Involvement of AMPA Receptors in Trigeminal Post-synaptic Potentials Recorded in Rat Abducens Motoneurons In Vivo

The pharmacology of trigeminal excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the vibrissal pad was investigated in vivo in rat abducens motoneurons using intracellular recordings combined with microionophoretic applications of excitatory amino acid agonists [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), NMDA, kainate] and a selective non-NMDA receptor antagonist (GYKI-52466). Intravenous applications of GYKI-52466 were also performed during synaptic and amino acid excitations. GYKI-52466, applied intravenously or microionophoretically, reversibly antagonized AMPA-induced depolarizations and trigeminal EPSPs in rat abducens motoneurons without affecting NMDA and kainate responses. The inhibition of AMPA-induced depolarizations was similar following i.v. and ionophoretic applications of GYKI-52466. Intravenous applications of GYKI-52466 (0.3–4 mg/kg) reversibly and dose-dependently reduced trigeminal EPSPs, which could be totally suppressed at the highest doses of GYKI-52466 (2–4 mg/kg). The antagonist effect, which developed very quickly, could last several minutes and recovered gradually. The effect of GYKI-52466 on the EPSPs and AMPA responses were compared in the same motoneurons. The partial inhibition of trigeminal EPSPs during microionophoretic applications of GYKI-52466 was probably due to the distribution of the synapses in the dendritic arborization of abducens motoneurons. Our results show that AMPA receptors are involved in the generation of trigeminal EPSPs in rat abducens motoneurons in vivo.     

NMDA Actions on Rat Abducens Motoneurons

Intracellular recordings were made from rat abducens motoneurons in vivo during local extracellular micro-ionophoretic application of N-methyl-d-aspartate (NMDA) and NMDA receptor antagonists. Typical NMDA responses, at a resting potential of -60 mV, consisted of a slow depolarization with an apparent increase in membrane resistance, bursts of action potentials followed by stable repetitive firing. Ionophoretic applications of aminophosphonovalerate (APV), kynurenate or MK801 reduced or blocked the NMDA-induced responses. The NMDA responses were voltage-dependent. NMDA responses induced by short (< 30 s) NMDA application pulses were blocked by hyperpolarizing the neuron. Long duration (> 30 s) NMDA applications induced rhythmic plateau potentials in hyperpolarized abducens motoneurons. The rhythmic depolarizations (15 - 30 mV) were modulated in both frequency and duration by current injection. They were abolished by further hyperpolarization or replaced by stable repetitive firing when hyperpolarization was removed. Our data show that NMDA receptors are present in rat abducens motoneurons and may be involved in the induction of rhythmic activities. The voltage-dependent blockade of somatic NMDA receptor-associated ion channels by cell hyperpolarization may be important for these oscillations. It is suggested that the rhythmic behaviour is due to the activation of dendritic NMDA receptors.     

NMDA receptors mediate poly- and monosynaptic potentials in motoneurons of rat embryos

We determined the contribution of glutamate receptor subtypes to developing excitatory synaptic transmission in isolated spinal cord of rat embryos. Using electrophysiological and morphological techniques, we studied the pattern of development of synapses between dorsal root afferents and motoneurons in lumbar spinal cords of 15- to 21-d-old rat embryos. Motoneuron dendritic fields and afferent projections onto motoneurons were identified by labeling with HRP. Afferents first entered the gray matter at Day 15 of gestation, and by Day 16 they terminated close to motoneuron dendritic trees. Afferent axons projected onto motoneuron dendritic fields at Day 17, when boutons were detected on motoneuron dendrites that were crossed by afferent axons. To determine the time course of formation of functional sensorimotor synapses and their pharmacological properties, a dorsal root was stimulated while recording intracellularly from segmental motoneurons. At Day 16, excitatory postsynaptic potentials (EPSPs) with long latencies, slow rates of rise, and long durations were recorded. The amplitudes of these EPSPs increased with membrane depolarization and in the absence of extracellular Mg2+. These EPSPs were blocked by D-2-amino-5-phosphonovalerate (APV) and ketamine, which are selective antagonists of N-methyl-D-aspartate (NMDA) receptors. These findings suggest that initial synaptic transmission in embryonic motoneurons is mediated solely by NMDA receptors. Short-latency EPSPs with fast rates of rise were first recorded in most motoneurons by Day 17. These EPSPs were composed of fast- and slow-rising potentials. The slow component was blocked by APV, while the fast component was eliminated by 6-cyano-7-nitroquinoxaline-2,3-dione and kynurenate. This indicates that the short-latency EPSPs are mediated by both NMDA and non-NMDA receptors. Dose-response curves of motoneurons to L-glutamate, NMDA, and kainate demonstrated that motoneurons are sensitive to these agonists prior to the formation of synapses between afferents and motoneurons. Motoneuron responses to NMDA and kainate increased immediately after the onset of short-latency EPSPs. This increased sensitivity could be due to extracellular factors influenced by growing sensory axons or intrinsic properties of differentiating motoneurons.     

