Electrical stimulation of the ventral tegmental area restores consciousness from sevoflurane-, dexmedetomidine-, and fentanyl-induced unconsciousness in rats

Abstract

Background: Dopaminergic neurons in the ventral tegmental area (VTA) are crucially involved in regulating arousal, making them a potential target for reversing general anesthesia. Electrical deep brain stimulation (DBS) of the VTA restores consciousness in animals anesthetized with drugs that primarily enhance GABA_A receptors. However, it is unknown if VTA DBS restores consciousness in animals anesthetized with drugs that target other receptors.

Objective: To evaluate the efficacy of VTA DBS in restoring consciousness after exposure to four anesthetics with distinct receptor targets.

Methods: Sixteen adult Sprague-Dawley rats (8 female, 8 male) with bipolar electrodes implanted in the VTA were exposed to dexmedetomidine, fentanyl, ketamine, or sevoflurane to produce loss of righting, a proxy for unconsciousness. After receiving the dopamine D1 receptor antagonist, SCH-23390, or saline (vehicle), DBS was initiated at 30 μA and increased by 10 μA until reaching a maximum of 100 μA. The current that evoked behavioral arousal and restored righting was recorded for each anesthetic and compared across drug (saline/SCH-23390) condition. Electroencephalogram, heart rate and pulse oximetry were recorded continuously.

Results: VTA DBS restored righting after sevoflurane, dexmedetomidine, and fentanyl-induced unconsciousness, but not ketamine-induced unconsciousness. D1 receptor antagonism diminished the efficacy of VTA stimulation following sevoflurane and fentanyl, but not dexmedetomidine.

Conclusions: Electrical DBS of the VTA restores consciousness in animals anesthetized with mechanistically distinct drugs, excluding ketamine. The involvement of the D1 receptor in mediating this effect is anesthetic-specific.

Keywords: Dexmedetomidine; Dopamine D1 receptor; Fentanyl; General anesthesia; Ketamine; Ventral tegmental area.

Fig. 1.. Study design.

One week following electrode implantation surgery, all animals underwent DBS during continuous sevoflurane anesthesia (N = 19). Animals that did not right under sevoflurane (“Non-responders,” N = 3) were removed from the study and brains collected. The remaining 16 animals that righted with DBS during sevoflurane anesthesia were then tested with dexmedetomidine, ketamine, and fentanyl in a blocked randomized order with either saline or SCH-23390. Rats underwent DBS a maximum of twice per week, with 72 h between tests. After receiving each intravenous anesthetic twice, rats were tested under sevoflurane twice with either saline or SCH-23390 in a randomized order. Finally, rats were euthanized, and tissues collected.

Fig. 2.. VTA DBS restores righting during continuous sevoflurane anesthesia.

A) Left: Representative histology of the electrode DiI track in a rat that completed the study with the VTA outlined in white. Right: Deepest region with DiI staining depicted in red. B) Left: Histology from the three rats that did not respond to DBS during sevoflurane anesthesia. DiI tracks confirm that electrode placement was outside the VTA. Right: Depiction of the deepest region with DiI from each of the three rats removed from the study shown in yellow. C) Probability of righting during VTA DBS at indicated currents during continuous sevoflurane anesthesia. Survival curve depicts the first sevoflurane test (blue), and the last two sevoflurane tests following saline (black) or SCH-23390 pre-treatment (red) (N = 16). D) Representative spectrograms from the same rat under continuous sevoflurane anesthesia, pretreated with either saline (left) or SCH-23390 (right) followed by DBS (indicated by green bars). E) Power spectral density (PSD) plots comparing 1 min prior to DBS (black) and during DBS at the maximum current delivered (red) after saline pre-treatment. Mean ± SEM, N = 4. F) PSD plots comparing 1 min prior to DBS (black) and during DBS at the maximum current delivered (red) after SCH-23390 pre-treatment. Mean ± SEM, N = 5. G-H) Median spectral power (and 95%CI) at indicated frequency bands during continuous sevoflurane anesthesia after pretreatment with (G) saline or (H) SCH-23390. * = p < 0.05, ** = p < 0.01.

