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PubMed2002

Tetanus toxin induces long-term changes in excitation and inhibition in the rat hippocampal CA1 area.

Intrahippocampal tetanus toxin induces a period of chronic recurrent limbic seizures in adult rats, associated with a failure of inhibition in the hippocampus. The rats normally gain remission from their seizures after 6-8 weeks, but show persistent cognitive impairment. In this study we assessed which changes in cellular and network properties could account for the enduring changes in this model, using intracellular and extracellular field recordings in hippocampal slices from rats injected with tetanus toxin or vehicle, 5 months previously. In CA1 pyramidal neurones from toxin-injected rats, the slope of the action potential upstroke was reduced by 32%, the fast afterhyperpolarisation by 32% and the slow afterhyperpolarisation by 54%, suggesting changes in voltage-dependent conductances. The excitatory postsynaptic potential slope was reduced by 60% and the population synaptic potential slope was reduced at all stimulus intensities, suggesting a reduced afferent input in CA1. Paired-pulse stimulation showed an increase of the excitability ratio and an increase of cellular excitability only for the second pulse, suggesting a reduced inhibition. The polysynaptic inhibitory postsynaptic potential was reduced by 34%, whereas neither the inhibitory postsynaptic potential at subthreshold stimulus intensities,nor the pharmacologically isolated monosynaptic inhibitory postsynaptic potential were different in toxin-injected rats, suggesting a reduced synaptic excitation of interneurones. Stratum radiatum stimuli in toxin-injected rats, and not in controls, evoked antidromic activation of CA1 neurones, demonstrating axonal sprouting into areas normally devoid of CA1 pyramidal cell axons.We conclude that this combination of enduring changes in cellular and network properties, both pro-epileptic (increased recurrent excitatory connectivity, reduced recurrent inhibition and reduced afterhyperpolarisations) and anti-epileptic (impaired firing and reduced excitation), reaches a balance that allows remission of seizures, perhaps at the price of persistent cognitive impairment.

Action Potentials↗
PubMed1996

Glutamate as a putative neurotransmitter in the mollusc, Lymnaea stagnalis.

Bath-applied glutamate (10-1000 microM) produced excitatory and inhibitory responses on numerous identified neurons of the mollusc Lymnaea stagnalis. Using both in situ and in vitro preparations, glutamate or glutamate agonists produced a depolarization in identified neurons right pedal dorsal 1 and right pedal dorsal 2 and 3. However, attempts to block glutamate-evoked responses with glutamate antagonists were unsuccessful. We examined a potential glutamatergic neuron, visceral dorsal 4. Exogenous application of the peptides (GDPFLRFamide and SDPFLRFamide) could mimic the inhibitory, but not the excitatory effects of visceral dorsal 4 on its postsynaptic cells, implying the presence of a second transmitter. We tested the possibility that glutamate is this second neurotransmitter by using excitatory synapses between visceral dorsal 4 and postsynaptic cells right pedal dorsal 2 and 3, right pedal dorsal 1, visceral F group and right parietal B group neurons. Of all the putative neurotransmitters tested, only glutamate had consistent excitatory effects on these postsynaptic cells. Also, the amplitude of the right pedal dorsal 2 and 3 excitatory postsynaptic potentials was reduced in the presence of N-methyl-D-aspartate and other glutamate agonists, suggesting desensitization of the endogenous transmitter receptor. In conclusion, some identified Lymnaea neurons respond to glutamate via a receptor with novel pharmacological properties. Furthermore, a Lymnaea interneuron may employ glutamate as a transmitter at excitatory synapses.

Animals↗
PubMed1993

Characteristics of spontaneous and evoked EPSPs recorded from dentate spiny hilar cells in rat hippocampal slices.

