Noradrenaline blocks accommodation of pyramidal cell discharge in the hippocampus

Long-term potentiation in the dentate gyrus: effects of noradrenaline depletion in the awake rat

The chronic rat preparation was utilized to study the effects of noradrenaline (NA) depletion on field potentials recorded from the hilus of the fascia dentata. Both single pulses and high-frequency trains were applied to the perforant path (PP). The effects of NA depletion on baseline responses as well as on long-term potentiation (LTP) were examined. Reduced NA levels resulted in an increase in the population spike amplitude and a depression of the population excitatory postsynaptic potential (EPSP). Depleted animals showed significantly higher levels of LTP of the population EPSP, but reduced levels of population spike LTP (measured 13-15 min after tetanization). There were, however, no differences in LTP levels 1 week after the potentiation tests. These results demonstrate that NA levels do not affect that component of LTP which can persist for several weeks.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Adenosine enhances afterhyperpolarization and accommodation in hippocampal pyramidal cells

Adenosine added to the perfusion fluid of rat hippocampal slices at 10 mumol X l-1 enhanced long lasting afterhyperpolarizations after single action potentials, bursts of action potentials or calcium spikes. Accommodation of firing during a depolarizing pulse was potentiated. An increase in calcium dependent potassium conductance is likely to mediate these effects. Adenosine at 50 mumol X l-1 induced a hyperpolarization accompanied by a reduction in input resistance. The hyperpolarization could be reversed at -85 mV. In TTX and TTX-barium treated slices the amplitude of the slow spike was decreased. This may result from a shunting of inward current in the dendrites due to an adenosine induced increase in potassium conductance. It is suggested that adenosine reduces pre- and postsynaptic excitatory signals principally by enhancing one or more potassium conductances. This effect is a powerful means for modulation of neuronal excitability and synaptic efficacy and can explain the antiepileptic activity of adenosine.

Frequency-dependent noradrenergic modulation of long-term potentiation in the hippocampus

Norepinephrine, briefly superfused during high-frequency stimulation of the mossy fibers in the rat hippocampal slice in vitro, produced a reversible increase in the magnitude, duration, and probability of induction of long-term synaptic potentiation in the CA3 subfield. Similar results were obtained with isoproterenol, whereas propranolol or timolol reversibly blocked long-term potentiation. Norepinephrine had little apparent effect on responses obtained during low-frequency stimulation of the mossy fibers. These data suggest that norepinephrine can mediate long-lasting, frequency-dependent modulation of synaptic transmission in the mammalian brain. Furthermore, the results suggest a plausible mechanism for some of the known associative interactions between synaptic inputs to hippocampal neurons.

Dopamine attenuates the effects of GABA on single unit activity in the globus pallidus

Studies were conducted to assess whether stimulation of dopamine receptors located in the globus pallidus might play a role in mediating the enhanced pallidal activity seen after systemic administration of dopamine agonists or D-amphetamine. Dopamine, applied iontophoretically, had modest effects on the activity of pallidal neurons; the baseline firing rates of 32% of cells recorded increased by an average of 23 +/- 2%, 18% decreased in rate and the remaining cells showed no significant rate change. More significantly, dopamine consistently attenuated the inhibitory actions of gamma-aminobutyric acid (GABA) in the globus pallidus. When dopamine was simultaneously iontophoresed with GABA, GABA's effectiveness at inhibiting pallidal activity was reduced by an average of 50%. Norepinephrine or acetylcholine, applied iontophoretically at equimolar concentrations and ejected at the same current as dopamine, caused no consistent attenuation of pallidal responses to GABA's rate effects. To determine whether the attenuation of GABA's inhibitory action by iontophoresed dopamine could be mimicked by systemic drug administration, apomorphine, 80 micrograms/kg, or D-amphetamine, 0.8 mg/kg, was given i.v. while GABA was iontophoresed. Apomorphine markedly decreased pallidal responses to the inhibitory effects of GABA in 75% of the cells by an average of 50%; haloperidol reversed this effect. Modulatory interactions between GABA and D-amphetamine were also observed in 5 of the 11 pallidal cells tested; GABA's inhibitory effect on pallidal cell activity was reduced by an average of 66% on these neurons. These results suggest that one way in which dopamine and dopamine agonists may affect basal ganglia function is by modulating GABAergic transmission in the globus pallidus.

