Ionic mechanisms of cholinergic excitation in mammalian hippocampal pyramidal cells

Mechanisms of action of acetylcholine in the guinea-pig cerebral cortex in vitro

The mechanisms of action of acetylcholine (ACh) in the guinea-pig neocortex were investigated using intracellular recordings from layer V pyramidal cells of the anterior cingulate cortical slice. At resting membrane potential (Vm = -80 to -70 mV), ACh application resulted in a barrage of excitatory and inhibitory post-synaptic potentials (p.s.p.s) associated with a decrease in apparent input resistance (Ri). ACh, applied to pyramidal neurones depolarized to just below firing threshold (Vm = -65 to -55 mV), produced a short-latency hyperpolarization concomitant with p.s.p.s and a decrease in Ri, followed by a long-lasting (10 to greater than 60 s) depolarization and action potential generation. Both of these responses were also found in presumed pyramidal neurones of other cortical regions (sensorimotor and visual) and were blocked by muscarinic, but not nicotinic, antagonists. The ACh-induced hyperpolarization possessed an average reversal potential of -75.8 mV, similar to that for the hyperpolarizing response to gamma-aminobutyric acid (GABA; -72.4 mV) and for the i.p.s.p. generated by orthodromic stimulation (-69.6 mV). This cholinergic inhibitory response could be elicited by ACh applications at significantly greater distance from the cell than the slow depolarizing response. Blockade of GABAergic synaptic transmission with solution containing Mn2+ and low Ca2+, or by local application of tetrodotoxin (TTX), bicuculline or picrotoxin, abolished the ACh-induced inhibitory response but not the slow excitatory response. In TTX (or Mn2+, low Ca2+) the slow excitatory response possessed a minimum onset latency of 250 ms and was associated with a voltage-dependent increase in Ri. Application of ACh caused short-latency excitation associated with a decrease in Ri in eight neurones. The time course of this excitation was similar to that of the inhibition seen in pyramidal neurones. Seven of these neurones had action potentials with unusually brief durations, indicating that they were probably non-pyramidal cells. ACh blocked the slow after-hyperpolarization (a.h.p.) following a train of action potentials, occasionally reduced orthodromically evoked p.s.p.s, and had no effect on the width or maximum rate of rise or fall of the action potential. It is concluded that cholinergic inhibition of pyramidal neurones is mediated through a rapid muscarinic excitation of non-pyramidal cells, resulting in the release of GABA. In pyramidal cells ACh causes a relatively slow blockade of both a voltage-dependent hyperpolarizing conductance (M-current) which is most active at depolarized membrane potentials, and the Ca2+-activated K+ conductance underlying the a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro

CA1 pyramidal neurones were studied in rat in vitro hippocampal slices using standard intracellular and single-electrode voltage-clamp recording techniques to examine the actions of noradrenaline (NA). NA had two different effects on the resting membrane potential of pyramidal neurones; either a hyperpolarization accompanied by a decrease in membrane input resistance, or less commonly, a depolarization accompanied by an increase in input resistance. In many cells, both effects, a hyperpolarization followed by a depolarization were observed. The depolarization was mediated by a noradrenergic beta-receptor. The hyperpolarization was more difficult to characterize, but may result from alpha-receptor activation. NA reduced the amplitude and duration of the slow calcium-activated potassium after-hyperpolarization (a.h.p.) that follows depolarization-induced action potentials. This action of NA was mediated by beta 1-noradrenergic receptors. NA, in the presence of tetrodotoxin and tetraethylammonium, reduced the a.h.p. without reducing the size of the calcium action potential which preceded it. This was unlike the action of the calcium channel blocker, cadmium, which reduced the calcium action potential and the a.h.p. in parallel. Furthermore, NA did not reduce the amplitude of calcium or barium currents recorded under voltage clamp after blockade of potassium currents. A functional consequence of this blockade of the calcium-activated a.h.p. was a reduction of the accommodation of action potential discharge such that the excitatory responses of the neurone to depolarizing stimuli, such as glutamate application or current passed through the recording electrode, were enhanced. We conclude that the effects of NA on calcium-activated potassium conductance and on resting membrane potential can interact to increase the signal-to-noise ratio of hippocampal pyramidal neurone responsiveness.

