The cholinergic neurons and fibers of the hypothalamus could not be revealedsuccessfully in the past,therefore,there has been general agreement that the hypotha-lamus is very poorly innervated by cholinergic system.In this study,choline acetyl-trans ferase (ChAT)-like immunoreactive positive neurons and fibers of the hypo-thalamus were revealed successfully by using avidin-biotin immunocytochemical me-thod.This study demonstrated for the first time that the cat hypothalamus is richlyinnervated by cholinergic system.We found cholinergic neurons of varying numbersin the following areas:dorsomedial hypothalamic nucleus,paraventricular nucleus,dorsal hypothalamic area,the area of the tuber cinereum surrounding ventromedialhypothalamic nucleus,lateral hypothalamic area,anterior hypothalamic area,anteriorhypothalamic nucleus,parvocellular hypothalamic nucleus,tuber-mammillary nucl-eus,posterior hypothalamic area,anterior mammillary nucleus and supramammillarynucleus.There were a lot of ChAT-like positive fibers in the lateral hypothalamicarea,mammillary area,dorsal hypothalamic area,paraventricular nucleus,parvocel-lular hypothalamic nucleus,the area of the tuber cinereum.Three kinds of neuronperikarya related to cholinergic system were identified in the hypothalamus of thecat:1.cholinergic perikarya;2.noncholinergic-cholinoceptive perikarya;3.choli-nergic-cholinoceptive perikarya.There were also immunoreactive positive fiberswhich were non-varicose and varicose.Two kinds of varicose-fibers,one withstrong immunoreactivity and the other with weak immunoreactivity were distingui-shed.

A cerebellar afferent connection from the periaqueductal grey (PAG) has been demonstrated in the rat by means of retrograde transport of horseradish peroxidase (HRP) in the present study. The projection is bilateral, but the projection from the ipsilateral side is predominant (3:1). Its main origin is the ventromedial and ventrolateral regions of middle and caudal parts of PAG (98.8％), and the fibers reach different cerebellar cortical regions: culmen, declive, folium vermis, tuber vermis, pyramis vermis, uvula vermis, lobulus quadrangularis, crus Ⅰ, crus Ⅱ, and paraflocculus. Most labelled neurons are medium sized, but some small neurons also appear to project to cerebellum. Only a few large neurons are retrogradely labelled at the most caudal end of the caudal part. Functionally, both cerebellum and PAG are related to visceral activities. Consulting the present experiment, we discussed the significant role of the PAG-cerebellar projection.

In order to study the bifurcate projections of midbrain periaqueductal gray and nucleus raphe dorsalis to nucleus accumbens and nucleus raphe magnus, the fluorescence double-labeled method was used in the present study. Bisbenzimide (Bb) and propidium iodide (PI) were injected into nucleus raphe magnus and unilateral nucleus accumbens stereotaxically according to the time period necessary for their axonal transport. The percentages of double-labeled neurons were 21％; PI single labeled neurons were 32％; Bb single labeled neurons were 47％. Most of the labeled neurons were located in the middle and caudal parts of periaqueductal gray and the nucleus raphe dorsalis, and most were medium sized and fusiform and triangular in shape.

The distributions of 5-HT-like and Met-ENK-like immunoreactive (5-HT-LI and Met-ENK-LI) structures in the nucleus accumbens (Acb) of rats were studied by immunohistochemical technique in the present study. Under light microscope, 5-HT-LI fibers and terminals could be seen in each subnucleus at different planes of Acb, but the 5-HT-LI fibers and terminals in the medial and ventral subnuclei were more than the dorsal and lateral subnuclei, the amount of 5-HT-LI fibers and terminals in the caudal segment were more than the rostral segment. According to the diameter, pathway, and number of varicosity, 5-HT-LI fibers could be divided into 3 types: (A) thick fiber (0.35—0.40?m); (B) medium fiber(0.20—0.30?m); (C) thin fiber (about 0.10?m). These 3 types of 5-HT-LI fibers were remarkable in the medial and ventral subnuclei of Acb. 5-HT-LI neuronal bodies did not observed in the Acb. A few scattered Met-ENK-LI neuronal bodies were seen in the ventral subncleus and ventral part of the medial subnucleus. Met-ENK-LI fibers and terminals distributed in all subnuclei and predominant in the medial and ventral subnuclei. The distributions of Met-ENK-LI structures were no differences between the rostral and caudal segments. All of the Met-ENK-LI fibers were thin and irregular and villi-like in shape. There were only a few varicosities on the MetENK-LI fibers. Part of Met-ENK-LI fibers looked like discontinued varicosities. Under electron microscope, 5-HT-LI axonal boutons formed symmetric and asymmetric synapses with non-5-HT-LI dendrites. Met-ENK-LI dendrites formed symmetric and asymmetric axo-dendritc synapses with non-Met-ENK-LI axonal boutons. These synapses were mainly observed in the medial and ventral subnuclei of Acb. The identity of 5-HT-LI and Met-ENK-LI structures, especially in the medial and ventral subnuclei, supported the physiological studies that 5-HT-LI ascending efferent fibers activated the Met-ENK-LI neurons and then the latter sent descending efferent fibers to lower brainstem structures to take part in antinociceptive functions.

