Components Of Postsynaptic Potentials Research Articles

Active integration of glutamatergic input to the inferior olive generates bidirectional postsynaptic potentials.

Key points We establish experimental preparations for optogenetic investigation of glutamatergic input to the inferior olive.Neurones in the principal olivary nucleus receive monosynaptic extra‐somatic glutamatergic input from the neocortex.Glutamatergic inputs to neurones in the inferior olive generate bidirectional postsynaptic potentials (PSPs), with a fast excitatory component followed by a slower inhibitory component.Small conductance calcium‐activated potassium (SK) channels are required for the slow inhibitory component of glutamatergic PSPs and oppose temporal summation of inputs at intervals ≤ 20 ms.Active integration of synaptic input within the inferior olive may play a central role in control of olivo‐cerebellar climbing fibre signals. The inferior olive plays a critical role in motor coordination and learning by integrating diverse afferent signals to generate climbing fibre inputs to the cerebellar cortex. While it is well established that climbing fibre signals are important for motor coordination, the mechanisms by which neurones in the inferior olive integrate synaptic inputs and the roles of particular ion channels are unclear. Here, we test the hypothesis that neurones in the inferior olive actively integrate glutamatergic synaptic inputs. We demonstrate that optogenetically activated long‐range synaptic inputs to the inferior olive, including projections from the motor cortex, generate rapid excitatory potentials followed by slower inhibitory potentials. Synaptic projections from the motor cortex preferentially target the principal olivary nucleus. We show that inhibitory and excitatory components of the bidirectional synaptic potentials are dependent upon AMPA (GluA) receptors, are GABAA independent, and originate from the same presynaptic axons. Consistent with models that predict active integration of synaptic inputs by inferior olive neurones, we find that the inhibitory component is reduced by blocking large conductance calcium‐activated potassium channels with iberiotoxin, and is abolished by blocking small conductance calcium‐activated potassium channels with apamin. Summation of excitatory components of synaptic responses to inputs at intervals ≤ 20 ms is increased by apamin, suggesting a role for the inhibitory component of glutamatergic responses in temporal integration. Our results indicate that neurones in the inferior olive implement novel rules for synaptic integration and suggest new principles for the contribution of inferior olive neurones to coordinated motor behaviours.

Saffron extract and trans-crocetin inhibit glutamatergic synaptic transmission in rat cortical brain slices

Saffron, the dried stigmata of Crocus sativus L., is used in traditional medicine for a wide range of indications including cramps, asthma, and depression. To investigate the influence of hydro-ethanolic saffron extract (CSE) and trans-crocetin on synaptic transmission, postsynaptic potentials (PSPs) were elicited by focal electrical stimulation and recorded using intracellular placed microelectrodes in pyramidal cells from rat cingulate cortex. CSE (10–200 μg/ml) inhibited evoked PSPs as well as the isolated NMDA and non-NMDA component of PSPs. Glutamate (500 μM) added into the organ bath induced membrane depolarization. CSE decreased glutamate-induced membrane depolarization. Additionally, CSE at 100 μg/ml decreased NMDA (20 μM) and kainate (1 μM)-induced depolarization, whereas AMPA (1 μM)-induced depolarization was not affected. Trans-crocetin (1–50 μM) showed inhibition of evoked PSPs and glutamate-induced membrane depolarization comparable to CSE. Trans-crocetin at 10 μM decreased NMDA (20 μM)-induced membrane depolarization, but did not inhibit the isolated non-NMDA component of PSPs. We conclude that trans-crocetin is involved in the antagonistic effect of CSE on NMDA but not on kainate receptors.

Interleukin‐1 inhibits firing of serotonergic neurons in the dorsal raphe nucleus and enhances GABAergic inhibitory post‐synaptic potentials

In vitro electrophysiological data suggest that interleukin-1 may promote non-rapid eye movement sleep by inhibiting spontaneous firing of wake-active serotonergic neurons in the dorsal raphe nucleus (DRN). Interleukin-1 enhances GABA inhibitory effects. DRN neurons are under an inhibitory GABAergic control. This study aimed to test the hypothesis that interleukin-1 inhibits DRN serotonergic neurons by potentiating GABAergic inhibitory effects. In vitro intracellular recordings were performed to assess the responses of physiologically and pharmacologically identified DRN serotonergic neurons to rat recombinant interleukin-1beta. Coronal slices containing DRN were obtained from male Sprague-Dawley rats. The impact of interleukin-1 on firing rate and on evoked post-synaptic potentials was determined. Evoked post-synaptic potentials were induced by stimulation with a bipolar electrode placed on the surface of the slice ventrolateral to DRN. Addition of interleukin-1 (25 ng/mL) to the bath perfusate significantly decreased firing rates of DRN serotonergic neurons from 1.3 +/- 0.2 Hz (before administration) to 0.7 +/- 0.2 Hz. Electrical stimulation induced depolarizing evoked post-synaptic potentials in DRN serotonergic neurons. The application of glutamatergic and GABAergic antagonists unmasked two different post-synaptic potential components: a GABAergic evoked inhibitory post-synaptic potentials and a glutamatergic evoked excitatory post-synaptic potentials, respectively. Interleukin-1 increased GABAergic evoked inhibitory post-synaptic potentials amplitudes by 30.3 +/- 3.8% (n = 6) without affecting glutamatergic evoked excitatory post-synaptic potentials. These results support the hypothesis that interleukin-1 inhibitory effects on DRN serotonergic neurons are mediated by an interleukin-1-induced potentiation of evoked GABAergic inhibitory responses.

