Latency EPSPs Research Articles

β2-Adrenergic Receptor Activation Suppresses the Rat Phenethylamine Hallucinogen-Induced Head Twitch Response: Hallucinogen-Induced Excitatory Post-synaptic Potentials as a Potential Substrate.

5-Hydroxytryptamine2A (5-HT2A) receptors are enriched in layers I and Va of the rat prefrontal cortex and neocortex and their activation increases the frequency of glutamatergic excitatory post-synaptic potentials/currents (EPSP/Cs) onto layer V pyramidal cells. A number of other G-protein coupled receptors (GPCRs) are also enriched in cortical layers I and Va and either induce (α1-adrenergic and orexin2) or suppress (metabotropic glutamate2 [mGlu2], adenosine A1, μ-opioid) both 5-HT-induced EPSCs and head twitches or head shakes induced by the phenethylamine hallucinogen 2,5-dimethoxy-4-iodoamphetamine (DOI). Another neurotransmitter receptor also localized to apparent thalamocortical afferents to layers I and Va of the rat prefrontal cortex and neocortex is the β2-adrenergic receptor. Therefore, we conducted preliminary electrophysiological experiments with rat brain slices examining the effects of epinephrine on electrically-evoked EPSPs following bath application of DOI (3 μM). Epinephrine (0.3–10 μM) suppressed the late EPSPs produced by electrical stimulation and DOI. The selective β2-adrenergic receptor antagonist ICI-118,551 (300 nM) resulted in a rightward shift of the epinephrine concentration-response relationship. We also tested the selective β2-adrenergic receptor agonist clenbuterol and the antagonist ICI-118,551 on DOI-induced head twitches. Clenbuterol (0.3–3 mg/kg, i.p.) suppressed DOI (1.25 mg/kg, i.p.)-induced head twitches. This clenbuterol effect appeared to be at least partially reversed by the selective β2-adrenergic receptor antagonist ICI-118,553 (0.01–1 mg/kg, i.p.), with significant reversal at doses of 0.1 and 1 mg/kg. Thus, β2-adrenergic receptor activation reverses the effects of phenethylamine hallucinogens in the rat prefrontal cortex. While Gi/Go-coupled GPCRs have previously been shown to suppress both the electrophysiological and behavioral effects of 5-HT2A receptor activation in the mPFC, the present work appears to extend this suppressant action to a Gs-coupled GPCR. Furthermore, the modulation of 5-HT2A receptor activation-induced glutamate release onto mPFC layer V pyramidal neurons apical dendrites by a range GPCRs in rat brain slices appears to results in behaviorally salient effects of relevance when screening for novel CNS therapeutic drugs.

Differential effects of phytotherapic preparations in the hSOD1 Drosophila melanogaster model of ALS

The present study was aimed at characterizing the effects of Withania somnifera (Wse) and Mucuna pruriens (Mpe) on a Drosophila melanogaster model for Amyotrophic Lateral Sclerosis (ALS). In particular, the effects of Wse and Mpe were assessed following feeding the flies selectively overexpressing the wild human copper, zinc-superoxide dismutase (hSOD1-gain-of-function) in Drosophila motoneurons. Although ALS-hSOD1 mutants showed no impairment in life span, with respect to GAL4 controls, the results revealed impairment of climbing behaviour, muscle electrophysiological parameters (latency and amplitude of ePSPs) as well as thoracic ganglia mitochondrial functions. Interestingly, Wse treatment significantly increased lifespan of hSDO1 while Mpe had not effect. Conversely, both Wse and Mpe significantly rescued climbing impairment, and also latency and amplitude of ePSPs as well as failure responses to high frequency DLM stimulation. Finally, mitochondrial alterations were any more present in Wse- but not in Mpe-treated hSOD1 mutants. Hence, given the role of inflammation in the development of ALS, the high translational impact of the model, the known anti-inflammatory properties of these extracts, and the viability of their clinical use, these results suggest that the application of Wse and Mpe might represent a valuable pharmacological strategy to counteract the progression of ALS and related symptoms.