1. Excitation of the spiny subtype of hilar neurons in the fascia dentata was characterized by intracellular recording from hilar cells in hippocampal slices. Stimulation of the outer molecular layer was used to activate the perforant path. Evoked responses were examined, as well as the large spontaneous excitatory potentials that are a distinctive characteristic of spiny hilar cells. 2. Excitatory potentials that occurred spontaneously, as well as those that occurred in response to outer molecular layer stimulation, were similar among the cells that were sampled, regardless of morphological variations such as the presence or absence of thorny excrescences. Spontaneous and evoked excitatory postsynaptic potentials (EPSPs) were complex depolarizations that often had several discrete peaks. Spontaneous EPSPs increased in amplitude slightly with hyperpolarization, and evoked EPSPs clearly increased with hyperpolarization. 3. Applications of selective antagonists of excitatory amino acid receptors were used to determine which excitatory amino acid receptor mediates EPSPs of these cells. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was used to block the receptor subtype selective for the agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (the "AMPA/kainate" receptor). 2-amino-5-phosphonovaleric acid (APV) was used to block receptors specific for the agonist N-methyl-D-aspartate (NMDA; the "NMDA" receptor). Perfusion with CNQX (5-25 microM) completely blocked all spontaneous and evoked excitation, even when activity was examined at relatively depolarized membrane potentials and a low concentration of extracellular magnesium (0.5 mM) was used. Under these conditions, APV (25-50 microM) had no detectable effect on spontaneous activity but did increase the stimulus strength required to elicit responses to outer molecular layer stimulation. 4. When extracellular magnesium was lowered to 0 mM (nominally), there was strong evidence for a contribution of NMDA receptors to spontaneous and evoked EPSPs. Thus, when cells were perfused with 0 mM extracellular magnesium and 5 microM CNQX, spontaneous depolarizations were present and EPSPs could be triggered by stimulation of the outer molecular layer. Both the spontaneous and evoked EPSPs were blocked by 25 microM APV. 5. Because gamma-aminobutyric acid (GABA)A receptors can cause depolarizations in hippocampal neurons, the GABAA receptor antagonist bicuculline was used to determine whether some of the EPSPs were mediated by GABAergic neurons that are normally activated by spontaneous release of excitatory amino acids. Bicuculline (5-25 microM) had no effect on spontaneous depolarizations, and led to an enhancement of evoked depolarizations. Therefore it does not appear that GABAA receptor-mediated depolarizations contribute to hilar cell depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1991

Neurophysiological maturation of cat substantia nigra neurons: evidence from in vitro studies.

The membrane properties and synaptic physiology of developing cat substantia nigra (SN) neurons were studied in in vitro slice preparations. Stable intracellular recordings were obtained from 46 neurons in 20 kittens ranging in age from fetal day (F) 51 to postnatal day (P) 120. Only two of these properties changed with development. The percentage of cells displaying inward rectification and the percentage of cells that generated low-threshold Ca++ spikes increased with age. Properties that did not change included resting membrane potentials, action potential amplitudes and durations, and input resistances. At all ages locally evoked synaptic responses consisted of sequences of excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. Most of the cells recorded had the electrophysiological properties which have been attributed to SN dopamine-containing neurons. To identify neurons morphologically, and verify the recording site, cells were filled with Lucifer yellow at the end of each experiment. Somatic shapes varied widely from oval to fusiform to triangular. Somatic diameters and dendritic length increased with development. Filopodial processes and growth cones were present up to the first postnatal month. Dye-coupling occurred only in the fetal group. These results indicate that cat SN neurons have many mature physiological properties during late fetal and early postnatal development. This contrasts with the significant maturation that occurs in cat caudate neurons during the same developmental period.

Action Potentials↗
PubMed1996

Inhibition of TEA-induced LTP by aluminum.

Brief application of tetraethylammonium (TEA) to hippocampal slices causes long-term potentiation (TEA LTP) at synapses of CA1 pyramidal neurons characterized by a long-lasting increase of field excitatory postsynaptic potential (fEPSP) slope and population spike (PS) amplitude. Since this kind of potentiation requires the activation of voltage-dependent calcium channels, we examined the effect of the inorganic calcium channel blocker aluminum, which has been shown to impair tetanus-induced LTP (eLTP). We found that Al inhibited in a concentration-dependent manner both fEPSP slope and PS amplitude potentiation by TEA; 0.68 microgram/ml A1 attenuated TEA LTP, while a complete block of long-lasting potentiation was obtained for 2.7 micrograms/ml Al. Occlusion experiments revealed that both concentrations of Al allowed the induction of eLTP 60 min after TEA/Al exposure. However, longer application (15 min) of 2.7 micrograms/ml Al before the induction of TEA LTP prevented the subsequent induction of eLTP although no significant differences concerning the action on TEA LTP were observed. This indicates a general loss of neuronal plasticity which might be due to progressive neuronal cell damage. Since the effective concentration range of Al is directly comparable to the action of Al on eLTP, our data provide evidence for shared mechanisms of both potentiations. Although based on different induction mechanisms, Ca2+ is assumed to be a general intracellular trigger for both forms of LTP and thus it can be hypothesized that the neurotoxic action of Al is due to interference with Ca(2+)-dependent processes by inhibition of calcium conductances.

