Excitatory actions of norepinephrine on multiple classes of hippocampal CA1 interneurons.

Norepinephrine (NE) causes an increase in the frequency of inhibitory postsynaptic potentials in CA1 pyramidal neurons in vitro. The possibility that this increase in tonic inhibition is caused by an excitatory effect on inhibitory interneurons was investigated through whole-cell recordings from pyramidal cells and both whole-cell and cell-attached patch recordings from visualized interneurons in acute slices of rat hippocampus. Adrenergic agonists caused a large increase in the frequency and amplitude of spontaneous IPSCs recorded from pyramidal cells in the presence of ionotropic glutamate receptor blockers, but they had no effect on either the frequency or the amplitude of action potential-independent miniature IPSCs recorded in tetrodotoxin. This effect was mediated primarily by an alpha adrenoceptor, although a slight beta adrenoceptor-dependent increase in IPSCs was also observed. NE caused interneurons located in all strata to depolarize and begin firing action potentials. Many of these cells had axons that ramified throughout the stratum pyramidale, suggesting that they are responsible for the IPSCs observed in pyramidal neurons. This depolarization was also mediated by an alpha adrenoceptor and was blocked by a selective alpha 1- but not a selective alpha 2-adrenoceptor antagonist. However, a slight beta adrenoceptor-dependent depolarization was detected in those interneurons that displayed time-dependent inward rectification. In the presence of a beta antagonist, NE induced an inward current that reversed near the predicted K+ equilibrium potential and was not affected by changes in intracellular Cl- concentration. In the presence of an alpha 1 antagonist, NE induced an inwardly rectifying current at potentials negative to approximately -70 mV that did not reverse (between -130 and -60 mV), characteristics similar to the hyperpolarization-activated current (lh). However, the depolarizing action of NE is attributable primarily to the alpha 1 adrenoceptor-mediated decrease in K+ conductance and not the beta adrenoceptor-dependent increase in lh. These results provide evidence that NE increases action potential-dependent IPSCs in pyramidal neurons by depolarizing surrounding inhibitory interneurons. This potent excitatory action of NE on multiple classes of hippocampal interneurons may contribute to the NE-induced decrease in the spontaneous activity of pyramidal neurons and the antiepileptic effects of NE observed in vivo.

Calcium channel involvement in GABAB receptor-mediated inhibition of GABA release in area CA1 of the rat hippocampus.

1. Experiments were performed in rat hippocampal slices to examine the nature of GABAergic inhibition of inhibitory synaptic transmission. In these experiments the effects of the gamma-aminobutyric acid-B (GABAB) receptor agonist, baclofen, and of subtype-selective calcium channel blockers were tested with the use of intracellular recordings of evoked inhibitory postsynaptic potentials (IPSPs) and whole cell recordings of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs). 2. Baclofen inhibited evoked and spontaneous (action-potential-dependent) monosynaptic GABAA-mediated IPSPs and IPSCs but had no effect on the frequency of tetrodotoxin-resistant (action-potential-independent) miniature IPSCs recorded in CA1 pyramidal neurons. 3. Depolarizing GABAergic synaptic terminals by raising the extracellular potassium concentration caused an increase in action-potential-independent miniature IPSC frequency that could be inhibited by either baclofen or cadmium, a blocker of voltage-dependent calcium channels. In addition, under these depolarizing conditions, cadmium occluded the baclofen inhibition of miniature IPSCs. These data suggest that baclofen reduces only depolarization-induced, not quantal, GABA release and that it does so by decreasing presynaptic voltage-dependent calcium influx. 4. Experiments with subtype-selective calcium channel blockers demonstrate that the presynaptic action of baclofen was mediated through both omega-conotoxin-GVIA-sensitive and omega-agatoxin-IVA-sensitive, but not dihydropyridine-sensitive calcium channels.

Opioid inhibition of GABA release from presynaptic terminals of rat hippocampal interneurons.

