The Actions of Propofol on γ-Aminobutyric Acid-A and Glycine Receptors in Acutely Dissociated Spinal Dorsal Horn Neurons of the Rat

Effects of nitrous oxide on glycinergic transmission in rat spinal neurons

We investigated the effects of nitrous oxide (N₂O) on glycinergic inhibitory whole-cell and synaptic responses using a "synapse bouton preparation," dissociated mechanically from rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique can evaluate pure single- or multi-synaptic responses from native functional nerve endings and enable us to accurately quantify how N₂O influences pre- and postsynaptic transmission. We found that 70 % N₂O enhanced exogenous glycine-induced whole-cell currents (I_Gly) at glycine concentrations lower than 3 × 10^-5 M, but did not affect I_Gly at glycine concentrations higher than 10^-4 M. N₂O did not affect the amplitude and 1/e decay-time of both spontaneous and miniature glycinergic inhibitory postsynaptic currents recorded in the absence and presence of tetrodotoxin (sIPSCs and mIPSCs, respectively). The decrease in frequency induced by N₂O was observed in sIPSCs but not in mIPSCs, which was recorded in the presence of both tetrodotoxin and Cd²⁺, which block voltage-gated Na⁺ and Ca²⁺ channels, respectively. N₂O also decreased the amplitude and increased the failure rate and paired-pulse ratio of action potential-evoked glycinergic inhibitory postsynaptic currents. N₂O slightly decreased the Ba²⁺ currents mediated by voltage-gated Ca²⁺ channels in SDCN neurons. We found that N₂O suppresses glycinergic responses at synaptic levels with presynaptic effect having much more predominant role. The difference between glycinergic whole-cell and synaptic responses suggests that extrasynaptic responses seriously modulate whole-cell currents. Our results strongly suggest that these responses may thus in part explain analgesic effects of N₂O via marked glutamatergic inhibition by glycinergic responses in the spinal cord.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Anesthetics: from modes of action to unconsciousness and neurotoxicity

Modern anesthetic compounds and advanced monitoring tools have revolutionized the field of medicine, allowing for complex surgical procedures to occur safely and effectively. Faster induction times and quicker recovery periods of current anesthetic agents have also helped reduce health care costs significantly. Moreover, extensive research has allowed for a better understanding of anesthetic modes of action, thus facilitating the development of more effective and safer compounds. Notwithstanding the realization that anesthetics are a prerequisite to all surgical procedures, evidence is emerging to support the notion that exposure of the developing brain to certain anesthetics may impact future brain development and function. Whereas the data in support of this postulate from human studies is equivocal, the vast majority of animal research strongly suggests that anesthetics are indeed cytotoxic at multiple brain structure and function levels. In this review, we first highlight various modes of anesthetic action and then debate the evidence of harm from both basic science and clinical studies perspectives. We present evidence from animal and human studies vis-à-vis the possible detrimental effects of anesthetic agents on both the young developing and the elderly aging brain while discussing potential ways to mitigate these effects. We hope that this review will, on the one hand, invoke debate vis-à-vis the evidence of anesthetic harm in young children and the elderly, and on the other hand, incentivize the search for better and less toxic anesthetic compounds.

Propofol Inhibits Cerebellar Parallel Fiber-Purkinje Cell Synaptic Transmission via Activation of Presynaptic GABAB Receptors in vitro in Mice

Propofol is a widely used intravenous sedative-hypnotic agent, which causes rapid and reliable loss of consciousness via activation of γ -aminobutyric acid A (GABA_A) receptors. We previously found that propofol inhibited cerebellar Purkinje cells (PC) activity via both GABA_A and glycine receptors in vivo in mice. We here examined the effect of propofol on the cerebellar parallel fiber (PF)-PC synaptic transmission in mouse cerebellar slices by whole-cell recording technique and pharmacological methods. We found that following blockade of GABA_A and glycine receptors activity, propofol reversely decreased the amplitude of PF-PC excitatory postsynaptic currents (PF-PC EPSCs), and significantly increased paired-pulse ratio (PPR). The propofol-induced decrease in amplitude of PF-PC EPSCs was concentration-dependent. The half-inhibitory concentration (IC₅₀) of propofol for inhibiting PF-PC EPSCs was 4.7 μM. Notably, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by GABA_B receptor antagonist, saclofen (10 μM), but not blocked by N-methyl-D-aspartate receptor (NMDA) receptor antagonist, D-APV (50 μM). Application of the GABA_B receptor agonist baclofen induced a decrease in amplitude and an increase in PPR of PF-PC EPSCs, as well masked the propofol-induced changes in PF-PC EPSCs. Moreover, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by a specific protein kinase A (PKA) inhibitor, KT5720. These results indicate that application of propofol facilitates presynaptic GABA_B receptors, resulting in a depression of PF-PC synaptic transmission via PKA signaling pathway in mouse cerebellar cortex. The results suggest that the interaction with GABA_B receptors may contribute to the general anesthetic action of propofol.