The projections from the nucleus tractus solitarii (NTS) and the nuclei parabrachiales (PB) to the nucleus accumbens (Acc) in the rat have been visulized using anterograde and retrograde HRP tracing techniques. After injecting HRP into the Acc, retrogradely labelled neurons were observed in bilateral PB and NTS with ipsilateral predominance. The labelled neurons were concentrated in the following areas of the PB and NTS: the waist area of the caudal PB, the external subnucleus and other part of the nucleus medialis parabrachialis (PBm), the external, central, and internal subnuclei of the nucleus lateralis parabrachialis (PB1) and the medial subnucleus of the caudal NTS (NTSm). After injecting HRP into the NTSm and commissural nucleus of the NTS, anterogradely labelled terminals were found bilaterally in the PBm and the ventral 3/4 area of the PB1. The densest sites occupied the waist area, the external, central, and internal subnuclei of the PB1. The density of the labelled terminals on the ipsilateral side was little higher than that on the contralateral side. The results indicate that there are two possible pathways from the NTS to the Acc, the one is the direct projection from the medial subnucleus of the caudal NTS to the Ace, the other is an indirect one, i. e. from the medial subnucleus and commissural nucleus of the caudal NTS to the waist area of the caudal PB, the external subnucleus, the dorsal part of the PBm, the external, central, and internal subnuclei of the PB1, where the NTS projection is. presumed to be relayed, and then, project from the PB to the Acc. This connection may be involved in the neural regulation of visceral and locomotor activities.

By using of combined HRP retrograde tracing and immunocytoche mistry methods, the projection from the caudal part of the ventrolateral medulla (VLM) to the nucleus accumbens (Acb) was examined. When HRP was injected into the ventral and medial area of the caudal part of Acb, the labeled cell bodies were mostly found in the bilateral (predominantly ipsilateral) caudal part of the VLM. When HRP technique (HRP injected into the Acb)was combined with immunocytochemical method, many HRP-TH, HRP-NT and HRP-CCK double labeled cell bodies were found in the VLM. The number of the HRP-TH double labeled cell bodies were numerous, while HRP-NT and HRP-CCK doublelabeled cells were less. HRP-TH double labeled neurons were also found in the reticular formation between the solitary tract nucleus and VLM.

The projection from the nucleus of solitary tract (Sol) to the nucleus accumbens (Acb) was examined by using HRP retrograde and anterograde tracing combined with immunocytochemical double-labeling technique. The following results were obtained: (1) when WGA-HRP was injected into the caudal part of the Sol, the labeled fibers, terminals and cell bodies were found in the ventromedial area of the caudal Acb. After injecting HRP into the ventromedial area of the caudal Acb, labeled cell bodies and terminals were found in the ipsilateral and contralateral caudal part of the Sol,mainly in the commissure nucleus and the medial subnucleus of nucleus of solitary tract. (2) After injecting HRP into the Acb and combined with immunocytochemical method, many HRP-TH, HRP-NT, and HRP-CCK double-labeled cell bodies were found in the caudal part of the Sol. The number of the HRP-TH double-labeled cell bodies was most numerous, HRP-NT cells was next and HRP-CCK cells was even less.

