Modulation of Kv1.5 potassium channel gating by extracellular zinc.

Zinc ions are known to induce a variable depolarizing shift of the ionic current half-activation potential and substantially slow the activation kinetics of most K(+) channels. In Kv1.5, Zn(2+) also reduces ionic current, and this is relieved by increasing the external K(+) or Cs(+) concentration. Here we have investigated the actions of Zn(2+) on the gating currents of Kv1.5 channels expressed in HEK cells. Zn(2+) shifted the midpoint of the charge-voltage (Q-V) curve substantially more (approximately 2 times) than it shifted the V(1/2) of the g-V curve, and this amounted to +60 mV at 1 mM Zn(2+). Both Q1 and Q2 activation charge components were similarly affected by Zn(2+), which indicated free access of Zn(2+) to channel closed states. The maximal charge movement was also reduced by 1 mM Zn(2+) by approximately 15%, from 1.6 +/- 0.5 to 1.4 +/- 0.47 pC (n = 4). Addition of external K(+) or Cs(+), which relieved the Zn(2+)-induced ionic current reduction, decreased the extent of the Zn(2+)-induced Q-V shift. In 135 mM extracellular Cs(+), 200 microM Zn(2+) reduced ionic current by only 8 +/- 1%, compared with 71% reduction in 0 mM extracellular Cs(+), and caused a comparable shift in both the g-V and Q-V relations (17.9 +/- 0.6 mV vs. 20.8 +/- 2.1 mV, n = 6). Our results confirm the presence of two independent binding sites involved in the Zn(2+) actions. Whereas binding to one site accounts for reduction of current and binding to the other site accounts for the gating shift in ionic current recordings, both sites contribute to the Zn(2+)-induced Q-V shift.

Eicosatetraynoic acid (ETYA), a non-metabolizable analogue of arachidonic acid, blocks the fast-inactivating potassium current of rat pituitary melanotrophs.

The effects of arachidonic acid (5,8,11,14-eicosatetraenoic acid, AA) and 5,8,11,14-eicosatetraynoic acid (ETYA), a non-metabolizable analogue of AA, were examined on the transient [I(K)(f)] and the delayed rectifier-like [I(K)(S)] voltage-gated potassium currents in rat pituitary melanotrophs. The main questions addressed were whether AA and ETYA blocked I(K)(f) and if any blocking action was specific. Macroscopic currents were measured using the patch clamp technique. Bath application of 20 microM AA reduced I(K)(f), however, the degree of the block varied between cells. In contrast, ETYA consistently inhibited I(K)(f). Fitting of the charge transfer or the peak current amplitude yielded KD estimates for ETYA of 1.2 microM and 3.3 microM, respectively. The reduction by ETYA of peak I(K)(f) was always associated with an increased rate of current decay, but there was no detectable change of the kinetics of activation. ETYA caused a small left shift of the I(K)(f) steady-state inactivation curve and significantly slowed recovery from inactivation. At 20 microM, ETYA also reduced I(K)(s), indicating that it is not specific. The possibility that ETYA acts as an open-channel blocker is discussed.

External K(+) relieves the block but not the gating shift caused by Zn(2+) in human Kv1.5 potassium channels.

1. We used the whole-cell recording technique to examine the effect of extracellular Zn(2+) on macroscopic currents due to Kv1.5 channels expressed in the human embryonic kidney cell line HEK293. 2. Fits of a Boltzmann function to tail current amplitudes showed that 1 mM Zn2+ shifted the half-activation voltage from -10.2 +/- 0.4 to 21.1 +/- 0.7 mV and the slope factor increased from 6.8 +/- 0.4 to 9.4 +/- 0.7 mV. The maximum conductance in 1 mM Zn2+ and with 3.5 mM K(+)o was 33 +/- 7 % of the control value. 3. In physiological saline the apparent KD for the Zn(2+) block was 650 +/- 24 M and was voltage independent. A Hill coefficient of 1.0 +/- 0.03 implied that block is mediated by the occupation of a single binding site. 4. Increasing the external concentration of K(+) ([K(+)]o) inhibited the block by Zn(2+). Estimates of the apparent K(D) of the Zn(2+) block in 0, 5 and 135 mM K(+) were 69, 650 and 2100 M, respectively. External Cs(+) relieved the Zn(2+) block but was less effective than K(+). Changing [K(+)]o did not affect the Zn(2+)-induced gating shift. 5. A model of allosteric inhibition fitted to the relationship between the block by Zn(2+) and the block relief by external K(+) gave KD estimates of approximately 70 M for Zn(2+) and approximately 500 M for K(+). 6. We propose that the gating shift and the block caused by Zn(2+) are mediated by two distinct sites and that the blocking site is located in the external mouth of the pore.