Aluminum↗
PubMed1999

Neuronal pathways from low-threshold hindlimb cutaneous afferents to motoneurons innervating trunk muscles in low-spinal cats.

Postsynaptic potentials (PSPs) evoked in motoneurons innervating the back and abdominal muscles in the lumbar part of the body by stimulating hindlimb cutaneous afferents were investigated in unanesthetized decerebate and spinal cats. Various types of PSP: pure excitatory postsynaptic potential (EPSP), pure inhibitory postsynaptic potential (IPSP), and mixed PSP (i.e., EPSP followed by IPSP, EPSP/IPSP; and IPSP followed by EPSP, IPSP/EPSP) were observed. The weak stimulation at 2 times threshold (2T) produced predominantly the EPSP, while at 5T the incidence of IPSP or EPSP followed by IPSP was increased. In about 20-50% of the various groups of motoneurons, PSPs evoked by ipsi- and contralateral nerves were qualitatively and quantitatively similar. For the other motoneurons, PSPs evoked by ipsi- and contralateral nerves were markedly different with respect to magnitude and/or polarity. These findings suggest that, within each motoneuron pool, some neurons act to increase stiffness of the trunk or to move vertically in response to an increased activity of cutaneous afferents, while the other motoneurons act to produce lateral bending of the trunk.

Afferent Pathways↗
PubMed1988

Effects of the NMDA receptor/channel antagonists CPP and MK801 on hippocampal field potentials and long-term potentiation in anesthetized rats.

The effects of the competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK801) were tested on hippocampal field potentials and long-term potentiation (LTP) in urethane-anesthetized rats. Neither drug had any significant effects on the dentate hilar population excitatory postsynaptic potential (EPSP) evoked by perforant path stimulation 30 or 150 min postinjection. However, both drugs produced a dose-dependent decrease in population spike amplitude at these times. Both drugs (at the highest doses) also blocked LTP when induced 150 min after administration, and this was related to a smaller response evoked during tetanization. CPP exerted similar effects on commissural-CA1 evoked responses and LTP. CPP remained an effective blocker of LTP for 6-8 h, and was still partially effective after 20-24 h. MK801 washed out more rapidly. The effect of MK801 on LTP did not depend on stimulus-evoked transmitter release during the pretetanization period. The results indicate that both CPP and MK801 have potent effects on LTP in the in vivo preparation, but that this is accompanied by an independent effect on evoked cell discharge.

Action Potentials↗
PubMed1988

Reticulospinal pacemaker neurons of the rat rostral ventrolateral medulla with putative sympathoexcitatory function: an intracellular study in vitro.

Extra- and intracellular recordings of tonically active neurons were obtained in slices of the rat rostral ventrolateral medulla maintained at 31 degrees C. The predominant type consisted of cells with a regular non-bursting discharge rate of 9 +/- 0.3 spikes/s (mean +/- S.E.M., n = 84). Intracellular recordings revealed that these neurons (n = 43) exhibited typical pacemaker potentials reset after a single spike, and an input resistance of 138 +/- 10 M omega (n = 21). No excitatory postsynaptic potentials were detected even during hyperpolarization (5-10 mV) which invariably resulted in silencing the cells (n = 28). Eighteen cells were injected intracellularly with Lucifer yellow, and the tissue was subsequently processed for the immunohistochemical detection of the adrenergic marker phenylethanol-amine N-methyltransferase (PNMT). None of the 12 dye-marked cells recovered exhibited any PNMT-like immunoreactivity, but all were surrounded by numerous adrenergic neurons. In 7 rats subjected to intraspinal injections (T3) of rhodamine-tagged microbeads, 4 out of 9 pacemaker cells marked intracellularly with Lucifer yellow were found labeled with the retrograde marker. It is concluded that the rostral ventrolateral medulla contains non-adrenergic reticulospinal cells with intrinsic pacemaker properties. These neurons probably represent a group of sympathoexcitatory cells on which the basal sympathetic tone depends.

