Growth restriction induced by chronic prenatal hypoxia affects breathing rhythm and its pontine catecholaminergic modulation.

Impaired transplacental supply of oxygen leads to intrauterine growth restriction, one of the most important causes of perinatal mortality and respiratory morbidity. Breathing rhythm depends on the central respiratory network modulated by catecholamines. We investigated the impact of growth restriction, using prenatal hypoxia, on respiratory frequency, on central respiratory-like rhythm, and on its catecholaminergic modulation after birth. At birth, respiratory frequency was increased and confirmed in en bloc medullary preparations, where the frequency of the fourth cervical (C4) ventral root discharge was increased, and in slice preparations containing the pre-Bötzinger complex with an increased inspiratory rhythm. The inhibition of C4 burst discharge observed in pontomedullary preparations was stronger in the growth-restricted group. These results cannot be directly linked by the tyrosine hydroxylase activity increase of A1/C1 and A2/C2 cell groups in the medulla since blockade of α1- and α2-adrenergic receptors did not abolish the difference between both groups. However, in pontomedullary preparations, the stronger inhibition of C4 burst discharge is probably supported by an increased inhibition of A5, a respiratory rhythm inhibitor pontine group of neurons displaying increased tyrosine hydroxylase activity, because blockade of α2-adrenergic receptors abolished the difference between the two groups. Taken together, these results indicate that growth restriction leads to a perturbation of the breathing frequency, which finds, at least in part, its origin in the modification of catecholaminergic modulation of the central breathing network.

Anandamide centrally depresses the respiratory rhythm generator of neonatal mice.

Endogenous cannabinoid receptors are widely distributed throughout the CNS, including the brainstem, and modulate a variety of functions, including breathing. In adult rats, activation of cannabinoid 1 receptors has been shown to depress breathing. Here in neonatal mice, we used in vitro electrophysiology, pharmacology, and immunohistochemistry to analyse the central effects of the endocannabinoid anandamide (AEA) on the activity of the medullary respiratory rhythm generator (RRG). First of all, in vitro electrophysiology on medullary preparations has revealed that bath application of AEA (30 µM, 15 min) significantly depressed respiratory activity. Secondly, applying pre-treatments with alpha-1 (Prazosin, 5 µM, 10 min) and alpha-2 (Yohimbine, 5 µM, 10 min) adrenoceptor antagonists prior to AEA application abolished the AEA-induced depression of the RRG. Finally, immunostaining revealed a dense network of fibres positive for the cannabinoid 1 receptor in the ventrolateral medulla (VLM), a region known to contain both the RRG and the modulatory A1/C1 catecholaminergic group. Moreover, cannabinoid 1 receptor positive fibres were found in close apposition with A1/C1 catecholaminergic cells, identified by the presence of tyrosine hydroxylase. In regard of our electrophysiological, pharmacological and immunostaining results, we conclude that AEA has a central depressive effect on the neonatal RRG, probably via the medullary A1/C1 catecholaminergic neurons which are already known to modulate the respiratory rhythm generator.