Acute modulation of synaptic transmission to motoneurons by BDNF in the neonatal rat spinal cord

We investigated the acute effects of bath applied BDNF on synaptic input to motoneurons in the hemisected spinal cord of the neonatal rat. Motoneurons were recorded intracellularly, and BDNF-induced modulation of the synaptic response to stimulation of the homologous dorsal root (DR) and the ventrolateral funiculus (VLF) was examined. All motoneurons exhibited long-lasting (up to several hours) depression of the DR-activated monosynaptic AMPA/kainate-receptor mediated EPSP in response to BDNF but in about half of the motoneurons this was preceded by facilitation. VLF-evoked AMPA/kainate EPSPs in the same motoneurons were unaffected. BDNF effects were blocked by K252a and were not observed in neonates older than 1 week. Bath applied NMDA antagonists APV and MK-801 abolished both facilitatory and inhibitory actions of BDNF on the AMPA/kainate responses indicating the requirement for functional NMDA receptors. The pharmacologically isolated, DR-evoked, NMDA receptor-mediated response exhibited the same pattern of changes after BDNF superfusion. When introduced into the motoneuron through the recording microelectrode, MK-801 selectively blocked the facilitatory action of BDNF. Furthermore, BDNF enhanced NMDA-induced depolarization of the motoneuron in the presence of tetrodotoxin (TTX), thus, confirming its facilitatory effect on motoneuron NMDA receptors. Bath application of either BDNF or NMDA depressed the monosynaptic EPSP after selective blockade of postsynaptic NMDA receptors indicating a role for presynaptic NMDA receptors in BDNF-induced inhibitory action. Thus, BDNF-induced facilitation of monosynaptic EPSPs in neonatal rats is mediated by direct effects on postsynaptic NMDA receptors, while its inhibitory action occurs presynaptically.     

Blocking the trigeminal EPSP in rat abducens motoneurons in vivo with the AMPA antagonists NBQX and GYKI 53655

In pentobarbitone-anaesthetized rats, the effects of two AMPA receptor antagonists, the competitive antagonist 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (NBQX) and the non-competitive 2,3-benzodiazepine GYKI 53655, were compared on excitatory synaptic transmission of trigeminal origin in intracellularly-recorded abducens motoneurons. The effects of both antagonists were also investigated on the alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA)-, kainate-, and N-methyl-D-aspartate (NMDA)-induced depression of extracellular antidromic field potentials in the abducens motor nucleus. Microiontophoretic application (< or =100 nA) or intravenous injection of NBQX (< or =5 mg/kg) affected both AMPA- and kainate-induced depressions whereas GYKI 53655 (< or =100 nA; < or =4 mg/kg) blocked only the AMPA-induced depression. Neither NBQX or GYKI 53655 affected NMDA-induced depressions of antidromic field potentials. Using low intravenous (i.v.) doses of the antagonists NBQX or GYKI 53655 (2-2.5 mg/kg), a complete blockade of the composite disynaptic trigeminal excitatory post-synaptic potential (EPSP) was obtained without any changes in membrane potential, input resistance and antidromic action potentials in abducens motoneurons. GYKI 53655 was more potent at low i.v. doses (0.5-1.8 mg/kg) but NBQX had longer-lasting effects. The results show the existence of differences between the blocking action of NBQX and GYKI 53655 on AMPA-mediated receptor EPSP in abducens motoneurons.     

