Peter A. Pahapill

Actions of ketamine, phencyclidine and MK-801 on NMDA receptor currents in cultured mouse hippocampal neurones.

1. Stable N‐methyl‐D‐aspartic acid (NMDA) receptor‐mediated currents in cultured mouse hippocampal neurons were evoked by 20 ms pressure pulse applications of L‐aspartate, repeatedly applied at 30 or 40 s intervals, to the cell body region of the neurone. We have characterized the voltage‐ and use‐dependent blockade of the currents by three dissociative anaesthetics: ketamine, phencyclidine (PCP) and MK‐801 in mouse hippocampal neurones grown in dissociated tissue culture. 2. We have used a simple model of the blockade, based on the ‘guarded receptor hypothesis’ to interpret our data. The model assumes that receptors are maximally activated at the peak of the response with an open probability (Po) approaching 1, that there is no desensitization and that the blocking drug only associates with, or dissociates from, receptor channels which have been activated by agonist (e.g. open channels). 3. The model allows us to estimate forward and reverse rate constants for binding of the blockers to open channels from measurements of the steady‐state level of blockade and the rate of change of the current amplitude per pulse during onset and offset of blockade. As predicted by the model, the estimated reverse rate was independent of blocker concentration while the forward rate increased with concentration. Changing the level of positively charged ketamine (pKa 7.5) tenfold by changing pH from 6.5 to 8.5 caused a corresponding change in the forward rate while having no effect on the reverse rate. Most of the voltage dependence of the blockade could be accounted for by reduction of the reverse rate by depolarization. 4. Estimated forward rate constants for ketamine, PCP and MK‐801 were similar to one another when measured under similar conditions and were 3 x 10(4) ‐ 3 x 10(5) M‐1 S‐1. Most of the differences in potency of the three blockers could be accounted for by differences in the reverse rate constants which were approximately 0.2, 0.03 and 0.003 s‐1 for ketamine, PCP and MK‐801, respectively. The estimated rate constants actually are the product of the rate constants and 1/Po. Suggestions that maximum Po is much less than 1 for NMDA channels imply that both forward and reverse rate constants of blockade may in fact be larger than we have calculated. However, their magnitudes, relative to one another, are unaffected by this consideration. 5. The reverse rate constant of blockade increased at positive potentials. This increase was prevented when the neurone was loaded with N‐methyl‐D‐glucamine, an impermeant cation which prevented outward currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Pleomorphic xanthoastrocytoma: case report and analysis of the literature concerning the efficacy of resection and the significance of necrosis.

The case of a patient with a pleomorphic xanthoastrocytoma (PXA), a low-grade glioma of adolescence, is presented. A literature review of 79 patients with PXAs is described and confirms a favorable prognosis in 80% of patients. The sex ratio in the reported cases was almost equal, and the median age at time of diagnosis was 14 years. Seventy-nine percent of the patients presented with seizures. Nine of the 15 deaths from PXA are associated with histological evidence of necrosis at initial presentation or in a recurrent tumor, confirming the poor prognosis associated with the presence of necrosis in these neoplasms. Survival curves confirm that the optimal treatment for PXAs without necrosis is primary surgical resection with subsequent operation for recurrent tumor. The roles of surgery or radiotherapy in necrotic PXA are not clear from the literature.

Tremor arrest with thalamic microinjections of muscimol in patients with essential tremor

Six patients undergoing stereotactic procedures for essential tremor received microinjections of muscimol (a γ‐aminobutyric acid‐A [GABAA] agonist) into the ventralis intermedius thalamus in areas where tremor‐synchronous cells were identified electrophysiologically with microelectrode recordings and where tremor reduction occurred with electrical microstimulation. Injections of muscimol but not saline consistently reduced tremor in each patient. The effect had a mean latency of 7 minutes and lasted an average of 9 minutes. We propose that GABA‐mediated thalamic neuronal inhibition may represent a mechanism underlying the effectiveness of surgery for tremor and that GABA analogues could potentially be used therapeutically. Ann Neurol 1999;46:249–252

A large, multiple-conductance chloride channel in normal human T lymphocytes.

