Biomedical subjects

G R Siggins

Publications and source records attributed to G R Siggins.

At least 19 recordsLinked to original sources

PubMed1992

Concentrations of carbachol stimulating phosphoinositide hydrolysis cause a sustained decrease in membrane potential and firing rate: role of inositol and inositol polyphosphate second messengers.

We have investigated the relationship between muscarinic agonist-stimulated phosphoinositide (PI) hydrolysis and electrophysiological responses in rat hippocampal slice preparations. In a previous extracellular study, we found that muscarinic agonists at concentrations that stimulate PI hydrolysis result in a biphasic firing response; an initial increase in firing followed by loss of firing at higher concentrations. To test the hypothesis that variability in obtaining consistent loss of firing is related to depletion of intracellular inositol, we investigated the effects of adding exogenous inositol to the buffer. We now report that concentrations of inositol similar to those in cerebral spinal fluid (30-100 microM) augment carbamylcholine (carbachol, CCh) mediated loss of firing and [3H]inositol-1,3,4,5-tetrakisphosphate ([3H]Ins(1,3,4,5)P4) formation. Inhibition of firing produced by 30 microM CCh in the presence of inositol was associated with a sustained depolarization of 20-25 mV, an increased slope resistance in the depolarized range (-60 to -40 mV), and a parallel shift in the hyperpolarized (-100 to -70 mV) range of the voltage-current curve and increased frequency of spontaneous IPSPs. Under voltage-clamp, measurements of the M-current (IM) showed sustained inactivation by CCh with reversal after washout of CCh. Manual depolarization of cells by current injection to the same level of depolarization as attained with CCh did not usually lead to the same loss of firing. These findings suggest that IM, and possibly other voltage-independent currents or ion pumps, may cause loss of firing only in part through a depolarization blockade of firing and not through desensitization. Furthermore, CCh treatment without inositol did not depolarize neurons as much as CCh with inositol, and usually did not cause a delayed loss of firing. Brain slice preparations may thus require physiological concentrations of inositol to show consistent or maximum phosphoinositide-mediated electrophysiological responses.

Action Potentials↗

PubMed1992

Interleukin-1 beta increases synaptic inhibition in rat hippocampal pyramidal neurons in vitro.

Interleukin-1 (IL-1), a cytokine with a broad spectrum of biological activity, modulates electrical properties of central neurons in the brain. The effects of IL-1 beta (143 pM) on conductances opened by synaptic stimulation of the Schaffer collaterals were studied by intracellular recording of hippocampal pyramidal cells of the CA1 region. IL-1 beta enhanced and prolonged synaptic inhibition by about 2 to 3-fold. Heat-inactivated IL-1 beta had no effect. This finding implies that IL-1 beta changes interneuronal communication in the hippocampus with a possible impact on neuronal plasticity.

Animals↗

PubMed1992

Opioid peptides reduce synaptic transmission in the nucleus accumbens.

Behavioral studies implicate the nucleus accumbens (NAcc) as a brain area pivotal for the rewarding effects of opiates like heroine and morphine. Therefore, we studied the effect of a variety of opioids on membrane properties and responses to synaptic stimulation in a slice preparation of the NAcc using intracellular recording. Superfusion of opioid peptides did not affect the membrane potential or input resistance of NAcc neurons, but significantly reduced both depolarizing and hyperpolarizing synaptic potentials. Naloxone superfusion significantly reversed the depressant effects of the mu and delta receptor agonists (but not those of the kappa agonist) on synaptic transmission, suggesting involvement of opiate receptors. These results imply that the predominant effect of opiates in NAcc is a reduction of synaptic transmission.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗

PubMed1990

Arachidonic acid metabolites as mediators of somatostatin-induced increase of neuronal M-current.

The M-current (IM) is a time- and voltage-dependent K+ current that persists at slightly depolarized membrane potentials. IM is reduced by muscarinic cholinergic agonists and certain peptides, and is thought to be responsible in part for the slow and late slow excitatory postsynaptic potentials in sympathetic neurons. Recently, we reported that IM in hippocampal neurons was also augmented by somatostatin-14 and -28 suggesting that two different receptors reciprocally regulate one neuronal channel type. Muscarinic effects on IM may be mediated by various components of the phosphatidylinositol phosphate pathway. We now report the involvement of a different second messenger pathway, that generated by phospholipase A2, in the somatostatin-induced augmentation of IM in hippocampal cells. This pathway generates arachidonic acid from which leukotrienes can be produced by lipoxygenases. We find that the IM-augmenting effects of somatostatin are abolished by two substances that can inhibit phospholipase A2, quinacrine and 4-bromophenacyl bromide, and that both arachidonic acid and leukotriene C4 mimic the effects of somatostatin-14 on hippocampal pyramidal neurons in vitro. Arachidonic and somatostatin effects are blocked by a lipoxygenase inhibitor, implicating an arachidonic acid metabolite, perhaps a leukotriene, in the somatostatin effect.