Opiates and the opioid peptide enkephalin can cause indirect excitation of principal cortical neurons by reducing inhibitory synaptic transmission mediated by GABAergic interneurons. The mechanism by which opioids mediate these effects on interneurons is unknown, but enkephalin hyperpolarizes the somatic membrane potential of a variety of neurons in the brain, including hippocampal interneurons. We now report a new, more direct mechanism for the opioid-mediated reduction in synaptic inhibition. The enkephalin analog D-Ala2-Met5-enkephalinamide (DALA) decreases the frequency of miniature, action potential-independent, spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) without causing a change in their amplitude. Thus, we conclude that DALA inhibits the action potential-independent release of GABA through a direct action on interneuronal synaptic terminals. In contrast, DALA reduces the amplitude of action potential-evoked, GABA-mediated IPSCs, as well as decreases their frequency. This suggests that the opioid-mediated inhibition of non-action potential-dependent GABA release reveals a mechanism that contributes to reducing action potential-evoked GABA release, thereby decreasing synaptic inhibition.

Isoproterenol increases the phosphorylation of the synapsins and increases synaptic transmission in dentate gyrus, but not in area CA1, of the hippocampus.

Previous studies have shown that either norepinephrine (NE) or isoproterenol (ISO) enhances the slope of the field excitatory postsynaptic potential (EPSP) in the dentate gyrus of the rat hippocampal formation. In contrast, NE and ISO cause no increase in excitatory transmission in area CA1 of the hippocampus. The molecular mechanism underlying this brain region-specific increase in synaptic transmission is not known. The phosphorylation of synapsin I and synapsin II, two homologous presynaptic vesicle-associated proteins, is thought to promote neurotransmitter release. The authors have observed previously NE- and ISO-enhanced phosphorylation of synapsins I and II in the dentate gyrus. The purpose of this study was to determine whether ISO-stimulated phosphorylation also occurs in the CA1, where ISO has no effect on excitatory neurotransmission. These studies were correlated with electrophysiological studies in in vitro hippocampal slices. Superfusion of slices with ISO resulted in an increase in EPSP slope in the dentate but not in area CA1. The enhanced dentate EPSP returned to baseline levels within 30 minutes of washout of the drug. Isoproterenol produced corresponding increases in the phosphorylation of the synapsins in dentate slices but had no effect on these proteins in CA1 slices. Moreover, in dentate slices exposed to a 30-minute wash following incubation with ISO, phosphorylation of the synapsins returned to control levels. This close temporal and brain regional correlation between ISO stimulation of both synapsin phosphorylation and synaptic transmission suggests that the synapsin proteins may play a role in the synaptic potentiation produced by ISO in the dentate.

Synaptic localization of adrenergic disinhibition in the rat hippocampus.

Norepinephrine is an endogenous neurotransmitter that reduces synaptic inhibition onto pyramidal neurons in the hippocampus by an action at an alpha-adrenergic receptor. The physiological mechanism of this disinhibition was previously not known, except that it occurred at a site presynaptic to the inhibited pyramidal cell. In this paper we present evidence that adrenergic disinhibition is restricted to the early phase of the evoked inhibitory postsynaptic potential in area CA1 of the hippocampus. The locus of disinhibition does not appear to reside in the interneuronal terminal, axon, or cell body. Instead, adrenergic agonists appear to reduce evoked synaptic inhibition by depressing excitatory synapses that activate the interneuron.

The alpha 2-adrenoceptor agonist dexmedetomidine increases the apparent potency of the volatile anesthetic isoflurane in rats in vivo and in hippocampal slice in vitro.

Alpha 2-adrenoceptor agonists such as clonidine are sedatives and enhance the effectiveness of several different kinds of anesthetics. This study was performed to quantitate the effect of dexmedetomidine, a novel alpha 2-adrenoceptor agonist, on the action of the volatile anesthetic agent isoflurane in rats in vivo. A separate set of experiments in rat hippocampal slices was designed to determine whether isoflurane and dexmedetomidine exerted similar effects on synaptic transmission in vitro and to examine the interaction between the two agents. In vivo, dexmedetomidine (100 micrograms/kg i.p.) reduced isoflurane minimum alveolar anesthetic requirement (MAC), determined by loss of response to tail pinch, by approximately 90%. In hippocampal CA1 neurons, on the other hand, there was a relatively small potentiation of the effects of isoflurane at the maximally effective dexmedetomidine concentration (1 nM). The hippocampal CA1 area, at least in the slice preparation, may thus not be representative of the CNS site(s) at which alpha 2 adrenoceptor agonists lessen anesthetic requirement in vivo.

