Ortho-substituted 2,2',3,5',6-pentachlorobiphenyl (PCB 95) alters rat hippocampal ryanodine receptors and neuroplasticity in vitro: evidence for altered hippocampal function.

The effects of PCBs on hippocampal function were studied in vitro, by radioligand-receptor binding analysis and electrophysiological measurements of the hippocampal slice preparation. [3H]Ryanodine, a conformation-sensitive probe for ryanodine receptors, was employed to determine how PCBs influence specific high-affinity occupancy to receptors found in microsomes isolated from rat hippocampus. PCB 95 (2,2',3,5'6-pentachlorobiphenyl) exhibited a dose-dependent enhancement of [3H]ryanodine receptor occupancy with an EC50 of 12 microM. In contrast, PCB 66 (2,3'4,4'-tetrachlorobiphenyl) showed no activity toward ryanodine receptors, up to its solubility limit (> or = 200 microM. Population spike (PS) and excitatory postsynaptic potential (EPSP) responses were recorded from striatum pyramidale of the CA1 region, which were generated from single pulse orthodromic stimulation of Schaffer collateral/commissural (SC/C) fibers at striatum radiatum of the hippocampal slice preparation. After the introduction of PCB 95 to the perfusion medium, PCB 95 depressed PS amplitude, especially at high stimulus intensities. Significant reductions in PS and EPSP maxima were seen, even after induction of long term potentiation, a model of neuroplasticity. However, these actions were not observed with PCB 66 which lacks ryanodine receptor activity, implicating a ryanodine receptor-mediated mechanism in the general depression of pyramidal cell excitability seen with PCB 95. Taken together, these results reveal a novel, arylhydrocarbon (Ah) receptor-independent, mechanism by which PCB 95 alters neuronal Ca2+ signaling and neuroplasticity in adult brain.

The effects of lindane and long-term potentiation (LTP) on pyramidal cell excitability in the rat hippocampal slice.

An in vitro orthodromic stimulation technique was used to examine the effects of lindane and long-term potentiation (LTP) inducing stimuli, alone or in combination, on the excitatory afferent terminal of CA1 pyramidal cells and on recurrent collateral evoked inhibition using the rat hippocampal slice model. Hippocampal slices of 400 microns thickness were perfused with oxygenated artificial cerebrospinal fluid. Stimulation of Schaffer collateral/commissural fibers produced extracellular excitatory postsynaptic potential (EPSP) and/or populations spike (PS) responses recorded from electrodes in the CA1 region. A paired-pulse technique was used to measure gamma-aminobutyric acid (GABAA)-mediated recurrent inhibition before and after treatments. After both lindane and LTP, larger PS amplitudes for a given stimulus intensity were seen. The resulting leftward shift in the curve of the PS amplitude versus stimulus intensity was larger after LTP than after 25 microM lindane. Both lindane and LTP treatments reduced PS thresholds and reduced or eliminated recurrent inhibition as measured by paired-pulse stimulation at the 15 msec interval. The reduction of recurrent inhibition after both treatments was more pronounced at lower stimulus intensities. When LTP stimuli were applied after lindane exposure a further large shift to the left was seen in the PS amplitude versus stimulus intensity curve. A smaller shift to the left was seen in the PS amplitude versus stimulus intensity curve only at the higher stimuli when lindane exposure occurred after LTP. Only at low stimulus intensities were further argumentations seen in PS amplitudes when the LTP stimuli was followed by a second LTP stimuli. Previous exposure to 25 microM lindane stimuli does not block the development of a further robust LTP in this in vitro model.

The effect of propofol on CA1 pyramidal cell excitability and GABAA-mediated inhibition in the rat hippocampal slice.