Acetophenones↗

PubMed1990

Ethanol diminishes a voltage-dependent K+ current, the M-current, in CA1 hippocampal pyramidal neurons in vitro.

Previous in vivo studies showed that systemic ethanol enhanced hippocampal neuronal responses to iontophoretically applied acetylcholine and somatostatin while having little or no effect on responses to other transmitters. We previously reported that these two agonists reciprocally regulate the non-inactivating, voltage-dependent K+ current called the M-current. Therefore, we tested ethanol superfusion on this current in rat hippocampal pyramidal neurons in vitro, using intracellular recording and single electrode voltage-clamp methods. Tetrodotoxin (TTX) was used to block Na+ spikes and synaptic transmitter release. Ethanol in low concentrations (22-44 mM), like muscarinic agonists, greatly reduced the M-current amplitude at depolarized membrane potentials and at 44 mM antagonized its augmentation by somatostatin. These changes were often accompanied by an inward baseline current with a conductance decrease. Other than a small inward current in some cells there was little or no consistent ethanol effect at resting membrane potentials. Atropine 1 microM (and TTX) did not alter the ethanol effect on the M-current. Therefore, the site of ethanol action is most likely distal to the muscarinic receptor. Ethanol reduction of the M-current, by summation of like effects, may account for the potentiation of acetylcholine responses seen in vivo and in vitro, and provides a mechanism for the excitatory effects of ethanol on some central neurons.

Animals↗

PubMed1990

Somatostatin immunohistochemistry of hippocampal slices with lucifer yellow-stained pyramidal neurons responding to somatostatin.

We have combined electrophysiology and immunohistochemistry to study the somatostatin (SS) innervation of neurons in the rat hippocampal slice. After recording the intracellular response of a pyramidal CA1 neuron in vitro to SS, Lucifer Yellow was injected into the cell and the slice fixed and processed for immunohistochemical localization of SS in the vicinity of the recorded neuron. Most pyramidal neurons (70%) responded to SS with a hyperpolarization associated with marked slowing of spontaneous discharge and reduced input resistance. SS-containing elements either crossed, ran parallel or seemingly terminated on the Lucifer Yellow-filled SS-responsive cell. These occurrences of close proximity of apparent pre- and postsynaptic elements were observed in all layers of the CA1 region and may represent synaptic terminations of SS elements on a pyramidal neuron that are likely to elicit membrane hyperpolarizations.

Animals↗

PubMed1988

Enkephalin analogues depress synaptic potentials in rat dentate granule cells recorded intracellularly in vitro.

Enkephalin analogues were superfused onto hippocampal slices during intracellular recording of dentate granule cells. The enkephalins elicited either weak depolarizations, weak hyperpolarizations, or no effect on membrane potential, in about equal numbers of cells tested. Similarly, input resistance either decreased, increased or did not change, and was not well correlated with the potential changes. However, at all concentrations tested (2-10 microM) the enkephalins reduced evoked depolarizing synaptic potentials by up to 70%. We speculate that a major function of endogenous enkephalin-containing fibers projecting to the dentate is to dampen afferent synaptic transmission.

Action Potentials↗

PubMed1988

Somatostatin augments the M-current in hippocampal neurons.

Immunocytochemical and electrophysiological evidence suggests that somatostatin may be a transmitter in the hippocampus. To characterize the ionic mechanisms underlying somatostatin effects, voltage-clamp and current-clamp studies on single CA1 pyramidal neurons in the hippocampal slice preparation were performed. Both somatostatin-28 and somatostatin-14 elicited a steady outward current and selectively augmented the noninactivating, voltage-dependent outward potassium current known as the M-current. Since the muscarinic cholinergic agonists carbachol and muscarine antagonized this current, these results suggest a reciprocal regulation of the M-current by somatostatin and acetylcholine.