Voltage Oscillations in Xenopus Spinal Cord Neurons: Developmental Onset and Dependence on Co-activation of NMDA and 5HT Receptors

The development of intrinsic, N-methyl-D-aspartate (NMDA) receptor-mediated voltage oscillations and their dependence on co-activation of 5-hydroxytryptamine (5HT) receptors was explored in motor neurons of late embryonic and early larval Xenopus laevis. Under tetrodotoxin, 100 μM NMDA elicited a membrane depolarization of around 20 mV, but did not lead to voltage oscillations. However, following the addition of 2–5 μM 5HT, oscillations were observed in 12% of embryonic and 70% of larval motor neurons. The voltage oscillations depended upon co-activation of NMDA and 5HT receptors since they were curtailed by selectively blocking NMDA receptors with D-2-amino-5-phosphonovaleric acid (APV) or by excluding Mg²⁺ from the experimental saline. 5HT applied in the absence of NMDA also failed to elicit oscillations. Oscillations could be induced by the non-selective 5HT_1a receptor agonist, 5-carboxamidotryptamine (5CT) and both 5HT- and 5CT-induced oscillations were abolished by pindobind-5HT₁, a selective 5HT_1a receptor antagonist. To test whether 5HT enables voltage oscillations by modulating the voltage-dependent block of NMDA channels by Mg²⁺, membrane conductance was monitored under tetrodotoxin. Although 5HT caused membrane hyperpolarization of 4–8 mV, there was little detectable change in conductance. NMDA application caused an approximate 20 mV depolarization and an ‘apparent’ decrease in conductance, presumably due to the conductance pulse bringing the membrane into a voltage region where Mg²⁺ blocks the NMDA ionophore. 5HT further decreased conductance, which we propose is due to its enhancement of the voltage-dependent Mg²⁺ block. When the membrane potential was depolarized by ～20 mV via depolarizing current injection (to mimic the NMDA-induced depolarization), 5HT increased rather than decreased membrane conductance. Furthermore, 5HT did not affect the increase in membrane conductance following NMDA applications in zero Mg²⁺ saline. The results suggest that intrinsic, NMDA receptor-mediated voltage oscillations develop in a brief period after hatching, and that they depend upon the co-activation of 5HT and NMDA receptors. The enabling function of 5HT may involve the facilitation of the voltage-dependent block of the NMDA ionophore by Mg²⁺ through activation of receptors with 5HT_1a-like pharmacology.     

Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture

We used brainstem motoneurons recorded in organotypic slice co-cultures maintained for more than 18 days in vitro, together with multibarrel ionophoretic applications of glutamate receptor agonists and bath applications of specific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) iontophoresis, and express NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-amino-5-phosphonovaleric acid (APV)- and (-)1-(4-aminophenyl)-3-methyl-carbamoyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine [GYKI 53784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials with latencies distributed in three regularly spaced groups. Excitatory postsynaptic potentials (EPSPs) in the earliest group have a similar latency and time course and correspond to monosynaptic activation. EPSPs in later groups have more scattered latencies and time courses and correspond to polysynaptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-competitive AMPA receptor antagonist GYKI 53784 (LY303070). Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the frequency of spontaneous EPSPs without modifying their shape or amplitude. We conclude that excitatory synapses on brainstem motoneurons in vitro are mainly activated through AMPA receptors (AMPA-Rs). NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic transmission within the premotor interneuronal network.     

Modulation of kainate-induced responses by pentobarbitone and GYKI-53784 in rat abducens motoneurons in vivo

The modulation of kainate-induced responses by pentobarbitone and the 2,3-benzodiazepine GYKI-53784 (LY303070), a potent non-competitive AMPA antagonist, was studied in vivo using both extracellular recordings of antidromic field potentials and intracellular recordings from abducens motoneurons in ketamine/diazepam-anesthetized rats. In previous studies on pentobarbitone-anesthetized rats [M. Ouardouz, J. Durand, GYKI-52466 antagonizes glutamate responses but not NMDA and kainate responses in rat abducens motoneurons, Neurosci. Lett. 125 (1991) 5-8; M. Ouardouz, J. Durand, Involvement of AMPA receptors in trigeminal postsynaptic potentials recorded in rat abducens motoneurons in vivo, Eur. J. Neurosci. 6 (1994) 1662-1668; A. Ruiz, J. Durand, Blocking the trigeminal EPSPs in rat abducens motoneurons in vivo with the AMPA antagonists, NBQX and GYKI-53655, J. Neurophysiol. (1998) submitted], we showed that 2,3-benzodiazepines do not affect kainate-induced depolarizations in abducens motoneurons. Here, we tested whether pentobarbitone is involved in the pharmacological discrimination by 2,3-benzodiazepines between AMPA- and kainate-induced responses. Kainate-induced depolarizations were reversibly depressed after application of either GYKI-53784 and pentobarbitone. However, kainate-induced depolarizations were not inhibited by GYKI-53784 with pentobarbitone; they were even potentiated sometimes. Using extracellular recordings, we confirmed that in the presence of pentobarbitone, GYKI-53784 counteracts the effects of AMPA but not of kainate on antidromic field potentials in the abducens nucleus. Blockade of kainate-induced responses by GYKI-53784 was reversed with pentobarbitone, which appears relevant to the discrimination between AMPA- and kainate receptor-mediated responses in vivo. In the presence of pentobarbitone, kainate would depolarize motoneurons mainly via kainate receptors since kainate-induced responses were not depressed by 2,3-benzodiazepines. This finding strongly favors the existence of kainate receptors in adult motoneurons but their role is still unknown.     