An in vitro paired-pulse orthodromic stimulation technique was used to examine the effects of propofol on excitatory afferent terminals, CA1 pyramidal cells and recurrent collateral evoked inhibition in the rat hippocampal slice. Hippocampal slices 400 microns thick were perfused with oxygenated artificial cerebrospinal fluid, and electrodes were placed in the CA1 region to record extracellular field population spike (PS) or excitatory postsynaptic potential (EPSP) responses to stimulation of Schaffer collateral/commissural fibers. Gamma-aminobutyric acid (GABA)-mediated recurrent inhibition was measured using a paired-pulse technique. The major effect of propofol (7-28 microM) was a dose and time dependent increase in the intensity and duration of GABA-mediated inhibition. This propofol effect could be rapidly and completely reversed by exposure to known GABAA antagonists, including picrotoxin, bicuculline and pentylenetetrazol. It was also reversed by the chloride channel antagonist, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). It was not antagonized by central (flumazenil) or peripheral (PK11195) benzodiazepine antagonists. Reversal of endogenous inhibition was also noted with the antagonists picrotoxin and pentylenetetrazol. Input/output curves constructed using stimulus propofol caused only a small enhancement of EPSPs at higher stimulus intensities but had no effect on PS amplitudes. These studies are consistent with propofol having a GABAA-chloride channel mechanism causing its effect on recurrent collateral evoked inhibition in the rat hippocampal slice.

Lindane blocks GABAA-mediated inhibition and modulates pyramidal cell excitability in the rat hippocampal slice.

An in vitro paired-pulse orthodromic stimulation technique was used to examine the effects of lindane on excitatory afferent terminals, CA1 pyramidal cells and recurrent collateral evoked inhibition in the rat hippocampal slice. This was done to establish simultaneous effects on a simple neural network and to develop procedures for more detailed analyses of the effects of lindane. Hippocampal slices 400 microns thick were perfused with oxygenated artificial cerebrospinal fluid. Electrodes were placed in the CA1 region to record extracellular population spike (PS) or excitatory postsynaptic potential (EPSP) responses to stimulation of Schaffer collateral/commissural (SC/C) fibers. Gamma-aminobutyric acid (GABA)-mediated recurrent inhibition was measured using a paired-pulse technique. Perfusion with lindane produced both time and dose dependent changes in a number of the responses measured. The most striking effect produced by lindane was the loss of GABAA-mediated recurrent collateral inhibition. This tended to occur rapidly, often before changes in EPSP or PS responses could be detected. With longer exposures to lindane, repetitive discharge of pyramidal cells developed resulting in multiple PSs to single stimuli. Lindane (50 microM) also completely reversed the effects of the injectable anesthetic, propofol, a compound known to potentiate GABAA-mediated inhibition via a direct action on the GABAA receptor-chloride channel complex. An analysis of input/output relationships at varying stimulus intensities showed that lindane increased EPSP and PS response amplitudes at any given stimulus intensity resulting in a leftward shift in the EPSP amplitude/stimulus intensity, PS amplitude/stimulus intensity and PS amplitude/EPSP amplitude relationships. This effect was most noticeable with low intensity stimuli and became progressively less so as stimulus intensities approached those yielding maximal responses. In addition lindane significantly increased paired pulse facilitation of EPSPs during paired stimulus presentation.

NMDA receptors have a dominant role in population spike-paired pulse facilitation in the dentate gyrus of urethane-anesthetized rats.

