Mechanism underlying bupivacaine inhibition of G protein-gated inwardly rectifying K+ channels

Local anesthetics, commonly used for treating cardiac arrhythmias, pain, and seizures, are best known for their inhibitory effects on voltage-gated Na(+) channels. Cardiovascular and central nervous system toxicity are unwanted side-effects from local anesthetics that cannot be attributed to the inhibition of only Na(+) channels. Here, we report that extracellular application of the membrane-permeant local anesthetic bupivacaine selectively inhibited G protein-gated inwardly rectifying K(+) channels (GIRK:Kir3) but not other families of inwardly rectifying K(+) channels (ROMK:Kir1 and IRK:Kir2). Bupivacaine inhibited GIRK channels within seconds of application, regardless of whether channels were activated through the muscarinic receptor or directly via coexpressed G protein G(beta)gamma subunits. Bupivacaine also inhibited alcohol-induced GIRK currents in the absence of functional pertussis toxin-sensitive G proteins. The mutated GIRK1 and GIRK2 (GIRK1/2) channels containing the high-affinity phosphatidylinositol 4,5-bisphosphate (PIP(2)) domain from IRK1, on the other hand, showed dramatically less inhibition with bupivacaine. Surprisingly, GIRK1/2 channels with high affinity for PIP(2) were inhibited by ethanol, like IRK1 channels. We propose that membrane-permeant local anesthetics inhibit GIRK channels by antagonizing the interaction of PIP(2) with the channel, which is essential for G(beta)gamma and ethanol activation of GIRK channels.

Ion selectivity filter regulates local anesthetic inhibition of G-protein-gated inwardly rectifying K+ channels

The weaver mutation (G156S) in G-protein-gated inwardly rectifying K+ (GIRK) channels alters ion selectivity and reveals sensitivity to inhibition by a charged local anesthetic, QX-314, applied extracellularly. In this paper, disrupting the ion selectivity in another GIRK channel, chimera I1G1(M), generates a GIRK channel that is also inhibited by extracellular local anesthetics. I1G1(M) is a chimera of IRK1 (G-protein-insensitive) and GIRK1 and contains the hydrophobic domains (M1-pore-loop-M2) of GIRK1 (G1(M)) with the N- and C-terminal domains of IRK1 (I1). The local anesthetic binding site in I1G1(M) is indistinguishable from that in GIRK2(wv) channels. Whereas chimera I1G1(M) loses K+ selectivity, although there are no mutations in the pore-loop complex, chimera I1G2(M), which contains the hydrophobic domain from GIRK2, exhibits normal K+ selectivity. Mutation of two amino acids that are unique in the pore-loop complex of GIRK1 (F137S and A143T) restores K+ selectivity and eliminates the inhibition by extracellular local anesthetics, suggesting that the pore-loop complex prevents QX-314 from reaching the intrapore site. Alanine mutations in the extracellular half of the M2 transmembrane domain alter QX-314 inhibition, indicating the M2 forms part of the intrapore binding site. Finally, the inhibition of G-protein-activated currents by intracellular QX-314 appears to be different from that observed in nonselective GIRK channels. The results suggest that inward rectifiers contain an intrapore-binding site for local anesthetic that is normally inaccessible from extracellular charged local anesthetics.

Defective gamma-aminobutyric acid type B receptor-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from weaver and Girk2 null mutant mice

Stimulation of inhibitory neurotransmitter receptors, such as gamma-aminobutyric acid type B (GABAB) receptors, activates G protein-gated inwardly rectifying K+ channels (GIRK) which, in turn, influence membrane excitability. Seizure activity has been reported in a Girk2 null mutant mouse lacking GIRK2 channels but showing normal cerebellar development as well as in the weaver mouse, which has mutated GIRK2 channels and shows abnormal development. To understand how the function of GIRK2 channels differs in these two mutant mice, we compared the G protein-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from Girk2 null mutant and weaver mutant mice with those from wild-type mice. Activation of GABAB receptors in wild-type granule cells induced an inwardly rectifying K+ current, which was sensitive to pertussis toxin and inhibited by external Ba2+ ions. The amplitude of the GABAB receptor-activated current was severely attenuated in granule cells isolated from both weaver and Girk2 null mutant mice. By contrast, the G protein-gated inwardly rectifying current and possibly the agonist-independent basal current appeared to be less selective for K+ ions in weaver but not Girk2 null mutant granule cells. Our results support the hypothesis that a nonselective current leads to the weaver phenotype. The loss of GABAB receptor-activated GIRK current appears coincident with the absence of GIRK2 channel protein and the reduction of GIRK1 channel protein in the Girk2 null mutant mouse, suggesting that GABAB receptors couple to heteromultimers composed of GIRK1 and GIRK2 channel subunits.