Robustness of multiplicative processes in auditory spatial tuning.

Auditory space-specific neurons in the owl's inferior colliculus selectively respond to the direction of sound propagation, which is defined by combinations of interaural time (ITD) and level (ILD) differences. Mathematical analyses show that the amplitude of postsynaptic potentials in these neurons is a product of two components that vary with either ITD or ILD. Temporal correlation in the fine structure of signals between the ears is essential for detection of ITD. By varying the degree of binaural correlation, we could accurately change the amplitude of the ITD component of postsynaptic potentials in the space-specific neurons. Multiplication worked for the entire range of postsynaptic potentials created by manipulation of ITD.

Modulation of cholinergic transmission in the neuronal network of the gill and siphon withdrawal reflex in Aplysia

Inhibitory interneurons are important elements of the network underlying the gill and siphon withdrawal reflex in Aplysia, and a large component of this inhibition is cholinergic. In this study, we investigated one key identified cholinergic inhibitory interneuron of the network, neuron L16, and studied some properties of its synaptic transmission and its modulation. We found that a slow inhibitory postsynaptic potential evoked in sensory neurons by L16 has two components. An earlier inhibitory postsynaptic potential component is sensitive to curare (100 μM) and has a reversal potential near the Cl − equilibrium potential (−54.5 mV). A later inhibitory postsynaptic potential component is sensitive to tetraethylammonium (0.5–1 mM); it is decreased by membrane hyperpolarization and becomes undetectable near the K + equilibrium potential (between −80 and −90 mV). Input to sensory neurons from L16 can be altered by two neuromodulators of the reflex, the small cardioactive peptide and serotonin. Small cardioactive peptide (10 μM) facilitates the connections between L16 and the sensory neurons, while serotonin (5–10 μM) inhibits them. Part of the effect of serotonin on the transmission between L16 and the sensory neurons is due to a postsynaptic mechanism, since responses to acetylcholine application in these cells are decreased by serotonin. These results indicate an additional site of synaptic plasticity in the withdrawal reflex network, the inhibitory cholinergic transmission, by two major neuromodulatory transmitters, small cardioactive peptide and serotonin.

Input patterns and pathways from the six semicircular canals to motoneurons of neck muscles. II. The longissimus and semispinalis muscle groups.

To reveal patterns of input from the six semicircular canals to motoneurons of various neck muscles and their relationship to the mechanical actions of individual neck muscles, patterns of input to neck motoneurons of the longissimus and the semispinalis muscle groups were investigated in the upper cervical spinal cord of anesthetized cats. Intracellular potentials were recorded from motoneurons of the longissimus muscle group (obliquus capitis superior muscle, OCS; splenius muscle, SPL; longissimus muscle, LONG) and the semispinalis muscle group (biventer cervicis muscle, BIV; complexus muscle, COMP), and effects of separate electrical stimulation of the six ampullary nerves on them were analyzed in each preparation. Neck motoneurons usually received convergent inputs from all of the six ampullary nerves, and motoneurons that supplied a particular muscle had a homogeneous pattern of input from the six ampullary nerves. Two different patterns of input were identified for motoneurons of these two muscle groups; one pattern for motoneurons of the longissimus muscle group and the other pattern for motoneurons of the semispinalis muscle group. Motoneurons of the OCS, the SPL, and the LONG muscles received excitation from the three contralateral ampullary nerves and inhibition from the three ipsilateral ampullary nerves. BIV and COMP motoneurons received excitation from the bilateral anterior canal nerves (ACNs) and the contralateral canal nerve (LCN) and inhibition from the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. Latencies of postsynaptic potentials (PSPs) evoked by stimulation of each of the six ampullary nerves indicated that the earliest component of excitatory PSPs (EPSPs) and inhibitory PSPs (IPSPs) was disynaptic in these motoneurons. However, trisynaptic IPSPs were evoked by stimulation of the contralateral PCN in a considerable number of BIV and COMP motoneurons. In OCS, SPL, and LONG motoneurons, all of the excitation from the contralateral and all of the inhibition from the ipsilateral ampullary nerves were mediated through the ipsilateral medial longitudinal fascicle (MLF). In BIV and COMP motoneurons, disynaptic excitation from the contralateral ACN and LCN and disynaptic inhibition from the ipsilateral LCN and bilateral PCNs were mediated through the ipsilateral MLF, whereas disynaptic excitation from the ipsilateral ACN was mediated through the ipsilateral lateral vestibulospinal tract. The patterns of semicircular canal input to neck motoneurons of these two muscle groups are related closely to the mechanical actions of the individual neck muscles and the optimal stimulus to the semicircular canals such that the connections will tend to stabilize head positions in response to head perturbations.