Paired-pulse facilitation was studied at the perforant path-granule cell synapses in the dentate gyrus of urethane-anesthetized rats. Extracellular field potentials comprising excitatory postsynaptic potentials (EPSPs) and population spikes (PSs) were used to compare facilitation of both responses at interpulse intervals between 10 and 1000 ms. In this model system EPSPs, produced at stimulus intensities well below the PS threshold, exhibited paired-pulse facilitation (PPF) at intervals less than 40 ms. Between 40 and 100 ms both responses were of equal size and from 200 to 1000 ms the second response exhibited paired-pulse depression (PPD). With higher stimulus intensities, PSs exhibited early (10-30 ms) and late (400-1000 ms) PPD and intermediate interval (40-200 ms) PPF. The enhanced PS amplitude during facilitation was accompanied by a modest decrease in EPSP slope and increase in PS latencies to onset and to peak. If PS amplitude was increased to the same extent by simply increasing stimulus intensity, an increase in EPSP slope and decrease in PS latencies to onset and peak were observed. Current source-density analysis revealed that the current sink generated by synaptic activation of granule cell dendrites was indistinguishable between control and facilitated responses up to the onset of the PS. However, the facilitated response exhibited a marked increase in the duration and amplitude of current flowing into the dendrites during the late phase of the EPSP. PPF of the PS was attenuated or blocked by the administration of the known NMDA receptor-ion channel blockers, MK-801, dextromethorphan and ketamine. The depressant effects of these pharmacological agents on facilitation shared the same time course as facilitation itself, peaking at 60 ms and lasting approximately 200 ms. These data suggest that facilitation in this situation is due to postsynaptic rather than presynaptic modulations, and is based upon an increase in the NMDA-mediated component of the evoked response. No increase in transmitter release, per se, could be demonstrated.

Modification of GABA-mediated inhibition by various injectable anesthetics.

Increasing doses of the injectable anesthetics etomidate, Saffan, thiopental, ketamine, and xylazine and the vehicles saline and propylene glycol were administered to urethane-anesthetized rats. Their effects in vivo on perforant pathway-evoked field excitatory post-synaptic potentials and population spikes in the hippocampal dentate gyrus were determined. The primary purpose was to ascertain whether these compounds affect hippocampal excitability in a manner consistent with their proposed mechanisms of action. Compared with their respective vehicles, thiopental, etomidate, and xylazine reduced the amplitude of population spikes to single perforant pathway stimulation by 20-30% at the highest doses tested. Xylazine also increased the latency to onset of the population spike. No other effects were observed. Using paired pulse paradigms, it was determined that etomidate produced a dramatic, prolonged reduction in granule cell excitability at interpulse intervals of 10-100 ms. The magnitude of the effect was dose related and was reversible with the discontinuance of administration of the drug. Similar changes occurred with Saffan (althesin) and thiopental. Ketamine produced a small but significant depression in granule cell excitability during intervals of 10-200 ms. Xylazine had no effect. These data corroborate the importance of a prolongation of gamma-aminobutyric acid A-mediated inhibition to the mechanism of actions of etomidate, thiopental, and Saffan at relevant exposure concentrations in vivo.

Stereoselective modulation of ryanodine-sensitive calcium channels by the delta isomer of hexachlorocyclohexane (delta-HCH).

delta-Hexachlorocyclohexane (delta-HCH) is shown to be 30-fold more potent as a positive inotropic agent with rat atrial strips compared with lindane (gamma-HCH). Threshold and ED50 values for enhanced contractile force at a pacing frequency of 0.5 Hz are less than 1 microM and 2.2 microM for delta-HCH and 40 microM and 63 microM for gamma-HCH, respectively. Contracture developed in atria exposed to greater than 4 microM delta-HCH (ED50 = 11 microM) but not in atria exposed to gamma-HCH. Uptake and release of Ca++ measured from actively loaded cardiac sarcoplasmic reticulum (SR) vesicles is measured with antipyrylazo III. Although delta-HCH (30 microM) decreases Ca(++)-dependent ATPase by 20%, it does not significantly alter Ca++ loading in the presence of ruthenium red. Addition of delta-HCH (5-50 microM) after loading is complete causes rapid, dose-dependent release of Ca++ from SR. Ca++ release induced by delta-HCH is markedly stereoselective. Compared with gamma-HCH (50 microM), delta-HCH (50 microM) induces a nearly 20-fold higher initial rate of Ca++ release (4.3 nmol of Ca++/mg/sec). Studies with [3H]ryanodine demonstrate that delta-HCH sharply inhibits Ca(++)- or daunorubicin-activated radioligand binding (IC50 = 37 and 25 microM, respectively, logit slope = 2). Inhibition of [3H]ryanodine-binding by delta-HCH is stereoselective inasmuch as IC50 values for alpha, beta and gamma isomers are greater than 100 microM. The delta-HCH modified Ca++ channel appears to proceed by a noncompetitive mechanism (reducing Bmax in equilibrium experiments) with respect to the conformationally sensitive binding site for [3H]ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Propofol modification of evoked hippocampal dentate inhibition in urethane-anesthetized rats.

