THE DISTRIBUTIONS OF 5-HT-LIKE AND MET-ENKLIKE IMMUNOREACTIVE STRUCTURES IN THE NUCLEUS ACCUMBENS OF RAT / 解剖学报

ABSTRACT

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.