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

Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis

The present study was conducted to examine the actions of norepinephrine (NE) and serotonin (5-HT) on the multiphasic, visually evoked discharges of cells recorded from the visual cortex (area 17) of anesthetized Long-Evans pigmented rats. Visual responses of 51 cells, evoked by computer controlled presentation of moving visual stimuli, were examined before, during and after low level microiontophoretic application of NE (1-55 nA) or 5-HT (1-50 nA). Drug-induced changes in stimulus-evoked and spontaneous discharges were quantitatively assessed by computer analysis of peri-event histograms. In the majority of cases tested, NE produced a net enhancement of visually evoked responses by facilitating excitatory and inhibitory components of stimulus-bound discharges. By contrast, 5-HT tended to suppress stimulus-evoked excitation and inhibition in many cases to the extent that neurons were no longer responsive to appropriate visual stimuli. In selected cases we were able to demonstrate additional effects of NE and 5-HT on response threshold, direction selectivity and discrimination of receptive field borders. For example, in some cells NE was capable of revealing evoked responses to visual stimuli which were previously ineffective in eliciting stimulus-bound discharges. In other instances, changes in cell activity evoked by stimulus movement across the visual field were accentuated during NE application in such a way that unit discharges at receptive field borders were more sharply defined in comparison to control conditions. 5-HT, on the other hand, was capable of decreasing the contrast between spontaneous and visually evoked discharge at receptive field boundaries. In summary, these results suggest that endogenously released NE and 5-HT may modulate, by complimentary actions, the magnitude of responses of visual cortical neurons to afferent synaptic inputs. Moreover, these monoaminergic projection systems may also have the capacity to modify the threshold of detection of afferent signals within a neuronal network as well as alter feature extraction properties of the circuit.

Modification of the visual response properties of cerebellar neurons by norepinephrine

Extracellular recordings were conducted in the paraflocculus of anesthetized Long-Evans pigmented rats to determine how ionotophoresis of norepinephrine (NE) affects the responsiveness of individual Purkinje cells and interneurons to presentations of visual stimuli within their visual receptive fields. Presentations of moving or stationary visual stimuli during the control (pre-NE) period elicited simple spike excitations or inhibitory responses in slightly more than one-half (55%, n = 32) of the cells tested (20 of 38 Purkinje cells, 12 of 20 interneurons). The predominant effect of NE iontophoresis was to improve visually evoked responses in those neurons which showed modulations in their simple spike discharge to control presentations of visual stimuli. A clear enhancement of visual responses by NE (i.e., absolute increase over control) was observed in 18 of the units, and in 12 of the 14 remaining cells, reductions in stimulus-bound discharge during catecholamine iontophoresis were accompanied by much larger depressions in background activity, resulting in a net enhancement in the ratio of signal-to-noise. NE differentially affected responses to stimulus movement in the preferred and non-preferred direction in one-third of these neurons, such that directional selectivity was increased. However, the orientation bias of individual units was unchanged by NE. Iontophoretic application of the beta-adrenergic antagonist sotalol but not the alpha-adrenergic antagonist phentolamine blocked these facilitating noradrenergic effects. An additional feature of noradrenergic action was revealed in tests conducted in 26 cells which did not respond to control presentations of visual stimuli. Iontophoresis of NE resulted in the elicitation of visual responses in 11 of these units, suggesting the possibility that NE might act in some cases to gate the efficacy of subliminal synaptic input conveyed by classical afferent channels. It is proposed that an important aspect of noradrenergic action within local cerebellar circuits might be to refine the receptive field properties of individual neuronal elements and thereby improve information flow through the cerebellum.

NMDA and quisqualate reduce a Ca-dependent K+ current by a protein kinase-mediated mechanism

Stimulation of N-methyl-D-aspartate (NMDA) or quisqualate (Quis) receptors by submicromolar concentrations of NMDA or Quis but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) reduced post-spike train after hyperpolarizations (AHPs) and blocked the underlying Iahp in dentate granule (DG) neurones in vitro. The NMDA but not Quis action was blocked by the NMDA receptor blocker 2-D,L-aminophosphonovaleric acid (APV). Actions of both NMDA and Quis were abolished by isoquinolinesulphonyl-2-methyl-piperazine dihydrochloride (H-7), an inhibitor of several protein kinases. These data suggest that there is a link between excitatory amino acid receptor activation, the protein kinase system, and neuronal excitability.

Auditory sensory gating in hippocampal neurons: a model system in the rat

Diminished evoked response to repeated auditory stimuli, an example of sensory gating normally present in human subjects, is often absent in schizophrenics. To examine the mechanism of the normal response and to delineate possible sites of its abnormality in psychosis, it would be desirable to reproduce the phenomenon in laboratory animals. In this study, we show that the pattern of diminished response to the second of paired auditory stimuli is found in activity recorded from the CA3 region of the hippocampus of anesthetized rats. The evoked potential recorded from this area is predominantly an N40 wave, at identical latency to the prominent negative wave recorded from the skull surface of unanesthetized rats. Similar responses were not found in other areas, including the auditory neocortex and the medial geniculate nucleus. Amphetamine, which diminished sensory gating in both animals and humans, diminished the gating of the evoked potential recorded in the hippocampus. The effect of amphetamine was reversed by haloperidol. The rat hippocampus may therefore contain neurons that can be used to study the neurobiology of sensory gating.

Immunological characterization of K+ channel components from the Shaker locus and differential distribution of splicing variants in Drosophila

Antibodies were raised to three portions of the predicted sequences of Shaker, a gene that encodes a family of K+ channel components that are produced by the alternative splicing of transcripts. On immunoblots, the protein products appear to be 65,000-85,000 daltons in size. No smaller products were detected. Immunocytochemistry has revealed a nonuniform distribution of Shaker products in the brain of the adult fly. By comparing antisera directed against regions shared by all the splicing variants to antisera that are directed against one particular group of splicing variants, we have determined that there is a differential distribution of that group of variants. Thus, the alternative splicing of Shaker transcripts appears to produce different subtypes of A-channels in different tissues.

Serotonin (5-HT) induces IPSPs in pyramidal layer cells of rat piriform cortex: evidence for the involvement of a 5-HT2-activated interneuron

In a slice preparation of rat piriform cortex, both intracellular and extracellular techniques were used to examine the pharmacological and electrophysiological actions of serotonin (5-HT). Bath application of 5-HT resulted in either depolarization (57%), hyperpolarization (34%) or no change (9%) in membrane potential of cells in the pyramidal cell layer (layer II) of piriform cortex. Additionally, when KCl-containing electrodes were used, 5-HT induced an increase in depolarizing synaptic potentials in 41% of these cells. It was concluded that these potentials were reverse inhibitory post-synaptic potentials (IPSPs) because they were blocked by bicuculline and tetrodotoxin. The induction of IPSPs by 5-HT was blocked by the 5-HT2-selective antagonist ritanserin. By recording extracellularly in the presence of 5-HT, a group of 5-HT-activated, putative interneurons was found at the border of layers II and III of piriform cortex, 5-HT but not norepinephrine activation was blocked by ritanserin. The actions of 5-HT were mimicked by the 5-HT2 agonist alpha-methyl-5-HT; the 5-HT2 partial agonist, 2,5-dimethoxy-4-methyl-amphetamine had a small agonist action of its own and blunted the effect of 5-HT. Activation of a larger group of putative interneurons by the more universal excitant N-methyl-D-aspartate showed that the 5-HT-activated interneurons represented 23% of the interneurons located on the border between layers II and III. We conclude that 5-HT induces IPSPs in layer II pyramidal cells by activating a subpopulation of interneurons at the border of layers II and III of piriform cortex.

