REM sleep-like atonia of hypoglossal (XII) motoneurons is caused by loss of noradrenergic and serotonergic inputs

Abstract

Rationale: Studies of hypoglossal (XII) motoneurons that innervate the genioglossus muscle, an upper airway dilator, suggested that the suppression of upper airway motor tone during REM sleep is caused by withdrawal of excitation mediated by norepinephrine and serotonin.

Objectives: Our objectives were to determine whether antagonism of aminergic receptors located in the XII nucleus region can abolish the REM sleep-like atonia of XII motoneurons, and whether both serotonergic and noradrenergic antagonists are required to achieve this effect.

Methods: REM sleep-like episodes were elicited in anesthetized rats by pontine carbachol injections before and at various times after microinjection of prazosin and methysergide combined, or of only one of the drugs, into the XII nucleus.

Measurements and main results: Spontaneous XII nerve activity was significantly reduced, by 35 to 81%, by each antagonist alone and in combination, indicating that XII motoneurons were under both noradrenergic and serotonergic endogenous excitatory drives. During the 32 to 81 min after microinjections of both antagonists, pontine carbachol caused no depression of XII nerve activity, whereas other characteristic effects (activation of the hippocampal and cortical EEG, and slowing of the respiratory rate) remained intact. A partial recovery of the depressant effect of carbachol then occurred parallel to the recovery of spontaneous XII nerve activity from the depressant effect of the antagonists. Microinjections of either antagonist alone did not eliminate the depressant effect of carbachol.

Conclusions: The REM sleep-like depression of XII motoneuronal activity induced by pontine carbachol can be fully accounted for by the combined withdrawal of noradrenergic and serotonergic effects on XII motoneurons.

Figure 1

Localization of pontine carbachol injection sites. Various symbols in A show the centers of carbachol injections placed on the corresponding closest standard sections of the pons (22). Circles, carbachol injection sites in the six rats with combined injections of prazosin and methysergide into the XII nucleus; triangles, sites in the six rats with only prazosin injected into the XII nucleus; diamonds, sites in the six rats with only methysergide injections. All carbachol injections were placed within a discrete region of the dorsal pontine tegmentum. (B) Carbachol injection site in one of the rats used in the present study, as seen in a coronal section of the pons. The darkened area in the dorsomedial pontine tegmentum (arrow) represents deposition of the blue dye injected with carbachol. LDT = laterodorsal tegmental nucleus; PO = nucleus pontis oralis; scp = superior cerebellar peduncle; VT = ventral tegmental nucleus.

Figure 2

Two responses to pontine carbachol elicited about 150 min apart in one experiment demonstrate long-term stability of hippocampal and cortical activations and respiratory rate slowing in response to pontine carbachol. In both records, 10 nl of carbachol was injected at the marker. Traces from the top are as follows: power of the hippocampal (Hipp) signal in the 3- to 5-Hz range, raw hippocampal recording, power of the cortical EEG in the 6- to 12-Hz range, raw cortical EEG, moving average (MA) of XII nerve activity (at this compressed time scale, the amplitude of the record represents the peak inspiratory activity in successive respiratory cycles), and the instantaneous central respiratory rate during successive 10-s intervals. The response in A was elicited before methysergide injections and the response in B was recorded about 150 min later and after methysergide injections into the XII nucleus. Note the similarity between the two responses except that the starting magnitude of XII nerve activity is different because of the methysergide actions. Two additional responses to pontine carbachol were elicited in this experiment between the two shown in A and B.

Figure 3

Average time course of the changes in spontaneous XII nerve activity after microinjections of prazosin and methysergide (A), prazosin only (B), and methysergide only (C) into the XII nucleus. (A) Combined antagonists were injected at three sites in the XII nucleus between time 0 and the point numbered 3; points 1–3 show the average activity levels measured 1 min after each of the three injections. XII nerve activity reached a minimum of 30.8 ± 2.7% of the control activity level about 60 min after the injections. A recovery to 65.3 ± 11% of the preantagonist level occurred over the next 2 h. (B) Prazosin only was injected at three sites in the XII nucleus (same format as in A). XII nerve activity declined to a minimum level corresponding to 19.1 ± 3.6% of the preantagonist level and then gradually recovered to 48.4 ± 7.7% over the next 2 h. (C) Methysergide only was injected at three sites in the XII nucleus (same format as in A). After these injections, XII nerve activity declined to 65.1 ± 6.6% of the control level during the first 20 min. It then transiently increased over the next 40 min to 89.2 ± 5.7% of the control level, suggesting that the drug diffused into a region where its action resulted in enhancement of XII nerve activity. Three hours after methysergide administration, XII nerve activity was still significantly depressed to 73.7 ± 7.4% of the control level. * Points at which XII nerve activity was significantly depressed (p < 0.05 relative to the level of activity at the beginning of these experiments); each curve is based on data from six animals. Pontine carbachol injections were made at greater than 30-min intervals during these experiments, both before and after antagonist injections, and each elicited characteristic responses, such as those illustrated in Figures 2 and 4, which lasted 3 to 6 min. The short horizontal bars mark the periods when carbachol responses described in text were elicited in individual animals.