Pertussis toxin and 4-aminopyridine differentially affect the hypnotic-anesthetic action of dexmedetomidine and pentobarbital.

Dexmedetomidine, a highly selective and potent agonist at alpha-2 adrenoceptors, produces a hypnotic-anesthetic action in rats. The mechanism for this response may involve an inhibitory G-protein and increased conductance through a potassium channel. To investigate this, the effects of pertussis toxin, a specific inactivator of inhibitory G-proteins, and 4-aminopyridine, a blocker of potassium channels, on the hypnotic-anesthetic response to dexmedetomidine were studied in rats. Pertussis toxin and 4-aminopyridine both decreased the hypnotic-anesthetic action of dexmedetomidine in a dose-dependent fashion. To preclude the possibility that pertussis toxin and 4-aminopyridine attenuated the hypnotic-anesthetic action of dexmedetomidine via indirect central nervous system excitation, the effects of pertussis toxin and 4-aminopyridine on the hypnotic-anesthetic action of pentobarbital also were assessed. Pentobarbital-induced hypnosis was not attenuated by either treatment. These results suggest that the receptor-effector mechanism for the hypnotic-anesthetic action of dexmedetomidine involves an inhibitory G-protein and increased conductance through a potassium channel.

Outpatient premedication: use of midazolam and opioid analgesics.

The perioperative effects of administering sedative and analgesic drugs prior to outpatient surgery were evaluated. One hundred fifty adult outpatients were randomly assigned to one of six study groups according to a double-blind protocol design. Patients were given placebo (saline) or midazolam (5 mg im) 30-60 min prior to surgery, and then either placebo, oxymorphone (1 mg iv), or fentanyl (100 micrograms iv) 3-5 min prior to a standardized anesthetic technique. Preoperatively, midazolam premedication was associated with a significantly lower anxiety level (37 +/- 29 mm vs. 50 +/- 32 mm, P less than 0.05), higher sedation level (254 +/- 136 mm vs. 145 +/- 109 mm, P less than 0.01), worsening of psychomotor skill (5 +/- 5 vs. 2 +/- 2 dots missed, P less than 0.01; midazolam vs. placebo), and impaired recall abilities. In addition, use of midazolam did not prolong the discharge time. Compared to control patients, those who received fentanyl had a decreased incidence of intraoperative airway difficulties such as coughing (28% vs. 0%, P less than 0.01). Although use of opioids increased the incidence of postoperative nausea (42% vs. 18%, P less than 0.01) and vomiting (23% vs. 2%, P less than 0.01; opioid vs. no opioid), average recovery times were not affected by opioid administration. Oxymorphone use was associated with a lower incidence of pain at home compared with that following fentanyl (46% vs. 74%, P less than 0.05). Finally, preoperative administration of both midazolam and fentanyl or oxymorphone prior to a standardized methohexital-nitrous oxide anesthetic technique did not adversely affect recovery after outpatient surgery.

Dexmedetomidine produces a hypnotic-anesthetic action in rats via activation of central alpha-2 adrenoceptors.