Reopening of single L-type Ca2+ channels in mouse cerebellar granule cells: dependence on voltage and ion concentration

1. We recorded the activity of single L-type Ca2+ channels from cell-attached patches on mouse cerebellar granule cells. The experiments investigated the mechanism of channel reopening at negative membrane potentials following a strong depolarization. 2. L-type channels that reopened following a strong depolarization showed a wide distribution of single-channel conductances, which ranged from 16 to 28 pS in the presence of 90 mM Ba2+. 3. The distribution of the latencies before reopening was fitted as the sum of two exponential components with time constants tau f approximately 1 and tau s approximately 12 ms at -70 mV. Hyperpolarization reduced the time constant of the slower component approximately e-fold per 43 mV, but had no effect on the faster component. 4. Raising the concentration of external Ba2+ reduced the time constant of the slower component of the reopening latency without altering the fast component. The time constant of the slow component was approximately 27 ms in 10 mM Ba2+ and decreased to 12 ms in 90 mM Ba2+ at -70 mV. The relation between the time constant and external Ba2+ saturated with an apparent KD of approximately 20 mM. 5. The distribution of reopening times was best fitted as the sum of two exponential components with time constants tau f approximately 0.5 ms and tau s approximately 4.5 ms at -70 mV. The conditional latencies before reopening into either the short or long open state were indistinguishable. 6. The results are consistent with the idea that a positively charged blocker occludes the pore during depolarization and channels reopen as the blocker dissociates following repolarization to negative potentials.

Functional effects of the mouse weaver mutation on G protein-gated inwardly rectifying K+ channels

The weaver mutation corresponds to a substitution of glycine to serine in the H5 region of a G protein-gated inwardly rectifying K+ channel gene (GIRK2). By studying mutant GIRK2 weaver homomultimeric channels and heteromultimeric channels comprised of GIRK2 weaver and GIRK1 in Xenopus oocytes, we found that GIRK2 weaver homomultimeric channels lose their selectivity for K+ ions, giving rise to inappropriate receptor-activated and basally active Na+ currents, whereas heteromultimers of GIRK2 weaver and GIRK1 appeared to have reduced current. Immunohistochemical localization indicates that GIRK2 and GIRK1 proteins are expressed in the cerebellar neurons of mice at postnatal day 4, at a time when these neurons normally undergo differentiation. Thus, the aberrant behavior of mutant GIRK2 weaver channels could affect the development of weaver mice in at least two distinct ways.

Identification of structural elements involved in G protein gating of the GIRK1 potassium channel

Chimeras of GIRK1 and IRK1, a G protein-insensitive inward rectifier, are activated by coexpression of G beta gamma if they contain either the N-terminal or part of the C-terminal hydrophilic domain of GIRK1. The N-terminal domain of GIRK1 also facilitates the fast rates of activation and deactivation following m2 muscarinic receptor stimulation. The hydrophobic core of GIRK1 (M1-H5-M2) is important for determining the brief single-channel open times typical of GIRK1 but not important for determining G beta gamma sensitivity. Coexpression with CIR revealed that the gating properties associated with different GIRK1 domains could not have arisen from altered ability to form heteromultimers. These results implicate specific regions of GIRK1 in G protein activation and suggest that GIRK1 may be closely linked to the m2 muscarinic receptor-G protein complex.

Evidence that direct binding of G beta gamma to the GIRK1 G protein-gated inwardly rectifying K+ channel is important for channel activation

Activation of G protein-gated K+ channels by G protein-coupled receptors contributes to parasympathetic regulation of heart rate in the atrium and inhibitory postsynaptic potentials in the peripheral and central nervous system. Having found that G beta gamma activates the cloned GIRK1 channel, we now report evidence for direct binding of G beta gamma to both the N-terminal hydrophilic domain and amino acids 273-462 of the C-terminal domain of GIRK1. These direct interactions are physiologically important because synthetic peptides derived from either domain reduce the G beta gamma binding as well as the G beta gamma activation of the channel. Moreover, the N-terminal domain may also bind trimeric G alpha beta gamma, raising the possibility that physical association of G protein-coupled receptors, G proteins, and K+ channels partially accounts for their compartmentalization and hence rapid and specific channel activation by receptors.

