Biomedical subjects

A Agmon

Publications and source records attributed to A Agmon.

11 recordsLinked to original sources

PubMed2001

Diverse types of interneurons generate thalamus-evoked feedforward inhibition in the mouse barrel cortex.

Sensory information, relayed through the thalamus, arrives in the neocortex as excitatory input, but rapidly induces strong disynaptic inhibition that constrains the cortical flow of excitation both spatially and temporally. This feedforward inhibition is generated by intracortical interneurons whose precise identity and properties were not known. To characterize interneurons generating feedforward inhibition, neurons in layers IV and V of mouse somatosensory ("barrel") cortex in vitro were tested in the cell-attached configuration for thalamocortically induced firing and in the whole-cell mode for synaptic responses. Identification as inhibitory or excitatory neurons was based on intrinsic firing patterns and on morphology revealed by intracellular staining. Thalamocortical stimulation evoked action potentials in approximately 60% of inhibitory interneurons but in <5% of excitatory neurons. The inhibitory interneurons that fired received fivefold larger thalamocortical inputs compared with nonfiring inhibitory or excitatory neurons. Thalamocortically evoked spikes in inhibitory interneurons followed at short latency the onset of excitatory monosynaptic responses in the same cells and slightly preceded the onset of inhibitory responses in nearby neurons, indicating their involvement in disynaptic inhibition. Both nonadapting (fast-spiking) and adapting (regular-spiking) inhibitory interneurons fired on thalamocortical stimulation, as did interneurons expressing parvalbumin, calbindin, or neither calcium-binding protein. Morphological analysis revealed that some interneurons might generate feedforward inhibition within their own layer IV barrel, whereas others may convey inhibition to upper layers, within their own or in adjacent columns. We conclude that feedforward inhibition is generated by diverse classes of interneurons, possibly serving different roles in the processing of incoming sensory information.

Action Potentials↗

PubMed2001

Vertical bias in dendritic trees of non-pyramidal neocortical neurons expressing GAD67-GFP in vitro.

The neocortical neuropil has a strong vertical (orthogonal to pia) orientation, constraining the intracortical flow of information and forming the basis for the functional parcellation of the cortex into semi-independent vertical columns or 'modules'. Apical dendrites of excitatory pyramidal neurons are a major component of this vertical neuropil, but the extent to which inhibitory, GABAergic neurons conform to this structural and functional design is less well documented. We used a gene gun to transfect organotypic slice cultures of mouse and rat neocortex with the enhanced green fluorescent protein (eGFP) gene driven by the promoter for glutamic acid decarboxylase 67 (GAD67), an enzyme expressed exclusively in GABAergic cells. Many GAD67-GFP expressing cells were highly fluorescent, and their dendritic morphologies and axonal patterns, revealed in minute detail, were characteristic of GABAergic neurons. We traced 150 GFP-expressing neurons from confocal image stacks, and estimated the degree of vertical bias in their dendritic trees using a novel computational metric. Over 70% of the neurons in our sample had dendritic trees with a highly significant vertical bias. We conclude that GABAergic neurons make an important contribution to the vertical neocortical neuropil, and are likely to integrate synaptic inputs from axons terminating within their own module.

Animals↗

PubMed2000

GABAergic inhibition suppresses paroxysmal network activity in the neonatal rodent hippocampus and neocortex.

