Pentobarbital: presynaptic effect in the squid giant synapse

A Comparative Study of Cell Specific Effects of Systemic and Volatile Anesthetics on Identified Motor Neurons and Interneurons of Lymnaea stagnalis (L.), Both in the Isolated Brain and in Single Cell Culture

1. A comparative descriptive analysis of systemic (sodium pentobarbital, sodium thiopentone, ketamine) and volatile (halothane, isoflurane, enflurane) general anesthetics revealed important differences in the neuronal responses of identified motor neurons and interneurons in the isolated central nervous system (CNS) and cultured identified neurons in single cell culture of Lymnaea stagnalis (L.). 2. At high enough concentrations all anesthetics eventually caused cessation of spontaneous or evoked action potentials, but volatile anesthetics were much faster acting. Halothane at low concentrations caused excitation, thought to be equivalent to the early excitatory phase of anesthesia. Strong synaptic inputs were not always abolished by pentobarbital. 3. There were cell specific concentration-dependent responses to halothane and pentobarbital in terms of membrane potential, action potential characteristics, the after hyperpolarization and patterned activity. Individual neurons generated specific responses to the applied anesthetics. 4. The inhalation anesthetics, enflurane, and isoflurane, showed little concentration dependence of effect, in contrast to results obtained with halothane. Enflurane was faster acting than halothane and isoflurane was particularly different, producing quiescence in all cells types studied at all concentrations studied. 5. Halothane, enflurane, the barbiturate general anesthetics, pentobarbital, and sodium thiopentone and the dissociative anesthetic ketamine, produced two distinctly different effects which could be correlated with cell type and their location in the isolated brain: either a decline in spontaneous and evoked activity prior to quiescence in interneurons or paroxysmal depolarizing shifts (PDS) in motor neurons, again prior to quiescence, which were reversed when the anesthetic was eliminated from the bath. In the strongly electrically coupled motor neurons, VD1 and RPD2, both types of response were observed, depending on the anesthetic used. Thus, with the exception isoflurane, all the motor neurons subjected to the anesthetic agents studied here were capable of generating PDS in situ, but the interneurons did not do so. 6. The effects of halothane on isolated cultured neurons indicates that PDS can be generated by single identified neurons in the absence of synaptic inputs. Further, many instances of PDS in neurons that do not generate it in situ have been found in cultured neurons. The nature of PDS is discussed.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Potentiating effects of L-type Ca2+ channel blockers on pentobarbital-induced hypnosis are influenced by serotonergic system

In order to elucidate the mechanism(s) behind the interactions between barbiturates and Ca(2+) antagonists, the effects of three structurally diverse types of Ca(2+) antagonists combined or not with 5-HT on pentobarbital-induced hypnosis in mice were investigated. The results showed that dihydropyridine derivative nifedipine (10.0 and 20.0 mg/kg, p.o.) and other types of Ca(2+) antagonist, verapamil (5.0 and 10.0 mg/kg, p.o.) and diltiazem (2.5, 5.0 and 10.0 mg/kg, p.o.) increased both the sleeping time in hypnotic dosage of pentobarbital (45 mg/kg, i.p.) treated mice and the rate of sleep onset in the sub-hypnotic dosage of pentobarbital (28 mg/kg, i.p.) treated mice in a dose-dependent manner, respectively, and these effects were significantly augmented by 5-hydroxytryptophan (5-HTP), the immediate precursor of 5-hydroxytryptamine (5-HT). Pretreatment with p-chlorophenylalanine (PCPA, 300 mg/kg, s.c.), an inhibitor of tryptophan hydroxylase, significantly decreased pentobarbital-induced sleeping time and nifedipine (10.0 mg/kg, p.o.), verapamil (5.0 mg/kg, p.o.) and diltiazem (2.5 mg/kg, p.o.) abolished this effect. From these results, it should be presumed that the augmentative effect of L-type Ca(2+) channel blockers on pentobarbital-induced sleep may be influenced by serotonergic system.

Increases in neuronal Ca2+ flux after withdrawal from chronic barbiturate treatment

Chronic barbital treatment significant increased the net K+-stimulated uptake of 45Ca2+ in cerebrocortical synaptosomal preparations, 24 h after withdrawal from chronic barbital administration. Basal uptake was not significantly changed. Hippocampal synaptosomal preparations showed a similar pattern, but the increase was not significant. The synaptosomal Ca2+ uptake was not affected by incubation with the dihydropyridine Ca2+ channel antagonist, nitrendipine, in controls or after chronic barbital treatment. Acute administration of a single dose of barbital did not alter the basal or stimulated uptake of 45Ca2+ in cortical synaptosomes, when this was measured 36 h after the barbital administration. Hippocampal slices prepared 24 h after withdrawal from chronic barbital treatment showed a significant increase in K+-stimulated uptake of 45Ca2+, and the basal uptake was significantly decreased. Both changes were prevented by nitrendipine. An increase in the density of dihydropyridine-sensitive binding sites was found in the cerebral cortex. The results indicate that both dihydropyridine-sensitive and insensitive neuronal Ca2+ channels are altered by chronic barbiturate treatment. These changes may be involved in physical dependence on barbiturates.