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.

Potassium currents in hippocampal pyramidal cells

The hippocampal pyramidal cells provide an example of how multiple potassium (K) currents co-exist and function in central mammalian neurones. The data come from CA1 and CA3 neurones in hippocampal slices, cell cultures and acutely dissociated cells from rats and guinea-pigs. Six voltage- or calcium(Ca)-dependent K currents have so far been described in CA1 pyramidal cells in slices. Four of them (IA, ID, IK, IM) are activated by depolarization alone; the two others (IC, IAHP) are activated by voltage-dependent influx of Ca ions (IC may be both Ca- and voltage-gated). In addition, a transient Ca-dependent K current (ICT) has been described in certain preparations, but it is not yet clear whether it is distinct from IC and IA. (1) IA activates fast (within 10 ms) and inactivates rapidly (time constant typically 15-50 ms) at potentials positive to -60 mV; it probably contributes to early spike-repolarization, it can delay the first spike for about 0.1 s, and may regulate repetitive firing. (2) ID activates within about 20 ms but inactivates slowly (seconds) below the spike threshold (-90 to -60 mV), causing a long delay (0.5-5 s) in the onset of firing. Due to its slow recovery from inactivation (seconds), separate depolarizing inputs can be "integrated". ID probably also participates in spike repolarization. (3) IK activates slowly (time constant, tau, 20-60 ms) in response to depolarizations positive to -40 mV and inactivates (tau about 5s) at -80 to -40 mV; it probably participates in spike repolarization. (4) IM activates slowly (tau about 50 ms) positive to -60 mV and does not inactivate; it tends to attenuate excitatory inputs, it reduces the firing rate during maintained depolarization (adaptation) and contributes to the medium after-hyperpolarization (mAHP); IM is suppressed by acetylcholine (via muscarinic receptors), but may be enhanced by somatostatin. (5) IC is activated by influx of Ca ions during the action potential and is thought to cause the final spike repolarization and the fast AHP (although ICT may be involved). Like IM, it also contributes to the medium AHP and early adaptation. It differs from IAHP by being sensitive to tetraethylammonium (TEA, 1 mM), but insensitive to noradrenaline and muscarine. Large-conductance (BK; about 200 pS) Ca-activated K channels, which may mediate IC, have been recorded. (6) IAHP is slowly activated by Ca-influx during action potentials, causing spike-frequency adaptation and the slow AHP. Thus, IAHP exerts a strong negative feedback control of discharge activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Imipramine alters beta-adrenergic, but not serotonergic, mediated responses in rat hippocampal pyramidal cells

Imipramine, a tricyclic antidepressant, acts acutely to block the reuptake of serotonin (5-HT) and norepinephrine (NE). However, imipramine's action as an antidepressant takes several weeks to develop. This study investigated acute and chronic effects of imipramine on intracellularly-recorded responses mediated by 5-HT and beta-adrenergic receptors on pyramidal cells from area CA1 of rat hippocampal slices maintained in vitro. Addition of 10 microM imipramine in the perfusion medium sinistrally shifted the 5-HT1A concentration-response curve for membrane hyperpolarization and the 5-HT concentration-response curve for the reduction in the amplitude of the slow afterhyperpolarization (AHP) elicited by a train of action potentials. After two weeks of treatment with imipramine (10 mg/kg daily i.p. injections or s.c. osmotic mini-pumps) the responses to 5-HT were not altered. In contrast the concentration-response curve for the beta-adrenergic mediated reduction in AHP amplitude was significantly altered; there was a reduction in Emax and a log unit dextral shift in EC50. There was no change in the concentration-response curve for the beta-adrenergic mediated depolarization. These data are in agreement with previous biochemical results reporting a decrease in beta-adrenergic receptor mediated stimulation in adenylyl cyclase and down-regulation of beta-receptor in cortex and hippocampus. These findings suggest that a consequence of long-term imipramine treatment is a decrease in the augmentation of cell excitation normally produced by beta-adrenergic receptor stimulation.

Direct evidence for acute and massive norepinephrine release in the hippocampus during transient ischemia

Recent studies suggest the norepinephrine (NE) may play a regulatory role in neuronal cell death in the hippocampus after transient ischemia. However, ischemia-induced changes in extracellular NE release have not been demonstrated. In the present study, we utilized the microdialysis technique to measure extracellular NE levels in the hippocampus before, during, and after 20 min of global ischemia induced by two-vessel occlusion combined with systemic hypotension in the rat. Stable basal concentrations of extracellular NE were detected in three consecutive samples collected prior to ischemia (1.86 +/- 1.21 pmol/ml of perfusate mean +/- SEM). During ischemia, NE levels increased to 30.1 +/- 5.5 pmol/ml, representing an 18-fold increase. The levels gradually returned to baseline by 40 min of reperfusion. These results are the first to demonstrate that acute and massive extracellular release of NE occurs in the hippocampus during ischemia and early recirculation. These results support the hypothesis that the activation of the noradrenergic system may play a significant role in modulating the development of ischemic neuronal damage.

Neurotensin-induced excitation of neurons of the rat's frontal cortex studied intracellularly in vitro

The actions of neurotensin (NT) on frontal pyramidal neurons were studied in vitro in slices of rat cerebral cortex using current clamp and single electrode voltage clamp (SEVC) techniques. Bath application of NT (0.1 microM-10 microM) induced a depolarization (2-13 mV) in 88% of the pyramidal cells, this effect was associated with a decrease in input conductance of 5-35% and its reversal potential was estimated at -88 +/ -9.7 mV. Typically, this depolarizing effect of NT was transient, since no cell responded to a second application of the peptide within 20 min after the first one. NT also induced an increase in the rate of firing of pyramidal cells evoked by direct stimulation, even when an hyperpolarizing current was applied to prevent the depolarization induced by NT. This effect could neither be explained by a decrease of the post-spike after-hyperpolarization, nor by an increase of the persistent sodium current which sustains the spiking of pyramidal cells, since the former was not affected consistently by NT and the later was insensitive to the peptide. This excitation of pyramidal neurons by NT persisted after blockade of synaptic transmission. On the other hand, NT also enhanced the synaptic noise recorded in pyramidal cells in standard perfusing medium. Furthermore, dopaminergic antagonists and noradrenergic antagonists failed to block these effects of NT. Finally, the inactive fragment of the peptide, NT(1-8), did not affect membrane properties of pyramidal cells. All together, these results suggest that NT excites frontal cortical neurons through the activation of specific NT receptors.

