Voltage-gated currents in identified rat olfactory receptor neurons

Whole-cell recording techniques were used to characterize voltage-gated membrane currents in neonatal rat olfactory receptor neurons (ORNs) in cell culture. Mature ORNs were identified in culture by their characteristic bipolar morphology, by retrograde labeling techniques, and by olfactory marker protein (OMP) immunoreactivity. ORNs did not have spontaneous activity, but fired action potentials to depolarizing current pulses. Action potentials were blocked by tetrodotoxin (TTX), which contrasts with the TTX-resistant action potentials in salamander olfactory receptor cells (e.g., Firestein and Werblin, 1987). Prolonged, suprathreshold current pulses evoked only a single action potential; however, repetitive firing up to 35 Hz could be elicited by a series of brief depolarizing pulses. Under voltage clamp, the TTX-sensitive sodium current had activation and inactivation properties similar to other excitable cells. In TTX and 20 mM barium, sustained inward current were evoked by voltage steps positive to -30 mV. This current was blocked by Cd (100 microM) and by nifedipine (IC50 = 368 nM) consistent with L-type calcium channels in other neurons. No T-type calcium current was observed. Voltage steps positive to -20 mV also evoked an outward current that did not inactivate during 100-msec depolarizations. Tail current analysis of this current was consistent with a selective potassium conductance. The outward current was blocked by external tetraethylammonium but was unaffected by Cd or 4-aminopyridine (4-AP) or by removal of external calcium. A transient outward current was not observed. The 3 voltage-dependent conductances in cultured rat ORNs appear to be sufficient for 2 essential functions: action potential generation and transmitter release. As a single odorant-activated channel can trigger an action potential (e.g., Lynch and Barry, 1989), the repetitive firing seen with brief depolarizing pulses suggests that ORNs do not integrate sensory input, but rather act as high-fidelity relays such that each opening of an odorant-activated channel reaches the olfactory bulb glomeruli as an action potential.     

Characterization of the neurons of the mouse hypogastric ganglion: morphology and electrophysiology

The somata of mouse hypogastric ganglion cells injected with Lucifer yellow were ovoid in shape, lacked dendritic processes but gave rise to a single axonal process. Antidromic activation demonstrated that some of the cells contained in this ganglion innervated the vas deferens. The passive and active membrane properties of the ganglion cells were determined in current clamp experiments. Cells fired tetrodotoxin-sensitive action potentials in response to intracellularly-applied depolarizing current. In voltage clamp evidence was obtained for both a persistent inward calcium current at potentials between -30 and -40 mV and a transient calcium current evoked by step depolarizations to around -20 mV. In current clamp, however, cells did not fire calcium action potentials in the presence of tetrodotoxin. Three potassium currents, IM (blocked by 1 mM barium and by 30 microM bethanechol), IA (blocked by 2 mM 4-aminopyridine) and IK(Ca) fast (blocked by 100 microM cadmium and by 5 mM tetraethylammonium) were characterized in these neurons. In addition, IK(Ca) slow was observed in a small proportion of cells. Fast, all-or-nothing, excitatory synaptic potentials were recorded in response to single stimuli applied to the afferent fibres running to the ganglion. In most cells the excitatory synaptic potentials were suprathreshold for action potential initiation and were markedly reduced or abolished by 100 microM mecamylamine, 1 mM hexamethonium and following desensitization to 100 microM nicotine. Excitatory synaptic potentials arose from stimulation of a single presynaptic nerve process and are typical of strong synaptic inputs.     

Mercury modulation of GABA-activated chloride channels and non-specific cation channels in rat dorsal root ganglion neurons.