To learn more about the site and mode of action of propofol, the authors anesthetized rats with urethane, and implanted a stimulating electrode in the perforant pathway and a recording electrode in the hippocampal dentate gyrus to measure evoked field responses. Catheters were placed in the jugular vein and carotid artery to measure blood pressure, take blood samples for measurement of blood gases, and inject drugs. Rat lungs were ventilated, and increasing doses of propofol or its 10% fat emulsion vehicle were administered. Over a dose range of 2.5-40 mg/kg, propofol had little effect on the field excitatory postsynaptic potential (EPSP) or the field population spike (PS), suggesting no important actions on perforant pathway fibers or intrinsic granule cell excitability. With the use of various paired-pulse stimulation paradigms, it was demonstrated that propofol administration led to a dramatic decrease in granule cell excitability over interpulse intervals of 10-100 ms. The magnitude of this effect clearly was related to dose, and the effect was reversible with discontinuance of drug. The vehicle had marginal effects on granule cell excitability. These were significantly less than those produced by propofol. Transient reductions in blood pressure followed by transient increases to above baseline were observed after administration of propofol at most doses. Increased heart rates accompanied the slight increase in blood pressure. Except at the largest dose of propofol tested, cardiovascular measures had returned to near control levels by 8 min, when the evoked potentials were evaluated. Arterial blood gases showed little change throughout the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of GABAA antagonists with deltamethrin, diazepam, pentobarbital, and SKF100330A in the rat dentate gyrus.

Activation of dentate gyrus granule cells leads to a period of depressed excitability mediated primarily by recurrent inhibition. This response can be increased in intensity and duration by drugs that enhance GABAA-mediated inhibition including GABA uptake blockers (SKF100330A), benzodiazepines (diazepam), and barbiturates (pentobarbital). It can be attenuated or abolished by administration of the GABAA antagonists picrotoxin and bicuculline. Type II pyrethroids, such as deltamethrin, produce effects in vitro which lead one to expect that they would decrease GABA-mediated inhibition. However, administration of deltamethrin to animals intensifies the inhibitory consequences of granule cell activation by many fold. To determine the role of GABAA-mediated mechanisms in the action of deltamethrin, we have looked at its antagonism by bicuculline and picrotoxin. Neither GABAA antagonist reduced the duration of the inhibition produced by deltamethrin. The only effect observed was a small but significant reduction in intensity of inhibition during the period of 10-40 msec after activation of the granule cells. For comparative purposes we have also examined the antagonism produced by bicuculline and picrotoxin on agents known to enhance GABAA-mediated inhibition. These antagonists were quite effective in antagonizing the enhancement of inhibition produced by diazepam and SKF 100330A. Bicuculline was also effective in antagonizing pentobarbital while picrotoxin was not. These results suggest that the prolongation of inhibition produced by deltamethrin in dentate granule cells does not result simply from an extension of GABAA-mediated inhibition, but rather derives from a different source.

Characteristics of the prolonged inhibition produced by a range of pyrethroids in the rat hippocampus.

Eight different synthetic pyrethroids were examined to determine their effects on the excitability of hippocampal granule cells in urethane-anesthetized rats. A paired stimulus approach was used. All eight prolonged the depression of granule cell excitability that follows stimulation of their major synaptic input, the perforant path. The magnitude of this effect depended upon the class to which the pyrethroid belonged. Type I pyrethroids (those primarily producing tremor) prolonged the depression of granule cell excitability for shorter periods than did type II pyrethroids (those primarily producing salivation and choreoathetosis) or pyrethroids producing a mixed type of intoxication. No overlap was found between groups. To determine whether the difference observed between type I and type II pyrethroids was the result of an infelicitous selection of doses, cismethrin (type I) was tested over a dose range of 1.5-24 times the conscious rat iv LD50. Even at the highest dose, the prolongation remained well below that produced by type II pyrethroids. The effect of deltamethrin was shown to be consistent with the production or potentiation of a surmountable inhibitory response. This action of deltamethrin was antagonizable by mephenesin and lidocaine, but not by picrotoxin or halothane. The type of effect, its time course, and the antagonism data suggest that type II pyrethroids enhance inhibition in the dentate gyrus. This action does not appear to be mediated by GABAA receptors.