Chloride (Cl) channels have been proposed to play roles in lymphocyte functions including volume regulation and cellular cytotoxicity; however, direct studies of such channels in normal human lymphocytes are lacking. In the present study we describe a large conductance Cl channel observed in about 50% of excised, inside-out patches from normal human peripheral T lymphocytes. The channel has multiple conductance states with linear single-channel current-versus-voltage relationships in symmetrical Cl solutions. The most prevalent state is the largest, which has a conductance of about 365 pS. The channel closes in a voltage-dependent manner at both negative and positive potentials, but does not show voltage-dependent inactivation. The probability of opening is maximal between −15 mV and +15 mV and the voltage dependence is well described by two Boltzmann equations with half-maximal probabilities at −22.8 mV and +18.0 mV. The slopes of the voltage dependence suggest two gates in series with 5.7 and 9.6 equivalent charges. The channel was about 30 times more selective for Cl− than for Na+ or K+ under balanced osmolarity but less selective (approx. 11∶1) under a large osmotic gradient. The single-channel conductance increased with Cl concentration with an apparent saturation at about 581 pS and a Michaelis-Menten constant of about 120 mM. The selectivity sequence among anions, determined from changes in reversal potential was: I− > NO3−> Br−, Cl− > F−, isethionate, HCO3−> SO42−> gluconate, propionate > aspartate ≫ Na+, K+ and was apparently the same for subconductance states. The sequence determined from measurable values of single-channel conductance was: I− > NO3−> Br− > Cl− > F− > HCO3−, isethionate. The channel was rapidly and reversibly blocked by 1 mM Zn2+ or 1 mM Ni2+ added to the cytoplasmic face. Possible roles of this maxi-Cl channel in lymphocyte function are discussed.

Cl− channels in intact human T lymphocytes

SummaryWe recently described a large, multiple-conductance Cl− channel in excised patches from normal T lymphocytes. The properties of this channel in excised patches are similar to maxiCl− channels found in a number of cell types. The voltage dependence in excised patches permitted opening only at nonphysiological voltages, and channel activity was rarely seen in cell-attached patches. In the present study, we show that Cl− channels can be activated in intact cells at physiological temperatures and voltages and that channel properties change after patch excision.Maxi-Cl− channels were reversibly activated in 69% of cellattached patches when the temperature was above 32°C, whereas fewer than 2% of patches showed activity at room temperature. Upon excision, the same patches displayed large, multiple-conductance Cl− channels with characteristics like those we previously reported for excised patches. After patch excision, warm temperatures were not essential to allow channel activity; 37% (114/308) of inside-out patches had active channels at room temperature. The voltage dependence of the channels was markedly different in cell-attached recordings compared with excised patches. In cell-attached patches, Cl− channels could be open at cell resting potentials in the normal range. Channel activation was not related to changes in intracellular Ca2+ since neither ionomycin nor mitogens activated the channels in cell-attached patches, Ca2+ did not rise in response to warming and the Cl− channel was independent of Ca2+ in inside-out patches. Singlechannel currents were blocked by internal or external Zn2+ (100–200 μm), 4-acetamido-4′ isothiocyanostilbene-2,2′-disulfonate (SITS, 100–500 μm) and 4,4′-diisothiocyanostilbene 2,2′disulfonate (DIDS, 100 μm). NPPB (5-nitro-2-(3-phenylpropylamino)-benzoate) reversibly blocked the channels in inside-out patches.

Modulation of potassium channels in intact human T lymphocytes.

1. A voltage‐dependent K+ channel called the ‘n’ type (for ‘normal’) is the most prevalent ion channel found in whole‐cell recordings from T lymphocytes. In whole‐cell patch‐clamp recordings activity of the n‐type channel is affected by mitogenic agents, pH, Ca2+ and temperature but not by cyclic nucleotides. Because channel properties and regulation can depend on cytoplasmic components we sought to reassess the properties of K+ channels in intact, normal human T lymphocytes using cell‐attached, patch‐clamp recordings. In the present study, we show that the predominant K+ channel in resting, intact cells is the n type and is affected by voltage, temperature and Ca2+ in ways similar to the disrupted cell. Moreover, K+ channels are activated by agents that raise cyclic AMP in intact cells. 2. In cell‐attached recordings, we found voltage‐activated K+ channels in about 60% of patches at room temperature. The channel was K+ selective as judged from the reversal potential under different Ka(+)‐K+ gradients and at different resting membrane potentials. Some patches were subsequently excised and the selectivity further confirmed. The current‐voltage relation was inwardly rectifying under symmetrical K+ concentrations and had a slope conductance of 9.4 pS at 50 mV depolarized and 23.8 pS at 50 mV hyperpolarized from the resting potential. From the reversal potentials under various conditions the cell resting potential was ‐51 +/‐ 1 mV in normal NaCl saline and about 0 mV when the bath contained 150 mM‐KCl saline. Two other types of K+ channel were seen in resting, intact cells, but were much less common (less than 5% and 11% of patches). A large‐conductance K+ channel was seen in less than 1% of inside‐out patches. 3. The predominant K+ channel in intact, resting T lymphocytes was confirmed as the n type underlying the whole‐cell K+ current evoked by voltage steps. In cell‐attached patches there was a low, steady‐state level of activity at the resting potential but activity was greatly increased by depolarizing voltage jumps. Steady‐state inactivation could be removed by a hyperpolarizing pre‐pulse. Ensemble currents constructed by summing channel openings during repeated voltage jumps showed sigmoid kinetics of current activation and a monoexponential decay phase. These kinetics were well fitted by a Hodgkin‐Huxley‐type n4j kinetic model with time constants very similar to the whole‐cell current of disrupted cells. Moreover, the kinetics depended on the external K+ concentration as previous research has shown.(ABSTRACT TRUNCATED AT 400 WORDS)