The effects of mercuric chloride and methylmercury chloride on the rat dorsal root ganglion neurons in primary culture were studied by the whole-cell patch clamp technique. gamma-Aminobutyric acid-induced chloride currents were augmented by mercuric chloride in a potent and efficacious manner; at concentrations of 1 and 10 microM, the current amplitude was increased to 130% and 200% of the control. Methylmercury even at 100 microM did not augment but rather decreased the GABA-induced chloride current. Both mercuric chloride and methylmercury generated slow inward currents by themselves. These currents are not mediated by the GABA-activated chloride channels or by voltage-activated sodium, potassium or calcium channels, and are likely to be due to non-specific cation channels.     

Serotonergic modulation of persistent sodium currents and membrane excitability via cyclic AMP-protein kinase A cascade in mesencephalic V neurons

In rat mesencephalic trigeminal (Mes V) neurons, persistent sodium currents in conjunction with low-threshold potassium currents are critical for generation of subthreshold membrane oscillations and onset of burst behavior. Here we demonstrate that the cAMP/protein kinase A (PKA) signaling pathway modulates persistent sodium currents. In particular, we show that elevation of cAMP suppresses a low-threshold I(NaP) via a PKA intracellular pathway. Bath application of forskolin (20 microM), a stimulant for the production of cAMP, reduced the peak I(NaP). 1,9-Dideoxy-forskolin (20 microM), an inactive form of forskolin, produced minimal effects on I(NaP), and the membrane-permeable cAMP analogue 8-bromo-cAMP (500 microM) mimicked the effect of forskolin. Additionally, preapplication of H89 (2 microM), a specific PKA inhibitor, suppressed the effect of forskolin, suggesting the involvement of the cAMP/PKA intracellular signaling pathway in this modulation. 5-HT receptor stimulation (20 microM) also mimicked the modulation of I(NaP) by forskolin via the cAMP/PKA-dependent signaling pathway. Current clamp analysis demonstrated that voltage-dependent membrane resonance in response to a ZAP input current at depolarized holding potentials (approximately -50 mV) was specifically suppressed by forskolin or 5-HT. Moreover, drug application enhanced frequency adaptation in response to a 1-sec current pulse. These results indicate that modulation of persistent sodium currents by a cAMP/PKA pathway can significantly alter the membrane excitability and discharge characteristics of Mes V neurons.     

Blockade of U50488H on sodium currents in acutely isolated mice hippocampal CA3 pyramidal neurons

The effect of opioid agonist U50488H on Na(+) currents was examined in freshly dissociated hippocampal neurons of mice using the whole-cell patch clamp technique. U50488H (1-100 microM) caused a concentration dependent reversible inhibition of the voltage-activated sodium currents. IC50 of 15.5 microM and Hill constant of 1.4 were calculated respectively. The inhibitory actions of U50488H on I(Na) were still observed in the presence of 30 microM naloxone. Moreover, under the action of U50488H, repetitive stimulation induced further inhibition which was frequency-dependent. The activation curve did not change before and after application of 10 microM U50488H. However, after exposure to 10 microM U50488H and repetitive depolarizing at 10 Hz, frequency-dependent inhibition occurred, and a mean shift of half-activation membrane potential by +20 mV could be induced. The inactivation curve was significantly changed toward negative membrane potential with 10 microM U50488H, and further negative shift was observed after repetitive depolarizing at 10 Hz. Our results indicate that U50488H could directly inhibit neuronal Na(+) currents without involvement in the activation of kappa-opioid receptors.     

Blockade of the sodium and potassium channels of a myelinated nerve fiber by quinidine

The effects of quinidine on sodium (INa) and potassium (IK) currents in the Ranvier node of frog myelinated nerve fibre was studied by means of voltage clamp technique. When applied externally quinidine (5.10(-5) M) suppresses both INa and IK. Inhibition of INa can be greatly increased by repetitive membrane depolarization. After the end of stimulation the INa value recovers slowly up to the initial level (time constant being about 30 s at 12 degrees C). Unlike repetitive stimulation a single depolarizing pulse of long (1s) duration does not enhance appreciably the quinidine block, which permits a conclusion that quinidine interacts preferently with open sodium channels. Batrachotoxin protects the channels from the blocking action of 5.10(-5) M quinidine. The outward IK is blocked by quinidine in time- and voltage-dependent manner suggesting the interaction of the drug with open potassium channels. The results are consistent with the notion that tertiary amine quinidine, like amine local anesthetics penetrates through the membrane in the neutral form and blocks open sodium and potassium channels from inside in charged (protonated) form. Quinidine and local anesthetics are supposed to share a common receptor in the inner mouth of the sodium channel.     