A metabolically stable analog of 1,4,5-inositol trisphosphate activates a novel K+ conductance in pyramidal cells of the rat hippocampal slice

IP(s)3, a metabolically stable analog of 1,4,5-inositol trisphosphate (IP3), inhibited action potential firing when injected into hippocampal pyramidal cells. This effect was associated with decreased input resistance, a more negative resting potential, outward rectification at depolarized potentials, and an afterhyperpolarization. The response to IP(s)3 was unaffected by antagonists of Na+, Ca2+, and Cl- conductances, but was sensitive to changes in extracellular K+ concentration. The IP(s)3-induced conductance was voltage-dependent, was activated in 10 ms with depolarization, and was blocked by extracellular Ba2+ or intracellular Ca2+ chelation. It was not suppressed by other K+ conductance antagonists. Thus, IP(s)3 may activate a novel K+ conductance in CA1 pyramidal cells. IP3 itself did not elicit this conductance, suggesting it may be rapidly metabolized in these cells.

Effects of glucocorticoids and norepinephrine on the excitability in the hippocampus

The CA1 pyramidal neurons in the hippocampus contain a high density of adrenal corticosteroid receptors. By intracellular recording, CA1 neurons in slices from adrenalectomized rats have been found to display a markedly reduced afterhyperpolarization (that is, the hyperpolarizing phase after a brief depolarizing current pulse) when compared with their sham controls. No differences were found for other tested membrane properties. Brief exposure of hippocampal slices from adrenalectomized rats to glucocorticoid agonists, 30 to 90 minutes before recording, greatly enhanced the afterhyperpolarization. In addition, glucocorticoids attenuated the norepinephrine-induced blockade of action potential accommodation in CA1 neurons. The findings indicate that glucocorticoids can reduce transmitter-evoked excitability in the hippocampus, presumably via a receptor-mediated genomic action.

Antidepressant drugs potentiate the alpha 1-adrenoceptor effect in hippocampal slices

The effect of prolonged treatment with antidepressant drugs on the phenylephrine- and norepinephrine (NE)-evoked reaction in hippocampal slices was examined by extracellular recording of the spontaneous activity of CA1 layer neurons. The alpha 1-adrenoceptor agonists, phenylephrine and methoxamine, depressed the neuronal discharges of most of the units tested, while NE evoked both excitatory and inhibitory effects which were blocked by propranolol and phentolamine or prazosin, respectively. Imipramine, mianserin, (+)- and (-)-oxaprotiline administered subchronically (10 mg/kg p.o., twice daily for 14 days, withdrawal 48 h), potentiated the inhibitory reaction to phenylephrine. Mianserin was the only drug tested in the acute dose to effectively augment the reaction to alpha 1-adrenoceptor stimulation. Prolonged administration of mianserin and imipramine attenuated the excitatory effect to NE, which probably reflects beta-receptor down-regulation; however, only mianserin, but not imipramine, enhanced the NE-induced inhibition. The observed potentiation of the alpha 1-adrenoceptor-related inhibitory reaction to phenylephrine produced by antidepressant drugs may reflect the development of the alpha 1-adrenergic system supersensitivity in the hippocampus.

Participation of cyclic adenosine monophosphate and beta-adrenergic receptors in the facilitatory effect of noradrenaline on the response of cultured cerebellar neurons to glutamate

For the purpose of examining possible involvement of the cyclic adenosine monophosphate (cAMP) system and adrenergic receptors in the modulatory effect of noradrenaline (NA) on the glutamate-induced depolarizing response, the effects of dibutyryl cAMP (DBcAMP), forskolin, theophylline, clonidine, isoproterenol and propranolol were intracellularly investigated in the cerebellar neurons cultured from chick embryos. Not only NA-induced hyperpolarization and increase in input resistance but also the facilitatory effect of NA on the glutamate response were mimicked by DBcAMP and isoproterenol. This facilitatory effect of DBcAMP was enhanced by theophylline or forskolin, while that of isoproterenol was antagonized by propranolol. Clonidine suppressed glutamate-induced depolarization. These results that the enhancing action of NA on the responsiveness of cultured cerebellar neurons to excitatory amino acids is mediated by beta-adrenergic receptors and the intracellular cAMP system.

Electrophysiological responsiveness to isoproterenol in rat hippocampal slices correlates with changes in beta-adrenergic receptor density induced by chronic morphine treatment

The effects of chronic morphine treatment and morphine withdrawal on beta-adrenergic receptor density and electrophysiological responsiveness in rat hippocampus were examined. Chronic treatment of rats with morphine for 14 days resulted in a 19% increase in the number of beta-adrenergic receptors in hippocampus, as measured by the binding of the specific antagonist [3H]dihydroalprenolol (DHA). In comparison, the number of specific binding sites for [3H]DHA was decreased 27% in hippocampus in morphine-withdrawn animals, compared to saline-treated controls. These alterations in beta-adrenergic receptor density were not accompanied by a significant change in the dissociation constant (Kd) for [3H]DHA or in the inhibitory constants (Ki) for the displacement of the [3H]-antagonist by either norepinephrine or isoproterenol. Electrophysiological experiments in the in vitro hippocampal slice preparation revealed that responses to threshold as well as maximal concentrations of isoproterenol were significantly enhanced in morphine-dependent animals, compared to controls, whereas electrophysiological responsiveness to maximal concentrations of isoproterenol was decreased in slices from morphine-withdrawn rats. The results of this study indicate that beta-adrenergic receptors in hippocampus are up-regulated during the development of morphine dependence and down-regulated during opiate withdrawal. These changes in hippocampal beta-adrenergic receptor density are likely to be of functional relevance since they are manifested in a corresponding increase and decrease, respectively, in electrophysiological responsiveness to an exogenously administered beta-adrenergic receptor agonist.

Inescapable versus escapable shock modulates long-term potentiation in the rat hippocampus

A group of rats was trained to escape low-intensity shock in a shuttle-box test, while another group of yoked controls could not escape but was exposed to the same amount and regime of shock. After 1 week of training, long-term potentiation (LTP) was measured in vitro in hippocampal slices. Exposure to uncontrollable shock massively impaired LTP relative to exposure to the same amount and regime of controllable shock. These results provide evidence that controllability modulates plasticity at the cellular-neuronal level.

