Domoic acid induces a long-lasting enhancement of CA1 field responses and impairs tetanus-induced long-term potentiation in rat hippocampal slices

Domoic acid (DOM) is known to cause hippocampal neuronal damage and produces amnesic effects. We examined synaptic plasticity changes induced by DOM exposure in rat hippocampal CA1 region. Brief bath application of DOM to hippocampal slices produces a chemical form of long-term potentiation (LTP) of CA1 field synaptic potentials. The potentiation cannot be blocked by NMDA receptor antagonist MK-801 but can be blocked by the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 or cAMP-dependent protein kinase (PKA) inhibitor H-89. DOM-potentiated slices show decreased autophosphorylated CaMKII (p-Thr286), an effect that is also dependent on the activity of CaMKII and PKA. Increased phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit GluR1 (p-Ser831) was seen in DOM-potentiated slices. Therefore, aberrant regulation of CaMKII and GluR1 phosphorylation occurs after DOM application. In addition, tetanus-induced LTP as well as the increase of phosphorylation of CaMKII (p-Thr286) were reduced in DOM-potentiated slices. Compared with brief exposure, slices recovering from prolonged exposure did not show potentiation or altered levels of CaMKII (p-Thr286) or GluR (p-Ser831). However, decreased phosphorylation of GluR1 at Ser845 was seen. These results describe a new chemical form of LTP and uncover novel molecular changes induced by DOM. The observed impairment of tetanus LTP and misregulation of CaMKII and GluR1 phosphorylation may partially account for DOM neurotoxicity and underlie the molecular basis for DOM-induced memory deficit.

Activation of metabotropic glutamate receptors by repetitive stimulation in auditory cortex

To determine whether metabotropic glutamate receptors (mGluRs) contribute to the responses of neurons to repetitive stimulation in the rat auditory cortex in vitro, five stimulus pulses were delivered at 2-100 Hz which elicited five depolarizing synaptic responses, f-EPSPs: f-EPSPs(1-5). Stimulus pulses 2-5 delivered at low frequencies (2-10 Hz) elicited f-EPSPs(2-5) that were about 15% smaller than the response elicited by the first pulse (f-EPSP(1)). In the presence of the nonspecific mGluR agonist, ACPD, the amplitude of all f-EPSPs was 40% smaller than predrug responses. APV, CNQX, or bicuculline (antagonists of NMDA-, AMPA/kainate-, and GABA(A)-receptors, respectively) did not change this effect of ACPD. The mGluR antagonist, MCPG, had no effect on f-EPSPs but did reduce the effect of ACPD. High-frequency stimulation (50-100 Hz) elicited f-EPSPs that were smaller with each successive stimulus. In ACPD, f-EPSP(1) was 40% smaller than predrug, but f-EPSPs(3-5) were not changed compared to pre-ACPD f-EPSPs(3-5), indicating that ACPD occludes the effect of repetitive stimulation. MCPG increased f-EPSP(5) by 15%, indicating that a portion of the reduction of f-EPSPs during high-frequency stimulation is mediated by mGluRs. MCPG also partially blocked the effect of ACPD. In CNQX, ACPD only decreased EPSPs, but APV or bicuculline did not change the effect of ACPD. These results suggest that the successive reduction of f-EPSPs during a high-frequency train is partially a result of mGluR activation.

Cholinergic synaptic potentials in the supragranular layers of auditory cortex

Receptive-field plasticity within the auditory neocortex is associated with learning, memory, and acetylcholine (ACh). However, the interplay of elements involved in changing receptive-fields remains unclear. Herein, we describe a depolarizing and a hyperpolarizing potential elicited by repetitive stimulation (20-100 Hz, 0.5-2 sec) and dependent on ACh, which may be involved in modifying receptive-fields. These potentials were recorded, using whole cell techniques, in layer II/III pyramidal cells in the rat auditory cortex in vitro. Stimulation at low stimulus intensities can give rise to a hyperpolarizing response and stimulation at higher stimulus intensities can elicit a depolarizing response. The depolarizing response had a reversal potential of -35 mV, and was reduced by the combination of AMPA/kainate and NMDA glutamate receptor antagonists (AMPA/kainate: CNQX, DNQX, and GYKI 52466; NMDA: APV, MK-801) and by the muscarinic ACh receptor antagonist atropine. The hyperpolarizing response had a reversal potential of -73 mV and could be reduced by atropine, GABA(A) receptor antagonists (bicuculline and a Cl(-) channel blocker picrotoxin), and to a small extent a GABA(B) receptor antagonist (saclofen). This suggests that the hyperpolarizing response is likely to be mediated by ACh acting on GABAergic interneurons. Extracellular recordings, also made from layer II/III of cortical slices, yielded a negative-going potential which was reduced by ionotropic glutamate receptor antagonists (same as above) and by the ACh receptor antagonists atropine and scopolamine, suggesting that this potential was the extracellular representation of the depolarizing response.

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.

Metabotropic glutamate receptors modify ionotropic glutamate responses in neocortical pyramidal cells and interneurons

In neocortex glutamate activates ionotropic and metabotropic receptors (mGluRs). Whole-cell current-clamp recordings in the in vitro rat auditory cortex at 32 degrees C were used to explore the role that mGluRs have in regulation of AMPA/kainate, NMDA, and GABA receptor-mediated synaptic transmission. Our findings are: (a) The fast EPSP (AMPA/kainate), slow EPSP (NMDA), and IPSPs (GABAA, GABAB), elicited in pyramidal neurons are reduced in the presence of (1S,3R)-ACPD (mGluR agonist) with greatest effect on the slow IPSP>fast IPSP>>fast EPSP. The effect is likely the result of ACPD acting at presynaptic mGluRs because the probability of release of glutamate and GABA is reduced in the presence of ACPD, intracellular infusion of a G protein antagonist (GDPPS) did not block the effect of ACPD, nor were iontophoretic kainic acid or NMDA-induced depolarizations reduced by ACPD. (b) The slow EPSP is enhanced following washout of ACPD and enhancement is not due to disinhibition because it is present in the absence of IPSPs, but if IPSPs are present, its magnitude can be influenced. Iontophoretic NMDA responses are enhanced in the presence of ACPD, an effect blocked by GDPbetaS and heparin (intracellular inositol 1,4,5-trisphosphate receptor antagonist). Taken together, this evidence suggests that enhancement is a result of group I postsynaptic mGluR activation. (c) In fast-spiking cells ACPD reduces the EPSP (AMPA/kainate and NMDA-mediated). This action is likely presynaptic because it persists when GDPbetaS is in the cells. (d) The rate of spike discharge recorded from fast-spiking cells is accelerated in ACPD but does not change in the presence of GDPbetaS, suggesting a postsynaptic effect. Our data indicate that mGluRs can influence neocortical synaptic transmission in complex ways by acting presynaptically and postsynaptically.