Voltage-activated calcium channels: targets of antiepileptic drug therapy?

Voltage-gated calcium currents play important roles in controlling neuronal excitability. They also contribute to the epileptogenic discharge, including seizure maintenance and propagation. In the past decade, selective calcium channel blockers have been synthesized, aiding in the analysis of calcium channel subtypes by patch-clamp recordings. It is still a matter of debate whether whether any of the currently available antiepileptic drugs (AEDs) inhibit these conductances as part of their mechanism of action. We tested oxcarbazepine, lamotrigine, and felbamate and found that they consistently inhibited voltage-activated calcium currents in cortical and striatal neurons at clinically relevant concentrations. Low micromolar concentrations of GP 47779 (the active metabolite of oxcarbazepine) and lamotrigine reduced calcium conductances involved in the regulation of transmitter release. In contrast, felbamate blocked nifedipine-sensitive conductances at concentrations significantly lower than those required to modify N-methyl-D-aspartate (NMDA) responses or sodium currents. Aside from contributing to AED efficacy, this mechanism of action may have profound implications for preventing fast-developing cellular damage related to ischemic and traumatic brain injuries. Moreover, the effects of AEDs on voltage-gated calcium signals may lead to new therapeutic strategies for the treatment of neurodegenerative disorders.

Action of general anaesthetics on unclamped Ca(2+)-mediated currents in unmyelinated axons of rat olfactory cortex

Na+ and Ca2+ currents were monitored using a suction electrode in unclamped presynaptic axons of rat olfactory cortex pretreated with 0.1 mM 3,4-diaminopyridine and 5 mM tetraethylammonium. The effects of anaesthetics on these currents were compared with tetrodotoxin or cadmium. Ketamine (0.1-1 mM), ether (20-200 mM), diisopropylphenol (0.01-0.5 mM) and lignocaine (0.01-0.2 mmol/l) all depressed both the initial Na+ component and the Ca(2+)-mediated tail of the response. Urethane (5-100 mM), halothane (1-5 mM) and pentobarbitone (0.1-2 mM) showed slight selectivity for the axonal Ca2+ tail. Diisopropylphenol apparently enhanced the Ca2+ tail at low concentrations. The alphaxalone (1-50 microM) depression was very weak. In a few cases the depression may contribute to anaesthesia but with others, high concentrations may contribute to the toxicity of the substances in vivo.

The role of neuronal calcium channels in dependence on ethanol and other sedatives/hypnotics

This review discusses the importance of neuronal calcium currents in dependence on ethanol, barbiturates, benzodiazepines and opiates. The main sections describe the actions of ethanol on control of intracellular calcium and on calcium and calcium-dependent conductance mechanisms. In particular, the effects of both acute and chronic ethanol treatment on dihydropyridine-sensitive, voltage-dependent, calcium channels are described. The later sections cover the effects of barbiturates, benzodiazepines and opiates on these systems. The conclusions suggest that dihydropyridine calcium channel antagonists may offer a new therapeutic approach to the treatment of ethanol and opiate dependence.

Different actions of a short-acting barbiturate on sodium and potassium conductances in invertebrate and vertebrate neurons

In this study, the effects of methohexital are compared on the voltage-gated sodium (Na+) and potassium ion (K+) conductances of Retzius cells in the leech Macrobdella and of dorsal root cells of the chick in culture. Under current-clamp conditions methohexital prolonged the Na+-dependent action potential of neurons in the leech. This prolongation occurred in the absence of changes in resting membrane potential or the maximum rate of depolarization of the spike. The prolonged action potentials were identical to those recorded in the same neurons in the absence of outward currents [i.e. in Ca2+-free Ringer's solution containing Mn2+, tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP)]. They consisted of an initial spike, followed by a plateau lasting several hundreds of milliseconds. Both components of the action potential were Na+-dependent and resistant to tetrodotoxin (TTX), while the plateau was selectively blocked by saxitoxin (STX), suggesting that it originated from the flow of Na+ through a conductance different from that underlying the spike potential (Johansen and Kleinhaus, 1987). Similarly, the plateau of the action potential prolonged by methohexital, described in this study was abolished by 50 microM saxitoxin. These results suggest that the action of the drug resulted from a block of repolarizing K+-conductances. This was confirmed by voltage-clamp experiments which showed that methohexital (100-1000 microM) reduced both IK and IA in the Retzius cell, essential mimicking the combined effects of TEA and 4-AP (Johansen and Kleinhaus, 1986b). In contrast, in dorsal root cells, methohexital decreased the amplitude of Na+ and K+ currents equally. This modulation of ionic conductances by methohexital may be important for the sedative and anesthetic actions of the drug.

