Effects of cAMP on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rat

Abstract

Involvement of cAMP in the generation of respiratory rhythm was studied in newborn rat brainstem-spinal cord preparations. The respiratory rhythm was monitored by C4 inspiratory activity and/or pre-inspiratory (Pre-I) activity of neurons in the rostral ventrolateral medulla; previously suggested to be primary rhythm generating neurons which have pacemaker properties. The effects of four cAMP-increasing agents (forskolin, IBMX, Db-cAMP, and 8-Br-cAMP) on this neuronal activity were examined. Perfusion with forskolin (3–10 μM) increased the burst rate of C4 inspiratory activity in 20 of 32 preparations, but in 8 of those the increase was preceded by transient depression. The facilitation of the respiratory rhythm was greater whenever the burst rate before forskolin treatment was lower. The Pre-I neuron burst rate, which was recorded together with C4 activity, predominantly increased with forskolin treatment. The effects of IBMX, Db-cAMP and 8-Br-cAMP were similar to those of forskolin, but they were slightly less potent. Long-lasting depression of the respiratory rhythm (C4 and Pre-I activity) by clonidine, which might decrease intracellular cAMP level viaμ 2-receptors, was reversed by forskolin. To investigate the direct effects of the cAMP-increasing agents on Pre-I neurons, Pre-I activity was isolated by blocking the chemical synaptic transmission by incubation in a low Ca solution (0.2 mM Ca 2+, 5 mM Mg 2+). Forskolin (5–10 μM), IBMX (5–10 μM), Db-cAMP (0.2–0.4 mM), and 8-Br-cAMP (0.4–0.75 mM) all enhanced the burst rate of isolated Pre-I neurons. Results suggest that cAMP is important in respiratory rhythm generation; this may be due to its regulation of the intrinsic burst generating properties of Pre-I neurons.