Noradrenaline blocks accommodation of pyramidal cell discharge in the hippocampus

Correlation between electroencephalogram isoelectric time and hippocampal norepinephrine levels, measured by microdialysis, during ischemia in rats

It is suggested that norepinephrine (NE) plays a role during transient forebrain ischemia. NE may have a protective action against neuronal cell death in the hippocampus, or it may be one of the causes of injurious ischemic effects. We used the microdialysis technique to study extracellular NE levels in the rat hippocampus before, during, and after 30 min of transient incomplete forebrain ischemia (induced by four-vessel occlusion) to describe the time course of NE in this condition. There was a maximal increase (fivefold) in extracellular NE after 10 min of reflow only when the electroencephalogram was isoelectric. NE levels returned to baseline 40 min after release of the carotid clamps and remained constant for the next 80 min. Thus there appears to be a transient NE overflow in the hippocampus during ischemia, closely related to the complete loss of brain electrical activity.

Functional significance of cyclic AMP secretion in cerebral cortex

Cyclic AMP secretion in response to beta adrenergic receptor stimulation has been demonstrated in glioma-derived cell lines, in cerebral cortex in dissociated cell culture, and in the frontal cortex of living animals. The possible functions of cAMP secretion are considered; in particular, a role for this phenomenon in mediating some of the actions of norepinephrine as a neuromodulator in cerebral cortex.

Dibutyryl cyclic AMP has epileptogenic potential in the hippocampus of freely behaving rats: a combined EEG-intracerebral microdialysis study

The effects of dibutyryl cyclic AMP were studied with the combined EEG-intracerebral microdialysis technique in the hippocampus of freely behaving rats. It was found that intrahippocampal microdialysis with this drug produced epileptiform EEG events associated with limbic type behavioral seizures. The dibutyryl cyclic AMP-induced seizures developed with a long latency, and persisted for a prolonged period even after the removal of the drug from the microdialysis fluid. Similar EEG or behavioral manifestations did not occur during intrahippocampal microdialysis with artificial cerebrospinal fluid or ATP solutions. These data suggest that in the hippocampus, in vivo, the cyclic AMP second messenger system may be involved in potentially epileptogenic excitatory processes.

Pyramidal neurons in immature rat hippocampus are sensitive to beta-adrenergic agents

The development of hippocampal neuronal sensitivities to the beta-noradrenergic agent, isoproterenol, was examined in tissue from immature rats. The in vitro hippocampal slice preparation was used to assess intracellularly recorded responses from hippocampal neurons to pressure-pulse and bath application of noradrenergic drugs. Effects of the drug on individual hippocampal CA3 pyramidal neurons were compared across several stages of development, ranging from postnatal day 4-5 (P4-5) to maturity. Isoproterenol, pressure-pulse applied to CA3c pyramidal cells, produced a depolarization of membrane potential and an increase in cell input resistance in tissue as young as P7. Spike frequency adaptation (in trains of action potentials triggered by depolarizing pulses) was reduced, as were the slow after-hyperpolarizations following the spike trains. All agonist effects were blocked by timolol, a beta-antagonist. Drug-induced changes in cell membrane and firing properties in immature tissue were qualitatively similar to beta-receptor-mediated noradrenergic effects in adult tissue. These results indicate that the beta-receptor-mediated component of the noradrenergic effect in rat hippocampus is physiologically functional by the seventh day of postnatal life; at earlier times (P4-5) these beta-receptor-mediated noradrenergic actions are, at best, equivocal.

Action potentials produce a long-term enhancement of M-current in frog sympathetic ganglion

M-current is a voltage-gated K+ current that can be turned off by the muscarinic action of acetylcholine. We examined the effects of postsynaptic action potential firing on the level of M-current in B-cells of the bullfrog sympathetic ganglion. High frequency stimulation of action potentials induced an approximately two-fold increase in the level of the M-current that could last up to 35 min. The 'enhanced' M-current was similar to the 'resting' one in its time-dependence, voltage-dependence and sensitivity to neurotransmitters. Experiments were undertaken to examine the functional consequences of the enhanced M-current. Following high frequency stimulation the number of spikes evoked by depolarizing current was reduced. In addition, the excitatory postsynaptic potential (EPSP) evoked by maximal input became subthreshold, thereby blocking information flow through the ganglion cell. These results indicate that the enhancement of M-current by spikes provides a negative feedback mechanism for the control of excitability. It has been reported that postsynaptic stimulation of ganglion cells also produces a long-term increase in the nicotinic EPSP, but we were unable to confirm this observation.

Clonidine prevents corticotropin releasing factor-induced epileptogenic activity in rats

Studies have shown that intracerebroventricular (i.c.v.) injection (10-20 micrograms) of corticotropin releasing factor (CRF) in rats induces epileptiform activity characterized by a regular (pacemaker-like) spiking pattern located in hippocampal leads. CRF has also been shown to increase the firing rate of noradrenergic neurons in the locus ceruleus. Our experiments clarified the possible role of norepinephrine (NE) in mediating hippocampal activity of CRF. Intraperitoneal (i.p.) injection of the alpha 2-agonist clonidine at a dose of 0.5-5 micrograms/kg prevented, in a dose-related manner, the hippocampal epileptiform activity induced by CRF (20 micrograms i.c.v.). Our results suggest a possible role of NE in CRF-induced spiking activity.

Beta-adrenergic receptors: astrocytic localization in the adult visual cortex and their relation to catecholamine axon terminals as revealed by electron microscopic immunocytochemistry

It has long been recognized that noradrenaline, the most abundant catecholamine within the visual cortex, plays important roles in modulating the sensitivity of cortical neurons to visual stimuli. However, whether or not these noradrenaline effects are confined to a discrete synaptic specialization or mediated by diffuse modulation of a group of synapses has remained an issue open for debate. The aim of this study was to examine the cellular basis for noradrenaline action within the visual cortex of adult rats and cats. To this end, I used electron microscopic immunocytochemistry to examine the relationship between (1) catecholamine axon terminals and beta-adrenergic receptors (beta AR), which, together, may define the effective sphere of noradrenaline modulation; and then (2) these putative sites for catecholamine modulation and axospinous asymmetric junctions where excitatory neurotransmission is likely to dominate. Antibodies against beta AR were used at light and electron microscopic levels on the visual cortex of rat and cat. Rat visual cortex was also labeled simultaneously for beta AR and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), to determine the ultrastructural relationships between catecholamine terminals and beta AR. Immunoperoxidase labeling revealed that beta AR404, a polyclonal antibody directed against the C-terminal tail of hamster lung beta AR (beta 2-type), recognized astrocytic processes predominantly. In contrast, beta AR248, a polyclonal antibody directed against the third cytoplasmic loop, recognized neuronal perikarya as observed in previous studies. Dual labeling for beta AR404 and TH revealed that catecholamine axon terminals that contained numerous vesicles formed direct contacts with astrocytic processes exhibiting beta AR404 immunoreactivity. However, some catecholamine axon terminals that lacked dense clusters of vesicles were positioned away from beta AR404-immunoreactive astrocytes. Frequently, beta AR-immunoreactive astrocytic processes surrounded asymmetric axospinous junctions while also contacting catecholamine axon terminals. These observations support the possibility that, through activation of astrocytic beta AR, noradrenaline modulates astrocytic uptake mechanism for excitatory amino acids, such as L-glutamate. Astrocytic beta AR might also define the effective sphere of catecholamine modulation through alterations in the morphology of distal astrocytic processes and the permeability of gap junctions formed between astrocytes.

