Amplitude Of Excitatory Postsynaptic Potentials Research Articles

Epidermal growth factor administered in the periphery influences excitatory synaptic inputs onto midbrain dopaminergic neurons in postnatal mice

Epidermal growth factor (EGF) has a neurotrophic activity on developing midbrain dopaminergic neurons. We investigated developmental effects of peripheral EGF administration on dopaminergic neurons in midbrain slice preparations containing ventral tegmental area (VTA). Subcutaneous EGF administration to mouse neonates triggered phosphorylation of EGF receptors (ErbB1 and ErbB2) in the midbrain region, suggesting its penetration through the blood–brain barrier. We repeated EGF administration in postnatal mice and examined synaptic transmission in the VTA with electrophysiological recordings. Subchronic EGF treatment increased the amplitude of field excitatory postsynaptic potentials evoked by stimulation of the anterior VTA. To analyze the EGF effect at a single cell level, dopaminergic neurons were identified by their characteristic hyperpolarizing activated currents in whole cell recording. In these dopaminergic neurons, EGF effects the amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs) without affecting their frequency. In agreement, EGF also enhanced the AMPA/NMDA ratio of evoked EPSCs in the dopaminergic neurons. In contrast, EGF effects on mEPSCs of neighboring neurons not exhibiting hyperpolarizing activated currents were modest or insignificant. Thus, these results suggest that circulating EGF substantially influences the physiological properties of developing midbrain dopaminergic neurons in perinatal and postnatal mice.

Long-Term Synaptic Plasticity in Cerebellar Stellate Cells

Inhibitory transmission controls the action potential firing rate and pattern of Purkinje cell activity in the cerebellum. A long-term change in inhibitory transmission is likely to have a profound effect on the activity of cerebellar neuronal circuits. However, little is known about how neuronal activity regulates synaptic transmission in GABAergic inhibitory interneurons (stellate/basket cells) in the cerebellar cortex. We have examined how glutamate released from parallel fibers (the axons of granule cells) influences postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in stellate cells and modulates gamma-aminobutyric acid (GABA) release from these neurons. First, we found that burst stimulation of presynaptic parallel fibers changes the subunit composition of post-synaptic AMPA receptors from GluR2-lacking to GluR2-containing receptors. This switch reduces the Ca(2+) permeability of AMPA receptors and the excitatory postsynaptic potential amplitude and prolongs the duration of the synaptic current, producing a qualitative change in synaptic transmission. This switch in AMPA receptor phenotype can be induced by activation of extrasynaptic N-methyl-D: -aspartate (NMDA) receptors and involves PICK1 and the activation of protein kinase C. Second, activation of presynaptic NMDA receptors triggers a lasting increase in GABA release from stellate cells. These changes may provide a cellular mechanism underlying associative learning involving the cerebellum.

Alternating low frequency stimulation of medial septal and commissural fibers induces NMDA‐dependent, long‐lasting potentiation of hippocampal synapses in urethane‐anesthetized rats

Recent evidence indicates that some synapses exhibit long-lasting synaptic potentiation in response to low frequency (1 Hz) stimulation, similar to long-term potentiation (LTP) following high frequency induction protocols. Here, the authors characterize a form of long-lasting synaptic potentiation in the hippocampal CA1 area following alternating, single pulse stimulation of the medial septum (MS) and hippocampal CA3 commissural fibers (MS-H LTP). In urethane-anesthetized rats, alternating single pulse stimulation of the MS and CA3 (50 pulses each at 0.5 Hz, 1,000 ms interstimulus interval [ISI]) produced gradual increases of field excitatory postsynaptic potential (fEPSP) amplitude in CA1 ( approximately 123% of baseline), while MS or CA3 stimulation alone was ineffective. The fEPSP enhancement was long-lasting (>4h) and repeated episodes of alternating MS-CA3 stimulation tended to result in greater levels of potentiation than those elicited by a single episode. Surprisingly, ISIs of 500, 750, and 1,500 ms did not result in significant changes in fEPSP amplitude, while an ISI of 100 ms produced synaptic depression. MS-H LTP was resistant to systemic administration of nicotinic and muscarinic receptor antagonists (scopolamine, mecamylamine), but abolished by systemic MK-801 (0.5 mg/kg, i.p.) or local CA1 application of AP-V (10 mM), indicative of a critical role of hippocampal NMDA receptors in this effect. Paired-pulse facilitation experiments revealed a gradually developing, significant inverse correlation between fEPSP enhancement and decrease in paired-pulse facilitation ratio, suggesting a role of changes in presynaptic transmitter release. Together, these data demonstrate a novel form of long-lasting synaptic enhancement in CA1 neurons in response to low frequency activity in separate afferent systems, an activity that might mimic some aspects of natural discharge patterns during the acquisition or consolidation of memory processes in hippocampal circuits.