Absence of long-term potentiation in the subcortically deafferented dentate gyrus

All subcortical afferents to the dorsal hippocampus, running in the fimbria-fornix and supracallosal path, were removed by aspiration. Three to 5 months later the rats were implanted with chronic recording electrodes in the dentate gyrus and CA1 region, and stimulating electrodes in the angular bundle. In non-lesioned rats, high-frequency trains delivered to the angular bundle gave rise to a sustained increase of the evoked population spike in the dentate gyrus. In lesioned animals, high-frequency stimulation resulted in only short-lasting changes, and by 15 min after the conditioning trains the amplitude of both the population spike and field postsynaptic potentials returned to baseline. In lesioned rats large amplitude interictal spikes (less than 40 ms, 3-8 mV) occurred spontaneously. These findings suggest that either (1) coactivation of entorhinal and subcortical inputs is essential for the induction of long-lasting plastic changes in the dentate gyrus, or (2) the long-term potentiation mechanism is saturated by the chronically occurring interictal discharges in the subcortically denervated dentate gyrus.

Postischemic administration of idazoxan, an alpha-2 adrenergic receptor antagonist, decreases neuronal damage in the rat brain

The effect of an alpha-2 receptor antagonist, idazoxan, on ischemic neuronal damage in the hippocampus and neocortex was studied in rats following 10 min of forebrain ischemia. Idazoxan was given 0.1 mg/kg i.v. immediately after recirculation, followed by 48 h of continuous infusion at a rate of 10 micrograms/kg/min. A histopathological examination of the CA1 region of the dorsal hippocampus and neocortex from each hemisphere was made on paraffin-embedded sections following 7 days of survival. In ischemic animals receiving an infusion of saline, 71% of the neurons in the hippocampal CA1 region were degenerated. In contrast, in the idazoxan-treated animals only 31% of the neurons were irreversibly damaged (p less than 0.01). We conclude that postischemic administration of the alpha-2 antagonist idazoxan protects neurons against damage following cerebral ischemia. Rapid postischemic administration of alpha-2 adrenergic receptor antagonists could be an effective treatment after stroke and cardiac arrest.

Ultrastructural localization of tyrosine hydroxylase-like immunoreactivity in the rat hippocampal formation

The light and electron microscopic localization of antigenic sites for a polyclonal antiserum directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), was examined in the hippocampal formation of the rat brain with a double-bridged peroxidase-antiperoxidase method. By light microscopy, the majority of varicose processes with intense TH-like immunoreactivity (LI) were contained in the hilus of the dentate gyrus (DG) and strata radiatum and lacunosum-moleculare of the CA3 region of the hippocampus. Only a few immunoreactive fibers were observed in the molecular and granule cell layers of the DG, in strata oriens and pyramidale of CA3, and in all layers of CA1. Electron microscopy confirmed that these labeled processes were primarily axons and axon terminals. Terminals with TH-LI were 0.4-1.1 micron in diameter and contained many small clear vesicles and from 0 to 3 larger dense-core vesicles. The number and types of associations formed by terminals with TH-LI were remarkably similar in the DG and hippocampus proper despite known differences in intrinsic cells and function. In both regions, the majority of terminals with TH-LI formed junctions on small (distal dendrites (52% of 112 in the DG; 67% of 116 in CA3) and dendritic spines (30% in the DG; 18% in CA3) that were both asymmetric and symmetric. In the DG, axosomatic junctions (2% of 112) were symmetric and occurred exclusively on the perikarya of granule cells, whereas junctions on large (proximal) dendrites were more numerous (16%), exhibited symmetric as well as asymmetric membrane specializations, and were of both granule (molecular layer) and nongranule (hilus) cell origin. In CA3, synaptic contacts on perikarya (5% of 116) and large (proximal) dendrites (10%) of both pyramidal cell and nonpyramidal cell origin were few and all symmetric. The distribution and types of synaptic associations formed by terminals with TH-LI in the CA1 region paralleled that seen in the CA3 region. In both the dentate and hippocampus proper, 10% of the terminals with TH-LI were observed closely apposed to unlabeled terminals that formed asymmetric synapses with dendrites and dendritic spines. In rare instances, TH-immunoreactive terminals were found in close association with the basement membrane of blood vessels, astrocytic processes, or with other unlabeled terminals not forming recognizable junctions. In addition TH-LI was occasionally detected within the cytoplasm of a minority of astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of calcium in the repetitive firing of neostriatal neurons

The Ca++ -dependence of the repetitive firing of neostriatal neurons was studied in an in vitro slice preparation of the rat neostriatum. Neuronal firing was evoked by injecting depolarizing currents of 100-200 ms duration. In normal conditions, the mode of firing was tonic and showed very little adaptation. The frequency-current relation was linear over a wide range of frequencies. The repetitive firing was first enhanced and later suppressed by Co++, Mn++ and Cd++. These effects on the repetitive firing by the Ca++ -channel blockers paralleled the suppression of the slow afterhyperpolarizing potential. The lowering (0.2 mM) of Ca++ had similar effects. In the presence of TEA (up to 10 mM), the cell fired both Na+ and Ca+ action potentials. The results suggest that, as in other CNS neurons of the vertebrate, in neostriatal neurons the slow afterhyperpolarizing potential (AHP) is due to a Ca++ -activated K+ -conductance, and that the AHP plays a crucial role in the repetitive firing of these neurons.

The transient potassium current, the A-current, is involved in spike frequency adaptation in rat amygdala neurons

The possible functional roles of the transient K+ current, IA, in basolateral amygdala (BLA) neurons were studied using a rat brain slice preparation and conventional intracellular recording techniques. Conditioning depolarization, which inactivates IA, slowed the action potential repolarization while conditioning hyperpolarization accelerated the action potential repolarization. 4-Aminopyridine (4-AP, 100 microM), a specific IA antagonist, also caused a clear delay in spike repolarization similar to the effect of conditioning depolarization suggesting that IA is involved in the action potential repolarization. When BLA neurons were excited by injecting long depolarizing current pulses (500 ms), they responded with an initial rapid discharge of action potentials which slowed or accommodated; an afterhyperpolarization (AHP) followed the depolarizing current pulses. Superfusion of 4-AP (100 microM) blocked accommodation resulting in an increase in action potential discharge in 74% (32 out of 43) neurons tested. The remaining 11 cells responded with an increased frequency of discharge of the first few action potentials. Unlike the effect of cadmium (Cd2+, 100 microM), a calcium channel blocker, 4-AP did not reduce the AHP. In the presence of norepinephrine (NE, 10 microM), a neurotransmitter which has been shown to block calcium-activated potassium conductance, 4-AP caused a further increase in the number and frequency of action potential discharge. In addition, in BLA neurons, spontaneous interictal and ictal-like events were observed at low and high concentrations of 4-AP, respectively. We conclude that IA is involved in the action potential repolarization as well as spike frequency adaptation in BLA neurons and that these actions may contribute to the convulsant effect of 4-AP.

Multiple voltage-sensitive K+ channels regulate dendritic excitability in cerebellar Purkinje neurons

Ionic conductances present in the dendritic region of the cerebellar Purkinje neuron were studied using the single-channel and whole-cell recording methods. Several types of voltage-sensitive K+ channels including a Ca2+ activated K+ channel were found to be a prominent components of the dendritic membrane. All patches studied contained K+ channel types and most patches contained more than one K+ channel type. In cell attached recordings, K+ channel activity was associated with the late phase of spontaneous action potentials suggesting a functional relationship. These data demonstrate that voltage-sensitive ion channels contribute to dendritic excitability and suggest that the transduction and integration of synaptic signals may involve both active and passive ionic conductances.