Electrophysiological evidences for the contribution of NMDA receptors to the inhibition of clonidine on the RVLM presympathetic neurons

The main objective of this study is to test the hypothesis that N-methyl-D-aspartate (NMDA) receptors within the rostral ventrolateral medulla (RVLM) are involved in the inhibition of clonidine on the RVLM presympathetic neurons. Totally, 22 presympathetic neurons were recorded in anesthetized and paralyzed rats. The majority of these neurons (n=16 of 22) were significantly inhibited by iontophoretic (30 nA) clonidine, the other 6 neurons were insensitive to clonidine. In seven clonidine-sensitive neurons, iontophoretic clonidine (30 nA) antagonized the neuronal excitation of iontophoretic NMDA receptor agonist NMDA (20 nA). In remaining nine clonidine-sensitive neurons, iontophoretic NMDA receptor antagonist MK801 (60 nA) significantly attenuated the neuronal inhibition of iontophoretic (30 nA) clonidine. In conclusion, these results suggest that NMDA receptors contribute to the inhibition of clonidine on the RVLM presympathetic neurons.     

Influence of MK-801 on brain extracellular calcium and potassium activities in severe hypoglycemia

The purpose of the present study was to examine the effect of blockade of N-methyl-D-aspartate (NMDA) receptors on the depolarization associated with severe hypoglycemia, which is commonly preceded by one or a few transient depolarizations reminiscent of cortical spreading depression (CSD). In the cerebral cortices of rats [K+]e and [Ca2+]e were measured with ion-selective microelectrodes. NMDA blockade was achieved by injection of MK801 in doses that block CSD. In control rats, the latency from the time point when blood glucose reached minimal levels to onset of ionic shifts was 33.2 +/- 3.5 min, and [K+]e rose from 3.2 +/- 0.2 to 55 +/- 5 mM. All variables remained unchanged in rats treated with MK801. In another four rats treated with MK801, [Ca2+]e declined from 1.06 +/- 0.22 to 0.12 +/- 0.02 mM. Plasma glucose measurements indicated that the cortex depolarized at a plasma glucose concentration between 0.7 and 0.8 mM, i.e., within a narrow range, suggesting a threshold phenomenon. In conclusion, activation of NMDA receptors seems of minor importance for hypoglycemic depolarization. The ionic transients that precede the persistent hypoglycemic depolarization are probably mediated by mechanisms distinct from those of electrically induced CSD.     

Electrotonic and chemical EPSPs in lamprey motor neurons following stimulation of the descending tract and posterior root afferents]

In experiments carried out on the isolated spinal cord of the lamprey (Lampetra fluviatilis) post-synaptic responses produced in spinal motoneurons by stimulation of the descending tract and dorsal roots were investigated by means of the intracellular recording technique. It is found that, in addition to giant reticulospinal (Müller's) axons, many other descending fibres as well as dorsal root afferents establish synaptic linkage with both chemical and electrical mode of transmission as revealed by their sensitivity to calcium-deficient and magnesium-rich perfusing solutions. Complex electrotonic EPSPs could have a very fast time course characteristic for the elementary responses but could also produce slow depolarization of the post-synaptic membrane, thus suggesting an integrative function of electrical synapses.     