The Effect of Lidocaine on Cholinergic Neurotransmission in an Identified Reconstructed Synapse

The presynaptic effect of lidocaine on cholinergic synaptic transmission is unclear because of the difficulty in identifying presynaptic neurons and the complexity of the central nervous system in vivo. To clarify the effect of lidocaine on cholinergic synapse, we reconstructed a cultured soma-soma chemical synapse model consisting of two identified visceral dorsal 4 (VD4) and left pedal e-1 (LPeD1) neurons from the snail, Lymnaea stagnalis, in vitro, and used it to determine how lidocaine affects cholinergic synaptic transmission. The response to acetylcholine and excitatory postsynaptic potential (EPSP) amplitude was recorded in the reconstructed chemical synaptic transmission model composed of VD4 and LPeD1 neurons. The currents for acetylcholine measurements were made under voltage-clamp in the presynaptic VD4 and postsynaptic LPeD1 neurons. Lidocaine inhibited both EPSP and the response for acetylcholine of the postsynaptic neuron. EPSP amplitude was reduced in a voltage-dependent manner in the presynaptic neuron, and lidocaine induced a hyperpolarization shift of the voltage-dependent inactivation curves of EPSP amplitude. Lidocaine inhibits cholinergic synaptic transmission with a voltage-dependent inactivation of EPSP amplitude through the membrane potential depolarization of presynaptic neurons.

How the nonlinear voltage dependence of the amplitude of excitatory postsynaptic potentials induced by cerebral cortical neurons affects cortical rhythmic activity

A nonlinear dependence of the amplitude of excitatory postsynaptic potentials on the membrane potentials was derived. The existence of a region of oscillation stability with an increase in the mean value of nonspecific afferent inflow was demonstrated. A high-frequency oscillation component (40–60 Hz) appears with a pronounced increase in the afferent inflow; this can cause instability in oscillations and abnormal brain activity.

Ethanol Decreases Purkinje Neuron Excitability by Increasing GABA Release in Rat Cerebellar Slices

Cerebellar Purkinje neurons (PNs) receive inhibitory GABAergic input from stellate and basket cells, which are located in the outer and inner portions of the molecular layer, respectively. Ethanol (EtOH) was recently shown to increase GABAergic transmission at PNs via a mechanism that involves enhanced calcium release from presynaptic internal stores (J Pharmacol Exp Ther 323:356-364, 2007). Here, we further characterized the effect of EtOH on GABA release and assessed its impact on PN excitability. Using whole-cell patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that EtOH acutely increases the frequency but not the amplitude or half-width of miniature and spontaneous inhibitory postsynaptic currents (IPSCs). EtOH significantly increased the amplitude and decreased the paired pulse ratio of IPSCs evoked by stimulation in the outer but not inner molecular layer. In current clamp, EtOH decreased both the amplitude of excitatory postsynaptic potentials evoked in PNs by granule cell axon stimulation and the number of action potentials triggered by these events; these effects depended on GABA(A) receptor activation because they were not observed in presence of bicuculline. Loose-patch cell-attached PN recordings revealed that neither the spontaneous action potential firing frequency nor the coefficient of variation of the interspike interval was altered by acute EtOH exposure. These findings suggest that EtOH differentially affects GABAergic transmission at stellate cell- and basket cell-to-PN synapses and that it modulates PN firing triggered by granule cell axonal input. These effects could be in part responsible for the cerebellar impairments associated with acute EtOH intoxication.