Block of BK (maxi K) channels of rat pituitary melanotrophs by Na+ and other alkali metal ions.

The block of large-conductance calcium-activated potassium (BK) channels by internal and external alkali metal ions was studied in adult rat melanotrophs. Internal but not external 20 mM Na+ produced a strongly voltage-dependent, flickery block that was well-fitted to the Woodhull model by using a value of 140 mM for the dissociation rate constant at 0 mV [Kd(0)] and an equivalent valence (zdelta) of 0.9. At a concentration of 20 mM external K+, Cs+ and Rb+, but not Li+, caused a rightward shift of the voltage dependence of the intracellular Na+ (Na+i ) block. This effect of K+, Cs+ and Rb+ was modelled by an equilibrium knock-out mechanism in which the block-relieving ion binds to a site located within the voltage field and consequently increases the off-rate of Na+. Internal Li+ caused little or no block whereas internal Cs+ caused a voltage-dependent block [Kd(0) approximately 150 mM]. Flickery channel block observed in cell-attached patches was consistent with a cytoplasmic Na+ activity between 1 and 10 mM.

Large-conductance calcium-activated potassium channels of cultured rat melanotrophs.

A large conductance, Ca(2+)-activated K+ channel of the BK type was examined in cultured pituitary melanotrophs obtained from adult male rats. In cell-attached recordings the slope conductance for the BK channel was approximately 190 pS and the probability (Po) of finding the channel in the open state at the resting membrane potential was low (< < 0.1). Channels in inside-out patches and in symmetrical 150 mM K+ had a conductance of approximately 260 pS. The lower conductance in the cell-attached recordings is provisionally attributed to an intracellular K+ concentration of approximately 113 mM. The permeability sequence, relative to K+, was K+ > Rb+ (0.87) > NH4+ (0.17) > Cs+ > or = Na+ (< or = 0.02). The slope conductance for Rb+ was much less than for K+. Neither Na+ nor Cs+ carried measurable currents and 150 mM internal Cs+ caused a flickery block of the channel. Internal tetraethylammonium ions (TEA+) produced a fast block for which the dissociation constant at 0 mV (KD(0 mV)) was 50 mM. The KD(0 mV) for external TEA+ was much lower, 0.25 mM, and the blocking reaction was slower as evidenced by flickery open channel currents. With both internal and external TEA+ the blocking reaction was bimolecular and weakly voltage dependent. External charybdotoxin (40 nM) caused a large and reversible decrease of Po. The Po was increased by depolarization and/or by increasing the concentration of internal Ca2+. In 0.1 microM Ca2+ the half-maximal Po occurred at approximately 100 mV; increasing Ca2+ to 1 microM shifted the voltage for the half-maximal Po to -75 mV. The Ca2+ dependence of the gating was approximated by a fourth power relationship suggesting the presence of four Ca2+ binding sites on the BK channel.

Changes of activation and inactivation gating of the transient potassium current of rat pituitary melanotrophs caused by micromolar Cd2+ and Zn2+.

We studied the effects of Zn2+ and Cd2+ on the behaviour of IK(f), a transient outward potassium current in acutely dissociated melanotrophs of the pars intermedia of the rat pituitary gland. Micromolar concentrations of external Cd2+ or Zn2+ caused parallel and nearly equal rightward shifts along the voltage axis of the activation and steady-state inactivation curves for IK(f). The KD for the half-maximal shift of the activation curve was 278 microM for Cd2+ and 93 microM for Zn2+; the maximal shifts of the activation curve were 32.5 and 34 mV, for Cd2+ and Zn2+, respectively. The times to half-activation and half-inactivation were shifted rightward by 30-60 mV in both 500 microM Cd2+ and 500 microM Zn2+. We suggest that Cd2+ and Zn2+ interact specifically with a binding site on or electrically close to the IK(f) channel and in so doing modify the electric field "seen" by the voltage sensors.