Desensitization of muscarinic acetylcholine receptors: possible relation to receptor heterogeneity and phosphoinositides

The increases in firing rates of hippocampal cells were examined following microiontophoretic application of several muscarinic cholinergic receptor agonists. The agonists studied had been pharmacologically characterized previously and divided into two classes: class A agonists (e.g. acetylcholine, carbamylcholine, and oxotremorine-M) which maximally stimulate PI turnover and reveal mAChR heterogeneity, and class B agonists (e.g. bethanecol and oxotremorine-1) which poorly stimulate PI turnover and do not alter mAChR conformation/orientation in the hippocampus. While comparable stimulatory effects on hippocampal pyramidal cell firing rates were seen with both classes of agonists during short (20 s) ejection periods, longer applications (greater than 25 s) produced class-dependent differential firing patterns. Prolonged ejection of class A agonists selectively desensitized cells to further, continued application in the same ejection period, and the firing rates declined. Class B agonists produced stimulatory responses in hippocampal cells during the entire ejection period, and DE was not observed. This desensitization effect (DE) was observed only for bursts and not for simple spikes.

An analysis of cholinoceptive neurons in the hippocampal formation by direct microinfusion

Microinfusions of cholinergic agents were made in various sites in the dorsal hippocampal formation of urethane anaesthetized rats. Infusions of eserine or carbachol elicited hippocampal theta activity when made in areas containing high levels of cholinergic markers: the stratum oriens and radiatum of the CA1 and CA3, the stratum moleculare and stratum granulosum of the dentate gyrus and the infragranular region of the hilus. Subsequent infusions of atropine sulfate antagonized the theta activity. Control infusions of equal volumes of saline in active sites were without effect. Infusions of eserine or carbachol in the vicinity of the hippocampal fissure, the stratum lacunosum/moleculare of the CA1 or CA3, in the deep regions of the hilus, and in the lateral ventricle and overlying neocortex, were also without effect. Furthermore, in active sites, the latency to onset of theta and subsequent theta frequency, were both directly related to the total amount of carbachol infused. Thus, areas in which theta could be elicited with a cholinergic agonist (carbachol), or an anticholinesterase (eserine) and antagonized with atropine, were found to correspond well to areas previously found to contain a high density of cholinoceptive neurons, using autoradiographic and immunohistochemical techniques. These results provide further support for the involvement of acetylcholine as a neurotransmitter in the generation of type 2 theta in the hippocampal formation.

Synaptic block of a transmitter-induced potassium conductance in Aplysia neurones

A voltage-clamp study was made of a slow excitatory post-synaptic potential (slow e.p.s.p.) that can be elicited in the medial cells of the left pleural ganglion of Aplysia californica by the firing of at least three different presynaptic neurones (labelled I, II and III). Each of these three neurones elicits other permeability changes in addition to the slow e.p.s.p., and all elements of these synaptic responses were shown to be mediated monosynaptically. The slow e.p.s.p., associated with an increase in membrane resistance, was shown to be due to a decrease in K permeability. When the slow e.p.s.p. was present spontaneously, it could be blocked by three compounds (tetraethylammonium (TEA), phenyltrimethylammonium (PTMA), or methylxylocholine (beta-TM 10], all previously shown to block the cholinergic receptor that mediates an increase in K conductance in the medial cells (see Kehoe, 1972b). Furthermore, in ganglia in which no slow e.p.s.p. was seen in response to firing of the neurones I, II, and III, such a response became manifest when agonists capable of activating the cholinergic receptor were applied (e.g. acetylcholine (ACh), carbachol, arecoline, or F2268). The slow e.p.s.p. thus appears to result from the reduction, induced by any one of three 'blocking neurones', of a cholinergically controlled K conductance. Finally, when presynaptic neurone I (the only neurone tested) was fired shortly before or during the activation of presynaptic neurone IV, previously shown to be cholinergic (Kehoe, 1972b), the K component of the cholinergic post-synaptic inhibitory potential was markedly reduced. The concentration at which a given agonist caused the manifestation of the synaptic diminution in K conductance (i.e. the slow e.p.s.p.) was found to be the same as that at which it caused a reduction in the synaptically activated, cholinergic, K-dependent conductance elicited by presynaptic neurone IV. Intracellularly injected adenosine 3',5'-cyclic monophosphate (cyclic AMP) imitated the effect of the 'blocking neurones' on the K conductance activated by bath-applied cholinomimetics. This effect was superimposed on a cyclic-AMP-induced, voltage-dependent inward current that disappeared when the cell was bathed in Na-free sea water, or when the extracellular Ca concentration was increased to 60 mM. The effect of cyclic AMP on the cholinergic K conductance remained even after this cyclic-AMP-activated inward current was eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic block of a calcium-activated potassium conductance in Aplysia neurones