Region-specific loss of alpha 1-adrenergic receptors in rat brain with aging: a quantitative autoradiographic study

The effects of aging on the density and affinity of alpha 1-adrenergic receptors (alpha 1-ARs) were studied in several circumscribed areas of the Fischer 344 male rat brain. Computer-assisted quantitative autoradiography was used to analyze saturation binding isotherms of [125I]BE-2254, a selective alpha 1-AR antagonist. Significant decreases in receptor density of 15 and 29% were observed in the thalamus at 16-18 and 24-28 months of age, respectively, when compared to 3-4-month-old controls. Progressive declines in receptor density of 24 and 44% were also found in the olfactory tubercle. In the cerebral cortex, a significant 26% loss in receptors occurred only in the oldest age group. No changes were found in any of the other brain areas investigated, including the cerebellum, brainstem, caudate-putamen, and several subregional areas of the hippocampal formation. Kd values ranged from 12 +/- 1.8 pM in the brainstem to 23 +/- 1.6 pM in the thalamus and were not affected by aging in any area examined. It is concluded that the density of alpha 1-ARs in the Fischer 344 rat brain is diminished with aging in a region-specific manner and that loss of these receptors may account for age-related functional deficits only in a few brain areas.

Modulatory actions of serotonin on ionic conductances of hippocampal dentate granule cells

Pressure ejection of serotonin (2 x 10(-4) M) onto dentate granule neurons in vitro produced a short-lasting membrane hyperpolarization associated with a 10-30% decrease in the input resistance. The hyperpolarization magnitude depended on the extracellular K+ concentration but not on the extra or intracellular Ca2+ concentration. It was followed by a depolarization, especially when serotonin was applied onto the perisomatic area of the neuron. The post-spike-train afterhyperpolarization, which represents a Ca2+-dependent K+ conductance, was decreased by serotonin by 10-100% and remained reduced for 2-10 min following the serotonin-induced hyperpolarization. Decreased adaptation of cell firing was also observed following serotonin application. Ca2+ action potentials evoked by intracellular depolarizing current pulses in the presence of the Na+ channel blocker tetrodotoxin and the K+ channel blocker tetraethylammonium were followed by a large afterhyperpolarization, which was markedly reduced for several minutes following serotonin application. The preceding Ca2+ action potential was either unaffected or prolonged. The hyperpolarization occurring in response to localized application of serotonin, and the reduction of the afterhyperpolarization, may represent two different mechanisms of serotonin action, probably mediated by different mechanisms. The slow time course of the late depolarization and the afterhyperpolarization depression represent modulatory effects of serotonin on dentate granule neurons.

Mechanisms underlying the serotonergic modulation of the spinal circuitry for locomotion in lamprey

The central nervous system of the lamprey contains serotonergic (5-hydroxytryptamine, 5-HT) neurones both in the spinal cord and in the brainstem. Endogenously released 5-HT from these systems modulates the pattern of fictive locomotion induced in the in vitro preparation; the burst rate is lowered and burst discharges become longer and of higher intensity. Local application of 5-HT, mimicking activation of the 5-HT systems, has a specific effect on the late phase of the afterhyperpolarization (AHP) in motoneurones and interneurones. 5-HT markedly reduces the amplitude of the late AHP without affecting passive membrane properties or the shape or threshold of the action potential. This 5-HT effect appears to result from a direct action on the calcium-dependent potassium channels underlying the late phase of the AHP. A reduction of the amplitude of the AHP will result in altered spike discharge characteristics, with potentiation of the response (discharge rate) to a given excitatory input in all neurones influenced by 5-HT. It is suggested that the modulatory effect of 5-HT on fictive locomotion can be attributed to its action on the late AHP and thereby to the potentiation of excitability in excitatory and inhibitory interneurones in the generator circuitry. This has been further corroborated in computer simulation studies of a network model, where the action of 5-HT was simulated by decreasing AHP amplitude, resulting in a slowing of the rhythm analogous to the effect demonstrated experimentally.

The intrinsic electrophysiological properties of mammalian neurons: insights into central nervous system function

This article reviews the electroresponsive properties of single neurons in the mammalian central nervous system (CNS). In some of these cells the ionic conductances responsible for their excitability also endow them with autorhythmic electrical oscillatory properties. Chemical or electrical synaptic contacts between these neurons often result in network oscillations. In such networks, autorhythmic neurons may act as true oscillators (as pacemakers) or as resonators (responding preferentially to certain firing frequencies). Oscillations and resonance in the CNS are proposed to have diverse functional roles, such as (i) determining global functional states (for example, sleep-wakefulness or attention), (ii) timing in motor coordination, and (iii) specifying connectivity during development. Also, oscillation, especially in the thalamo-cortical circuits, may be related to certain neurological and psychiatric disorders. This review proposes that the autorhythmic electrical properties of central neurons and their connectivity form the basis for an intrinsic functional coordinate system that provides internal context to sensory input.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. I. Receptors mediating the effect of microiontophoretically applied norepinephrine

The rat hippocampus receives a dense noradrenergic innervation originating exclusively from the locus coeruleus. The present electrophysiological study was undertaken to characterize the adrenoceptor mediating the suppressant effect of microiontophoretically applied norepinephrine (NE) on CA1 and CA3 dorsal hippocampus pyramidal neurons of the rat. The rank order of potency of microiontophoretically applied agonists, in suppressing the firing rate of hippocampus pyramidal neurons was: oxymetazoline greater than NE greater than phenylephrine greater than isoproterenol greater than clonidine. In the hippocampus, oxymetazoline was more potent than NE, whereas it was ineffective in the lateral geniculate nucleus where the effect of NE is mediated by an alpha 1-adrenoceptor. Low currents of clonidine antagonized the effect of NE suggesting that clonidine may exert a partial agonistic effect. The rank order of potency of i.v. administered adrenergic antagonists in blocking the suppressant effect of microiontophoretically applied NE was: idazoxan much greater than prazosin much greater than propranolol. Idazoxan also blocked the effect of oxymetazoline, phenylephrine, and isoproterenol but did not modify the effect of microiontophoretically applied gamma-aminobutyric acid (GABA). In addition, idazoxan, applied by microiontophoresis, readily blocked the suppressant effect of NE without affecting that of GABA. These results suggest that the suppressant effect of microiontophoretically applied NE on rat dorsal hippocampus pyramidal neurons is primarily mediated by alpha 2-adrenoceptors.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. II. Receptors mediating the effect of synaptically released norepinephrine

