Direct And Indirect Projections From The Nucleus Tractus Solitarius To The Periaqueductal Gray In The Rat

Periaqueductal gray (PAG) is the midline struture of the brain stem that encircles the mesencephalic aqueduct from the posterior commissure to the rostral locus coeruleus. It has been considered to be an important structure for antinociptive effect in the desending pain inhibitory system and had important roles in intrinsic antinociceptive, it also participated in regulating vocalization, fear, anxiety, sex behavior and regulating circulation, respiration, and so on. Nucleus tractus solitarius(NTS), lies in dorsomedial medulla, shaped Y cellular columns along medulla span. Rostral 1/3 segment received gustatory primary afferent, caudal 2/3 segments received primary afferent from cervical, thoracic and abdominal organs. Physiological investigation suggested that the NTS was not only the relay nucleus of visceral senses but also the primary regulatory center of cardiovascular reflex, respiration, vomit, swallow and gastrointestinal reflex, and the NTS had close relationships with introduction of the noxious visceral informations and vagual antinociception. Physiological and pharmacological studies proved that the PAG and the NTS had close relationships with regulating analgesia and visceral senses. On morphological, Guan-zhenglong had investigated the direct projections from the PAG to the NTS detailedly. However it was also lack of systemic morphological information on orientational projections from the NTS to the PAG. Parabrachial nucleus(PBN) is the important relay station of visceral reactivity. Besides participating in transferring the taste, it participated in the regulating the respiration, cardiovascular function, gastrointestinal activity, balance of water and salt, economical metabolize and regulating pain, mental behavior and neuro-endocrine activity, and so on. Previous studies suggested that PBN, PAG and NTS had reciprocal relations. In the present study, the direct projections from the NTS to the PAG and indirect projections via the PBN from the NTS to the PAG were investigated using anterograde and retrograde tract tracing method combining immunohistochemical technique. 1. Direct projections from the NTS to the PAGRetrograde labeling (FG) and anterograde tract tracing (PHA-L) were combined to systematically investigate the projections from the NTS to the PAG in the rat. After microinjection of PHA-L into the caudal (including area postrema, AP) and middle part of the NTS, respectively, we found that the distributions of PHA-L-labeled fibers and terminals in the PAG have the following characteristics. (1) In the coronal sections, the most PHA-L-labeled fibers and terminals were frequently found in the ventrolateral region of the PAG, and more were found in the lateral of the PAG, and a few were found in the dorsomedial region of the PAG , no labeled fibers and terminals was found in the dorsolateral PAG. At the rostral-caudal orientations, the number of the PHA-L-labeled fibers and terminals was increasing gradually fromthe rostral part to the caudal part of the PAG. (2) In the two experimental groups, the PHA-L-labeled fibers and terminals in the PAG was more in the groups when the injection sites were in the middle part in the NTS than the groups when the PHA-L was injected into the caudal part of the NTS. (3) The PHA-L-labeled fibers and terminals in above areas showed an ipsilateral dominance manner.In FG retrograde tracing experiments, after injecting FG into ventrolateral, lateral and dorsomedial regions of the rostral, middle and caudal PAG, the distribution pattern of the retrograde labeled neurons in the NTS were as followed. (1) the retrograde labeled neurons was most in the middle part of the NTS, more in the caudal part of the NTS, few in the rostral part of the NTS. Within the subnucleui of the NTS, the labeled neurons were the most in the medial subnucleus of the NTS, moderate in the commissural subnucleus of the NTS, a few in the ventral subnucleus of the NTS and dorsal subnucleus of the NTS, and the others subnucleus of the NTS had no labeled neurons. In the AP, more labeled neur...