Interaction Of Amino Acidergic Inputs Of Preganglionic Cardiac Vagal Neurons And Modulation Of These Inputs By Orexin-A And Substance P

The physiological control of heart rate and cardiac functions is dominated by the activity of the parasympathetic nerves. Preganglionic cardiac vagal neurons (the CVNs) are primarily located in the nucleus ambiguus (NA), although some can also be found in the dorsal motor nucleus of the vagus (DMNV) and in the intermediate zone between the NA and the DMNV. The CVNs are intrinsically silent, and their activity relies completely on their synaptic inputs. The CVNs receive excitatory glutamatergic and cholinergic inputs, and inhibitory GABAergic and glycinergic inputs.Studying the interaction and modulation of the synaptic inputs of the CVNs are critically important in understanding neural control of these neurons. Previous both excitatory and inhibitory amino acidergic inputs of the CVNs have been proved to be facilitated by nicotinic cholinergic receptors, and the nicotinic facilitation of the GABAergic inputs has been proved to be related to the genesis of respiratory sinus arrhythmia. However the amino acidergic inputs, the major synaptic inputs of the CVNs, have not been well understood with regard to whether and how they interact each other in controlling these neurons. This issue becomes particularly significant if considered under the background of epilepsy or seizures, since little is known about the firing behavior of the CVNs during epileptic attacks, which are well known to be related to decreased function of inhibitory neurotransmission. Thus, the first purpose of the present study is to investigate the interaction between the excitatory and the inhibitory amino acidergic inputs of the CVNs, and to test the hypothesis that the CVNs fire in seizure-like pattern under seizure-evoking condition.Many neuropeptides regulate heart rate and cardiac functions through their central actions; and altered central release and binding of certain neuropeptides are related to some diseases including cardiovascular disorders. Some neuropeptides have been proved to affect cardiac vagal activity by modulating the amino acidergic inputs of the CVNs.Orexin-A and orexin-B, also known as hypocretin-1 and hypocretin-2, are members of a new family of neuropeptides synthesized in the lateral hypothalamus and perifornical area neurons. Some evidence suggests that orexins might have a role in the parasympathetic control of heart rate and cardiac functions. However, the mechanisms by which orexins affect the CVNs are not clear at the synaptic level, and it is not known whether the GABAergic inputs and the glycinergic inputs are differentially regulated by orexins, and by which to play different roles in the control of the CVNs. The second purpose of the present study is to evaluate the effects of orexin-A on the glycinergic and the GABAergic inputs of the CVNs.Substance P (SP) has long been indicated to be involved in the vagal control of heart rate and cardiac functions. However, little is known about how SP alters the activity of the CVNs at synaptic level. This issue is more significant if considered with some important cardiovascular diseases such as myocardial ischemia and hypertension. Myocardial ischemia and hypertension are known to have diminished cardiac vagal tone and heart rate variability (including the respiratory-related), which have been well proved to be correlated with increased incidence of sudden cardiac death. It has also been found that these diseases have increased SP release from visceral afferents and enhanced SP binding in brainstem nuclei. However whether the increased central SP concentration and enhanced central SP binding are related to the diminished respiratory-related heart rate variability is not known. The third purpose of the present study is to test whether SP affects the CVNs directly or by acting on their amino acidergic inputs, and whether SP affects the respiratory-related enhancement of the inhibitory amino acidergic inputs, and by which to alter the respiratory-related heart rate variability.The CVNs were retrogradedly labeled by injecting rhodamine into the fat pads of the heart or into the cardiac sac of newborn rats. The CVNs were identified by presence of fluorescence in brainstem slices and studied using patch-clamp. In some slices with respiratory-like rhythm the CVNs were functionally identified by presence of inspiratory-related augmentation of their inhibitory inputs.Following are the results and conclusions:1. The GABAergic neurons preceding the CVNs in the DMNV are tonically excited by glutamatergic inputs, whereas the GABAergic neurons preceding the CVNs in the NA receive little, if any, glutamatergic control. These findings indicated that the CVNs in the DMNV are differentially regulated from the CVNs in the NA, which may be a possible mechanism that enables the CVNs in the DMNV to play different roles from those in the NA in the parasympathetic control of heart rate and cardiac functions.2. Blockade of inhibitory neurotransmission evoked intermittent seizure-like activation of the CVNs, as was recorded as intermittent excitatory inward currents (IEICs) under voltage clamp and intermittent seizure-like firing under current clamp. The seizure-like activation of the CVNs was in synchrony with that of the hypoglossal motor neurons. These results have given new evidence that the CVNs might fire in a seizure-like pattern during epileptic attack, which might be responsible for the neurogenic ictal bradyarrhythmia, cardiac asystole, or even the sudden deaths of patients of epilepsy.3. Orexin-A dose-dependently increased the frequency of both the glycinergic and the GABAergic spontaneous inhibitory postsynaptic currents (sIPSC) of the CVNs; the glycinergic neurons preceding the CVNs were more sensitive to orexin-A than the GABAergic neurons preceding the CVNs, which might indicate that these two kinds of inhibitory neurons play different roles in the synaptic control of the CVNs.4. SP enhanced the GABAergic and the glycinergic inputs of the CVNs, and diminished the inspiratory-related augmentation of these inputs. These results suggested that SP inhibits cardiac vagal activity and diminishes its respiratory-related variability, which might be related to the weakened cardiac vagal tone and heart rate variability in myocardial ischemia and hypertension.