In the adult cerebral cortex, the neurotransmitter GABA is strongly inhibitory, as it profoundly decreases neuronal excitability and suppresses the network propensity for synchronous activity. When fast, GABA(A) receptor (GABA(A)R)-mediated neurotransmission is blocked in the mature cortex, neuronal firing is synchronized via recurrent excitatory (glutamatergic) synaptic connections, generating population discharges manifested extracellularly as spontaneous paroxysmal field potentials (sPFPs). This epileptogenic effect of GABA(A)R antagonists has rarely been observed in the neonatal cortex, and indeed, GABA in the neonate has been proposed to have an excitatory, rather than inhibitory, action. In contrast, we show here that when fast GABAergic neurotransmission was blocked in slices of neonatal mouse and rat hippocampus and neocortex, sPFPs occurred in nearly half the slices from postnatal day 4 (P4) to P7 neocortex and in most slices from P2 to P7 hippocampus. In Mg(2+)-free solution, GABA(A)R antagonists elicited sPFPs in nearly all slices of P2 and older neocortex and P0 and older hippocampus. Mg(2+)-free solution alone induced spontaneous events in the majority of P2 and older slices from both regions; addition of GABA(A)R antagonists caused a dramatic increase in the mean amplitude, but not frequency, of these events in the hippocampus and in their mean frequency, but not amplitude, in the neocortex. In the hippocampus, GABA(A)R agonists suppressed amplitudes, but not frequency, of sPFPs, whereas glutamate antagonists suppressed frequency but not amplitudes. We conclude that neonatal rodent cerebral cortex possesses glutamatergic circuits capable of generating synchronous network activity and that, as in the adult, tonic GABA(A)R-mediated inhibition prevents this activity from becoming paroxysmal.

Action Potentials↗

PubMed1996

Functional GABAergic synaptic connection in neonatal mouse barrel cortex.

Intracortical inhibition is crucial to proper functioning of the mature neocortex, yet, paradoxically, is reported to be rare or absent in the neonatal animal. We reexamined this issue by recording whole-cell postsynaptic currents (PSCs) of barrel cortex neurons in thalamocortical brain slices from neonatal mice. Monosynaptic, excitatory thalamocortical responses were elicited in layers V/VI neurons as early as postnatal day 0 (P0, the first 24 hr after birth) and in presumptive layer IV as early as P2. At very low stimulation frequencies, the monosynaptic response was invariably followed by a prolonged (up to 1 sec) synaptic barrage, which fatigued at stimulus repetition rates of 2/min or higher. This barrage consisted of postsynaptic responses to spiking activity in neighboring cortical cells, because (1) it could also be evoked by intracortical stimulation in coronal slices and (2) it was abolished by antagonists to NMDA receptors (NMDARs), even when NMDARs on the recorded cell were under a voltage-dependent block. Some of the larger polysynaptic events changed polarity at a negative reversal potential and were blocked by GABAA receptor (GABAAR) antagonists, with a concurrent enhancement of the extracellular field potential, indicating that they were GABAAR- mediated, CI-dependent inhibitory PSCs (IPSCs). We conclude that a network of functional intracortical GABAAR-mediated synaptic connections exists from the earliest postnatal ages, although it gives rise to responses that differ from mature IPSCs in reversal potential and latency.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1995

Topological precision in the thalamic projection to neonatal mouse barrel cortex.

Somatosensory thalamus and cortex in rodents contain topological representations of the facial whisker pad. The thalamic representation of a single whisker ("barreloid") is presumed to project exclusively to the cortical representation ("barrel") of the same whisker; however, it was not known when this correspondence is established during early development, nor how precise the thalamocortical projection is at birth, before formation of barrels and barreloids. To answer these questions, we retrogradely labeled thalamocortical projection neurons in fixed brain slices from 0-8 d old (P0-P8) mice, by placing paired deposits of two fluorescent dyes in adjacent barrels or (before barrel formation) in adjacent loci in upper cortical layers. At all ages studied, a negligible fraction of the retrogradely labeled cells was double labeled, implying that branches of single thalamocortical axons never extended within layer IV over an area wider than a single barrel. In P0 preparations, 70% of paired dye deposits placed 75-200 microns apart resulted in statistically significant segregation of labeled cell clusters in the thalamus. Quantitative analysis indicated that on P0 about 70% of thalamocortical axons were within 1.3 presumptive barrel diameters from their topologically precise target. In P4-P8 preparations, the great majority of thalamic cells retrogradely labeled from a single barrel were found in a single barreloid, implying a 1:1 projection of barreloids to barrels. The postnatal increase in topological precision was reproduced by a computer simulation, which assumed that many aberrant axons corrected their initial targeting error by extending terminal arborizations asymmetrically, towards the center of their appropriate barrel.

Aging↗

PubMed1994

Oscillatory synaptic interactions between ventroposterior and reticular neurons in mouse thalamus in vitro.