The novel NTPDase inhibitor sodium polyoxotungstate (POM-1) inhibits ATP breakdown but also blocks central synaptic transmission, an action independent of NTPDase inhibition

Understanding the mechanisms and properties of purinergic signalling would be greatly assisted by the discovery of subtype selective and potent inhibitors of the NTPDase enzymes, which metabolise nucleotides such as ATP and ADP in the extracellular space. Currently ARL 67156 is the best available NTPDase inhibitor, but its relatively poor efficacy means that negative results are difficult to interpret. POM-1 (sodium polyoxotungstate) is a novel NTPDase inhibitor, which has shown promising results with the inhibition of recombinant NTPDases 1, 2 and 3. We have tested the effectiveness and physiological effects of POM-1 with cerebellar and hippocampal slices. Using the malachite green phosphate assay, HPLC and biosensor measurements we have found that POM-1 is more effective at blocking ATP breakdown in cerebellar slices than ARL 67156. The site of inhibition is at the first step of the breakdown cascade (conversion of ATP to ADP) and the effects of POM-1 appear readily reversible. However, POM-1 has multiple effects on synaptic transmission. At the cerebellar parallel fibre-Purkinje cell (PF) synapse POM-1 produced a long lasting inhibition of transmission, which was preceded in a minority of synapses by a transient increase in PF excitatory postsynaptic potential (EPSP) amplitude (∼20%). This increase in PF EPSP amplitude appears to result from a reduction in the tonic activation of presynaptic A 1 receptors, consistent with POM-1 preventing the breakdown of ATP to adenosine. The reduction in PF EPSP amplitude does not however appear to result from NTPDase inhibition as it persists when both adenosine and ATP (P2Y and P2X) receptors are blocked. An increase in paired pulse ratio and a reduction in presynaptic volley amplitude suggest that there is a presynaptic component of POM-1 action which reduces glutamate release. POM-1 produced similar inhibition at climbing fibre synapses and at hippocampal CA1 pyramidal synapses. Thus although POM-1 is more effective than ARL 67156 at blocking ATP breakdown its usefulness is limited by off-target actions on synaptic transmission.

Differential effect of carbamazepine and oxcarbazepine on excitatory synaptic transmission in rat hippocampus

In this study, we have compared the effects of two structurally related compounds carbamazepine (CBZ) and oxcarbazepine (OXC), both in current use for the treatment of epilepsy and bipolar disorder, on fast excitatory transmission in rat hippocampal slices. Using electrophysiological recordings, we have investigated the effects of CBZ and OXC on repetitive action potential discharge of CA1 pyramidal neurons demonstrating that both compounds produced firing inhibition with similar IC(50) values. Moreover, we show that bath applied CBZ (0.01-1 mM) exerted a concentration-dependent decrease in the amplitude of the field excitatory postsynaptic potentials with an IC(50) of approximately 194.3 microM. When OXC was used at the same concentrations, the concentration-response curve was shifted to the right (IC(50) of approximately 711.07 microM). In addition, we demonstrated that CBZ and OXC reduced, to a different extent, both evoked excitatory postsynaptic currents and NMDA-, AMPA-, and KA-mediated inward currents, CBZ being more potent than OXC. These data highlight distinct presynaptic and postsynaptic sites of action for both compounds and suggest that CBZ, by markedly depressing postsynaptic ionotropic glutamate receptors-mediated responses, may produce more severe cognitive and memory impairment. Thus, we assume that relatively high doses of OXC could be better tolerated than therapeutically equivalent doses of CBZ, justifying the preferential use of OXC as first-line treatment in the therapy of neurological and psychiatric disorders, particularly when compared with CBZ.