Intracortical Circuits in Rat Anterior Cingulate Cortex Are Activated by Nociceptive Inputs Mediated by Medial Thalamus

We investigated the afferents and intracortical synaptic organization of the anterior cingulate cortex (ACC) during noxious electrical stimulation. Extracellular field potentials were recorded simultaneously from 16 electrodes spanning all layers of the ACC in male Sprague-Dawley rats anesthetized by halothane inhalation. Laminar-specific transmembrane currents were calculated with the current source density analysis method. Two major groups of evoked sink currents were identified: an early group (latency = 54.04 +/- 2.12 ms; 0.63 +/- 0.07 mV/mm(2)) in layers V-VI and a more intense late group (latency = 80.07 +/- 4.85 ms; 2.16 +/- 0.22 mV/mm(2)) in layer II/III and layer V. Multiunit activities were evoked mainly in layer V and deep layer II/III with latencies similar to that of the early and late sink groups. The evoked EPSP latencies of pyramidal neurons in layers II/III and V related closely with the sink currents. The sink currents were inhibited by intracortical injection of CNQX (1 mM, 1 microl), a glutaminergic receptor antagonist, and enhanced by intraperitoneal (5 mg/kg) and intracortical (10 microg/microl, 1 microl) injection of morphine, a mu-opioid receptor agonist. Paired-pulse depression was observed with interpulse intervals of 50 to 1,000 ms. High-frequency stimulation (100 Hz, 11 pulses) enhanced evoked responses in the ACC and evoked medial thalamic (MT) unit activities. MT lesions blocked evoked responses in the ACC. Our results demonstrated that two distinct synaptic circuits in the ACC were activated by noxious stimuli and that the MT is the major thalamic relay that transmits nociceptive information to the ACC.

Efficacy and connectivity of intracolumnar pairs of layer 2/3 pyramidal cells in the barrel cortex of juvenile rats

Synaptically coupled layer 2/3 (L2/3) pyramidal neurones located above the same layer 4 barrel ('barrel-related') were investigated using dual whole-cell voltage recordings in acute slices of rat somatosensory cortex. Recordings were followed by reconstructions of biocytin-filled neurones. The onset latency of unitary EPSPs was 1.1 +/- 0.4 ms, the 20-80% rise time was 0.7 +/- 0.2 ms, the average amplitude was 1.0 +/- 0.7 mV and the decay time constant was 15.7 +/- 4.5 ms. The coefficient of variation (c.v.) of unitary EPSP amplitudes decreased with increasing EPSP peak and was 0.33 +/- 0.18. Bursts of APs in the presynaptic pyramidal cell resulted in EPSPs that, over a wide range of frequencies (5-100 Hz), displayed amplitude depression. Anatomically the barrel-related pyramidal cells in the lower half of layer 2/3 have a long apical dendrite with a small terminal tuft, while pyramidal cells in the upper half of layer 2/3 have shorter and often more 'irregularly' shaped apical dendrites that branch profusely in layer 1. The number of putative excitatory synaptic contacts established by the axonal collaterals of a L2/3 pyramidal cell with a postsynaptic pyramidal cell in the same column varied between 2 and 4, with an average of 2.8 +/- 0.7 (n = 8 pairs). Synaptic contacts were established predominantly on the basal dendrites at a mean geometric distance of 91 +/- 47 mum from the pyramidal cell soma. L2/3-to-L2/3 connections formed a blob-like innervation domain containing 2.8 mm of the presynaptic axon collaterals with a bouton density of 0.3 boutons per mum axon. Within the supragranular layers of its home column a single L2/3 pyramidal cell established about 900 boutons suggesting that 270 pyramidal cells in layer 2/3 are innervated by an individual pyramidal cell. In turn, a single pyramidal cell received synaptic inputs from 270 other L2/3 pyramidal cells. The innervation domain of L2/3-to-L2/3 connections superimposes almost exactly with that of L4-to-L2/3 connections. This suggests that synchronous feed-forward excitation of L2/3 pyramidal cells arriving from layer 4 could be potentially amplified in layer 2/3 by feedback excitation within a column and then relayed to the neighbouring columns.

Neuronal relays in double crossed pathways between feline motor cortex and ipsilateral hindlimb motoneurones

