Serotonergic and noradrenergic effects on respiratory neural discharge in the medullary slice preparation of neonatal rats

Abstract

Rhythmically active medullary slice preparations isolated from neonatal rats (postnatal days 0-3, P0-P3) were used to study the modulation of respiraory rhythmogenesis and hypoglossal (XII) nerve discharge by serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NA). 5-HT, NA and their respective receptor agonists and antagonists were applied either to the bathing medium or focally via pressure injection into regions encompassing the pre-Botzinger complex or XII motoneurons. The effects of endogenously released 5-HT were also studied by chemical stimulation of neurons within the raphe obscurus. The frequency of respiratory burst discharge was increased when 5-HT was applied: (1) to the bathing medium (37+/-16%; 30 "mu"M; P < 0.05); (2) via pressure injection into the region of the pre-Botzinger complex (22 +/- 14%; < 25 pmol; P < 0. 05); or (3) endogenously released in response to activation of neurons within the raphe obscurus via pressure injection of (R,S)- "alpha"-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA, 34 +/- 15%; P < 0.05) or 5-HT (33 +/- 5%; P < 0. 05). All of these effects were antagonized by bath application of methysergide (30-40 "mu"M). NA caused a reduction of respiratory burst frequency when applied to the bathing medium (40 +/- 15%; 100 "mu"M; P < 0.05) or when pressure injected into the region of the pre-Botzinger complex (22 +/- 11%; < 25 pmol; P < 0.05). These effects were blocked by the bath application of the "alpha"2-receptor antagonist idazoxan (2 "mu"M). 5-HT and NA both caused an augmentation of tonic discharge of XII nerves when applied either to the bathing medium or via pressure injection into the XII motoneuron pool. The 5-HT-induced XII nerve tonic discharge was mimicked by the 5-HT2 receptor agonist R(-)2-(2, 5-dimethoxy-4-iodophenyl) (DOI.HCl, 5 "mu"M) and blocked by the 5-HT2 receptor antagonist ketanserine tartrate (30-40 "mu"M). The NA-induced XII nerve tonic discharge was mimicked by the "alpha"1-receptor agonist phenylephrine HCl (500 nM) and blocked by the "alpha"1-receptor antagonist prozasin HCl (1 "mu"M).

Endogenous Activation of Serotonin-2A Receptors Is Required for Respiratory Rhythm GenerationIn Vitro

“…At the cellular level, ionophoresis of 5-HT 2A agonists (␣-Me-5-HT and DOI) depolarizes membrane potential and increases action potential frequency in respiratory neurons (Lalley et al, 1995;Onimaru et al, 1998), which results in an increase in respiratory rhythmic activity in the brainstem slice preparation (Al-Zubaidy et al, 1996;Onimaru et al, 1998). This is consistent with the DOI effects described in the present study (Fig.…”

“…Exogenous applications of specific 5-HT 2A agonists are known to exert an excitatory effect on respiratory activity in vitro (Onimaru et al, 1998) in anesthetized cats (Lalley et al, 1995) and in conscious rats (Cayetanot et al, 2002). This effect can be mimicked in transverse slices (Al-Zubaidy et al, 1996) by stimulating raphe neurons, the primary source of endogenous 5-HT. Because 5-HT-containing neurons fire tonically (Richerson et al, 2001), continuously released 5-HT acting on the 5-HT 2A receptor can modulate the respiratory network.…”

A5 cells are silenced when REM sleep-like signs are elicited by pontine carbachol

“…Local application of clonidine inhibits noradrenergic A5 neurons (15,26,28). Because norepinephrine excites XII motoneurons (5,22,40,49), the silencing of A5 neurons by clonidine should reduce XII nerve activity. However, no changes occurred, suggesting that silencing A5 neurons (and presumed decrement in norepinephrine release in their target areas) has no effect on the activity of XII motoneurons.…”

“…They have extensive axonal projections that include cardiorespiratory and motor regions of the brain stem and spinal cord (1,8,9,23,36). Through these projections, they may control, among others, sympathetic and respiratory outputs and, via projection to the hypoglossal (XII) motor nucleus, may mediate noradrenergic excitation of XII motoneurons (5,22,27,40,49). XII motoneurons are of particular interest because they innervate the genioglossus muscle of the tongue, an important airway dilator.…”

Transient attenuation of CO2 sensitivity after neurotoxic lesions in the medullary raphe area of awake goats

“…Among others, the pre-Bötzinger complex contains overlapping subpopulations of neurons expressing serotonin and neurokinin-1 receptors (11,17). Chemical and/or electrical stimulation of the raphe, or endogenous and/or exogenous increases in serotonin at the pre-Bötzinger complex, increases respiratory frequency (2,7,34). On the other hand, serotonin antagonists block these effects and can elicit respiratory arrests and rhythm irregularity (2,7,31).…”