Effects of propofol on glycinergic neurotransmission in a single spinal nerve synapse preparation

The effects of the intravenous anesthetic, propofol, on glycinergic transmission and on glycine receptor-mediated whole-cell currents (IGly) were examined in the substantia gelatinosa (SG) neuronal cell body, mechanically dissociated from the rat spinal cord. This "synaptic bouton" preparation, which retains functional native nerve endings, allowed us to evaluate glycinergic inhibitory postsynaptic currents (IPSCs) and whole-cell currents in a preparation in which experimental solution could rapidly access synaptic terminals. Synaptic IPSCs were measured as spontaneous (s) and evoked (e) IPSCs. The eIPSCs were elicited by applying paired-pulse focal electrical stimulation, while IGly was evoked by a bath application of glycine. A concentration-dependent enhancement of IGly was observed for ≥10µM propofol. Propofol (≥3µM) significantly increased the frequency of sIPSCs and prolonged the decay time without altering the current amplitude. However, propofol (≥3µM) also significantly increased the mean amplitude of eIPSCs and decreased the failure rate (Rf). A decrease in the paired-pulse ratio (PPR) was noted at higher concentrations (≥10µM). The decay time of eIPSCs was prolonged only at the maximum concentration tested (30µM). Propofol thus acts at both presynaptic glycine release machinery and postsynaptic glycine receptors. At clinically relevant concentrations (<1μM) there was no effect on IGly, sIPSCs or eIPSCs suggesting that at anesthetic doses propofol does not affect inhibitory glycinergic synapses in the spinal cord.

Propofol depresses cerebellar Purkinje cell activity via activation of GABA(A) and glycine receptors in vivo in mice

Propofol is an intravenous sedative-hypnotic agen, which causes rapid and reliable loss of consciousness. Under in vitro conditions, propofol activates GABAA and glycine receptors in spinal cord, hippocampus and hypothalamus neurons. However, the effects of propofol on the cerebellar neuronal activity under in vivo conditions are currently unclear. In the present study, we examined the effects of propofol on the spontaneous activity of Purkinje cells (PCs) in urethane-anesthetized mice by cell-attached recording and pharmacological methods. Our results showed that cerebellar surface perfusion of propofol (10-1000 μM) induced depression of the PC simple spike (SS) firing rate in a dose-dependent manner, but without significantly changing the properties of complex spikes (CS). The IC50 of propofol for inhibiting SS firing of PCs was 144.5 μM. Application of GABAA receptor antagonist, SR95531 (40 μM) or GABAB receptor antagonist, saclofen (20 μM), as well as glycine receptor antagonist, strychnine (10 μM) alone failed to prevent the propofol-induced inhibition of PCs spontaneous activity. However, application the mixture of SR95531 (40 μM) and strychnine (10 μM) completely blocked the propofol-induced inhibition of PC SS firing. These data indicated that cerebellar surface application of propofol depressed PC SS firing rate via facilitation of GABAA and functional glycine receptors activity in adult cerebellar PCs under in vivo conditions. Our present results provide a new insight of the anesthetic action of propofol in cerebellar cortex, suggesting that propofol depresses the SS outputs of cerebellar PCs which is involved in both GABAA and glycine receptors activity.

