| The longitudinal nucleus of the solitary tract (NTS) is mainly located in the dorsomedial part of the medulla oblongata. NTS is a major site of primary termination for many visceral afferents. It receives visceral afferent from most organs in the neck, chest and abdomen through the VIIth, IXth, and Xth cranial nerves. The NTS can be divided into different subnuclei that are associated with different functions. The rostral part of NTS receives the special visceral, gustatory afferents, while the ventrolateral and intermedial subnuclei of the mid-caudal part are associated with the respiratory control. The dorsomedial, parvocellular, medial and commissural subnuclei of the mid-caudal part are associated with the regulation of gastrointestinal and cardiovascular functions. The NTS is regarded as the initial regulatory point of peripheral and central nervous autonomic reflexes. Besides the visceral inputs, the NTS can receive projections from cells in lamina I of spinal cord that relaying peripheral somatic nociceptive information, it also receives somatic orofacial information through the branches of the trigeminal nerve. Clinical studies found that electrical acupuncture stimulating the orofacial acupuncture points could affect visceral activities. Vagal stimulation inhibits tooth pulpnerve-evoked digastric reflexes. These data suggest that the NTS as a major site of primary termination for visceral afferents may make a significant contribution to somatovisceral signal processing. Imbe.H et al observed that FOS positive neurons existed in bilateral NTS after the complete Freund's adjuvant (CFA) injection into the masseter muscle, but unilateral vagotomy resulted in a selective reduction of inflammation-induced FOS protein expression at the NTS. These findings indicated that the inflammation of the masseter muscle, an injury of orofacial deep tissue, effects widespread change of the neuronal activities in NTS that depends in part on the integrity of the vagus afferents.The NTS is rich in neuroactive substances. The medial, commissural, and intermedial subnuclei contain catecholaminergic (CA) neurons belonging to the noradrenergic group A2 and adrenergic group C2. Tyrosine hydroxylase (TH) is the key and rate-limiting enzyme in the biosynthesis of CA, and is a special marker of the CA neurons. Up to now, it has drawn attention that the CA neurons in the medulla oblongata have an important effect in regulation of the visceral activation. Hypertensive haemorrhage and nociceptive stimulation to gastrointestinal have been shown to induce the expression of FOS protein in CA neurons of NTS. Further more, it has found by using electromicroscope that these neurons can make synapse with afferent inputs of the vagus. Since CA is a type of inhibitory neuroactive substance, the CA neurons of NTS could directly regulate the visceral afferent signals. Besides visceral inputs, these neurons also receive somatic orofacial nociceptive information. These studies indicted that the CA neurons of NTS might be involved in the regulation of somatovisceral reflex. The projection of the CA neurons is very extensive, with the lateral parabrachial nucleus (LPB) as the major projectiontarget. Anatomic studies have recognized that the LPB receives visceral information from NTS and somatic one from the spinal cord and medulla dorsal horn. Therefore, the parabrachial nucleus (PBN) is an important area to integrate the somatic and visceral afferent signals. Up to date, it is unclear whether CA neurons projecting to PBN in NTS could receive nociceptive information from somatic orofacial deep tissue, and whether nociceptive information from somatic orofacial deep tissue and visceral inputs carried by vagus nerve could converge onto the CA neurons projecting to PBN in NTS. If these questiones could be resolved, it will be very helpful to clearify the mechanism of convergence and regulation of somatic and visceral information in central nervous system.The present study use methods of retrograde and transganglionic tracing, formalin stimulating masseter muscle, combined with immunofluorescence histochemistry techniques, to address these questions through a series of experiments on Sprague-Dawley (SD) rat.In the first series of experiments, 2% fluoro-gold (FG) was unilaterally injected into the LPB for retrograde tracing and 2% formalin was administered into the ipsilateral masseter muscle, combined with immunofluorescence for the detection of FOS and TH in the rat. Under fluorescent microscope, it was shown that 3 kinds of single-labeled neurons (FG, TH, FOS), 3 kinds of double-labelad neurons (FG/TH, FOS/TH, FOS/FG) and FG/FOS/TH triple-labeled neurons were mainly located ipsilaterally in the commissural, medial, intermedial and ventral subnucleus of NTS. The percentage of neurons, which receive stimulating information from masseter muscle and project to LPB, accounted for 26.4% of the ipsilateral FG retrograde-labeled neurons. The percentage of TH positive neurons, which receive orofacial stimulatinginformation and project to LPB, were 23.7% and 8.4% of the ipsilateral total numbers of TH and FG single-labeled neurons, respectively. In addition, 10% tetramethyl rhodamine (TMR) instead of FG was unilaterally injected into the LPB. Under laser scanning confocal microscope (LSCM), the result is consistent with above data. These results indicate that some of catecholaminergic neurons of NTS might receive nociceptive inputs from somatic orofacial deep tissue and transmit this information to the lateral parabrachial nucleus.In the second series, 10% TMR was unilaterally injected into LPB for retrograde labeling, 10% biotinylated dextran amine (BDA) was ipsilaterally injected into the cervical trunk of vagus nerve for transganglionic tracing , and ipsilateral injection of 2% formalin into the masseter muscle , immunofluorescence was made to detect TH and FOS. Under LSCM, it was shown that ? part of TH positive neurons expressed FOS protein, and formed close apposition with immunolabeled terminals from vagus afferents, ? in NTS, part of the projecting neurons to LPB expressed FOS protein, and formed close apposition with vagus primary inputs. These results suggest that somatic nociceptive message from orofacial deep tissue and visceral message carried by vagus nerve might converge onto some CA neurons in NTS; In NTS, some neurons projecting to LPB might receive visceral afferents and nociceptive information from orofacial deep tissue, and part of that are CA neurons.Taken together, the present results indicate that nociceptive information from orofacial deep tissue and visceral message carried by vagus nerve might converge onto CA neurons in NTS. As an inhibitory neurotransmitter, CA might regulate the two types of information, and transmit integrated signals to...
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