NMDA Receptor Activation Triggers Voltage Oscillations, Plateau Potentials and Burstinq in Neonatal Rat Lumbar Motoneurons ln Vitro

Whole-cell recordings of lumbar motoneurons in the intact neonatal rat spinal cord in vitro were undertaken to examine the effects of Kmethyl-D-aspartate (NMDA) receptor activation on membrane behaviour. Bath application of NMDA induced rhythmic voltage oscillations of 5.9 ± 2.1 mV (SD) at a frequency of 4.4 ± 1.5 Hz. Amplitude, but not frequency, of the voltage oscillations was membrane potential-dependent. Voltage oscillations could recruit action potentials and/or plateau potentials with or without superimposed bursting. Blockade of synaptic transmission with tetrodotoxin (TTX) sometimes resulted in a loss of oscillatory activity which could then be restored by increasing the NMDA concentration. After application of TTX, the trajectory of NMDA-induced oscillations was similar to the trajectory induced in the presence of intact synaptic networks, although the mean oscillation duration was longer and the oscillation frequency was slower (1.8 ± 1.1 Hz). Current ramps delivered after bath application of NMDA demonstrated bistable membrane properties which may underlie the plateau potentials. Injection of intracellular current pulses could initiate, entrain and terminate individual plateau potentials. The results suggest that membrane depolarization produced by oscillations may activate other intrinsic conductances which generate plateau potentials, thereby providing the neuron with enhanced voltage sensitivity, compared to that produced by NMDA receptor activation alone. These oscillatory events may have a role in the regulation of motor output in a variety of rhythmic behaviours including locomotion.     

Small Aβ1–42 oligomer‐induced membrane depolarization of neuronal and microglial cells: Role of N‐methyl‐D‐aspartate receptors

Although it is well documented that soluble beta amyloid (Aβ) oligomers are critical factors in the pathogenesis of Alzheimer's disease (AD) by causing synaptic dysfunction and neuronal death, the primary mechanisms by which Aβ oligomers trigger neurodegeneration are not entirely understood. We sought to investigate whether toxic small Aβ_1–42 oligomers induce changes in plasma membrane potential of cultured neurons and glial cells in rat cerebellar granule cell cultures leading to neuronal death and whether these effects are sensitive to the N‐methyl‐D‐aspartate receptor (NMDA‐R) antagonist MK801. We found that small Aβ_1–42 oligomers induced rapid, protracted membrane depolarization of both neurons and microglia, whereas there was no change in membrane potential of astrocytes. MK801 did not modulate Aβ‐induced neuronal depolarization. In contrast, Aβ_1?42 oligomer‐induced decrease in plasma membrane potential of microglia was prevented by MK801. Small Aβ_1–42 oligomers significantly elevated extracellular glutamate and caused neuronal necrosis, and both were prevented by MK801. Also, small Aβ_1–42 oligomers decreased resistance of isolated brain mitochondria to calcium‐induced opening of mitochondrial permeability transition pore. In conclusion, the results suggest that the primary effect of toxic small Aβ oligomers on neurons is rapid, NMDA‐R‐independent plasma membrane depolarization, which leads to neuronal death. Aβ oligomers‐induced depolarization of microglial cells is NMDA‐R dependent. © 2014 Wiley Periodicals, Inc.     

Synaptic actions of vagal afferents on facial motoneurons in the cat

Synaptic potentials in facial motoneurons of cats were intracellularly recorded on stimulation of the vagal nerve, superior laryngeal nerve, solitary tract nucleus and spinal trigeminal tract nucleus. A possible disynaptic excitation was elicited in the facial motoneurons by stimulation of the vagal nerves and superior laryngeal nerves on both sides. Activation of the neurons in the solitary tract nucleus and/or trigeminal tract nucleus induced monosynaptic excitatory postsynaptic potentials (EPSPs) in the facial motoneurons.     

A plateau potential mediated by the activation of extrasynaptic NMDA receptors in rat hippocampal CA1 pyramidal neurons