Low-calcium field burst discharges of CA1 pyramidal neurones in rat hippocampal slices

Incubation of rat hippocampal slices in solutions containing low Ca2+ and increased Mg2+ rapidly blocked synaptic responses and increased spontaneous firing of all the principal neurones. More remarkably, a rhythmic and synchronous bursting discharge developed, which was restricted to the CA1 population of pyramidal neurones. These 'field bursts' or 'spreading excitation' were rapidly abolished by restoring the Ca2+ to 2 mM, by increasing the Mg2+ to 6 mM or by decreasing K+ from 6 to 3 mM. The CA1 pyramidal cells depolarized after the change to the low-Ca2+ solution by about 10-20 mV. Individual field bursts were associated with a further depolarization of 10-12 mV surmounted by a burst of action potentials at about 20/s. This transient depolarization shift, recorded extracellularly as a negative field, could be attributed to the increase of [K+]o during the bursts, reaching 9-10 mM as measured by ion-sensitive electrodes. The bursts were followed by a hyperpolarization, seen extracellularly as a small soma-positive field, which was attributed to an electrogenic pump and/or a Ca2+-activated K+ conductance. Stimulation of the tightly packed pyramidal cell axons in the alveus elicited a train of population spikes, instead of the single spike normally seen, and could trigger a full field burst. Recordings of the alvear tract volley suggested that the repeated spikes arose within the pyramidal cells. Multiple recordings from CA1 revealed that field bursts usually, but by no means always, started near the caudal (subicular) end of the area. They spread through the cell layer at 0.04-0.12 m/s. The most rapid propagation was seen when the bursts had an abrupt onset; slower propagation (1-10 mm/s) occurred when the bursts started gradually, which generally was the case near the sites of burst initiation and termination. Usually the action potentials within each burst were synchronized into population spikes which spread across CA1 at 0.04-0.15 m/s. The site of initiation and the extent of the spread of these population spikes varied during each burst, as did their amplitude. The degree of spike synchronization was enhanced by various treatments expected to increase neuronal excitability. Measurements of transmembrane potential during the burst confirmed the role in the generation of population spikes of ephaptic or field interactions between the pyramidal cells. It is proposed that the increased firing of all neurones is due to the block of tonic inhibition, depression of after-hyperpolarization and to increased membrane excitability.(ABSTRACT TRUNCATED AT 400 WORDS)

A novel membrane sodium current induced by injection of cyclic nucleotides into gastropod neurones

Injection of cyclic AMP (cAMP) or cyclic GMP into identifiable neurones from several different gastropod species immediately depolarized the cell membranes in a dose-dependent manner. Doses were monitored photometrically and evidence is presented for depolarizing effects following nucleotide injections of as little as 30-35 mumol. The depolarizing effect was reversible and was demonstrated under voltage clamp to be primarily the result of a nucleotide-induced, transient increase in a membrane Na current, INa (cAMP). The charge-carrying species was identified by using ion-substituted salines, reversal potential in low-Na saline, and intracellular ion-sensitive electrode measurements. The current was resistant to tetrodotoxin, ouabain and amiloride. Substituting Trisma, tetramethylammonium or bis-tris propane for Na prevented the induced current, whereas Li substitution did not. Duration of the induced current was greatly prolonged in neurones bathed in the phosphodiesterase inhibitor isobutylmethylxanthine, or following injection of any of several cAMP analogues, indicating that the reversible nature of the current stems primarily from in situ hydrolysis of the injected dose and not current inactivation. Amplitude of the induced current either remained constant or decreased over the voltage range where it could be easily measured, i.e. -30 greater than Vm greater than -100 mV, reflecting a voltage as well as a chemical sensitivity of INa (cAMP).

