Isoflurane abolishes spontaneous firing of serotonin neurons and masks their pH/CO₂chemosensitivity

Abstract

Serotonin (5-hydroxytryptamine, 5-HT) neurons from the mouse and rat rostral medulla are stimulated by increased CO2 when studied in culture or brain slices. However, the response of 5-HT neurons has been variable when animals are exposed to hypercapnia in vivo. Here we examined whether halogenated inhalational anesthetics, which activate TWIK-related acid-sensitive K(+) (TASK) channels, could mask an effect of CO2 on 5-HT neurons. During in vivo plethysmography in mice, isoflurane (1%) markedly reduced the hypercapnic ventilatory response (HCVR) by 78-96% depending upon mouse strain and ambient temperature. In a perfused rat brain stem preparation, isoflurane (1%) reduced or silenced spontaneous firing of medullary 5-HT neurons in situ and abolished their responses to elevated perfusate Pco2. In dissociated cell cultures, isoflurane (1%) hyperpolarized 5-HT neurons by 6.52 ± 3.94 mV and inhibited spontaneous firing. A subsequent decrease in pH from 7.4 to 7.2 depolarized neurons by 4.07 ± 2.10 mV, but that was insufficient to reach threshold for firing. Depolarizing current restored baseline firing and the firing frequency response to acidosis, indicating that isoflurane did not block the underlying mechanisms mediating chemosensitivity. These results demonstrate that isoflurane masks 5-HT neuron chemosensitivity in vitro and in situ and markedly decreases the HCVR in vivo. The use of this class of anesthetic has a particularly potent inhibitory effect on chemosensitivity of 5-HT neurons.

Role of A5 noradrenergic neurons in the chemoreflex control of respiratory and sympathetic activities in unanesthetized conditions

“…However, acute pharmacological inhibition and chronic lesion of noradrenergic cells in the A5 did not reduce baseline arterial pressure levels (Koshiya and Guyenet, 1994a; Schreihofer and Guyenet, 2000; Taxini et al, 2011). It is important to mention that all these previous studies were performed on anesthetized rats – an experimental condition that may interfere with the normal functioning of sympathetic nervous system (Machado and Bonagamba, 1992; Accorsi-Mendonca et al, 2007; Massey et al, 2015). In this regard, we demonstrated that lesions of the noradrenergic neurons in the A5 did not alter baseline arterial pressure of conscious rats and the sympathetic activity levels of unanesthetized in situ rat preparations.…”

“…Moreover, although studies reported that stimulation of peripheral chemoreceptors promotes the activation of A5 cells (Huangfu et al, 1991; Hirooka et al, 1997; Kanbar et al, 2011), lesions of the A5 noradrenergic neurons did not modify the sympathoexcitatory response to hypoxia (Taxini et al, 2011). These contrasting data may be associated with the fact that some of these studies were performed on anesthetized animals – an experimental condition that may affect the mechanisms responsible for the control of sympathetic activity and the processing of reflex responses (Machado and Bonagamba, 1992; Accorsi-Mendonca et al, 2007; Massey et al, 2015), or due to the lesion protocol, which was not limited to the A5 noradrenergic neurons.…”

Functional link between the hypocretin and serotonin systems in the neural control of breathing and central chemosensitivity

“…For example, although pH/CO 2 sensitivity in the regions such as the retrotrapezoid nucleus is contributed to by astrocytes (Moreira et al 2015), glia are believed to play no role in raphe 5-HT neuron chemosensory transduction (Sobrinho et al 2014;Wu et al 2013). Similarly, despite the implication of acid-sensitive TASK channels in this process (Bayliss et al 2015;Putnam et al 2004), chemosensitivity of raphe 5-HT neurons does not appear to be dependent on TASK channels (Massey et al 2015;Wu et al 2008). Our observations suggest that input from hypocretin synthesizing neurons somehow modulates chemotransduction on 5-HT-synthesizing neurons.…”

Inner ear lesion and the differential roles of hypoxia and hypercarbia in triggering active movements: Potential implication for the Sudden Infant Death Syndrome

“…Isoflurane (1%) is known to mask the CO2 response of serotonergic chemosensitive neurons and the hypercapnic response (Massey et al 2015). Thus, it is conceivable that the lack of an arousal response could be attributed to the light anesthesia used in our experimental paradigm.…”

“…Anesthesia, even at low concentration, is arguably different than natural sleep. We therefore included the natural sleep assessments as a closer corollary to the circumstances surrounding SIDS and also to remove any potential suppressive effect of anesthesia on chemosensitive Raphe neurons and the hypoxic response (Buchanan, Richerson 2010, Massey et al 2015.…”