Immunohistochemical localization of alpha1a-adrenoreceptors in muscle spindles of rabbit masseter muscle

The expression of α1a-adrenoreceptors (α1a-ARs) within the muscle spindles of rabbit masseter muscle was investigated. The α1a-ARs were detected by immunohistochemical fluorescent method and examined along the entire length of 109 cross serially sectioned spindles. The sympathetic fibers were visualized by the immunofluorescent labeling of the noradrenaline synthesizing enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). In order to recognize the intrafusal muscle fiber types, antibodies for different myosin heavy chain isoforms (MyHCI) were used. TH and DBH immunolabeled nerve fibers have been observed within the capsule lamellar layers, in the periaxial fluid space and close to intrafusal muscle fibers. The α1a-ARs were detected on the smooth muscle cells of the blood vessels coursing in the muscle and in the capsule lamellar layers or within the periaxial fluid space of the spindles. Moreover, at the polar regions of a high percentage (88.1%) of muscle spindles a strong α1a-ARs immunoreactivity was present on the intrafusal muscle fibers. In double immunostained sections for α1a-ARs and MyHCI it was evidenced that both bag, and nuclear chain fibers express α1a-ARs. The receptors that we have detected by immunofluorescence may support a direct control by adrenergic fibers on muscle spindle.