β adrenergic receptor modulation of neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Abstract

β-adrenergic receptors are a class of G protein-coupled receptors that have essential roles in regulating heart rate, blood pressure, and other cardiorespiratory functions. Although the role of β adrenergic receptors in the peripheral nervous system is well characterized, very little is known about their role in the central nervous system despite being localized in many brain regions involved in autonomic activity and regulation. Since parasympathetic activity to the heart is dominated by cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA), β adrenergic receptors localized in the NA represent a potential target for modulating cardiac vagal activity and heart rate. This study tests the hypothesis that activation of β adrenergic receptors alters the membrane properties and synaptic neurotransmission to CVNs. CVNs were identified in brainstem slices, and membrane properties and synaptic events were recorded using the whole-cell voltage-clamp technique. The nonselective β agonist isoproterenol significantly decreased inhibitory GABAergic and glycinergic as well as excitatory glutamatergic neurotransmission to CVNs. In addition, the β(1)-selective receptor agonist dobutamine, but not β(2) or β(3) receptor agonists, significantly decreased inhibitory GABAergic and glycinergic and excitatory glutamatergic neurotransmission to CVNs. These decreases in neurotransmission to CVNs persisted in the presence of tetrodotoxin (TTX). These results provide a mechanism by which activation of adrenergic receptors in the brainstem can alter parasympathetic activity to the heart. Likely physiological roles for this adrenergic receptor activation are coordination of parasympathetic-sympathetic activity and β receptor-mediated increases in heart rate upon arousal.

Fig. 1

Application of the non-selective β agonist isoproterenol (100μM) significantly decreased GABAergic and glycinergic IPSC frequency as well as glutamatergic EPSC frequency to CVNs. A Representative traces from a typical experiment isolating for GABAergic events is shown on the left with summary data shown on the right (n=7). B On the left, typical traces are shown from an experiment isolating for glycinergic IPSCs with summary results on the right (n=10). C Raw data from an experiment isolating for glutamatergic EPSCs is shown on the left with summary data on the right (n=8).

Fig. 2

Application of the β₁ selective agonist dobutamine (10 μM) significantly inhibited GABAergic and glycinergic IPSC frequency and amplitude in addition to glutamatergic EPSC frequency and amplitude to CVNs while application of the β₂ selective agonist albuterol (10 μM) and β₃ selective agonist BRL 37344 (1 μM) did not significantly effect neurotransmission to CVNs. A On the left, representative traces from a typical experiment isolating for GABAergic IPSCs with summary data shown on the right (n=7). B Raw data from a typical experiment isolating for glycinergic IPSCs is shown on the left with summary data on the right (n=7). C Representative traces from an experiment isolating for glutamatergic EPSCs are shown on the left with summary data on the right (n=7). D Albuterol had no significant effect on GABAergic or glycinergic IPSC frequency or glutamatergic EPSC frequency to CVNs. E Application of BRL 37344 had no significant effect on inhibitory or excitatory neurotransmission to CVNs.

Fig. 3

Application of the β₁ selective antagonist atenolol (100 μM) prior to and during application of dobutamine prevented the significant decrease in GABAergic IPSCs (n=8) and glycinergic IPSCs (n=8) as well as glutamatergic EPSCs to CVNs (n=7) seen when dobutamine is applied by itself. While application of atenolol increased glutamatergic EPSCs, this change was not significant.

Fig. 4

Increasing concentrations of the β₁ selective agonist dobutamine significantly inhibited GABAergic and glycinergic IPSCs as well as glutamatergic EPSCs to CVNs. A GABAergic IPSC frequency was significantly inhibited from the control frequency starting at 0.1 μM with further inhibition as the dobutamine concentration was increased. The amplitude of GABAergic IPSCs were significantly inhibited at 10 and 100 μM B Glycinergic IPSC frequency was significantly inhibited by 0.01 μM dobutamine with higher concentrations further inhibiting neurotransmission to CVNs. The amplitude of glycinergic IPSCs were significantly decreased at 100 μM. C Glutamatergic EPSC frequency and amplitude to CVNs were significantly inhibited by 0.01 μM dobutamine with increased inhibition as dobutamine concentrations were increased. D Summary data isolating for GABAergic, glycinergic and glutamatergic neurotransmission are shown (n=7) E The summary results for amplitude data is shown for GABAergic, glycinergic, and glutamatergic neurotransmission to CVNs (n=7).

Fig. 5

Miniature IPSCs and miniature EPSCs were isolated by inclusion of TTX in the perfusate prior to and during application of dobutamine (10 μM). A Dobutamine significantly inhibited GABAergic mIPSC frequency but not amplitude with representative traces shown on the left and summary data on the right (n=7). B Glycinergic mIPSC frequency but not amplitude was significantly decreased in the presence of dobutamine. Representative traces from a typical experiment are depicted on the left with summary data on the right (n=7). C Dobutamine significantly decreased glutamatergic mEPSC frequency but not amplitude to CVNs. On the left, representative traces from a typical experiment are shown with summary data on the right (n=7).