Central and Peripheral GABA(A) Receptor Regulation of the Heart Rate Depends on the Conscious State of the Animal

Abstract

Intuitively one might expect that activation of GABAergic inhibitory neurons results in bradycardia. In conscious animals the opposite effect is however observed. GABAergic neurons in nucleus ambiguus hold the ability to control the activity of the parasympathetic vagus nerve that innervates the heart. Upon GABA activation the vagus nerve will be inhibited leaving less parasympathetic impact on the heart. The picture is however blurred in the presence of anaesthesia where both the concentration and type of anaesthetics can result in different effects on the cardiovascular system. This paper reviews cardiovascular outcomes of GABA activation and includes own experiments on anaesthetized animals and isolated hearts. In conclusion, the impact of changes in GABAergic input is very difficult to predict in these settings, emphasizing the need for experiments performed in conscious animals when aiming at determining the cardiovascular effects of compounds acting on GABAergic neurons.

Figure 1

Model of how activation of GABAergic input to the cardiac vagal neurons increases heart rate. GABAergic neurons from the nucleus tractus solitarius inhibits the preganglionic cardiac vagal neurons, which leads to reduced postganglionic vagal input to the heart. Consequently, the muscarinic acetylcholine receptor (M₂) activity is reduced. Because the G_i protein no longer inhibits the production of cAMP by the adenylyl cyclase, HCN channel activity is increased. In addition the G protein-coupled inwardly rectifying potassium channel (GIRK) is no longer activated. Together this will cause the heart rate to increase.

Figure 2

Effect of diazepam on heart rate in artificially ventilated (1% isoflurane, filled circles; 2.5% isoflurane, open circles; O₂ : N₂O 1 : 1) female guinea pigs (538 ± 27 g). The animals were placed on heated mats and the temperature was monitored and kept constant at 37 ± 1°C throughout the experiment. Electrocardiographic (ECG) recordings were obtained using 2 electrodes placed in the subcutaneous layer of the forelimbs (left and right), and 1 electrode placed in the subcutaneous layer of the left hind limb. ECG recordings were analysed using Chart ADinstrument software and Graphpad Prism 5. A stabilization period of minimum 20 min was performed, followed by NaCl 0.9% i.p. Subsequently the animals were injected intraperitoneally every 25th min with increasing concentrations of diazepam (1, 3 and 10 mg/kg i.p.).

Figure 3

Effect of GABA 100 μM (left column) or picrotoxin 100 μM (right column) on isolated retrograde perfused female guinea pig hearts. Hearts were excised, mounted in a Langendorff apparatus, instrumented, and perfused with krebs-henseleit solution at a constant pressure of 60 mmHg as previously described [29]. Hearts were left to stabilize for a minimum of 30 min. After 20 min baseline recordings, where flow and heart rate was monitored, the hearts were paced from the right atrium for 2 min at 240 BPM. This protocol was repeated in the presence of GABA 100 μM or picrotoxin 100 μM (n = 5; GABA: 677 ± 99 g; Picrotoxin: 672 ± 100 g). GABA 100 μM or picrotoxin 100 μM produced no significant effect on action potential duration (a), flow (b), and heart rate (c).

Figure 4

Effect of diazepam on isolated retrograde perfused female guinea pig hearts (868 ± 45 g). After a minimum of 30 min stabilization, the hearts underwent 20 min of baseline recording. Subsequently the hearts were exposed to diazepam at increasing concentrations in 20 min intervals. Diazepam induced a dose-dependent reduction of heart rate and significantly shortened the heart rate at 30 μM (n = 5).

Figure 5

The effect of diazepam (10 μM), diazepam (10 μM) + picrotoxin (100 μM) (co-administration), and picrotoxin (100 μM) on heart rate (a), action potential duration (b), and coronary flow (c). Hearts were left to stabilize for a minimum of 30 min. 20 min of baseline recordings were performed, where heart rate and flow were monitored. The hearts were then paced from the right atrium at 240 BPM for 2 min in order to measure the action potential duration at a fixed heart rate. This (20+2) protocol was repeated for the different drugs investigated (n = 5; *P ≤ 0.05 (1-way ANOVA); (715 ± 129 g)).