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

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

Contrasting effects of neurohypophysial peptides on pyramidal and non-pyramidal neurones in the rat hippocampus

Oxytocin and vasopressin increased the rate of firing of a class of presumed non-pyramidal neurones located in the CA1 area of rat hippocampal slices. This excitatory effect persisted in conditions of synaptic uncoupling. In contrast, pyramidal neurones were either unaffected by neurohypophysial peptides or showed one or several of the following effects: a decrease in firing rate in cells which were spontaneously active; a slight membrane hyperpolarization; and an increase in the rate of occurrence of spontaneous inhibitory postsynaptic potentials. We therefore propose that oxytocin and vasopressin excite directly a class of non-pyramidal inhibitory interneurones, whereas their observed effect on pyramidal neurones is indirect and inhibitory.

A single gene error of noradrenergic axon growth synchronizes central neurones

One strategy for deciphering inherited neurological disease is to examine the expression of individual genes controlling the assembly and physiology of specific cell groups within the developing mammalian central nervous system (CNS). This neurogenetic approach, using defined single-locus mutations arising on coisogeneic mouse strains, has recently been used to analyse a major class of neuronal membrane diseases involving abnormal excitability, the epilepsies, and to identify examples of hereditary variation in signalling properties at central synapses. An interesting mutation, the Tottering (tg) gene, causes a delayed onset, recessive neurological disorder in the mouse featuring a stereotyped triad of ataxia, intermittent myoclonus and cortical spike-wave discharges accompanied by behavioural absence seizures which resemble petit mal epilepsy. Axon branches of the locus coeruleus, a noradrenergic brain-stem nucleus, hyperinnervate specific target regions of the tg brain. The number of parent coerulean perikarya is unaffected, indicating a true proliferation of the terminal axonal arbor. With the exception of this unusually precise error of axonal growth, no other cytopathology has been identified in the tg brain. Here I present evidence that selective lesions of the central noradrenergic axons early in development limit the expression of the disease.

The pharmacology of cholinergic excitatory responses in hippocampal pyramidal cells

The pharmacology of excitatory cholinergic responses in CA1 pyramidal cells was examined in detail using intracellular recording from the hippocampal slice preparation. Acetylcholine (ACh), carbachol, muscarine and pilocarpine depolarized the membrane potential with an associated increase in input resistance. In addition, these agonists increased cell firing and depressed the afterhyperpolarization (AHP) that is due to a calcium-activated potassium conductance. The weak effects of ACh (20-200 microM) were considerably enhanced by addition of eserine (1-10 microM). All excitatory effects were completely antagonized by atropine (0.1-1 microM) but unaffected by dihydro-beta-erythroidine (DHBE) and gallamine (1-50 microM). In contrast to the muscarinic agonists, the nicotinic agonists nicotine and dimethylphenylpiperazinium (DMPP) had no excitatory effects on CA1 pyramidal cells. Phenyltrimethylammonium (PTMA), at high concentrations did depolarize cells and depress the AHP but these effects were antagonized by atropine and not DHBE or gallamine. The action of the analogue of cyclic GMP, 8-bromo-cyclic GMP, although variable, mimicked the membrane effects of ACh in some cells and depressed the AHP in most cells. Intracellular injection of cyclic GMP routinely depressed the AHP. In summary, we have demonstrated two cholinergic responses of hippocampal pyramidal cells that are mediated purely by muscarinic receptors. We could find no evidence to support a mixed-type receptor or the involvement of nicotinic receptors in the excitation of hippocampal pyramidal cells to cholinergic agents.

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

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

The role of inhibitory mechanisms in hippocampal long-term potentiation

The role of inhibitory processes in long-term potentiation (LTP) was investigated in hippocampal slices of the rat. GABAergic inhibition, tested by double shock experiments and by intracellular recording of inhibitory postsynaptic potentials, was not reduced in CA 1 pyramidal neurones after the induction of LTP. In cells recorded with caesium chloride-filled electrodes LTP could not be elicited. A reduction of intrinsic potassium-dependent inhibition may thus be responsible for LTP.