Block by capsaicin of voltage-gated K+ currents in melanotrophs of the rat pituitary.

1. Whole-cell recordings of macroscopic K+ currents were made from acutely dissociated and cultured melanotrophs isolated from the pars intermedia of the adult rat pituitary. 2. In acutely dissociated cells, external capsaicin reversibly decreased the amplitude both of the fast-activating, fast-inactivating potassium current IK(f) and the slowly-activating, slowly-inactivating potassium current IK(s). To simplify the investigation of the mechanism of action of capsaicin experiments were conducted on cultured melanotrophs that express only IK(s). 3. In control cells the activation rate and the amplitude of IK(s) increased with depolarization and the current showed very little inactivation at any voltage during pulses lasting for 100-300 ms. In capsaicin, the decrease of the current amplitude was associated with an increased rate of current decay ('inactivation'). At a given voltage, the extent and the rate of the capsaicin-induced inactivation was proportional to the capsaicin concentration; and, at a given concentration, the extent and rate of the inactivation increased with membrane depolarization. 4. The fit of the Hill equation to data derived from the steady-state block of IK(s) evoked at 10 mV indicated an equilibrium dissociation constant (KD) of 17.4 microM (95% confidence limits 15.8-19.0) and a Hill coefficient of 1.8 (95% Cl 1.5-2.2) suggesting that at least two molecules of capsaicin must bind to the channel to block it. 5. Analysis of the voltage-dependence of the steady-state block in 100 microM capsaicin showed that half-maximal block occurred at -29 +/- 2 mV (n = 10). Two-pulse experiments designed to study the time-dependence of channel block in 100 MicroM capsaicin indicated that the blocking kinetics were well fitted by a single exponential and that the rate of block increased with depolarization. The value for Tblock at 0mV was 24 +/-7ms (n=4).6. Recovery from block in 100 MicroM capsaicin was also well fitted by a single exponential. The recovery time constant ( was 708 +/- 140 ms at - 50 mV, 70 +/- 6 ms at - 70 mV and 19 +/- 1.3 ms at-90 mV (n = 4).7. In 50-100 MicroM capsaicin, the decay of the tail current was biexponential, the values for fast and Tslow being, respectively, less than and greater than the single time constant fitted to the control tail current.Inward and outward K+ currents were equally affected by capsaicin.8. Most of these effects of capsaicin on the IK(S) of melanotrophs can be accounted for by a kinetic scheme in which capsaicin binds to and blocks open K+ channels.

Voltage-clamp analysis of the voltage-gated sodium current of the rat pituitary melanotroph.

By using the whole-cell recording technique the Na+ current in cultured melanotrophs of the adult rat pituitary was studied. The Na+ current was eliminated by 1 microM TTX and its equilibrium potential confirmed that it was carried predominantly by Na+. The activation threshold was near -40 mV and half-maximal activation occurred at approximately -23 mV. The peak amplitude of 640 +/- 110 pA (n = 8) occurred near -10 mV. Steady-state half-inactivation occurred near -50 mV. Recovery from inactivation at -70 mV was biexponential: approximately half of the channels recovered with a time constant of 13 ms whereas the slower phase of recovery had a time constant of 430 ms. These properties of the Na+ current are discussed in relation to its role in cell firing and hormone secretion.

Quinidine-induced inhibition of the fast transient outward K+ current in rat melanotrophs.

