Synaptic interactions of retrogradely labeled motoneurons that innervate the genioglossus muscle with substance P-like and delta opioid receptor-like immunoreactive nerve terminals

Obstructive sleep apnea (OSA) is a respiratory disorder that is experienced by 12 million adults (according to the American Academy of Family Physicians) when the upper airway collapses during sleep. Although the causes of sleep apneas are not yet clearly defined, failure of the genioglossus muscles (GG) to contract upon inspiration (during sleep) is believed to be one cause of obstructive sleep apnea. The GG muscles pull the tongue forward, and when they do not contract, the tongue falls back and obstructs the airway. The dysfunction of hypoglossal motoneurons results in the loss of tone in these muscles (this occurs in OSA). Therefore, it is important to identify the neurochemical input to the motoneurons which control the GG activity. By determining this input, drugs can be designed to modify GG activity in the loss of tone (during OSA) and improve airway patency by restoring normal GG activity. All of the neurotransmitters/neuromodulators that regulate motoneuronal activity associated with the genioglossus muscles have not been identified. Since substance P and the opiates are included in the series of neurotransmitters/neuromodulators that affect respiration, this study investigated whether synaptic interactions existed between hypoglossal motoneurons that project to the GG muscles and axon terminals immunoreactive for substance P (SP) and immunoreactive terminals for mu (MOR) and delta opioid receptors (DOR).; Cholera toxin B conjugated to horseradish peroxidase (CTB-HRP) was injected into the genioglossus on the right side of four isoflurane anesthetized cats and four pentobarbital anesthetized rats. Two to three days later, the animals were sacrificed and histochemical analysis was performed utilizing tetramethylbenzidine as the chromogen. The tissues were then processed for immunocytochemistry using antisera raised against substance P, the mu opioid receptor, and the delta opioid receptor. The chromogen used in this case was diaminobenizidine. Terminals immunoreactive for SP, MOR, and DOR were observed in the trigeminal nucleus. However, only SP and DOR-like terminals were observed in the hypoglossal nucleus. At the electron microscopic level, SP-like (21% ± 6.9; mean % standard error) and DOR-like (23% ± 1.9; mean % ± standard error) immunoreactive terminals formed synaptic contacts with retrogradely-labeled dendrites and perikarya in the hypoglossal nucleus of the cat. In order to further explore the effect of SP on genioglossus motoneurons, electromyography was used to determine the effect of an agonist [Sar₉ Met (O₂)11]-SP (SP_(NK1)) on genioglossus activity in nine rats. The microinjections of SP_(NK1) made into the hypoglossal nucleus produced a statistically significant increase in GG muscle activity as evidenced by the Wilcoxon Signed Ranks statistical test (control vs. SP _(NK1), 0.051 ± 0.014 to 0.130 ± 0.052, mean ± standard error, p < 0.05, N = 6, with 60 observations).; These studies demonstrate that the hypoglossal motoneurons which innervate the major protrusor muscle of the tongue, the genioglossus muscle, were modulated by terminals containing SP and DOR, but not MOR. Additionally, it was been shown that microinjections of an NK1 agonist into the hypoglossal nucleus caused a rise in GG muscle activity. By identifying the neurochemical input to the GG motoneurons, this information may aid in developing effective treatments to restore the loss of activity in the GG during OSA episodes.