Direct neural effect of lateral hypothalamic stimulation on insulin secretion by pancreases of normal and obese rats.

Perfusion of CNS intact pancreases with 200 mg/dl glucose with concomitant lateral hypothalamic area (LHA) stimulation significantly inhibited insulin secretion both in normal and obese rats. Sprague-Dawley, Zucker lean (FaFa) and Zucker obese (fafa) rats all responded in a similar manner, suggesting a general effect unrelated to metabolic state. Insulin secretion during mins 25-40 of perfusion was inhibited in Sprague Dawley, lean Zucker and obese Zucker rats by 31%, 42% and 33%, even though LHA stimulation took place from mins 20-25. Thus, the duration of inhibition was greater than the period of LHA stimulation, indicating that this pathway can induce prolonged changes in the responsiveness of the pancreas. The data presented in this study demonstrate that LHA stimulation, in the absence of humoral factors, results in a direct CNS-mediated suppression of insulin secretion which is relatively long lasting. This effect may illustrate a basic control mechanism by the CNS to regulate the endocrine pancreas.

Modification of evoked hippocampal dentate inhibition by diazepam and three antagonists in urethane-anesthetized rats.

Urethane-anesthetized rats with perforant pathway stimulating electrodes and recording electrodes placed in the hippocampal dentate gyrus were exposed to increasing doses of either the benzodiazepine agonist diazepam or an antagonist (PK-11195, CGS-8216, and RO15-1788). Analysis of the monosynaptic evoked field potentials indicated that none of the four compounds altered the threshold for eliciting the excitatory postsynaptic potential (EPSP). Reductions in field population spike (PS) amplitudes were seen after exposure to RO15-1788, CGS-8216, and diazepam, but not PK-11195. Using a paired-pulse paradigm, diazepam significantly increased early gamma-aminobutyric acid (GA-BAA)-mediated recurrent inhibition. The antagonist RO15-1788, but not CGS-8216 or PK-11195, also significantly increased early GABAA-mediated inhibition. The increase in GABAA-mediated inhibition after diazepam was reversed by the subsequent administration of the central antagonists RO15-1788 or CGS-8216, but not the peripheral antagonist PK-11195. Pretreatment with CGS-8216 or RO15-1788 prevented diazepam-induced inhibition. These data support the important modulatory role of the central benzodiazepine receptor in early GABAA-mediated inhibition at this synapse. They also suggest that basal granule cell excitability is not importantly modulated by this benzodiazepine receptor.

Type I and type II pyrethroids increase inhibition in the hippocampal dentate gyrus of the rat.

Urethane-anesthetized rats were prepared for stimulation of the perforant path and for recording from the granule cell region of the hippocampal dentate gyrus. Subjects were administered varying doses of allethrin (a prototype type I pyrethroid) or deltamethrin (a prototype type II pyrethroid), and the excitability of the perforant path and granule cells was tested. Both pyrethroids produced a dose-dependent decrease in the responsiveness of granule cells, following stimulation of the perforant path, that lasted up to 100 msec. Analysis suggested that the pyrethroid-induced effects were associated with an increase in interneuronally mediated inhibition. Neither the perforant path axon or terminal nor the granule cell was affected by doses which appeared to affect interneurons. Basal excitability of the granule cells was also decreased by deltamethrin. This effect may have been secondary to an increase in tonic inhibition evoked by the same mechanisms responsible for the increase in phasic inhibition.