The present studies were undertaken to determine the nature of the receptors mediating the effects of endogenous norepinephrine (NE) released by stimulation of the locus coeruleus (LC) on the firing activity of dorsal hippocampus pyramidal neurons in the rat. Unitary activity of CA3 pyramidal neurons was recorded extracellularly. In most neurons, the LC stimulation produced a period of suppression, followed by a period of activation. The suppression was selectively blocked by prazosin, an alpha 1-adrenoceptor antagonist, whereas the activation was selectively blocked by propranolol, a beta-adrenoceptor antagonist. Idazoxan, an alpha 2-adrenoceptor antagonist, increased the period of suppression without affecting the period of activation. The effectiveness of microiontophoretic applications of NE on the same neurons was reduced by idazoxan, but was modified neither by propranolol nor prazosin. Lesion of the central noradrenergic system by intracerebroventricular 6-hydroxydopamine markedly decreased the NE content in the hippocampus in all rats but the effectiveness of the LC stimulation was reduced only in rats with a depletion greater than 90%. These results demonstrate that the suppressant effect of endogenous NE released by LC stimulation on hippocampus pyramidal neurons is mediated by an alpha 1-adrenoceptor and suggest that its late excitatory effect might involve beta-adrenoceptors. Since the effect of microiontophoretically applied NE on the same neurons is mediated by alpha 2-adrenoceptors, these data provide evidence that, in the rat hippocampus, postsynaptic alpha 1-adrenoceptors are intrasynaptic, whereas postsynaptic alpha 2-adrenoceptors are extrasynaptic.

Suppression and induction of epileptic activity by neuronal grafts

Fetal rat brain cell suspensions prepared from either the locus coeruleus region or hippocampus were implanted bilaterally into the subcortically denervated seizure-prone hippocampus of adult rats. Animals with locus coeruleus grafts were protected against picrotoxin-induced behavioral seizures and had significantly fewer interictal spikes. In contrast, in rats with fetal hippocampal grafts the incidence of interictal spikes was significantly higher than in lesion-only controls, and spontaneous behavioral seizures occurred in almost half of the animals. We suggest that neuronal grafting offers an alternative method for studying the mechanisms and control of epileptic brain activity.

Commentary on neurophysiology, plasticity, and morphology of aging: the search for a critical substrate

It is becoming increasingly apparent that several issues must be considered in interpreting the neural bases of behavioral differences in aged animals. Prior assumptions need to be modified to account for the potential increase in number of age-related brain changes likely to be uncovered in the coming years as investigation of the aging process proceeds. It will therefore be necessary to rule out processes irrelevant for the particular behavioral/physiological mechanisms under study. Thus, a simple correlation between any one anatomical or physiological measure will not suffice in the future to adequately explain the nature of the age-related behavioral deficits in the human brain.

New evidence for a gating action of norepinephrine in central neuronal circuits of mammalian brain

Many previous studies have examined the effects of norepinephrine (NE) on neuronal responsiveness to synaptic inputs and putative transmitter substances and have described differential depressant actions of NE on stimulus evoked versus spontaneous discharge such that the "signal to noise" ratio of threshold responses was increased. In the present studies, similar experimental strategies employing a combination of microiontophoresis, single unit recording and afferent pathway stimulation in intact anesthetized and brain tissue slice preparations have revealed noradrenergic "gating" actions whereby weak or subthreshold synaptic stimuli can evoke threshold neuronal responses in the presence of iontophoretically applied NE or following electrical stimulation of the locus coeruleus. Overall, these results suggest that potentially threshold excitatory and inhibitory synaptic inputs may normally arrive at central neurons but appear weak or absent except during behavioral conditions favoring the synaptic release of NE. As such, these findings provide evidence that signal to noise ratio may not be the only potential modulatory action expressed by NE in noradrenergic target circuits of the mammalian brain.

Antagonism of the responses to isoproterenol in the rat hippocampal slice with subtype-selective antagonists

The electrophysiological and cAMP responses to the beta-adrenoceptor agonist isoproterenol were measured in the in vitro hippocampal slice preparation. Subtype-selective antagonists were used to evaluate the specificity of these responses. The beta 1-selective antagonist ICI 89,406 was 60-fold more potent than was the beta 2-selective antagonist ICI 118,551 at antagonizing the electrophysiological response. ICI 89,406 was 200 times more potent in its antagonism of the cAMP response. These results suggest that the electrophysiological and cAMP responses in this preparation are primarily mediated by beta 1-adrenoceptors.

Inhibitory processes in development of seizure activity in hippocampal slices

The possibility that changes in inhibitory processes are necessary for development of seizure activity was examined using guinea pig hippocampal slices. After repeated tetanic stimulation to the stratum radiatum in region CA3, seizure discharges were evoked in the stratum pyramidale by test stimuli. The latency of the seizure discharges was shortened and the duration was prolonged progressively with the number of tetanic stimulations. The latency ranged from 30 to 100 ms and it was decreased successively as the distance between the recording site and the site of tetanic stimulation was decreased. This suggests that the foci of the seizure discharges existed near the site of tetanic stimulation. In neurons within the foci, inhibitory postsynaptic potentials and the suppressing action of alveus stimulation on glutamate-induced single cell discharges remained unchanged during development of seizure activity, although excitatory postsynaptic potentials were potentiated. In addition, no marked changes were detected in the input resistance, resting membrane potential and the amplitude, threshold and afterhyperpolarization of action potentials. These results suggest that suppression of inhibitory processes do not necessary for tetanus-induced seizure activity.

The coupling of neurotransmitter receptors to ion channels in the brain

Recent studies on the action of neurotransmitters on hippocampal pyramidal cells indicate that different neurotransmitter receptors that use either the same or different coupling mechanisms converge onto the same ion channel. Conversely, virtually all of the neurotransmitters act on at least two distinct receptor subtypes coupled to different ion channels on the same cell. The existence of both convergence and divergence in the action of neurotransmitters results in a remarkable diversity in neuronal signaling.

Inhibitory action of serotonin in CA1 hippocampal neurons in vitro

The ionic mechanism of the inhibitory effect of serotonin was investigated in vitro in the CA1 region of the rat hippocampus by extra- and intracellular recordings. Local or bath applications of serotonin induced a long-lasting reduction of extracellularly recorded synaptic potentials and orthodromic population spikes without affecting the afferent volley or the antidromic population spike. Serotonin can also reduce the frequency of occurrence of spontaneous excitatory and inhibitory postsynaptic potentials without any reduction of input resistance of the pyramidal neuron. During the response to serotonin, the conductance increase evoked by GABA, the inhibitory neurotransmitter, was not changed. A direct postsynaptic effect of serotonin was demonstrated: local or bath applications of serotonin induced a tetrodotoxin-resistant hyperpolarization and conductance increase. The conductance change was not reduced by manual clamp of the neurons to the control resting membrane potential; therefore, a possible involvement of the sodium-potassium electrogenic pump is unlikely. When neurons were loaded with chloride, serotonin could still induce a hyperpolarization with an apparent reversal more negative than the resting membrane potential. When neurons were loaded with caesium, the hyperpolarization and the conductance increase evoked by serotonin were blocked. It is therefore concluded that serotonin increases potassium permeability. Similar effects were induced by a 5-HT1A ligand. The slow after hyperpolarization was reduced by serotonin; the calcium spike was reduced at the same time. In caesium loaded neurons, the spike duration was not modified by serotonin. In the presence of extracellular caesium (4-5 mM), the serotonin-induced hyperpolarization and the conductance change were blocked, but the effect of serotonin on calcium spikes persisted. Tetraethylammonium (5-10 mM) or 4-aminopyridine (0.5 mM) had no effect on the response to serotonin. These data indicate that serotonin has a postsynaptic inhibitory action by an activating potassium conductance. The possibility of a regulation of calcium currents is discussed. The possible role of serotonin on intrinsic synaptic transmission is also discussed.