Dexmedetomidine, a highly selective and potent alpha-2 adrenoceptor agonist, reduces halothane anesthetic requirements by over 90% in rats. The present study examined whether dexmedetomidine produces a hypnotic-anesthetic action in rats. Dexmedetomidine induced a hypnotic-anesthetic state in rats characterized by loss of righting reflex at doses greater than or equal to 0.1 mg/kg. This response was dose-dependent between 0.1 and 3 mg/kg. Alpha-2 adrenoceptor antagonists that cross the blood-brain barrier (antipamezole and idazoxan) decreased the hypnotic-anesthetic action of dexmedetomidine in a dose-dependent fashion. In contrast, the alpha-2 antagonist, L-659,066, which does not penetrate into the CNS did not affect dexmedetomidine-induced hypnosis. Antagonists for the other adrenoceptors not only failed to reduce the hypnotic-anesthetic action of dexmedetomidine but in some cases even potentiated this effect. Thus, prazosin, an alpha-1 adrenoceptor antagonist, significantly enhanced the hypnotic-anesthetic property of dexmedetomidine. Antagonists with beta-2 receptor blocking properties also enhanced dexmedetomidine-induced hypnosis. Selective beta-1 receptor antagonists did not affect the hypnotic action of dexmedetomidine. These results suggest that dexmedetomidine produces a hypnotic-anesthetic action in rats via activation of central alpha-2 adrenoceptors.

Benzodiazepine antagonism does not provoke a stress response.

Acute anxiety reactions have been reported following antagonism of benzodiazepine-induced sedation. In this study, the level of sedation and anxiety was assessed in 30 patients randomly assigned to receive either saline or flumazenil (a benzodiazepine antagonist) after midazolam sedation according to a double-blind protocol. Carefully titrated doses of flumazenil, 0.8 +/- 0.2 mg (mean +/- SD), effectively reversed residual midazolam-induced sedation without producing significant changes in the patients' level of anxiety. In addition, plasma epinephrine, norepinephrine, vasopressin, and beta-endorphin concentrations were measured in a subset of patients (n = 5) from each group. The levels of these stress hormones did not acutely change following flumazenil (or saline). These results indicate that flumazenil, 0.6-1.0 mg iv, can antagonize midazolam sedation without producing acute anxiety or evidence of a stress response.

Dexmedetomidine diminishes halothane anesthetic requirements in rats through a postsynaptic alpha 2 adrenergic receptor.

The effect of 4(5)-[1-(2,3-dimethylphenyl)ethyl]imidazole (medetomidine), the alpha 2 adrenergic agonist, on anesthetic requirements was investigated in rats anesthetized with halothane. Halothane MAC was determined before and after either dexmedetomidine (d-enantiomer) or levomedetomidine (l-enantiomer) 10, 30, and 100 micrograms/kg or vehicle ip. There was a dose-dependent decrease in MAC with the d-, but not the l-, stereoisomer. At the highest dose of dexmedetomidine (100 micrograms/kg), halothane could be discontinued for up to 30 min with no response to tail clamping. To determine whether alpha 2 adrenoreceptors mediated this effect of dexmedetomidine on MAC, cohorts of rats were pretreated with idazoxan, 10 mg/kg ip, a highly selective alpha 2 antagonist. This completely prevented the reduction of MAC caused by dexmedetomidine. To determine whether the reduction of MAC caused by dexmedetomidine was mediated in part through either opiate or adenosine receptors, groups of rats were pretreated with either naltrexone, 5 mg/kg ip, an opiate antagonist, or 8-phenyltheophylline, 2.5 mg/kg ip, an A1 adenosine antagonist. These two pretreatments did not alter the reduction of MAC by dexmedetomidine. To determine whether postsynaptic mechanisms mediate the anesthetic effect of dexmedetomidine, rats were depleted of central catecholamine stores with either n-(2-chloroethyl)-n-ethyl-2-bromobenzylamine (DSP-4) or reserpine and alpha-methyl-para-tyrosine and MAC was determined before and after each dose of dexmedetomidine. While the catecholamine-depleted rats had a lower basal MAC than the vehicle controls, there was still a profound reduction in halothane MAC after administration of dexmedetomidine. The reduction of MAC by dexmedetomidine was blocked with idazoxan in the catecholamine depleted rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and pharmacodynamics of propofol infusions during general anesthesia.