In the preceding paper (Kehoe, 1985) it was shown that the firing of any one of three neurones (I, II, III) presynaptic to the medial cells of the pleural ganglion of Aplysia californica causes a diminution of the cholinergically controlled K conductance in those cells. Firing of the same three presynaptic neurones was shown here to cause a similar diminution in a depolarization-induced K-dependent conductance in the same post-synaptic cells. The depolarization-induced K conductance was found to disappear when Ca ions were removed from the sea water bathing the ganglion or when the cell was injected with the Ca chelator ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetra-acetic acid (EGTA). The diminution in this Ca-activated, K-dependent current occurred even when the presynaptic neurone was fired a few seconds after the end of the depolarizing voltage step to the post-synaptic neurone, showing that the diminution in K conductance was not an indirect effect of a transmitter-induced diminution in Ca influx during the depolarizing pulse. The two K conductances affected by the 'blocking neurones' could be selectively eliminated. The cholinergic conductance could be blocked by receptor-specific cholinergic antagonists (e.g. 1 mM concentrations of phenyltrimethylammonium (PTMA), choline and tetraethylammonium (TEA]. Even at 10 mM concentrations, none of these compounds (including TEA, which is known to block certain Ca-activated K conductances) had an effect on the depolarization-induced, Ca-activated K conductance studied here. This latter conductance, on the other hand, was selectively blocked by an intracellular injection of EGTA. The three blocking neurones continued to diminish the K conductance (cholinergic or depolarization induced) that remained intact under these different experimental conditions. The depolarization-induced influx of Ca was shown to block the cholinergically controlled K conductance, but Ca was excluded as the possible mediator of the diminution in K conductance caused by the three blocking neurones. An intracellular injection of Ca ions into the medial cells was shown to activate a variety of changes in membrane conductance; in particular, two K-conductance increases: an early, TEA-sensitive one, and a slowly developing, TEA-insensitive one. Both the permeant cyclic AMP analogue p-chlorophenylthioadenosine 3',5'-monophosphate (CPT-cyclic AMP) and the phosphodiesterase inhibitors amino-phylline and isobutyl-1-methylxanthine (IBMX) were shown to block the depolarization-induced K conductance, and to reduce, though not eliminate, the slowly developing K conductance activated by an intracellular injection of Ca.(ABSTRACT TRUNCATED AT 400 WORDS)

Two types of muscarinic response to acetylcholine in mammalian cortical neurons

Applications of acetylcholine (AcCho) to pyramidal cells of guinea pig cingulate cortical slices maintained in vitro result in a short latency inhibition, followed by a prolonged increase in excitability. Cholinergic inhibition is mediated through the rapid excitation of interneurons that utilize the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). This rapid excitation of interneurons is associated with a membrane depolarization and a decrease in neuronal input resistance. In contrast, AcCho-induced excitation of pyramidal cells is due to a direct action that produces a voltage-dependent increase in input resistance. In the experiments reported here, we investigated the possibility that these two responses are mediated by different subclasses of cholinergic receptors. The inhibitory and slow excitatory responses of pyramidal neurons were blocked by muscarinic but not by nicotinic antagonists. Pirenzepine was more effective in blocking the AcCho-induced slow depolarization than in blocking the hyperpolarization of pyramidal neurons. The two responses also varied in their sensitivity to various cholinergic agonists, making it possible to selectively activate either. These data suggest that AcCho may produce two physiologically and pharmacologically distinct muscarinic responses on neocortical neurons: slowly developing voltage-dependent depolarizations associated with an increase in input resistance in pyramidal cells and short-latency depolarizations associated with a decrease in input resistance in presumed GABAergic interneurons.