Coupling between pyramidal tract (PT) neurones and ipsilateral hindlimb motoneurones was investigated by recording from commissural interneurones interposed between them. Near maximal stimulation of either the left or right PT induced short latency EPSPs in more than 80% of 20 commissural interneurones that were monosynaptically excited by reticulospinal tract fibres in the medial longitudinal fascicle (MLF). The EPSPs were evoked at latencies that were only 1-2 ms longer than those of EPSPs evoked from the MLF, compatible with a disynaptic coupling between PT fibres and these commissural interneurones. EPSPs evoked by PT stimulation were frequently associated with IPSPs which either followed or preceded the EPSPs. The latencies of the IPSPs (on average about 1 ms longer than latencies of the earliest EPSPs) indicated that they were mediated via single additional inhibitory interneurones. Records from a sample of nine commissural interneurones from a different population (with monosynaptic input from group I and/or II muscle afferents, and disynaptically excited from the MLF) suggest that actions of PT fibres on such interneurones are weaker because only four of them were excited by PT stimuli and at longer latencies. By demonstrating disynaptic coupling between PT neurones and commissural interneurones via reticulospinal fibres, the results provide a direct demonstration of trisynaptic coupling in the most direct pathways between PT neurones and ipsilateral motoneurones, and thereby strengthen the proposal that the double crossed pathways between PT neurones and ipsilateral motoneurones might be used to replace crossed actions of damaged PT neurones.

Neurotrophic Regulation of Calcium Channels by the Peptide Neurotransmitter Luteinizing Hormone Releasing Hormone

We exploited the simple organization of bullfrog paravertebral sympathetic ganglia (BFSG) to test whether the neurotransmitter peptide luteinizing hormone releasing hormone (LHRH), which generates the late slow EPSP, could also exert long-term neurotrophic control of ion channel expression. Whole-cell recordings from B-cells in BFSG showed that removal of all of the sources of ganglionic LHRH for 10 d by cutting preganglionic C-fibers in vivo caused a 28% reduction in Ca2+ current density. When BFSG B-neurons were dissociated from adult bullfrogs and maintained in a defined-medium, neuron-enriched, low-density, serum-free culture, the ICa density was increased by 49% after 6-7 d in the presence of 0.45 microm LHRH. This increase was not associated with alterations in the voltage dependence of Ca2+ current activation or inactivation and reflected a selective increase in N-type Ca2+ channel current. The increase in ICa density induced by LHRH was blocked by the transcription inhibitor actinomycin D. These results suggest that chronic exposure to a neurotransmitter that acts through G-protein-coupled receptors exerts long-term control of ion channel expression in a fully differentiated, adult sympathetic neuron in vitro or in vivo.

Differential presynaptic inhibition of actions of group II afferents in di- and polysynaptic pathways to feline motoneurones.

The aim of this study was to investigate differences in the effects of presynaptic inhibition of transmission from group II muscle afferents to neurones in the dorsal horn and in the intermediate zone and the consequences of these differences for reflex actions of group II afferents upon alpha-motoneurones. The degree of presynaptic inhibition was estimated from the degree of depression of monosynaptic components of population EPSPs (field potentials) evoked by group II muscle afferents in deeply anaesthetized cats. The decrease in the area of field potentials was considerably larger and longer lasting in the intermediate zone, where they were often obliterated, than in the dorsal horn, where they were reduced to about two-thirds. Presynaptic inhibition of field potentials evoked by other afferents at the same locations was much weaker. Intracellular records from alpha-motoneurones revealed that short latency EPSPs and IPSPs evoked from group II afferents are considerably reduced by conditioning stimuli that effectively depress intermediate zone field potentials of group II origin. The results of this study lead to the conclusion that strong presynaptic inhibition of transmission to intermediate zone interneurones allows a selective depression of disynaptic actions of group II muscle afferents on alpha- and gamma-motoneurones, mediated by these interneurones, and favours polysynaptic actions of these afferents.

Effects of acute amphetamine in the mediodorsal nucleus of the thalamus: possible relevance for psychiatric disorders Amphetamine administration alters thalamic activity

Disruptions in the thalamocortical circuit have been related to several psychiatric disorders, such as schizophrenia and obsessive-compulsive disorders. The dopaminergic/GABAergic afferents to the limbic thalamocortical circuit originate in the ventral tegmental area (VTA) and play a crucial role in the regulation of the normal cognitive functions mediated by this circuit. Moreover, it has been shown that the dopaminergic modulatory system is altered in schizophrenics. Despite the strong clinical and behavioral evidence of the importance of the thalamocortical circuit and the mesocortical dopamine (DA) system for normal cognitive function, little is known about the basic synaptic interactions between the thalamus and VTA. This research hypothesized that VTA stimulation would evoke DAergic and/or GABAergic-mediated responses in the MD thalamic neurons. Using intracellular recordings in vivo, it was found that VTA stimulation evoked short latency EPSPs in 73% of the recorded MD neurons and IPSPs-like responses in 16% of the MD cells. The nature of the VTA-MD projection was explored using DAergic and GABAergic drugs. It was found that the indirect DAergic agonist amphetamine affects the spontaneous and VTA-mediated responses recorded in the MD thalamus. These results indicate that the VTA projection contributes to the modulation of MD activity and that the disruption of such modulation may be relevant in neuropsychiatry disorders.

