Excitatory Responses to Microinjection of Glutamate Depend on Dose Not Volume: A Meta-Analysis of Studies in Rat RVLM

Activation of µ-opioid receptors in the rostral ventrolateral medulla blocks the sympathetic counterregulatory response to glucoprivation

Activation of neurons in the rostral ventrolateral medulla (RVLM) following glucoprivation initiates sympathoadrenal activation, adrenaline release, and increased glucose production. Here, we aimed to determine the role of RVLM µ-opioid receptors in the counterregulatory response to systemic glucoprivation. Experiments were performed in pentobarbital sodium anesthetized male Sprague-Dawley rats ( n = 30). Bilateral activation of RVLM µ-opioid receptors with [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (8 mM, 50 nl) depressed adrenal sympathetic nerve activity for ~60 min ( n = 6; Δ49.9 ± 5.8%, P < 0.05). The counterregulatory response to glucoprivation (measured by adrenal sympathetic efferent nerve activity) induced by 2-deoxyglucose (2-DG) ( n = 6; Δ63.6 ± 16.5%, P < 0.05) was completely blocked 60 min after DAMGO microinjections ( n = 6; Δ10.2 ± 3.5%, P < 0.05). Furthermore, DAMGO pretreatment attenuated the increase in blood glucose levels after 2-DG infusion ( n = 6; 6.1 ± 0.7mmol/l vs. baseline 5.2 ± 0.3mmol/l, P > 0.05) compared with 2-DG alone ( n = 6; 7.6 ± 0.4mmol/l vs. baseline 6.0 ± 0.4mmol/l, P < 0.05). Thus, activation of RVLM µ-opioid receptors attenuated the neural efferent response to glucoprivation and reduced glucose production.

Inhibitory control of the cough reflex by galanin receptors in the caudal nucleus tractus solitarii of the rabbit

The caudal nucleus tractus solitarii (NTS) is the main central station of cough-related afferents and a strategic site for the modulation of the cough reflex. The similarities between the characteristics of central processing of nociceptive and cough-related inputs led us to hypothesize that galanin, a neuropeptide implicated in the control of pain, could also be involved in the regulation of the cough reflex at the level of the NTS, where galanin receptors have been found. We investigated the effects of galanin and galnon, a nonpeptide agonist at galanin receptors, on cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nl) into the caudal NTS of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Galnon antitussive effects on cough responses to the mechanical stimulation of the airway mucosa via a custom-built device were also investigated. Bilateral microinjections of 1 mM galanin markedly decreased cough number, peak abdominal activity, and increased cough-related total cycle duration. Bilateral microinjections of 1 mM galnon induced mild depressant effects on cough, whereas bilateral microinjections of 10 mM galnon caused marked antitussive effects consistent with those produced by galanin. Galnon effects were confirmed by using the cough-inducing device. The results indicate that galanin receptors play a role in the inhibitory control of the cough reflex at the level of the caudal NTS and provide hints for the development of novel antitussive strategies.