Electrophysiological evidence that noradrenergic afferents selectively facilitate the activity of supraoptic vasopressin neurons

The functional role of the ascending projection from A1 noradrenergic neurons of the caudal ventrolateral medulla to the supraoptic nucleus of the hypothalamus was investigated by examining the effects of electrical stimulation of the A1 region on the activity of supraoptic neurons deemed to be vasopressinergic or oxytocinergic on the basis of basal firing patterns and responsivity to baroreceptor activation. A1 stimulation enhanced the activity of all putative vasopressin-secreting supraoptic neurons tested. This effect appeared to be selective in that no putative oxytocin-secreting neurons were excited by A1 stimulation. Destruction of the supraoptic noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of A1 stimulation but did not noticeably alter basal activity patterns, nor the influence of baroreceptor inhibitory pathways. These findings suggest a facilitatory role for noradrenergic afferents in regulating the activity of neurohypophysially-projecting vasopressin neurons of the supraoptic nucleus.

Serotonin and Retzius cell depress the hyperpolarization following impulses of leech touch cell

Intracellular recordings from T mechanosensory cells of Hirudo medicinalis showed, as previously demonstrated, that repetitive firing is followed by a long-lasting hyperpolarization. Serotonin application at two concentrations (1 microM and 50 microM) depressed this hyperpolarization by up to 2/3; the effect was dose-dependent, long-lasting and reversible. Intracellular stimulation of giant serotonergic neurons (Retzius cells, Rz) mimicked serotonin perfusion: the effect was proportional to the number of spikes fired by Retzius cells. The combined use of intracellular iontophoretic injection of horseradish peroxidase and lucifer yellow indicated the possible sites of contact between Rz and T cells. The effect of serotonin, released by Rz cells, is discussed with respect to its possible physiological significance.

Neurophysiological evidence for tricyclic antidepressant-induced decreased beta-adrenergic responsiveness in the rat hippocampus

A study was made in rats of the effects of chronic treatment with the tricyclic antidepressant agent imipramine on the increase in the population spike amplitude produced by the beta-adrenergic receptor agonist isoproterenol in the 'in vitro' slice preparation of the hippocampus. A significant reduction of 39% and 44% of the population spike increase was observed at 5 X 10(-7) M and 1 X 10(-6) M isoproterenol respectively in rats treated for 28 days with 10 mg/kg i.p. imipramine.

The actions of noradrenaline on neurones of the rat substantia gelatinosa in vitro

Intracellular recordings were made from substantia gelatinosa (s.g.) neurones in slices cut from adult rat spinal cord and maintained in vitro. Noradrenaline applied by superfusion (1-50 microM), or by brief pressure ejection from a micropipette, reversibly hyperpolarized 80% of the s.g. cells. The noradrenaline induced hyperpolarization was associated with an increase in conductance and it reversed in polarity at -88 mV. The reversal potential changed when the external potassium concentration was changed, as predicted by the Nernst equation. The noradrenaline hyperpolarization was antagonized by phentolamine and yohimbine but not by propranolol and prazosin. The hyperpolarization was probably a direct action on the impaled cell and not due to release or block of release of other transmitters, because the effects persisted during a perfusion with a low calcium/high magnesium solution or in a solution containing cobalt and high magnesium. In 35 of 148 cells, noradrenaline caused a dose-related increase of spontaneous excitatory post-synaptic potentials (e.p.s.p.s). This effect was blocked by tetrodotoxin. The noradrenaline induced increase in e.p.s.p.s was blocked by phentolamine and prazosin but not by the alpha 2-blockers yohimbine and RX 781094. A few cells were depolarized by noradrenaline, and this was blocked by prazosin but not by yohimbine. It is suggested that noradrenaline may inhibit nociceptive input to the spinal cord by increasing the potassium conductance of s.g. neurones.

Muscarinic agonists depress calcium-dependent gK in bullfrog sympathetic neurons

Intracellular recordings were made from 'fast B' [9] neurons in bullfrog sympathetic ganglia. A single soma action potential was followed by a prolonged after-hyperpolarization lasting for several hundred milliseconds up to 2 s. The spike afterhyperpolarization, which is generated by calcium-dependent potassium conductance increase (gKCa) [3,20-24], was shortened by the muscarinic action of acetylcholine and oxotremorine (30-300 nM). These concentrations of muscarinic agonists were too low to cause any detectable changes in resting membrane potential, input resistance or action potential wave form. ACh released from presynaptic terminal under a physiological condition also caused the shortening of the calcium-dependent hyperpolarization. The results suggested that the shortening of calcium-dependent spike afterhyperpolarization may permit the neuron to pass the high frequency of discharge during the muscarinic excitation.