Phorbol esters mimic some cholinergic actions in hippocampal pyramidal neurons

Muscarinic receptor stimulation in the hippocampus has been associated with inositol phospholipid breakdown. In other systems this leads to the formation of inositol trisphosphate and diacylglycerol, which promotes the activation of protein kinase C. Phorbol esters, which directly activate protein kinase C, exhibit high and specific binding in the hippocampus. This, along with the advantages of the hippocampal slice preparation, including direct pharmacological access to a cell population (CA1 pyramidal cells) having clearly defined muscarinic responses, makes this an ideal preparation to examine whether protein kinase C serves as the intracellular signal for muscarinic receptor occupation. Like muscarinic agonists, phorbol esters abolish the slow calcium-activated potassium afterhyperpolarizing potential (AHP) and its underlying current without reducing calcium action potentials. Those phorbol analogs that do not activate kinase C have no effect, suggesting that activation of this enzyme is required to reduce the AHP. The accommodation of spike discharge normally seen during a long depolarizing stimulus is also markedly reduced by phorbol esters as well as by muscarinic receptor activation. However, unlike muscarinic agonists, phorbol esters have no effect on the muscarine-sensitive, voltage-dependent, potassium current termed IM, nor do they consistently cause an increase in input resistance. Moreover, unlike ACh, they do not appear to have a presynaptic inhibitory action on the fast EPSP elicited by orthodromic stimulation. The slow cholinergic EPSP was blocked by phorbol esters, but this could be accounted for by a postsynaptic action. Thus, if inositol phospholipid turnover is involved in mediating muscarinic responses in the hippocampus, the activation of protein kinase C can account for only part of the electrophysiological response.     

Evidence for the involvement of G-proteins in the generation of the slow poststimulus afterdepolarisation (sADP) induced by muscarinic receptor activation in rat olfactory cortical neurones in vitro

The involvement of G-proteins in generating the slow poststimulus afterdepolarising potential (sADP) induced by muscarinic receptor activation in immature (P10-20) rat olfactory cortical brain slice neurones was investigated under whole-cell patch clamp, using GTP-gamma-S (G-protein activator) or GDP-beta-S (G-protein blocker)-filled electrodes. In control experiments using K methylsulphate electrodes, cell resting potential (V(m)) and spike firing properties were unaffected over 10-15 min recording, although input resistance (R(N)) was slightly increased ( approximately 14%). Oxotremorine-M (OXO-M; 10 microM) produced a reversible slow depolarisation, an increase in R(N) ( approximately 90%) and induction of a slow poststimulus inward tail current (I(ADP)) (measured under voltage clamp at -60 mV) that was sustained during drug exposure (up to 15 min); the amplitude of slow inward rectifier (I(h)) currents activated from -50 mV were also apparently increased. By contrast, in GTP-gamma-S-loaded cells, R(N) was consistently decreased ( approximately 22%) and spike firing threshold (V(th)) was raised ( approximately 5 mV) after 10 min recording. In approximately 60% of loaded cells, a persistent muscarinic slow inward current and I(ADP) were induced by OXO-M; I(h) relaxation amplitude was also significantly decreased. The effects of GTP-gamma-S on R(N), V(th) and I(h) were partly counteracted by adding Ba(2+) (100 microM) to the bathing medium or mimicked by adding baclofen (GABA(B) receptor agonist; 100 microM) to normally-recorded cells. Intracellular GDP-beta-S (up to 30 min) had no effect on cell membrane properties or I(h), but irreversibly blocked the muscarinic slow inward current and I(ADP) induced by OXO-M. We conclude that both muscarinic responses require G-protein-linked transduction mechanisms for their generation.     