Figure 4

Combined microinjections of prazosin and methysergide into the XII nucleus eliminate the REM sleep–like depression of XII nerve activity. In each panel, the top trace shows hippocampal activity (Hipp) and the bottom trace shows the moving average (MA) of XII nerve activity, both recorded at the same gains in all three panels. Each peak in the XII nerve MA represents one compressed inspiratory burst. In each record, 10-nl carbachol injections were made at the markers. (A) The preantagonist response to pontine carbachol, during which XII nerve activity is profoundly depressed and the respiratory rate is reduced from 52.5 to 47.5 min⁻¹. (B) A response to carbachol elicited 42 min after antagonists were injected into the XII nucleus; at this time, the precarbachol level of XII nerve activity was reduced to a level similar to that observed at the time of maximal response to carbachol before antagonist administration (A), and no further depression occurred after carbachol injection. The characteristic hippocampal activation and slowing of the respiratory rate from 52.5 to 46.7 min⁻¹ indicate that carbachol was effective but did not depress XII nerve activity. (C) By 137 minutes after administration of the antagonists, the precarbachol level of XII nerve activity increased compared with that in B, and XII nerve activity was visibly depressed after pontine carbachol injection, both indicating a partial recovery from the effect of the antagonists.

Figure 5

Individual and average effects of combined prazosin and methysergide microinjections into the XII nucleus on the carbachol-induced depression of XII nerve activity (A) and slowing of the respiratory rate (B). (A) Individual (top graph) and average (bottom graph) responses of the XII nerve to pontine carbachol. Each data triplet shows the level of XII nerve activity just before, at the peak of, and just after the response to carbachol. XII nerve activity is normalized relative to the activity level at the beginning of each experiment. The three data sets illustrate the effects of carbachol before and during two periods after antagonist administration, as indicated at the bottom. XII nerve activity was significantly depressed by pontine carbachol under the control conditions and when tested 135 to 199 min after the antagonists. However, during the tests with carbachol conducted 32 to 81 min after antagonist administration, XII nerve activity was on average not altered, and only minimally increased or decreased in individual animals (middle data set, top graph). Thus, during this period, the combined antagonism of adrenergic and serotonergic receptors reversibly eliminated the depressant effect of pontine carbachol on XII nerve activity. (B) Average changes in respiratory rate elicited by carbachol at successive stages of these experiments, shown using the same format as in the bottom graph in A. The baseline respiratory rate and its carbachol-induced decreases were well maintained at all times after antagonist administration. * Significantly different from the precarbachol level at p < 0.05.

Figure 6

Microinjections into the XII nucleus of prazosin only did not eliminate the depressant effect of pontine carbachol on XII nerve activity. (A) The effect of pontine carbachol on XII nerve activity before, and at two times after, prazosin microinjections, shown in the same format as in Figure 5A. Each data set shows XII nerve activity just before, at the peak of the response to pontine carbachol, and just after the response. The three data sets illustrate the effects of carbachol, as tested under control conditions and then during two periods after prazosin administration (indicated at the bottom). Note that pontine carbachol injections made 40–85 minutes after prazosin had a significant depressant effect on XII nerve activity; during the same period, combined microinjections of prazosin and methysergide eliminated the carbachol-induced depression (cf. Figure 5A). XII nerve activity is normalized relative to its level at the beginning of the experiments. (B) The baseline respiratory rate and its carbachol-induced changes were well maintained at all times after prazosin administration. * Significantly different from the precarbachol level at p < 0.05.

Figure 7

Microinjections into the XII nucleus of methysergide only did not eliminate the depressant effect of pontine carbachol on XII nerve activity. (A) The effect of pontine carbachol on XII nerve activity before, and at two times after, methysergide administration, shown in the same format as in Figure 5. Each data set shows XII nerve activity levels just before, at the peak of the response to pontine carbachol, and just after the response. The three data sets illustrate the effects of carbachol under control conditions and then during two periods after methysergide administration (indicated at the bottom). XII nerve activity is normalized relative to the activity level at the beginning of the experiments. Note that pontine carbachol had a significant depressant effect on XII nerve activity when tested 40 to 47 min after methysergide administration, whereas the combined microinjections of prazosin and methysergide eliminated the depression (cf. Figure 5A). (B) The baseline respiratory rate and its carbachol-induced changes were well maintained throughout these experiments. * Significantly different from the precarbachol level at p < 0.05. (C) Mean systolic arterial blood pressure measured at the same times as XII nerve activity and respiratory rate shown in A and B. Neither the small arterial pressure increases after carbachol injections nor the gradual increasing trend across the entire duration of these experiments were significant. See the last section of RESULTS for more details on blood pressure across all conditions.