Histamine potentiates neuronal excitation by blocking a calcium-dependent potassium conductance

Histaminergic neurones send their axons to the whole forebrain. The diffuse projection is consistent with a modulatory role of these pathways. In hippocampal slices from rats a mechanism for this modulation is described, on pyramidal neurones of the CA 1 area: Strong excitations induced by intracellular current injection, ionophoretic administration of glutamate or synaptic stimulation normally restrict themselves by the activation of the calcium-dependent potassium current (gK(Ca) ). This current causes a long lasting afterhyperpolarization and an accommodation of firing. Their block by histamine and impromidine (reversed by metiamide and cimetidine) leads to a profound potentiation of excitatory signals. It is suggested that HA, through H2 receptors, accelerates the removal of intracellular free Ca++ ions.

Norepinephrine elicits both excitatory and inhibitory responses from Purkinje cells in the in vitro rat cerebellar slice

Superfusion of Purkinje neurons in the in vitro rat cerebellar slice with norepinephrine caused increases and decreases of spontaneous Purkinje cell firing. Excitations were evoked by low concentrations of norepinephrine (0.5-10 microM) and by the beta receptor agonist isoproterenol (0.1-5 microM). These excitations were reduced by timolol (1-2 microM), a beta receptor antagonist. Perfusion with higher concentrations of norepinephrine (greater than 16 microM), caused a depression of Purkinje neuron spontaneous activity. This inhibitory response was blocked by the alpha receptor antagonist phentolamine. The alpha 1 selective agonist phenylephrine had no effect on spontaneous activity at concentrations up to 100 microM, but the alpha 2 selective agonist clonidine (1-50 microM) elicited decreases in firing rate. These responses appeared to be due to a direct action on Purkinje cells, because neither the excitation nor the depression of Purkinje neuron activity elicited by norepinephrine was substantially altered when tested in a medium which substantially blocked synaptic transmission within the slice. Under these in vitro conditions, norepinephrine appears to increase the firing rate of Purkinje neurons via an interaction with beta adrenergic receptors, while norepinephrine induced depressions may be linked to alpha adrenergic receptor interactions; both receptors appear to be located directly on the Purkinje neurons.

The functional significance of neurotransmitter diversity

A major continuing theoretical issue in neurobiology is the nature of the information encoded into chemical neurotransmitters. This problem has been intensified by major recent advances in cellular neurobiology in three specific areas of work: neuronal connectivity, synaptic mechanisms, and neuronal transmitters. Countless new interneuronal connections have been revealed through the use of sensitive new cellular tracing methods, revealing a far more detailed picture of the complexities and principles of brain organization. A whole range of ionic conductance mechanisms has been detailed by the application of new methods of electrophysiological analysis, particularly useful with the in vitro preparations. Many of these mechanisms were not previously recognized, and together they provide a much richer repertoire of receptor response mechanisms upon which neurotransmitters can operate. Last, the modern methods of chemical analysis have provided an ever-increasing list of new neurotransmitter molecules, which fall conveniently into three chemical categories: amino acids, monoamines, and neuropeptides. This review considers some possible underlying principles by which the rich signaling capacity of the central nervous system may be approached conceptually and experimentally.

A bicuculline-resistant inhibitory post-synaptic potential in rat hippocampal pyramidal cells in vitro