Action Potentials↗
PubMed1989

On the properties and origin of the GABAB inhibitory postsynaptic potential recorded in morphologically identified projection cells of the cat dorsal lateral geniculate nucleus.

Intracellular recordings were performed from projection cells of the cat dorsal lateral geniculate nucleus in vitro to investigate the properties and origin of optic tract evoked inhibitory postsynaptic potentials mediated by GABAB receptors and their relationship to the physiologically different cell classes present in this nucleus. In all three main laminae of the dorsal lateral geniculate nucleus, stimulation of the optic tract evoked an excitatory postsynaptic potential followed by two inhibitory postsynaptic potentials. The first is a GABAA receptor mediated inhibitory postsynaptic potential since it was blocked by bicuculline, reversed in polarity following intracellular Cl- injection and had a reversal potential similar to the bicuculline sensitive hyperpolarizing effect of GABA. The second is a GABAB receptor mediated inhibitory postsynaptic potential. Its amplitude was not linearly related to membrane potential (maximal amplitude at -60 mV), it decreased when using frequencies of stimulation higher than 0.05 Hz and it was reversibly increased by addition of bicuculline to the perfusion medium. The reversal potential of GABAB inhibitory postsynaptic potentials was dependent on the extracellular K+ concentration but did not change in the presence of bicuculline or when recording with Cl- filled microelectrodes. While GABAA inhibitory postsynaptic potentials always abolished repetitive firing of projection cells, GABAB inhibitory postsynaptic potentials were able to block weak firing but unable to decrease strong activation of projection cells evoked by direct current injection. Optic tract evoked GABAB (as well as GABAA) inhibitory postsynaptic potentials could be recorded in slices which did not include the perigeniculate nucleus, thus indicating that they are generated by the interneurons of the dorsal lateral geniculate nucleus. Using intracellular injection of horseradish peroxidase, we have found that the GABAB inhibitory postsynaptic potentials are present in projection cells showing many different types of neuronal morphologies. In conclusion, GABA released from interneurons in the dorsal lateral geniculate nucleus is capable of evoking an early, short-lasting GABAA and a late, long-lasting GABAB inhibitory postsynaptic potential in projection cells with diverse morphology, indicating that the late inhibition in the dorsal lateral geniculate nucleus can no longer be associated exclusively with the recurrent inhibitory pathway through the perigeniculate nucleus.

Animals↗
PubMed1994

Long-term adrenalectomy reduces hippocampal granule cell excitability in vivo.

Ten days after bilateral adrenalectomy (ADX), neural transmission between the perforant path and hippocampal dentate granule cells was severely impaired in the anaesthetized rat, in that the slope of the stimulus-response curve was reduced to less than half the value in sham controls, the stimulation current necessary to elicit a standard population spike (PS) field potential was increased approximately threefold, the amplitude of PS and its ratio to the slope of the field excitatory postsynaptic potential (EPSP) were reduced, and high-frequency tetanization (TET) of the perforant path resulted in potentiated PS with smaller amplitude and higher onset latency in ADX rats than in sham controls. However, the fractional increase of PS amplitude and its decay following TET were similar in 10 day-ADX and sham groups, from which it is inferred that long-term ADX entails a general decrease of dentate granule cell excitability, particularly at somatic membrane level, without specifically affecting the mechanism of long-term potentiation. None of the above changes occurred 24 h after ADX.

Adrenalectomy↗
PubMed1997

The pathophysiology of giant SEPs in cortical myoclonus: a scalp topography and dipolar source modelling study.