Modulation of synaptic efficacy in field CA1 of the rat hippocampus by forskolin

Activation of cAMP-dependent protein kinase (kinase A) has recently been shown to enhance responses evoked by stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in cultured hippocampal pyramidal neurons. Here we report results of experiments designed to determine if activation of the cAMP cascade potentiates synaptic strength in field CA1 of rat hippocampal slices. We find that bath application of the direct adenylate cyclase activator forskolin (50 microM) enhances the field excitatory postsynaptic potential (EPSP) slope and population spike amplitude evoked by stimulation of Schaffer/commissural afferents. This effect is potentiated by the phosphodiesterase inhibitor and adenosine receptor antagonist 3-isobutyl-1-methylxanthine (IBMX). The enhancement produced by forskolin is suppressed in the presence of adenylate cyclase inhibitors and is not mimicked by the inactive forskolin analogue 1,9-dideoxyforskolin, indicating that, indeed, activation of adenylate cyclase mediates the effects of forskolin in field CA1. Our observations support the idea that changes in intracellular cAMP levels can modulate synaptic efficacy of excitatory glutamatergic synapses in the mammalian hippocampus.

Vasoactive intestinal polypeptide modulates neuronal excitability in hippocampal slices of the rat

Vasoactive intestinal polypeptide added at submicromolar concentrations to the perfusion fluid of rat hippocampal slices and slice cultures enhanced the excitability of CA1 and CA3 pyramidal cells in several ways. Specifically, cells were depolarized and the Ca(2+)- and cyclic AMP-dependent potassium conductance was blocked as demonstrated by reduction of the long-lasting afterhyperpolarization and the accommodation of firing. This was also found in tetrodotoxin-containing medium. In low Ca(2+)-high Mg2+ medium (in synaptic isolation) the firing rate was increased. Synaptic transmission was potentiated: extracellularly registered excitatory postsynaptic potentials and population spikes in response to stratum radiatum stimulation and intracellularly recorded excitatory postsynaptic potential-inhibitory postsynaptic potential sequences were enhanced. These results are in keeping with the known stimulation of adenylate cyclase by vasoactive intestinal polypeptide.

Agonists of cholinergic and noradrenergic receptors facilitate synergistically the induction of long-term potentiation in slices of rat visual cortex

Acetylcholine (ACh) and noradrenaline (NA) have been shown to facilitate experience-dependent modifications of synaptic connectivity during postnatal development of the kitten visual cortex. To further investigate the mechanisms of this facilitation we studied the effects of these neuromodulators in an in vitro model of use-dependent synaptic plasticity. We have chosen long-term potentiation (LTP) in rat visual cortex slices because it shares several features with the in vivo model. In both cases induction of synaptic modifications requires that postsynaptic activation reaches a critical threshold and in both cases changes are induced more easily in young animals and when N-methyl-D-aspartate (NMDA) receptor-gated conductances are activated. Intracellular recordings were obtained from regular spiking cells in supragranular layers of rat visual cortex and LTP was induced by tetanic stimulation of the underlying white matter. Both cholinergic and noradrenergic agonists raised the probability that tetanic stimuli induced LTP and as in vivo they acted synergistically. These effects were mediated by agonists of muscarinic and beta-receptors, respectively. The agonists of both receptor systems enhanced the depolarizing response to the tetanus and increased NMDA receptor-gated conductances during this response. We suggest that this mode of action also accounts for the facilitatory effects which ACh and NA have on use-dependent synaptic plasticity in the developing visual cortex.

Norepinephrine activates potassium conductance in neurons of the turtle cerebral cortex

Whole-cell voltage and current clamp recordings were obtained from cortical neurons of the pond turtle, Pseudemys scripta elegans. Norepinephrine (NE) induced an outward current in 50% of pyramidal neurons. This current had a reversal potential of -88.3 +/- 3.2 mV, consistent with a K+ conductance increase, and had a mean amplitude of 18.3 +/- 7.2 pA at -40 mV. The ionic dependence and pharmacological analyses are both consistent with alpha 2 adrenergic receptor stimulation. Inhibition of Na(+)-dependent action potentials with TTX did not diminish the NE-induced K+ conductance, indicating that NE acts directly on the postsynaptic neuron. In addition to effects on postsynaptic conductance, NE dramatically decreased the amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in 55% of pyramidal neurons. The decrease in spontaneous IPSCs was observed both in those neurons which exhibited an increase in K+ conductance in response to NE administration (81%) and in those which did not (33%). Thus, NE modulates neuronal excitability both directly by activating a postsynaptic K+ conductance and indirectly by decreasing spontaneous IPSCs.

Presynaptic Glutamate Receptors Regulate Noradrenaline Release from Isolated Nerve Terminals

The wide-ranging neuronal actions of excitatory amino acids, such as glutamate, are thought to be mediated mainly by postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. We now report the existence of presynaptic glutamate receptors in isolated nerve terminals (synaptosomes) prepared from hippocampus, olfactory bulb, and cerebral cortex. Activation of these receptors by NMDA or non-NMDA agonists, in a concentration-dependent manner, resulted in Ca(2+)-dependent release of noradrenaline from vesicular transmitter stores. The NMDA-stimulated release was potentiated by glycine and was blocked by Mg2+ and selective NMDA antagonists. In contrast, release stimulated by selective non-NMDA agonists was blocked by 6-cyano-7-nitroquinoxaline-2,3- dione, but not by Mg2+ or NMDA antagonists. Our data suggest that the presynaptic glutamate receptors can be classified pharmacologically as both the NMDA and non-NMDA types. These receptors, localized on nerve terminals of the locus ceruleus noradrenergic neurons, may play an important role in interactions between noradrenaline and glutamate.

Suppression of amygdala kindling with massed stimulation: effect of noradrenaline antagonists

Afterdischarge (AD) triggered by brief, daily stimulation of the amygdala progressively increases in complexity and duration and, over days, develops into generalized convulsions. This progression, called kindling, is delayed by noradrenaline (NA). When brief stimulation of the amygdala occurs too frequently (massed), there is a suppression of AD growth and little evidence of kindling. Previously we showed that depletion of NA before massed amygdala stimulation prevented the suppression of AD growth described above, and readily precipitated generalized seizures. In the present report, we examined the role of NA in maintaining this suppression of AD growth, after it was well established. We showed that suppression of AD development during the first 15 massed stimulations (interstimulus interval of 5 min) was reduced by subsequent injection of the NA alpha 2 antagonist, yohimbine, with most rats exhibiting occasional generalized convulsions. Conversely, rats exposed to the beta antagonist, propranolol, like controls, not only showed suppressed AD growth, but also elevated AD thresholds. Three weeks later, only a small positive transfer to daily kindling was observed in all groups. We conclude that alpha 2 NA receptors help maintain suppression of AD growth induced by massed stimulation of the amygdala, while beta receptors provide only a small proepileptic influence. These results and those from the 'rapid' kindling model (Lothman et al., Brain Research, 360 (1985) 83-91) are compared, and related to NA receptor subtype variations in the amygdala and hippocampus.

