Thiopental directly depresses lumbar dorsal horn neuronal responses to noxious mechanical stimulation in goats

Involvement of NMDA Receptors in Thiopental-Induced Loss of Righting Reflex, Antinociception and Anticonvulsion Effects in Mice

Potentiation of GABA(A) receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 microg i.c.v.) or bicuculline (0.6 microg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA(A) and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA(A) receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.

Etomidate depresses lumbar dorsal horn neuronal responses to noxious thermal stimulation in rats

Etomidate is a widely used IV anesthetic, but little is known about its analgesic properties, in particular, its effects on spinal cord neuronal responses to noxious stimuli. We hypothesized that etomidate would depress lumbar neuronal responses to noxious heat. Rats (n = 15) were anesthetized with isoflurane (1.2%) and laminectomy was performed to record single unit activity. Lumbar neuronal responses to noxious thermal (52 degrees C, 12 s) stimulation of the hindpaw were recorded before and every 2 min (up to 13 min postinjection) after administration of etomidate. The responses at peak effect of etomidate (as a percentage of the control response) were 63% +/- 16%, 63% +/- 16%, 38% +/- 25%, 36% +/- 30%, and 41% +/- 26% for the 0.125, 0.25, 0.5, 1 and 2 mg/kg doses, respectively. The responses quickly recovered, usually by the 10-min period postinjection. Similar responses were obtained in decerebrate, isoflurane-free rats administered etomidate and in isoflurane-anesthetized rats administered propofol. These data demonstrate that etomidate depresses spinal cord neuronal responses to noxious stimulation and is a possible mechanism by which this drug might produce analgesia.

Modulation of gamma-aminobutyric acid A receptor function by thiopental in the rat spinal dorsal horn neurons

To assess the actions of thiopental at the spinal dorsal horn level, we examined the effects of thiopental using the whole cell patch-clamp technique on mechanically dissociated rat spinal dorsal horn neurons. Thiopental, at large concentrations, elicited a current (I(Thio)) through activation of chloride conductance, and its threshold concentration was approximately 50 microM. I(Thio) was sensitive to bicuculline, a gamma-aminobutyric acid (GABA)A receptor antagonist, but not to strychnine, a glycine receptor antagonist. At a clinically relevant concentration (30 muM), thiopental markedly enhanced the peak amplitude of a subsaturating GABA-induced current (I(GABA)) but not that of a saturating GABA-induced current. Furthermore, thiopental prolonged the time constants of both desensitization and deactivation of I(GABA). At a large concentration (300 muM), it inhibited the peak amplitude of I(GABA), which may be the result of open-channel blockade. In addition, at 30 microM, thiopental increased the duration and decreased the frequency of GABAergic miniature inhibitory postsynaptic currents. These results indicate that thiopental enhances GABAergic inhibitory transmission and suggest that GABA(A) receptors in the spinal cord are a potential target through which thiopental causes immobility and depresses the response to noxious stimuli.

