Genetically Defined Subtypes of Somatostatin-Containing Cortical Interneurons

Inhibitory interneurons play a crucial role in proper development and function of the mammalian cerebral cortex. Of the different inhibitory subclasses, dendritic-targeting, somatostatin-containing (SOM) interneurons may be the most diverse. Earlier studies used GFP-expressing and recombinase-expressing mouse lines to characterize genetically defined subtypes of SOM interneurons by morphologic, electrophysiological, and neurochemical properties. More recently, large-scale studies classified SOM interneurons into 13 morpho-electric transcriptomic (MET) types. It remains unclear, however, how these various classification schemes relate to each other, and experimental access to MET types has been limited by the scarcity of specific mouse driver lines. To address these issues, we crossed Flp and Cre driver lines with a dual-color intersectional reporter, allowing experimental access to several combinatorially defined SOM subsets. Brains from adult mice of both sexes were retrogradely dye labeled from the pial surface to identify layer 1-projecting neurons and immunostained against several marker proteins, revealing correlations between genetic label, axonal target, and marker protein expression in the same neurons. Lastly, using whole-cell recordings ex vivo, we analyzed and compared electrophysiological properties between different intersectional subsets. We identified two layer 1-targeting subtypes with nonoverlapping marker protein expression and electrophysiological properties, which, together with a previously characterized layer 4-targeting subtype, account for >50% of all layer 5 SOM cells and >40% of all SOM cells, and appear to map onto 5 of the 13 MET types. Genetic access to these subtypes will allow researchers to determine their synaptic inputs and outputs and uncover their roles in cortical computations and animal behavior.

Ultrafast (400 Hz) network oscillations induced in mouse barrel cortex by optogenetic activation of thalamocortical axons

Oscillations of extracellular voltage, reflecting synchronous, rhythmic activity in large populations of neurons, are a ubiquitous feature in the mammalian brain and are thought to subserve important, if not fully understood cognitive functions. Oscillations at different frequency bands are hallmarks of specific brain and behavioral states. At the higher end of the spectrum, ultrafast (400-600 Hz) oscillations in the somatosensory cortex, in response to peripheral nerve stimulation or punctate sensory stimuli, were previously observed in humans and in a handful of animal studies; however, their synaptic basis and functional significance remain largely unexplored. Here we report that brief optogenetic activation of thalamocortical axons, in brain slices from mouse somatosensory (barrel) cortex, elicited in the thalamorecipient layer local field potential (LFP) oscillations which we dubbed 'ripplets', consisting of a sequence of precisely reproducible 2-5 negative transients at ~400 Hz which originated in the postsynaptic cortical network. Fast-spiking (FS) inhibitory interneurons fired ~400 Hz spike bursts entrained to the LFP oscillation, while regular-spiking (RS) excitatory neurons typically fired only 1-2 spikes per ripplet, preceding FS spikes by ~1.5 ms. Spike bursts were exquisitely synchronized between neighboring FS cells, while RS cells received synchronous, precisely repeating sequences of alternating excitatory and inhibitory postsynaptic currents (E/IPSCs) phase-locked to the LFP oscillation. Spikes in FS cells followed at short (~0.4 ms) latency onset of EPSCs and preceded (by ~0.8 ms) onset of IPSCs in simultaneously recorded RS cells, suggesting that FS cells were driven to fire by phasic inputs from excitatory cells, and in turn evoked volleys of inhibition which enforced synchrony on excitatory cells. We suggest that ripplets are an intrinsically generated cortical response to a strong, synchronous thalamocortical volley. Ripplets and the associated spike sequences in excitatory cells could provide increased bandwidth for encoding and transmitting sensory information. In addition, optogenetically induced ripplets are a uniquely accessible model system for studying synaptic mechanisms of fast and ultrafast cortical and hippocampal oscillations.