Catecholaminergic innervation of pyramidal and GABAergic nonpyramidal neurons in the rat hippocampus. Double label immunostaining with antibodies against tyrosine hydroxylase and glutamate decarboxylase

This study describes the catecholaminergic innervation of rat hippocampal neurons at the electron microscopic level by using an antibody against tyrosine hydroxylase (TH) and immunocytochemical techniques. In a first series of experiments, the course and distribution as well as the synaptic contacts of TH-immunoreactive fibers were analyzed with the peroxidase-antiperoxidase (PAP) method. Next, peroxidase immunostaining of TH fibers was combined with glutamate decarboxylase (GAD) immunostaining, using avidinated ferritin as a second electrondense marker. Our results demonstrate that TH-immunostained terminals establish asymmetric synaptic contacts with spines of pyramidal neurons, and symmetric synaptic contacts with cell bodies and dendritic shafts of ferritin-labeled GAD-immunoreactive nonpyramidal cells.

Norepinephrine decreases synaptic inhibition in the rat hippocampus

The effects of norepinephrine (NE) on inhibitory synaptic potentials were studied on CA1 pyramidal neurons in the hippocampal slice in vitro. Norepinephrine caused the appearance of multiple population spikes in the CA1 region of the hippocampal slice, reminiscent of the actions of gamma-aminobutyric acid (GABA) antagonists. Intracellular recording revealed that NE causes a marked and reversible reduction in inhibitory postsynaptic potentials (IPSPs) recorded in CA1 pyramidal cells. This reduced IPSP results in a larger intracellular excitatory postsynaptic potential (EPSP), which can cause the cell to fire more than one action potential. This disinhibitory effect of NE appears to be mediated by an alpha-receptor, and occurs at a site presynaptic to the pyramidal cell, since NE does not change the reversal potential of the IPSP nor does it affect the amplitude or the reversal potential of iontophoretic GABA responses. In addition to reducing evoked IPSPs, NE causes an increase in the frequency of spontaneous IPSPs, suggesting that inhibition of interneuronal firing may not account for this disinhibitory action of NE.

Excitatory effect of noradrenaline on pacemaker cells in spinal cord primary cultures

Intracellular recordings were made from dissociated fetal mouse spinal cord neurons in primary culture. One particular type of neuron, with a large cell body (40-50 micron) and three to five thick neurites, exhibited rhythmic electrical activity of two different types, consisting of either spontaneous burst discharges or tonic action potential firing. Both types of activity appeared to be triggered by an endogenous membrane potential oscillation. Micropressure application of noradrenaline (10(-5) M in the delivery pipette) onto the surface of such cells evoked, in a dose-dependent manner, an increase in the input resistance with a depolarization of the membrane potential. The response to NA was potential-dependent. The maximum change in input resistance was observed at membrane potential values between -60 mV and -45 mV and the response was suppressed at membrane potentials lower than -80 mV. No modification of the response was observed in the presence of 50 mM of tetraethylammonium. The extrapolated reversal potential, close to -90 mV, was modified by increasing extracellular K+ concentration and unaltered by increasing the intracellular Cl- concentration. The decrease in K+ conductance induced by noradrenaline was Ca2+-dependent and reversibly suppressed by Ba2+ (6 mM) and Cd2+ (0.1 mM). This response to noradrenaline was suppressed in the presence of muscarine (10 microM) suggesting that noradrenaline decreases a K+ conductance related to M current. The noradrenaline evoked increase in input resistance was mediated by activation of an alpha 1 receptor site. Prazosin, an alpha 1 antagonist and phentolamine, an alpha 1 alpha 2 antagonist, reversibly suppressed the response in a competitive manner. Yohimbine, a competitive alpha 2 antagonist, also blocked the response, but in a noncompetitive manner. Clonidine, an alpha 2 agonist, isoprenaline, a beta agonist and L-alprenolol, a beta antagonist, had no effect.

How should one study brain adrenergic receptors in aging?

Contradictory findings and lack of definitive information regarding adrenergic receptors in aging results in part from problems related to the methodology that has been used to study this question. Limitations of available techniques and new biochemical, molecular biological, and physiological methods that may prove particularly helpful for future studies are discussed.

Actions of serotonin recorded intracellularly in rat dorsal lateral septal neurons

The actions of serotonin (5HT) on passive and active membrane properties of neurons in the rat dorsal lateral septal nucleus (LSN) were studied by using intracellular recordings in transverse, septal slices. Superfusion with 10 microM 5HT induced a hyperpolarization of the membrane in almost all neurons tested in the dorsolateral part of the LSN. The hyperpolarization was accompanied by a decrease in membrane resistance. These effects of 5HT persisted in a low-Ca2+/high-Mg2+-containing medium or medium with tetrodotoxin, indicating a post-synaptic site of action for 5HT. The reversal potential for the hyperpolarizing effect was ca. -95 mV. If the extracellular K+-concentration was raised, the reversal potential became less negative. These data suggest that 5HT hyperpolarizes LSN neurons by increasing a K+-conductance. Spontaneous, synaptically evoked action potentials and action potentials induced in LSN neurons by a depolarizing current step typically display a fast Na+-spike with a subsequent K+-afterhyperpolarization, followed by a much slower Ca2+-dependent afterdepolarization. The amplitude of the K+-afterhyperpolarization was decreased by 5HT, while at the same time the afterdepolarization became more pronounced. The Ca2+-spike of LSN neurons was not affected by 5HT. Synaptic responses that were evoked in LSN neurons by stimulation of the dorsal part of the LSN consisted of a fast EPSP or spike, followed by a Cl(-)-dependent fast IPSP and a K+-dependent late IPSP. Of these synaptic responses, 5HT suppressed particularly the late IPSP. The present data indicate that 5HT affects the conductance for active and passive K+-channels in LSN neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 microM-1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2-20 mV). Under voltage-clamp conditions, Ad (100 microM) produced fast, slow and mixed types of inward currents (AdIs) with amplitude of 2.9 +/- 1.3 nA. beta-Adrenoceptors may be responsible for the generation of these AdDs. The slow AdI which lasted for 1-5 min was associated with a decreased membrane conductance. The slow AdI decreased at hyperpolarized potential level and eventually nullified at -70 mV. No reversal of the slow AdI polarity was observed in the Ringer solution. Injection of Cs2+ into the ganglion cells produced a marked depression of the amplitude of the slow AdI. The slow AdI was blocked by bath-applied Ba2+ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range -35 to -55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast AdI decreased and reversed its polarity at -36 +/- 8.3 mV. Removal of Cl ion from superfusing solution depressed the fast AdI, suggesting that activation of Cl- conductances may be involved in the generation of the fast AdI. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.