Clindamycin-induced alteration of ganglionic function. II. Effect of nicotinic receptor-channel function

The postsynaptic effects of clindamycin have been analyzed in bullfrog sympathetic ganglion B cells using single electrode current and voltage clamp recordings and two electrode voltage clamp measurements. Clindamycin added to the bathing solution in the concentration range, 2.5 x 10(-4) to 5 x 10(-4) M, inhibited fast ganglionic transmission. In addition, local application of clindamycin decreased depolarizations produced by direct application of acetylcholine and decreased the amplitude of miniature excitatory postsynaptic potentials (MEPSPs) evoked by tetanic stimulation of the preganglionic trunk. In contrast, clindamycin did not change the amplitude or time course of the slow EPSP elicited by preganglionic stimulation (30 Hz for 10 s) or muscarinic depolarizations produced by local acetylcholine application to preparations pretreated with 25-50 microM (+)-tubocurarine. In voltage-clamped ganglion cells, excitatory postsynaptic current (EPSC) amplitude initially was increased and then decreased with increasing concentrations of clindamycin (0.5 x 10(-5) to 2.5 x 10(-4) M). The EPSC time course in control cells was exponential. After exposure to clindamycin, the EPSC decay was composed of two exponential components. The time constant of the fast component decreased and the time constant of the slow component increased with increasing concentrations of clindamycin. The two time constants of EPSCs obtained in clindamycin were independent of membrane voltage between -50 and -100 mV. We concluded that the block of fast ganglionic transmission is primarily due to a postsynaptic site of action, at least part of which is due to a concentration-dependent, but voltage-independent blockade of open nicotinic receptor channel complexes.     

Topiramate depresses carbachol-induced plateau potentials in subicular bursting cells

Intracellular recordings were made in an in vitro slice preparation to establish whether the antiepileptic drug topiramate reduces the depolarizing plateau potentials (PPs) induced in the rat subiculum by intracellular pulses of depolarizing current, in the presence of the cholinergic agonist carbachol (CCh, 70-100 microM). PPs lasted up to about 2 s, and disappeared during application of the muscarinic receptor antagonist atropine. Topiramate (10-100 microM, n = 22 neurons) decreased and eventually abolished in a dose-dependent manner these PPs, even when the function of excitatory amino acid and GABAA receptors was blocked. Hence, topiramate depresses muscarinic receptor-dependent PPs in the rat subiculum, thus suggesting that this form of excitation may represent a target for the mechanism of action of this antiepileptic compound.     

Activation of calcium channel currents in rat sensory neurons by large depolarizations: effect of Guanine nucleotides and (-)-baclofen

Calcium channel currents have been recorded from cultured rat sensory neurons at clamp potentials of between -30 and +120 mV. At large depolarizing potentials between +50 and +120 mV, the current was outward. This outward current was shown to be largely due to ions passing through calcium channels, because it was substantially although generally incompletely blocked by Cd2+ (1 mM) and omega-conotoxin (1 microM). Internal GTP-gamma-S (100 microM) and to a lesser extent GTP (1 mM) reduced the amplitude and slowed the activation of the outward, as well as the inward calcium channel current. Baclofen (100 microM) reversibly inhibited both the inward and outward currents. These results suggest that the effect of baclofen and G protein activation on calcium channel currents is not due to a shift in the voltage-dependence of channel availability.     