Long-term potentiation in the hippocampus is blocked by tyrosine kinase inhibitors

Long-term potentiation (LTP) in the hippocampus is thought to contribute to memory formation. In the Ca1 region, LTP requires the NMDA (N-methyl-D-aspartate) receptor-dependent influx of Ca2+ and activation of serine and threonine protein kinases. Because of the high amount of protein tyrosine kinases in hippocampus and cerebellum, two regions implicated in learning and memory, we examined the possible additional requirement of tyrosine kinase activity in LTP. We first examined the specificity in brain of five inhibitors of tyrosine kinase and found that two of them, lavendustin A and genistein, showed substantially greater specificity for tyrosine kinase from hippocampus than for three serine-threonine kinases: protein kinase A, protein kinase C, and Ca2+/calmodulin kinase II. Lavendustin A and genistein selectively blocked the induction of LTP when applied in the bath or injected into the postsynaptic cell. By contrast, the inhibitors had no effect on the established LTP, on normal synaptic transmission, or on the neurotransmitter actions attributable to the actions of protein kinase A or protein kinase C. These data suggest that tyrosine kinase activity could be required postsynaptically for long-term synaptic plasticity in the hippocampus. As Ca2+ calmodulin kinase II or protein kinase C seem also to be required, the tyrosine kinases could participate postsynaptically in a kinase network together with serine and threonine kinases.

Amphetamine actions on pre- and postpubertal rat hippocampal dentate granule neurons

Clinical evidence suggests different actions of amphetamine (AMPH) in children and adults. Using intracellular recording techniques, the actions of AMPH at 10 and 40 microM were investigated in granule neurons of hippocampal slices from pre- and postpubertal rats. AMPH (10-40 microM) caused depolarization of most postpubertal neurons, often with increased spontaneous activity, whereas most prepubertal neurons were hyperpolarized. In both age groups, AMPH caused increased neuronal excitability by reducing spike threshold, attenuating the postspike train afterhyperpolarization, reducing spike frequency adaptation, and potentiating excitatory postsynaptic potentials. Changes in cell input resistance were variable and Ca2+ currents were unaffected. AMPH actions took 10-15 min to appear and became maximal 30-55 min after application. The effects were reversible at 10 microM, but at 40 microM, prolonged washout for up to 2 h did not completely reverse these actions. The beta-adrenergic blocker, propranolol, partially blocked AMPH actions. The dopamine (D2) blocker, haloperidol, did not block AMPH actions. Mature neurons were also tested with 2.5 microM AMPH showing similar but more reversible effects as the higher concentrations. Depleting catecholamines by reserpine partly attenuated the effects of 40 microM AMPH in mature neurons. Perfusion of neurons with 10 and 20 microM cocaine did not produce effects similar to those of AMPH. It is suggested that AMPH produces its effects on granule neurons only in part through the release of norepinephrine. The involvement of other neurotransmitters and/or neuromodulators released by AMPH, or direct postsynaptic actions of AMPH are also possible.

Calcium-activated K+ channels: metabolic regulation

Calcium-activated potassium (KCa) channels are highly modulated by a large spectrum of metabolites. Neurotransmitters, hormones, lipids, and nucleotides are capable of activating and/or inhibiting KCa channels. Studies from the last few years have shown that metabolites modulate the activity of KCa channels via: (1) a change in the affinity of the channel for Ca2+ (K 1/2 is modified), (2) a parallel shift in the voltage axis of the activation curves, or (3) a change in the slope (effective valence) of the voltage dependence curve. The shift of the voltage dependence curve can be a direct consequence of the change in the affinity for Ca2+. Recently, the mechanistic steps involved in the modulation of KCa channels are being uncovered. Some interactions may be direct on KCa channels and others may be mediated via G-proteins, second messengers, or phosphorylation. The information given in this review highlights the possibility that KCa channels can be activated or inhibited by metabolites without a change in the intracellular Ca2+ concentration.

Electrophysiological actions of VIP in rat somatosensory cortex

Electrophysiological and biochemical studies suggest that VIP may exert a facilitating action in the neocortical local circuitry. In the present study, we examined the actions of VIP and VIP + norepinephrine (NE) on somatosensory cortical neuron responses to direct application of the putative transmitters acetylcholine (ACh) and gamma-aminobutyric acid (GABA). Spontaneous and transmitter-induced discharges of cortical neurons from halothane-anesthetized rats were monitored before, during and after VIP, NE and VIP + NE iontophoresis. In 57 VIP-sensitive cells tested, VIP application (5-70 nA) increased (n = 18), decreased (n = 36) or had biphasic actions (n = 3) on background firing rate. In a group of 20 neurons tested for NE + VIP, the combined effect of both peptide and bioamine was predominantly (70%) inhibitory. On the other hand, inhibitory and excitatory responses of cortical neurons to GABA (11 of 15 cases) and ACh (10 of 18 cases), respectively, were enhanced during VIP iontophoresis. Concomitant application of VIP and NE produced additive (n = 2) or more than additive (n = 3) enhancing effects on GABA inhibition. NE administration reversed or enhanced further VIP modulatory actions on ACh-induced excitation. These findings provide electrophysiological evidence that NE and VIP afferents may exert convergent influences on cortical neuronal responses to afferent synaptic inputs such that modulatory actions are anatomically focused within the cortex.

Increased effect of noradrenaline on synaptic responses in rat CA1 hippocampal area after adrenalectomy

Pyramidal neurons in the rat CA1 hippocampal area contain intracellular receptors for the steroid hormone corticosterone, and membrane associated alpha- and beta-adrenergic receptors. We compared the effect of noradrenaline (NA) on the synaptically evoked CA1 field potential in adrenalectomized (ADX) rats, where corticosteroid receptors are not occupied, with sham-operated controls. It appeared that the increase in the amplitude of the field potential induced by 10 microM NA was more pronounced in slices from ADX rats than in sham operated controls. In vitro application of 30 nM corticosterone (20 min) to slices from ADX rats shifted the NA-evoked increases in synaptic efficacy towards the level of the sham operated rats. The incidence of a secondary population spike after application of NA was increased in slices from ADX rats compared to slices from sham controls or ADX rats treated with corticosterone. Occupation of receptors for corticosterone in the hippocampal CA1 area may therefore potentially lead to suppression of excitability.