Genetically Defined Subtypes of Somatostatin-Containing Cortical Interneurons

Inhibitory interneurons play a crucial role in proper development and function of the mammalian cerebral cortex. Of the different inhibitory subclasses, dendritic-targeting, somatostatin-containing (SOM) interneurons may be the most diverse. Earlier studies used transgenic mouse lines to identify and characterize subtypes of SOM interneurons by morphological, electrophysiological and neurochemical properties. More recently, large-scale studies classified SOM interneurons into 13 morpho-electro-transcriptomic (MET) types. It remains unclear, however, how these various classification schemes relate to each other, and experimental access to MET types has been limited by the scarcity of type-specific mouse driver lines. To begin to address these issues we crossed Flp and Cre driver mouse lines and a dual-color combinatorial reporter, allowing experimental access to genetically defined SOM subsets. Brains from adult mice of both sexes were retrogradely dye-labeled from the pial surface to identify layer 1-projecting neurons, and immunostained against several marker proteins, allowing correlation of genetic label, axonal target and marker protein expression in the same neurons. Using whole-cell recordings ex-vivo, we compared electrophysiological properties between intersectional and transgenic SOM subsets. We identified two layer 1-targeting intersectional subsets with non-overlapping marker protein expression and electrophysiological properties which, together with a previously characterized layer 4-targeting subtype, account for about half of all layer 5 SOM cells and >40% of all SOM cells, and appear to map onto 5 of the 13 MET types. Genetic access to these subtypes will allow researchers to determine their synaptic inputs and outputs and uncover their roles in cortical computations and animal behavior.

SIGNIFICANCE STATEMENT

Inhibitory neurons are critically important for proper development and function of the cerebral cortex. Although a minority population, they are highly diverse, which poses a major challenge to investigating their contributions to cortical computations and animal and human behavior. As a step towards understanding this diversity we crossed genetically modified mouse lines to allow detailed examination of genetically-defined groups of the most diverse inhibitory subtype, somatostatin-containing interneurons. We identified and characterized three somatostatin subtypes in the deep cortical layers with distinct combinations of anatomical, neurochemical and electrophysiological properties. Future studies could now use these genetic tools to examine how these different subtypes are integrated into the cortical circuit and what roles they play during sensory, cognitive or motor behavior.

Recruitment of GABAergic Interneurons in the Barrel Cortex during Active Tactile Behavior

Neural computation involves diverse types of GABAergic inhibitory interneurons that are integrated with excitatory (E) neurons into precisely structured circuits. To understand how each neuron type shapes sensory representations, we measured firing patterns of defined types of neurons in the barrel cortex while mice performed an active, whisker-dependent object localization task. Touch excited fast-spiking (FS) interneurons at short latency, followed by activation of E neurons and somatostatin-expressing (SST) interneurons. Touch only weakly modulated vasoactive intestinal polypeptide-expressing (VIP) interneurons. Voluntary whisker movement activated FS neurons in the ventral posteromedial nucleus (VPM) target layers, a subset of SST neurons and a majority of VIP neurons. Together, FS neurons track thalamic input, mediating feedforward inhibition. SST neurons monitor local excitation, providing feedback inhibition. VIP neurons are activated by non-sensory inputs, disinhibiting E and FS neurons. Our data reveal rules of recruitment for interneuron types during behavior, providing foundations for understanding computation in cortical microcircuits.

A Novel Method for Training Mice in Visuo-Tactile 3-D Object Discrimination and Recognition

Perceiving, recognizing and remembering 3-dimensional (3-D) objects encountered in the environment has a very high survival value; unsurprisingly, this ability is shared among many animal species, including humans. The psychological, psychophysical and neural basis for object perception, discrimination, recognition and memory has been extensively studied in humans, monkeys, pigeons and rodents, but is still far from understood. Nearly all 3-D object recognition studies in the rodent used the "novel object recognition" paradigm, which relies on innate rather than learned behavior; however, this procedure has several important limitations. Recently, investigators have begun to recognize the power of behavioral tasks learned through reinforcement training (operant conditioning) to reveal the sensorimotor and cognitive abilities of mice and to elucidate their underlying neural mechanisms. Here, we describe a novel method for training and testing mice in visual and tactile object discrimination, recognition and memory, and use it to begin to examine the underlying sensory basis for these cognitive capacities. A custom-designed Y maze was used to train mice to associate one of two 3-D objects with a food reward. Out of nine mice trained in two cohorts, seven reached performance criterion in about 20-35 daily sessions of 20 trials each. The learned association was retained, or rapidly re-acquired, after a 6 weeks hiatus in training. When tested under low light conditions, individual animals differed in the degree to which they used tactile or visual cues to identify the objects. Switching to total darkness resulted only in a transient dip in performance, as did subsequent trimming of all large whiskers (macrovibrissae). Additional removal of the small whiskers (microvibrissae) did not degrade performance, but transiently increased the time spent inspecting the object. This novel method can be combined in future studies with the large arsenal of genetic tools available in the mouse, to elucidate the neural basis of object perception, recognition and memory.