Role of muscarinic receptors, G-proteins, and intracellular messengers in muscarinic modulation of NMDA receptor-mediated synaptic transmission

Previously, we reported that activation of muscarinic receptors modulates N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission in auditory neocortex [Aramakis et al. (1997a) Exp Brain Res 113:484-496]. Here, we describe the muscarinic subtypes responsible for these modulatory effects, and a role for G-proteins and intracellular messengers. The muscarinic agonist oxotremorine-M (oxo-M), at 25-100 microM, produced a long-lasting enhancement of NMDA-induced membrane depolarizations. We examined the postsynaptic G-protein dependence of the modulatory effects of oxo-M with the use of the G-protein activator GTP gamma S and the nonhydrolyzable GDP analog GDP beta S. Intracellular infusion of GTP gamma S mimicked the facilitating actions of oxo-M. After obtaining the whole-cell recording configuration, there was a gradual, time-dependent increase of the NMDA receptor-mediated slow-EPSP, and of iontophoretic NMDA-induced membrane depolarizations. In contrast, intracellular infusion of either GDP beta S or the IP3 receptor antagonist heparin prevented oxo-M mediated enhancement of NMDA depolarizations. The muscarinic receptor involved in enhancement of NMDA iontophoretic responses is likely the M1 receptor, because the increase was prevented by pirenzepine, but not the M2 antagonists methoctramine or AF-DX 116. Oxo-M also reduced the amplitude of the pharmacologically isolated slow-EPSP, and this effect was blocked by M2 antagonists. Thus, muscarinic-mediated enhancement of NMDA responses involves activation of M1 receptors, leading to the engagement of a postsynaptic G-protein and subsequent IP3 receptor activity.

Muscarinic reduction of GABAergic synaptic potentials results in disinhibition of the AMPA/kainate-mediated EPSP in auditory cortex

The present study is concerned with the ability of muscarinic actions of acetylcholine (ACh) to modulate glutamate and gamma-aminobutyric acid (GABA)-mediated synaptic transmission in the in vitro rat auditory cortex. Whole-cell patch clamp recordings were obtained from layer II-III pyramidal neurons, and the fast-EPSP (AMPA/kainate), fast-IPSP (GABA(A)), and slow-IPSP (GABA(B)), were elicited following a stimulus to deep gray/white matter. Acetyl-beta-methylcholine (MCh), a muscarinic receptor agonist, applied by either superfusion or iontophoresis, produced an atropine-sensitive increase or decrease in the amplitude of the fast-EPSP. The effect of MCh could be predicted by the response of the fast-EPSP to paired-pulse stimulation (i.e. a conditioning pulse followed 300 ms later by a test pulse). The fast-EPSP was decreased in amplitude by MCh in cases where the test-EPSP was suppressed in the pre-MCh condition, and increased in amplitude when the test-EPSP was facilitated. The fast- and slow-IPSPs were always reduced by MCh. In several experiments, the strength of synaptic inhibition was systematically modified by varying stimulus intensity. When the fast-EPSP was elicited in the absence of IPSPs, it was decreased in amplitude by MCh. However, when the fast-EPSP was elicited in conjunction with large IPSPs it was increased in amplitude during MCh. Because the magnitude of the fast-EPSP is influenced by the degree of temporal overlap with IPSPs, it was hypothesized that enhancement of the fast-EPSP was the result of disinhibition produced as a consequence of muscarinic reduction of GABAergic IPSPs. This view was supported by the finding that MCh could reduce the amplitude of pharmacologically isolated GABAergic IPSPs (i.e. elicited in the absence of glutamatergic transmission). Our results suggest that ACh at muscarinic receptors can modify fast glutamatergic neurotransmission differently as a function of strength of inhibition, to suppress that produced by 'weak' inputs and enhance that produced by 'strong' inputs.

Activation of muscarinic receptors modulates NMDA receptor-mediated responses in auditory cortex

The present study examines the ability of muscarinic receptor activation to modulate glutamatergic responses in the in vitro rat auditory cortex. Whole-cell patch-clamp recordings were obtained from layer II-III pyramidal neurons and responses elicited by either stimulation of deep gray matter or iontophoretic application of glutamate receptor agonists. Iontophoresis of the muscarinic agonist acetyl-beta-methylcholine (MCh) produced an atropine-sensitive reduction in the amplitude of glutamate-induced membrane depolarizations that was followed by a long-lasting (at least 20 min) response enhancement. Glutamate depolarizations were enhanced by MCh when elicited in the presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 2,3-dihydroxy-6-nitro-7-sulfamoyl, benzo(F)quinoxaline (NBQX) but not the NMDA antagonists D-2-amino-5-phosphonovaleric acid (APV) or MK-801 hydrogen maleate. The magnitude of enhancement was voltage-dependent with the percentage increase greater at more depolarized membrane potentials. An involvement of NMDA receptors in these MCh-mediated effects was tested by using AMPA/kainate receptor antagonists to isolate the NMDA-mediated slow excitatory postsynaptic potential (EPSP) from other synaptic potentials. The slow EPSP and iontophoretic responses to NMDA were similarly modified by MCh, i.e., both being reduced during and enhanced (15-55 min) following MCh application. Cholinergic modulation of NMDA responses involves the engagement of G proteins, as enhancement was prevented by intracellular infusion with the nonhydrolyzable GDP analog guanosine-5'-O-(2-thiodiphosphate) trilithium salt (GDPbetaS). GDPbetaS was without effect on the early MCh-induced response suppression. Our results suggest that acetylcholine, acting at muscarinic receptors, produces a long-lasting enhancement of NMDA-mediated neurotransmission in auditory cortex, and that this modulatory effect is dependent upon a G protein-mediated event.

Neuropeptide Y receptor agonists: multiple effects on spontaneous activity in the paraventricular hypothalamus

In vitro rat hypothalamic slices were used to examine the ability of neuropeptide Y (NPY), and the putative Y1 and Y2 receptor agonists [Pro34]NPY and [C2]NPY, to modify spontaneous single-neuron discharge in the paraventricular nucleus (PVN). NPY and [Pro34]NPY, at high concentrations (1500 nM), decreased discharge rates. At intermediate concentrations (150 nM) these peptides produced multiple effects, including increases, decreases, and biphasic changes. At lower concentrations (0.15-15 nM), they typically increased discharge rates. In contrast, [C2]NPY, at all concentrations (1.5-1500 nM), predominantly increased discharge rates. Thus, these NPY subtype agonists have multiple effects on discharge rate, which may be due to actions on multiple NPY receptor subtypes.

GABAergic suppression prevents the appearance and subsequent fatigue of an NMDA receptor-mediated potential in neocortex

We have investigated the regulation of an N-methyl-D-aspartate (NMDA) receptor-mediated synaptic potential by gamma-aminobutyric acid (GABA)-mediated inhibition using extracellular and whole-cell voltage clamp recordings in rat auditory cortex in vitro. Single afferent stimulus pulses at low intensity elicited a slow extracellular negativity (Component C) that was mediated by NMDA receptors. At higher intensities, Component C was suppressed by recruitment of GABAergic inhibition. To understand the actions of GABAergic inhibition on Component C, we determined the effects of: (i) paired-pulse stimulation, which depresses GABAergic inhibition; (ii) pharmacological antagonism of GABA receptors; and (iii) afferent stimulation in slices from neonatal rats prior to the development of cortical inhibition. The results indicate that GABAergic inhibition prevents Component C from occurring, thereby preventing its reduction upon repeated stimulation. Whole-cell voltage clamp recordings were used to test the hypothesis that GABAergic suppression occurred by way of membrane hyperpolarization. At hyperpolarized holding potentials no NMDA receptor-mediated synaptic current was elicited, even with paired-pulse stimulation. At depolarized holding potentials a significant NMDA synaptic current was elicited despite the presence of GABAergic synaptic currents. We conclude that membrane hyperpolarization by GABAergic inhibition prevents the appearance and subsequent fatigue of an NMDA receptor-mediated synaptic potential. Reduction of inhibition can act as a 'switch' to fully release the NMDA potential as frequently as once every 10-20 s.