Acetylcholine↗

PubMed1988

Systemic phenoxybenzamine but not beta-adrenergic antagonists block noradrenergic inhibition of cerebellar Purkinje and hippocampal pyramidal neurons.

Previous pharmacological characterization of central noradrenergic receptors has been interpreted as favoring beta-type receptors on cerebellar Purkinje neurons and hippocampal pyramidal neurons. However, the recent development of additional noradrenergic antagonists suitable for single neuron analysis, prompted an initial re-evaluation of alpha and beta adrenergic receptors in these two cell populations. In contrast with earlier data based on local antagonism of iontophoretric or synaptically released norepinephrine (NE), we now find that systemic phenoxybenzamine, the alpha antagonist, was effective in blocking responses to NE in cerebellum and hippocampus, whereas systemic beta antagonists metoprolol, ICI 118.551 [correction of ICI 181.551], or proprandol did not interfere with local NE responses at systemic doses that altered spontaneous discharge rates. These preliminary data suggest that a more complete re-evaluation of the nature of central noradrenergic response mechanisms may be warranted.

Adrenergic beta-Antagonists↗

PubMed1988

Somatostatin depresses excitability in neurons of the solitary tract complex through hyperpolarization and augmentation of IM, a non-inactivating voltage-dependent outward current blocked by muscarinic agonists.

The synaptic function of somatostatin-containing fibers in the nervous system is controversial. Therefore, we used a slice preparation of the rat brain stem to test the electrophysiological effects of prosomatostatin-derived peptides on neurons of the solitary tract complex, which contains an abundance of somatostatin-containing fibers and cell bodies. Superfusion of both somatostatin-14 and somatostatin-28 (the precursor for somatostatin-14), but not somatostatin-28-(1-12) or -(1-10), predominantly inhibited spontaneous spike and subthreshold (probably synaptic) activity. In intracellular recordings, somatostatin-14 and -28 hyperpolarized most neurons in association with a slight (10-35%) but reproducible decrease in input resistance. These hyperpolarizing responses were augmented in depolarized cells and persisted in cells in which spontaneous inhibitory postsynaptic potentials became depolarizing after Cl- injection. These data suggest that somatostatin receptors regulate a K+ conductance. In voltage-clamp studies, somatostatin-28 and -14 induced a steady outward current and augmented the voltage-dependent, nonactivating outward K+ conductance (IM) shown to be blocked by activation of muscarinic cholinergic receptors. These results suggest (i) that somatostatin-containing elements in the solitary tract complex play an inhibitory role through the activation of postsynaptic permeability to potassium ions and (ii) that the same ion channel type may be coregulated by two neurotransmitter candidates, somatostatin and acetylcholine, through a reciprocal control mechanism.

Animals↗

PubMed1987

Effects of ethanol on CA1 and CA3 pyramidal cells in the hippocampal slice preparation: an intracellular study.

Superfusion of ethanol (10-350 mM) sometimes caused weak hyperpolarization, but more often elicited weak depolarization or biphasic depolarizing, hyperpolarizing responses in CA1 and CA3 pyramidal neurons of the hippocampal slice. The occasional polarizations were sometimes accompanied by, but not always correlated with, small increases or decreases in input resistance. However, many cells in both areas showed no detectable change in membrane potential (36% of cells) or input resistance (57% of cells), even at very high ethanol concentrations (86-200 mM). Spontaneous spiking, when present, was occasionally accelerated or decelerated, although in CA3 a biphasic speeding-slowing sequence was often seen. The afterhyperpolarizations following bursts of action potentials evoked by current (CA1) or occurring spontaneously (CA3) were most often either slightly reduced in amplitude (CA3) or not affected (CA1) by ethanol superfusion. In contrast, synaptic potentials evoked by stimulation of the hilar mossy fiber pathway (for CA3) or the stratum radiatum (for CA1) were more sensitive to ethanol: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were most often reduced in amplitude in both CA1 and CA2, even at low ethanol concentrations (10-50 mM). The action on IPSPs may be exerted presynaptically, because responses to locally applied GABA were little affected. These results suggest that hippocampal evoked synaptic activity may be more sensitive than postsynaptic membrane properties to physiologically relevant ethanol concentrations.

Action Potentials↗

PubMed1987

Electrophysiology of ethanol on central neurons.