Neurons dissociated from rat myenteric plexus retain differentiated properties when grown in cell culture. II. Electrophysiological properties and responses to neurotransmitter candidates

We have used intracellular recordings to study the electrophysiological and pharmacological properties of neurons that have been grown in cell cultures after having been dissociated from the myenteric plexus of the small intestine of newborn rats. Studies of action potential mechanisms revealed that all of the neurons could generate Na+-dependent action potentials in the presence of Ca2+-channel blockers and that about 70% could generate Ca2+-dependent action potentials when Na+ channels were blocked with tetrodotoxin. No neurons generated long afterhyperpolarizations after single action potentials but about 50% of neurons did so following trains of action potentials. Over 95% of the neurons tested accommodated rapidly to sustained depolarization. The effects of several enteric neurotransmitter candidates were studied by superfusing or pressure-ejecting test solutions while recording neuronal responses. All of the cultured neurons tested had nicotinic responses to acetylcholine. Subsets of neurons responded to muscarinic cholinergic agonists (slow depolarization and increased excitability), serotonin (fast depolarization or slow depolarization and increased excitability), gamma-aminobutyrate (fast depolarization), substance P (slow depolarization, biphasic fast and slow depolarization or increased excitability without a change in membrane potential), vasoactive intestinal peptide (slow depolarization and increased excitability), or [Met]enkephalin (slow hyperpolarization and/or decreased action potential duration). We conclude that myenteric neurons grown in cell culture retain many of the physiological and pharmacological properties that they have in situ. Such cultures will permit detailed biophysical and pharmacological studies of the mechanisms of action of enteric neurotransmitter candidates.

Transplanted septal neurons make viable cholinergic synapses with a host hippocampus

Cell suspensions from the fetal septal region were injected stereotaxically into the hippocampus of fornix-fimbria-transected adult rats. The host rats were sacrificed up to 3 months after the operation and the hippocampus sliced into 350 microns transverse slices. Intracellular recording was made from CA1 neurons adjacent to the graft. Electrical stimulation of the graft produced a voltage-dependent depolarization in some recorded neurons. This was associated with an increase in spontaneous and anodal break action potential discharges. In addition, a slow after-hyperpolarization (AHP) which typically follows a burst discharge was blocked during the depolarization indicating that the stimulation may block a Ca2+-dependent K+ current. The effects of the stimulation were antagonized by atropine. A response to the stimulation was seen 2 weeks but not 1 week after grafting. Over time, cells that were located away from the graft became activated by the stimulation. This was correlated with the extent of proliferation of acetylcholinesterase-containing fibers around the graft. These results suggest that grafted septal neurons make viable cholinergic connections with a host hippocampus.

Seizures produced by pilocarpine in mice: a behavioral, electroencephalographic and morphological analysis

Increasing doses of pilocarpine, 100-400 mg/kg, were given intraperitoneally to mice and the resulting behavioral, electroencephalographic and neuropathological alterations were studied. No behavioral phenomena were observed in mice treated with the lowest dose of pilocarpine. Occasional tremor and myoclonus of hindlimbs were found in animals which received pilocarpine in a dose of 200 mg/kg. At doses of 300, 325 and 350 mg/kg, pilocarpine produced a sequence of behavioral alterations including staring spells, limbic gustatory automatisms and motor limbic seizures that developed over 15-30 min and built up progressively into a limbic status epilepticus lasting for several hours. The highest dose of pilocarpine, 400 mg/kg, was generally lethal to mice. Pilocarpine produced both interictal and ictal epileptiform activity in the electroencephalogram (EEG). The earliest EEG alterations appeared in the hippocampus and then spread to cortical areas. EEG seizures started 10-15 min after injection of large doses of pilocarpine, 300-350 mg/kg. Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the EEG activity. By 30-45 min paroxysmal activity resulted in a status epilepticus. Examination of frontal forebrain sections with light microscopy revealed a widespread damage to several brain regions including the hippocampus, amygdala, thalamus, olfactory cortex, neocortex and substantia nigra. Scopolamine, 10 mg/kg, and diazepam, 10 mg/kg, prevented the development of convulsive activity and brain damage produced by pilocarpine. The results emphasize that excessive and sustained stimulation of cholinergic receptors can lead to seizures and seizure-related brain damage in mice. It is proposed that systemic pilocarpine in mice provides a useful animal model for studying mechanisms of and therapeutic approaches to temporal lobe epilepsy.