1. The effect of quinidine on the fast-activating, fast-inactivating potassium current (IK(f] in acutely dissociated melanotrophs of the adult rat pituitary was examined. Macroscopic currents were measured by use of the whole-cell configuration of the patch clamp technique. 2. Bath application of quinidine caused a dose-dependent reduction of the peak amplitude of IK(f). The Kd for blockade of IK(f) at 0 mV was estimated to be 41 +/- 5.6 microM. 3. Quinidine elicited a dose-dependent increase of the rate of the decay of IK(f) and this effect was enhanced by membrane depolarization. The possibility that this phenomenon reflects an open channel blocking reaction is discussed. 4. Quinidine also caused a 5 mV hyperpolarizing shift of the steady-state inactivation curve and increased the half-time for recovery from inactivation. Quinidine did not affect the onset of inactivation measured at -30 mV. 5. Internal quinidine did not appear substantially to affect either the peak amplitude or kinetics of IK(f). 6. A study of some structural analogues showed that hydroquinidine and quinacrine had effects similar to those of quinidine. The effect of quinacrine on the amplitude and kinetics of IK(f) was also pH-dependent. Cinchonine, which bears a close structural resemblance to quinidine, was much less effective as a blocker of IK(f).

Catechol blocks the fast outward potassium current in melanotrophs of the rat pituitary.

The effect of catechol on the fast voltage-gated K+ current (IK(f)) of acutely dissociated rat melanotrophs was investigated in whole-cell recordings. Half-maximal inhibition of IK(f) occurred at an external concentration of 1.7 mM and this effect was associated with a decrease of the rate of the current decay. Internal catechol had no measurable effect on IK(f). Catechol appeared to be equally effective as a blocker of the slow voltage-gated K+ current (IK(s)). Despite this lack of specificity the blocking action of catechol was voltage- and frequency-independent and was rapidly reversible. Catechol therefore represents a useful alternative to 4-aminopyridine as a blocker of IK(f).

4-Aminopyridine causes a voltage-dependent block of the transient outward K+ current in rat melanotrophs.

1. Whole-cell voltage-clamp recordings were made from acutely dissociated melanotrophs obtained from adult rats. 2. In the presence of external Na+ and Ca2+ channel blockers and 20 mM-tetraethylammonium (TEA) depolarizations to -40 mV or more evoked a fast-activating fast-inactivating outward K+ current (IK(f)). Double-pulse experiments showed that steady-state half-inactivation occurred near -37 mV; half-maximal activation of IK(f) occurred at -15 mV. Recovery from inactivation in most cells fitted a single exponential with a time constant of 40-50 ms. 3. When applied either internally or externally, 1-2.5 mM-4-aminopyridine (4-AP) substantially reduced IK(f) but the degree of block was affected by the intensity, duration and frequency of depolarizing commands. 4. Analysis of the steady-state voltage dependence of the block by 4-AP showed that half-maximal blocking occurred at approximately -31 mV. This implied that 4-AP binds to the resting state of the IK(f) channel. 5. Studies of the time dependence for the blocking or unblocking of IK(f) showed that both processes were exponential with mean time constants of 1942 ms (at -70 mV) and 726 ms (at 20 mV), respectively. Recovery from inactivation was apparently unaffected by 4-AP. 6. A four-state sequential model in which 4-AP reversibly binds to the resting state of the channel replicates the frequency dependence of the 4-AP blockade.

Cultured melanotrophs of the adult rat pituitary possess a voltage-activated fast transient outward current.

1. Whole-cell voltage-clamp recordings were made from cultured melanotrophs obtained from adult rats and maintained in vitro using conventional cell culture procedures. 2. The outward current recorded in the presence of Na+ and Ca2+ channel blockers was normally comprised of two components: a slowly activating, slowly inactivating current (IK(s] and a fast transient current (IK(f]. The selective blockade of IK(s) by 20 mM-tetraethylammonium (TEA+) allowed the properties of IK(f) to be analysed in isolation. 3. The activation threshold for IK(f) was normally between -20 and -10 mV and the current-voltage relationship was linear. At positive potentials the decay of IK(f) was well fitted by a single exponential having a time constant of 20-35 ms. At -70 mV recovery from inactivation was best described by a single-exponential function with a time constant of 20-40 ms. IK(f) was fully activatable at -60 mV and was fully inactivated at -10 mV; the half-inactivation potential was approximately -25 mV. 4. Since IK(f) was reduced by raising the external concentration of K+, was blocked by Ba2+ and Cs+, and persisted in Ca2+-free medium it is attributed to a voltage-activated K+ conductance. The amplitude of IK(f) was unaffected either by 5 mM-4-aminopyridine (4-AP) or 50 microM-quinidine. 5. The electrical properties of IK(f) suggest that by affecting the amplitude and/or duration of the action potential IK(f) may modulate Ca2+ influx and consequently hormone release.

