Activation of neurons of zone A5 of the rat brain upon hypoxic and thermonociceptive stimulation and switching off of the central respiratory generator

We recorded spike activity of noradrenergic neurons of zone A5 (n = 89) in the brain of anesthetized rats under conditions of hypoxic stimulation (breathing with pure N₂, 10 sec), thermonociceptive stimulation (tail-flick test), and reversible hypothermal blocking of the central respiratory activity. Hypoxic stimulation of peripheral O₂-sensitive chemoreceptors considerably increased the discharge frequency in all the examined neurons and induced tachypnea and a hypotensive reaction. Sixty-nine (77.5%) neurons of the studied group were tested using nociceptive stimulation (thermal stimulation of the tail); such stimulation resulted in a multifold increase in their discharge frequency. This was accompanied by tachypnea and a hypertensive response. Thus, we first demonstrated the role of nociception in the control of activity of noradrenergic neurons in zone A5 and the role of nociceptive afferent signals in the modulation of functions of the respiratory and cardiovascular systems mediated by neurons of the above zone. Under conditions of blocking of the central respiratory activity, we examined 36 (40.4%) neurons of zone A5 and first observed the effect of strong activation of a significant proportion of these cells upon switching off of respiration. This fact shows that there is an activating “respiratory” drive on neurons of zone A5 (probably, from the side of an expiratory neuronal population of the respiratory center) and allows us to hypothesize on the genesis of “respiratory” modulation of these cells. The activity of 16 (18.0%) cells was recorded under conditions of consecutive applications of the above stimuli; all the neurons were activated by the respective afferent influences. The simultaneously induced effects of hypoxic and nociceptive stimulations on the activity of neurons of zone A5 were additive. Thus, we first obtained proofs in favor of the multimodality of noradrenergic neurons of the above zone. This feature is a significant factor providing integrative interaction between the respiratory and cardiovascular systems and the system of nociception. Neirofiziologiya/Neurophysiology, Vol. 38, No. 4, pp. 305–313, July–August, 2006.