Respiratory activity in medulla oblongata and its modulation by adenosine and opioids

Abstract

From the moment of birth the complex neuronal networks generating breathing has to function continuously and adapt to the new postnatal environmental demands. This thesis aims at studying the perinatal development of respiratory control and its modulation by adenosine and opioids. Respiratory activity was studied in vitro using brainstem spinal cord preparations and in vivo with a barometric plethysmograph. In vitro whole-cell patch clamp recordings of respiratory related neurones in medulla oblongata was performed while simultaneously recording the respiratory output at cervical ventral roots. Foetal (embryonic day 18-21; E18-21) and neonatal (postnatal day 0-14; PO-14) rats were used.

Continuous but irregular respiratory activity and its modulation by pontine structures was established in foetal rats at E18. Respiratory activity became successively more regular with age but the basic central respiratory rhythm generation, as studied in the isolated brainstern preparation, did not undergo any major changes from the foetal to the neonatal stage.

In the neonatal rat opioids depressed respiration through K- and µ-opioid receptors (including µ-1) in the ventral medulla oblongata. The ability of morphine to depress respiration increased during the first postnatal days. The inhibitory effect of at- but not K- opioid receptors on respiration was temperature dependent and decreased dramatically between 320C to 220C.

Adenosine depressed respiratory activity through presynaptic adenosine A1-receptor mediated inhibition of synaptic transmission in the medulla oblongata as well as via postsynaptic inactivation of expiratory neurones. Theophylline attenuated the effects of adenosine by antagonising adenosine A1-receptors in medulla oblongata. This may be one mechanism underlying the therapeutic effects of theophylline on neonatal apnoea.

The effects of adenosine was inversely correlated to perinatal age with strong inhibitory effects at E18 that gradually declined with increasing foetal and postnatal age. Chronic matemal intake of caffeine (an adenosine antagonist) during gestation increased the pontine inhibition of medullary respiratory activity in perinatal pups (E18-P3). In addition, the anoxia-induced gasping was reduced in the caffeine treated pups (P7). No changes in the development of adenosine A1-receptor binding and A1-receptor mRNA expression were detected in pups exposed to caffeine compared to controls. This suggests that caffeine induced changes in brainstem neuronal networks may have long term functional consequences without altering adenosine A1-receptor development.

The fact that postsynaptic adenosine A1-receptors hyperpolarised and inactivated expiratory neurones but only modulated the generation of respiratory rhythm in vitro, indicates that these neurones per se are not necessary for respiratory rhythm generation.