Synaptic localization of adrenergic disinhibition in the rat hippocampus

Norepinephrine is an endogenous neurotransmitter that reduces synaptic inhibition onto pyramidal neurons in the hippocampus by an action at an alpha-adrenergic receptor. The physiological mechanism of this disinhibition was previously not known, except that it occurred at a site presynaptic to the inhibited pyramidal cell. In this paper we present evidence that adrenergic disinhibition is restricted to the early phase of the evoked inhibitory postsynaptic potential in area CA1 of the hippocampus. The locus of disinhibition does not appear to reside in the interneuronal terminal, axon, or cell body. Instead, adrenergic agonists appear to reduce evoked synaptic inhibition by depressing excitatory synapses that activate the interneuron.

Acute monoaminergic depletion in the rat potentiates the excitatory effect of the subthalamic nucleus in the substantia nigra pars reticulata but not in the pallidal complex

Recent neurochemical evidence suggests that chemical or electrical stimulation of the subthalamic nucleus (STH) increases dopamine release in the substantia nigra (SN) with a subsequent decrease in the striatum. In a previous paper, we reported that bicuculline-induced activation of the STH increases neuronal activity in the substantia nigra pars reticulata (SNpr) and in the pallidal complex. In order to investigate the role played by the dopaminergic system in the observed activation, the neuronal responses of subthalamic nucleus target structures were studied in amine depleted rats following subthalamic stimulation. Amine depletion was accomplished by pretreating the rats with reserpine (2 mg/kg; S.C.) and with alpha-methyl-para-tyrosine (alpha-mpt; 50 mg/kg; I.P.). Following this treatment, dopamine levels were reduced by 94% in the striatum as measured by HPLC. Amine depletion significantly increased the spontaneous activity of subthalamic cells by 53%. In the SNpr, no significant changes in the spontaneous neuronal activity were observed, but the excitatory responses to bicuculline-induced stimulation of the STH were potentiated as compared to non-treated animals. In the pallidial complex (GP-EP), no potentiation was found. The data suggest that the spontaneous pattern of discharge of the STH is probably under monoaminergic control. They also suggest a reciprocal interaction between dopamine and glutamatergic afferent terminals from the STH within the SNpr, but not in the pallidal complex.

Glutamate mediates a slow synaptic response in hippocampal slice cultures

Glutamate (GLU) mediates its 'fast' excitatory transmitter action in the brain by directly gating cation-selective ion channels ('ionotropic' receptors). However, GLU can also activate another type of receptor, coupled to phospholipase C ('metabotropic' receptor). In hippocampal cells, stimulation of this metabotropic receptor by GLU, or by a racemic mixture of (1S-3R and 1R-3S) 1-aminocyclopentyl-1,3-dicarboxylate (ACPD), induces a slower excitation mediated by inhibition of K+ currents. We have assessed whether this slow form of metabotropic receptor excitation can contribute to the effects of synaptically released GLU in hippocampal slice cultures, by recording the responses of CA3 pyramidal cells to afferent mossy fibre stimulation. When the fast ionotropic response was blocked pharmacologically, mossy fibre stimulation produced a slow depolarizing postsynaptic potential associated with a decrease in membrane conductance, a depression of the slow after-hyperpolarization following a train of action potentials, and reduced accommodation during the action potential train. Under voltage-clamp, mossy fibre stimulation produced a slow voltage-dependent inward current which resembled that produced by application of exogenous ACPD or quisqualate (QUIS), and which was occluded by these metabotropic agonists. We therefore suggest that synaptically released GLU can induce two types of postsynaptic responses: a fast excitation through activation of ionotropic receptors and a slower excitation associated with inhibition of K+ conductances through activation of metabotropic receptors. This is analogous to the dual action of acetylcholine on ionotropic (nicotinic) and metabotropic (muscarinic) receptors.

Nonlinearity of the inhibition underlying retinal directional selectivity

An important mechanism for the discrimination of direction of motion in the retina is a spatially asymmetric inhibition. This inhibition has been postulated to operate either as a subtraction, like in difference-of-Gaussians' models, or as a division, like in shunting-inhibition models of directional selectivity. The latter, but not the former, is nonlinear. This raises the question of whether the inhibitory mechanism involved in directional selectivity is nonlinear. To investigate this issue, we studied the linearity of the contrast dependence of the extracellularly recorded responses to apparent motions in ON-OFF directionally selective ganglion cells of the rabbit retina. The results show that the inhibition underlying directional selectivity is nonlinear and fits shunting-inhibition models well. Other biophysical mechanisms that might account for the type of nonlinearity observed in the data are also considered.

Regulation of spontaneous activity and oscillatory spike firing in rat midbrain dopamine neurons recorded in vitro

Intracellular recordings were obtained from identified dopamine (DA) neurons in rat midbrain slices maintained in vitro. DA neuron membranes exhibited pronounced instantaneous and time-dependent anomalous rectification that showed evidence of maximal activation at average membrane potentials of -63 and -78 mV, respectively. Action potentials were followed by prominent afterhyperpolarizations (AHP) that consisted of two components. The fast component showed evidence of inactivation at -63 mV independent of the initial membrane potential, whereas the longer-duration, later component increased in amplitude at hyperpolarized potentials. Unlike DA neurons recorded in vivo, there was no evidence of spike frequency adaptation or summation of AHPs with prolonged depolarization-induced spike trains. Spontaneous spike discharge occurred via an endogenous pacemaker potential that was dependent on both TTX-sensitive and cobalt-sensitive processes. Hyperpolarizing prepulses could activate rebound pacemaker discharge, but this rebound activity was progressively blocked with larger-amplitude hyperpolarizing prepulses. DA neurons recorded in the anesthetized animal, freely moving animal, and in vitro preparations have been shown to exist in two states of activity: 1) spontaneously discharging action potentials or 2) hyperpolarized, quiescent, and nonfiring. Furthermore, although it is rare to find DA neurons in the untreated animal in transitional states of activity, quiescent neurons can be activated by stimuli that place a demand on the DA system. The evidence presented here is consistent with the hypothesis that the special combination of membrane properties of DA neurons contribute to the segregation of their activity into active or inactive states.

Effect of corticosteroid hormones on electrical activity in rat hippocampus

Pyramidal neurons in the rat CA1 hippocampal area contain both mineralocorticoid (MR) and glucocorticoid receptors (GR) which bind the endogenous adrenal steroid corticosterone with differential affinity. With intracellular electrophysiological recording techniques we have investigated how corticosterone affects the membrane properties of these cells. We observed that low doses (1 nM) of corticosterone or aldosterone can, through MR, reduce the spike frequency accommodation and afterhyperpolarization (AHP) evoked by a short depolarizing current in pyramidal neurons. As the accommodation/AHP can be considered as an intrinsic mechanism of CA1 neurons to attenuate transmission of excitatory input, the MR-mediated action might potentially enhance cellular excitability in the CA1 area. Higher doses of corticosterone or selective glucocorticoids were able to reverse the MR-mediated effect on accommodation/AHP, eventually increasing particularly the amplitude of the AHP. GR-mediated events may thus potentially suppress excitability in the hippocampal CA1 area. Not only current- but also transmitter-induced membrane effects were affected by the steroids. Firstly, GR-ligands were able to suppress a temporary noradrenaline-evoked decrease in accommodation/AHP. Secondly, membrane hyperpolarizations induced by serotonin were reduced by MR-agonists. We propose that cellular excitability in the hippocampus is at least partly under control of coordinative, antagonistic MR- and GR-mediated effects on electrical activity.