Properties of precise firing synchrony between synaptically coupled cortical interneurons depend on their mode of coupling

Precise spike synchrony has been widely reported in the central nervous system, but its functional role in encoding, processing, and transmitting information is yet unresolved. Of particular interest is firing synchrony between inhibitory cortical interneurons, thought to drive various cortical rhythms such as gamma oscillations, the hallmark of cognitive states. Precise synchrony can arise between two interneurons connected electrically, through gap junctions, chemically, through fast inhibitory synapses, or dually, through both types of connections, but the properties of synchrony generated by these different modes of connectivity have never been compared in the same data set. In the present study we recorded in vitro from 152 homotypic pairs of two major subtypes of mouse neocortical interneurons: parvalbumin-containing, fast-spiking (FS) interneurons and somatostatin-containing (SOM) interneurons. We tested firing synchrony when the two neurons were driven to fire by long, depolarizing current steps and used a novel synchrony index to quantify the strength of synchrony, its temporal precision, and its dependence on firing rate. We found that SOM-SOM synchrony, driven solely by electrical coupling, was less precise than FS-FS synchrony, driven by inhibitory or dual coupling. Unlike SOM-SOM synchrony, FS-FS synchrony was strongly firing rate dependent and was not evident at the prototypical 40-Hz gamma frequency. Computer simulations reproduced these differences in synchrony without assuming any differences in intrinsic properties, suggesting that the mode of coupling is more important than the interneuron subtype. Our results provide novel insights into the mechanisms and properties of interneuron synchrony and point out important caveats in current models of cortical oscillations.

What is the right timing for ultrasound evaluation after pregnancy termination with mifepristone?

OBJECTIVE

To determine the timing for ultrasound evaluation after medical termination of pregnancy (MTOP).

STUDY DESIGN

The records of 301 consecutive women who underwent MTOP between July 2010 and July 2011 were studied retrospectively. The follow-up protocol included ultrasound evaluation 2 weeks after MTOP. Surgical termination was offered when pregnancy was found to be ongoing, and either hysteroscopy/curettage or a repeat ultrasound 2 weeks later was offered when the ultrasound findings were suspicious for retained products of conception. Pathology reports were used to confirm the presence of retained products of conception.

RESULTS

Women with ultrasound findings suspicious for retained products of conception were significantly older than women with negative ultrasound findings (30.9±7.7 years vs 24.8±6 years, p<0.0001). Two weeks after MTOP, ultrasound findings were negative in 236 women and suspicious in 66 women. This rate declined as the interval between ultrasound evaluation and MTOP increased (up to 10 weeks). Of the 18 women (5.98%) who underwent hysteroscopy/curettage, pathology reports indicated that 15 (83.3%) had true residua.

CONCLUSIONS

At 2 weeks after MTOP, ultrasound findings suspicious for retained products of conception do not conclusively indicate failure of the procedure. Ultrasound evaluation should be repeated 4-6 weeks later (6-8 weeks after MTOP) in women with suspected residua before diagnosing failure of the procedure.

A novel, jitter-based method for detecting and measuring spike synchrony and quantifying temporal firing precision

BACKGROUND

Precise spike synchrony, at the millisecond or even sub-millisecond time scale, has been reported in different brain areas, but its neurobiological meaning and its underlying mechanisms remain unknown or controversial. Studying these questions is complicated by the lack of a validated, well-normalized and robust index for quantifying synchrony. Previously used measures of synchrony are often improperly normalized and thereby are not comparable between different experimental conditions, are sensitive to variations in firing rate or to the firing rate differential between the two neurons, and/or rely on untenable assumptions of firing rate stationarity and Poisson statistics. I describe here a novel measure, the Jitter-Based Synchrony Index (JBSI), that overcomes these issues.