Synaptic interactions involving acetylcholine, glutamate, and GABA in rat auditory cortex

Using electrophysiological techniques in the in vitro rat auditory cortex, we have examined how spontaneous acetylcholine (ACh) release modifies synaptic potentials mediated by glutamate and gamma-aminobutyric acid (GABA). Single stimulus pulses to lower layer VI elicited in layer III a four-component (A-D) extracellular field response involving synaptic potentials mediated by glutamate and GABA. The cholinesterases inhibitor eserine (10-20 microM) or the cholinergic agonist carbachol (25-50 microM) depressed by 10-50% the glutamatergic components A and C, and the GABAergic components B and D. Atropine reversed the depressive effects of eserine and carbachol. A novel finding was that the degree of depression of component A varied inversely with stimulus intensity. However, during partial pharmacological antagonism of GABAA receptors, depression of A varied directly, not inversely, with stimulus intensity. Normally, then, depression of A is offset by reduced GABAergic inhibition of A. We also tested for differential depression of responses mediated by N-methyl-D-aspartate (NMDA) versus non-NMDA glutamate receptors. Following physiological and pharmacological isolation of the responses, eserine depressed the non-NMDA, but not the NMDA, receptor-mediated potential. Since the isolated NMDA potential still could be depressed by carbachol, the data suggested that activation of NMDA receptors may reduce spontaneous ACh release. In support of this, preincubation of slices in NMDA (10-20 microM) largely prevented eserine's, but not carbachol's, depression of components A and B. These results permit three conclusions of relevance to cortical information processing: (1) spontaneous ACh release tonically depresses synaptic potentials mediated by glutamate and GABA; (2) ACh depresses responses to weak inputs to a greater degree than responses to strong inputs: (3) activation of NMDA receptors may "feedback" to reduce ACh release, a mechanism that could place regulation of local ACh release under glutamatergic afferent control.

Facilitation of an NMDA receptor-mediated EPSP by paired-pulse stimulation in rat neocortex via depression of GABAergic IPSPs

1. Tight seal, whole-cell recordings from auditory cortex in vivo and in vitro were obtained to investigate modification of N-methyl-D-aspartate (NMDA) receptor-mediated synaptic activity by paired-pulse afferent stimulation. 2. In recordings from urethane-anaesthetized rats (at 37 degrees C), or from cortical slices maintained in vitro (32 degrees C), afferent stimulation elicited a monosynaptic early EPSP and polysynaptic early and late IPSPs. In addition, a late EPSP could be elicited when the stimulus was preceded by an identical priming stimulus (interval approximately 200 ms). The late EPSP was attenuated by the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV, 50 microM). 3. Bath application of the gamma-aminobutyric acid-B (GABAB) receptor antagonist 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH-saclofen; 50 microM) attenuated the late IPSP and clearly revealed a late EPSP. However, 2-OH-saclofen had lesser effects on the second late EPSP elicited during paired-pulse stimulation. Membrane depolarization in 2-OH-saclofen increased the magnitude of the early IPSP, which suppressed the late EPSP once again. Since pharmacological blockade of EPSPs revealed paired-pulse depression of monosynaptically elicited early and late IPSPs, these data indicate that (1) both early and late IPSPs were capable of suppressing the late EPSP, and (2) these effects were reduced during paired-pulse stimulation. 4. Pharmacological isolation of the late EPSP allowed testing of the direct effect of paired-pulse stimulation. Application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 microM), picrotoxin (10 microM) and 2-OH-saclofen (50 microM) isolated the late EPSP (onset, 3 ms; peak latency, 28 ms; peak amplitude, 7 mV; duration, 240 ms), which grew in magnitude with membrane depolarization and was largely (> 90%) blocked by APV. Paired-pulse stimulation depressed the isolated late EPSP by 30%. 5. Thus, apparent paired-pulse facilitation of the late EPSP is attributable to release from GABAergic inhibition, and not to direct facilitation. Facilitation of the late EPSP is a functional consequence of IPSP depression. The results indicate the importance of inhibition in regulating synaptic activity mediated by NMDA receptors.

Modulation of cellular excitability in neocortex: muscarinic receptor and second messenger-mediated actions of acetylcholine

Muscarinic-type acetylcholine (ACh) receptor are involved in a variety of cortical functions. ACh "activates" neocortex; simultaneously modifying spontaneous subthreshold activity, intrinsic neuronal oscillations and spike discharge modes, and responsiveness to fast (putative glutamatergic) synaptic inputs. However, beyond the general involvement of muscarinic receptors, a mechanistic understanding of integrated cholinergic actions, and interactions with non-cholinergic transmission, is lacking. We have addressed this problem using intracellular recordings from the in vitro auditory neocortex. First, we investigated cholinergic modification of responses to the excitatory amino acid glutamate. ACh, or the muscarinic agonist methacholine, produced a lasting enhancement of glutamate-mediated membrane depolarizations. Muscarinic receptors of the M1 and/or M3 subtype, rather than M2 or nicotinic receptors, mediated this enhancement. Subsequently, we investigated whether second messenger systems contribute to observed muscarinic actions. Activation of protein kinase C with phorbol 12,13-dibutyrate (4 beta-PDBu), enhanced neuronal responses to glutamate. The effect of 4 beta-PDBu was attenuated by the kinase antagonist H7. Finally, we attempted to identify postsynaptic actions of endogenous ACh. Tetanic stimulation of cholinergic afferents elicited voltage-dependent effects, including reduced spike frequency adaptation and reduced slow afterhyperpolarization (sAHP) elicited by transmembrane depolarizing stimuli. These effects were mimicked by methacholine, enhanced by eserine, and antagonized by muscarinic receptor antagonists. These data suggest that cholinergic modulation in neocortex likely involves the integrated actions of diverse mechanisms, primarily gated by muscarinic receptors, and at least partly involving second messenger systems.

