Rationale:
The DBA/1 mouse of sudden unexpected death in epilepsy (SUDEP) displays generalized seizure-induced respiratory arrest (S-IRA), which is the most common precipitating event in SUDEP in patients with epilepsy.
The PAG is implicated as a critical brain structure in S-IRA in the DBA/1 mouse and in human SUDEP.
Stimulation of the PAG is known to increase respiration, and this structure is proposed to function as a suffocation alarm. Therefore, we evaluated the effect of electrical stimulation in the PAG on the respiration of anesthetized DBA/1 mice.
Methods:
/1 mice (22-28 days old) were primed [four audiogenic seizures (AGSz)] at daily intervals), using an electric bell (122 dB SPL for 60 sec), and the mice were resuscitated after each S-IRA. Subsequently, the primed DBA/1 mice underwent stereotaxic surgery under isoflurane anesthesia. A bipolar concentric electrode (NEX-100-01) was inserted into the PAG, and trains of electrical stimuli were presented (1 msec pulse duration, 15 sec duration) at increasing stimulus intensities (200-1000mA at 25 Hz). The effects on respiration were evaluated using an abdominal piezo-electric transducer, which was connected to a PowerLab 8/sp, and the data were analyzed using Lab Chart software, displayed visually, and analyzed digitally off-line.
Results:
In anesthetized mice (N=5-10) there was a significant effect of PAG stimulation on both respiratory rate and amplitude, which both increased progressively as current intensity was increased. Thus, breathing frequency was increased at 200 mA to 165
+12.4% (SEM), at 600 mA to 435
+25% and at 1000 mA 890
+50% of control, which were significant at p< 0.05. The amplitude of respiration was also increased, but reached maximum at or near the end of the stimulation period. Thus, at 200 mA the increase was 201
+25.7%, at 600 mA 349
+50%, and at 1000mA 390+50% of control, which were significant at p< 0.05.
There was also a large transient increase in respiration amplitude at stimulus offset which is associated with a sigh response in previous studies.
When the concentration of isoflurane was increased to near-fatal concentrations (5%) the respiratory rate and amplitude were greatly reduced, but PAG stimulation under these conditions again increased both respiratory parameters.
Conclusions:
A robust effect of PAG stimulation was seen in DBA/1 mice that extensively enhanced both the rate and amplitude of respiration in DBA/1 mice, and the degree of increase was dependent on the intensity of the current.
The need for relatively high stimulus intensities to produce significant increases in respiration is consistent with previous studies, which indicated that the sensitivity of DBA/1 mice to PAG stimulation is significantly below that seen in non-epileptic mice. Chronically implanted electrodes in PAG are planned, which will attempt to resuscitate the mice and prevent death due to S-IRA.
Funding:
NIH NS126870