Noradrenergic and locus coeruleus modulation of the perforant path-evoked potential in rat dentate gyrus supports a role for the locus coeruleus in attentional and memorial processes

The perforant path-dentate gyrus synapse has provided a model system for functional neural plasticity in adult mammalian brain. NMDA-dependent long-term changes in neural connectivity occur at this synapse in response to high-frequency input. Norepinephrine (NE) applied exogenously or released endogenously can initiate both a short- and a long-term potentiation (LTP) of the dentate gyrus response to perforant path input. Triggering of the potentiated response depends on beta-receptor activation and does not require a high-frequency stimulus. An increase in locus coeruleus (LC) activity can initiate both short and LTP of the perforant path response, although a reduction in LC activity does not alter baseline perforant path responses. This chapter considers differences between NE modulation in vitro and in vivo, differences and similarities between NE-LTP and frequency-induced LTP, and the surprising specificity of NE effects at the perforant path synapse. Studies of NE in the dentate gyrus support a role for the LC in promoting both short- and long-term enhancement of responses to complex sensory inputs and are consistent with a role for the LC in memorial as well as attentional processes.

Actions of norepinephrine in the rat hippocampus

Acting at postsynaptic alpha 1- and beta 1-receptors, norepinephrine (NE) exerts a complex action in rat hippocampus. It is currently believed that beta 1-receptor activation enhances excitability of recorded neurons, whereas alpha 1 activation suppresses reactivity to afferent stimulation. These reported effects of alpha-agonists are not consistent with alpha 1 effects found elsewhere in the brain. We have conducted experiments in the anesthetized rat and found that an amphetamine-induced increase in the dentate gyrus population spike can be blocked by a beta-antagonist but also by an alpha 1-antagonist. We have conducted experiments in the brain slide preparation and found that an alpha-agonist, phenylephrine (PHE), selectively enhances responses to N-methyl-D-aspartate (NMDA) but not to quisqualate. We propose that the product of activation of both alpha- and beta-receptor types will enhance reactivity of hippocampal cells to afferent stimulation.

Regional heterogeneity in the distribution of neurotransmitter markers in the rat hippocampus

A detailed neurochemical analysis of the distribution of markers for the most relevant neurotransmitter systems within the rat hippocampal formation has been performed. The hippocampi, obtained from unfrozen brains of male Sprague-Dawley rats were subdissected into tissue parts containing mainly CA1, CA3 or the dentate gyrus, respectively. Each part was further divided into ventral and dorsal halves. In these six hippocampal subregions the concentrations of noradrenaline, dopamine, serotonin, 3-methoxy-4-hydroxyphenylglycol, 5-hydroxyindoleacetic acid and the putative neurotransmitter amino acids glutamate, aspartate, GABA, glycine and taurine, and the levels of somatostatin and neuropeptide Y and the activities of choline acetyltransferase, acetylcholinesterase and glutamate decarboxylase were measured. A marked heterogeneity in the subregional distribution of markers for various neurotransmitter systems within the hippocampal formation was observed. Each neuronal marker was characterized by an individual pattern of distribution. Most of the markers showed a concentration-gradient, increasing from dorsal to ventral; only taurine was more abundant in the dorsal than in the ventral parts and no dorsoventral difference was seen for aspartate, glycine and neuropeptide Y. The highest molar ratios of total 3-methoxy-4-hydroxyphenylglycol to noradrenaline and 5-hydroxyindoleacetic acid to serotonin were found in the dorsal hippocampus. The levels of noradrenaline, GABA and glutamate decarboxylase activity were highest in the dentate gyrus and lowest in CA1. The concentrations of somatostatin were highest in CA1; those of serotonin were highest in CA3. Highest activities of choline acetyltransferase and acetylcholinesterase were found in the dentate gyrus; lowest activities were found in CA3. In CA3 the lowest values of glutamate, aspartate, taurine and somatostatin were also found. The heterogeneity in the distribution of individual neurochemical markers allows insights into possible functional differences of hippocampal subregions and provides a relevant basis for future neurochemical investigations in this brain area.

Splanchnic input to thoracic spinal neurons and its supraspinal modulation in the rat

The urinary responses of 62 T8-T11 spinal neurons were recorded extracellularly following electrical stimulation of the greater splanchnic nerve (GSN) in chloralose-anesthetized rats. Recorded neurons were found in both the dorsal and ventral horns. Fifty-seven neurons increased their firing rate in response to GSN stimulation; 8 of these exhibited biphasic responses consisting of excitations followed by inhibitions. Excitatory responses to GSN stimulation consisted of either one or two bursts with latencies consistent with activation by either A delta or C fibers. GSN stimulation inhibited 5 neurons. The effects of reversible spinalization on spontaneous activity and on both synchronous and non-synchronous (afterdischarge) GSN-evoked responses were investigated using a cooling probe on the spinal cord between C1 and C2. Of 19 neurons tested in this way, 9 exhibited opposite directional changes in their spontaneous activities and their GSN-evoked responses upon spinalization. Differential effects of cold-block on first and second bursts, or on A delta- and C-fiber mediated responses, were not usually observed. However, differential effects of cold-block on synchronous and non-synchronous portions of the overall GSN-evoked response were often observed in that their magnitudes often changed independently of one another. Supraspinal pathways contributed to GSN-evoked responses of several neurons because their responses were diminished during cooling while spontaneous activity was increased or unchanged. These decreases in the magnitude of the GSN-evoked response were not always accounted for by decreases in the synchronous portions of the responses. However, most neurons did exhibit decreases in the number of non-synchronous responses, or afterdischarges, during spinal cooling, exhibiting in some cases biphasic responses. This study provides evidence for strong supraspinal regulation of splanchnic afferent input to the spinal cord of the rat. Further, this regulation exhibits some specificity toward different portions of splanchnic-evoked responses in spinal neurons.

Potassium conductances in hippocampal neurons blocked by excitatory amino-acid transmitters

Excitatory amino acids mediate fast synaptic transmission in the central nervous system through the activation of at least three distinct ionotropic receptors: N-methyl-D-aspartate (NMDA), the alpha-amino-3-hydroxy-5-methyl-isoxasole-4-propionate (AMPA)/quisqualate (QUIS) and the kainate subtypes (for reviews, see refs 1, 2). They also activate the additional QUIS 'metabotropic' receptor (sensitive to trans-1-amino-cyclopentyl-1,3-dicarboxylate, ACPD) linked to inositol phospholipid metabolism. We have used hippocampal slice cultures to study the electrophysiological consequences of the metabotropic response. We find that activation of an ACPD-sensitive QUIS receptor produces a 'slow' excitation of CA3 pyramidal cells, resulting from depression of a Ca2(+)-dependent K+ current and a voltage-gated K+ current. Combined voltage-clamp and microfluorometric recordings show that, although these receptors can trigger an increase in intracellular Ca2+ concentration, suppression of K+ currents is independent of changes in intracellular Ca2+. These effects closely resemble those induced by activating muscarinic acetylcholine receptors in the same neurons and suggest that excitatory amino acids not only act as fast ionotropic transmitters but also as slow neuromodulatory transmitters.