Fig. 3.. VTA DBS restores righting following dexmedetomidine-induced unconsciousness, independent of D1R antagonism.

A) Probability of righting following dexmedetomidine-induced unconsciousness in rats pre-treated with either saline of SCH-23390 (N = 16). B) Final latency to return of righting following dexmedetomidine-induced unconsciousness in rats pretreated with either saline or SCH-23390 (N = 16). C) Representative spectrograms from a rat treated with dexmedetomidine and either saline (left) or SCH-23390 (right) followed by DBS (in green). D) PSD plots comparing epochs from the pre-dexmedetomidine awake state (black), following dexmedetomidine delivery and saline treatment (red), and the maximum current delivered (blue). Mean ± SEM, N = 6. E) PSD plots comparing epochs from the pre-dexmedetomidine awake state (black), following dexmedetomidine delivery and SCH-23390 treatment (red), and the maximum current delivered (blue). Mean ± SEM, N = 6. F-G) Median spectral power (and 95%CI) at indicated frequency bands of the same epochs in D-E in (F) saline-treated and (G) SCH-23390-treated rats. * = p < 0.05, ** = p < 0.01 versus awake epoch. † = p < 00.05, †† = p < 00.01 versus Dex+drug epoch. N = 6.

Fig. 4.. VTA DBS restores righting following fentanyl-induced unconsciousness, which is attenuated by D₁R antagonism.

A) Probability of righting following fentanyl-induced unconsciousness in rats treated with either saline or SCH-23390 (N = 16). B) Final latency to return of righting following fentanyl-induced unconsciousness in rats pretreated with either saline or SCH-23390 (N = 16). C) Representative spectrograms from a rat treated with fentanyl and either saline (left) or SCH-23390 (right) followed by DBS (in green). D) PSD plots comparing epochs from the pre-fentanyl awake state (black), following fentanyl delivery and saline treatment (red), and during the maximum current delivered (blue). Mean ± SEM, N = 5. E) PSD plots comparing epochs from the pre-fentanyl awake state (black), following fentanyl delivery and SCH-23390 treatment (red), and the maximum current delivered (blue). Mean ± SEM, N = 5. F-G) Median spectral power (and 95% CI) at indicated frequency bands of the same epochs in D-E in (F) saline-treated and (G) SCH-23390-treated rats. * = p < 0.05, *** = p < 0.001, **** = p < 0.0001 versus awake epoch. † = p < 00.05, ††† = p < 00.001, †††† = p < 00.0001 versus Fentanyl+drug epoch. N = 5.

Fig. 5.. VTA DBS does not elicit arousal or restore righting following ketamine-induced unconsciousness.

A) Probability of righting following ketamine-induced unconsciousness in rats treated with either saline of SCH-23390 (N = 16). B) Final latency to return of righting following ketamine-induced unconsciousness in rats pretreated with either saline or SCH-23390 (N = 16). C) Sample spectrograms from the same rat treated with ketamine and either saline (left) or SCH-23390 (right) followed by DBS (in green). D) PSD plots comparing epochs from the pre-ketamine awake state (black), following ketamine delivery and saline treatment (red), and during the maximum current delivered (blue). Mean ± SEM, N = 5. E) PSD plots comparing epochs from the pre-ketamine awake state (black), following ketamine delivery and SCH-23390 treatment (red), and the maximum current delivered (blue). Mean ± SEM, N = 5. F-G) Mean spectral power at indicated frequency bands of the same epochs in D-E in (F) saline-treated and (G) SCH-23390-treated rats. * = p < 0.05, ** = p < 0.01, *** = p < 0.001, **** = p < 0.0001 versus awake epoch. N = 5.