Ionic flux contributions to neocortical slow waves and nucleus basalis-mediated activation: whole-cell recordings in vivo

Slow, rhythmic membrane potential (Vm) fluctuations occur spontaneously in cortical neurons of urethane-anesthetized rats, and likely underlie EEG activity in the same low-frequency (1-4 Hz or delta) range. Nucleus basalis (NB) stimulation elicits neocortical activation, simultaneously modifying Vm and EEG fluctuations, by way of cortical muscarinic ACh receptors (Metherate et al., 1992). To investigate the nature of spontaneous fluctuations and their modification by NB stimulation, we have obtained intracellular recordings from auditory cortex using the whole-cell recording technique in vivo. Spontaneous Vm fluctuations appeared to contain three components whose polarity and time course resembled the EPSP, putative Cl(-)-mediated IPSP, and putative K(+)-mediated, long-lasting IPSP elicited by thalamic stimulation. The spontaneous, long-lasting hyperpolarization, whose rhythmic occurrence appeared to set the slow-wave rhythm, was associated with an increased conductance that could shunt the thalamocortical EPSP. We hypothesized that spontaneous Vm fluctuations arise from intermixed rapid depolarizations, rapid Cl(-)-mediated hyperpolarizations, and long-lasting, K(+)-mediated hyperpolarizations. NB-mediated cortical activation might then result from muscarinic suppression of K+ permeability, allowing the rapid depolarizations and Cl- fluxes to continue uninterrupted. Tests of this hypothesis showed that (1) intracellular blockade of K+ channels by rapid diffusion of Cs+ from the recording pipette resulted in suppression of spontaneous, long-lasting hyperpolarizations, mimicking the effect of NB stimulation, and reducing shunting of the thalamocortical EPSP; (2) effects of Cs+ and NB stimulation suggested overlapping, or converging, mechanisms of action; however, there were important differential effects on the spontaneous, long-lasting hyperpolarizations and the K(+)-mediated IPSP; and (3) modifying Cl- fluxes with intracellular picrotoxin or high intracellular Cl- concentrations resulted in spontaneous and NB-elicited large-amplitude depolarizations. We conclude that spontaneous, long-lasting hyperpolarizations are K+ fluxes, but are not "spontaneous" K(+)-mediated IPSPs. Since NB-mediated reduction of spontaneous hyperpolarizations implies muscarinic suppression of a K+ conductance, the spontaneous hyperpolarizations more likely result from the calcium-activated K+ current, IK(Ca). Finally, Cl- fluxes form an important component of activated Vm fluctuations that acts to restrain excessive depolarization.

Nucleus basalis stimulation facilitates thalamocortical synaptic transmission in the rat auditory cortex

Nucleus basalis (NB) neurons are a primary source of neocortical acetylcholine (ACh) and likely contribute to mechanisms of neocortical activation. However, the functions of neocortical activation and its cholinergic component remain unclear. To identify functional consequences of NB activity, we have studied the effects of NB stimulation on thalamocortical transmission. Here we report that tetanic NB stimulation facilitated field potentials, single neuron discharges, and monosynaptic excitatory postsynaptic potentials (EPSPs) elicited in middle to deep cortical layers of the rat auditory cortex following stimulation of the auditory thalamus (medial geniculate, MG). NB stimulation produced a twofold increase in the slope and amplitude of the evoked short-latency (onset 3.0 +/- 0.13 ms, peak 6.3 +/- 0.21 ms), negative-polarity cortical field potential and increased the probability and synchrony of MG-evoked unit discharge, without altering the preceding fiber volley. Intracortical application of atropine blocked the NB-mediated facilitation of field potentials, indicating action of ACh at cortical muscarinic receptors. Intracellular recordings revealed that the short-latency cortical field potential coincided with a short-latency EPSP (onset 3.3 +/- 0.20 ms, peak 5.6 +/- 0.47 ms). NB stimulation decreased the onset and peak latencies of the EPSP by about 20% and increased its amplitude by 26%. NB stimulation also produced slow membrane depolarization and sometimes reduced a long-lasting IPSP that followed the EPSP. The combined effects of NB stimulation served to increase cortical excitability and facilitate the ability of the EPSP to elicit action potentials. Taken together, these data indicate that NB cholinergic neurons can modify neocortical functions by facilitating thalamocortical synaptic transmission.

Cellular bases of neocortical activation: modulation of neural oscillations by the nucleus basalis and endogenous acetylcholine

In the mammalian neocortex, the EEG reflects the state of behavioral arousal. The EEG undergoes a transformation, known as activation, during the transition from sleep to waking. Abundant evidence indicates the involvement of the neurotransmitter acetylcholine (ACh) in EEG activation; however, the cellular basis of this involvement remains unclear. We have used electrophysiological techniques with in vivo and in vitro preparations to demonstrate actions of endogenous ACh on neurons in auditory neocortex. In vivo stimulation of the nucleus basalis (NB), a primary source of neocortical ACh, (1) elicited EEG activation via cortical muscarinic receptors, (2) depolarized cortical neurons, and (3) produced a change in subthreshold membrane potential fluctuations from large-amplitude, slow (1-5 Hz) oscillations to low-amplitude, fast (20-40 Hz) oscillations. The NB-mediated change in pattern of membrane potential fluctuations resulted in a shift of spike discharge pattern from phasic to tonic. Stimulation of afferents in the in vitro neocortex elicited cholinergic actions on putative layer 5 pyramidal neurons. Acting via muscarinic receptors, endogenous ACh (1) reduced slow, rhythmic burst discharge and facilitated higher-frequency, single-spike discharge in burst-generating neurons, and (2) facilitated the appearance and magnitude of intrinsic membrane potential oscillations. These in vivo and in vitro observations suggest that neocortical activation results from muscarinic modulation of intrinsic neural oscillations and firing modes. Rhythmic-bursting pyramidal neurons in layer 5 may act as cortical pacemakers; if so, then modifying their discharge characteristics could alter local cortical networks. Larger, intercortical networks could also be modified, due to the widespread projections of NB neurons. Thus, NB cholinergic neurons may play a critical role in producing different states of neocortical function.

Synaptic potentials and effects of amino acid antagonists in the auditory cortex

Neurons of in vitro guinea pig and rat auditory cortex receive a complex synaptic pattern of afferent information. As many as four synaptic responses to a single-stimulus pulse to the gray or white matter can occur; an early-EPSP followed, sequentially, by an early-IPSP, late-EPSP, and late-IPSP. Paired pulse stimulation and pharmacological studies show that the early-IPSP can modify information transmission that occurs by way of the early-EPSP. Each of these four synaptic responses differed in estimated reversal potential, and each was differentially sensitive to antagonism by pharmacological agents. DNQX (6,7-dinitroquinoxaline-2,3-dione), a quisqualate/kainate receptor antagonist, blocked the early-EPSP, and the late-EPSP was blocked by the NMDA receptor antagonist APV (D-2-amino-5-phosphonovalerate). The early-IPSP was blocked by the GABA-a receptor antagonist bicuculline, and the late-IPSP by the GABA-b receptor antagonists 2-OH saclofen or phaclofen. Presentation of stimulus trains, even at relatively low intensities, could produce a long-lasting APV-sensitive membrane depolarization. Also discussed is the possible role of these synaptic potentials in auditory cortical function and plasticity.

Basal forebrain stimulation modifies auditory cortex responsiveness by an action at muscarinic receptors

We have hypothesized that auditory cortex plasticity involves modification of thalamocortical transmission by basal forebrain (BF) cholinergic neurons, and that this action may involve muscarinic receptors. In a first test of this hypothesis, we report that BF stimulation can suppress or facilitate, depending on the intensity of stimulation, auditory cortical responses elicited by thalamic stimulation. BF-mediated facilitation is antagonized by atropine, implicating muscarinic receptors. These data suggest that BF cholinergic neurons functionally modify auditory cortex by regulating thalamocortical transmission.

