Role of 5-HT1A receptors in induction and preservation of phrenic long-term facilitation in rats

Role of raphe magnus 5-HT1A receptor in increased ventilatory responses induced by intermittent hypoxia in rats

Background

Intermittent hypoxia induces increased ventilatory responses in a 5-HT-dependent manner. This study aimed to explore that effect of raphe magnus serotonin 1A receptor (5-HT_1A) receptor on the increased ventilatory responses induced by intermittent hypoxia.

Methods

Stereotaxic surgery was performed in adult male rats, and acute and chronic intermittent hypoxia models were established after recovery from surgery. The experimental group received microinjections of 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the raphe magnus nucleus (RMg). Meanwhile, the control group received microinjections of artificial cerebrospinal fluid instead of 8-OH-DPAT. Ventilatory responses were compared among the different groups of oxygen status. 5-HT expressions in the RMg region were assessed by immunohistochemistry after chronic intermittent hypoxia.

Results

Compared with the normoxia group, the acute intermittent hypoxia group exhibited higher ventilatory responses (e.g., shorter inspiratory time and higher tidal volume, frequency of breathing, minute ventilation, and mean inspiratory flow) (P < 0.05). 8-OH-DPAT microinjection partly weakened these changes in the acute intermittent hypoxia group. Further, compared with the acute intermittent hypoxia group, rats in chronic intermittent hypoxia group exhibited higher measures of ventilatory responses after 1 day of intermittent hypoxia (P < 0.05). These effects peaked after 3 days of intermittent hypoxia treatment and then decreased gradually. Moreover, these changes were diminished in the experimental group. 5-HT expression in the RMg region increased after chronic intermittent hypoxia, which was consistent with the changing trend of ventilatory responses. While activation of the 5-HT_1A receptor in the RMg region alleviated this phenomenon.

Conclusions

The results indicate that RMg 5-HT_1A receptor, via changing the expression level of 5-HT in the RMg region, is involved in the modulation of the increased ventilatory responses induced by intermittent hypoxia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-01970-6.

Phrenic long-term depression evoked by intermittent hypercapnia is modulated by serotonergic and adrenergic receptors in raphe nuclei

Intermittent hypercapnia evokes prolonged depression of phrenic nerve activity (phrenic long-term depression, pLTD). This study was undertaken to investigate the role of 5-HT and α2-adrenergic receptors in the initiation of pLTD. Adult male urethane-anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats were exposed to a protocol of acute intermittent hypercapnia (AIHc; 5 episodes of 15% CO2in air, each episode lasting 3 min). The experimental group received microinjection of the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), the broad-spectrum 5-HT antagonist methysergide, or the α2-adrenergic antagonist yohimbine, whereas the control group received microinjection of 0.9% saline into the caudal raphe region. Peak phrenic nerve activity (pPNA) and burst frequency ( f) were analyzed during baseline (T0), during 5 hypercapnic episodes (THc1–THc5), and at 15, 30, and 60 min after the end of the last hypercapnic episode. In the control group, pPNA decreased 60 min after the end of the last hypercapnic episode compared with baseline values, i.e., pLTD developed ( P = 0.023). In the 8-OH-DPAT group, pPNA significantly decreased at T15, T30, and T60 compared with baseline values, i.e., pLTD developed ( P = 0.01). In the methysergide and yohimbine groups, AIHc did not evoke significant changes of the pPNA at T15, T30, and T60 compared with baseline values. In conclusion, activation of 5-HT1Areceptors accentuated induction of pLTD, whereas blockade of α2-adrenergic receptors prevented development of pLTD following AIHc in anesthetized rats. These results suggest that chemical modulation of 5-HT and α2-adrenergic receptors in raphe nuclei affects hypercapnia-induced pLTD, offering important insights in understanding the mechanisms involved in development of respiratory plasticity.

NEW & NOTEWORTHY Hypercapnia is a concomitant feature of many breathing disorders, including obstructive sleep apnea. In this study, acute intermittent hypercapnia evoked development of phrenic long-term depression (pLTD) 60 min after the last hypercapnic episode that was preserved if the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide was microinjected in the caudal raphe region before the hypercapnic stimulus. This study highlights that both 5-HT and adrenergic receptor activation is needed for induction of pLTD in urethane-anesthetized rats following intermittent hypercapnia exposure.

Periodicity During Hypercapnic and Hypoxic Stimulus Is Crucial in Distinct Aspects of Phrenic Nerve Plasticity

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.

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

Obstructive sleep apnoea (OSA) is characterized by periods of upper airway collapse accompanied by repeated episodes of hypoxia. In experimental animals repeated bouts of hypoxia may evoke sustained augmentation of phrenic nerve activity, known as phrenic long-term facilitation (pLTF). This form of physiological compensation might contribute to stable breathing, minimizing the occurrence of apnoeas and/or hypopnoeas during sleep in patients with OSA. Serotonin (5-HT) has been shown to modulate respiratory neuronal activity, possibly via projections originating in the raphe nuclei. Our model focuses on the effects of 5-HT1A receptors blockade by selective antagonist WAY-100635 into the caudal raphe region on phrenic long-term facilitation after exposure to acute intermittent hypoxia (AIH) episodes. Adult, male, urethane-anaesthetized, vagotomized, paralyzed and mechanically ventilated Sprague-Dawley rats were exposed to AIH protocol. Experimental group received microinjection of WAY-100635 into the caudal raphe nucleus, whereas the control group received saline into the same site. Peak phrenic nerve activity and respiratory rhythm parameters were analysed during five hypoxic episodes, as well as at 15, 30 and 60 min after the end of hypoxias. In the control group, 1 h post-hypoxia pLTF was developed. Microinjections of selective 5-HT1A receptor antagonist WAY-100635 into the raphe nuclei prior to the AIH protocol prevented induction of pLTF. These results suggest that 5-HT1A receptor activation at supraspinal level is important for induction of pLTF, which is suggested to be an important respiratory neuroplasticity model in animal studies that possibly correlates with OSA in humans.