1. The thalamic reticular nucleus (RTN) has reciprocal connections with relay neurons in the dorsal thalamus. We used whole cell recording in a mouse in vitro slice preparation maintained at room temperature to study the synaptic interactions between the RTN and the ventroposterior thalamic nucleus (VP) during evoked low-frequency oscillations. 2. After a single electrical stimulus of the internal capsule, postsynaptic potentials (PSPs) were recorded in all VP and RTN neurons. In 76% of slices, there was an initial response followed by recurrent PSPs lasting for up to 8 s and with a frequency of approximately 2 Hz in both the VP and RTN. 3. In RTN neurons the initial response consisted of a fast excitatory postsynaptic potential (EPSP) that generated a burst of action potentials. Recurrent PSPs consisted of barrages of EPSPs that often reached burst threshold. The structure of subthreshold EPSP barrages in RTN neurons suggested that they were generated by bursting VP neurons. 4. In VP neurons the stimulus usually evoked a small EPSP followed by a large inhibitory postsynaptic potential (IPSP) that was often followed by a rebound burst. This initial response was often followed by a series of recurrent IPSPs presumably generated by RTN bursts, because intrinsic inhibitory neurons are absent in rodent VP. 5. IPSPs in VP neurons and recurrent EPSPs in RTN neurons were completely abolished by application of a gamma-aminobutyric acid-A (GABAA) receptor antagonist. A GABAB receptor antagonist produced no or little change in either the initial or recurrent response. 6. Recurrent IPSPs in VP neurons were abolished by glutamate receptor antagonists before the initial IPSP, which always remained stimulus dependent. 7. The dependency of recurring IPSPs in VP and recurring EPSPs in RTN upon GABA-mediated inhibition and excitatory amino acid-mediated excitation, plus the character of recurring EPSPs in the RTN strongly suggest that the recurring events were generated through reverse-reciprocal synaptic interactions between VP and RTN neurons. These synaptic interactions most likely play an important role in thalamic oscillations in behavior.

Animals↗

PubMed1993

Organized growth of thalamocortical axons from the deep tier of terminations into layer IV of developing mouse barrel cortex.

The thalamocortical projection to rodent somatosensory ("barrell") cortex is highly ordered in both the radial and the tangential dimensions. During a brief period of postnatal development, thalamocortical axons establish two tiers of terminations, in the deep layers and in layer IV, and form whisker-specific clusters within layer IV; however, little is known about the cues that guide them to their appropriate radial and tangential positions. To gain insight into potential mechanisms underlying this process, we studied the development of thalamocortical termination patterns in mouse barrel cortex at high spatial resolution. Developing thalamocortical axons were labeled in fixed slices with the lipophilic carbocyanine dye Dil and imaged with a laser scanning confocal microscope. On the day of birth (postnatal day 0, P0) axons coursed through layers VI and V, with little or no branching. By P2 the lower tier of terminations, at the border of layers VI and V, was clearly identifiable. Below this tier axons coursed obliquely or tangentially, forming a dense meshwork of intersecting fibers, but with no apparent branching. By P4 the upper tier of terminations, in layer IV, was clearly recognizable, and consisted of periodic, dense clusters of terminal arborizations. In marked contrast to the oblique and apparently disorderly course followed by axons in layer VI and lower layer V, axons in upper layer V heading toward the upper tier were organized in loose bundles running radially, suggesting that axons destined to terminate in a particular layer IV barrel had already reached their appropriate tangential coordinates within the lower tier. Thus, the pattern of thalamocortical terminations in layer IV seems to be projected from the deep tier of terminations, and does not develop from an initially profuse arborization pattern through pruning of inappropriate branches.

Animals↗

PubMed1992

NMDA receptor-mediated currents are prominent in the thalamocortical synaptic response before maturation of inhibition.

1. The N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) is thought to underlie synaptic plasticity in both adult and developing CNS; however, its involvement in the thalamocortical synapse has not yet been directly demonstrated. 2. Whole-cell, thalamus-evoked synaptic currents were recorded from layer IV cells in slices of immature mouse somatosensory cortex. 3. Earlier than postnatal day 9 the majority of responses were monosynaptic and purely excitatory, with both non-NMDAR and NMDAR-mediated glutamatergic components. 4. In older animals, disynaptic inhibitory currents summated with the excitatory ones and lowered the reversal potential of the response to voltages at which the NMDAR conductance is mostly blocked. 5. These findings suggest a cellular basis for the transient plasticity observed in layer IV during early postnatal development.