Thiopental produces immobility primarily by supraspinal actions in rats

The spinal cord mediates most of the immobilizing action of inhaled anesthetics. In the present study we investigated whether spinal or supraspinal sites mediate the immobilizing action of thiopental in rats. Thiopental was administered IV, intrathecally (IT), intracerebroventricularly (ICV), or simultaneously IT and ICV. Only the IV infusion produced anesthesia, defined as immobility in response to application of a tail clamp (i.e., the equivalent of minimum alveolar concentration, MAC). Consequently, the MAC-sparing effect (for isoflurane) of thiopental was used to assess the immobilizing contribution of IT and ICV infusions of thiopental. Thiopental concentrations were determined in whole brain, spinal cord, and a slice of cerebral cortex distant from the infusion sites. These concentrations were correlated with the MAC-sparing effect of the thiopental infusions in a multiple regression model. To assess the rate at which thiopental penetrates the cord, rat spinal cords were equilibrated in a bath of thiopental ex vivo and the concentration of thiopental in the cord was measured as a function of equilibration time. This was repeated in vivo with IT infusions of thiopental spanning the time of the behavioral studies. We found that IT or ICV infusion of thiopental 25 microg/min decreased isoflurane MAC <25%. The associated thiopental concentrations in the spinal cord after IT infusion, and in the whole brain after ICV infusion of 25 microg/min thiopental, exceeded by 500% and 680%, respectively, the concentrations found in the spinal cord and in the whole brain after IV infusion of thiopental in a dose that produced anesthesia in the absence of isoflurane. The percentage decrease in the MAC of isoflurane correlated primarily with the concentration of thiopental found in cerebral tissue not in contact with the cerebral ventricles. The spinal cord infusion produced an approximately 20% decrease in MAC. Ex vivo IT thiopental readily diffused into the spinal cord, with a time constant of approximately 1 h. We conclude that, unlike inhaled anesthetics, the immobilizing action of thiopental is largely supraspinal. Centers in the brain other than those near the third and fourth ventricles produce the greatest effect.

Effects of pentobarbital on purinergic P2X receptors of rat dorsal root ganglion neurons

Purinergic P2X receptors are ligand-gated ion channels that are activated by extracellular adenosine triphosphate (ATP) and are widely expressed not only in the central and peripheral nervous system but also in tissues throughout the body, playing an important role in the transfer of nociceptive information. Since the influence of barbiturates on P2X receptor subtypes is not known, we studied the effects of pentobarbital sodium (PB) on ATP responses in dorsal root ganglion (DRG) neurons. DRG neurons were dissected from 10- to 14-day-old rats and dissociated after enzyme treatment. Electrical measurements were performed using the nystatin-perforated patch recording mode under voltage-clamp conditions. Drugs were applied using the Y-tube method. ATP evoked three types of inward current at -60 mV: fast desensitizing, slow desensitizing, and mixed. The fast-type current was attributed to activation of P2X3 subtype and the slow type to the P2X2 subtype. PB suppressed the fast-type current in a concentration-dependent manner, while the slow type was slightly reduced. A noncompetitive inhibition was suggested by a downward shift of the ATP concentration-response curves. The current-voltage relationships showed inward rectification, and the extent of suppression was not affected by the holding potential. The reduction was greater in external solutions of higher pH. PB had subtype-specific effects on P2X receptors. The ionized form is likely to be responsible for the suppression of the P2X3 receptor current, which may result in a reduction of the excitability of central and peripheral neurons and may contribute to the anesthetic and analgesic actions of the agent.

Spinal anaesthesia indirectly depresses cortical activity associated with electrical stimulation of the reticular formation

BACKGROUND

Neuraxial blockade reduces the requirements for sedation and general anaesthesia. We investigated whether lidocaine spinal anaesthesia affected cortical activity as determined by EEG desynchronization that occurs following electrical stimulation of the midbrain reticular formation (MRF).

METHODS

Six goats were anaesthetized with isoflurane, and cervical laminectomy performed to permit spinal application of lidocaine. The EEG was recorded before, during and after focal electrical stimulation (0.1, 0.2, 0.3 and 0.4 mA) in the MRF while keeping the isoflurane concentration constant.

RESULTS

During lidocaine spinal anaesthesia, the spectral edge frequency (SEF) after MRF electrical stimulation (13.6 (SD 1.0) Hz, averaged across all stimulus currents) was less than the SEF during control and recovery periods (18.6 (3.6) Hz and 17.2 (2.2) Hz, respectively; P<0.05). Bispectral index values were similarly affected: 69 (10) at control compared with 55 (6) during the spinal block (P<0.05).

CONCLUSIONS

These results suggest that lidocaine spinal anaesthesia blocks ascending somatosensory transmission to mildly depress the excitability of reticulo-thalamo-cortical arousal mechanisms.