Acute Enhancement of Synaptic Transmission and Chronic Inhibition of Synaptogenesis Induced by Perfluorooctane Sulfonate through Mediation of Voltage-Dependent Calcium Channel

Perfluorooctane sulfonate (PFOS) is a persistent and bioaccumulative pollutant ubiquitous in wildlife and humans. Although the distribution and fate of PFOS have been widely studied, its potential neurotoxicity remains largely unknown. In the present study, the acute and chronic effects of PFOS on the development and synaptic transmission of hippocampal neurons was examined. Perfusion with PFOS markedly increased the frequency of miniature postsynaptic currents (mPSCs) and slightly elevated the amplitude of mPSCs in cultured hippocampal neurons. Perfusion with PFOS also increased the amplitude of field excitatory postsynaptic potentials (fEPSPs) recorded in the CA1 region of hippocampal slices. Both of these effects were largely blocked by the L-type Ca2+ channel antagonist nifedipine. Further studies showed that PFOS enhanced inward Ca2+ currents and increased intracellular Ca2+ in cultured neurons; these effects were also substantially inhibited by nifedipine. Moreover, prolonged treatment with PFOS moderately inhibited neurite growth and dramatically suppressed synaptogenesis in cultured neurons in a nifedipine-sensitive manner. Thus, through enhancement of Ca2+ channels, PFOS may exhibit both acute excitotoxic effects on synaptic function and chronically inhibit synaptogenesis in the brain.

Cellular and Synaptic Actions of Acetylcholine in the Lamprey Spinal Cord

This study investigated cellular and synaptic mechanisms of cholinergic neuromodulation in the in vitro lamprey spinal cord. Most spinal neurons tested responded to local application of acetylcholine (ACh) with depolarization and decreased input resistance. The depolarization persisted in the presence of either tetrodotoxin or muscarinic antagonist scopolamine and was abolished with nicotinic antagonist mecamylamine, indicating a direct depolarization through nicotinic ACh receptors. Local application of muscarinic ACh agonists modulated synaptic strength in the spinal cord by decreasing the amplitude of unitary excitatory and inhibitory postsynaptic potentials. The postsynaptic response to direct application of glutamate was unchanged by muscarinic agonists, suggesting a presynaptic mechanism. Cholinergic feedback from motoneurons was assessed using stimulation of a ventral root in the quiescent spinal cord while recording intracellularly from spinal motoneurons or interneurons. Mainly depolarizing potentials were observed, a portion of which was insensitive to removal of extracellular Ca2+, indicating electrotonic coupling. Hyperpolarizing potentials were also observed and were attenuated by the glycinergic antagonist strychnine, whereas depolarizing responses were potentiated by strychnine. Mecamylamine also reduced hyperpolarizing responses. The pharmacology of these responses suggests a Renshaw-like feedback pathway in lamprey. Immunohistochemistry for choline acetyltransferase, performed in combination with retrograde filling of motoneurons, demonstrated a population of nonmotoneuron cholinergic cells in the lamprey spinal cord. Thus endogenous cholinergic modulation of the lamprey spinal locomotor network is likely produced by both motoneurons and cholinergic interneurons acting via combined postsynaptic and presynaptic actions.