Effects of acute amphetamine in the mediodorsal nucleus of the thalamus: possible relevance for psychiatric disorders: Amphetamine administration alters thalamic activity

Disruptions in the thalamocortical circuit have been related to several psychiatric disorders, such as schizophrenia and obsessive-compulsive disorders. The dopaminergic/GABAergic afferents to the limbic thalamocortical circuit originate in the ventral tegmental area (VTA) and play a crucial role in the regulation of the normal cognitive functions mediated by this circuit. Moreover, it has been shown that the dopaminergic modulatory system is altered in schizophrenics. Despite the strong clinical and behavioral evidence of the importance of the thalamocortical circuit and the mesocortical dopamine (DA) system for normal cognitive function, little is known about the basic synaptic interactions between the thalamus and VTA. This research hypothesized that VTA stimulation would evoke DAergic and/or GABAergic-mediated responses in the MD thalamic neurons. Using intracellular recordings in vivo, it was found that VTA stimulation evoked short latency EPSPs in 73% of the recorded MD neurons and IPSPs-like responses in 16% of the MD cells. The nature of the VTA-MD projection was explored using DAergic and GABAergic drugs. It was found that the indirect DAergic agonist amphetamine affects the spontaneous and VTA-mediated responses recorded in the MD thalamus. These results indicate that the VTA projection contributes to the modulation of MD activity and that the disruption of such modulation may be relevant in neuropsychiatry disorders.

Differential Effect of TEA on Long-Term Synaptic Modification in Hippocampal CA1 and Dentate Gyrus in vitro

The effectiveness of tetraethylammonium (TEA) and high-frequency stimulation (HFS) in inducing long-term synaptic modification is compared in CA1 and dentate gyrus (DG) in vitro. High-frequency stimulation induces long-term potentiation (LTP) at synapses of both perforant path-DG granule cell and Schaffer collateral-CA1 pyramidal cell pathways. By contrast, TEA (25 mM) induces long-term depression in DG while inducing LTP in CA1. The mechanisms underlying the differential effect of TEA in CA1 and DG were investigated. It was observed that T-type voltage-dependent calcium channel (VDCC) blocker, Ni2+ (50 μM), partially blocked TEA-induced LTP in CA1. A complete blockade of the TEA-induced LTP occurred when Ni2+ was applied together with the NMDA receptor antagonist, D-APV. The L-type VDCC blocker, nifidipine (20 μM), had no effect on CA1 TEA-induced LTP. In DG of the same slice, TEA actually induced long-term depression (LTD) instead of LTP, an effect that was blocked by D-APV. Neither T-type nor L-type VDCC blockade could prevent this LTD. When the calcium concentration in the perfusion medium was increased, TEA induced a weak LTP in DG that was blocked by Ni2+. During exposure to TEA, the magnitude of field EPSPs was increased in both CA1 and DG, but the increase was substantially greater in CA1. Tetraethylammonium application also was associated with a large, late EPSP component in CA1 that persisted even after severing the connections between CA3 and CA1. All of the TEA effects in CA1, however, were dramatically reduced by Ni2+. The results of this study indicate that TEA indirectly acts via both T-type VDCCs and NMDA receptors in CA1 and, as a consequence, induces LTP. By contrast, TEA indirectly acts via only NMDA receptors in DG and results in LTD. The results raise the possibility of a major synaptic difference in the density and/or distribution of T-type VDCCs and NMDA receptors in CA1 and DG of the rat hippocampus.

Regulation of Limbic Information Outflow by the Subthalamic Nucleus: Excitatory Amino Acid Projections to the Ventral Pallidum