Modulation of tetrodotoxin-resistant sodium channels by dihydropyrazole insecticide RH-3421 in rat dorsal root ganglion neurons

The effects of the dihydropyrazole insecticide RH-3421 on the retrodotoxin-resistant (TTX-R) voltage-gated sodium channels in rat dorsal root ganglion (DRG) neurons were studied using the whole-cell patch clamp technique. RH-3421 at 10 nM to 1 microM completely blocked action potentials. The sodium currents were irreversibly suppressed by 1 microM RH-3421 in a time- and a dose-dependent manner and the IC50 value of RH-3421 was estimated to be 0.7 microM after 10 min of application. RH-3421 blocked the sodium currents to the same extent over the entire range of test potentials. The sodium conductance-voltage curve was not shifted along the voltage axis by 1 microM RH-3421 application In contrast, both fast and slow steady-state sodium channel inactivation curves were shifted in the hyperpolarizing direction in the presence of 1 microM RH-3421. It was concluded that RH-3421 bound to the resting and inactivated sodium channels to cause block with a higher affinity for the latter state.     

Acetylcholine-activated currents in mouse neuroblastoma cells

The nicotinic and muscarinic responses of differentiated mouse neuroblastoma cells from the clonal line N1E 115 to applied cholinergic agents were recorded using single channel and whole cell patch clamp techniques. An inward macroscopic current induced by acetylcholine (ACh) at the resting potential was blocked by curare; cell-attached recordings revealed a single channel conductance of 18 pS and a lifetime of 36 ms at 30°C, with 200 nM ACh. The zero current potential was close to 0 mV. The kinetics of these nicotinic currents were described by multiexponential functions for both the open and closed time distributions. An outward single channel current, present at resting and slightly depolarized potentials, was also observed and has been tentatively described as being dependent on muscarinic receptor activation, as it was usually blocked by atropine. Under our conditions of whole cell clamp, no macroscopic outward current sensitive to ACh was observed.     

Interaction of Noradrenergic and Cholinergic Agonists with Ligands Increasing K-conductance of Guinea Pig Hippocampal Neurons,in vitro

Single electrode current clamp and voltage clamp recordings were employed to study the effects of noradrenergic agonists and a cholinergic agonist (carbachol, Cch) on the resting membrane potential of CA3 neurons in guinea pig hippocampal slices. Stimulation of muscarinic and beta-adrenergic receptors depolarized, and stimulation of alpha1-adrenergic receptor hyperpolarized, CA3 neurons but the membrane potential changes were small. Hyperpolarizations or outward currents induced by baclofen, adenosine or serotonin (5-HT) were strongly potentiated by alpha-noradrenergic agonists and suppressed by Cch at concentrations ten times lower than those having any direct effects on membrane potential. Both the enhancement of the baclofen-induced hyperpolarization by phenylephrine and its suppression by Cch were pronounced at low concentrations of baclofen, but diminished at higher concentrations. The modulatory effects persisted after blockade of sodium spikes by tetrodotoxin and after blockade of fast inhibitory and excitatory synaptic transmission by picrotoxin and 6-cyano-7-nitroquinoxaline-2,3-dione. Our data suggest that, through the postsynaptic interaction with ligands activating potassium conductance, noradrenergic and muscarinic receptor stimulation can exert a stronger inhibitory and excitatory effect on CA3 pyramidal neurons at their resting membrane potential than would be expected from the changes in membrane potential induced by these neuromodulators on their own.     

