Immunohistochemical demonstration of serotonin nerve fibers in the subthalamic nucleus of the rat, cat and monkey.

Distribution of serotonin-immunoreactive nerve fibers (5-HT fibers) in the subthalamic nucleus (STN) of the rat, cat and monkey was examined with a modified peroxidase-anti-peroxidase method using 5-HT antiserum without pretreatment. Fine varicose 5-HT fibers formed a fine network in the STN. Overall density of 5-HT fibers in the STN was highest in the monkey, and lowest in the rat. In the rat and cat, these fibers were distributed almost diffusely in the STN, whereas in the monkey they were particularly abundant in the ventral and medial parts of the STN. Many thick 5-HT fibers ran through the monkey STN.