Repetitive excitation of bursts of action potentials in the rat hippocampus following single shock stimulation

1. Post-stimulus time (PST) histograms of rat hippocampal cells were recorded in vivo following single-shock stimulation of the fornix. 2. The PST histograms displayed a series of peaks of decreasing amplitude, similar to damped oscillatory responses previously recorded in cats and rabbits. 3. The effect of increased background activity was investigated by recording histograms with concurrent pulse train stimulation of the contralateral hippocampus. The histograms showed a decreased latency to the onset of the second peak. 4. Damped oscillatory activity seen in the in vivo rat preparation could not be elicited in the in vitro rat slice preparation. Thus species differences cannot account for the absence in slice studies of this type of damped oscillatory activity. 5. We conclude that the level of spontaneous activity is one factor contributing to the genesis of multiple peaks in histograms in the in vivo preparation.

The effects of noradrenaline on neurones in the rat dorsal motor nucleus of the vagus, in vitro

1. Intracellular recordings were made from vagal motoneurones identified by antidromic stimulation in the dorsal motor nucleus of the vagus (d.m.v.) in slice preparations of rat medulla oblongata. 2. Noradrenaline (NA) applied by perfusion (0.01 microM to 1 mM) depolarized 55%, hyperpolarized 32% and produced a biphasic response (hyperpolarization followed by depolarization) in 9% of the d.m.v. neurones tested. 3. The NA effects persisted after complete elimination of synaptic inputs during perfusion with Ca2+-free high-Mg2+ solution, and therefore probably resulted from a direct action on the postsynaptic membranes. 4. The NA depolarization was blocked by prazosin and the NA hyperpolarization by yohimbine, but neither was blocked by propranolol or timolol. Phenoxybenzamine blocked both responses. The results indicate that NA depolarization is mediated by alpha 1-adrenoceptors and hyperpolarization by alpha 2-adrenoceptors. 5. The neurones which were depolarized by NA were also hyperpolarized by NA when the alpha 1-adrenoceptors were blocked by prazosin (all of seven neurones tested). This result suggests that most vagal motoneurones in the d.m.v. have both alpha 1-and alpha 2-adrenoceptors. 6. The NA depolarization was accompanied by a decrease in membrane conductance and the hyperpolarization by an increase in membrane conductance, both of which were measured under manual-clamp conditions. 7. The reversal potentials for the NA responses were around -85 mV in normal Ringer solution, and shifted as predicted by the Nernst equation when the extracellular K+ concentration was changed. 8. The inhibitory postsynaptic potentials evoked by focal electrical stimulation on the slice surface of the commissural part of the nucleus of the tractus solitarius (n.t.s.), which contains an A2 catecholaminergic cell group, were abolished by yohimbine. 9. The results suggest that NA modulates vagal output by decreasing or increasing the K+ conductance of d.m.v. neurones through alpha 1- or alpha 2-adrenoceptors. In addition, the A2 noradrenergic cell group within the n.t.s. may send inhibitory inputs to the d.m.v.

Pharmacologically distinct actions of serotonin on single pyramidal neurones of the rat hippocampus recorded in vitro

1. The actions of serotonin (5-HT) on pyramidal cells of the CA1 region of the rat hippocampus were characterized using intracellular recording in in vitro brain slices. 2. 5-HT typically evokes a biphasic response consisting of a hyperpolarization which is followed by a longer-lasting depolarization. These effects on membrane potential are accompanied by a decrease in the calcium-activated after-hyperpolarization (a.h.p). 3. Detailed analysis using 5-HT antagonists and agonists indicates that the hyperpolarization is mediated by a 5-HT1A receptor. Spiperone is the most effective antagonist of the response and the selective 5-HT1A agonist, 8-OHDPAT, behaves as a partial agonist at this receptor. In agreement with the distribution of 5-HT1A binding sites, responses to 5-HT were most prominent in the stratum radiatum. 4. The hyperpolarizing response is associated with a decrease in input resistance, is blocked by extracellular barium and intracellular caesium, is unaffected by the chloride gradient, and its reversal potential shifts with the extracellular concentration of potassium as predicted for a response mediated by a selective increase in potassium permeability. 5. The depolarizing response and reduction in the a.h.p. could be studied in isolation by blocking the hyperpolarizing response with either pertussis toxin or spiperone. The pharmacology of these responses did not correspond to that of any of the 5-HT binding sites reported in C.N.S. tissue. Although the depolarization and blockade of the a.h.p. have the same time course it is unclear if they are mediated by the same or different receptors. 6. The depolarization most likely results from a decrease in resting potassium conductance. However, neither a blockade of the M current nor the a.h.p. current can account for the depolarization. 7. Blockade of phosphodiesterase activity by 3-isobutyl-1-methylxanthine (IBMX) did not enhance the depressant action of 5-HT on the a.h.p., making it unlikely that this action is mediated by cyclic AMP. 8. Blockade of the a.h.p. by 5-HT reduces spike frequency adaptation and counteracts the inhibitory action of 5-HT on 5-HT1A receptors. This excitatory action outlasts the hyperpolarizing action. 9. In summary 5-HT acts on at least two distinct receptors on hippocampal pyramidal cells, one coupled to the opening of potassium channels and a second coupled to a decrease in a resting potassium conductance and a decrease in the a.h.p.

Differential effects of norepinephrine on hippocampal complex-spike and theta-neurons

The central noradrenergic system has long been postulated to modulate learning and memory. A brain structure known to be important in these functions is the hippocampus. Since the hippocampus receives a noradrenergic projection from the locus coeruleus, knowledge of norepinephrine's actions in the hippocampus may help determine its role in learning and memory. In the present study, the effects of norepinephrine were examined on two hippocampal cell types: complex-spike and theta-neurons. In the hippocampus, there is good evidence that complex-spike cells are pyramidal neurons, while theta-neurons are interneurons. Extracellular action potentials from hippocampal neurons were recorded using multibarrel glass micropipettes. Drugs were locally applied using pressure micro-ejection. Norepinephrine inhibited the spontaneous firing of complex-spike cells, while theta-neurons were excited. The inhibitory response of complex-spike neurons was mediated by an alpha 1-receptor. However, selective agonists for the alpha 2- and beta-noradrenergic receptors excited the complex-spike cells. The noradrenergic-induced excitatory response of theta-neurons was also mediated by alpha 2- and beta-receptors. This study provides evidence that locally applied norepinephrine produces different responses on two types of hippocampal neurons. Furthermore, these differential responses arise primarily from the activation of distinct populations of noradrenergic receptors.