Experiments were performed on rat hippocampal CA1 pyramidal cells in vitro in order to elucidate the origin of the late hyperpolarizing potential, which follows the gamma-aminobutyric acid (GABA)-mediated inhibitory post-synaptic potential (GABA-i.p.s.p.). The late hyperpolarizing potential could be evoked by orthodromic stimulation via stratum radiatum or stratum oriens but not by selective antidromic stimulation. The membrane soluble analogue of cyclic AMP, 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br cyclic AMP), which blocks calcium-activated potassium hyperpolarizations (GK(Ca], did not reduce the late hyperpolarizing potential. The enkephalin analogue, (D-ala2-met5)-enkephalinamide (DALA) reversibly reduced both the GABA-i.p.s.p. and the late hyperpolarizing potential. The late hyperpolarizing potential and GABA-i.p.s.p. were more sensitive to low doses of the calcium antagonist, cadmium, than the excitatory post-synaptic potential (e.p.s.p.). The local application of cadmium to the pyramidal cell layer blocked the antidromic i.p.s.p. but the orthodromically evoked late hyperpolarizing potential was less affected. In contrast to the GABA-i.p.s.p., the late hyperpolarizing potential was not reversed by chloride injection and was enhanced, rather than depressed, by bicuculline. We conclude that the late hyperpolarizing potential is a bicuculline-resistant i.p.s.p. The unidentified transmitter for this i.p.s.p. is released from feed-forward interneurones primarily onto the dendrites of the pyramidal cell and may act by increasing the potassium permeability of the membrane. The epileptiform burst after-hyperpolarization evoked in the presence of GABA antagonists is composed of at least two components, a long-duration hyperpolarization mediated GK(Ca) and an earlier and shorter late hyperpolarizing potential. Blockade of the GK(Ca) by 8-Br cyclic AMP did not alter the duration of epileptiform bursts but did markedly increase the frequency of their occurrence. This suggests that GK(Ca) is involved in controlling the interval between bursts.

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.

Examination of the role of the electrogenic sodium pump in the adrenaline-induced hyperpolarization of amphibian neurones

The effect of adrenaline and acetylcholine (ACh) on the membrane potential of Rana pipiens sympathetic ganglia was examined by means of the sucrose gap recording technique. Adrenaline (1-50 microM) consistently produced a hyperpolarization (Adrh) which was not reduced by Ringer solution containing 10 mM-Mn2+, nor by Ringer solution where the Na+ concentration was reduced from 100 to 30 mM. High doses of ACh (10 mM) produced a biphasic response, a depolarization (AChd) followed by an after-hyperpolarization (ACha.h.p.). Ringer solution containing 100 mM-Li+ (rather than 100 mM-Na+) or 10 microM-ouabain blocked the ACha.h.p. and reduced the Adrh. The AChd was essentially unchanged. Ringer solution containing 0.2 mM-K+ (rather than 2 mM-K+) blocked part of the ACha.h.p. whereas the Adrh was enhanced. Ringer solution containing 6 mM-K+ reduced the amplitude of the Adrh. The Adrh and the antidromically evoked action potential after-hyperpolarization (antidromic a.h.p.) reversed polarity at approximately the same membrane potential. These data do not support the hypothesis that the Adrh results from activation of the electrogenic sodium pump. It is tempting to speculate that the response may be generated by an increase in potassium conductance (gK) which is especially sensitive to manipulations which result in sodium pump inhibition.

Modulation of low calcium induced field bursts in the hippocampus by monoamines and cholinomimetics

The influence of monoamine transmitter candidates, acetylcholine and related substances on rhythmic depolarization shifts (field bursts) in the CA 1 area of hippocampal slices from rats in low calcium (0.2 mmol X 1(-1) ) high magnesium (4 mmol X 1(-1) ) was investigated. Acetylcholine (ACh), histamine (HA) and H2-agonists, noradrenaline (NA) and beta-agonists at nano- to micromolar concentrations as well as dopamine (DA) and 8-bromo-cyclic AMP at 100 mumol X 1(-1) accelerated the field bursts. H2-antagonists blocked HA actions, beta-antagonists blocked NA actions selectively; muscarinic antagonists blocked ACh, HA and NA actions. H1-agonists, serotonin, dopamine and adenosine slowed the field bursts at micromolar concentrations. These effects parallel the action of the tested substances on afterhyperpolarizations in CA 1 pyramidal cells. High sensitivity and specificity make this response of the field bursts an excellent model to study postsynaptic transmitter actions in the central nervous system.

Noradrenergic actions of Purkinje and locus coeruleus neurons in culture

Explant cultures prepared from neonatal mice were used to study the actions of iontophoretically applied noradrenaline (NA) on Purkinje and locus coeruleus neurons, with extracellular and intracellular recording respectively. NA depressed spontaneous activity of Purkinje neurons and enhanced excitatory responses to glutamate in 14/16 cells. In cultures older than 26 days, NA hyperpolarized LC neurons but had no effect on or depressed depolarizing responses to glutamate. The hyperpolarizations were blocked by the selective alpha 2 antagonist yohimbine. Enhancement of glutamate responses is not a ubiquitous characteristic of NA action and appears not to be associated with alpha 2 adrenergic receptors.