Differential effects of the muscarinic M2 antagonists, AF-DX 116 and gallamine, on single neurons of rabbit sympathetic ganglia

Intracellular recording techniques were used to compare the effects of the M2 muscarinic antagonists, AF-DX 116 and gallamine, on membrane potential (Vm), input resistance (Ri), responses induced by methacholine, muscarinic slow postsynaptic potentials and action potentials in the superior cervical ganglion of the rabbit. Gallamine or AF-DX 116 antagonized methacholine-induced or synaptically-evoked muscarinic hyperpolarization, without having significant effect on depolarization induced by methacholine or synaptically. The drug AF-DX 116 reduced evoked muscarinic hyperpolarizing potentials, without significant change in Vm or Ri, recorded in the absence of muscarinic stimulation. In contrast to AF-DX 116, gallamine elicited a concentration-dependent depolarization of the membrane, with a corresponding increase in Ri, when tested in the absence of muscarinic stimulation. These effects of gallamine were accompanied by an increase in duration and decrease in the slope of the descending phase of the action potential. Blockade by gallamine of evoked hyperpolarization was independent of membrane depolarization and readily occurred when gallamine-induced depolarization was prevented by clamping Vm at its pre-gallamine level. The effects of gallamine were maintained during its presence and reversed upon washing with gallamine-free physiological solution. These results indicate that AF-DX 116 and gallamine have a specificity for antagonism of muscarinic responses, mediated by receptors of the M2 type in the superior cervical ganglion. However, gallamine, while an effective antagonist of M2 responses, also has the ability to modify the electrical characteristics of ganglion cells and thus may modify ganglionic transmission by mechanisms other than antagonism of receptors.

Acetylcholine modifies neuronal acoustic rate-level functions in guinea pig auditory cortex by an action at muscarinic receptors

Cholinergic modification of neuronal responsiveness in auditory cortex includes alteration of spontaneous and tone-evoked neuronal discharge. Previously it was suggested that the effects of acetylcholine (ACh) and muscarinic agonists on neuronal discharge resembled those due to increases in the intensity of acoustic stimuli (Ashe et al. 1989). To determine the relationship between neuronal modifications due to ACh acting at muscarinic receptors and those due to changes in stimulus intensity, we determined acoustic rate-level functions for neurons in the auditory cortex of barbiturate-anesthetized guinea pigs before, during and after administration of ACh. ACh facilitated acoustic rate-level functions in 82% of the cells tested. In addition, during ACh administration 66% of neurons responded to stimuli that were previously subthreshold, that is, ACh decreased the response threshold. Cholinergic facilitation of rate-level functions was attenuated by the general muscarinic antagonist atropine. The nature of the muscarinic receptors involved in the actions of ACh was further examined by presenting single tones before, during, and after administration of ACh and specific muscarinic receptor subtype antagonists, either pirenzepine (M1) or gallamine (M2). ACh-induced facilitation of spontaneous and tone evoked neuronal discharge was antagonized by pirenzepine, but not by gallamine, suggesting the involvement of the M1 muscarinic receptor subtype. These data indicate that ACh can facilitate stimulus-evoked responses and decrease response thresholds for neurons in auditory cortex, possibly via activation of M1 muscarinic receptors. Such effects of ACh acting at muscarinic receptors could underly cholinergic regulation of information processing in the auditory cortex.

Argiotoxin-636 blocks excitatory synaptic transmission in rat hippocampal CA1 pyramidal neurons

Argiotoxin 636, (AR636), a synaptic antagonist from orb weaver spider venom, is shown to produce reversible blockade of excitatory transmission in CA1 pyramidal neurons of the in vitro rat hippocampus. Microtopical application of AR636 (5-50 nM) resulted in a concentration-dependent suppression of the amplitude of the dendritic field EPSP recorded from stratum radiatum, and the amplitude of the population spike recorded from stratum pyramidale in response to stimulation of the Schaffer collaterals. The maximum effect of AR636 occurred at about 15-25 min. These effects were reversible after washing with toxin-free physiological solution with the rate of recovery having an inverse relationship to the concentration of AR636. In contrast to the effects observed with orthodromic stimulation, the amplitude of the antidromic spike was not affected by exposure to AR636. The temporal pattern of GABAergic paired-pulse inhibition was unaffected by exposure to AR636. Neuronal discharge elicited by pressure ejection of L-glutamate was abolished by AR636, whereas, responses to L-aspartate were not significantly affected. These data suggest that AR636 functions as a selective antagonist of glutamate-mediated synaptic transmission in rat hippocampus.

Cholinergic modulation of frequency receptive fields in auditory cortex: II. Frequency-specific effects of anticholinesterases provide evidence for a modulatory action of endogenous ACh

Exogenously applied muscarinic agonists--for example, acetylcholine (ACh) and acetyl-beta-methacholine (MCh)--modify frequency receptive fields in auditory cortex of unanesthetized animals in a frequency-specific rather than global manner. The present study sought to relate these findings to endogenous actions of ACh by using the anticholinesterase agents eserine sulphate and soman (0-1,2,2-trimethylpropylmethylphosphonofluoridate) to facilitate the effects of endogenous ACh. Frequency receptive fields (FRF) were determined by presenting sequences of different isointensity tones before, during, and after application of ACh, MCh, eserine, or soman; also the cholinesterase blockers were applied between applications of ACh or MCh. The major effects produced by the inhibitors were similar to those of the agonists. Predominant effects were frequency-specific changes in FRF. Further, eserine and soman, similar to ACh and MCh, produced shifts in the best frequency (BF) of FRF due mainly to coordinated depression of responses to the BF and increased responses to adjacent, non-BF. The results indicate that exogenous and endogenous ACh, acting via muscarinic receptors, can significantly influence the physiological functioning of cortical neurons and consequently their processing of sensory information.

Cholinergic modulation of frequency receptive fields in auditory cortex: I. Frequency-specific effects of muscarinic agonists

Previously we reported that acetylcholine (ACh) and acetyl-beta-methacholine (MCh) modify responses of neurons in auditory cortex to individual frequencies. The purpose of this study was to determine whether muscarinic agonists produce frequency-specific alterations or general changes in cellular responses. Frequency-specific modifications would be evident in alterations of frequency receptive fields (FRF) that differed across frequencies while general effects would be seen as changes that were more or less the same over frequencies. Responses of single neurons to designated sets of tones were recorded in the auditory cortex of chronically prepared awake cats before, during, and following ejection of ACh or MCh by iontophoresis or micropressure using multibarrel micropipettes. Frequency receptive fields were determined by presenting isointensity tones across a range of frequencies including the cell's best frequency (BF) to tone onset. FRF for "off" and "sustained (through)" responses were also determined quantitatively. The effects of ACh and MCh were predominantly frequency-specific (77%, 39/51 cells); general changes (19%, 10/51) and no effects (4%, 2/51) were less likely. Frequency-specific effects involved both facilitation and reduction of the same response component to different frequencies within the same neuron. For responses to tone onset (but not "through" and "off" responses), agonists were more likely to produce a decrease at the BF while simultaneously increasing responses to other frequencies. Agonists could increase or decrease frequency selectivity. Effects of agonists could be blocked by atropine, suggesting involvement of muscarinic receptors.

