Research reportThe enkephalinergic innervation of the genioglossus musculature in the rat: implications for the respiratory control of the tongue
Introduction
The enkephalins (Leu- and Met-), two naturally occurring opioid peptides first identified in the central nervous system in 1975 [30], are involved in the control and regulation of a variety of behaviors and functions, most notably pain modulation and analgesia (see 23, 47for reviews). Of particular interest to this laboratory, however, is the purported functional role of ENK in the regulation of respiration 16, 17, 21, 44, 50. ENK-containing neurons have been identified in several respiratory-related brainstem sites 20, 27, 28, 33, 42, and studies have shown that systemically and iontophoretically applied ENK or opioid agonists markedly effect respiration 16, 17, 21, 50.
Previous investigations have demonstrated that the protrusor musculature in the base of the tongue, i.e. the paired genioglossi (GG), discharges rhythmically during the inspiratory phase of respiration 41, 46, 53, and that this activity is important in maintaining the patency of the upper airway 13, 51. However, a review of the literature has revealed no investigations into possible connectional relationships between the endogenous opioid system and the lower motoneurons in the hypoglossal nucleus (nXII) that specifically control the GG musculature.
Thus, the present study sought to determine if the GG motoneuron pool in the rat, whose location in the ventrolateral subcompartment of the nXII has been recently defined 5, 57, was preferentially targeted by ENK afferents. Previous mapping studies have shown that the nXII in the rat is the recipient of ENK inputs 8, 15, 19, 34, 52, 54, 59, 61, and that the ENK innervation may be unequally distributed throughout the nucleus [59]. The present investigation represents an extension of these earlier studies.
Section snippets
Materials and methods
Eighteen, young adult (175–200 g), female, Sprague–Dawley rats (Charles Rivers) were used in this study. Animals were anesthetized (Nembutal, 60 mg/kg, i.p.) and perfused-fixed transcardially with 4% paraformaldehyde and 1% sucrose in 0.1 M phosphate buffer (pH 7.4) that was preceded by a 0.9% saline rinse. The brainstem was postfixed overnight at 4°C, blocked, sectioned serially on a vibratome (Oxford) at 40–60 μm in the coronal plane, and processed for immunocytochemistry 1, 2, 4, 6.
Briefly,
Results
All regions of the nXII contained immunoreactive profiles; however, the density of innervation varied within different compartments and subcompartments. Fig. 1 summarizes the distribution of ENK immunoreactivity in the nXII of the rat at the three major levels – caudal, middle, and rostral.
Along the caudal one-third of the nXII, immunoreactivity was most heavily concentrated in the ventral compartment, mainly throughout the ventrolateral subcompartment and along the lateral margin of the
Discussion
The present findings confirm the results of previous investigations that the nXII is the target of ENK inputs 8, 15, 19, 34, 52, 54, 59, 61, and extend these observations by demonstrating that ENK afferents do, in fact, terminate differentially within the nXII and preferentially among select, tongue muscle-controlling motoneuron pools. More specifically, the ventral (protrusor) compartment was consistently found to be the most heavily labeled component of the nXII, with the ventrolateral
Acknowledgements
This research was supported by a Grant-in-Aid (AL-G-970020) from the American Heart Association.
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