New Findings r What is the central question of this study?Intermittent hypercapnia is a concomitant feature of breathing disorders. Hypercapnic stimuli evoke a form of respiratory plasticity known as phrenic long-term depression in experimental animals. This study was performed to investigate the putative role of serotonin receptors in the initiation of phrenic long-term depression in anaesthetized rats. r What is the main finding and its importance?Phrenic nerve long-term depression was revealed in animals pretreated with the serotonin broad-spectrum antagonist, methysergide. This study highlights that serotonin receptors modulate respiratory plasticity evoked by acute intermittent hypercapnia in anaesthetized rats.This study was performed to test the hypothesis that intermittent hypercapnia can evoke a form of respiratory plasticity known as long-term depression of the phrenic nerve (pLTD) and that 5-HT receptors play a role in the initiation of pLTD. Adult male urethane-anaesthetized, vagotomized, paralysed, mechanically ventilated Sprague-Dawley rats were exposed to an acute intermittent hypercapnia protocol. One group received i.v. injection of the non-selective 5-HT receptor antagonist methysergide and another group received i.v. injection of the selective 5-HT 1A receptor antagonist WAY-100635 20 min before exposure to intermittent hypercapnia. A control group received i.v. injection of saline. Peak phrenic nerve activity and respiratory rhythm parameters were analysed at baseline (T0), during each of five hypercapnic episodes, and 15, 30 and 60 min (T60) after the last hypercapnia. Intravenous injection of methysergide before exposure to acute intermittent hypercapnia induced development of amplitude pLTD at T60 (decreased by 46.1 ± 6.9%, P = 0.003). Conversely, in control and WAY-100635-pretreated animals, exposure to acute intermittent hypercapnia did not evoke amplitude pLTD. However, a long-term decrease in phrenic nerve frequency was evoked both in control (42 ± 4 breaths min −1 at T0 versus 32 ± 5 breaths min −1 at T60; P = 0.036) and in methysergide-pretreated animals (42 ± 2 breaths min −1 at T0 versus 32 ± 3 breaths min −1 at T60; P = 0.028). In WAY-100635 pretreated animals, frequency pLTD was prevented. These results suggest that 5-HT receptors modulate respiratory plasticity induced by acute intermittent hypercapnia in anaesthetized rats.

This study was undertaken to determine pattern sensitivity of phrenic nerve plasticity in respect to different respiratory challenges. We compared long-term effects of intermittent and continuous hypercapnic and hypoxic stimuli, and combined intermittent hypercapnia and hypoxia on phrenic nerve plasticity. Adult, male, urethane-anesthetized, vagotomized, paralyzed, mechanically ventilated Sprague-Dawley rats were exposed to: acute intermittent hypercapnia (AIHc or AIHcO2), acute intermittent hypoxia (AIH), combined intermittent hypercapnia and hypoxia (AIHcH), continuous hypercapnia (CHc), or continuous hypoxia (CH). Peak phrenic nerve activity (pPNA) and burst frequency were analyzed during baseline (T0), hypercapnia or hypoxia exposures, at 15, 30, and 60 min (T60) after the end of the stimulus. Exposure to acute intermittent hypercapnia elicited decrease of phrenic nerve frequency from 44.25±4.06 at T0 to 35.29±5.21 at T60, (P=0.038, AIHc) and from 45.5±2.62 to 37.17±3.68 breaths/min (P=0.049, AIHcO2), i.e. frequency phrenic long term depression was induced. Exposure to AIH elicited increase of pPNA at T60 by 141.0±28.2 % compared to baseline (P=0.015), i.e. phrenic long-term facilitation was induced. Exposure to AIHcH, CHc, or CH protocols failed to induce long-term plasticity of the phrenic nerve. Thus, we conclude that intermittency of the hypercapnic or hypoxic stimuli is needed to evoke phrenic nerve plasticity.

This study was performed to investigate long‐term changes of peak phrenic (pPNA) and renal sympathetic nerve (RSNA) activities during and following acute intermittent hypercapnic exposures. Urethane anesthetized, vagotomized, paralyzed and mechanically ventilated male Sprague‐Dawley rats, were exposed to the acute intermittent hypercapnia protocol consisting of 5 hypercapnic episodes (15% CO2). Experimental groups were pretreated with either methysergide (3 mg/kg iv, MeHC) or WAY‐100635 (3 mg/kg iv, WAY) prior to the onset of the first hypercapnia. PPNA or RSNA and blood pressure were recorded throughout the protocol and analyzed before and during hypercapnic exposures, and 15 (T15), 30 (T30) and 60 (T60) minutes after the last hypercapnic episode. PPNA did not significantly change at T15, T30 and T60 compared to baseline. In the MeHC group, pPNA significantly decreased at T30 and T60 (by 41.3 ± 5.3%; and by 46.1 ± 6.9%, P<0.005), compared to baseline values. In the WAY group, there were no significant changes in the pPNA at T15, T30, and T60 compared to baseline values. RSNA increased during hypercapnia and remained elevated at T15, T30 and T60 compared to baseline. In the WAY group RSNA increased during hypercapnia but returned to the baseline level at T60 (95.4 ± 23.1%). Acute intermittent hypercapnia induced long‐term depression of the pPNA following acute intermittent hypercapnia exposure only after blockade of serotonin receptors in anesthetized rats. On the other hand, acute intermittent hypercapnia evoked long‐term facilitation of the RSNA that was attenuated after blockade of serotonin receptors. Support: Croatian Science Foundation 09/165.