Control of the repetitive discharge of rat CA 1 pyramidal neurones in vitro

Experiments using intracellular recording techniques were performed on rat hippocampal neurones in vitro, to study the discharge properties of these cells. When CA 1 pyramidal cells were excited by injecting long depolarizing current pulses (approximately 600-800 ms), they responded with an initial rapid action potential discharge which slowed, or accommodated, and then stopped after 200-300 ms. The train of action potentials was followed by a hyperpolarization which was due primarily to calcium-activated potassium conductance (GK(Ca]. The amplitude of this hyperpolarization increased with an increasing number of action potentials in the initial discharge. Blocking the calcium-activated potassium conductance, by injecting EGTA into the cell, by bathing the cell in cadmium, a calcium channel blocker, or by bathing the cell in calcium-free medium, reduced the after-hyperpolarization (a.h.p.) and accommodation such that the frequency of action potential discharge increased and the duration of this discharge was prolonged. Blocking the calcium-activated potassium conductance had a greater effect on discharge frequency later in the action potential train, as late interspike intervals were shortened more than early ones by the application of cadmium or of calcium-free medium. This was presumably because the calcium-activated potassium conductance was more developed later in the train. Accommodation was not completely abolished in the absence of calcium and presence of cadmium, suggesting that other factors, in addition to calcium-activated potassium conductance, contributed to this process. This remaining accommodation was reduced by low doses of carbachol, suggesting that the M-current also plays a role in accommodation. We conclude that accommodation of the action potential discharge of hippocampal pyramidal cells may be regulated by at least two potassium currents: the calcium-activated potassium current and the M-current. Both of these currents are turned on during excitation of the neurone and act in an inhibitory manner on that neurone to limit further action potential discharge.

Characterization of a slow cholinergic post-synaptic potential recorded in vitro from rat hippocampal pyramidal cells

Intracellular recording from CA1 pyramidal cells in the hippocampal slice preparation was used to compare the action of exogenously applied acetylcholine (ACh) and cholinomimetics to the effect of electrically stimulating sites in the slice known to contain cholinergic fibres. ACh depolarized pyramidal cells with an associated increase in input resistance, blocked a calcium-activated potassium conductance (GK(Ca], and blocked accommodation of action potential discharge. All of these actions were blocked by the muscarinic antagonist, atropine. Repetitive electrical stimulation of stratum (s.) oriens evoked a series of fast excitatory post-synaptic potentials (e.p.s.p.s) followed by an inhibitory post-synaptic potential. These potentials were followed by a slow e.p.s.p. that lasted 20-30 s. The slow e.p.s.p. was selectively enhanced by eserine and blocked by atropine. Ionophoretic application of ACh closely mimicked the time course of the slow e.p.s.p. The slow e.p.s.p. was blocked by tetrodotoxin and cadmium, indicating that it was dependent on propagated action potentials and on calcium. Considerably higher stimulus strengths were needed to elicit a slow e.p.s.p. than to elicit the earlier synaptic potentials. The size of the slow e.p.s.p. was markedly increased by repetitive stimulation. Stimulation of the alveus, s. oriens, s. pyramidale and fimbria all evoked a slow e.p.s.p., while stimulation of s. radiatum was relatively ineffective. The input resistance of the cell increased during the slow e.p.s.p. Hyperpolarizing the cell decreased the size of the slow e.p.s.p. and at membrane potentials of -70 mV or greater, little response was recorded. Stimulation of s. oriens blocked GK(Ca) and accommodation of action potential discharge. These effects, which could be seen in the absence of any change in membrane potential, were enhanced by eserine and blocked by atropine. The present electrophysiological results establish that CA1 pyramidal cells receive a cholinergic input and demonstrate that this input can dramatically alter the firing properties of these neurones for tens of seconds in the absence of any marked effect on membrane potential. Such an action contrasts with previously characterized synaptic potentials in this region of the brain.