Somatosensory evoked potential (SEP) recordings in patients suffering from cortical myoclonus (CM) are characterised by evidence of abnormally enhanced scalp components. Our aim was to verify whether enhanced activity in giant SEPs arises from the same generators as in healthy subjects. We used the brain electrical source analysis (BESA) to compare scalp SEP generators of healthy subjects to those calculated in 3 patients with CM of varying causes. Firstly, we built a 4-dipole model explaining scalp distribution of early SEPs in normal subjects and then applied it to traces recorded from CM patients. Our model, issued from the right median nerve grand average and applied also to recordings from single individuals, included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potentials and, with opposite polarity, the P24/ N24 responses; the second dipole explained the central P22 distribution and the third had a peak of activity corresponding to the N30 component. When we applied our 4-dipole model to CM recordings, the first perirolandic dipole had a third peak of activity in all patients at the same latency as a parietal negativity and a frontal positivity, both following giant P24/N24 components; on the other hand, in one patient the second perirolandic dipole showed a later activation corresponding to a high central negativity, following a giant P22 response. We suggest that only the initial giant SEPs correspond to physiological potentials evoked in healthy subjects. The occurrence of late giant SEPs could be explained by hyperpolarization, following the postsynaptic excitatory potentials responsible for the early giant components.

Adult↗
PubMed2003

Anatomy and physiology of horizontal cells in layer 5b of the chicken optic tectum.

In the visual midbrain of birds, a variety of cell types has recently been characterized with both anatomical and physiological techniques to gain insight into the mechanisms of visual information processing. Here we present data from a horizontal cell type located in the retinorecipient layer 5b of the chick optic tectum. Intracellular labeling revealed that these neurons are multipolar, have no axonal structures and arborize completely within the layer 5b where they extend over considerable distances. Immunohistochemistry with an antibody against calbindin labeled a population of horizontal cells in layer 5b; however, double labeling showed that these neurons represent a subpopulation of approximately one third of the neurons in that layer. Whole-cell patch recordings with additional cell filling from horizontal cells revealed that the physiological responses to depolarization changes with maturation, from a comparatively slow oscillatory pattern reminiscent of hair cell physiology at embryonal stages to a damped series of small action potentials at posthatching. In response to electrical stimulation in the vicinity of the neurons, cells responded with either excitatory postsynaptic potentials or small action potentials. Horizontal cell types are found in the visual midbrain of both avian and mammalian species. On the basis of the data presented here and data from the literature, the functional role of these cells is discussed. As in layer 5b of the chick optic tectum specific synaptic glomeruli have been found, the horizontal cells might constitute local inhibitory circuits within the retino-tectal synapses and, in addition, contribute to mechanisms of directional selectivity in these projections.

Animals↗
PubMed1996

Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex.

Cerebral responses evoked by peripheral stimuli are known to depend critically on the interstimulus interval (ISI). Here we report on the effects of ISI on somatosensory evoked magnetic fields (SEFs) to right median nerve stimulation, obtained in 9 healthy adults with ISIs of 0.15 0.3, 1,3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between the ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0.15 s. In contrast, the P35m and P60m deflections were very sensitive to changes of the ISI, declining steadily with shortening of the ISI throughout the entire range. These deflections were frequently undetectable at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parietal cortex (PPC) were seen only with ISIs of 1 s or greater, being strongest at the 5 s ISI. Based on known effects of the ISI on intracellular evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excitatory postsynaptic potentials (EPSPs), P35m early inhibitory postsynaptic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyramidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.

Adult↗
PubMed1989

Physiological and Ultrastructural Characterization of a Central Synaptic Connection between Identified Motor Neurons in the Locust.