AF-DX 116: a selective antagonist of the slow inhibitory postsynaptic potential and methacholine-induced hyperpolarization in superior cervical ganglion of the rabbit

AF-DX 116 [11-([2-[(diethylamino)methyl]-1-piperdinyl]acetyl)-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiaze pine-6-one], a muscarinic receptor antagonist that divides the M2-type muscarinic receptor into additional functional classes, modified muscarinic responses recorded from the superior cervical ganglion of the rabbit with sucrose or air gap techniques. Incubation of ganglia with AF-DX 116 suppressed the amplitude of the slow-inhibitory postsynaptic potential (s-IPSP) in a concentration-dependent and highly specific manner. At concentrations which reduced the amplitude of the s-IPSP by 80 to 90%, there was no significant reduction of the amplitudes of the muscarinic slow-excitatory postsynaptic potential or the nicotinic fast-excitatory postsynaptic potential. In addition, superfusion of ganglia with AF-DX 116 resulted in the concentration-dependent suppression of ganglionic hyperpolarization induced by methacholine without suppression of methacholine-induced depolarization. Ganglionic hyperpolarization that was produced by norepinephrine was unaffected by AF-DX 116. Increasing the level of acetylcholine available for interaction with muscarinic receptors by increasing the number of stimulus volleys that were applied to the preganglionic nerve resulted in a parallel shift, to the right, of the concentration-response curve for suppression of the s-IPSP by AF-DX 116. Similarly, incubation of ganglia with the specific antiacetylcholinesterase, BW 284 (1-5-bis[4-allyl dimethylammonium phenyl]pentan-3 one dibromide), increased the concentration of AF-DX 116 that was required to produce a comparable suppression of the s-IPSP. These results indicate that the s-IPSP in mammalian superior cervical ganglion involves an action of acetylcholine at the M2 type receptor that is preferentially blocked by AF-DX 116.

Muscarinic agonists modulate spontaneous and evoked unit discharge in auditory cortex of cat

The present experiments studied the effects of cholinergic agonists and antagonists on the spontaneous and acoustic-evoked discharge of auditory cortical neurons and examined whether these effects were mediated by muscarinic cholinergic receptors. A primary focus of this report is the analysis of specific effects of these agents on the spontaneous and tone-evoked discharge and on different temporal components of the evoked discharge. Single neurons were recorded in the auditory cortex of chronically prepared, awake cats with multibarrel micropipette electrodes. The responses to acoustic stimuli were obtained before, during, and following continuous ejection of cholinergic agonist or antagonists by micropressure. The mean rate of discharge of the neurons was analyzed quantitatively for spontaneous discharge and for different peaks of the tone-evoked PSTH corresponding to tone "on," "through," and "off" responses. Acetylcholine (ACh) and acetyl-beta-methacholine (MCh) produced significant effects on spontaneous activity in 72% and 68% of neurons tested, respectively. Tone-evoked responses were effected in 92% and 82% of cells tested, respectively. The ability of these agonists to modify spontaneous or evoked activity was dose-dependent. Agonist effects on spontaneous and evoked activity were often different in the same cell; however, effects on spontaneous activity did predict effects on "through" responses. The most common effect of ACh or MCh on evoked activity was facilitation of the tone "on" response. For neurons with multicomponent discharge patterns in response to tones, the agonists had nonuniform effects on different response components. However, the effects of ACh on the "on" and "off" responses covaried. Hence cholinergic agonists produce heterogeneous, selective effects on different components of the responses of auditory cortical neurons rather than simple increases or decreases in discharge level. The effects of cholinergic agonists were modified in the presence of atropine. The effects of MCh were blocked by atropine in a higher proportion of cases than those of ACh.

Modification of nicotinic ganglionic transmission by muscarinic slow postsynaptic potentials in the in vitro rabbit superior cervical ganglion

The influence of slow muscarinic postsynaptic potentials, i.e., the s-IPSP and s-EPSP, on synaptic transmission mediated through nicotinic receptors was studied in the superior cervical ganglion of the rabbit. Postganglionic spikes and synaptic potentials were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. When paired stimulus volleys were separated by brief intervals (20-100 msec) or long intervals (1,000-8,000 msec), the population spike elicited by the test stimulus was larger in amplitude than that elicited by the conditioning volley. When paired stimulus volleys were separated by 250-500 msec, the amplitude of the population spike elicited by the test volley was smaller than that elicited by the conditioning stimulus. Gallamine, which selectively blocks the s-IPSP, reduced the suppression of the test spike which occurred when stimulus IPIs ranged between 250-500 msec. Pirenzepine, which selectively blocks the s-EPSP, reduced the late facilitation of test postganglionic spikes which occurred with stimulus IPIs greater than 1,000 msec. The non-selective muscarinic antagonist QNB, produced changes in postganglionic spike amplitude that were similar to the combined effects of gallamine and pirenzepine. The evidence indicates that the s-IPSP and s-EPSP modified the excitability state of the ganglionic neurons and subsequent synaptic transmission that was mediated through nicotinic receptors.

Antagonist discrimination of two muscarinic responses elicited by applied agonists and orthodromic stimuli in superior cervical ganglion of rabbit

Pirenzepine and gallamine selectively and differentially antagonized two muscarinic responses, in the superior cervical ganglion of the rabbit, whether elicited by the muscarinic agonist methacholine or by orthodromic stimulation. Methacholine elicited a biphasic ganglionic response, consisting of hyperpolarizing and depolarizing components that were the agonist-induced equivalents of the slow-inhibitory and slow-excitatory postsynaptic potentials elicited by orthodromic stimulation. Superfusion of ganglia with pirenzepine resulted in a concentration-dependent suppression of depolarization induced by methacholine with no suppressant action on ganglionic hyperpolarization. In contrast, superfusion of ganglia with gallamine resulted in a concentration-dependent suppression of ganglionic hyperpolarization and the slow-inhibitory postsynaptic potential. These effects occurred without appreciable suppression of ganglionic depolarization or the slow-excitatory postsynaptic potential. The action of gallamine was specific for muscarinic hyperpolarization. Hyperpolarizations produced by superfusion with dopamine or norepinephrine were unaffected by gallamine, at concentrations that suppressed the muscarinic slow-inhibitory post-synaptic potential. Incubation with anti-cholinesterases produced a parallel shift, to the right, of concentration-response curves for suppression by gallamine of the slow-inhibitory postsynaptic potential. This was presumably the consequence of an increase in the acetylcholine available for interaction with the muscarinic receptor. The evidence suggests that the ability of gallamine and pirenzepine to suppress selectively the slow-inhibitory and slow-excitatory postsynaptic potentials, as previously demonstrated, is through an action at muscarinic receptors. Furthermore, the data suggest that these pharmacological agents produce their effects by interaction at different muscarinic recognition sites.