The subthalamic nucleus (STN), a component of the basal ganglia motor system, sends an excitatory amino acid (EAA)-containing projection to the ventral pallidum (VP), a major limbic system output region. The VP contains both NMDA and AMPA subtypes of EAA receptors. To characterize the physiology of the subthalamic pathway to the VP, and to determine the influence of EAA receptor subtypes, in vivo intracellular recordings, and in vivo extracellular recordings combined with microiontophoresis, were made from VP neurons in anesthetized rats. Of the intracellularly recorded neurons, 86% responded to STN stimulation, and these displayed EPSPs with an onset of 8.7 msec, consistent with a monosynaptic input. The EPSPs evoked in spontaneously firing neurons were nearly twice the amplitude of those in nonfiring cells (13.1 vs 6.8 mV, respectively). As neurons were depolarized by current injection, the latency for spiking decreased from 24.2 to 14.2 msec, although EPSP latency was unaffected. Eighty-seven percent of the extracellularly recorded VP neurons responded to STN stimulation with a rapid and robust enhancement of spiking; the response onset, like the EPSP onset, equaled 8.7 msec. Firing rate was enhanced by NMDA in 94% of the STN-excited cells, and AMPA increased firing in 94% as well. The NMDA-selective antagonist AP-5 attenuated 67% of the STN-evoked excitatory responses, and the AMPA-selective antagonist CNQX attenuated 52%. Both antagonists attenuated 33% of responses, and 78% were attenuated by at least one. This evidence suggests that a great majority of VP neurons are directly influenced by STN activation and that both NMDA and non-NMDA receptors are involved. Moreover, the VP response to STN stimulation appears to be strongly dependent on the depolarization state of the neuron.

Neuronal pathways from group-I and -II muscle afferents innervating hindlimb muscles to motoneurons innervating trunk muscles in low-spinal cats

Synaptic inputs to motoneurons innervating the back and abdominal muscles in the lumbar part of the body from low-threshold hindlimb muscle afferents were studied in unanesthetized low-spinal cats. At a stimulus intensity of 1.2-1.5x threshold (T), which was sufficient to activate only group-I afferents, the incidence of post-synaptic potentials (PSPs) was higher when stimulating proximal muscle nerves than when stimulating distal muscle nerves (e.g., 52% versus 22% for motoneurons innervating m. iliocostalis lumborum: Ilio MNs; 38% versus 18% for motoneurons innervating m. obliquus externus: OEA MNs). At 2-5 T, at which group-II as well as group-I muscle afferents were presumably stimulated, the PSP incidence increased irrespective of nerves stimulated (e.g., 76% for Ilio MNs; 60% for OEA MNs). The minimal central latencies of EPSPs evoked at 1.2-1.5 T ranged 0.8-16.7 ms for Ilio motoneurons and 1.4 -14.2 ms for OEA motoneurons, indicating that the connection between back and abdominal motoneurons and low-threshold afferents from the hindlimb muscle include a monosynaptic one. The latencies of IPSPs were longer and ranged 1.9-18.8 ms for Ilio motoneurons and 2.4-15.8 ms for OEA motoneurons. Input patterns from various hindlimb muscles varied among individual motoneurons, even though they were within the same motoneuron pool. Such synaptic organization seems to differ from that for the leg motoneuron pool. The overall projection pattern of low-threshold afferents from leg muscles to lumbar back and abdominal motoneurons nevertheless suggests that group-I afferent inputs are related to lateral and vertical movements, and that group-II afferent inputs control the stiffness of the trunk.

Striking Differences in Transmission of Corticospinal Excitation to Upper Limb Motoneurons in Two Primate Species

There is considerable debate as to the relative importance, for cortical control of upper limb movements, of direct cortico-motoneuronal (CM) versus indirect, propriospinal transmission of corticospinal excitation to cervical motoneurons. In the cat, which has no CM connections, a significant proportion of corticospinal excitation reaches forelimb motoneurons via a system of C(3)-C(4) propriospinal neurons (PN). In contrast, in the macaque monkey most motoneurons receive direct CM connections, and, under the same experimental conditions as in the cat, there is little evidence for PN transmission. We have investigated corticospinal transmission in the New World squirrel monkey (Saimiri sciureus) because its CM projections are weaker than in the macaque. Intracellular recordings were made from motoneurons identified from the ulnar, median, and deep radial (DR) nerves in four adult squirrel monkeys under chloralose anesthesia and neuromuscular paralysis. Responses to stimulation of the contralateral medullary pyramid were recorded before and after a lesion to the dorsolateral funiculus (DLF) at C(5), designed to interrupt direct corticospinal inputs to the lower cervical segments and unmask PN-mediated effects. This lesion greatly reduced the proportion of motoneurons showing either CM EPSPs or disynaptic IPSPs, but the proportion showing late EPSPs with segmental latencies beyond the monosynaptic range, evoked by repetitive but not single PT stimuli, was unaffected: 23 of 29 motoneurons (79%) before and 32 of 37 (86%) after the lesion; 41% of these late EPSPs had strictly disynaptic latencies after the lesion, only 14% before. These results are in striking contrast to the macaque (late EPSPs in only 18% of motoneurons before a C(5) lesion, 19% after it). Transmission of the late EPSPs via C(3)-C(4) PNs in the squirrel monkey was indicated by their absence after an additional C(2) DLF lesion. Nearly all tested motoneurons also responded with short latency EPSPs to stimulation in the ipsilateral lateral reticular nucleus. By analogy with the cat, these EPSPs probably reflect antidromic activation of ascending collaterals of C(3)-C(4) PNs with monosynaptic connections to motoneurons; the EPSPs were significantly smaller than in the cat but larger than in the macaque. These results suggest that the positive correlation across species between more advanced hand function and the strength of the CM system is accompanied by a negative correlation between hand function and the strength of the PN system. We hypothesize that in primates with more advanced hand function, the CM system effectively replaces PN-mediated control. This would include a contribution to the control of reaching movements, which are said to be specifically under the control of the PN system in the cat, and we speculate that these differences may be related to the degree of dexterity exhibited by the different species. This interpretation of the results predicts that in man, where the CM system is highly developed, the PN system is unlikely to be responsible for significant transmission of cortical commands to upper limb motoneurons.