Gastrointestinal delivery of propofol from fospropofol: its bioavailability and activity in rodents and human volunteers

BACKGROUND

Propofol is a safe and widely used intravenous anesthetic agent, for which additional clinical uses including treatment of migraine, nausea, pain and anxiety have been proposed (Vasileiou et al. Eur J Pharmacol 605:1-8, 2009). However, propofol suffers from several disadvantages as a therapeutic outside anesthesia including its limited aqueous solubility and negligible oral bioavailability. The purpose of the studies described here was to evaluate, in both animals and human volunteers, whether fospropofol (a water soluble phosphate ester prodrug of propofol) would provide higher propofol bioavailability through non-intravenous routes.

METHODS

Fospropofol was administered via intravenous, oral and intraduodenal routes to rats. Pharmacokinetic and pharmacodynamic parameters were then evaluated. Based on the promising animal data we subsequently conducted an oral and intraduodenal pharmacokinetic/pharmacodynamic study in human volunteers.

RESULTS

In rats, bioavailability of propofol from fospropofol delivered orally was found to be appreciable, in the order of around 20-70%, depending on dose. Availability was especially marked following fospropofol administration via the intraduodenal route, where bioavailability approximated 100%. Fospropofol itself was not appreciably bioavailable when administered by any route except for intravenous. Pharmacologic effect following oral fospropofol was confirmed by observation of sedation and alleviation of thermal hyperalgesia in the rat chronic constrictive injury model of neuropathic pain. The human data also showed systemic availability of propofol from fospropofol administration via oral routes, a hereto novel finding. Assessment of sedation in human volunteers was correlated with pharmacokinetic measurements.

CONCLUSIONS

These data suggest potential utility of oral administration of fospropofol for various therapeutic indications previously considered for propofol.

Central depressant effects and toxicity of propofol in chicks

Propofol is an ultra-short acting anesthetic agent. The information on the pharmacological and toxicological effects of propofol in the chicken is rather limited. This study examines the toxicity and pharmaco-behavioral effects of propofol given intraperitoneally in 7–10 day-old chicks. The median effective doses of propofol for the induction of sedation, analgesia to electric stimulation and sleep in the chicks were 1.82, 2.21 and 5.71 mg/kg, respectively. The 24-h median lethal dose of propofol in chicks was 57.22 mg/kg. The therapeutic indices of propofol for sedation, analgesia and sleep were 31.4, 25.9 and 10, respectively. Propofol at 0.5 and 1 mg/kg reduced the locomotor activity and increased the duration of tonic immobility in chicks. Propofol at 2 and 4 mg/kg caused analgesia to electric stimulation as well as analgesia and anti-inflammatory responses against formalin test in chicks. Propofol at 5, 10 and 20 mg/kg induced sleep in chicks for 8.4 to 25 min. Physostigmine shortened the sleep duration of propofol. Data suggest that propofol induces anti-inflammatory action and central nervous system depression in chicks resulting in sedation, analgesia and anesthesia with wide safety margin. These effects could form the basis of further pharmacological and toxicological studies on propofol in the young chick model, and the drug could be safely applied clinically in the chicken.

Ovariectomy in Jenny using subarachnoid propofol administration

OBJECTIVE

To evaluate the effects of subarachnoid propofol administration (SPA) on analgesia, sedation, ataxia, cardiopulmonary, blood gas variables, and its application in ovariectomy in Jennies.

STUDY DESIGN

Prospective, randomized, clinical trial.

ANIMALS

Adult Jennies (n = 20; 100-120 kg; 5-7 years old).

METHODS

In study 1, 5 Jennies had SPA (n = 5; group 1) through Tuohy catheter, and another 5 control Jennies had normal saline subarachnoid administration (n = 5; group 2). In study 2, 10 Jennies had SPA and ovariectomy (n = 10; group 3). In groups 1 and 3 analgesia, sedation and ataxia scores, cardiopulmonary and blood gas variables were recorded at different intervals before and after PSA. In group 2, only cardiopulmonary and blood gas variables were recorded.

RESULTS

SPA produced pronounced analgesia, mild sedation and minimal ataxia. Heart rate (HR), respiratory rate (RR), pulse rate (PR), rectal temperature (RT), arterial blood pH, PaO2 , and PaCO2 were not significantly different from base line values after subarachnoid administration of either propofol or normal saline.

CONCLUSION

SPA was found to be useful, efficient and feasible procedure for ovariectomy through standing flank laparotomy in Jennies.