Altered hippocampal network excitability in the hypernoradrenergic mutant mouse tottering

A latent, gene-linked alteration of hippocampal network excitability in tg/tg mutant mice was unmasked in vitro by convulsant-activated synchronous neuronal discharges. Exposure to elevated extracellular potassium ions or 4-aminopyridine, but not picrotoxin, revealed an abnormally prolonged network discharge duration in the mutant CA3 pyramidal cell region. In both phenotypes, noradrenaline, and a selective beta-noradrenergic receptor agonist, isoproterenol, reversibly accelerated the frequency of the discharges. These findings identify an intrinsic alteration in the excitability of an isolated neuronal network in a model of inherited generalized spike-wave epilepsy, and further implicate noradrenergic mechanisms in the temporal modulation of hippocampal synchronization and epileptogenesis.

Effects of pentetrazol on neuronal activity and on extracellular calcium concentration in rat hippocampal slices

Effects of pentetrazol (PTZ) were studied on neuronal responses in dentate granule cells and area CA1 hippocampal pyramidal cells with intra- and extracellular recording techniques. PTZ induced spontaneous epileptiform field potential transients in areas CA3 and CA1, but not in the dentate gyrus. The concentration optimum for induction of spontaneous epileptiform activity was 2 mM. The epileptiform activity compared in many respects to that induced by GABA antagonists such as picrotoxin, bicuculline and penicillin. Paired pulse stimulus induced responses were affected by concentrations of 0.5 mM. In the concentration range 0.5-2 mM mostly disinhibitory effects were noted. Stimulus induced Ca2+ concentration changes were found to be maximally augmented at concentrations of 2-5 mM. In this range, intracellular studies revealed a block of frequency habituation and an increase in input resistance. The convulsant action of PTZ decreased at concentrations above 5 mM, probably due to a decrease of inward currents. We suggest that the action of PTZ in screening studies for anticonvulsants is mostly due to a decrease of GABAA-receptor mediated IPSPs.

Pharmacological and functional analysis of a novel serotonin receptor in the rat hippocampus

Administration of serotonin (5-hydroxytryptamine, 5-HT) to pyramidal cells of the CA1 region of the hippocampus results in a hyperpolarizing response which is followed by a rebound depolarization and a decrease in the calcium-activated afterhyperpolarization (AHP). While the hyperpolarizing response has been previously shown to be mediated by receptors of the 5-HT1A subtype, the identity of the receptor(s) involved in the depolarizing response and decrease of the AHP have not been identified. In the present study the effectiveness of a series of 5-HT receptor antagonists in blocking the membrane depolarization and reduction of the AHP was assessed. While a variety of 5-HT1 and 5-HT2 antagonists were found to be ineffective, the substituted benzamide BRL 24924 was found to be a potent and selective antagonist of the 5-HT-induced depolarization and decrease in the AHP in this region. This effect however appeared unrelated to the ability of this compound to block 5-HT3 receptors, as ICS 205-930 and MDL 72222 were markedly less efficacious in blocking these effects of 5-HT. Upon blockade of 5-HT1A receptors, 5-HT elicits a depolarization which is accompanied by a marked increase in excitability. These effects were also dose-dependently antagonized by BRL 24924. The present results thus suggest the presence in the CA1 region of the hippocampus of a novel 5-HT receptor at which BRL 24924 functions as a selective antagonist and which is capable of mediating slow excitatory responses in central neurons.

Immunocytochemical localization of the neural-specific regulatory subunit of the type II cyclic AMP-dependent protein kinase to postsynaptic structures in the rat brain

The cellular and subcellular distribution of a major cyclic AMP binding protein in the central nervous system, the neural-specific regulatory subunit of the type II cyclic AMP-dependent protein kinase (RII-B), was analyzed in rat brains with light and electron microscopic immunocytochemical methods. The distribution of the non-neural isoform of the regulatory subunit of the enzyme (RII-H) was also analyzed. It was found that RII-B immunoreactivity was predominantly localized to neurons whereas glial and endothelial cells were unlabeled. In the neurons the RII-B immunoreactivity occurred in the perikaryal cytoplasm and in the dendrites; there was no significant accumulation of immunoreaction product in nuclei, myelinated axons and axon terminals. Although immunoreactivity was never detected in axon terminals, it was characteristically associated with the postsynaptic densities and the surrounding non-synaptic sites in somata, dendrites and dendritic spines. The localization of RII-B antigenic sites did not show specificity to any type of neuron or synapse, but the amount of immunoreactivity varied. The distribution of RII-H immunoreactivity was similar to that of RII-B except that RII-H immunoreaction product was also observed in glial cells and occurred more frequently in myelinated axons. Our data confirm that RII-B is one of the major cyclic AMP binding proteins in neurons, and provide morphological support for the involvement of the type II cyclic AMP-dependent protein kinase in postsynaptic neural functions.

Inhibition of a cAMP-dependent Ca-activated K conductance by forskolin prolongs Ca action potential duration in lamprey sensory neurons

Intracellular recordings from primary mechanosensory neurons (dorsal cells) of the lamprey spinal cord were made to test the membrane effects of forskolin, an activator of adenylate cyclase in these cells. At a concentration of 50 microM, forskolin was found to have a pronounced broadening effect on calcium action potentials (Ca APs) produced in the presence of voltage-activated K channel blockers (TEA, 3,4-DAP). Forskolin had no effect on passive membrane properties of the cells, such as resting potential or input resistance. Nor did it affect delayed rectification or Na APs and thus appeared not to block voltage-activated K channels. Forskolin's effect was evident only when a significant Ca component to the AP was present. It appeared not to increase the conductance of the Ca channel since its action was accompanied by a decrease in membrane conductance during the Ca AP, indicating instead an inhibition of a repolarizing Ca-activated conductance, other than a Ca-activated Cl conductance. The prolongation of Ca APs by forskolin, barium or the neurotransmitter GABA were all correlated in voltage-clamp with a decrease in outward current. Under the conductions used here, the major outward conductance in dorsal cells is a Ca-activated K conductance (gK(Ca]28, and it is concluded that the most probable mode of action for forskolin is via a cyclic AMP-mediated inhibition of this conductance. GABA has also been shown to prolong Ca APs in lamprey dorsal cells by inhibiting a repolarizing gK(Ca)28. Thus, the action of this transmitter may be mediated by an increase in intracellular cyclic AMP.

Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In vivo electrochemical measurements of chronoamperometric recordings from Nafion-coated electrodes were used to investigate monoamine overflow from selected regions of the rat hippocampal slice. Concurrent electrophysiological measurements of evoked CA1 pyramidal cell population spike responses were used to characterize changes in the electrical activity in the slices that occur during potassium-induced neurotransmitter overflow. Superfusion with elevated K+ (10-50 mM, 5 min) elicited consistent concentration-dependent increases in the electrochemical responses recorded from the dentate gyrus. At the onset of K+ perfusion, there was an initial increase in the population spike response, followed by electrical silence, which usually lasted 5-10 min following the return to normal medium, and required 20-30 min for complete recovery of the response. The potassium-induced electrochemical signal always increased following the decline in the electrophysiological response. Although the electrochemical signal usually returned to baseline much before the electrophysiological response (usually within 5 min), both signals remained refractory for some time. Cocaine pretreatment (10-50 microM) caused a dose-dependent augmentation of the electrochemical responses. Local pressure ejection of K+ via a micropipette elicited dose-dependent increases in the electrochemical signals that were of relativity brief duration as compared to superfusion with K+. Such potassium-evoked responses were highly localized, and were attenuated in amplitude in animals that had been previously treated with the selective noradrenergic neurotoxin, DSP-4. In addition to K+, local applications of methyl-amphetamine, tyramine and veratridine also elicited electrochemical signals, and the time courses of these responses were specific to the releasing agent that was used. Preliminary data obtained using high-speed electrochemical recordings of both oxidation and reduction current suggested that tyramine ejections evoked primarily norepinephrine overflow, while K+ evoked the overflow of both norepinephrine and serotonin. The present experiments demonstrate that simultaneous electrophysiological and electrochemical experiments can be used in an isolated preparation of brain such as the hippocampal slice to characterize the electrophysiological events that occur during stimulated transmitter release.

Muscarinic and beta-adrenergic depression of the slow Ca2(+)-activated potassium conductance in hippocampal CA3 pyramidal cells is not mediated by a reduction of depolarization-induced cytosolic Ca2+ transients

Combined intracellular and microfluorometric recording techniques were used to evaluate whether the inhibition by cholinergic or adrenergic transmitters of the Ca2(+)-activated potassium current (IAHP) in hippocampal CA3 pyramidal cells was mediated by an alteration of depolarization-induced change in cytosolic free Ca2+ concentration [(Ca2+]i). Low concentrations of isoproterenol (1-10 microM) and muscarine (0.25-1 microM) reversibly abolished IAHP without affecting concomitant Ca2+ transients or the steady-state [Ca2+]i. Only after application of higher concentrations of muscarine, [Ca2+]i increased; in the presence of potassium channel blockers, muscarine depressed Ca2+ currents and concomitant Ca2+ transients. These observations provide direct evidence that the inhibition of IAHP by isoproterenol and muscarine are not mediated by an alteration of Ca2+ dynamics.

Electrophysiological analysis of synaptic transmission between intraocular hippocampus/locus coeruleus co-transplants

We have examined the establishment of functional connectivity between hippocampal and locus coeruleus co-transplants in oculo. Co-transplants were allowed to mature in oculo for 8-54 weeks following grafting and were subsequently removed from the anterior eye chamber for in vitro electrophysiological studies. Single hippocampal transplants in oculo have been shown to exhibit prolonged synaptic responses to local electrical stimulation, and similar responses were observed in hippocampal neurons following stimulation of the hippocampal portion of hippocampus-locus coeruleus co-transplants. Electrical stimulation of the locus coeruleus attenuated the afterhyperpolarization in hippocampal neurons elicited by the injection of depolarizing current, an effect that has been described previously in hippocampal slices following direct application of norepinephrine, and this effect was antagonized by pretreatment with the beta-adrenergic antagonist timolol. Stimulation of the locus coeruleus also produced both hyperpolarizing and depolarizing changes in the resting membrane potential in hippocampal neurons in 2- and 6-month-old co-transplants. In the 2-month-old co-transplants the responses were primarily depolarizing, and appeared to be mediated by a beta-adrenergic receptor, whereas in the 6-month-old co-transplants the responses were more varied. The results suggest that functional alpha- as well as beta-adrenergic receptors develop in oculo, and that the release of norepinephrine at nerve terminals in double grafts produces effects in the hippocampal neurons which are similar to those observed during superfusion of the hippocampal slice preparation with exogenous norepinephrine.

Changes in excitability of CA1 pyramidal neurons in slices prepared from AlCl3-treated rabbits

Intracellular recordings in 'in vitro' hippocampal slices, prepared from intracisternally AlCl3-intoxicated rabbits, were obtained from 43 CA1 pyramidal neurons. The experiments were performed 12-20 days after aluminum administration. The electrotonic length was significantly shorter than that of 33 control neurons, in agreement with morphological evidence of an Al-induced dendritic impairment. Both postsynaptic and Ca2(+)-dependent K+ hyperpolarizing potentials were also found to be significantly decreased, with reciprocal enhancement of excitatory postsynaptic potentials and depolarizing after-potentials. The former finding is ascribed to a selective neurotoxic effect of aluminum on GABAergic interneurons; the latter can be accounted for by an Al-induced increase in cyclic AMP, which is known to block the Ca2(+)-activated K+ conductance responsible for after-hyperpolarizing potentials. It is concluded that aluminum can exert its epileptogenic effect through multiple neurotoxic mechanisms involving membrane electrotonic properties, K+ conductances, and synaptic influences, thus resulting in a neuronal hyperexcitable state. Such changes are detectable in the early stages of the Al-induced encephalopathy, when there is only slight evidence of cytoskeleton alterations (i.e., neurofibrillary degeneration).

Learning-induced afterhyperpolarization reductions in hippocampus are specific for cell type and potassium conductance

Hippocampal slices were prepared from rabbits trained in a trace eye-blink conditioning task and from naive and pseudoconditioned controls. Measurements of the post-burst afterhyperpolarization (AHP), action potential, and other cellular properties were obtained from intracellular recordings of CA1 pyramidal (N = 49) and dentate gyrus granule cells (N = 52). A conditioning-specific reduction in the amplitude of the AHP was found in CA1 cells but not in dentate granule cells. This reduction in the AHP was apparent at 50 ms after the end of a depolarizing current pulse, and was maintained for at least 650 ms. Other measured cell characteristics (input resistance, resting membrane potential, action potential shape, inward rectification, spike threshold) were not affected by training, in either CA1 pyramidal or dentate granule cells. Time-course measures indicate that both the medium, Ca2(+)-independent AHP and the slow, Ca2(+)-dependent AHP are reduced by conditioning. The slow AHP largely reflects the Ca2(+)-dependent K+ current, IAHP. Rising and falling slopes, peak amplitude, and width of individual action potentials were not changed by learning. This contrasts with observations from invertebrates in which action potential broadening was reported following learning. We conclude that the reduction in AHP that follows hippocampally-dependent associative learning occurs in specific hippocampal cell types and not others, and is mediated by changes in a Ca2(+)-independent AHP and a particular Ca2(+)-dependent K+ current, IAHP.