Responses of opossum and rat hippocampal CA1 cells to paired stimulus volleys

Short-term synaptic plasticity was studied in the in vitro hippocampus of the North American opossum (Didelphis virginiana) and rat (Rattus norvegicus). Conditioning and test stimulus pulses were delivered to fibers in stratum radiatum, and intracellular and extracellular recordings were obtained from area CA1 pyramidal cells. In rat, the amplitude of the population spike in response to the second (test) of two stimulus impulses is suppressed at short inter-pulse-intervals (IPI's). In opossum, the amplitude of the test population spike is facilitated at comparable IPI's. Facilitation of the test population spike in rat occurs only when the test stimulus is separated from the first stimulus (conditioning) by a longer IPI. Peak values of facilitation do not significantly differ between species. Intracellular responses, elicited by stimulus pulses that were subthreshold for spike production, indicate that the amplitude of test EPSP's recorded from opossum pyramidal cells are facilitated at IPI's that result in suppression of test EPSP's in rat pyramidal cells.

Differential and selective antagonism of the slow-inhibitory postsynaptic potential and slow-excitatory postsynaptic potential by gallamine and pirenzepine in the superior cervical ganglion of the rabbit

Two cholinergic antagonists, gallamine and pirenzepine, agents that have been shown to bind selectively to different subpopulations of the muscarinic receptor, were found to antagonize selectively and differentially the amplitudes of the slow-inhibitory and slow-excitatory postsynaptic potentials in the superior cervical ganglion of the rabbit. Incubation of ganglia with gallamine resulted in a concentration-dependent suppression of the slow-inhibitory postsynaptic potential. The pharmacological action of gallamine was highly specific. At concentrations which reduced the amplitude of the slow-inhibitory postsynaptic potential by as much as 70-90%, there was no reduction of the amplitudes of the muscarinic slow-excitatory postsynaptic potential, the nicotinic fast-excitatory postsynaptic potential, noncholinergic slow-slow-excitatory postsynaptic potential, or post-stimulus hyperpolarizing afterpotentials. The amplitude of the slow-excitatory postsynaptic potential was actually facilitated in the presence of gallamine, presumably as a result of suppression of the overlapping slow-inhibitory postsynaptic potential. In contrast to the action of gallamine, pirenzepine produced a selective suppression of the amplitude of the slow-excitatory postsynaptic potential. Pirenzepine had very little influence on the amplitude of the slow-inhibitory postsynaptic potential at concentrations sufficient to reduce the amplitude of the slow-excitatory postsynaptic potential by as much as 50%, and had no effect on the amplitudes of the nicotinic fast-excitatory postsynaptic potential or noncholinergic slow-slow-excitatory postsynaptic potential. The evidence presented suggests that multiple muscarinic recognition sites, previously identified by studies of the affinities of pharmacological agents for the muscarinic receptor, may actually be involved in synaptic transmission and functionally coupled to cellular effector mechanisms.

Modulation by dopamine of population spikes in area CA1 hippocampal neurons elicited by paired stimulus pulses

Extracellular recording techniques were used to study the effects of dopamine on postactivation excitability of rat area CA1 hippocampal neurons maintained in vitro. Population spikes were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. The interval between the conditioning and test volley was set to separate delivery of stimuli by 10 to 80 msec. The effect of superfusion or microtopical application of dopamine (DA) on population responses to test stimulus pulses was studied. When paired stimulus volleys, separated by brief intervals (up to 40 msec), were delivered to afferent fibers, paired-pulse suppression (PPS) was indicated by the amplitude of the population spike elicited by the test volley being smaller than that elicited by the conditioning volley. When paired volleys were separated by longer intervals (40 to 80 msec), the response elicited by the test volley was larger in amplitude than that elicited by the conditioning volley, indicating paired-pulse facilitation (PPF). Following exposure to DA, the amplitude of the population response elicited by the conditioning volley was larger than the amplitude before exposure to DA. This effect was long-lasting, enduring for tens of minutes. However, when the amplitude of the conditioning population response was held constant, the PPS was decreased, indicating disinhibition. It is suggested that dopamine produces a long-lasting attenuation of an intervening inhibitory influence onto CA1 pyramidal neurons.

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.

Effect of inhibitors of protein synthesis on dopamine modulation of the slow-EPSP in rabbit superior cervical ganglion

The protein synthesis inhibitors, anisomycin and cycloheximide, were tested for their ability to prevent dopamine-induced long-term enhancement of the slow-EPSP in rabbit superior cervical ganglion. Exposure of ganglia to either inhibitor of protein synthesis, at a concentration that suppressed [3H]leucine incorporation into ganglionic protein by at least 95%, had no effect on the development of dopamine-induced enhancement of the slow-EPSP. Incubation of ganglia with dopamine, without an inhibitor of protein synthesis, was without effect on [3H]leucine incorporation into ganglionic protein. It is concluded that synthesis of new protein is not required for the development of long-term enhancement of the slow-EPSP induced by dopamine.

Ganglionic slow postsynaptic potentials and muscarinic asynchronous discharge in postganglionic nerve elicited by orthodromic stimulation

Simultaneous recordings of the slow postsynaptic potentials of in vitro rabbit superior cervical ganglion and the muscarinic asynchronous discharge in the postganglionic nerve were obtained in order to examine their relationship. Analyses for linear correlation indicated that the peak amplitude of the S-EPSP cannot fully determine the maximum rate of muscarinic asynchronous discharge. The rate of asynchronous discharge in the postganglionic nerve systematically increased with increases in either the frequency or duration of preganglionic stimulation even after the S-EPSP was at asymptotic levels. We suggest that this deviation from linearity is the result of repetitive spike discharge elicited by the S-EPSP. The S-IPSP had no apparent influence on generation of asynchronous afterdischarge, but may have had an influence on asynchronous discharge occurring during a train of stimuli. Incubation of ganglia with gallamine, an antagonist of the ganglionic muscarinic receptors mediating hyperpolarization, resulted in the selective blockade of the S-IPSP. Neither the S-EPSP nor the muscarinic afterdischarge were suppressed by muscarinic receptor blockade by gallamine, nor did gallamine produce any effect on the nicotinic F-EPSP or the noncholinergic SS-EPSP. Temporary exposure of ganglia to dopamine, in the presence of an inhibitor of catechol-O-methyltransferase, was followed by a potentiation of the muscarinic afterdischarge in accordance with the long-term enhancement reported for the S-EPSP.

Pharmacological properties and monoaminergic mediation of the slow IPSP, in mammalian sympathetic ganglion

Phenoxybenzamine can selectively eliminate the s-IPSP, in the presence of anti-cholinesterases that enhance s-IPSP and s-EPSP; and the alpha 2-antagonist, yohimbine, can partially but consistently depress s-IPSP selectively. The results provide positive pharmacological support for the monoaminergic nature of the transmitter for s-IPSP in mammalian sympathetic ganglia and argue against suggestions that the s-IPSP is a direct hyperpolarizing response to acetylcholine.