Connection changes in somatosensory cortex induced by different doses of propofol

BACKGROUND

The mechanism by which general anesthetics, widely used in clinical practice for over 160 years, effects on sensory responsiveness has been unclear until now. In the present study, the authors sought to explore the effect of different doses of propofol on somatosensory cortex by whisker stimulation in rats.

METHODS

In a fixed cage, rats were anesthetized with propofol 80 mg/kg intraperitoneally and then cathetered tail vein with 23-gauge metal needle connected with a pump. Two holes (2 mm diameter) were drilled and recording electrodes implantated in the primary somatosensory cortex barrel field (S1BF) and secondary somatosensory cortex (S2). The extracellular (20 rats) and intracellular (8 rats) recordings were used to test the neuron activity in both cortices at different doses of propofol (20, 40 and 80 mg/kg/h) through tail vein by pump. Meantime, vibrissal, olfactory, corneal responses (VOCR, sedation), and tail-pinch response (TRP, analgesia) were tested every 10 min during the doses of propofol 20, 40 and 80 mg/kg/h.

RESULTS

VOCR and TRP were depressed by propofol in a dose-dependent manner. The amplitude by whisker stimulation in S1BF was stronger and the peak latency was shorter compared with that of in S2. The response latency of S1BF and S2 was increased by raising infusion rate of propofol with the response latency in S2 being longer than that in S1BF at the same doses of propofol. The cross-correlation between S1BF and S2 decreased as the propofol infusion rate increased. The input resistance was higher by increasing infusion rate of propofol.

CONCLUSION

The sedation and analgesia effects of propofol were dose-dependent. Both the connectivity and instinctive oscillation between S1BF and S2 were proportionally modulated by the different doses of propofol.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Allosteric Modulation of αβδ GABAA Receptors

GABA_A receptors mediate the majority of the fast inhibition in the mature brain and play an important role in the pathogenesis of many neurological and psychiatric disorders. The αβδ GABA_A receptor localizes extra- or perisynaptically and mediates GABAergic tonic inhibition. Compared with synaptically localized αβγ receptors, αβδ receptors are more sensitive to GABA, display relatively slower desensitization and exhibit lower efficacy to GABA agonism. Interestingly, αβδ receptors can be positively modulated by a variety of structurally different compounds, even at saturating GABA concentrations. This review focuses on allosteric modulation of recombinant αβδ receptor currents and αβδ receptor-mediated tonic currents by anesthetics and ethanol. The possible mechanisms for the positive modulation of αβδ receptors by these compounds will also be discussed.

Effects of anesthetic propofol on release of amino acids from the spinal cord during visceral pain

As one of general anesthetics, propofol, has been used for surgical procedures of visceral organs. However, the mechanisms underlying the action of propofol on visceral nociception remain controversial. The aim of this study is to test whether the antinociception of systemic administration of propofol against visceral stimuli is related to the changes in release of excitatory and inhibitory amino acids in the spinal cord. The spinal microdialysis catheters were implanted subarachnoidally via the atlanto-occipital membrane in healthy SD rats. The rats received an intraperitoneal injection of acetic acid for visceral pain induction 10min after intraperitoneal pretreatment with vehicle or propofol (100mg/kg). The acetic acid-induced writhing assay was used to determine the degree of antinociception. Cerebrospinal fluid dialysate was collected by microdialysis from the spinal subarachnoid space before pretreatment and after visceral pain induction. Visceral pain-induced release of amino acids into the dialysate, including glutamate, aspartate, and γ-amino butyric acid was evaluated by measuring the changes in the concentrations of these amino acids. Acetic acid increased release of aspartate and glutamate, and decreased release of γ-amino butyric acid in the cerebrospinal fluid as measured by microdialysis. Pretreatment with propofol significantly decreased writhing responses induced by visceral pain, suppressed the visceral pain-induced aspartate and glutamate release, and reversed the decreased release of γ-amino butyric acid in the cerebrospinal fluid. These data provide evidence for a potential mechanism for the antinociceptive effects of propofol on visceral nociception.

