Changes in GABAergic and non-GABAergic synapses during chronic ethanol exposure and withdrawal in the dentate fascia of LS and SS mice

Gamma-hydroxybutyrate withdrawal syndrome

STUDY OBJECTIVE

Gamma-hydroxybutyrate (GHB) withdrawal syndrome is increasingly encountered in emergency departments among patients presenting for health care after discontinuing frequent GHB use. This report describes the characteristics, course, and symptoms of this syndrome.

METHODS

A retrospective review of poison center records identified 7 consecutive cases in which patients reporting excessive GHB use were admitted for symptoms consistent with a sedative withdrawal syndrome. One additional case identified by a medical examiner was brought to our attention. These medical records were reviewed extracting demographic information, reason for presentation and use, concurrent drug use, toxicology screenings, and the onset and duration of clinical signs and symptoms.

RESULTS

Eight patients had a prolonged withdrawal course after discontinuing chronic use of GHB. All patients in this series were psychotic and severely agitated, requiring physical restraint and sedation. Cardiovascular effects included mild tachycardia and hypertension. Neurologic effects of prolonged delirium with auditory and visual hallucinations became episodic as the syndrome waned. Diaphoresis, nausea, and vomiting occurred less frequently. The onset of withdrawal symptoms in these patients was rapid (1 to 6 hours after the last dose) and symptoms were prolonged (5 to 15 days). One death occurred on hospital day 13 as withdrawal symptoms were resolving.

CONCLUSION

In our patients, severe GHB dependence followed frequent ingestion every 1 to 3 hours around-the-clock. The withdrawal syndrome was accompanied initially by symptoms of anxiety, insomnia, and tremor that developed soon after GHB discontinuation. These initial symptoms may progress to severe delirium with autonomic instability.

Withdrawal syndromes

The pathophysiology of substance withdrawal is elucidated by a review of classic and cutting-edge research. The manifestation and evaluation of the associated withdrawal syndromes from ethanol, sedative-hypnotics, opioids, and baclofen, are compared. The general management of and pharmacotherapy for these patients are discussed.

Epileptiform activity generated by endogenous acetylcholine during blockade of GABAergic inhibition in immature and adult rat hippocampus

We tested the effects of the acetylcholinesterase inhibitor eserine (10 microM), an indicator of the activity of endogenous ACh, on the generation of epileptiform discharges during blockade of inhibitory GABA(A)-mediated potentials by bicuculline (10 microM), in the CA3 area of hippocampal slices from postnatal days 4-20 (P4-P20) immature and adult rats. Eserine provoked or significantly increased the frequency of spontaneous synchronous epileptiform discharges, in 6/22 (27%) P4-P10 slices, 34/35 P11-P20 slices and 18/18 adult slices, an epileptogenic effect. In immature slices, spontaneous discharges showed a stable frequency throughout perfusion with eserine, while in 5/11 adult slices an initial fast frequency was followed by a slower steady-state one. The cholinergic agonist carbachol (CCh, 25 microM) provoked only transient or no spontaneous synchronous discharges in adult slices (n=8), thus suggesting that massive activation of cholinergic receptors may lead to suppression of epileptiform activity in adult brain. Stimulus-induced excitatory CA3 responses, were depressed by eserine in approximately half of 20 P4-P10, 45 P11-P20 and 11 adult slices. The depression consisted of a decrease in the amplitude, duration, and number of population spikes of the field potentials by about 30%, a minor neuroprotective effect, which did not change with maturation. The different developmental profiles of the epileptogenic and neuroprotective effects of endogenous ACh suggest that they are mediated by different mechanisms. These experiments demonstrate that, endogenous ACh is sufficient to induce epileptogenesis during a decrease or failure of GABAergic inhibition, in both >/=P10 immature and in adult hippocampus. We therefore suggest that clinical or behavioral conditions which raise the concentration of endogenous ACh may lower the threshold to seizures.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Fine structure of hippocampal dendrites in the dentate fascia of LS/SS-mice after chronic ethanol treatment

1. The effect of ethanol and its withdrawal on the dendritic microtubules in the dentate fascia of male mice was studied in the ethanol-sensitive, long-sleep (LS) line and the ethanol-insensitive, short-sleep (SS) line. 2. Both mouse lines were treated with a liquid ethanol diet. Dendrites in the dentate molecular layer (DML) of the right hippocampus were examined. 3. They revealed marked changes in microtubule density as compared with the controls. While the microtubule density in LS mice was significantly reduced by 15% and 18% in the middle and distal third of the DML, respectively, in SS mice the reduction (by 12%) took place in the distal third only. During withdrawal a recovery of the microtubule density has been observed in both lines.

Quantifying the role of inhibition in associative long-term potentiation in dentate granule cells with computational models

In the dentate gyrus, coactivation of a mildly strong ipsilateral perforant path (pp) input with a weak contralateral pp input will not induce associative long-term potentiation in the weak input path unless both inputs project to the same part of the molecular layer. This "spatial convergence requirement" is thought to arise from either voltage attenuation between input locations or inhibition. Simulations with a detailed model of a dentate granule cell were performed to rule out voltage attenuation and to quantify the inhibition necessary to obtain the spatial convergence requirement. Strong lateral and weak medial or strong medial and weak lateral pp input were activated eight times at 400 Hz. Calcium current through N-methyl-D-aspartate receptor channels and subsequent changes in calcium concentration and the concentration of calmodulin bound with four calcium ions ([Cal-Ca4]) in the spine head were computed for a medial and a lateral pp synapse. To satisfy the spatial convergence requirement, peak [Cal-Ca4] had to be much larger in the strongly activated path synapse than in the weakly activated path synapse. With no inhibition in the model, differences in peak [Cal-Cal4] at the two synapses were small, ruling out voltage attenuation as the explanation of the spatial convergence requirement. However, with shunting inhibition, modeled by reducing membrane resistivity to 1,600 omega cm2 in the distal two-thirds of the dendritic tree, peak [Cal-Ca4] was 3-5 times larger in the strongly activated path synapse than in the weakly activated path synapse. The magnitude of shunting inhibition was varied to determine the level that maximized this difference in peak [Cal-Ca4]. For strong lateral and weak medial pp input, the optimal level was one that prevented the cell from firing an action potential. For strong medial and weak lateral pp input, the optimal level was one at which the cell fired two action potentials. The distribution of shunting inhibition that best satisfied the spatial convergence requirement was inhibition on the distal two-thirds of the dendritic tree with or without inhibition at the soma, with inhibition stronger in the distal third than in the middle third. It was estimated that the number of inhibitory synapses involved in the shunting inhibition should be 25-50% of the number of excitatory synapses activated by the eight-pulse, 400-Hz tetanus. This number could be 20-50% of the total number of inhibitory synapses in the distal two-thirds of the dendritic tree. The addition of a single inhibitory synapse on a dendrite had a significant effect on peak spine head [Cal-Ca4] in nearby spines. Inhibitory synapses had to be activated four or more times at 100 Hz for effective shunting to take place, and the inhibition had to begin no later than 2-5 ms after the first excitatory input. The results suggest that inhibition can isolate potentiated synapses to particular dendritic domains and that the location of activated inhibitory synapses may affect potentiation of individual synapses on individual dendrites.