Conservative Treatment of Acute Spontaneous Spinal Epidural Hematoma

BACKGROUND In cases of acute spontaneous epidural hematoma producing neurological deficits, emergency surgical evacuation is the standard treatment. METHODS Such a case is presented in which complete resolution of neurological deficits occurred without surgical intervention. RESULTS This is the fifth reported case of complete recovery in a patient managed conservatively. In most reports, significant and sustained neurological recovery had occurred with 12 hours of impairment of walking. CONCLUSION In cases of acute spontaneous spiral epidural hematoma in which neurological deterioration is followed by early and sustained recovery, non-operative therapy may be considered.

Asymmetry of motor impulses and neuromuscular synapses produced in crayfish claws by unilateral immobilization

SummaryThe ‘fast’ axon supplying the closer muscle in crayfish (Procambarus clarkii) normally fires few impulses and generates large excitatory postsynaptic potentials (EPSPs) that fatigue rapidly with repeated stimulation. When the dactyl of one claw is immobilized in the closed position, impulse production in the fast axon decreases on the immobilized side and increases on the contralateral side. On the immobilized side, EPSPs become larger but more readily depressed with repeated stimulation, while converse changes occur on the contralateral side.In order to establish whether the smaller number of impulses on the immobilized side was responsible for the changes in EPSPs, extra impulses were generated in the fast axon of immobilized claws by implanting electrodes in the claw. Raising the impulse production to equal or exceed that of the contralateral side did not prevent the changes in EPSPs produced by immobilization. Thus, it is probable that changes in the level of synaptic input to central parts of the fast closer excitor neuron are mainly responsible for altered physiological properties of peripheral synapses, rather than the fast axons impulse traffic per se.

Long-term follow-up study of chronic deep brain stimulation of the subthalamic nucleus for cervical dystonia.

Objectives. Medically refractory cervical dystonia has recently been treated using deep brain stimulation (DBS), targeting the subthalamic nucleus (STN). There has been limited literature regarding short‐term outcomes and no literature regarding long‐term outcomes for refractory cervical dystonia following DBS of the STN. Materials and Methods. Two patients with medically refractory cervical dystonia underwent STN DBS. Patients were rated using the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) preoperatively and immediately postoperatively as well as just prior to turning on the stimulators and subsequently at 24–48 hours, six months, one, two, and three years after stimulation. Microrecordings were used to identify the STN and substantia nigra reticulata (SNr). Results. Significant immediate and sustained long‐term improvements were seen in motor, disability, pain, and total TWSTRS scores. In one patient, only unilateral stimulation was required. The STN and SNr were easily identified as having activity similar to off‐state Parkinsons patients. Conclusions. DBS therapy for cervical dystonia utilizing the STN as the surgical target may be novel and may be an alternative target to the globus pallidus internus as supported by this first long‐term outcome report. Further studies need to be performed to confirm these conclusions.

Long-term facilitation and low-frequency depression in a crayfish phasic motor axon

Summary1.Stimulation of the fast axon to the closer muscle of the isolated crayfish claw preparation at 0.1 Hz leads to low-frequency depression (LFD) of the excitatory postsynaptic potentials (EPSPs) recorded from single muscle fibers. EPSP amplitude is reduced by 50% after only 30 stimuli at 0.1 Hz, and declines during maintained 0.1 Hz stimulation with a time constant of 10–15 min.2.LFD is not affected by temperature changes ranging from 10 °C to 25 °C. However, ouabain (10−4M-10−3M) in the extracellular solution slightly reduces the rate of LFD.3.When 0.1 Hz stimulation is delivered following a period of conditioning stimulation (5–10 Hz) which causes depression, a facilitated EPSP appears. Facilitation persists for 1/2 h or more after induction. This long-term facilitation (LTF) has some features in common with that seen after stimulation of a tonic motoneuron in crayfish and crabs.4.The amplitude of LTF is directly related to the number of conditioning impulses. However, the effect is pattern-sensitive: the same number of impulses delivered in a bursting pattern enhances the expression of LTF.5.LTF is enhanced by lowering the temperature or applying ouabain (10−3M) during conditioning.6.The results implicate sodium loading of phasic axon nerve terminals during stimulation as a factor in enhancing EPSP amplitude. This suggests that the mechanism for induction of LTF at phasic neuromuscular synapses is similar to that in crustacean tonic neuromuscular synapses.