Propofol allows precise quantitative arterial spin labelling functional magnetic resonance imaging in the rat

Functional magnetic resonance imaging (fMRI) techniques highlight cerebral vascular responses which are coupled to changes in neural activation. However, two major difficulties arise when employing these techniques in animal studies. First is the disturbance of cerebral blood flow due to anaesthesia and second is the difficulty of precise reproducible quantitative measurements. These difficulties were surmounted in the current study by using propofol and quantitative arterial spin labelling (QASL) to measure relative cerebral blood volume of labelled water (rCBV(lw),) mean transit time (MTT) and capillary transit time (CTT). The ASL method was applied to measure the haemodynamic response in the primary somatosensory cortex following forepaw stimulation in the rat. Following stimulation an increase in signal intensity and rCBV(lw) was recorded, this was accompanied by a significant decrease in MTT (1.97+/-0.06s to 1.44+/-0.04s) and CTT (1.76+/-0.06s to 1.39+/-0.07s). Two animals were scanned repeatedly on two different experimental days. Stimulation in the first animal was applied to the same forepaw during the initial and repeat scan. In the second animal stimulation was applied to different forepaws on the first and second days. The control and activated ASL signal intensities, rCBVlw on both days were almost identical in both animals. The basal MTT and CTT during the second scan were also very similar to the values obtained during the first scan. The MTT recorded from the animal that underwent stimulation to the same paw during both scanning sessions was very similar on the first and second days. In conclusion, propofol induces little physiological disturbance and holds potential for longitudinal QASL fMRI studies.

Comparative molecular field analysis and synthetic validation of a hydroxyamide-propofol binding and functional block of neuronal voltage-dependent sodium channels

Voltage gated sodium channels represent an important therapeutic target for a number of neurological disorders including epilepsy. Unfortunately, medicinal chemistry strategies for discovering new classes of antagonist for trans-membrane ion channels have been limited to mostly broad screening compound arrays. We have developed new sodium channel antagonist based on a propofol scaffold using the ligand based strategy of comparative molecular field analysis (CoMFA). The resulting CoMFA model was correlated and validated to provide insights into the design of new antagonists and to prioritize synthesis of these new structural analogs (compounds 4 and 5) that satisfied the steric and electrostatic model. Compounds 4 and 5 were evaluated for [(3)H]-batrachotoxinin-A-20-alpha-benzoate ([(3)H]-BTX-B) displacement yielding IC(50)'s of 22 and 5.7 microM, respectively. We further examined the potency of these two compounds to inhibit neuronal sodium currents recorded from cultured hippocampal neurons. At a concentration of 50 microM, compounds 4 and 5 tonically inhibited sodium channels currents by 59+/-7.8% (n=5) and 70+/-7.5% (n=7), respectively. This clearly demonstrates that these compounds functionally antagonize native neuronal sodium channel currents. In summary, we have shown that CoMFA can be effectively used to discover new classes of sodium channel antagonists.

The correlation between the effects of propofol on the auditory brainstem response and the postsynaptic currents of the auditory circuit in brainstem slices in the rat

BACKGROUND

Although there have been reports showing the changes of the auditory brainstem response (ABR) waves by propofol, no detailed studies have been done at the level of brainstem auditory circuit. So, we studied the effects of propofol on the postsynaptic currents of the medial nucleus of the trapezoid body (MNTB)-lateral superior olive (LSO) synapses by using the whole cell voltage clamp technique and we compared this data with that obtained by the ABR.

METHODS

5 rats at postnatal (P) 15 days were used for the study of the ABR. After inducing deep anesthesia using xylazine 6 mg/kg and ketamine 25 mg/kg, the ABRs were recorded before and after intraperitoneal propofol injection (10 mg/kg) and the effects of propofol on the latencies of the I, III, and V waves and the I-III and III-V interwave intervals were evaluated. Rats that were aged under P11 were used in the voltage clamp experiments. After making brainstem slices, the postsynaptic currents (PSCs) elicited by MNTB stimulation were recorded at the LSO, and the changes of the PSCs by the bath application of propofol (100 microM) were monitored.

RESULTS

We found small, but statistically significant increases in the latencies of ABR waves III and V and the interwave intervals of I-III and III-V by propofol. However, no significant changes were observed in the glycinergic or glutamatergic PSCs of the MNTB-LSO synpases by the application of propofol (100 microM).

CONCLUSIONS

Glycinergic or glutamatergic transmission of the MNTB-LSO synapses might not contribute to the propofol-induced changes of the ABR.