Activation of the cloned muscarinic potassium channel by G protein beta gamma subunits

Acetylcholine released during parasympathetic stimulation of the vagal nerve slows the heart rate through the activation of muscarinic receptors and subsequent opening of an inwardly rectifying potassium channel. The activation of these muscarinic potassium channels is mediated by a pertussis toxin-sensitive heterotrimeric GTP-binding protein (G protein). It has not been resolved whether exogenously applied G alpha or G beta gamma, or both, activate the channel. Using a heterologous expression system, we have tested the ability of different G protein subunits to activate the cloned muscarinic potassium channel, GIRK1. We report here that coexpression of GIRK1 with G beta gamma but not G alpha beta gamma in Xenopus oocytes results in channel activity that persists in the absence of cytoplasmic GTP. This activity is reduced by fusion proteins of the beta-adrenergic receptor kinase and of recombinant G alpha i-GDP, both of which are known to interact with G beta gamma. Moreover, application of recombinant G beta gamma, but not G alpha i-GTP-gamma S, activates GIRK1 channels. Thus G beta gamma appears to be sufficient for the activation of GIRK1 muscarinic potassium channels.

The S4-S5 loop contributes to the ion-selective pore of potassium channels

Mutagenesis experiments on voltage-gated K+ channels have suggested that the ion-selective pore is comprised mostly of H5 segments. To see whether regions outside of the H5 segment might also contribute to the pore structure, we have studied the effect of single amino acid substitutions in the segment that connects the S4 and S5 putative transmembrane segments (S4-S5 loop) on various permeation properties of Shaker K+ channels. Mutations in the S4-S5 loop alter the Rb+ selectivity, the single-channel K+ and Rb+ conductances, and the sensitivity to open channel block produced by intracellular tetraethylammonium ion, Ba2+, and Mg2+. The block of Shaker K+ channels by intracellular Mg2+ is surprising, but is reminiscent of the internal Mg2+ blockade of inward rectifier K+ channels. The results suggest that the S4-S5 loop constitutes part of the ion-selective pore. Thus, the S4-S5 loop and the H5 segment are likely to contribute to the long pore characteristic of voltage-gated K+ channels.

Primary structure and functional expression of a rat G-protein-coupled muscarinic potassium channel

Parasympathetic nerve stimulation causes slowing of the heart rate by activation of muscarinic receptors and the subsequent opening of muscarinic K+ channels in the sinoatrial node and atrium. This inwardly rectifying K+ channel is coupled directly with G protein. Based on sequence homology with cloned inwardly rectifying K+ channels, ROMK1 (ref. 11) and IRK1 (ref. 12), we have isolated a complementary DNA for a G-protein-coupled inwardly rectifying K+ channel (GIRK1) from rat heart. The GIRK1 channel probably corresponds to the muscarinic K+ channel because (1) its functional properties resemble those of the atrial muscarinic K+ channel and (2) its messenger RNA is much more abundant in the atrium than in the ventricle. In addition, GIRK1 mRNA is expressed not only in the heart but also in the brain.

Dihydropyridine- and omega-conotoxin-sensitive Ca2+ currents in cerebellar neurons: persistent block of L-type channels by a pertussis toxin-sensitive G-protein

The inhibition of high-threshold Ca2+ channel currents by activated G-proteins was studied in mouse cerebellar granule cells making use of the hydrolysis-resistant GTP analog GTP-gamma-S. When individual granule cells were internally dialyzed with GTP-gamma-S, the high-threshold Ca2+ current decreased to approximately 20% of its initial value within approximately 2 min. The GTP-gamma-S-resistant current was reduced further by the subsequent addition of either omega-conotoxin or dihydropyridine antagonist, indicating that both N- and L-type Ca2+ channels carried the remaining current. Continuous exposure to the dihydropyridine agonist +(S)-202-791 caused a rapid increase in the GTP-gamma-S-resistant current. The L-type current evoked by the agonist subsequently decreased to the level observed prior to adding the drug following a time course similar to the initial inhibition of the total high-threshold current. A second application of the drug at a later time failed to increase the current a second time, indicating a persistent blockade of the agonist-evoked L-current. Pretreating cells with pertussis toxin prevented the initial inhibition of the total whole-cell Ca2+ channel current as well as the subsequent inhibition of the agonist-evoked L-current. The results show that a pertussis toxin-sensitive G-protein produces a persistent inhibition of L-type Ca2+ channels in these central neurons.

