Laryngeal Electromyographic, Cardiovascular, and Respiratory Effects of Neuropeptide Injections into the Nucleus Tractus Solitarius of Rats

Abstract

Identifying central neurotransmitters that mediate laryngeal adductor activity may aid in managing pathological laryngeal adduction as occurs in laryngospasm or apparent life-threatening events in infants. We studied the effect on cricothyroid (CT) and thyroarytenoid (TA) electromyography (EMG) and on cardiovascular parameters of neurotransmitter injections into the rat nucleus tractus solitarius (NTS), the primary sensory relay center of the larynx. Twenty nanoliters of vasoactive intestinal peptide (VIP; 2.5 pmol), neurokinin B (NKB, 16 pmol), calcitonin gene-related peptide (3.0 pmol), neurokinin A (NKA; 35 pmol), or artificial cerebrospinal fluid (control solution) was stereotactically injected into the region of the NTS and the control nucleus gracilis in 119 studies performed in 24 mature, anesthetized Sprague-Dawley rats. Changes in diaphragm, CT, and TA EMG activity and blood pressure (BP) were compared. Injection sites were verified histologically. Injections of both VIP and NKB into the region of the NTS, but not the nucleus gracilis, induced life-threatening changes, including apnea, a marked decline in BP (p < .05), and increases in EMG activity of the CT and TA adductor muscles ranging from sustained contraction to mild phasic increases during inspiration. Calcitonin gene-related peptide injection also decreased BP (p < .05), but caused only mild increases in CT and TA EMG activity. NKA and control injections into the NTS did not alter respiration, BP, or CT or TA EMG activity. VIP and NKB may play important roles in modulating EMG activity of the CT and TA muscles. This information may prove useful in evaluating pharmacological targets of central reflex activity to manage life-threatening laryngeal adduction.