Spinal 5-HT7 receptor activation induces long-lasting phrenic motor facilitation

Non-technical summary Acute intermittent hypoxia elicits a serotonin-dependent form of respiratory plasticity known as phrenic long-term facilitation via synthesis of brain-derived neurotrophic factor (BDNF) and activation of its receptor TrkB. Recently we demonstrated that spinal activation of Gs protein-coupled adenosine A2A receptors 'trans-activates' the BDNF receptor, TrkB, and induces phrenic motor facilitation (PMF). Therefore we hypothesized that perhaps other Gs protein-coupled serotonin receptors elicit PMF, specifically serotonin receptor type-7 (5-HT7) which are expressed in phrenic motor neurons and underlie multiple forms of spinal respiratory plasticity. We demonstrate that spinal 5-HT7 receptor activation in anaesthetized rats elicits long-lasting PMF. 5-HT7 agonist-induced PMF appears to require TrkB receptor activity and/or new synthesis of TrkB protein, in addition to activity of the signalling molecule PI3K. A more complete understanding of signalling mechanisms involved in PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders.Acute intermittent hypoxia elicits a form of serotonin-dependent respiratory plasticity known as phrenic long term facilitation (pLTF). Episodic spinal serotonin-2 (5-HT2) receptor activation on or near phrenic motor neurons is necessary for pLTF. A hallmark of pLTF is the requirement for serotonin-dependent synthesis of brain-derived neurotrophic factor (BDNF), and activation of its high affinity receptor, TrkB. Activation of spinal Gs protein-coupled adenosine 2A receptors (GsPCRs) elicits a unique form of long-lasting phrenic motor facilitation (PMF), but via unique mechanisms (BDNF independent TrkB trans-activation). We hypothesized that other GsPCRs elicit PMF, specifically serotonin-7 (5-HT7) receptors, which are expressed in phrenic motor neurons. Cervical spinal (C4) injections of a selective 5-HT7 receptor agonist, AS-19 (10 mu m, 5 mu l; 3 x 5 min), in anaesthetized, vagotomized and ventilated male Sprague-Dawley rats elicited long-lasting PMF (> 120 min), an effect prevented by pretreatment with a 5-HT7 receptor antagonist (SB 269970; 5 mm, 7 mu l). GsPCR activation 'trans-activates' TrkB by increasing synthesis of an immature TrkB isoform. Spinal injection of a TrkB inhibitor (k252a) and siRNAs that prevent TrkB (but not BDNF) mRNA translation both blocked 5-HT7 agonist-induced PMF, confirming a requirement for TrkB synthesis and activity. k252a affected late PMF (>= 90 min) only. Spinal inhibition of the PI3K/AKT pathway blocked 5-HT7 agonist-induced PMF, whereas MEK/ERK inhibition delayed, but did not block, PMF. An understanding of signalling mechanisms giving rise to PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders associated with respiratory insufficiency, such as spinal injury and motor neuron disease.