The pharmacokinetic and pharmacodynamic properties of propofol were studied in 50 surgical patients. Propofol was administered as a bolus dose, 2 mg/kg iv, followed by a variable-rate infusion, 0-20 mg/min, and intermittent supplemental boluses, 10-20 mg iv, as part of a general anesthetic technique that included nitrous oxide, meperidine, and muscle relaxants. For a majority of the patients (n = 30), the pharmacokinetics of propofol were best described by a two-compartment model. The propofol mean total body clearance rate was 2.09 +/- 0.65 1/min (mean +/- SD), the volume of distribution at steady state was 159 +/- 57 l, and the elimination half-life was 116 +/- 34 min. Elderly patients (patients older than 60 yr vs. those younger than 60 yr) had significantly decreased clearance rates (1.58 +/- 0.42 vs. 2.19 +/- 0.64 l/min), whereas women (vs. men) had greater clearance rates (33 +/- 8 vs. 26 +/- 7 l.kg-1.min-1) and volumes of distribution (2.50 +/- 0.81 vs. 2.05 +/- 0.65 l/kg). Patients undergoing major (intraabdominal) surgery had longer elimination half-life values (136 +/- 40 vs. 108 +/- 29 min). Patients required an average blood propofol concentration of 4.05 +/- 1.01 micrograms/ml for major surgery and 2.97 +/- 1.07 micrograms/ml for nonmajor surgery. Blood propofol concentrations at which 50% of patients (EC50) were awake and oriented after surgery were 1.07 and 0.95 microgram/ml, respectively. Psychomotor performance returned to baseline at blood propofol concentrations of 0.38-0.43 microgram/ml (EC50). This clinical study demonstrates the feasibility of performing pharmacokinetic and pharmacodynamic analyses when complex infusion and bolus regimens are used for administering iv anesthetics.

Propofol-nitrous oxide versus thiopental-isoflurane-nitrous oxide for general anesthesia.

One hundred and twenty patients undergoing elective operations were randomly assigned to receive anesthesia with either thiopental, 4 mg/kg-isoflurane, 0.2-3%-nitrous oxide, 60-70% (control) or propofol, 2 mg/kg-propofol infusion, 1-20 mg/min-nitrous oxide, 60-70% (propofol). Although anesthetic conditions were similar during the operation, differences were noted in the recovery characteristics. For non-major (superficial) surgical procedures, the times to awakening, responsiveness, orientation, and ambulation were significantly shorter in the propofol group (4 +/- 3, 5 +/- 4, 6 +/- 4, and 104 +/- 36 min) than in the control group (8 +/- 7, 9 +/- 7, 11 +/- 9, and 142 +/- 61 min, respectively). In addition, less nausea and vomiting (20 vs. 45%) and significantly less psychomotor impairment was noted in the non-major propofol (vs. control) group. Following major abdominal operations, recovery characteristics did not differ between propofol and control groups. Delayed emergence (greater than 20 min), significant psychometric impairment, and a high overall incidence of postoperative side effects (55-60%) were noted in both drug treatment groups. The authors conclude that propofol-nitrous oxide compares favorably to thiopental-isoflurane-nitrous oxide for maintenance of anesthesia during short outpatient procedures. However, for major abdominal operations, propofol anesthesia does not appear to offer any clinically significant advantages over a standard inhalational anesthetic technique.

Comparison of propofol with methohexital for outpatient anesthesia.

Propofol is an intravenous anesthetic currently available for clinical investigative use. The intraoperative and postoperative effects of propofol were compared to methohexital when used as an adjuvant to nitrous oxide for outpatient anesthesia. Sixty healthy young women were randomly assigned to receive either methohexital, 1.5 mg/kg intravenously (IV), or propofol, 2.5 mg/kg IV, for induction of anesthesia. Both drugs produced transient cardiovascular and respiratory depression after induction. Maintenance of anesthesia consisted of either methohexital, 6 +/- 2 mg/min, or propofol, 7 +/- 2 mg/min (mean +/- SD) by continuous infusion in combination with nitrous oxide, 70% in oxygen. Use of a propofol infusion was associated with lower blood pressures and heart rates during maintenance. Propofol was associated with fewer side effects (e.g., hiccoughing, nausea, and vomiting) intra- and postoperatively. Recovery times for awakening, orientation, and ambulation were consistently shorter with propofol. We conclude that propofol is a useful alternative to methohexital for induction and maintenance of outpatient anesthesia.