Glycine receptors contribute to hypnosis induced by ethanol

BACKGROUND

Glycine is a major inhibitory neurotransmitter in the adult central nervous system (CNS), and its receptors (GlyRs) are well known for their effects in the spinal cord and the lower brainstem. Accumulating evidence indicates that GlyRs are more widely distributed in the CNS, including many supraspinal regions. Previous in vitro studies have demonstrated that ethanol potentiates the function of these brain GlyRs, yet the behavioral role of the brain GlyRs has not been well explored.

METHODS

Experiments were conducted in rats. The loss of righting reflex (LORR) was used as a marker of the hypnotic state. We compared the LORR induced by systematic administration of ethanol and of ketamine in the absence and presence of the selective glycine receptor antagonist strychnine. Ketamine is a general anesthetic that does not affect GlyRs.

RESULTS

Systemically administered (by intraperitoneal injection) ethanol and ketamine dose-dependently induced LORR in rats. Furthermore, systemically administered (by subcutaneous injection) strychnine dose-dependently reduced the percentage of rats exhibiting LORR induced by ethanol, increased the onset time, and decreased the duration of LORR. Strychnine had no effect, however, on the LORR induced by ketamine.

CONCLUSIONS

Given that hypnosis is caused by neuronal depression in upper brain areas, we therefore conclude that brain GlyRs contribute at least in part to the hypnosis induced by ethanol.

Effects of general anesthetics on visceral pain transmission in the spinal cord

Current evidence suggests an analgesic role for the spinal cord action of general anesthetics; however, the cellular population and intracellular mechanisms underlying anti-visceral pain by general anesthetics still remain unclear. It is known that visceral nociceptive signals are transmited via post-synaptic dorsal column (PSDC) and spinothalamic tract (STT) neuronal pathways and that the PSDC pathway plays a major role in visceral nociception. Animal studies report that persistent changes including nociception-associated molecular expression (e.g. neurokinin-1 (NK-1) receptors) and activation of signal transduction cascades (such as the protein kinase A [PKA]-c-AMP-responsive element binding [CREB] cascade)-in spinal PSDC neurons are observed following visceral pain stimulation. The clinical practice of interruption of the spinal PSDC pathway in patients with cancer pain further supports a role of this group of neurons in the development and maintenance of visceral pain. We propose the hypothesis that general anesthetics might affect critical molecular targets such as NK-1 and glutamate receptors, as well as intracellular signaling by CaM kinase II, protein kinase C (PKC), PKA, and MAP kinase cascades in PSDC neurons, which contribute to the neurotransmission of visceral pain signaling. This would help elucidate the mechanism of antivisceral nociception by general anesthetics at the cellular and molecular levels and aid in development of novel therapeutic strategies to improve clinical management of visceral pain.

Modulation of Remifentanil-Induced Postinfusion Hyperalgesia by Propofol

BACKGROUND

Experimental and clinical studies suggest that brief opioid exposure can enhance pain sensitivity. During anesthesia, however, opioids are commonly administered in combination with either IV or inhaled hypnotic drugs. In this investigation we sought to determine the analgesic and antihyperalgesic properties of propofol in subhypnotic concentrations on remifentanil-induced postinfusion hypersensitivity in an experimental human pain model.

METHODS

Fifteen healthy volunteers were included in this randomized, double-blind, and placebo-controlled study in a cross-over design. Transcutaneous electrical stimulation at high current densities (41.7 +/- 14.3 mA) induced spontaneous acute pain (numerical rating scale = 6 of 10) and stable areas of hyperalgesia. Pain intensities and areas of hyperalgesia were assessed before, during and after a 30 min target-controlled infusion of propofol (1.5 microg/mL) and remifentanil (0.05 microg x kg(-1) x min(-1)), either alone or in combination (propofol 1.5 microg/mL with remifentanil 0.025 or 0.05 microg x kg(-1) x min(-1)).

RESULTS

During infusion, propofol significantly reduced the electrically evoked pain to 72% +/- 21% of control. Subhypnotic concentrations of propofol did not lead to any hyperalgesic effects. Coadministration of remifentanil led to synergistic analgesic effects (to 62% +/- 26% and 58% +/- 25% of control, for 0.025 or 0.05 microg x kg(-1) x min(-1), respectively), but upon withdrawal, pain and hyperalgesia increased above control level.