Reopening of Ca2+ channels in mouse cerebellar neurons at resting membrane potentials during recovery from inactivation

Recordings of single-channel activity from cerebellar granule cells show that a component of Ca2+ entry flows through L-type Ca2+ channels that are closed at negative membrane potentials following a strong depolarization, but then open after a delay. The delayed openings can be explained if membrane depolarization drives Ca2+ channels into an inactivated state and some channels return to rest through the open state after repolarization. Whole-cell recordings show that the charge carried by Ca2+ during the tail increases as inactivation progresses, whereas the current during the voltage step decreases. Voltage-dependent inactivation may be a general mechanism in central neurons for enhancing Ca2+ entry by delaying it until after repolarization, when the driving force for ion entry is large. Modifying the rate and extent of inactivation would have large effects on Ca2+ entry through those channels that recover from inactivation by passing through the open state.

Inactivating and non-inactivating dihydropyridine-sensitive Ca2+ channels in mouse cerebellar granule cells

1. Granule cells were dissociated from mouse cerebellum and grown in vitro. Currents through single Ca2+ channels were recorded from the cell body with the patch clamp technique. 2. Voltage steps to 0 mV produced brief channel openings with a mean open time of approximately 0.5 ms. The single-channel conductance measured from the amplitude of the single-channel current with 90 mM-Ba2+ in the patch electrode was 22 pS. 3. The probability of Ca2+ channel opening increased with test potentials more positive than -30 mV, with half-activation near 0 mV, and followed the Boltzmann relation for the activation of whole-cell Ca2+ current. 4. Voltage steps to potentials more positive than 0 mV produced more channel activity at the beginning than at the end of the voltage step. The average of the single-channel currents decayed to a non-zero level with a time course similar to that of the whole-cell Ca2+ current. 5. The amplitude as well as the decay of the mean current measured during a test pulse to 0 mV was reduced as the holding potential was made more positive than approximately -90 mV. The change in the open channel probability with holding potential followed the Boltzmann relation which described the inactivation of the whole-cell Ca2+ current. 6. Ca2+ channel activity persisted for over several minutes after excising the patch from the cell body when intracellular cyclic AMP was increased. After patch excision, the number of functional channels decreased to a level where only one channel at a time was active. Ca2+ channel openings appeared as either short bursts at the beginning of the voltage step or long bursts that lasted throughout the pulse. 7. Exposing the cell to the dihydropyridine agonist +(S)-202-791 markedly increased the fraction of sweeps with long openings and produced a non-decaying mean current that was approximately 5 times larger than control. In a fraction of the sweeps, however, long openings occurred more frequently at the beginning than at the end of the voltage step and these produced a decaying mean current. 8. Shifting the holding potential to more positive potentials in the presence of the dihydropyridine agonist preferentially reduced the number of brief openings while sparing the long openings. The amplitude of the mean current was similar to that obtained from the more negative holding potential and there was no change in the fraction of sweeps with long openings that occurred at the beginning of the voltage pulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation of calcium currents in granule cells cultured from mouse cerebellum

1. Cells dissociated from mouse cerebellum were grown in vitro. Ca2+ channel currents were recorded from granule cells with the patch-clamp technique under conditions which suppressed currents through Na+ and K+ channels and minimized run-down of current through Ca2+ channels. 2. A strong depolarizing voltage step from a hyperpolarized holding potential produced inward Ca2+ channel current that decayed exponentially to a non-zero level. Inward current decayed to approximately 40% of its peak amplitude (range 20-90%). 3. The inward current increased in amplitude when Ca2+ was replaced with Ba2+ or after raising the concentration of extracellular Ba2+, but the rate of decay of current was unaffected. 4. The current-voltage (I-V) relation showed that peak or sustained current increased with voltage pulses more positive than approximately -30 mV, reached a maximum amplitude near +20 mV and became progressively smaller with larger depolarizations. 5. The tail currents produced after rapidly repolarizing the membrane potential to -70 mV from a positive test pulse decayed along a single exponential time course with a time constant of approximately 0.5 ms. The amplitude of tail current measured at a fixed repolarization potential increased as the pre-pulse was made more positive and reached a maximum with pre-pulses more positive than +40 mV. A plot of normalized amplitude of the tail current as a function of the pre-pulse potential was fitted with a Boltzmann relation with V1/2 = approximately + 8 mV and steepness k = 14 mV. 6. Shifting the holding potential to more positive potentials reduced the amplitude of the Ca2+ channel current elicited by the fixed voltage step and abolished the decay of the inward current. The peak current was normalized to the maximum peak current elicited from a very negative holding potential and plotted as a function of holding potential. The points were fitted with a Boltzmann relation for inactivation with V1/2 = approximately -57 mV and steepness k = 14 mV. 7. The onset of inactivation was studied in two-pulse experiments in which the duration of conditioning pre-pulse was varied. Increasing the duration of a pre-pulse to a fixed potential reduced the peak inward current evoked by the second test pulse. Plotting normalized current as a function of pre-pulse duration showed that inactivation developed along a double exponential time course. Both fast and slow time constants decreased as the pre-pulse potential was made more positive.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartment-specific changes in the density of choline and dopamine uptake sites and muscarinic and dopaminergic receptors during the development of the baboon striatum: a quantitative receptor autoradiographic study