Hippocampal pyramidal neurons express various extrasynaptic glutamate receptors. When glutamate spillover was facilitated by blocking glutamate uptake and fast synaptic transmission was blocked by antagonists of AMPA- and NMDA-type glutamate receptors and an ionotropic GABA receptor blocker, repetitive synaptic stimulation evoked a persistent membrane depolarization that consisted of an early Ca²⁺-independent component and a late Ca²⁺-dependent component. The early component, which we refer to as a plateau potential, had a half-width of 770 ± 160 ms and a steady peak level of −9.54 ± 3.50 mV. It was accompanied by an increase in membrane conductance, the I–V relationship of which showed a peak at −19.91 ± 2.18 mV and reversal of the current at −4.32 ± 2.13 mV, and was suppressed by high concentration of an NMDA receptor (NMDAR) antagonist d -APV, or an NMDAR glycine-binding site antagonist 5,7-dCK. After blocking synaptically located NMDARs using MK801, the potential was still evoked synaptically when spillover was facilitated. A sustained depolarization was evoked by iontophoretic application of glutamate in the presence or absence of a glutamate uptake blocker. This potential was not affected by Na⁺ or Ca²⁺ channel blockers, but was suppressed by 5,7-dCK, leaving an unspecified depolarizing potential. Iontophoresis of NMDA evoked a sustained depolarization that was blocked by a high concentration of d -APV or 5,7-dCK. The I–V relationship of the current during this potential was similar to that obtained during the synaptically induced plateau potentials. These results show that CA1 pyramidal neurons generate plateau potentials mediated most likely by activation of extrasynaptic NMDARs.     

Glutamate and glycine induce a negative wave on hippocampal field response through NMDA receptors

In rats under urethane anesthesia, iontophoresis of large amounts (30-300 nA) of glutamate in the hippocampus induced a negative wave on the field potential evoked by stimulation of fimbria/commissura or perforant pathway. The amplitudes of the negative waves ranged between 0.2 and 9.8 mV and their mean duration was 341 +/- 12 ms. This activity was antagonized by iontophoresis of N-methyl-D-aspartate (NMDA) antagonists: Mg2+ (80-100 nA), ketamine (50-150 nA), MK-801 (50-150 nA) and by systemic ketamine (5 mg/kg, i.v.) administration. Iontophoresis of N-methyl-DL-aspartate (NMDLA) (20-40 nA) and glycine (25-100 nA) also elicited a negative wave which was blocked by NMDA antagonists. The negative waves were induced in all hippocampal layers except the dentate hilus by glutamate, NMDLA and glycine. Pyramidal regions were found to be as sensitive as dendritic layers; the mean amplitudes of glutamate-induced negative waves on the field response were 4.1 +/- 0.6 and 4.2 +/- 0.5 mV for CA1 stratum pyramidale and radiatum, respectively. These data suggest that large amounts of glutamate activate NMDA receptor/ion channels causing appearance of a long-lasting negative wave on the hippocampal field response. The data also demonstrate that glycine leads to a significant participation of NMDA receptors during glutamatergic transmission which is largely mediated through non-NMDA receptors.     

NK-3 receptor activation depolarizes and induces an after-depolarization in pyramidal neurons in gerbil cingulate cortex

The involvement of tachykinins in cortical function is poorly understood. To study the actions of neurokinin-3 (NK3) receptor activation in frontal cortex, whole cell patch clamp recordings were performed from pyramidal neurons in slices of cingulate cortex from juvenile gerbils. Senktide (500nM), a selective NK3 receptor agonist, induced a transient increase in spontaneous EPSPs in layer V pyramidal neurons, accompanied by a small depolarization ( approximately 4 mV). EPSPs during senktide had a larger amplitude and faster 10-90% rise time than during control. Senktide induced a transient depolarization in layer II/III pyramidal neurons, which often reached threshold for spikes. The depolarization ( approximately 6 mV) persisted in TTX, and was accompanied by an increase in input resistance. Senktide also transiently induced a slow after-depolarization, which appeared following a depolarizing pulse. The slow after-depolarization persisted in TTX. These data suggest that activation of NK3 receptors on layer II/III pyramidal neurons induce post-synaptic depolarization and an after-depolarization, which could be mediated by blockade of a leak potassium conductance and a non-selective cation conductance, respectively.     

Organization of hindlimb muscle afferent projections to lumbosacral motoneurons in the chick embryo.