Intrinsic firing patterns and whisker-evoked synaptic responses of neurons in the rat barrel cortex.

We have used whole cell recording in the anesthetized rat to study whisker-evoked synaptic and spiking responses of single neurons in the barrel cortex. On the basis of their intrinsic firing patterns, neurons could be classified as either regular-spiking (RS) cells, intrinsically burst-spiking (IB) cells, or fast-spiking (FS) cells. Some recordings responded to current injection with a complex spike pattern characteristic of apical dendrites. All cell types had high rates of spontaneous postsynaptic potentials, both excitatory (EPSPs) and inhibitory (IPSPs). Some spontaneous EPSPs reached threshold, and these typically elicited only single action potentials in RS cells, bursts of action potentials in FS cells and IB cells, and a small, fast spike or a complex spike in dendrites. Deflection of single whiskers evoked a fast initial EPSP, a prolonged IPSP, and delayed EPSPs in all cell types. The intrinsic firing pattern of cells predicted their short-latency whisker-evoked spiking patterns. All cell types responded best to one or, occasionally, two primary whiskers, but typically 6-15 surrounding whiskers also generated significant synaptic responses. The initial EPSP had a relatively fixed amplitude and latency, and its amplitude in response to first-order surrounding whiskers was approximately 55% of that induced by the primary whisker. Second- and third-order surrounding whiskers evoked responses of approximately 27 and 12%, respectively. The latency of the initial EPSP was shortest for the primary whiskers, longer for surrounding whiskers, and varied with the neurons' depth below the pia. EPSP latency was shortest in the granular layer, longer in supragranular layers, and longest in infragranular layers. The receptive field size, defined as the total number of fast EPSP-inducing whiskers, was independent of each cell's intrinsic firing type, its subpial depth, or the whisker stimulus parameters. On average, receptive fields included >10 whiskers. Our results show that single neurons integrate rapid synaptic responses from a large proportion of the mystacial vibrissae, and suggest that the whisker-evoked responses of barrel neurons are a function of both synaptic inputs and intrinsic membrane properties.

Does a C3-C4 propriospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey.

1. Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose-anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3-C4 propriospinal interneurones similar to that in the cat. 2. In monkeys with intact spinal cords, only a small proportion of motoneurones (19%) responded with late EPSPs to repetitive stimulation of the pyramid; only 3% had segmental latencies that were appropriate for a disynaptic pathway. 3. From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3-C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18%); 14% of motoneurones had EPSPs within the disynaptic range. 4. In addition, some EPSPs with relatively long segmental latencies (> 1.1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions. 5. In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1.2-2.5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3-C4 propriospinal neurones to the LRN, as demonstrated in the cat. 6. These short-latency EPSPs evoked from the LRN were just as common before (77%) as after (75%) the C5 lesion. They had small amplitudes both before (mean +/- s.d. 1.1 +/- 0.59 mV) and after (1.2 +/- 0.72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16%) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid. 7. The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3-C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.