Comparison of effects induced by toxic applications of kainate and glutamate and by glucose deprivation on area CA1 of rat hippocampal slices

Baseline and stimulus-induced changes in [Ca 2+] o and [K +] o as well as field potentials (fp's) were studied during application of the excitatory amino acids kainate or glutamate, or during glucose deprivation in area CA1 and CA3 of rat hippocampal slices. Bath application of kainate in concentrations of 1, 2, 5, 8 and 10 mM induced a sudden rapid fall of [Ca 2+] o in area CA1, associated with a negative shift of the slow fp. Kainate induced disappearance of stratum radiatum (SR) as well as alveus stimulation-evoked postsynaptic fp's, with partial recovery after application of up to 2 mM kainate, but no recovery after 5 mM kainate. Only afferent volleys and repetitive SR stimulation-induced decreases of [Ca 2+] o recovered after 5 mM kainate. Similar observations were made with glutamate. Only when glutamate was applied with 20 mM, irreversible disappearance of postsynaptic fp's was noted. Glucose deprivation for 60–90 min led to an initial slow decline of [Ca 2+] o in area CA1 and CA3, associated with increases in [K +] o, but no significant changes in the fp baseline. Before reaching the lowest level in [Ca 2+] o, stimulation of afferent and efferent fibres in area CA1 and CA3 evoked epileptiform discharges. After reaching the lowest level in [Ca 2+] o, all postsynaptic potential components were irreversibly abolished, sparing afferent volleys and SR stimulation-induced decreases in [Ca 2+] o. The application of the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 30 μM) and l-2-amino-5-phosphonovalerate (2APV, 30 μM) during glucose deprivation did not prevent irreversible loss of alveus and SR stimulation-induced postsynaptic signals. These findings suggest that glutamate release during glucose deprivation is not the main factor of acute cell damage.

Modulation of short latency cutaneous excitation in flexor and extensor motoneurons during fictive locomotion in the cat.

We examined modulation of transmission in short-latency, distal hindlimb cutaneous reflex pathways during fictive locomotion in 19 decerebrate cats. Fictive stepping was produced either by electrical stimulation of the mesencephalic locomotor region (MLR) or by administration of Nialamide and 1-DOPA to acutely spinalized animals. Postsynaptic potentials (PSPs) produced by electrical stimulation of low threshold afferents (less than 2.5 times threshold) in the superficial peroneal (SP), sural, saphenous or medial plantar nerves were recorded intracellularly from various extensor (n = 28) and flexor (n = 24) motoneurons and averaged throughout the step cycle, together with voltage responses to intrasomatic constant current pulses (in order to monitor relative cell input resistance). Each motoneuron studied displayed rhythmic background oscillations in membrane potential and correlated variations in input resistance. The average input resistance of extensor motoneurons was lowest during mid-flexion, when the cells were relatively hyperpolarized and silent. Conversely, average input resistance of flexor motoneurons was highest during mid-flexion, when they were depolarized and active. The amplitude of the minimum-latency excitatory components of PSPs produced by cutaneous nerve stimulation were measured from computer averaged records representing six subdivisions of the fictive step cycle. Oligosynaptic EPSP components were consistently modulated only in the superficial peroneal responses in flexor motoneurons, which exhibited enhanced amplitude during the flexion phase. With the other skin nerves tested (sural, saphenous, and plantar), no consistent patterns of modulation were observed during fictive locomotion. We conclude that transmission through some, but not all, oligosynaptic excitatory cutaneous pathways is enhanced by premotoneuronal mechanisms during the flexion phase of fictive stepping in several cat hindlimb motor nuclei. The present results suggest that the patterns of interaction between the locomotor central pattern generator and excitatory cutaneous reflex pathways depend on the source of afferent input and on the identity of the target motoneuron population.