CONCLUSIONS

The results suggest clinically relevant interactions of propofol and remifentanil in humans, since propofol led to a delay and a weakening of remifentanil-induced postinfusion anti-analgesia in humans. Nevertheless, pronociceptive effects were not completely antagonized by propofol, which may account for the increased demand for analgesics after remifentanil-based anesthesia in clinical practice.

Changes of K+ -Cl- cotransporter 2 (KCC2) and circuit activity in propofol-induced impairment of long-term potentiation in rat hippocampal slices

Enhancing inhibition via gamma-aminobutyric acid type A (GABA(A)) receptors contributes to anesthetic-induced impairment of long-term potentiation (LTP) of excitatory synaptic transmission, which may account for general anesthesia-associated memory impairment (amnesia). The neuron-specific K+ -Cl- cotransporter 2 (KCC2) is necessary for fast synaptic inhibition via maintaining the low intracellular chloride concentration required for the hyperpolarizing actions of GABA via GABA(A) receptors. To explore a possible role of KCC2-dependent inhibition in anesthetic-induced impairment of LTP, we used field excitatory postsynaptic potentials (fEPSP) recording and immunoblotting to study the effect of propofol on LTP maintenance and KCC2 expression in CA1 region of rat hippocampal slices. We found that propofol (30 microM) not only impaired LTP expression but also prevented LTP-accompanied downregulation of KCC2 without affecting the basal transmission of glutamatergic synapses. Moreover, the recurrent inhibition in hippocampal slices was enhanced by propofol. These propofol-induced effects were completely abolished by picrotoxin, a specific GABA(A) receptor-chloride channel blocker. Thus, enhancement of GABAergic inhibition and suppression of neuronal excitability may account for the sustained expression of KCC2 and the impairment of LTP by propofol. Together, this study supports a novel role for KCC2 in LTP expression and gives hints to a molecular mechanism, by which anesthetics might cause impairment of LTP.

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.

Structural features of phenol derivatives determining potency for activation of chloride currents via alpha(1) homomeric and alpha(1)beta heteromeric glycine receptors

Phenol derivatives constitute a family of neuroactive compounds. The aim of our study was to identify structural features that determine their modulatory effects at glycine receptors. We investigated the effects of four methylated phenol derivatives and two halogenated analogues on chloride inward currents via rat alpha(1) and alpha(1)beta glycine receptors, heterologously expressed in HEK 293. All compounds potentiated the effect of a submaximal glycine concentration in both alpha(1) homomeric and alpha(1)beta glycine receptors. While the degree of maximum potentiation of the glycine 10 microM effect in alpha(1)beta receptors was not different between the compounds, the halogenated compounds achieved half-maximum potentiating effects in the low microM range -- at more than 20-fold lower concentrations compared with their nonhalogenated analogues (P<0.0001). The coactivating effect was over-ridden by inhibitory effects at concentrations >300 microM in the halogenated compounds. Neither the number nor the position of the methyl groups significantly affected the EC(50) for coactivation. Only the bimethylated compounds 2,6 and 3,5 dimethylphenol (at concentrations >1000 microM) directly activated both alpha(1) and alpha(1)beta receptors up to 30% of the maximum response evoked by 1000 microM glycine. These results show that halogenation in the para position is a crucial structural feature for the potency of a phenolic compound to positively modulate glycine receptor function, while direct activation is only seen with high concentrations of compounds that carry at least two methyl groups. The presence of the beta subunit is not required for both effects.

Intrathecal propofol has analgesic effects on inflammation-induced pain in rats

PURPOSE

Propofol is thought to act on gamma-aminobutyric acid receptors, which have some role in pain transmission in the spinal cord. In this study, we examined the effects of intrathecal propofol on acute thermally- or inflammation-induced pain in rats.

METHODS

Lumbar intrathecal catheters were implanted in Male Sprague-Dawley rats. The tail withdrawal response to thermal stimulation (tail flick test) or paw flinching and shaking response by sc formalin injection into the hind paw (formalin test) were tested. Propofol 1000, 300 or 100 microg or saline (control) was administered as 10 microL intrathecally. Motor disturbance and behavioural side effects were also monitored in the rats during the tail flick test. Eight rats were used for each dose in each test.