Sensorimotor pathways involved in interjoint reflex action of an insect leg

Coordination of motor output between leg joints is crucial for the generation of posture and active movements in multijointed appendages of legged organisms. We investigated in the stick insect the information flow between the middle leg femoral chordotonal organ (fCO), which measures position and movement in the femur-tibia (FT) joint and the motoneuron pools supplying the next proximal leg joint, the coxa-trochanteral (CT) joint. In the inactive animal, elongation of the fCO (by flexing the FT joint) induced a depolarization in eight of nine levator trochanteris motoneurons, with a suprathreshold activation of one to three motoneurons. Motoneurons of the depressor trochanteris muscle were inhibited by fCO elongation. Relaxation signals, i.e., extension of the FT joint, activated both levator and depressor motoneurons; i.e., both antagonistic muscles were coactivated. Monosynaptic as well as polysynaptic pathways contribute to interjoint reflex actions in the stick insect leg. fCO afferents were found to induce short latency EPSPs in levator motoneurons, providing evidence for direct connections between fCO afferents and levator motoneurons. In addition, neuronal pathways via intercalated interneurons were identified that transmit sensory information from the fCO onto levator and/or depressor motoneurons. Finally, we describe two kinds of alterations in interjoint reflex action: (a) With repetitive sensory stimulation, this interjoint reflex action shows a habituation-like decrease in strength. (b) In the actively moving animal, interjoint reflex action in response to fCO elongation, mimicking joint flexion, qualitatively remained the same sign, but with a marked increase in strength, indicating an increased influence of sensory signals from the FT joint onto the adjacent CT joint in the active animal.

Instrumental conditioning of the activity of putative command neurons in the mollusk Helix

Gaining insight into the mechanism of generation of goal-directed actions is important in understanding neural function. In this study we examined the role of the action potential (AP) in a single molluscan neuron (responsible for a defensive response) in an instrumental behavior. The intracellular electrical activity of two neurons was recorded simultaneously. One neuron was trained and the other served as a control neuron. When the trained neuron produced an AP in response to a conditioned stimulus (CS), the mollusc did not receive a painful stimulus. Delivery of the painful stimulus did not depend on the response of the control neuron. The number of AP's in a trained neuron, the AP latency and the threshold revealed a bell-shaped dependence on learning, whereas the response of the control neuron to a CS decreased during learning. It is apparently feasible to elaborate this type of instrumental reflex, so that the discharge of a single neuron may serve as an instrumental action for the entire animal. The membrane potential in a trained neuron varies significantly during instrumental learning, but the changes do not correspond to the dynamics of the instrumental reaction in the response to a CS. The control neuron exhibited weak but significant hyperpolarization during learning. The onset of the EPSP is determined by the timing of AP generation in presynaptic neurons. However, it changed in the trained neuron during the elaboration of an instrumental reflex. The alterations in the latency of EPSP's during learning were significant, but were not consistent with the time history of the conditioned response. Therefore, although the learning procedure was directed to only one neuron, the presynaptic neurons and neurons at the same neuronal level (command-like neurons of the same behavior) participated in the learning. The sign of the participation was not necessarily the same as that in the trained neuron.

Inhibition of Neurotransmission by Peptides Containing the Synaptic Protein Interaction Site of N-Type Ca 2+ Channels

N-type Ca 2+ channels bind directly to the synaptic core complex of VAMP/synaptobrevin, syntaxin, and SNAP-25. Peptides containing the synaptic protein interaction (“synprint”) site caused dissociation of N-type Ca 2+ channels from the synaptic core complex. Introduction of synprint peptides into presynaptic superior cervical ganglion neurons reversibly inhibited synaptic transmission. Fast EPSPs due to synchronous transmitter release were inhibited, while late EPSPs arising from asynchronous release following a train of action potentials were increased and paired-pulse facilitation was increased. The corresponding peptides from L-type Ca 2+ channels had no effect, and the N-type peptides had no effect on Ca 2+ currents through N-type Ca 2+ channels. These results are consistent with the hypothesis that binding of the synaptic core complex to presynaptic N-type Ca 2+ channels is required for Ca 2+ influx to elicit rapid, synchronous neurotransmitter release.

Facilitation of an NMDA receptor‐mediated EPSP by paired‐pulse stimulation in rat neocortex via depression of GABAergic IPSPs.