Attenuation of hippocampal mossy fiber long‐term potentiation by low micromolar concentrations of zinc

The role of zinc in long-term potentiation (LTP) at hippocampal mossy fiber synapses is controversial because of the contrary results obtained when using zinc chelators. On the basis of the postulation that exogenous zinc enhances the action of zinc released from mossy fibers, mossy fiber LTP after tetanic stimulation (100 Hz, 1 sec) was checked in the presence of exogenous zinc at low micromolar concentrations. Mossy fiber LTP was significantly attenuated in the presence of 5-30 microM ZnCl(2), and the amplitude of field excitatory postsynaptic potentials 60 min after tetanic stimulation was decreased to almost the basal level. Mossy fiber LTP was also attenuated in the presence of 5 microM ZnCl(2) 5 min after tetanic stimulation. The present study is the first to demonstrate that low micromolar concentrations of zinc attenuate mossy fiber LTP. When mossy fiber LTP was induced in the presence of CaEDTA and ZnAF-2 DA, a membrane-impermeable and a membrane-permeable zinc chelator, respectively, extracellular and intracellular chelation of zinc enhanced a transient posttetanic potentiation (PTP) without altering LTP. It is likely that zinc released by tetanic stimulation is immediately taken up into the mossy fibers and attenuates mossy fiber PTP. These results suggest that attenuation of PTP rather than LTP at mossy fiber synapses is a more physiological role for endogenous zinc. Targeting molecules of zinc in mossy fiber LTP seem to be different between during and after LTP induction because of the differential synaptic activity between them.

Glial-toxin-mediated disruption of spinal cord locomotor network function and its modulation by 5-HT

While it is established that glial cells actively influence neuronal and synaptic properties, the functional effects of glial–neuronal interactions are still not well understood. To address the role of glia at the network level we have examined the effects of the specific gliotoxin l-aminoadipic acid on the locomotor network output and cellular and synaptic properties in the lamprey spinal cord.The gliotoxic effect of aminoadipic acid was associated with a specific depolarization of glial cells. Aminoadipic acid depolarized the membrane potential of spinal cord neurons, suggesting a functional link between glia and neurons. The depolarization was significantly reduced by glutamate receptor antagonists in adults, but by gap junction blockers in larvae, suggesting a developmental difference in glial–neuronal interactions. Aminoadipic acid also reduced the amplitude of monosynaptic excitatory postsynaptic potentials (EPSPs), an effect that was not associated with changes in the presynaptic release probability or postsynaptic response to glutamate.These cellular and synaptic effects of aminoadipic acid were associated with disruption of the locomotor network output. This could not be accounted for by changes in glutamate uptake or potassium buffering by glia, suggesting a direct role for glia in the network. Interestingly, we found that the aminoadipic acid–evoked disruption of network activity and reduction of monosynaptic EPSP amplitudes did not occur in the presence of the endogenous spinal modulator 5-HT.These results thus provide evidence for an active functional role for glial cells in spinal cord locomotor networks, and suggest a potential glial modulatory effect of 5-HT.

Amnestic Concentrations of Sevoflurane Inhibit Synaptic Plasticity of Hippocampal CA1 Neurons through γ-Aminobutyric Acid–mediated Mechanisms

The cellular mechanisms of anesthetic-induced amnesia are still poorly understood. The current study examined sevoflurane at various concentrations in the CA1 region of rat hippocampal slices for effects on excitatory synaptic transmission and on long-term potentiation (LTP), as a possible mechanism contributing to anesthetic-induced loss of recall. Population spikes and field excitatory postsynaptic potentials were recorded using extracellular electrodes after electrical stimulation of Schaffer-collateral-commissural fiber inputs. Paired pulse facilitation was used as a measure of presynaptic effects of the anesthetic. LTP was induced using tetanic stimulation (100 Hz, 1 s). Sevoflurane at concentrations from amnestic (0.04 mm) to clinical concentrations (0.23-0.41 mm) were added to the perfusion solution. In the presence of 0.04 mm sevoflurane, the amplitude of population spikes was significantly depressed, and tetanic stimulation induced only posttetanic potentiation and then failure of LTP. These inhibitory effects were antagonized by bicuculline (10 microm), a gamma-aminobutyric acid type A receptor antagonist. Sevoflurane at 0.23-0.41 mm further depressed the amplitude of field excitatory postsynaptic potentials in a dose-dependent manner and completely blocked LTP. Bicuculline only partially antagonized 0.41 mm sevoflurane-induced profound inhibition of LTP. Sevoflurane at 0.23-0.41 mm, but not at 0.04 mm, significantly increased paired pulse facilitation, suggesting that sevoflurane has presynaptic actions to reduce glutamate release from nerve terminals. The current study provides evidence that amnestic concentrations of sevoflurane inhibit LTP of hippocampal CA1 neurons through gamma-aminobutyric acid-mediated mechanisms, and these actions seem to account for the effects of amnestic sevoflurane on synaptic plasticity.