The pharmacology of hippocampal theta cells: evidence that the sensory processing correlate is cholinergic

The firing repertoires of theta cells in the CA1 and dentate layers of the hippocampal formation of the freely moving rabbit were analyzed during 3 behavioral conditions: (1) voluntary motor patterns, termed type 1 theta behaviors; (2) automatic motor patterns, termed type 2LIA behaviors; (3) alert immobility with presentation of sensory stimuli, termed type 2 theta behavior. Cholinergic manipulations were shown to effect the firing repertoires of theta cells during the type 2 theta behavior condition (sensory processing) and not the other two behavioral conditions. A hypothesis of a sensorimotor processing function of the hippocampal formation is presented and discussed.

Facilitation by acetylcholine of tetrodotoxin-resistant spikes in rat hippocampal pyramidal cells

The electrical activity of hippocampal pyramidal cells was studied in slice cultures during blockade of the regenerative Na currents. In the presence of tetrodotoxin, these neurones had a mean resting potential of -68 mV, a membrane input resistance of 87 M omega and displayed marked non-linearities in their current voltage relationship. In response to depolarizing stimuli, pyramidal cells generated action potentials of small amplitude, slow rise and long duration. These tetrodotoxin-resistant spikes were abolished by calcium conductance blockers such as cobalt and cadmium ions. Acetylcholine applied to the bath or by iontophoresis depolarized pyramidal cells, elicited spontaneous tetrodotoxin-resistant spikes and facilitated spiking evoked by depolarizing rectangular current pulses or a current ramp. The effects of acetylcholine were not only slow in onset, but also prolonged; they were completely reversible and sensitive to atropine and calcium-antagonists such as cadmium and cobalt ions which, respectively, reduced and abolished these effects. After hyperpolarizations following injection of depolarizing current pulses were suppressed by acetylcholine and often transformed into depolarizing afterpotentials. Acetylcholine had no effect on voltage-independent conductances as determined by application of hyperpolarizing current pulses. These results could be explained by inhibition of the voltage-dependent K+-current, i.e. the M current (blockade of the calcium current could remove any depolarizing influence resulting from M current inhibition) or by a direct activation of a voltage-dependent calcium current by muscarinic agonists.

Cholinergic modulation of the functional organization of the cat visual cortex

The cortex receives a cholinergic input which is considered to be involved in mediating the effects of arousal. The experiments reported here have examined the nature of the cholinergic influence on the neuronal organization of the cat visual cortex. Out of 83 cells studied, 92% exhibited a modification in their visual response properties during the iontophoretic application of ACh. These comprised 61% in which responses were facilitated and 31% in which responses were depressed. The facilitatory effects were associated with a striking increase in stimulus specific responses without any concomitant loss in the selectivity. This comment applied equally to orientation and direction selectivity. It is argued that the facilitatory action of ACh on stimulus specific responses is consistent with a modulation of potassium conductance and most probably the conductance associated with a voltage dependent channel. We found no evidence to support the view that the facilitatory action involved disinhibition; the action of bicuculline, which blocks inhibitory influences in the visual cortex, was quite distinct to that of ACh. The facilitatory and depressive effects of ACh did not show any correlation with the simple-complex classification of cells or any other obvious parameter of receptive field organization, but there was a correlation with cortical lamination. Cells facilitated by ACh were found in all cortical laminae, but those depressed by ACh were found in laminae III and IV.

N-methyl aspartate activates voltage-dependent calcium conductance in rat hippocampal pyramidal cells