1. Tight seal, whole-cell recordings from auditory cortex in vivo and in vitro were obtained to investigate modification of N-methyl-D-aspartate (NMDA) receptor-mediated synaptic activity by paired-pulse afferent stimulation. 2. In recordings from urethane-anaesthetized rats (at 37 degrees C), or from cortical slices maintained in vitro (32 degrees C), afferent stimulation elicited a monosynaptic early EPSP and polysynaptic early and late IPSPs. In addition, a late EPSP could be elicited when the stimulus was preceded by an identical priming stimulus (interval approximately 200 ms). The late EPSP was attenuated by the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV, 50 microM). 3. Bath application of the gamma-aminobutyric acid-B (GABAB) receptor antagonist 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH-saclofen; 50 microM) attenuated the late IPSP and clearly revealed a late EPSP. However, 2-OH-saclofen had lesser effects on the second late EPSP elicited during paired-pulse stimulation. Membrane depolarization in 2-OH-saclofen increased the magnitude of the early IPSP, which suppressed the late EPSP once again. Since pharmacological blockade of EPSPs revealed paired-pulse depression of monosynaptically elicited early and late IPSPs, these data indicate that (1) both early and late IPSPs were capable of suppressing the late EPSP, and (2) these effects were reduced during paired-pulse stimulation. 4. Pharmacological isolation of the late EPSP allowed testing of the direct effect of paired-pulse stimulation. Application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 microM), picrotoxin (10 microM) and 2-OH-saclofen (50 microM) isolated the late EPSP (onset, 3 ms; peak latency, 28 ms; peak amplitude, 7 mV; duration, 240 ms), which grew in magnitude with membrane depolarization and was largely (> 90%) blocked by APV. Paired-pulse stimulation depressed the isolated late EPSP by 30%. 5. Thus, apparent paired-pulse facilitation of the late EPSP is attributable to release from GABAergic inhibition, and not to direct facilitation. Facilitation of the late EPSP is a functional consequence of IPSP depression. The results indicate the importance of inhibition in regulating synaptic activity mediated by NMDA receptors.

Large, deep layer pyramid-pyramid single axon EPSPs in slices of rat motor cortex display paired pulse and frequency-dependent depression, mediated presynaptically and self-facilitation, mediated postsynaptically

1. In slices of rat sensorimotor cortex, dual intracellular recordings were obtained from 1,952 pairs of deep layer pyramidal neurons. Where action potentials in one neurone elicited excitatory postsynaptic potentials (EPSPs, n = 56) in the other, responses to different presynaptic firing rates and patterns and at different postsynaptic membrane potentials were recorded and on some occasions both neurons were filled with biocytin. 2. Slices were fixed, sectioned again at 60 microns, and incubated with Avidin horseradish peroxidase (HRP), which was then visualized using the 3,3'-diaminobenzidine tetrahydrochloride (DAB) method. All neurones reported here that were identified histologically were pyramidal cells with their somata in the deep layers (V and VI). 3. One in 70 of the tests performed revealed a synaptic connection, 25 of which were studied in detail. Mean EPSP amplitude was 1.67 +/- 1.66 (SD) mV, with some single sweep events as large as 9 mV. For some of the smaller EPSPs the amplitude distributions contained a clear peak around 0 mV, the coefficient of variation (CV) was large, and paired pulse facilitation was apparent. EPSPs with large average amplitudes displayed no apparent failures of transmission, EPSP amplitudes were fairly evenly distributed around the mean, CVs were small, and paired pulse depression was apparent in 2.5 mM extracellular Ca2+. When single sweeps were selected according to the size of the first EPSP, large second EPSPs were found to follow small first EPSPs and small second EPSPs to follow large first EPSPs. Paired pulse effects appeared, in the majority of tests, to be due to a change in presynaptic release probability. 4. Two EPSPs were recorded in three different extracellular Ca2+ concentrations. In 1 mM Ca2+, the first EPSP of a short interval pair was small and paired pulse facilitation was apparent. In 5 mM Ca2+, first EPSPs were between 2.5 and 4 times larger than in 1 mM Ca2+ and paired pulse depression was apparent. In all Ca2+ concentrations however, averaged third and fourth EPSPs of brief bursts were of similar amplitudes and smaller than second EPSPs. If presynaptic inhibition does contribute to paired pulse effects here, it is not overcome by a combination of raised extracellular Ca2+ and repetitive presynaptic firing. 5. These EPSPs displayed a wide range of time courses. The mean 10-90% rise time was 2.49 +/- 1.08 ms, the mean width at half amplitude was 15.39 +/- 5.42 ms (n = 22), and the mean EPSP latency was 1.59 +/- 0.68 ms (n = 18). (ABSTRACT TRUNCATED AT 400 WORDS)