The firing patterns of rat melanotrophs recorded using the patch clamp technique.

Cell-attached and whole-cell recordings were made from adult rat melanotrophs maintained in vitro by standard cell culture techniques. In cell-attached recordings the cells showed small biphasic currents which reflected spontaneous cell firing. Single channel currents often had distinct relaxations and depolarizing currents through single channels could trigger the discharge of an action potential in the cell; both observations are consistent with the high input resistance (1-10 G omega) measured in the whole-cell configuration. The discharge of action potentials occurring either spontaneously or by current injection was eliminated by tetrodotoxin or by removing Na from the external medium. A Na-dependent plateau depolarization which activated near the spike threshold was also seen. In cells exposed to tetrodotoxin and K-channel blocking agents it was possible to evoke a long-lasting (up to 20 s) action potential which was enhanced and reduced, respectively, by Ba and Cd and thus appeared to reflect currents through voltage-activated Ca channels. Small amplitude Ca-dependent depolarizations could also be evoked at membrane potentials as low as -40 mV. In cell-attached and whole-cell recordings 10 mM Ba caused the discharge of tetrodotoxin-insensitive action potentials prior to a maintained depolarization of the membrane. The low threshold for Ca-dependent depolarizations suggest that Ca influx might occur in these cells even at the resting potential. Additionally, both a Ca current and the current underlying the Na-dependent plateau depolarization may influence the rate of cell firing and in doing so further increase Ca influx through voltage-activated channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiology of neurosecretory cells from the pituitary intermediate lobe.

One of the goals in studying the electrical properties of neurosecretory cells is to relate their electrical activity to the process of secretion. A central question in these studies concerns the role of transmembrane calcium ion flux in the initiation of the secretory event. With regard to the secretory process in pituitary cells, several research groups have addressed this question in vitro using mixed primary anterior pituitary cell cultures or clonal cell lines derived from pituitary tumours. Other workers, including ourselves, have used homogeneous cell cultures derived from the pituitary intermediate lobes of rats to examine the characteristics of voltage-dependent conductances, the contribution of these conductances to action potentials and their role in stimulus-secretion coupling. Pars intermedia (PI) cells often fire spontaneous action potentials whose frequency can be modified by the injection of sustained currents through the recording electrode. In quiescent cells action potentials can also be evoked by the injection of depolarizing current stimuli. At around 20 degrees C these action potentials have a duration of about 5 ms. Although most of the inward current during action potentials is carried by sodium ions, a calcium ion component can be demonstrated under abnormal conditions. Voltage-clamp experiments have revealed that the membrane of these cells contains high-threshold, L-type, Ca2+ channels and low-threshold Ca2+ channels. Since hormone release from PI cells appears not to be dependent on action potential activity but does depend on external calcium ions, it is not clear what role these Ca2+ channels play in stimulus-secretion coupling in cells of the pituitary pars intermedia. One possibility is that the low-threshold Ca2+ channels are more important to the secretory process than the high-threshold channels.

A patch clamp study of gamma-aminobutyric acid (GABA)-induced macroscopic currents in rat melanotrophs in cell culture.

1. The macroscopic currents induced in cultured rat melanotrophs by exogenous gamma-aminobutyric acid (GABA) were analysed using the patch clamp recording technique. 2. Using various concentrations of intra- and extracellular chloride it was demonstrated that the conductance activated by GABA was chloride selective. Since these currents were blocked with bicuculline and enhanced with chlordiazepoxide the involvement of GABAA receptors similar to those in the CNS is indicated. 3. When chloride was symmetrically distributed across the membrane the voltage/current relationship was linear; pronounced rectification of GABA mediated currents was evident when there was an asymmetrical distribution of chloride. 4. With concentrations of GABA greater than 10 microM a fading of the current was seen during prolonged (5-10 s) applications. This effect appeared to be due to a decline of conductance rather than a shift of the chloride equilibrium potential. 5. Values for the Hill coefficient derived from dose-response curves suggested that the binding of 2 molecules of GABA to the receptor is required for the activation of the chloride channel. 6. There was no indication of a direct, GABAB receptor-mediated change of conductance.