Effects of the two antidepressant drugs mianserin and indalpine on the serotonergic system: single-cell studies in the rat

Several antidepressant treatments enhance serotonergic neurotransmission. The present electrophysiological studies were undertaken to assess the effect of mianserin and indalpine, two antidepressant drugs with different pharmacological profiles, on serotonergic neurotransmission. In a first series of experiments, the responsiveness of hippocampal pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE) and gamma-aminobutyric acid (GABA) was assessed following mianserin, imipramine (5 mg/kg/day IP) or saline administration for 14 days. At 48 h after the last dose of mianserin, responsiveness to 5-HT was increased whereas that to NE and GABA was not modified. The degree of sensitization to 5-HT was the same as that produced by imipramine. Acute IV administration of mianserin (up to 10 mg/kg) did not decrease the firing rate of dorsal raphe 5-HT neurons. In a second series of experiments, long-term administration of indalpine (5 mg/kg/day IP for 14 days) did not modify the responsiveness of hippocampal pyramidal neurons to microiontophoretically applied 5-HT, NE and GABA whereas imipramine treatment (5 mg/kg/day IP) increased selectively their sensitivity to 5-HT when compared to indalpine-treated rats. In keeping with its potent reuptake-blocking property, acute IV indalpine produced a marked decrease in the firing rate of dorsal raphe 5-HT neurons (ED50 0.33 mg/kg). The firing rate of dorsal raphe 5-HT neurons was assessed following 2-, 7- and 14-day treatments with indalpine (5 mg/day IP). After 2 days, the firing rate of 5-HT neurons was greatly reduced, after 7 days it had recovered partially and after 14 days it had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenoceptor agonists increase membrane K+ conductance in cardiac Purkinje fibres

Hormonal modulation of the ionic conductance of cell membranes is a topic of considerable current interest; it has a major role, for example, in the improved performance of the vertebrate heart elicited by sympathetic nerve stimulation or by circulating catecholamines, an effect involving enhanced calcium influx. beta-Agonist catecholamines also abbreviate the action potential of cardiac Purkinje fibres, and increase the resting potential in a variety of cells, including cardiac cells, a hyperpolarization usually attributed to stimulation of the electrogenic Na+/K+ pump. We show here that nanomolar concentrations of beta-catecholamines cause hyperpolarization of cardiac Purkinje fibres, not by increasing Na+/K+ pump current, but by increasing resting membrane K+ conductance. The hyperpolarization and shortening of the action potential should increase availability of Na+ channels and reduce the refractory period, effects tending to safeguard impulse propagation through the ventricular conducting system despite the increased heart rate caused by beta-catecholamine action on the sinus node pacemaker.

Acetylcholine mediates a slow synaptic potential in hippocampal pyramidal cells

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

Histamine and noradrenaline decrease calcium-activated potassium conductance in hippocampal pyramidal cells

Ample evidence exists for histaminergic and noradrenergic projections to the hippocampus. Both amines exert neurotransmitter or modulator actions on principal neurones in the CA 1 and in the dentate area. A number of mechanisms have been proposed for these actions, including increased potassium conductance, increased chloride conductance and electrogenic pump stimulation, and reduction of the anomalous inward rectification. Action potentials, and particularly bursts of spikes, in CA 1 pyramidal cells, are followed by an afterhyperpolarization (AHP) which consists of two components. The late AHP depends on a calcium-activated potassium conductance gK+ (Ca2+), and has recently been shown to be increased by dopamine. We report here a rapid and reversible decrease of the late AHP component following a burst of sodium spikes or a calcium spike, during perfusion with micromolar concentrations of histamine and noradrenaline. This effect is mediated by H2 receptors and beta-receptors, respectively, and occurred in the absence of changes in the calcium spike. By such a mechanism histamine and noradrenaline can profoundly potentiate the excitatory impact of depolarizing signals.