Prostaglandins and vasoactive amines in cerebral vasospasm after aneurysmal subarachnoid hemorrhage.

Five consecutively admitted patients with aneurysmal subarachnoid hemorrhage were treated with an indwelling lumbar spinal catheter. Daily samples of cerebrospinal fluid were analyzed for erythrocyte, protein, glucose, dopamine, epinephrine, serotonin, 5-hydroxyindoleacetic acid, tryptophan, histamine, thromboxane, 6-ketoprostaglandin F1 alpha, prostaglandin E, and prostaglandin F2 alpha concentrations. The patients' neurologic grade on admission, hospital course, presence of vasospasm, level of consciousness, computed tomographic and angiographic findings, and outcome were compared with the concentrations of the above substances in the cerebrospinal fluid. All patients had elevated concentrations of serotonin, with the highest levels found early in the hospital course of the patients who developed vasospasm. Tryptophan content increased markedly in association with clinical and angiographic vasospasm. Concentrations of prostaglandin F2 alpha correlated highly with development of and fluctuations in clinical vasospasm, with angiographic findings, with neurologic grade on admission, and with outcome. Our results suggest that prostaglandin F2 alpha may be involved in delayed clinical vasospasm in patients with subarachnoid hemorrhage.

Effects of lindane on calcium fluxes in synaptosomes.

The effect of lindane on 45Ca fluxes in synaptosomes was examined to determine whether modulation of calcium regulation can explain the lindane-induced enhancement in neurotransmitter release observed in other preparations. Lindane (0.03 - 4.6 microM) enhanced calcium uptake in non-depolarizing (low K+) medium: the effect was maximal at 0.1 microM lindane, which increased uptake by 15.8%. Lindane (11.5 microM) decreased calcium uptake in depolarizing (high K+) medium; lower concentrations were ineffective. In addition, lindane (9.2 microM) increased early 45Ca release from synaptosomes. However, lindane (2.4 or 5.9 microM) had no effect on calcium uptake by lysed synaptosomes in the presence or absence of mitochondrial poisons and/or ATP, indicating that lindane does not affect sub-cellular calcium storage. It is concluded that the lindane-induced changes in synaptosomal calcium flux are due to an increase in non-specific synaptic membrane permeability to calcium.

Convulsant-induced changes in perforant path-dentate gyrus excitability in urethane anesthetized rats.

The dentate gyrus of intact urethane-anesthetized rats was employed to evaluate the effects of different convulsive agents on granule cell excitability. One purpose of this study was to determine whether mechanisms of action suggested from in vitro studies of these compounds could be demonstrated over clinically relevant dose ranges in vivo. The data demonstrate that drugs capable of antagonizing gamma amino butyric acid (GABA)-mediated inhibition produced similar effects on the intact hippocampus. Bicuculline, picrotoxin, pentylenetetrazol and the insecticide lindane increased granule cell excitability to perforant path stimulation. The primary cause of this was an increase in the innate excitability of the granule cells. These compounds also modified field potentials in a manner consistent with a reduction in early GABA-mediated inhibition. They did not affect synaptically mediated facilitation. The magnitudes of the effects were dose dependent, and changes were clearly measurable at exposures that produce clinical effects in nonanesthetized rats. Other convulsants could be readily differentiated from those affecting GABA-mediated inhibition. Kainic acid depressed granule cell responses to perforant path stimulation and severely depressed both inhibition and facilitation. A decrease in granule cell responsiveness was also produced by exposure to beta carboline. Exposure to strychnine had little effect on granule cell excitability. These data indicate that for bicuculline, picrotoxin, pentylenetetrazol and lindane, an apparently selective antagonism of GABA-mediated inhibition is clearly demonstrable at exposures that also produce clinical intoxication.(ABSTRACT TRUNCATED AT 250 WORDS)

Episodic release of insulin by rat pancreas: effects of CNS and state of satiety.