Synaptic modulation by dopamine of calcium currents in rat pars intermedia

Melanotrophs of the rat pars intermedia are innervated by dopaminergic fibers traveling through the pituitary stalk which inhibit secretion via an action on D-2 receptors. As secretion from the melanotroph has been shown to be calcium (Ca2+) dependent, it is possible that dopamine may have an action to inhibit Ca2+ currents in these cells. This possibility was tested by examining the effects of exogenously applied dopaminergic agonists or synaptically released dopamine upon Ca2+ currents recorded under single electrode voltage clamp in intact rat pars intermedia in vitro. Following blockade of sodium and potassium currents in melanotrophs, Ca2+ spikes were elicited with intracellular injection of depolarizing currents; electrical stimulation of the pituitary stalk caused an inhibition of the Ca2(+)-based action potentials which lasted for several seconds. Using single-electrode voltage-clamp techniques, we recorded inward Ca2+ currents corresponding to the T, N, and L types (see Williams et al., 1990). Stimulation of the pituitary stalk inhibited both the low- and high-threshold peak inward Ca2+ currents elicited from a holding potential of -90 mV. In contrast, when noninactivating Ca2+ currents were elicited from a holding potential of -30 mV, the currents were not altered by stalk stimulation. This pattern of inhibition of the Ca2+ currents was consistent with the preferential inhibition, by stalk stimulation, of the N and T Ca2+ currents, while sparing the L current. We observed that inhibition of Ca2+ currents due to stalk stimulation was completely reversed by bath perfusion of domperidone (1 microM), an antagonist of dopamine at the D-2 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of nerve membrane sodium channel activation by deltamethrin.

Deltamethrin is a highly potent pyrethroid insecticide that causes hypersensitivity, choreoathetosis, tremors, and paralysis in mammals. It is known to modify the sodium channel in such a way as to prolong the tail current associated with step repolarization following a depolarizing pulse. Using the axial-wire voltage-clamp technique with the giant axon of the squid Loligo pealei, we have demonstrated that deltamethrin also greatly slows the opening of the sodium channel. This was first observed as a decrease, by as much as 80%, in the peak sodium current flowing during a short, 10 ms depolarization. Current flowing through these slowly opening deltamethrin modified sodium channels was observed during the first depolarizing pulse after deltamethrin exposure and developed with a time constant of 320 ms. This supports the idea that deltamethrin can modify sodium channels when they are in the closed or resting state. Further, evidence of this hypothesis was provided by experiments using 0.1 and 10 microM deltamethrin and measuring the tail current amplitude after depolarizing pulses of varying duration (1-1200 ms). The mean time constant for the increase in tail current amplitude was almost concentration independent; 253 ms at 0.1 microM and 193 ms at 10 microM. We conclude that deltamethrin modifies the activation kinetics of sodium channels in such a way as to slow opening and that this modification occurs predominantly when channels are in the closed or resting state.     

Differential regulation of gamma-aminobutyric acid receptor channels by diazepam and phenobarbital

The anticonvulsant activity of diazepam and phenobarbital may be mediated in part by enhancement of inhibition involving gamma-aminobutyric acid (GABA). While both diazepam and phenobarbital increase GABA receptor chloride current, they may have different mechanisms of action, since they bind to different sites on the GABA receptor-chloride channel complex. We used the patch clamp technique to compare the effects of diazepam and phenobarbital on single GABA receptor currents. Outside-out patches were obtained from mouse spinal cord neurons grown in cell culture for 2 to 4 weeks. GABA (2 microM) evoked single channel currents that occurred as single brief openings or in bursts of multiple openings. Diazepam (20 nM) and phenobarbital (500 microM) both increased the GABA receptor current by increasing mean open time without altering channel opening frequency. However, the temporal grouping of openings into bursts suggested that the enhancement occurred via different mechanisms. Diazepam increased the frequency of bursting GABA receptor currents with minimal effect on the duration of bursts. Phenobarbital increased the duration of bursting GABA receptor currents without altering the frequency of bursts. These results suggest that diazepam binds to a site that may enhance single channel burst frequency by increasing the affinity of GABA binding, while phenobarbital may stabilize the bursting open state of the channel by binding to a different modulatory site at or near the chloride channel.     