Aging↗

PubMed1992

Correlation between intrinsic firing patterns and thalamocortical synaptic responses of neurons in mouse barrel cortex.

We used a thalamocortical slice preparation to record both spike trains and synaptically evoked responses from neurons of mouse barrel cortex. Cells were classified as regular spiking (RS), intrinsically bursting (IB), or fast spiking (FS) according to their temporal firing patterns when injected with current. RS cells were further separated into two subtypes, RS1 and RS2 cells, the latter encountered only in the infragranular layers. Synaptic responses were elicited by focal electrical stimuli in the ventrobasal nucleus of the thalamus (VB) while holding the cells at different membrane potentials. Postsynaptic potentials were classified as excitatory (EPSPs) or inhibitory (IPSPs), and their latencies were measured from the onset of the extracellularly recorded fiber volley in layer IV. EPSPs fell into three groups, according to latency. Those in the early cluster had latencies shorter than 1 msec and were coincident with the postsynaptic layer IV population response; they were considered monosynaptic. A second group, with latencies between 1.3 and 2.5 msec, were coincident with all IPSPs and were classified as disynaptic. The rest had latencies longer than 5 msec and were considered polysynaptic. The synaptic order of a cell was correlated with its laminar position and its electrophysiological class. Specifically, monosynaptic responses were restricted to infragranular RS cells and to FS cells, while disynaptic EPSPs were found in supragranular RS cells and in IB cells. Disynaptic IPSPs were found in both deep and superficial layers; in the deep layers they nearly always followed monosynaptic EPSPs, while in the superficial layers they were mostly found in isolation. We conclude that the intrinsic spiking characteristics of a neuron are an important determinant of its position in the cortical circuit and may have a substantial role in determining its response properties.

Action Potentials↗

PubMed1991

Thalamocortical responses of mouse somatosensory (barrel) cortex in vitro.

We have developed a novel slice preparation of the mouse somatosensory forebrain. This preparation is unique in including both the ventrobasal nucleus of the thalamus and the sensorimotor "barrel" cortex in a 400-microns-thick slice with the functional connectivity between them preserved, and in allowing direct visualization of the various components of the barrel system in unstained living tissue. Thalamocortical connectivity was demonstrated by recording the laminar profile of cortical field potentials evoked electrically from the ventrobasal nucleus. Current-source density analysis of this profile showed that the largest and earliest sinks were coextensive with the two known sites of thalamocortical terminals, layer IV and the junction of layers V and VI. The sink in layer IV could be dissociated experimentally into a small, early sink of presynaptic origin (most probably a presynaptic spike volley in the thalamocortical terminals) and a later, larger sink generated postsynaptically. By mapping the subcortical stimulation sites that elicited a response at different layer IV recording sites we concluded that the thalamus-to-cortex projection preserves the general dorsoventral relationship of the afferents. Intracellularly recorded responses elicited by thalamic stimulation included (but were not limited to) monosynaptic excitatory and disynaptic inhibitory postsynaptic potentials. The thalamus-to-cortex connections were also mapped with the axonal fluorescent tracer dioctadecyl-tetramethylindocarbocyanine perchlorate. The thalamo-cortical slice is a very suitable system for studying the physiology and pharmacology of the thalamocortical synapse and for exploring the synaptic circuitry of the somatosensory cortex.

Animals↗

PubMed1989

Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex.

Intracellular recordings were made from neurons of the mouse somatosensory cortex isolated in vitro. Two physiologically distinct classes of pyramidal cells were observed: regular-spiking cells were the majority, and generated accommodating trains of single spikes; bursting cells generated clusters of 2-5 action potentials, and clusters were separated by prolonged afterhyperpolarizations. The bursting cells of the mouse neocortex were unusual in producing repetitive bursting during sustained current pulses, and in being localized to a laminar zone straddling layers V and VI.

Animals↗