The depolarizing actions of N-methyl-DL-aspartate (NMA) and L-glutamate on pyramidal neurones were compared in a hippocampal slice preparation. Tetrodotoxin (1 microM) was added to the perfusion solution to suppress regenerative Na conductances. Depolarization evoked by ionophoretic application of NMA triggered slow, high-threshold regenerative spikes. These are considered to be Ca spikes since the amplitude and rate of rise could be reduced by verapamil, D-600, Co2+ and Mn2+, and increased by Ba2+. Multiple Ca-spike thresholds could be demonstrated in the same cell. In contrast, depolarizations evoked by L-glutamate only rarely triggered Ca-spikes. The minimum latency to the onset of depolarization evoked by NMA was less than 20 ms. The latency and amplitude of NMA-evoked responses were highly dependent on the position of the ionophoretic pipette; movements of the pipette by as little as 10-50 micron could markedly change the size of the response. Spatially separate hot spots for NMA and glutamate were not found. Depolarizations evoked by small to moderate ionophoretic currents of NMA were usually associated with an apparent rise in input resistance, as tested by the response to transmembrane current pulses. Ionophoresis of L-glutamate, or high NMA doses, however, usually caused a fall in input resistance. Both the depolarization and the conductance change evoked by NMA were highly voltage-dependent within the approximate range -50 to -80 mV; they could be increased by modest depolarization and reduced by hyperpolarization of the membrane. No reversal potential could be demonstrated in the hyperpolarizing direction. Rather, the NMA response approached zero asymptotically at sufficiently hyperpolarized membrane potentials. Subthreshold depolarizations and conductance changes elicited by NMA could be blocked by Co2+, Mn2+ and Cd2+, and reduced by D-600 and verapamil. These Ca2+ antagonists had little or no effect on resting membrane potential or input resistance, or on responses to L-glutamate. Ba2+ increased the amplitude of subthreshold NMA responses. Intracellular injection of Cs+ plus tetraethylammonium caused cells to fire large, prolonged (up to 15 s) Ca spikes, presumably because most K+ conductances were blocked. Under these conditions the effect of NMA was unchanged or enhanced. Raising [K+]o to 10.5 mM (from the normal 3.5 mM) caused a depolarization and fall in input resistance, but did not change the amplitude or voltage dependence of the NMA response. Reducing [Na+]o caused an initial increase, then usually a delayed decrease in the amplitude of the NMA response.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine mediates a slow synaptic potential in hippocampal pyramidal cells

The hippocampal slice preparation was used to study the role of acetylcholine as a synaptic transmitter. Bath-applied acetylcholine had three actions on pyramidal cells: (i) depolarization associated with increased input resistance, (ii) blockade of calcium-activated potassium responses, and (iii) blockade of accommodation of cell discharge. All these actions were reversed by the muscarinic antagonist atropine. Stimulation of sites in the slice known to contain cholinergic fibers mimicked all the actions. Furthermore, these evoked synaptic responses were enhanced by the cholinesterase inhibitor eserine and were blocked by atropine. These findings provide electrophysiological support for the role of acetylcholine as a synaptic transmitter in the brain and demonstrate that nonclassical synaptic responses involving the blockade of membrane conductances exist in the brain.

Depression of calcium-dependent potassium conductance of guinea-pig myenteric neurones by muscarinic agonists

Intracellular recordings were made from myenteric neurones of the guinea-pig ileum. Muscarinic agonists acetylcholine (ACh) and oxotremorine reduced membrane potassium conductance (gK). Calcium carried into the neurone by one or more action potentials increased membrane potassium conductance (gK, Ca). The time course of the muscarinic changes in gK was compared to that of the change in gK, Ca following an action potential. The time course of conductance decrease was similar in both cases, and both time courses had the same temperature coefficient. Concentrations of ACh (100 nM) which were too low to cause a detectable reduction in resting gK shortened the duration of the gK, Ca increase which followed an action potential. Low concentrations of barium (10-100 microM) had the same effect as ACh. This was not due to a reduction in calcium entry during the action potential. Higher concentrations of ACh and barium also reduced resting membrane conductance. The conductance changes during the muscarinic action and the action potential after-hyperpolarization did not add linearly. It is proposed that muscarinic agonists and barium may act by reducing the availability of calcium ions at a site within the membrane which controls gK.

Asymmetric distribution of acetylcholine receptors and M channels on prepyriform neurons