RESULTS AND DISCUSSION

The JBSI method is based on the introduction of virtual spike jitter. While previous implementations of the jitter method used it only to detect synchrony, the JBSI method also quantifies synchrony. Previous implementations of the jitter method used computationally intensive Monte Carlo simulations to generate surrogate spike trains, whereas the JBSI is computed analytically. The JBSI method does not assume any specific firing model, and does not require that the spike trains be locked to a repeating external stimulus. The JBSI can assume values from 1 (maximal possible synchrony) to -1 (minimal possible synchrony) and is therefore properly normalized. Using simulated Poisson spike trains with introduced controlled spike coincidences, I demonstrate that the JBSI is a linear measure of the spike coincidence rate, is independent of the mean firing frequency or the firing frequency differential between the two neurons, and is not sensitive to co-modulations in the firing rates of the two neurons. In contrast, several commonly used synchrony indices fail under one or more of these scenarios. I also demonstrate how the JBSI can be used to estimate the spike timing precision in the system.

CONCLUSIONS

The JBSI is a conceptually simple and computationally efficient method that can be used to compute the statistical significance of firing synchrony, to quantify synchrony as a well-normalized index, and to estimate the degree of temporal precision in the system.

Distinct subtypes of somatostatin-containing neocortical interneurons revealed in transgenic mice

GABA-releasing inhibitory interneurons in the cerebral cortex can be classified by their neurochemical content, firing patterns, or axonal targets, to name the most common criteria, but whether classifications using different criteria converge on the same neuronal subtypes, and how many such subtypes exist, is a matter of much current interest and considerable debate. To address these issues, we generated transgenic mice expressing green fluorescent protein (GFP) under control of the GAD67 promoter. In two of these lines, named X94 and X98, GFP expression in the barrel cortex was restricted to subsets of somatostatin-containing (SOM+) GABAergic interneurons, similar to the previously reported "GIN" line (Oliva et al., 2000), but the laminar distributions of GFP-expressing (GFP+) cell bodies in the X94, X98, and GIN lines were distinct and nearly complementary. We compared neurochemical content and axonal distribution patterns of GFP+ neurons among the three lines and analyzed in detail electrophysiological properties in a dataset of 150 neurons recorded in whole-cell, current-clamp mode. By all criteria, there was nearly perfect segregation of X94 and X98 GFP+ neurons, whereas GIN GFP+ neurons exhibited intermediate properties. In the X98 line, GFP expression was found in infragranular, calbindin-containing, layer 1-targeting ("Martinotti") cells that had a propensity to fire low-threshold calcium spikes, whereas X94 GFP+ cells were stuttering interneurons with quasi fast-spiking properties, residing in and targeting the thalamo-recipient neocortical layers. We conclude that much of the variability previously attributed to neocortical SOM+ interneurons can be accounted for by their natural grouping into distinct subtypes.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.

The role of the hyperpolarization-activated cationic current I(h) in the timing of interictal bursts in the neonatal hippocampus

Under both pathological and experimental conditions, area CA3 of the adult or juvenile hippocampus generates periodic population discharges known as interictal bursts. Whereas the ionic and synaptic basis of individual bursts has been comprehensively studied experimentally and computationally, the pacemaker mechanisms underlying interictal rhythmicity remain conjectural. We showed previously that rhythmic population discharges resembling interictal bursts can be induced in hippocampal slices from first postnatal week mice, in Mg2+-free solution with GABA(A) receptor-mediated inhibition blocked. Here we show that these neonatal bursts occurred with high temporal precision and that their frequency and regularity were greatly reduced by the bradycardic agent ZD-7288 when applied at concentrations and durations that selectively block the hyperpolarization-activated, cationic current I(h). Augmenting I(h) by elevating intracellular cAMP dramatically increased burst frequency in a protein kinase A-independent manner. Burst amplitudes were strongly correlated with the preceding, but not the following, interburst intervals. The experimentally observed distribution of interburst intervals was modeled by assuming that a burst was triggered whenever the instantaneous rate of spontaneous EPSPs (sEPSPs) exceeded a threshold and that the mean sEPSP rate was minimal immediately after a burst and then relaxed exponentially to a steady-state level. The effect of blocking I(h) in any given slice could be modeled by decreasing only the steady-state sEPSP rate, suggesting that the instantaneous rate of sEPSPs is governed by the level of I(h) activation and raising the novel possibility that interburst intervals reflected the slow activation kinetics of I(h) in the neonatal CA3.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.