Commissural inhibition and facilitation of granule cell discharge in fascia dentata.

Stimulation of the contralateral hippocampus in the hilar region had a marked effect on granule cell excitability in the fascia dentata. The primary effect was to block the population spike that otherwise occurred in response to perforant path stimulation. In contrast, the size the excitatory post-synaptic potential component of the perforant path-evoked field potential was only slightly reduced. The population spike diminution began at short latency (3.5 msec), beginning at about 1.0 msec after the onset of the slow component of the potential evoked by the contralateral stimulus. The completeness and duration of this population spike diminution depended on the contralateral stimulus intensity. The maximum duration was less than 40 msec in unanesthetized rats and more than 100 msec in rats under pentobarbital anaesthesia. Bicuculline did not diminish the field potential evoked by the contralateral stimulus but it did prevent the stimulus from blocking the perforant path population spike and, instead, permitted a weak facilitation of the population spike. The normal reduction of the population spike was not mediated by recurrent inhibition, secondary to granule cell activation, since it occurred whether or not the granule cells were inhibited at the time of contralateral stimulation. These results imply that the initial main effect on fascia dentata granule cells of activity in the contralateral hilus is a gamma-aminobutyric-acid-mediated inhibition. This effect most probably involves commissural excitation of local inhibitory interneurons. The direct excitatory action of commissural fibers on granule cells, by comparison, is very weak.

Snake alpha-toxin effects on cholinergic and noncholinergic responses of Aplysia californica neurons

The effects of alpha-toxins from Bungarus multicinctus (alpha BuTX) and Naja naja siamensis (alpha NTX) were studied on synaptic responses and on extrasynaptic responses to focally applied acetylcholine (ACh), histamine (Hm), gamma-aminobutyric acid (GABA), and glutamate (glu) in neurons of the marine mollusc, Aplysia californica. Binding of 125I-alpha BuTX to Aplysia ganglia homogenates was pharmacologically characterized and compared with the pharmacological sensitivity of 125I-alpha BuTX binding to rat brain membrane preparations. Both alpha BuTX and alpha NTX at 10(-5) M reversibly inhibited the ACh-induced increase in Cl- conductance (GCl) without affecting ACh responses mediated by an increase in Na+ conductance (GNa) or an increase in K+ conductance (GK). In addition, both alpha-toxin reversibly inhibited the GCl responses induced by Hm and, in some cases, glu. GABA-induced GCl responses were not affected by either toxin even at concentrations as high as 10(-3) M. Both toxins also inhibited synaptic cholinergic GCl responses as well as the GCl component of postsynaptic potentials mediated by noncholinergic presynaptic neurons. Studies of 125I-alpha BuTX binding to Aplysia ganglia homogenates demonstrated the presence of a single saturable, high affinity site with a dissociation constant (KD) of 3.6 X 10(-9) M. The pharmacological profile of agents which inhibit 125I-alpha BuTx binding to Aplysia ganglia was similar to the profile of vertebrate ganglionic and brain preparations; d-tubocurarine, nicotine, and carbamylcholine effectively and competitively inhibit binding, whereas hexamethonium and decamethonium are less effective and noncompetitive. Strychnine and bicuculline, agents known to block responses to a variety of agonists in Aplysia, were effective competitive inhibitors of toxin binding in Aplysia and in rat brain. Strychnine is not as potent in inhibiting toxin binding to electric organ preparations. Chloride channel blockers, such as penicillin and picrotoxin, and noncholinergic agonists, such as Hm and glu, had no effect on toxin binding. The alpha-toxins appear to inhibit noncholinergic GCl responses through a cholinergic receptor associated with the Cl- conductance mechanism for the noncholinergic agonist.

Effects of guanidine on synaptic transmission in the spinal cord of the frog

The effects of guanidine on motoneurons of the isolated frog spinal cord were studied by adding the drug to the solution bathing the cord during intracellular recording. Guanidine (5.10(-4) M) did not alter the membrane potential of motoneurons. The main effect was a marked increase of the amplitudes and frequencies of small spontaneously occurring inhibitory postsynaptic potentials. The hyperpolarizing component of postsynaptic potentials evoked by stimulation of dorsal roots was also enhanced by guanidine. Higher concentrations of guanidine (5.10(-3) M) resulted in a very large and irreversible increase of the small spontaneously occurring inhibitory potentials, which now appeared in a regular, rhythmic pattern. The effects of guanidine could easily be blocked by increasing the magnesium ions (15 mM) in the bath solution. These results indicate that guanidine facilitates the release of an inhibitory transmitter in afferent terminals of the frog spinal cord either by a direct action on these terminals or indirectly by an action on nerve endings impinging on inhibitory interneurons.