With respect to the theme of this volume, the results of our recent studies on three neuronal model systems point to several relevant conclusions: ethanol may interact electrophysiologically with certain anesthetics such as urethane; ethanol can selectively enhance responses to certain neurotransmitters; resting membrane properties of individual neurons show a wide range of sensitivities to ethanol and are generally fairly insensitive; the synapse--independent of specific transmitters--seems most sensitive to ethanol. As regards the first point, it has long been known that ethanol and anesthetics have features in common, including the ability to alter the lipid components of biological membranes (see R. A. Harris et al., L. L. M. van Deenen et al., M. J. Hudspith et al., E. Rubin et al., and C. C. Cunningham & P. I. Spach in this volume), so interactions between the two are not unexpected. However, our electrophysiological findings suggest great caution and appropriate controls be used in in-vivo studies of anesthetized animals, as the interactions derived may actually reverse the usual effect of ethanol. The enhancement of responses to ACh and SS (second point) might be assumed to arise postsynaptically in the target cells recorded and are seen with low, intoxicating doses of ethanol. Whether this potentiation involves enhancement of specific agonist binding to the receptor or facilitation of the function of the ionic channel linked to the receptor remains to be determined. It is not hard to imagine that ethanol could perturb membrane properties near receptors, to alter their conformation and ligand binding, or perhaps even uncover hidden receptors. The relative insensitivity of the resting membrane properties (third point) may suggest that membrane channels responsible for these functions (e.g., 'leak' channels for Na+ and K+ ions) do not usually interact with the lipid components affected by ethanol, at least at low, 'intoxicating' ethanol concentrations. Finally, the reduction of synaptic potentials by ethanol may indicate a presynaptic locus of action, as the response to the transmitter for at least one of these synaptic potentials (GABA) was not altered. These data would seem to indicate that synaptic release of the transmitter is reduced by ethanol, at least in the hippocampal slice. The high sensitivity of this presynaptic element for ethanol could indicate that the machinery for synaptic release, such as conductances for calcium entry (see REF. 39) or the action of second messenger systems (e.g., those leading to synapsin phosphorylation) are particularly sensitive to ethanol.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine↗

PubMed1986

Ethanol increases single unit activity in the inferior olivary nucleus.

Single unit recording of rat inferior olivary nucleus neurons reveals significantly elevated discharge after acute intraperitoneal injection of 2 g/kg ethanol. This effect is consistent across 3 different methods of anesthesia and immobilization: local Xylocaine plus intraperitoneal D-tubocurare, intraperitoneal chloral hydrate and halothane vapor. In contrast, under urethane anesthesia acute ethanol produces significant depression of olivary discharge. Since this effect is opposite to that found under the other anesthetic conditions (including topical Xylocaine only), urethane anesthesia may compromise generalizations of electrophysiologic studies of ethanol. Neurons of the inferior olivary nucleus excite cerebellar Purkinje cells through a powerful afferent circuit; our data therefore suggest that ethanol-induced increases in cerebellar Purkinje cell complex (climbing fiber burst) spikes, obtained in our previous studies, are secondary to olivary activation.

Anesthesia, General↗

PubMed1986

Systemic ethanol: selective enhancement of responses to acetylcholine and somatostatin in hippocampus.

In rat hippocampal pyramidal cells tested in situ by iontophoresis of several neurotransmitters, ethanol significantly enhanced excitatory responses to acetylcholine and inhibitory responses to somatostatin-14 but had no statistically significant effect on excitatory responses to glutamate or inhibitory responses to gamma-aminobutyric acid or, in preliminary tests, to norepinephrine or serotonin. The effects of ethanol on responses to acetylcholine and somatostatin-14 may provide insight into synaptic mechanisms underlying the behavioral consequences of ethanol intoxication.

Acetylcholine↗

PubMed1986

Somatostatin selectively enhances acetylcholine-induced excitations in rat hippocampus and cortex.

The neuronal effects of somatostatin-14 (SS-14) and its influence on responses to acetylcholine (AcCho) were studied in vivo in the rat parietal cortex and dorsal hippocampus, using single-unit recording and microiontophoresis. SS-14 inhibited spontaneous firing of nearly all cells tested, while AcCho facilitated their firing. In contrast to its direct slowing effect, sustained iontophoretic application of SS-14 enhanced AcCho-induced excitations in 78% of all cells tested. This AcCho-enhancing effect of SS-14 was dose dependent. SS-14 did not enhance the responsiveness to pulses of the excitatory amino acid glutamate. Neurons tonically driven by iontophoretic currents of AcCho responded to concurrent pulses of SS-14 with an increase in firing. Thus, iontophoretic application of SS-14 can produce qualitatively different effects on the spontaneous activity of its target cells depending on the simultaneous effects of other chemical messengers. These condition-dependent interactions may explain the diverse neuronal effects of SS-14 reported in the literature.