Monosynaptic Excitatory Connection from the Rostral Ventrolateral Medulla to Sympathetic Preganglionic Neurons Revealed by Simultaneous Recordings

To directly investigate whether a monosynaptic connection exists between neurons in the rostral ventrolateral medulla (RVLM) and sympathetic preganglionic neurons (SPNs), we used simultaneous extracellular recordings of RVLM neurons and whole-cell patch-clamp recordings of SPNs at the Th2 level and analyzed them by spike-triggered averaging. We averaged 200 sweeps of membrane potentials in SPN triggered by the spikes in the RVLM neuron. No clear postsynaptic potentials were detected in the averaged wave of SPNs before angiotensin II (Ang II) superfusion, whereas during superfusion with Ang II (6 micromol/L) on the medulla oblongata side alone excitatory postsynaptic potentials (EPSPs) were clearly found in the SPN of 3 out of 10 pairs at 40 +/- 1 ms after the averaged triggering spike in the RVLM neuron. We consider them to be monosynaptic EPSPs, because 1) the averaged EPSPs exhibited a sharp rise time, 2) the onset latency of the averaged EPSPs in the SPNs after the trigger spike in the RVLM was the same as the latency of the antidromic action potentials in the RVLM neurons in response to electrical stimulation of the SPNs, and 3) the amplitude of the averaged EPSPs was over 2 mV. In summary, combining simultaneous recording and spike-triggered averaging allowed us to demonstrate a monosynaptic excitatory connection between a single RVLM neuron and a single SPN in the thoracic spinal cord. Such connections provide the basis for the maintenance of sympathetic tone and the integrative reflex that relays through the RVLM. The results explain the mechanism by which Ang II in the RVLM area increases peripheral sympathetic activity and blood pressure.

Histamine depolarizes neurons in the dorsal vagal complex

We sought to determine whether histamine has effects on single neurons in the dorsal vagal complex of the brainstem since previous studies have suggested a role for histamine receptors in this region. Using whole-cell patch clamp recordings from neurons within the nucleus of the tractus solitarius (NTS) and the dorsal vagal nucleus (DVN), histamine (20 μM) depolarized a small proportion of neurons in these regions accompanied by a decrease in input resistance. Although few neurons were depolarized (21% of NTS neurons and 15% of DVN neurons), those that were affected showed robust depolarizations of 13 mV. These depolarizations were antagonized by the histamine H1 receptor antagonist triprolidine (2 μM) and were subject to a level of desensitization. Neither histamine nor the H3 receptor agonist imetit caused any change in the amplitudes of excitatory or inhibitory postsynaptic potentials elicited in NTS neurons by stimulation of the solitary tract. These data indicate that histamine has a restricted but profound effect on neurons in the dorsal vagal complex.