An excitatory connection between an extensor and several flexor tibiae motor neurons that innervate antagonistic muscles in the hind leg of a locust has been characterized using physiological and ultrastructural methods. Simultaneous intracellular recordings from the single fast extensor (FETi) motor neuron and up to three flexor motor neurons show that a spike in FETi is followed by a short latency depolarizing synaptic potential in the flexors that is powerful enough to evoke a burst of flexor spikes. The chemically mediated excitatory postsynaptic potential (EPSP) is caused centrally as it persists when sensory feedback from the leg is removed, and has a latency of 1.6-2.0 ms depending upon the position of the recording electrodes in the somata or neuropilar segments of the pre- and postsynaptic neurons. The amplitude of the EPSP declines gradually in a saline containing no calcium but high magnesium, indicating that no spiking interneuron is interposed in the pathway. With repetitive stimulation, the EPSP decrements markedly so that at intervals of 50 ms the second EPSP of a pair is reduced by 90%. The amplitude of the EPSP is also dependent on the amplitude of the presynaptic spike. The physiological evidence suggesting a monosynaptic connection is directly confirmed by electron microscopy of ganglia in which FETi and a flexor were both labelled with horseradish peroxidase. Direct chemical synapses between the two identified neurons, in which FETi is the presynaptic element, occur in three regions of the neuropil examined. At a synapse, the flexor motor neuron may be the only postsynaptic neuron or it may be one element in a dyad. The synaptic arrangements between the two neurons are complex with serial synapses through unlabelled processes linking FETi to flexor motor neurons and with frequent reciprocal synaptic connections between FETi and unlabelled processes. Unidentified processes also make input synapses on both neurons close to the synapses from FETi. The behavioural significance of the connection lies in the mechanical requirements for kicking and jumping. To prepare for these powerful movements the extensor and flexor tibiae muscles must co-contract. The connection from FETi enhances the depolarization and frequency of spikes in the flexors during the co-contraction.

Journal Article↗
PubMed1991

Optical mapping of the early development of the response pattern to vagal stimulation in embryonic chick brain stem.

1. In both intact and slice preparations of vagus-brain stem isolated from 3- to 8-day-old chick embryos, the spatial pattern of neural responses to vagal stimulation and its development were assessed by means of multiple-site optical recording of electrical activity, using a voltage-sensitive merocyanine-rhodanine dye (NK2761) and a 12 x 12-element photodiode array. 2. The first neural responses, viz. fast optical signals (related to the action potential), were recorded in the 4-day-old brain stem preparation, and slow optical signals (related to excitatory postsynaptic potentials) were detected from late 7- and 8-day-old brain stem preparations. 3. The evoked optical signals appeared to be concentrated longitudinally in the central region of the stimulated side of the intact brain stem preparation and in a limited dorsal area in the slice preparation. The signal size gradually increased and the response area expanded as development proceeded. 4. Based on the above results, we have constructed developmental maps of the spatial patterns of the fast and slow optical responses. In the maps, the positions of the peak-size regions of the fast and slow signals were assessed and we have found that there were differences in the location of these areas for the fast vs. the slow signals in the late 7- and 8-day-old embryonic brain stem preparations. 5. In the maps for the late 7- and 8-day-old embryonic brain stems, the fast signal response area seems to correspond to the dorsal motor nucleus of the vagus nerve and the slow response area to the nucleus tractus solitarii.

Action Potentials↗
PubMed1997

Analysis of the responses of myenteric neurons in the small intestine to chemical stimulation of the mucosa.

Responses of myenteric AH and S neurons to local application of chemicals to the mucosa of the guinea pig small intestine were obtained using conventional intracellular recording techniques. Preparations were dissected to reveal the myenteric plexus over one-half of the circumference of the gut with intact mucosa on the the half. Neurons were impaled within the exposed one-half, whereas potential stimulants, in buffered saline, were transiently applied to the mucosa, 1-1.5 mm circumferential from the impalement. The stimulants elicited action potentials (AP) in AH neurons that did not arise from synaptic activity. AH neurons also responded with slow excitatory postsynaptic potentials (EPSP). S neurons were activated synaptically, via fast and slow EPSP, but not nonsynaptically. Mucosal application of solutions of a low pH (3-5) or a high pH (9-11) were both effective stimulants. Solutions of a neutral pH, which was also a control for mild mechanical stimulation, were usually ineffective. Both a short-chain fatty acid, acetate (pH 7.2), and 5-hydroxytryptamine elicited responses in each neuronal type. We conclude that myenteric AH neurons of the guinea pig distal ileum are primary afferent neurons that respond to a variety of mucosally applied chemical stimuli with burst of AP. In addition, the physiologically evoked transmission of slow EPSP to AH neurons suggests that primary afferent neurons interconnect in a self-reinforcing network. S neurons are second or higher order neurons in the reflex pathways.

