Serotonergic Modulation Of The Synaptic Inputs Of Preganglionic Cardiopulmonary Vagal Neurons

5-hydroxytryptamine(serotonin,5-HT),an indolamine that is widely distributed in the peripheral tissue and central nervous system,is involved in a large variety of physiological functions and pathogenesis including pain modulation,sleep,endocrine, cardiopulmonary function and psychiatric disorders such as depression,anxiety and aggressiveness.Anatomical studies have found that 5-HT containing neurons are mainly within the raphe nuclei(pallidus,obscurus,magnus).Serotonergic neurons project to the whole brain and synapse upon many kind of neurons.In recent years, many in vivo electrophysiological studies have shown that 5-HT plays an important role in mediating the reflex control of cardiopulmonary functions,which involves many receptor subtypes including 5-HT₁,5-HT₂,5-HT₃,5-HT₄,5-HT₇.Blocking 5-HT_1A or 5-HT₇.receptors attenuated bradycardias evoked by stimulating baroreceptor and cardiopulmonary afferents.In the nucleus tractus solitarius(NTS), 5-HT₂ receptors had variable effects;5-HT_2B receptors excite and 5-HT_2C receptors inhibit.However,little is known about the neural pathway and the synaptic mechanisms.Using patch-clamp techniques to study the effect of 5-HT on the synaptic inputs of cardiopulmonary vagal preganglionic neurons is important to elucidate the reflex control of cardiopulmonary functions at the synaptic level.Previous studies have made much progress about the central distribution and the synaptic control of cardiac vagal preganglionic neurons(CVPNs).With respect to pulmonary vagal preganglionic neurons,the study is scarce.So,our aim is to investigate:(1) the effect of 5-HT on the synaptic inputs of CVPNs.(2) the central distribution and electrophysiological characteristics of airway-related parasympathetic preganglionic neurons(APPNs).(3)the effect of 5-HT on the synaptic inputs of APPNs.The CVPNs were retrogradely labeled by injecting rhodamine into the fat pads of the heart or into the cardiac sac of 2-6-day-old Sprague-Dawley rats and the APPNs were retrogradely labeled by injecting rhodamine into the recurrent laryngeal nerve (RLN) or tracheal wall.The CVPNs and APPNs were identified by the presence of fluorescence in brainstem slices and studied using whole cell patch-clamp recording technique.Following are the results and conclusions:1.5-HT_1A/7 receptor agonist 8-OH-DPAT significantly inhibited the spontaneous inhibitory postsynaptic currents(sIPSCs) of GABAergic and glycinergic inputs and also caused significantly amplitude decrease of the GABAergic currents evoked by stimulation of NTS,whereas it had no effect on the miniature inhibitory postsynaptic currents(mIPSCs) of GABAergic and glycinergic inputs.8-OH-DPAT had no effect on the currents evoked by exogenous GABA and glycine and had no effect on the glutamatergic spontaneous excitatory postsynaptic currents(sEPSCs).These findings suggested that 5-HT_1A/7 receptor agonist might excite CVPNs through inhibition of their GABAergic and glycinergic inputs.This conclusion at least in part revealed the synaptic mechanisms involved in the 5-HT mediated reflex control of cardiac vagal tone.2.Tracer application directly to the RLN only labeled the putative APPNs within the compact portion of the nucleus ambiguous(cNA),while tracer injection into the tracheal wall labeled the putative APPNs both in cNA and in the area ventro/ventrolateral to cNA(vNA).The putative APPNs within cNA receive mainly inhibitory synaptic inputs,which in some neurons showed an inspiratory-related augmentation and in others showed an inspiratory-related attenuation.The putative APPNs in vNA received both excitatory and inhibitory synaptic inputs,and central inspiratory activity excited some of them via augmentation of their excitatory inputs and inhibited others via augmentation of their inhibitory inputs.The APPNs inhibited during inspiration might be expiration-related or postinspiratory-related motoneurons and others excited during inspiration might be inspiratory-related motoneurons. Theses results provide evidence that APPNs controlling different segments of the airway might be dissociated in the ventrolateral medulla both anatomically and in functional control.In addition,we recorded the gap junction currents(GJCs) in single inspiratory APPN(I-APPN) by elimination of the transmembrane electrochemical gradients in voltage patch-clamp recording.The GJCs were rhythmically activated by central inspiration activity.These findings indicated that gap junctions exist in the APPNs and might play an important role in the synchronization of the central respiratory neurons. This method may be used as a fast way to detect GJCs within spontaneously active neuronal network.The study about the effect of 5-HT on the synaptic inputs of APPNs is still on the way.