Acetylcholine↗

PubMed1986

Opioid peptides and epileptogenesis in the limbic system: cellular mechanisms.

The localization of opioid peptides in the rat hippocampal formation and the epileptogenic action of beta-endorphin and certain enkephalin analogues have led to speculations that opioids may play a role in limbic seizures. These immunochemical and electroencephalographic data are compatible with single-unit electrophysiological studies showing predominant excitations of hippocampal pyramidal neurons in CA1 and CA3 fields produced by iontophoresis of endorphins or enkephalins. These excitations are naloxone sensitive and appear to arise from a disinhibitory mechanism due to inhibition of inhibitory interneurons. Thus, intracellular recordings in in vitro preparations of hippocampus usually show opioid-induced reduction of inhibitory postsynaptic potentials. However, more recent studies suggest that a major opioid-containing pathway in the hippocampus, the mossy fiber projection from the dentate gyrus to CA3 pyramidal neurons, contains more pro-dynorphin-derived peptides than pro-enkephalin. Intracerebroventricular dynorphin does not induce epileptiform activity in the rat, and single-unit and field-potential studies show mixed effects on CA3 neuronal excitability, with more inhibitory responses than are seen with the enkephalins. Selective inactivation of mu opioid receptors reveals that dynorphin, which was previously shown to express specificity for kappa receptors, can act on delta receptors in CA1. Furthermore, a specific kappa agonist, U50,488H, has inhibitory actions when applied directly to CA3 neurons. These data suggest the presence of multiple opioid receptor types in the hippocampus. These multiple receptors may point to heterogeneous functions of the different families of opioid peptides in various regions of the hippocampus, and could explain the divergent effects reported for the various opioids and naloxone to promote or prevent paroxysmal activity.

Animals↗

PubMed1985

Selective inactivation of opioid receptors in rat hippocampus demonstrates that dynorphin-A and -B may act on mu-receptors in the CA1 region.

Dynorphin-A1-17 and dynorphin-B increased the evoked response of hippocampal CA1 pyramidal cells, as did other opioids tested. Treatment of the hippocampal slice with beta-funaltrexamine, a mu-receptor selective antagonist, blocked the effects of normorphine, dynorphin-A and dynorphin-B, but did not change the response to D-Ala2, D-Leu5-enkephalin. The low potency of kappa selective agonists and the antagonism by beta-funaltrexamine of the dynorphins' effect indicate that kappa-opioid receptors may not be involved in these observed responses. Our data suggest that both mu- and delta-receptors are functionally represented and provide evidence that the dynorphins or their derivatives may also be agonists at the mu-receptor.

Animals↗

PubMed1985

Synaptic responses of neurons of the nucleus tractus solitarius in vitro.

Postsynaptic responses of neurons in the nucleus tractus solitarius (NTS) have been studied in an in vitro slice preparation using extra- and intracellular recording. Single or paired pulse stimulations were delivered to afferent fibers within the tractus solitarius (TS) to activate orthodromic responses in these neurons. Most NTS neurons displayed an initial synaptic excitation followed by inhibition of spontaneous or evoked firing lasting up to 150-200 ms after stimulation. Excitatory postsynaptic potentials (EPSPs), recorded intracellularly, were increased in amplitude by membrane hyperpolarization. Large afterhyperpolarizations followed action potentials triggered by the EPSPs or evoked by intracellular current injections. Intracellular evidence for synaptic inhibition within the NTS included: (1) the presence, after Cl-injection, of flurries of spontaneous PSPs likely to be inverted inhibitory postsynaptic potentials; (2) reduction of the size of a test EPSP by a previous subthreshold TS conditioning volley; and (3) hyperpolarizing PSPs recorded in some neurons. Other NTS neurons exhibited prolonged excitatory responses to TS stimulation and could be local inhibitory interneurons. These results may help specify synaptic mechanisms in the NTS that could play an integrative role in the relay of visceral sensory inputs to higher order effectors.

Animals↗