This study reports that insulin is secreted in an episodic manner in rats and that the characteristics of its release can be modified by the central nervous system (CNS) and state of satiety. The pancreata of male Sprague-Dawley rats were perfused using the in situ brain-pancreas technique under urethan anesthesia. Episodic insulin release under non-fasted conditions was not altered by the presence or absence of CNS innervation to the pancreas. Under these conditions the interpeak period was 5.9 and 6 min, respectively, and cycle length was 3.7 and 4 min. However, perfusions that were performed following an overnight fast demonstrated that the CNS is capable of modulating episodic insulin release. After fasting, when comparing CNS-ablated with -intact preparations, the period was shortened from 5.2 to 4.1 min (P less than 0.05), and the number of episodes per 90-min perfusion increased from 16.0 to 19.0 (P less than 0.05) when the pancreas was innervated by the CNS. Additionally, the effect of fasting on denervated pancreata resulted in a shortening of the cycle length, which was prevented when the CNS was functional. These results demonstrate that episodic insulin release can be modified by metabolic conditions and are subject to mediation by the CNS.

Exposure to lindane and two other hexachlorocyclohexane isomers increases free intracellular calcium levels in neurohybridoma cells.

NCB-20 neurohybridoma cells were exposed to isomers of hexachlorocyclohexane (HCH) for periods of time ranging from 5-45 min, and intracellular free calcium [Ca++]i levels were determined using the INDO1/AM method. Exposure to lindane (gamma HCH) produced a dose-dependent increase in [Ca++]i, significant increases being observed with exposures from 10-400 microM. During exposure, [Ca++]i increased to a peak and then declined over the next 45 min. After 45 min levels were still elevated above control levels. This action of the gamma isomer of HCH was shared by the alpha and beta isomers. The potencies of the isomers were alpha (0.5), beta (0.75) and gamma (1.0). The effects of HCH isomers on [Ca++]i in neurohybridoma cells are similar to those reported for the isomers using rat brain synaptosomes. The ability of lindane to increase [Ca++]i may explain the previously reported ability of lindane to increase spontaneous and evoked (under low quantal release conditions) release of transmitter from frog neuromuscular junction. The relevance of this effect to mammalian CNS hyperexcitability, however, remains unsettled. These effects occur at concentrations of lindane higher than those effective in antagonizing GABA-mediated inhibition, and they lack the specificity shown by the isomers under other circumstances.

Increased inhibition in dentate gyrus granule cells following exposure to GABA-uptake blockers.

Rats anesthetized with urethane had stimulating and recording electrodes placed in the perforant pathway and in the dentate gyrus. They were then exposed to increasing doses of either the vehicle control dimethylsulfoxide (DMSO) or one of two gamma-aminobutyric acid (GABA)-uptake blockers (SKF-100330A or SKF-89976A). Analysis of evoked field potentials from dentate granule cells indicated that the only effect of the GABA uptake blockers was to increase the threshold for evoking the field population spikes (PS). No other measure of excitatory postsynaptic potentials (EPSPs) or PS's were significantly affected. The lack of effect on evoked EPSP by these drugs suggests no direct effect on transmitter release at this synapse, while the increase in PS threshold suggests a slight decrease in granule cell excitability. The effects of the two GABA-uptake blockers on synaptically mediated facilitation and inhibition was tested by using paired-pulse paradigms. Both GABA-uptake blockers increased early GABA-mediated inhibition to a greater extent than they reduced synaptically mediated facilitation. Neither GABA uptake blocker appeared to effect the late inhibition seen at paired-pulse intervals of 400-1000 ms which is presumably associated with calcium-activated increases in potassium conductance. These effects on granule cell responses occurred at doses found previously not to be associated with side effects and yet to be anticonvulsant in unanesthetized rats. These data confirm in vivo that SKF-100330A and SKF-89976A increase GABA-mediated inhibition. The effect on granule cell excitability and late inhibition are minimal. Although facilitation was reduced by exposure to these drugs, the mechanism of this reduction (direct or prolongation of early inhibition) cannot be determined.