Microinjection of methysergide into the raphe nucleus attenuated phrenic long-term facilitation in rats

Abstract

Exposure to acute intermittent hypoxia (AIH) evokes persistent increase in respiratory activity that lasts up to 60 min after hypoxic episodes have ceased. This persistent increase in phrenic nerve activity (PNA) is known as phrenic long-term facilitation (LTF). AIH-induced phrenic LTF in anesthetized rats is serotonin dependant. The present study was performed to determine whether microinjection of methysergide (4 mM, 20 +/- 5 nl), a broad spectrum 5-HT receptor antagonist, into the caudal raphe nuclei influences phrenic LTF. Peak integrated PNA and respiratory frequency were recorded at 15, 30, and 60 min after five 3-min episodes of normocapnic hypoxia in urethane-anesthetized, vagotomized, paralyzed and ventilated male Sprague-Dawley rats. In control animals, phrenic nerve amplitude was elevated 66.7 +/- 8.6% from baseline 1 h after episodic hypoxia, indicating phrenic LTF. Experimental microinjections of methysergide prior to AIH exposure attenuated phrenic LTF (amplitude increase 2.62 +/- 2.9% over baseline). We conclude that methysergide microinjections into the caudal raphe region attenuated phrenic LTF induced by AIH, indicating involvement of 5-HT receptor activation at a supraspinal level.

Acute intermittent hypoxia induces phrenic long‐term facilitation which is modulated by 5‐HT1A receptor in the caudal raphe region of the rat

“…Selective glutamate receptor agonist d,l‐ homocysteic acid (DLH; 20 ± 5 nL; 10 m m ; Sigma‐Aldrich, St Louis, MO, USA) was microinjected into the raphe region. Adjustments of the micropipette position were made until a site was located at which a DLH‐microinjection evoked an increase in peak phrenic nerve activity (pPNA) with no significant change in the respiratory frequency (Valic et al. , 2010).…”

“…Mechanisms of phrenic LTF (pLTF) have been studied mainly in anaesthetized, vagotomized and mechanically ventilated rats (Baker‐Herman and Mitchell, 2008; Pavlinac et al. , 2011; Valic et al. , 2010).…”

“…This indicates that modulation of the 5‐HT 2 receptor subtype during but not following episodic hypoxias could abolish phrenic LTF. The action of methysergide as a partial agonist at 5‐HT receptors and its affinity to 5‐HT 1 and 5‐HT 2 receptors (Barnes and Sharp, 1999) has prevented identification of the 5‐HT receptor subtype and the exact mechanism of pLTF reduction (Valic et al. , 2010).…”

“…, 2010). The lack of specificity of these and other agents (Bach and Mitchell, 1996; Valic et al. , 2010) has hindered efforts to demonstrate the exact receptor subtype involved in the induction and maintenance of pLTF.…”

“…While previous studies have focused on postsynaptic effects of 5‐HT receptors at spinal motoneurones (Fuller et al. , 2001), recent study included additional sites of action at supraspinal level (Valic et al. , 2010), such as the raphe region.…”