cells (eIcells) were found to start firing up to 20 ms before dEMG activity and to cease activity through mid-inspiration, exhibiting a decrementing discharge pattern. Late inspiratory (lateI) cells commenced their activity midway through inspiration and stopped firing prior to the cessation of dEMG. These cells exhibited an incrementing discharge pattern. Cells firing in phase with the dEMG have been classified as I-all cells and exhibit a steady impulse pattern throughout inspiration. Neurons active during the expiratory phase have been categorised as early expiratory cells (earlyE), late expiratory cells (lateE) and E-all neurons according to their discharge pattern in relation to dEMG.

Cells in the dorso-lateral PAG were activated with microinjections of excitatory amino acid D,L-homocysteic acid (DLH0.2 M, 10-60 nl) and VLM respiratory neuronal and muscle responses were studied. PAG stimulation induced dose-dependent changes to respiratory output. Inspiratory and expiratory durations were shortened, respiratory frequency and dEMG activity increased along with increases in heart rate and blood pressure. Concurrently PAG stimulation led to increased activation of eI cells, I-all cells and E-all cells found in the NA/NRA complex. Abdominal muscle activity was not evident during quiet breathing though expiratory neuronal activity was seen. However, activation of PAG elicited synchronous aEMG bursts along with an increase in E-all cell activity.

Unilateral vagal nerve stimulation, in order to simulate activation of the pulmonary stretch receptors, was undertaken. I-all cell and dEMG activity were inhibited upon stimulation of the vagus. However, I-all neuronal firing as well as the diaphragm activity resurfaced whilst vagal stimulation was prolonged. Expiratory cells are yet to be tested for responses to vagal stimulation.

Our previous studies [1] demonstrated a physiological link between the PAG and NTS in the rat. The current investigation shows that chemical activation of PAG influences the discharge patterns of the VLM respiratory neurons within the NA/NRA complex suggesting a possible physiological link between the two centres. These results indicate that both NTS and NA/NRA respiratory neurons participate in the mediation of PAG induced respiratory changes. Roles for NA/NRA neurons in the brainstem control of respiration in the rat and the interactions between the neural centres in the midbrain are discussed.