AMPA receptors undergo channel arrest in the anoxic turtle cortex

Without oxygen, all mammals suffer neuronal injury and excitotoxic cell death mediated by overactivation of the glutamatergic N-methyl-D-aspartate receptor (NMDAR). The western painted turtle can survive anoxia for months, and downregulation of NMDAR activity is thought to be neuroprotective during anoxia. NMDAR activity is related to the activity of another glutamate receptor, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR). AMPAR blockade is neuroprotective against anoxic insult in mammals, but the role of AMPARs in the turtle's anoxia tolerance has not been investigated. To determine whether AMPAR activity changes during hypoxia or anoxia in the turtle cortex, whole cell AMPAR currents, AMPAR-mediated excitatory postsynaptic potentials (EPSPs), and excitatory postsynaptic currents (EPSCs) were measured. The effect of AMPAR blockade on normoxic and anoxic NMDAR currents was also examined. During 60 min of normoxia, evoked peak AMPAR currents and the frequencies and amplitudes of EPSPs and EPSCs did not change. During anoxic perfusion, evoked AMPAR peak currents decreased 59.2 +/- 5.5 and 60.2 +/- 3.5% at 20 and 40 min, respectively. EPSP frequency (EPSP(f)) and amplitude decreased 28.7 +/- 6.4% and 13.2 +/- 1.7%, respectively, and EPSC(f) and amplitude decreased 50.7 +/- 5.1% and 51.3 +/- 4.7%, respectively. In contrast, hypoxic (Po(2) = 5%) AMPAR peak currents were potentiated 56.6 +/- 20.5 and 54.6 +/- 15.8% at 20 and 40 min, respectively. All changes were reversed by reoxygenation. AMPAR currents and EPSPs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In neurons pretreated with CNQX, anoxic NMDAR currents were reversibly depressed by 49.8 +/- 7.9%. These data suggest that AMPARs may undergo channel arrest in the anoxic turtle cortex.

Dopamine and cyclic-AMP regulated phosphoprotein-32–dependent modulation of prefrontal cortical input and intercellular coupling in mouse accumbens spiny and aspiny neurons

The roles of dopamine and cyclic-AMP regulated phosphoprotein-32 (DARPP-32) in mediating dopamine (DA)-dependent modulation of corticoaccumbens transmission and intercellular coupling were examined in mouse accumbens (NAC) neurons by both intracellular sharp electrode and whole cell recordings. In wild-type (WT) mice bath application of the D2-like agonist quinpirole resulted in 73% coupling incidence in NAC spiny neurons, compared with baseline (9%), whereas quinpirole failed to affect the basal coupling (24%) in slices from DARPP-32 knockout (KO) mice. Thus, D2 stimulation attenuated DARPP-32-mediated suppression of coupling in WT spiny neurons, but this modulation was absent in KO mice. Further, whole cell recordings revealed that quinpirole reversibly decreased the amplitude of cortical-evoked excitatory postsynaptic potentials (EPSPs) in spiny neurons of WT mice, but this reduction was markedly attenuated in KO mice. Bath application of the D1/D5 agonist SKF 38393 did not alter evoked EPSP amplitude in WT or KO spiny neurons. Therefore, DA D2 receptor regulation of both cortical synaptic (chemical) and local non-synaptic (dye coupling) communications in NAC spiny neurons is critically dependent on intracellular DARPP-32 cascades. Conversely, in fast-spiking interneurons, blockade of D1/D5 receptors produced a substantial decrease in EPSP amplitude in WT, but not in KO mice. Lastly, in putative cholinergic interneurons, cortical-evoked disynaptic inhibitory potentials (IPSPs) were attenuated by D2-like receptor stimulation in WT but not KO slices. These data indicate that DARPP-32 plays a central role in 1) modulating intercellular coupling, 2) cortical excitatory drive of spiny and aspiny GABAergic neurons, and 3) local feedforward inhibitory drive of cholinergic-like interneurons within accumbens circuits.