RESULTS

No analgesic effects were observed in the tail flick test. In the formalin test, 50% of effective doses were 449 mug (95% confidence interval, 80-3180 microg) in phase 1 and 275 microg (146-519 microg) in phase 2. Motor disturbance was observed in one rat with 100 microg and agitation and allodynia were seen in one rat with 300 microg. However, both were reversible in 120 min.

CONCLUSIONS

Intrathecal administration of propofol had analgesic effects on inflammation-induced acute and facilitated pain but not on thermally-induced acute pain. Transient motor and sensory disturbance could not rule out the possibility of neurotoxicity.

Multiple actions of propofol on alphabetagamma and alphabetadelta GABAA receptors

GABAA receptors are predominantly composed of alphabetagamma and alphabetadelta isoforms in the brain. It has been proposed that alphabetagamma receptors mediate phasic inhibition, whereas alphabetadelta receptors mediate tonic inhibition. Propofol (2,6-di-isopropylphenol), a widely used anesthetic drug, exerts its effect primarily by modulating GABAA receptors; however, the effects of propofol on the kinetic properties of alphabetagamma and alphabetadelta receptors are uncertain. We transfected human embryonic kidney (HEK293T) cells with cDNAs encoding rat alpha1, alpha6, beta3, gamma2L, or delta subunits and performed whole-cell patch-clamp recordings to explore this issue. Propofol (3 microM) increased GABA concentration-response curve maximal currents similarly for both alpha1beta3gamma2L and alpha6beta3gamma2L receptors, but propofol increased those for alpha1beta3delta and alpha6beta3delta receptors differently, the increase being greater for alpha1beta3delta than for alpha6beta3delta receptors. Propofol (10 microM) produced similar alterations in alpha1beta3gamma2L and alpha6beta3gamma2L receptor currents when using a preapplication protocol; peak currents were not altered, desensitization was reduced, and deactivation was prolonged. Propofol enhanced peak currents for both alpha1beta3delta and alpha6beta3delta receptors, but the enhancement was greater for alpha1beta3delta receptors. Desensitization of these two isoforms was not modified by propofol. Propofol did not alter the deactivation rate of alpha1beta3delta receptor currents but did slow deactivation of alpha6beta3delta receptor currents. The findings that propofol reduced desensitization and prolonged deactivation of gamma2L subunit-containing receptors and enhanced peak currents or prolonged deactivation of delta subunit-containing receptors suggest that propofol enhancement of both phasic and tonic inhibition may contribute to its anesthetic effect in the brain.

The Effects of Propofol on Hypothalamic Paraventricular Nucleus Neurons in the Rat

The mechanism of hypotension induced by anesthetics is not completely understood. Because no electrophysiologic examination of the effects of propofol on the central nervous system has shown its involvement in the control of sympathetic and cardiovascular functions, we investigated the actions of propofol on rat hypothalamic paraventricular nucleus (PVN) neurons using the whole-cell mode of the patch-clamp technique in rat hypothalamic PVN slice preparations. Propofol induced Cl(-) currents at concentrations of 10(-5) and 10(-4) M, which were sensitive to picrotoxin and, to a lesser extent, to strychnine. Propofol (10(-6) M) enhanced gamma-aminobutyric acid(A) (GABA(A); 10(-6) M)-induced current synergistically. Moreover, propofol (10(-5) and 10(-4) M) significantly increased the decay time of evoked-inhibitory postsynaptic currents, which suggests a postsynaptic modulation of GABA(A) receptors. In addition, propofol (10(-5), 10(-4), and 2 x 10(-4) M) reversibly inhibited voltage-gated Ca(2+) currents. Taken together, these results suggest that propofol enhancement of GABA(A)-receptor mediated currents and inhibition of voltage-gated Ca(2+) currents at the central level, which is involved in the control of cardiovascular and sympathetic functions may be, at least in part, involved in general anesthetic-induced cardiovascular and sympathetic depression.

IMPLICATIONS

We investigated the actions of propofol on the rat hypothalamic paraventricular nucleus neurons, which are involved in the control of cardiovascular and sympathetic functions. The results suggest that propofol enhancement of gamma-aminobutyric acid(A)-receptor mediated currents and inhibition of voltage-gated Ca(2+) currents at the central level may be, at least in part, involved in general anesthetic-induced cardiovascular and sympathetic depression.