The responses of pyramidal neurons of rat prepyriform cortex to ionophoretic application of acetylcholine (ACh) were studied in a submerged, perfused brain slice. ACh excited some neurons but only if applied to an area near to the cut surface of the slice. This area contained the basal dendrites of the pyramidal cells and some cell bodies. No excitation was seen if ACh was applied at depths of 250 microns or more from the cut surface, an area which contained only apical dendrites, although the apical dendrites were very sensitive to excitatory amino acids such as aspartate (Asp) and glutamate (Glu). On all neurons which did not discharge to ionophoretic application of ACh, ACh potentiated the response to Glu and Asp. No potentiation of amino acid responses was obtained on apical dendrites. The potentiation had a time course similar to that of the discharge of neurons which fired to ACh. This observation suggests that pyramidal neurons have receptors for ACh on basal but not apical dendrites. The ACh response in the basal dendrite-soma region was elicted by pilocarpine and blocked by atropine but not curare. This was true whether the response studied was direct excitation or potentiation of the response to an amino acid. The ACh response was associated with a voltage-dependent increase in membrane resistance which had a slow time course and appeared to be due to a turning off of an M current, as described by Brown and Adams (1980) in sympathetic ganglion cells. The effects of ACh were minimal at the resting potential but increased with depolartization. ACh had no effect on the current-voltage relation of the cell, except at depolarized potentials of less than -60 mV. Ionophoretic application of Ba2+ to the basal dendritic region resulted in potentiation of the amino acid responses and sometimes induced a discharge similar to that of ACh. Since Ba2+ mimics the ACh response, presumably by a direct blockade of the M channel, the effects of Ba2+ on apical dendrites were tested to determine whether these dendrites contain M channels associated with a transmitter receptor other than ACh. However, Ba2+ did not induce potentiation in apical dendrites, suggesting that M channels are also restricted to the basal dendrites or cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic enhancement of penicillin-induced epileptiform discharges in pyramidal neurons of the guinea pig hippocampus

Acetylcholine (1-20 mM) was applied to guinea pig hippocampal slices bathed in normal and penicillin-containing media. Recordings in the CA 1 pyramidal cell layer in the presence of penicillin showed that acetylcholine caused a prolonged enhancement of the extracellular field potential. Intracellular recordings documented an increase in duration of cell bursting, a decrease in burst afterhyperpolarization, and a membrane depolarization lasting 1-5 min. These results suggest that the actions of acetylcholine to increase membrane excitability interact with penicillin-induced disinhibition to enhance hippocampal epileptogenesis.

Associative conditioning of single sensory neurons suggests a cellular mechanism for learning

A cellular analog of associative learning has been demonstrated in individual sensory neurons of the tail withdrawal reflex of Aplysia. Sensory cells activated by intracellular current injection shortly before a sensitizing shock to the animal's tail display significantly more facilitation of their monosynaptic connections to a tail motor neuron than cells trained either with intracellular stimulation unpaired to tail shock or with tail shock alone. This associative effect is acquired rapidly and is expressed as a temporally specific amplification of heterosynaptic facilitation. The results suggest that activity-dependent neuromodulation may be a mechanism underlying associative information storage and point to aspects of subcellular processes that might be involved in the formation of neural associations.

Cholinergic excitation of mammalian hippocampal pyramidal cells

Responses of CA1 pyramidal neurons to ACh were recorded with intracellular microelectrodes utilizing the in vitro guinea pig hippocampal slice preparation. ACh was delivered by drop or iontophoretic application to stratum oriens or stratum radiatum. Threshold dose for drop application was 1 mM. An initial hyperpolarization of 3.1 +/- 1.8 (S.D.) mV associated with a decrease in membrane input resistance (RN) of 21 +/- 9% (S.D.) occurred in about half the cells. This result is consistent with a presynaptic action of ACh mediated through excitation of inhibitory interneurons. This interpretation was supported by recordings of cholinergic excitatory responses from presumed interneurons, and repetitive spontaneous IPSPs from pyramidal neurons during the hyperpolarization. ACh evoked a slow depolarization (14.3 +/- 10.8 (S.D.) mV) accompanied by a peak increase in apparent input resistance (Ra) of about 60% in the majority of cells. Large increases in spike frequency were associated with these events but action potential shape was unchanged. Plots of Ra versus membrane potential following ACh application revealed that Ra increases were proportionately higher at depolarized membrane potential levels (less than or equal to -70 mV) in some neurons. In these cells Ra was increased significantly at -60 mV (28%), but only 6% at -75 mV. These results are consistent with the conclusion that ACh reduces a voltage-dependent gK, distinct from delayed rectification. ACh also induced a non-voltage-dependent increase in Ra in some cells. ACh-evoked changes in Ra were long-lasting and gave rise to alterations in firing mode, with development of burst generation. ACh also transiently blocked after hyperpolarizations which followed spike trains in pyramidal neurons and presumed interneurons, an action which may be related to effects on a Ca2+-activated gK.