Clindamycin-induced alteration of ganglionic function. I. Direct effects on ganglion cell properties

The influence of the lincosamide antibiotic, clindamycin, on the properties of bullfrog sympathetic ganglion B cells has been determined in vitro using conventional voltage recording methods or single microelectrode voltage-clamp recording techniques. Individual neurons were depolarized with both bath application or local perfusion of clindamycin. The amplitude of the depolarization was not altered by pretreatment with 50 microM (+)-tubocurarine, 10-microM atropine, or 1.5 microM tetrodotoxin (TTX), indicating that the clindamycin-induced depolarization does not result from either the activation of (1) nicotinic receptors, (2) muscarinic receptors, or (3) voltage-gated sodium channels. Clindamycin partially inhibited IM, an action which accounts for part of the clindamycin-induced depolarization. The duration of the hyperpolarizing afterpotential (HAP) following the action potential was decreased in the presence of clindamycin. Clindamycin decreased the amplitude and maximum rate of rise (MRR) of TTX-insensitive action potentials. As calcium influx is thought to contribute to the depolarizing phase of the TTX-insensitive spikes, we suggest that the decrease in HAP duration by clindamycin results from a decrease in the somal calcium current. Further, it is suggested that a decrease in IM and HAP duration may be responsible for the increased excitability exhibited during exposure to clindamycin.     

Whole-cell recording from honeybee olfactory receptor neurons: ionic currents, membrane excitability and odourant response in developing workerbee and drone

Whole-cell recording techniques were used to characterize ionic membrane currents and odourant responses in honeybee olfactory receptor neurons (ORNs) in primary cell culture. ORNs of workerbee (female) and drone (male) were isolated at an early stage of development before sensory axons connect to their target in the antennal lobe. The results collectively indicate that honeybee ORNs have electrical properties similar, but not necessarily identical to, those currently envisaged for ORNs of other species. Under voltage clamp at least four ionic currents could be distinguished. Inward currents were made of a fast transient, tetrodotoxin-sensitive sodium current. In some ORNs a cadmium-sensitive calcium current was detected. ORNs showed heterogeneity in their outward currents: either outward currents were made of a delayed rectifier type potassium current, which was partially blocked by tetraethyl ammonium or quinidine, or were composed of a delayed rectifier type and a transient calcium-dependent potassium current, which was cadmium-sensitive and abolished by removal of external calcium. The proportion of each of the two outward currents, however, was different within the ORNs of the two sexes suggesting a gender-specific functional heterogeneity. ORNs showed heterogeneity in action potential firing properties: depolarizing current steps elicited either one action potential or, as in most of the cells, it led to repetitive spiking. Action potentials were tetrodotoxin-sensitive suggesting they are carried by sodium. Odourant stimulation with different mixtures and pure substances evoked depolarizing receptor potentials with superimposed action potentials when spike threshold was reached. In summary, honeybee ORNs are remarkably mature at early stages in their development.     

Voltage clamp analysis of cholinergic action in the hippocampus

A slow muscarinic EPSP, accompanied by an increase in membrane input resistance, can be elicited in hippocampal CA1 pyramidal cells in vitro by electrical stimulation of cholinergic afferents in the slice preparation. Associated with the slow EPSP is a blockade of calcium-activated potassium afterhyperpolarizations (AHPs) (Cole and Nicoll, 1984a). In this study a single-electrode voltage clamp was used to examine the currents affected by activation of muscarinic receptors, using either bath application of carbachol or electrical stimulation of the cholinergic afferents. The 3 main findings of this study are that (1) of the 2 calcium-activated potassium currents (termed IAHP and IC) in hippocampal pyramidal cells, only IAHP is sensitive to carbachol; (2) IAHP is approximately 10-fold more sensitive to carbachol than is another muscarine-sensitive current, IM; and (3) neither blockade of IAHP nor of IM can account for the production of the slow EPSP. Rather, the slow EPSP appears to be generated by the blockade of a nonvoltage-dependent, resting potassium current. We propose that the muscarinic blockade of IAHP, which largely accounts for spike frequency adaptation, is primarily involved in enhancing action potential discharge to depolarizing stimuli, while the slow EPSP acts directly to cause action potential discharge.