Activity-dependent Regulation of h Channel Distribution in Hippocampal CA1 Pyramidal Neurons

The hyperpolarization-activated cation current, I(h), plays an important role in regulating intrinsic neuronal excitability in the brain. In hippocampal pyramidal neurons, I(h) is mediated by h channels comprised primarily of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits, HCN1 and HCN2. Pyramidal neuron h channels within hippocampal area CA1 are remarkably enriched in distal apical dendrites, and this unique distribution pattern is critical for regulating dendritic excitability. We utilized biochemical and immunohistochemical approaches in organotypic slice cultures to explore factors that control h channel localization in dendrites. We found that distal dendritic enrichment of HCN1 is first detectable at postnatal day 13, reaching maximal enrichment by the 3rd postnatal week. Interestingly we found that an intact entorhinal cortex, which projects to distal dendrites of CA1 but not area CA3, is critical for the establishment and maintenance of distal dendritic enrichment of HCN1. Moreover blockade of excitatory neurotransmission using tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione, or 2-aminophosphonovalerate redistributed HCN1 evenly throughout the dendrite without significant changes in protein expression levels. Inhibition of calcium/calmodulin-dependent protein kinase II activity, but not p38 MAPK, also redistributed HCN1 in CA1 pyramidal neurons. We conclude that activation of ionotropic glutamate receptors by excitatory temporoammonic pathway projections from the entorhinal cortex establishes and maintains the distribution pattern of HCN1 in CA1 pyramidal neuron dendrites by activating calcium/calmodulin-dependent protein kinase II-mediated downstream signals.

Isoflurane Bidirectionally Modulates the Paired-Pulse Responses in the Rat Hippocampal CA1 Field In Vivo

We studied the effects of isoflurane on hippocampal synaptic transmission and paired-pulse plasticity, under in vivo intact interneuron circuitry. Using rats chronically implanted with electrodes, excitatory postsynaptic potential (EPSP) and population spike amplitude (PSA) were measured in the hippocampal CA1 field by stimulating Schaffer collaterals. The lungs of the rats were mechanically ventilated with 0.25-1.5 minimum alveolar anesthetic concentration (MAC) isoflurane. A control value was obtained in the absence of isoflurane. Isoflurane depressed EPSP responses and enhanced synaptic efficacy. PSA was not depressed except under high concentrations, presumably reflecting a well-balanced combination with the decreased EPSP and enhanced synaptic efficacy. Low concentrations of isoflurane (0.25 and 0.5 MAC) increased paired-pulse facilitation (PPF), whereas a high concentration of isoflurane (1.5 MAC) prolonged the paired-pulse depression. Isoflurane appeared to affect multiple sites of CA1 synapses: 1) the depression of presynaptic glutamatergic transmission as shown by depressed EPSP and increased PPF; 2) the depression of pyramidal neurons as shown by prolonged PPF and depressed PSA under high concentration; and 3) the depression of interneurons as shown by the greater synaptic efficacy. The degree of each of these inhibitory effects seemed to vary at different concentrations, and the overall direction of the synaptic properties may depend on the balances between these inhibitory effects in vivo.

Functional studies of motor cortex.

Defining functions for neural elements becomes more difficult the more remote they are, in synaptic linkages, from motor neurons. The precentral motor cortex contains a corticomotoneuronal projection system, only one synapse removed from motor neurons. Corticomotoneuronal fibres produce monosynaptic excitation of spinal motor neurons, which is more powerful for those acting distally, innervating extensor muscles of the fingers and intrinsic hand muscles. Latencies and time-course of corticomotoneuronal excitation are defined. Amplitudes of unitary corticomotoneuronal excitatory postsynaptic potentials are very small. Many corticomotoneuronal cells converge on a given motor neuron. Some motor neurons innervating proximally acting muscles appear to receive no corticomotoneuronal excitation. Disynaptic inhibitory actions are produced by corticospinal volleys via the common Ia inhibitory interneuron--possibly reciprocal actions produced over collaterals of corticomotoneuronal fibres. Anatomical divergence of projections of collaterals of an identified corticomotoneuronal fibre is extensive enough to provide both for delivery of synapses to a large number of motor neurons and for the dispersion of specific projections to inhibitory interneurons, to fusimotor neurons and to other interneurons. Functions of corticomotoneuronal elements in motor cortex whose targets have been identified and whose excitatory or inhibitory actions have been specified by cross-correlation have been studied by Fetz & Cheney (J Neurophysiol 1980; 44:751-772).