In the fetal and young primate neostriatum, cholinergic and dopaminergic markers show patches of high density surrounded by a lower-density matrix. In the adult, the same markers display the opposite pattern, a lower density in striosomes, surrounded by a higher-density matrix. In order to understand the developmental sequences leading to the adult compartmental organization of the primate neostriatum, a quantitative technique was used to study the ontogeny of pre- and postsynaptic components of cholinergic and dopaminergic neurons in baboon caudate nucleus and putamen. The development of specific uptake mechanisms for choline and dopamine and receptors was studied by means of quantitative autoradiography of the specific binding of [3H]-hemicholinium-3 [( 3H]-HC3) and [3H]-mazindol [( 3H]-MAZ) to the choline and dopamine uptake systems, respectively. [3H]-pirenzepine [( 3H]-PIR) was used to label M1 muscarinic receptors and [3H]-spiroperidol [( 3H]-SPI) was used to label striatal dopamine D2 receptors. Serial sections were used for each ligand to determine the precise anatomical relationships between the binding patterns of the different markers. Our aim was to determine whether the adult striosomal distribution of the binding sites studied was due to 1) a selective decrease in patch/striosomal binding density or 2) a selective increase in matrix binding density. Our studies show that a postnatal decrease in the density of [3H]-HC3 sites in the patch/striosomes and an increase in the matrix density of [3H]-MAZ sites are the primary, but not the sole, changes in the compartmental distribution of these sites leading to the adult striosomal organization of the striatal cholinergic and dopaminergic innervation. D2 receptors follow the general developmental pattern of [3H]-MAZ and [3H]-HC3, changing their density of distribution in both compartments during the developmental period examined. In addition, M1 muscarinic receptors already display their adult pattern in the newborn baboon striatum, and therefore represent one of the first neurochemical makers to adopt its mature organization.

Effects of anticonvulsants on cholinergic and GABAergic properties in the neuronal cell clone NG108-15

The effects of anticonvulsant drugs on growth, cholinergic, and GABAergic properties were examined in the neuronal cell clone NG108-15. Cells were exposed for 4 days to valproic acid, phenobarbital, phenytoin, or carbamazepine in concentrations equivalent to therapeutic free levels in human serum. Experiments were also performed with varying concentrations of a recently proposed antiepileptic, gamma-vinyl GABA. Of these five anticonvulsants, cell growth (total protein and cell counts) was decreased with valproic acid and phenytoin but only valproic acid and gamma-vinyl GABA altered neurotransmitter markers. Therapeutic concentrations of valproic acid increased choline acetyltransferase activity to 142% of control but had no effect on either the activity of glutamate decarboxylase or the level of GABA. The effects of a higher (toxic) concentration of valproic acid (200 micrograms/ml) were similar to those induced by the differentiating agent dibutyryl cyclic AMP: both decreased cell growth, enhanced the activity of choline acetyltransferase and reduced the activity of glutamate decarboxylase. Gamma-vinyl GABA had no effect on cholinergic markers but, at 1300 micrograms/ml, increased GABA levels to 135% of control despite the reduction of glutamate decarboxylase to 68% of control. In the NG108-15 cell clone, anticonvulsants have varying effects on cell growth, differentiation, and neurotransmitter systems. Our findings do not support the proposal that the mechanism of action for valproic acid, phenobarbital, phenytoin, and carbamazepine is via alteration of GABA levels.

