Changes in inhibitory amino acid release linked to pontine-induced atonia: an in vivo microdialysis study

Abstract

We hypothesized that cessation of brainstem monoaminergic systems and an activation of brainstem inhibitory systems are both involved in pontine inhibitory area (PIA) stimulation-induced muscle atonia. In our previous study (Lai et al., 2001), we found a decrease in norepinephrine and serotonin release in motoneuron pools during PIA stimulation-induced muscle tone suppression. We now demonstrate an increase in inhibitory amino acid release in motor nuclei during PIA stimulation in the decerebrate cat using in vivo microdialysis and HPLC analysis techniques. Microinjection of acetylcholine into the PIA elicited muscle atonia and simultaneously produced a significant increase in both glycine and GABA release in both the hypoglossal nucleus and the lumbar ventral horn. Glycine release increased by 74% in the hypoglossal nucleus and 50% in the spinal cord. GABA release increased by 31% in the hypoglossal nucleus and 64% in the spinal cord during atonia induced by cholinergic stimulation of the PIA. As with cholinergic stimulation, 300 msec train electrical stimulation of the PIA elicited a significant increase in glycine release in the hypoglossal nucleus and ventral horn. GABA release was significantly increased in the hypoglossal nucleus but not in the spinal cord during electrical stimulation of the PIA. Glutamate release in the motor nuclei was not significantly altered during atonia induced by electrical or acetylcholine stimulation of the PIA. We suggest that both glycine and GABA play important roles in the regulation of upper airway and postural muscle tone. A combination of decreased monoamine and increased inhibitory amino acid release in motoneuron pools causes PIA-induced atonia and may be involved in atonia linked to rapid eye-movement sleep.

Fig. 1.

Histology showing electrical stimulation (squares) and acetylcholine injection (stars) sites. BC, Brachium conjunctivum;CNF, cuniformis nucleus; IC, inferior colliculus; LC, locus ceruleus; P, pyramidal tract; PIA, pontine inhibitory area;SO, superior olivary nucleus; TR, tegmental reticular nucleus.

Fig. 2.

Histology showing the dialysate collecting sites in the brainstem and spinal cord. Dialysates were collected from both sides of the motor nuclei with a total of 30 sites (1 case,h; 2 cases, a, b,d–f, i; 4 cases, c,g, j, k; 12 cases,l) in eight cats. Dialysates collected from the following sites were under both electrical and chemical stimulation administered into the PIA: a, b,d–f, and i–l. Collecting sites were reconstructed and presented on the right.L7, The seventh lumbar segment of the spinal cord;NPM, nucleus paramedianus; P11–P14, brainstem levels from 11–14 posterior to the interaural zero according to the atlas of Berman (1968); 5ST, spinal trigeminal tract; 12, hypoglossal nucleus; VII,VIII, IX, laminas VII, VIII, and IV in the ventral horn.

Fig. 3.

Change in EMG activity by ACh injection into the pontine inhibitory area. Polygraphic recording of neck EMG activity during control baseline, ACh injection, and recovery is shown at thetop. A significant reduction of genioglossus (GG) and neck (NM) muscle activity was seen after pontine ACh injection. Both genioglossus and neck EMG returned to the pre-ACh injection level at an average of 4.6 min after injection. The error bars shown in this and subsequent figures represent the SEM. B, Baseline. Calibration for EMG activity, 200 μV. *p < 0.05,t test relative to baseline.

Fig. 4.

Change in glycine release in the hypoglossal nucleus and spinal cord with pontine acetylcholine injection. A significant increase in glycine release in both hypoglossal nucleus and ventral horn was seen during pontine ACh injection-induced muscle tone suppression. SC, Spinal cord; XII, hypoglossal nucleus; B, baseline. *p< 0.05; **p < 0.01; t test relative to baseline.

Fig. 5.

Change in GABA release in the hypoglossal nucleus and spinal cord with pontine acetylcholine injection. A significant increase in GABA release in both hypoglossal nucleus and ventral horn was seen during pontine ACh injection-induced muscle tone suppression.SC, Spinal cord; XII, hypoglossal nucleus; B, baseline.

Fig. 6.

Change in glycine release in the hypoglossal nucleus and spinal cord during pontine electrical stimulation. Electrical stimulation (ES) in the PIA induces a significant increase in glycine release in both the hypoglossal nucleus and the ventral horn. SC, Spinal cord;XII, hypoglossal nucleus; B, baseline.

Fig. 7.

GABA release in the hypoglossal nucleus and spinal cord during electrical stimulation in the PIA. Train stimulations in the PIA elicit a significant increase in GABA release in the hypoglossal nucleus, whereas the same stimulation did not produce a significant change in GABA release in the ventral horn.SC, Spinal cord; XII, hypoglossal nucleus; B, baseline.

Fig. 8.

Glutamate release in the hypoglossal nucleus and spinal cord was unaltered by pontine acetylcholine injection. A nonsignificant increase in glutamate release in the ventral horn was seen during ACh injection-induced muscle tone suppression.SC, Spinal cord; XII, hypoglossal nucleus; B, baseline.

Fig. 9.

Glutamate release in the hypoglossal nucleus and spinal cord was unaltered by electrical stimulation in the PIA. As with ACh injection, electrical stimulation (ES) in the PIA did not change glutamate release in the motor nuclei.SC, Spinal cord; XII, hypoglossal nucleus; B, baseline.