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Developmental changes in pontine and adrenergic influences on the respiratory rhythm generator in neonatal rats Corcoran, Andrea E.

Abstract

In neonatal rats, the pons is known to modulate the respiratory rhythm. Much of the influence is inhibitory and has been attributed to adrenergic inputs originating from the A5 nuclei of the pons. Nonetheless, there are conflicting reports as to how pontine influences change throughout early development in rats. To identify the changes in pontine influence and adrenergic inputs, the in vitro brainstem-spinal cord preparation was used in rats aged PO, P2 and P4 (0, 2 and 4 post-natal days). First, a series of validation experiments was carried out in order to establish the viability and stability of the preparations. PO to P4 preparations were stable (both in Active breathing frequency and burst amplitude) during the first 3 hours of recording. In the second set of experiments, we transected the pons to determine breathing rates with and without pontine inputs. Thirdly, αl and α2 adrenoreceptor antagonists (Prazosin and Rauwolscine, respectively) were applied to either the pons or medulla, and in the presence or absence of pontine inputs, to determine if results found in the pons transections experiments could be explained by changes in adrenergic inputs. Our pontine transection results showed that at PO, the pons exerted an excitatory drive on the medullary respiratory rhythm generator. At P2, the pons appeared to have no effect on breathing frequency. At P4, the pons switched to providing an inhibition on the rhythm generator. The changes observed with transection of the pons cannot be explained solely by removal of adrenergic influences and must arise from non-adrenergic influences coming from the pons. Blocking α2 adrenoreceptors in the medulla both in the presence and absence of the pons resulted in a decrease in respiratory frequency at all ages. However, medullary αl-adrenoreceptor blockade had no significant effect when the pons was still present. Similarly, blocking pontine α2 adrenoreceptors (autoreceptors) did not significantly change breathing frequency. Furthermore, pharmacological blockade of both αl - and α2- medullary adrenoreceptors had a more pronounced effect when the pons was absent, suggesting that the adrenergic inputs are originating not only from the pons, but the medulla itself. Full explanation of the mechanisms underlying the switch from an excitatory pons to one that is inhibitory likely includes changes in several other neuromodulatory systems, and requires further studies using carefully and narrowly defined age groups.

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