Development of beta 1 and beta 2 adrenergic receptors in baboon brain: an autoradiographic study using [125I]iodocyanopindolol

[125I]iodocyanopindolol (ICYP) autoradiography was used to investigate the temporal development and distribution of beta 1 and beta 2 receptors in brains of baboons at ages embryonic day 100 (E100), full-term gestation (El80), and 3 years. In all brain regions examined, with the exception of the hippocampus, binding to beta 1 receptors exceeded that to beta 2 receptors. The highest densities of beta 1 receptors were found in the caudate nucleus, putamen, globus pallidus, substantia nigra, and cerebral cortex; intermediate receptor densities were observed in most nuclei of thalamus, and the lowest concentrations were in the hippocampus. At E100, beta receptors were identified in the striatum, globus pallidus, and thalamus. During maturation, the number of beta 1 receptors declined in cortical areas but increased in the head of the caudate and putamen. Significant differences in the developmental distribution of beta receptors during development were also detected: at E100 and E180 beta 1 receptors appeared as patches in the caudate and putamen, but by 3 years of age they were more homogeneously distributed in both regions; changes also occurred in the distribution of binding within cortical layers. Autoradiograms of [125I]ICYP and [3H]mazindol binding show overlapping patches of labeling in the E180 striatum, suggesting a possible developmental association between beta receptors and dopamine high-affinity uptake carrier sites. This study demonstrates that noradrenergic receptors in the primate forebrain undergo significant developmental reorganization with regional variations.

An autoradiographic study of the development of [3H]hemicholinium-3 binding sites in human and baboon basal ganglia: a marker for the sodium-dependent high affinity choline uptake system

The developmental distribution of the sodium-dependent high affinity choline uptake (SDHACU) system has been studied in the caudate and putamen of the baboon and of the human by in vitro autoradiography with the ligand, [3H]hemicholinium-3 [( 3H]HCh3). Our results show that [3H]HCh3 binding sites in the newborn baboon and fetal human neostriatum are localized to patches, and then adopt a 'striosome-like' distribution in the juvenile baboon brain. These findings indicate a reorganization of [3H]HCh3 binding site distribution during the ontogeny of the primate neostriatum.

Inactivation of GM1-ganglioside beta-galactosidase by a specific inhibitor: a model for ganglioside storage disease

This study was designed to establish an in vitro model with biochemical and morphological similarities to the human neurodegenerative disease GM1 gangliosidosis. Utilizing a specific inactivator of the lysosomal enzyme GM1-ganglioside beta-galactosidase (beta-D-galactopyranosylmethyl-p-nitrophenyltriazene [beta-GalMNT]) and neuroblastoma X glioma hybrid cells (NG108-15), we suppressed beta-galactosidase activity for up to 72 hours. Coincidental with suppression of this enzyme to levels less than 1% of control, we found up to a nine-fold accumulation of its substrate, the GM1-ganglioside, and the ultrastructural appearance of membranous cytoplasmic bodies. beta-GalMNT treatment suppressed growth but had little effect on the specific activity of choline acetyltransferase, lactate dehydrogenase, or other lysosomal enzymes including galactosylceramidase. This model should permit studies of the neurophysiological effects of increased ganglioside accumulation and their reversibility.

Effects of anticonvulsants on cell growth and enzymatic and receptor binding activity in a neuroblastoma x glioma hybrid cell culture

The effects of anticonvulsants on markers of growth, intracellular enzymes, and synaptic functions were evaluated using a rapidly dividing cholinergic neuroblastoma x glioma hybrid cell-line (NG108-15). Cell cultures were exposed for 4 days to phenobarbital, phenytoin, carbamazepine, or valproic acid. Anticonvulsant concentrations added to the media were selected to produce free levels in the cell media that were equivalent to free levels in humans ranging from therapeutic to very toxic. Free levels of anticonvulsants in the toxic range affected cell number, protein content, and neurochemical markers. However, only valproic acid and phenytoin reduced cell growth at therapeutic free drug concentrations. Valproic acid was the only medication to act as a differentiating agent, significantly increasing the activity of choline acetyltransferase, beta-galactosidase, and muscarinic cholinergic receptor binding. These results emphasize the importance of performing drug studies at appropriate free drug concentrations and suggest that valproic acid differs from other commonly prescribed anticonvulsants by having both a growth-suppressing and a differentiating effect.