We have examined the organization of muscle afferent projections to motoneurons in the lumbosacral spinal cord of chick embryos between stage 37, when muscle afferents first reach the motor nucleus, and stage 44, which is just before hatching. Connectivity between afferents and motoneurons was assessed by stimulating individual muscle nerves and recording the resulting motoneuron synaptic potentials intracellularly or electrotonically from other muscle nerves. Most of the recordings were made in the presence of DL-2-amino-5-phosphonovaleric acid (APV), picrotoxin, and strychnine to block long-latency excitatory and inhibitory pathways. Activation of muscle afferents evoked slow, positive potentials in muscle nerves but not in cutaneous nerves. These potentials were abolished in 0 mM Ca2+, 2mM Mn2+ solutions, indicating that they were generated by the action of chemical synapses. The muscle nerve recordings revealed a wide-spread pattern of excitatory connections between afferents and motoneurons innervating six different thigh muscles, which were not organized according to synergist-antagonist relationships. This pattern of connectivity was confirmed using intracellular recording from identified motoneurons, which allowed the latency of the responses to be determined. Short-latency potentials in motoneurons were produced by activation of homonymous afferents and the heteronymous afferents innervating the hip flexors sartorius and anterior iliotibialis. Stimulation of anterior iliotibialis afferents also resulted in some short-latency excitatory postsynaptic potentials (EPSPs) in motoneurons innervating the knee extensor femorotibialis, though other connections were of longer latency. Afferents from the adductor, a hip extensor, did not evoke short-latency EPSPs in any of these three types of motoneurons. Short-latency, but not long-latency EPSPs, persisted during repetitive stimulation at 5 Hz, suggesting that they were mediated monosynaptically. Long-latency, fatigue-sensitive potentials were maintained in the presence of APV, picrotoxin, and strychnine, suggesting that polysynaptic pathways utilize non-NMDA receptors as well as NMDA receptors. We found no difference in the pattern of inputs to femorotibialis motoneurons between stage 37-39 and near hatching at stage 44, suggesting muscle afferent projections to these motoneurons are correct at stage 37, when the afferents first reach the lateral motor column in substantial numbers.     

Plateau Potentials in Cat Neocortical Association Cells In Viva Synaptic Control of Dendritic Excitability

The dendrites of neocortical pyramidal cells are bombarded by myriads of synaptic inputs and express active conductances generating prominent plateau potentials. We have examined in vivo the possibility that spontaneous synaptic inputs trigger or terminate plateau potentials after blockage of K⁺ currents. Under barbiturate anaesthesia, pairs of cortical cells were intracellularly recorded with sharp electrodes from the cat's association cortex (areas 5–7). In one pyramidal cell, K⁺ channels were blocked with intracellular Cs⁺, while in the simultaneously impaled pyramidal cell the K⁺ conductances were left intact to act as a control; this second cell allowed recognition of spontaneous spindle-related synaptic activity. Depolarizing current pulses elicited single, all-or-none plateau potentials (60–70 mV, 0.1–0.4 s). Plateau potentials slowly repolarized towards a break point of fast repolarization around -20 mV. Thalamic-evoked inhibitory postsynaptic potentials consistently shut off the plateaus. Synchronized spontaneous activity, as occurring during thalamic-generated spindle oscillations, either triggered or blocked the plateaus. These results suggest that spontaneously occurring synaptic activation during synchronized oscillatory states, such as those that occur during sleep spindles in vivu , may exert a strong control over the dendritic excitability in neocortical pyramidal cells.     

The cortically evoked secondary depolarization affects the integrative properties of thalamic reticular neurons

Corticothalamic terminals on thalamic reticular (RE) neurons account for most synapses from afferent pathways onto this nucleus and these inputs are more powerful than those from axon collaterals of thalamocortical neurons. Given the supremacy of cortical inputs, we analysed here the characteristics and possible mechanisms underlying a secondary component of the cortically elicited depolarization in RE neurons, recorded in cats under barbiturate anesthesia. Electrical stimulation of corticothalamic axons in the internal capsule evoked fixed and short-latency excitatory postsynaptic potentials (EPSPs) that, by increasing stimulation intensity and at hyperpolarized levels (< -70 mV), developed into low-threshold spikes and spindle oscillations. The threshold for spindle oscillations was 60% higher than that required for evoking minimal EPSPs. The evoked EPSPs included a secondary depolarizing component, which appeared approximately 5 ms after the peak of the initial component and was voltage dependent, i.e. most prominent between -70 mV and -85 mV, while being greatly reduced or absent at more hyperpolarized levels. The secondary depolarizing component was sensitive to QX-314 in the recording micropipette. We suggest that the secondary component of cortically evoked EPSPs in RE neurons is due to the dendritic activation of T-currents, with a probable contribution of the persistent Na+ current. This late component affected the integrative properties of RE neurons, including their spiking output and temporal summation of incoming cortical inputs.