A5 and A6 Noradrenergic Cell Groups: Implications for Cardiorespiratory Control

Central pontine A5 and A6 noradrenergic cell groups are two of the main sources of nor- adrenaline release at the spinal cord, at the level of the superficial dorsal horn, the motoneu ron pools of the ventral horn, lamina X and the thoracic and sacral intermediolateral cell columns. Noradrenergic ascending or descending pathways originating in the A5 or A6 noradrenergic cell groups are highly sensitive to stress and to other high-arousal states. These noradrenergic groups present extensive projections that play a key role in the modulation of all antinociceptive and autonomic responses elicited by painful or threatening situations. Depending on the locations of these projections, different possible roles for each noradrener gic cell groups are suggested. The A6 noradrenergic cell group might have the greatest effect on somatosensory transmission and the A5 group on sympathetic function. Consistent with this, stimulation of central noradrenergic pathways evokes an array of stresslike and anti- nociceptive effects, including changes in blood pressure, heart rate and respiratory rate. In addition, it also produces an increase in excitability, which leads to a high degree of arousal and a potentiation of cortical and subcortical mechanism generating the necessary cognitive, behavioral and autonomic responses to confront these physical or psychological situations.