A pharmacological characterization of chloride- and potassium-dependent inhibitions in the CA3 region of the rat hippocampus in vitro.

Population spikes evoked in CA3 pyramidal cells of rat hippocampal slices by stimulation of the fimbria are subject to an early and a late inhibition following activation of the perforant path or the mossy fibres. The early inhibition is known to be GABA-mediated, and is blocked by addition of bicuculline to the superfusing medium; however the late inhibition is bicuculline-insensitive. Both inhibitions are reduced by the addition of (+/-)-baclofen or noradrenaline to the medium; the early inhibition only is blocked by D-Ala-D-Leu-enkephalinamide while the late inhibition is preferentially reduced by kainate. These data together with the results in the preceding paper suggest that both inhibitions are synaptically mediated, possibly by two distinct types of interneurone, one GABAergic and a second which may release an unidentified transmitter.

An electrophysiological characterization of inhibitions and postsynaptic potentials in rat hippocampal CA3 neurones in vitro.

Inhibitory processes in the CA3 region of the rat hippocampal slice were studied extracellularly using paired stimuli and with intracellular impalements of pyramidal neurones. As with mossy fibre (MF) or commissural (COMM) conditioning stimuli (Kehl and McLennan 1983), activation of the perforant path (PP) input caused a long-lasting inhibition of test orthodromic population spikes (PSs) evoked by shocks delivered to the fimbria. That at least a portion of this orthodromically-evoked inhibition reflected postsynaptic events was shown by the reduction both of the amplitude of antidromic PSs and the firing rate of spontaneously active single units. Experiments in which the extracellular concentration of chloride was reduced indicated that only an early component of the inhibition was due to a conductance for that anion. The existence of two inhibitory mechanisms distinguishable extracellularly by their sensitivity to bicuculline and manipulation of extracellular ion concentrations was correlated intracellularly with two hyperpolarising peaks occurring approximately 20 and 150 ms following MF, COMM or PP stimuli. The later hyperpolarisation had an equilibrium potential 20-25 mV more negative than the early IPSP, was unaffected by manipulations of extra- or intracellular concentrations of chloride and was associated with a decrease of membrane resistance suggesting that a potassium conductance was involved in its generation. The fact that it was recorded in the absence of any preceding depolarisation, was blocked by drugs acting presynaptically to cause disinhibition (Kehl and McLennan 1985) and, like the early inhibition, was reversibly reduced by hypoxia suggested that the late inhibition/hyperpolarisation was a synaptic phenomenon rather than an intrinsic membrane event. Because the late inhibition/IPSP could be shown to have a lower threshold for activation vis-à-vis the chloride- dependent early inhibition, it is possible that two distinct populations of interneurones mediate these two synaptic events.

The action of some analogues of the excitatory amino acids in the dentate gyrus of the rat.

We have confirmed that gamma-D-glutamylglycine and the L-isomer of 2-amino-4-phosphonobutyric acid, and have shown also that L-2-amino-5-phosphonovaleric (L-APV) acid, are antagonists of synaptic excitations of dentate granule cells induced from both lateral and medial perforant paths. The N-methyl-D-aspartic acid (NMDA) antagonist D-APV is without effect. The synaptic antagonists reduce the presynaptic fibre volley particularly in the lateral path, suggesting that a reduced transmitter output contributes to their action. NMDA receptors exist upon the granule cells, but they are not involved with these synaptic process.

Effects of folic and kainic acids on synaptic responses of hippocampal neurones.

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.

Evidence for a bicuculline-insensitive long-lasting inhibition in the CA3 region of the rat hippocampal slice.

Long-lasting inhibition (up to 2 s) of the commissurally-evoked response in the CA3 region of hippocampal slices was observed following a mossy fibre or commissural conditioning stimulus. Bicuculline applied iontophoretically or by superfusion (1-5 X 10(-6) M) blocked the early phase (20-40 ms) of the post-stimulus inhibition but either had no effect or potentiated the later inhibition.