Effects of pentobarbital on behavioral and synaptic plasticities in crayfish

Intra-abdominal injections of 90 mg/kg (3.5 x 10(-4) M estimated body concentration) sodium pentobarbital (PB) eliminated the cheliped closing response and eyestalk withdrawal response in crayfish (Procambarus clarkii). Repeated injections produced tolerance to both of these behavioral measures within several days. At glutamatergic synapses of the opener muscle, PB at dosages from 10(-7) to 10(-4) M had no significant effect on non-facilitated transmitter release evoked by 1 Hz stimuli. Facilitated transmitter release at 10 Hz stimuli was significantly decreased by 10(-3) M PB but was not significantly affected at lower concentrations. At 10(-4) M and 10(-3) M, PB significantly reduced the ratio of excitatory postsynaptic potential (EPSP) amplitudes at 10 Hz to those at 1 Hz. The frequency of spontaneous miniature EPSPs (MEPSPs) was reduced by 10(-4) M PB to about half of the control level, while MEPSP amplitudes and time constants were not significantly affected. These results suggest that the ability of PB to depress various crayfish behaviors is due, at least in part, to a presynaptic ability of the drug to depress facilitation of transmitter release at glutamatergic synapses.

Differential actions of pentobarbitone on calcium current components of mouse sensory neurones in culture

1. Using the single-electrode voltage clamp technique, three calcium current components were recorded at 35 degrees C from mouse dorsal root ganglion (DRG) neurones in culture. A transient low-threshold calcium current (T current) was recorded at clamp potentials (Vc) positive to -60 mV. Holding potentials (Vh) at or negative to -90 mV were required to fully remove inactivation. A large transient high-threshold calcium current component (N current) was recorded at Vc positive to -40 mV. Vh at or negative to -80 mV removed all steady-state inactivation. A slowly inactivating high-threshold calcium current component (L current) was recorded at Vc positive to -30 mV. Inactivation was removed by Vh at or negative to -60 mV. When currents were evoked at Vc positive to -20 mV from Vh negative to -60 mV, all three calcium current components were present. 2. Pentobarbitone (500 microM) had no effect on the isolated T current, but reduced the isolated L current 50-100% when evoked at Vc of -20 to 0 mV from Vh of -50 mV. Pentobarbitone had voltage-dependent effects on calcium currents containing all three calcium current components. Pentobarbitone produced small and equal reductions of the peak and late (greater than or equal to 300 ms) calcium currents evoked at -20 to 0 mV from Vh at or negative to -80 mV, but at more positive Vh there was a greater reduction in the peak current. The rate of current inactivation was increased in the presence of pentobarbitone. 3. Current-voltage plots were constructed from currents recorded in the absence and presence of 500 microM-pentobarbitone. Pentobarbitone reduced the magnitude of the calcium current without affecting the voltage dependence of the current-voltage relation. 4. Calcium current traces were fitted with a multiexponential function to determine the amplitudes and inactivation time constants (tau i) of the three calcium current components. Inactivation time constants decreased with more positive Vc for all three calcium current components. Pentobarbitone reduced only those tau i corresponding to the N current. 5. Recovery from inactivation of the N current was determined using a two-pulse protocol. In control neurones, recovery from inactivation occurring at 0 mV was slower at Vh = -65 mV than at Vh = -80 mV. In the presence of pentobarbitone, recovery from inactivation was faster, and occurred at a similar rate at both potentials. 6. Steady-state inactivation curves for the N current were derived from neurones in the absence and presence of pentobarbitone.(ABSTRACT TRUNCATED AT 400 WORDS)

Barbiturates block divalent cation action potentials in leech nociceptive cells

Phenobarbital (PNB), pentobarbital (PTB) and methohexital (MTX) decreased the maximum rate of depolarization Vmax and duration of divalent cation action potentials elicited in leech nociceptive neurons in Na+-free solutions containing the K+-channel blocker TEA, without significantly affecting resting membrane potential or conductance. The block of the divalent cation action potentials was reversible and dose-dependent, ED50 for inhibition of Vmax being 560 microM for MTX, 800 microM for PTB and 3000 microM for PNB. This order of potency correlated well with the ratio of unchanged/charged form of the drugs at physiological pH suggesting that in leech, as in other preparations, the non-ionized form was the active one. In Na+-containing Ringer, the 3 barbiturates depolarized and decreased membrane resistance in the lateral nociceptive cells, but not the medial nociceptive cells. These results provide additional information regarding the newly described pharmacological differences among closely related neurons. These membrane actions may be related to some of the excitatory properties described for other barbiturates in invertebrate and mammalian preparations.