Animals↗
PubMed1984

Characteristics of a slow hyperpolarizing synaptic potential in rat hippocampal pyramidal cells in vitro.

An orthodromically evoked late hyperpolarizing potential (LHP) was studied using intracellular recording techniques in rat hippocampal CA1 pyramidal cells in vitro. Several tests indicated that the LHP is not blocked by GABA antagonists, but rather comprises the initial portion of the stimulation-induced burst afterhyperpolarization (AHPs) produced in the presence of these antagonists. Bath application of magnesium (Mg) or 8-bromo adenosine 3',5'-cyclic monophosphate (cAMP), or intracellular injection of ethyleneglycol-bis (beta-amino-ethylether)-N,N'-tetraacetic acid (EGTA) blocked the late portion of the AHPs, at times when the early portion was only slightly or not at all affected. The late part of the AHPs was also associated with the voltage-dependent components of the burst, whereas the early part was not. Both the early part of the burst AHPs and the LHP in standard saline have similar time courses and dependence on membrane potential. The LHP was nullified by hyperpolarization of the membrane in extracellular potassium concentrations [( K]o) of 5.4 mM and below and could be reversed in [K]o above 5.4 mM. The apparent reversal potential for the LHP followed shifts in [K]o as predicted by the Nernst equation and is, therefore, probably a K-dependent potential. No specific antagonist of the LHP from among several K conductance blockers was found, however. An alternative hypothesis, that the LHP might be an electrogenic pump effect was not supported. Ouabain depressed the LHP; however this effect was probably nonspecific and due, in part, to a ouabain-induced increase in [K]o. Decreasing temperature in the range 37-22 degrees C prolonged but did not block the LHP. The LHP was enhanced by increases in extracellular calcium concentration and depressed by high [Mg]o or cadmium. It was associated with a small (14%) decrease in total resting input resistance. In cells depolarized to approximately 0 mV, regenerative voltage-dependent potentials were blocked; however, an LHP still occurred. The LHP was not found to be dependent on the excitatory postsynaptic potential (EPSP). With weak stimuli LHP and EPSP amplitudes were uncorrelated and the EPSP was more resistant than the LHP to block by high [Mg]o. The LHP continued to occur when the EPSP was reversed in depolarized cells. The LHP may be mediated by interneuronal circuitry within a slice. In GABA antagonists the LHP occasionally occurred spontaneously at regular intervals.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1998

Effects of quinine on the excitability and voltage-dependent currents of isolated spiral ganglion neurons in culture.

This work examined how quinine, a drug that induces both hearing loss and tinnitus, interfered with the excitability of spiral ganglion (SG) neurons in cultures. The membrane potential changes and the modification of the action-potential waveform induced by quinine were studied in SG neurons under current clamp. The effects of the drug on voltage-dependent currents in SG neurons were also investigated by the voltage-clamp method. Quinine did not appreciably affect either resting membrane potentials or input resistance at rest. However, action potentials fired by SG neurons were significantly broadened by the presence of quinine. With higher concentrations of quinine (>20 microM), the amplitude of action potentials was also reduced. Voltage-clamp results demonstrated that quinine primarily blocked the whole cell potassium currents (IK) in a voltage-dependent manner. Up to 100 microM of quinine did not appreciably block IK evoked by a test pulse to -35 mV. In contrast, IK was significantly reduced with more positive test pulses, e.g., the concentration needed to obtain 50% inhibition (IC50) was 8 microM for a test pulse to 65 mV. At higher concentrations (>20 microM), quinine also reduced the size of sodium currents (INa) in a use-dependent manner, while leaving calcium currents (ICa) relatively unaffected. Compared with the potency of quinine's effects on other targets in the inner ear, the relatively low IC50 and the voltage-dependent nature of quinine inhibition on IK suggested that its modulation of the waveform and threshold of action potentials of SG neurons probably was primarily responsible for its ototoxic effects. From the point of view of how neural signaling process is affected by the drug, quinine-induced tinnitus may be explained by its broadening of action potentials while the drug's inhibition on INa may result in hearing loss by making the conversion from excitatory postsynaptic potentials to the generation of action potentials more difficult.

Action Potentials↗