Opiates are widely used analgesics in anesthesiology, but they have serious adverse effects such as depression of breathing. This is caused by direct inhibition of rhythm-generating respiratory neurons in the Pre-Boetzinger complex (PBC) of the brainstem. We report that serotonin 4(a) [5-HT4(a)] receptors are strongly expressed in respiratory PBC neurons and that their selective activation protects spontaneous respiratory activity. Treatment of rats with a 5-HT4 receptor-specific agonist overcame fentanyl-induced respiratory depression and reestablished stable respiratory rhythm without loss of fentanyl's analgesic effect. These findings imply the prospect of a fine-tuned recovery from opioid-induced respiratory depression, through adjustment of intracellular adenosine 3',5'-monophosphate levels through the convergent signaling pathways in neurons.

The respiratory centre of neonatal mice (4 to 12 days old) was isolated in 700 μm thick brainstem slices. Whole‐cell K+ currents and single ATP‐dependent potassium (KATP) channels were analysed in inspiratory neurones. In cell‐attached patches, KATP channels had a conductance of 75 pS and showed inward rectification. Their gating was voltage dependent and channel activity decreased with membrane hyperpolarization. Using Ca2+‐containing pipette solutions the measured conductance was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by extracellular Ca2+. KATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3‐4 min after oxygen removal from the bath. Hypoxic potentiation of open probability was due to an increase in channel open times and a decrease in channel closed times. In inside‐out patches and symmetrical K+ concentrations, channel currents reversed at about 0 mV. Channel activity was blocked by ATP (300‐600 μM), glibenclamide (10‐70 μM) and tolbutamide (100‐300 μM). In the presence of diazoxide (10‐60 μM), the activity of KATP channels was increased both in inside‐out, outside‐out and cell‐attached patches. In outside‐out patches, that remained within the slice after excision, the activity of KATP channels was enhanced by hypoxia, an effect that could be mediated by a release of endogenous neuromodulators. The whole‐cell K+ current (IK) was inactivated at negative membrane potentials, which resembled the voltage dependence of KATP channel gating. After 3‐4 min of hypoxia, K+ currents at both hyperpolarizing and depolarizing membrane potentials increased. IK was partially blocked by tolbutamide (100‐300 μM) and in its presence, hypoxic potentiation of IK was abolished. We conclude that KATP channels are involved in the hypoxic depression of medullary respiratory activity.

The respiratory centre within the brainstem is one of the most active neuronal networks that generates ongoing rhythmic activity. Stabilization of such vital activity requires efficient processes for activity‐correlated adjustment of neuronal excitability. Recent investigations have shown that a regulatory factor coupling electrical activity with cell metabolism comprises ATP‐dependent K+ channels (KATP channels), which continuously adjust the excitability of respiratory neurons during normoxia and increasingly during hypoxia. We used the single‐cell antisense RNA amplification‐polymerase chain reaction (PCR) technique to demonstrate that respiratory neurons co‐express the sulphonylurea receptor SUR1 with the Kir6.2 potassium channel protein. Single channel measurements on rhythmically active inspiratory neurons of the brainstem slice preparation of newborn mice revealed that KATP channels are periodically activated in synchrony with each respiratory cycle. The Na+‐K+‐ATPase was inhibited with ouabain to demonstrate that oscillations of the channel open probability disappear, although respiratory activity persists for a longer time. Such findings indicate that KATP channel open probability reflects activity‐dependent fluctuations in the ATP concentration within submembrane domains. We also examined the effects of extracellular [K+] and hypoxia. All changes in the respiratory rhythm (i.e. changes in cycle length and burst durations) affected the periodic fluctuations of KATP channel activity. The data indicate that KATP channels continuously modulate central respiratory neurons and contribute to periodic adjustment of neuronal excitability. Such dynamic adjustment of channel activity operates over a high range of metabolic demands, starting below physiological conditions and extending into pathological situations of energy depletion.

In the rhythmic brain stem slice preparation, spontaneous respiratory activity is generated endogenously and can be recorded as output activity from hypoglossal XII rootlets. Here we combine these recordings with measurements of the intrinsic optical signal (IOS) of cells in the regions of the periambigual region and nucleus hypoglossus of the rhythmic slice preparation. The IOS, which reflects changes of infrared light transmittance and scattering, has been previously employed as an indirect sensor for activity-related changes in cell metabolism. The IOS is believed to be primarily caused by cell volume changes, but it has also been associated with other morphological changes such as dendritic beading during prolonged neuronal excitation or mitochondrial swelling. An increase of the extracellular K(+) concentration from 3 to 9 mM, as well as superfusion with hypotonic solution induced a marked increase of the IOS, whereas a decrease in extracellular K(+) or superfusion with hypertonic solution had the opposite effect. During tissue anoxia, elicited by superfusion of N(2)-gassed solution, the biphasic response of the respiratory activity was accompanied by a continuous rise in the IOS. On reoxygenation, the IOS returned to control levels. Cells located at the surface of the slice were observed to swell during periods of anoxia. The region of the nucleus hypoglossus exhibited faster and larger IOS changes than the periambigual region, which presumably reflects differences in sensitivities of these neurons to metabolic stress. To analyze the components of the hypoxic IOS response, we investigated the IOS after application of neurotransmitters known to be released in increasing amounts during hypoxia. Indeed, glutamate application induced an IOS increase, whereas adenosine slightly reduced the IOS. The IOS response to hypoxia was diminished after application of glutamate uptake blockers, indicating that glutamate contributes to the hypoxic IOS. Blockade of the Na(+)/K(+)-ATPase by ouabain did not provoke a hypoxia-like IOS change. The influences of K(ATP) channels were analyzed, because they contribute significantly to the modulation of neuronal excitability during hypoxia. IOS responses obtained during manipulation of K(ATP) channel activity could be explained only by implicating mitochondrial volume changes mediated by mitochondrial K(ATP) channels. In conclusion, the hypoxic IOS response can be interpreted as a result of cell and mitochondrial swelling. Cell swelling can be attributed to hypoxic release of neurotransmitters and neuromodulators and to inhibition of Na(+)/K(+)-pump activity.

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