Metoclopramide mimics a D-1 type of dopamine action in rabbit superior cervical ganglion

Metoclopramide (MCP) in a sufficiently high concentration (100 microM) induced a large and persisting potentiation of slow-excitatory postsynaptic potentials (s-epsp) and slow-inhibitory postsynaptic potentials (s-epsp) but depressed in fast epsp. This modulatory action of metoclopramide was markedly suppressed by (+)-butaclamol (7 microM) and, to a lesser extent, by spiroperidol (2.5-4 microM). Metoclopramide also possessed weak anti-acetylcholinesterase activity(I50% = 245 microM; measured by Dr N. Inestrosa), but this was shown not to account for the potentiating actions of metoclopramide. Thus, although metoclopramide is a D-2 antagonist, it appears to mimic the D-1 action of dopamine in modulating the slow psps.

Modulation of slow postsynaptic potentials by dopamine, in rabbit sympathetic ganglion

(1) Temporary exposure of rabbit's superior cervical ganglion (SCG) to dopamine (DA), in the presence of an inhibitor of catechol-o-methyltransferase (COMT) is consistently followed by a potentiation of the slow (s)-EPSP and s-IPSP, lasting for some hours. The fast (f)-EPSP is not significantly increased, but it is better maintained than in control ganglia. (2) Exposure to the COMT-inhibitor U-0521 alone induces less but substantial potentiations of both s-PSPs. This effect is explained as due to protection of DA released intraganglionically at rest. (3) This evidence suggests that COMT may significantly limit the access of catecholamines to postsynaptic receptors, for at least certain types of neuron-to-neuron synaptic actions. (4) The potentiation of both s-PSPs, whether induced by DA in the presence of U-0521 or by U-0521 alone, is depressed by DA-1 antagonists that have been found to depress DA-stimulation of adenyl cyclase in rabbit SCG; these are spiroperidol, butaclamol and, to a lesser extent, bromocriptine. The specific 'DA-2' antagonists metoclopramide and sulpiride, and the alpha-adrenergic antagonist dihydroergotamine, did not depress potentiation. (5) Potentiation of s-EPSP is viewed as identical in nature to the previously discovered DA-modulatory enhancement of direct muscarinic depolarizing actions (by acetylcholine or its agonists). Potentiation of s-IPSP may be due to a similar DA-modulation of other muscarinic response(s) involved in mediating the s-IPSP. The consistency and comparative ease with which these DA-modulatory effects can be induced, under presently described experimental conditions, should facilitate future study of this mode of synaptic action.

Orthodromic production of non-cholinergic slow depolarizing response in the superior cervical ganglion of the rabbit

1. A late slow depolarization in the rabbit superior cervical ganglion, recorded extracellularly as ;late late negative' (l.l.n.) response, can be elicited by suitably repetitive stimulation of cervical sympathetic nerve. The l.l.n. response is not blocked by strong nicotinic, muscarinic or adrenergic antagonists; it appears with latencies in seconds, rise times in minutes, durations of up to 20 min or more, and extracellular amplitudes that can exceed 1 mV when recorded in an air-gap chamber.2. The l.l.n. component is a graded post-synaptic response that decreases with a length constant similar to those of the known p.s.p.s (fast e.p.s.p., slow i.p.s.p., and slow e.p.s.p.). This and its other characteristics indicate that the l.l.n. response is neuronally generated and represents a non-cholinergic late slow depolarization. The term ;slow slow e.p.s.p.' is suggested for this response, to replace both ;slow depolarization' and ;late slow e.p.s.p.'.3. The amplitudes, evaluated relative to the compound action potentials, and the durations of l.l.n. responses recorded from intact neurones of rabbit superior cervical ganglion were considerably greater and more consistently producible than the non-cholinergic slow depolarizations recorded by others from impaled neurones of guinea-pig inferior mesenteric ganglion.4. The l.l.n. response does not exhibit the special sensitivity to sodium azide previously found for the muscarinic ;late negative' or slow e.p.s.p. response.5. The total number of orthodromic volleys is the chief determinant of the amplitude and duration of the l.l.n. response. Increases in pulse frequency, with no change in pulse number, exert only a minor influence on amplitude and duration of the l.l.n. response but can markedly decrease latency and rise time.6. Even very low pulse frequencies (e.g. 1/sec) are almost as effective as higher frequencies if a sufficiently large number of stimulus pulses is applied.7. The features of orthodromic production of the l.l.n., slow slow e.p.s.p. response, as well as the amplitudes and durations of this depolarization, indicate that this non-cholinergic post-synaptic response could, like the muscarinic slow e.p.s.p., play a significant role in mediating physiological activities of sympathetic ganglia.

Mesencephalic multiple-unit activity during acquisition of conditioned pupillary dilation

Multiple-unit recordings were obtained from the interstitial nucleus of Cajal, nucleus of Darkschewitsch and the superior colliculus of the cat during acquisition of classically conditioned pupillary dilation. Multiple-unit responses in all regions were enhanced by conditioning procedures. However, only the acquisition functions for the accessory oculomotor nuclei, i.e., interstitial nucleus of Cajal and nucleus of Darkschewitsch, were significantly correlated with the acquisition of conditioned pupillary dilation. These results were discussed in relation to the mechanism of autonomic control of conditioned pupillary dilation. It was concluded that inhibition of parasympathetic pupillomotor efferents via the accessory oculomotor nuclei may play a role in the acquisition of conditioned pupillary dilation.

Modification of auditory and somatosensory system activity during pupillary conditioning in the paralyzed cat

The role of sensory systems in the development of behavioral conditioned responses was investigated by recording multiple-unit activity in the auditory and somatosensory pathways during Pavlovian conditioning of the pupillary-dilation responses of paralyzed cats. Establishment of conditioned pupillary-dilation responses to a white noise CS+, pupillary discrimination between the CS+ and a tone CS-, and subsequent discrimination reversal provided the behavioral foundation for examining neural changes related to behavioral learning. Multiple-unit responses to the acoustic CS+ were significantly enhanced in the auditory cortex, cochlear nucleus, and somatic cortex, but not in the cuneate nucleus. The possibility that these effects could be due to changes in stimulus intensity at the sensory receptor, to mo-ement artifacts, or to feedback from skeletal responses were ruled out because the animals were immobilized. Nor could these neural changes be attributable to sensitization, as those brain areas which showed conditioned enhancement to the CS+ exhibited significantly larger responses to the CS+ than to the CS-. Furthermore, the changes in neural activity followed the significance of the CS; after reversal of the reinforcement contingencies, the amount of multiple-unit activity evoked by the stimuli gradually reversed too. Although the somatic cortex showed conditioning and discrimination, greater stimulus specificity was found in the auditory system. Only in the somatic cortex was there a significant increase in responses to the CS- as well as the CS4. Furthermore, both somatosensory loci exhibited enhanced responses to those tactile probes presented during the acoustic CS, suggesting a phasic increase in neural excitability to all stimuli. Analysis of the number of trials required to attain an acquisition criterion indicated that the neural changes occurred first in the auditory cortex, then the cochlear nucleus, followed in turn by the somatic cortex, and finally the cuneate nucleus. However, none of these neural changes preceded acquisition of conditioned pupillary dilations. These results suggest that sensory system changes are not essential for the initial associative process. These findings indicate that the study of autonomic conditioned responses may prove beneficial in seeking the critical neural events which underlie the initial association between two stimuli. A hypothetical model, which explains the development of pupillary and sensory system conditioned responses, was also presented.