5-HT-, SP- and L-ENK-like (5-HT-LI, SP-LI and L-ENK-LI) immunoreactive ultrastructures of the ventrolateral subdivision (VLS) of midbrain periaqueductal gray (PAG) in the rat were observed by immunoelectron microscopical technique in the present study. 5-HT-LI neuronal cell bodies were frequently seen in the VLS of the PAG. 5-HT-LI dendrites were found to form axo-dendritic synapses with non-immunoreactive axon terminals, and the major form of this kind of synapse was asymmetric. A few 5-HT-LI axon terminals formed axo-axonic synapses with non-5-HT-LI axon boutons. 5-HT-LI and non-5-HT-LI axon boutons formed axo-dendritic and axo-somatic synapses with 5-HT-LI dendrites, non-5-HT dendrites and 5-HT-LI neuronal cell bodies, respectively. SP-LI fusiform neuronal cell bodies were only a few and formed axo-somatic synapses with non-SP-LI synaptic boutons which contained pleomorphic vesicles. SP-LI dendrites formed axo-dendritic synapses with non-SP-LI axon terminals, this kind of synapse was the main form of synapses formed by SP-LI structures. SP-LI axon boutons formed axo-somatic and axo-dendritic synapses with non-SP-LI neuronal cell bodies and SP-LI dendrites. L-ENK-LI neuronal cell bodies were also limited. The most common form of synapses of L-ENK-LI structures was L-ENK-LI dendrites formed axo-dendritic synapses with non-L-ENK axon terminals. Non-L-ENK-LI axon terminals constituted axo-somatic synapses with L-ENK-LI neuronal cell bodies. A few L-ENK-LI axon terminals formedaxo-dendritic synapses with L-ENK-LI dendrites. The majority of the synapses mentioned above contained spherical clear vesicles, but some were mixed with a few flat, oval and granular vesicles. The immunoreacrive products were located on the surface of vesicles or on the surface of membranous organelles in the cytoplasm.

The ultrastructure of the rat nucleus raphe magnus (NRM) was observed under transmission electron microscope in the present study. Most of the neurons of the NRM were medium and small sized fusiform and triangular in shape, they had spherical or ellipsoid nuclei and rather high nucleo-cytoplasmic ratio. Nuclear rodlets which composed of parallel filaments could be seen in some fusiform NRM neurons. There were numerous organelles in the cytoplasm. Axonal terminals apposed to most neuronal bodies and formed axo-somatic synapses. The predominant type of these synapses was symmetric. Sometimes, rod-like or spine-like cytoplasmic protrusions could be seen on the neuronal bodies, they often made axo-somatic synapses with axonal terminals. The neuropil of the NRM was quite complex. It was formed by transverse sections of myelinated fibers, unmyelinated fibers, synapses, and neuroglia. The axo-dendritic synapses were the major synaptic type in the neuropil. The predominant type of these axo-dendritic synapses was also symmetric and asymmetric axo-dendritic synapses. Some axonal terminals were arranged parallel with dendrites and formed symmetric synapses. Beneath subsynaptic membrane of some postsynaptic bags, there were some electrical dense spherules or bands which formed subsynaptic dense bodies. There were. no typical axo-axonic synapses in the NRM, but the parallelly arranged axonal terminals were often seen. Most of the presynaptic bags contained clear spherical vesicles or mixed with flattened and granular vesicles. Some postsynaptic bags were filled with flattened vesicles, they were also often mixed with spherical and granular vesicles.

The submicroscopic structure of nucleus accumbens (Acb) was observed under transmission electron micrscope in the present study. Most of the Acb neurons were spherical, with diameter ranging from 10 ?m to 15 ?m. The nucleus were ellipsoid with, sometimes, nucleolus and deep invaginations of nuclear membrane. The nucleocytoplasmic ratio was always rather high. Axo-somatic synapses were formed on the surface of most neuronal perikarya, predominantly of the symmetric type. The percentage of axo-somatic synapses in total number of Acb synapses was 2%. Generally, the outline of the dendrites was regular. Most of the thick dendrites made symmetric axo-dendritic synapses while the thin dendrites usually made asymmetric axo-dendritic synapses. Some of the irregular dendrites had spines. The spines were usually long and thin, with enlarged end and markedly thin neck, and formed asymmetric synapses with axonal terminals. The axo-dendritic synapes composed 97% of the total synapses within Acb. Small sized axons were frequently observed. The majority of the axon terminal segments contained clear round vesicles and made synapses en passant with the dendrites. Synapses en passant were also observed on the collaterals of axon. Two or more axonal terminals were often closely contacted, but no typical synaptic formations were found. Only a few axo-axonic synapses could be observed. Axoaxonic-dendritic synaptic complexes were also seen. The percentage of axo-axonic synapse was 0.5%. Most of the synaptic boutons mentioned above were mainly filled by spherical vesicles, some of them were filled with pleomorphic vesicles.