Effects of repetitive activation and changes in external ionic environment on hippocampal CA1 pyramidal cell afterhyperpolarizations

Afterhyperpolarizations (AHPs) in hippocampal CA1 pyramidal neurons known to be generated predominantly by a Ca-dependent K conductance were examined to see if they could be inverted by changes in extracellular potassium of the magnitude observed during interictal or ictal discharges and for their liability during repetitive activation under normal ionic conditions or in the presence of elevated extracellular potassium and decreased extracellular calcium. Under all circumstances tested, the AHP remained hyperpolarizing and was associated with a conductance increase. Thus, the very liable hyperpolarizing event that follows a depolarizing shift in hippocampal pyramidal neurons in various epileptic foci (which disappears early during the transition between interictal and ictal activity) probably is not due to the same mechanism as that which underlies the AHP.

Mechanisms concerned in the direct effect of isoflurane on rat hippocampal and human neocortical neurons

The effect of isoflurane on postsynaptic neurons was studied by intracellular recordings from rat hippocampus and human neocortex in vitro. Isoflurane caused a hyperpolarization of the cell membrane. The hyperpolarization was reversed (although incompletely in some neurons) by increasing the membrane potential. The reversal potential was -80 +/- 12 mV (mean +/- S.D.) or 12 +/- 6 mV negative to the resting membrane potential. Potassium channel blockers reduced the isoflurane-induced hyperpolarization, while chloride loading was without effect. The transient depolarization preceding the hyperpolarization in some of the neurons was not reversed by hyperpolarization. The action potential was prolonged by 19 +/- 3% due to a slower rate of rise. The rise time was almost doubled. Firing threshold was increased by 4 +/- 3 mV (relative to the reference electrode). Subthreshold inward rectification was reduced or abolished. Some cells showed subthreshold outward rectification during isoflurane administration. These results suggest that isoflurane depressed neuronal excitability by (1) hyperpolarizing the cell membrane, at least partly by an increase in potassium conductance, (2) slowing the rate of rise of the action potential, presumably due to interference with the fast sodium channel, (3) decreasing subthreshold inward rectification and (4) increasing firing threshold.

Electrophysiological actions of luteinizing hormone-releasing hormone: intracellular studies in the rat hippocampal slice preparation

The electrophysiological effects of luteinizing hormone-releasing hormone (LHRH) on CA1 pyramidal cells were investigated utilizing intracellular recordings from the in vitro rat hippocampal slice preparation. Bath application of LHRH (10(-7) - 10(-12) M) resulted in several changes in the electrophysiological properties of CA1 neurons. LHRH induced a long-lasting depolarization associated with increased input resistance, a decrease in the afterhyperpolarization (AHP) following a train of action potentials, and a reduction in accommodation of repetitive cell discharge. These effects were blocked by the synthetic LHRH antagonist [Ac-delta-Pro1,pCl-D-Phe2,D-Trp3,6]-LHRH. These findings provide electrophysiological evidence for the role of LHRH as a neurotransmitter/neuromodulator in the hippocampus.

Effects of iontophoretically applied monoamines on somatosensory cortical neurons of unanesthetized rats

The response of somatosensory cortical neurons to iontophoretic applications of monoamines was studied in unanesthetized rats. The animal's head was held in a stereotaxic apparatus by means of a painless head-restraining system implanted 8 days prior to the recording sessions. The electrodes consisted of a recording micropipette attached to a multibarreled iontophoresis micropipette. The electrode penetrations were reconstructed on camera lucida drawings of frontal brain sections. The percentage of cortical neurons responding to application of monoamines were 76% after noradrenaline, 58% after dopamine and 66% after serotonin. The differences observed among percentages of responses seemingly correlate with the relative abundance of terminal axons and receptors for each of the three monoamines in the somatosensory cortex. The vast majority of the responding neurons were inhibited by monoamines and this inhibitory effect was independent of the level of spontaneous activity. The depressant effect of the monoamines on glutamate and acetylcholine-evoked responses supports a modulatory role for these substances. Serotonin was the most potent, followed by noradrenaline and dopamine. The present study shows that when the influence of anesthesia is eliminated, the predominant effect of monoamines on cortical first somatosensory neurons is one of inhibition. These findings contrast with results obtained under some anesthetic conditions, as well as under in vitro conditions.

Effects of kainate on the excitability of rat hippocampal neurones

Intracellular recordings from CA1 pyramidal neurones of the rat hippocampal slice preparation were used to study changes in neuronal excitability induced by the excitatory amino acid analogues kainate (KA) and N-methyl-D-aspartate (NMDA). Low concentrations of bath-applied KA (50-200 nM) or NMDA (1-3 microM) elicited a relatively small membrane depolarization and increased the number of spikes fired by a constant current pulse. The spike after-hyperpolarization (AHP) was depressed by KA but enhanced by NMDA. After blockade of the voltage-sensitive Na+ conductances with tetrodotoxin, intracellularly applied current pulses elicited Ca2+ spikes. Whereas NMDA always increased the duration (and number) of Ca2+ spikes and of their AHP, KA conversely reduced these spikes and (in almost half of the cells tested) the late phase of their AHP. When Ba2+ was used to replace extracellular Ca2+, prolonged plateau potentials developed and were also blocked by KA. NMDA had no effect on Ba2(+)-dependent responses. These results suggest that low concentrations of KA profoundly modified the electroresponsiveness of CA1 neurones perhaps by depressing a Ca2(+)-dependent K+ conductance mechanism responsible for dampening the excitability of these cells.

Cytosolic free calcium in hippocampal CA3 pyramidal cells

The dynamics of cytosolic free Ca2+ ([Ca2+]i) of single voltage-clamped CA3 pyramidal cells in hippocampal slice cultures is reviewed. [Ca2+]i amounts to about 30 nM at resting membrane potential and increases slowly when the membrane potential is clamped at more positive values (up to 500 nM at -30 mV). Short lasting depolarizations (40-100 ms) induce a transient rise in [Ca2+]i which activates a slow aftercurrent (IAHP). The muscarinic or beta-adrenergic depression of IAHP is not accompanied by any change in the dynamics of Ca2+ and appears, therefore, to result primarily from an inhibition of the K(+)-current itself or of the ability of Ca2+ to activate the current. At higher concentrations than those required to inhibit IAHP, muscarine produces a pronounced inward current and this is accompanied by a rise in resting [Ca2+]i concentration.

Effects of histamine on dentate granule cells in vitro

Hippocampal slices from rat brain were exposed to histamine and related substances in a perfusion chamber. Granule cells of the dentate gyrus were studied with conventional extra- and intracellular recording and a single electrode voltage clamp. Histamine caused, through activation of H(2)-receptors, a small depolarization, an increase in the number of synaptic and action potentials, a block of the long lasting (but not the early) component of spike afterhyperpolarizations and a reduction of the accommodation of action potential firing. These effects were mimicked by forskolin (suggests activation of adenylate cyclase). In voltage clamp, histamine blocked a long lasting calcium-dependent outward tail current without any reduction of inward current. Thus histamine selectively blocks the late calcium-dependent potassium current in dentate granule cells which receive histaminergic input from the posterior hypothalamus. Histamine also reduces the field excitatory postsynaptic potential evoked by perforant path stimulation. These actions allow for a powerful modulation of excitatory signals and an effective regulation of hippocampal excitability.