Scott J. Cruikshank

Synaptic basis for intense thalamocortical activation of feedforward inhibitory cells in neocortex

The thalamus provides fundamental input to the neocortex. This input activates inhibitory interneurons more strongly than excitatory neurons, triggering powerful feedforward inhibition. We studied the mechanisms of this selective neuronal activation using a mouse somatosensory thalamocortical preparation. Notably, the greater responsiveness of inhibitory interneurons was not caused by their distinctive intrinsic properties but was instead produced by synaptic mechanisms. Axons from the thalamus made stronger and more frequent excitatory connections onto inhibitory interneurons than onto excitatory cells. Furthermore, circuit dynamics allowed feedforward inhibition to suppress responses in excitatory cells more effectively than in interneurons. Thalamocortical excitatory currents rose quickly in interneurons, allowing them to fire action potentials before significant feedforward inhibition emerged. In contrast, thalamocortical excitatory currents rose slowly in excitatory cells, overlapping with feedforward inhibitory currents that suppress action potentials. These results demonstrate the importance of selective synaptic targeting and precise timing in the initial stages of neocortical processing.

Pathway-specific feedforward circuits between thalamus and neocortex revealed by selective optical stimulation of axons

Thalamocortical and corticothalamic pathways mediate bidirectional communication between the thalamus and neocortex. These pathways are entwined, making their study challenging. Here we used lentiviruses to express channelrhodopsin-2 (ChR2), a light-sensitive cation channel, in either thalamocortical or corticothalamic projection cells. Infection occurred only locally, but efferent axons and their terminals expressed ChR2 strongly, allowing selective optical activation of each pathway. Laser stimulation of ChR2-expressing thalamocortical axons/terminals evoked robust synaptic responses in cortical excitatory cells and fast-spiking (FS) inhibitory interneurons, but only weak responses in somatostatin-containing interneurons. Strong FS cell activation led to feedforward inhibition in all cortical neuron types, including FS cells. Corticothalamic stimulation excited thalamic relay cells and inhibitory neurons of the thalamic reticular nucleus (TRN). TRN activation triggered inhibition in relay cells but not in TRN neurons. Thus, a major difference between thalamocortical and corticothalamic processing was the extent to which feedforward inhibitory neurons were themselves engaged by feedforward inhibition.

Differential modulation of auditory thalamocortical and intracortical synaptic transmission by cholinergic agonist.

To investigate synaptic mechanisms underlying information processing in auditory cortex, we examined cholinergic modulation of synaptic transmission in a novel slice preparation containing thalamocortical and intracortical inputs to mouse auditory cortex. Extracellular and intracellular recordings were made in cortical layer IV while alternately stimulating thalamocortical afferents (via medial geniculate or downstream subcortical stimulation) and intracortical afferents. Either subcortical or intracortical stimulation elicited a fast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive, monosynaptic EPSP followed by long-duration, polysynaptic activity. The cholinergic agonist carbachol suppressed each of the synaptic potentials to different degrees. At low concentrations (5 microM) carbachol strongly reduced (>60%) the polysynaptic slow potentials for both pathways but did not affect the monosynaptic fast potentials. At higher doses (10-50 microM), carbachol also reduced the fast potentials, but reduced the intracortically-elicited fast potential significantly more than the thalamocortically-elicited fast potential, which at times was actually enhanced. Atropine (0.5 microM) blocked the effects of carbachol, indicating muscarinic receptor involvement. We conclude that muscarinic modulation can strongly suppress intracortical synaptic activity while exerting less suppression, or actually enhancing, thalamocortical inputs. Such differential actions imply that auditory information processing may favor sensory information relayed through the thalamus over ongoing cortical activity during periods of increased acetylcholine (ACh) release.

Thalamocortical inputs trigger a propagating envelope of gamma-band activity in auditory cortex in vitro.

Abstract To investigate how auditory cortex responds to thalamic inputs, we have used electrophysiological and anatomical techniques to characterize a brain slice containing functionally linked thalamocortical and intracortical pathways. In extracellular recordings, stimulation of thalamic afferents elicited a short-latency field potential and current sink in layer IV of the cortex, followed by 100–500 ms of polysynaptic activity containing rapid (gamma-band, 20–80 Hz) fluctuations. Paired intracellular and extracellular recordings showed that a short-latency excitatory postsynaptic potential (EPSP) corresponded to the fast extracellular potential, and that a slow intracellular depolarization with superimposed rapid fluctuations corresponded to the polysynaptic extracellular activity. Pharmacological manipulations demonstrated that glutamate receptors contributed to mono- and polysynaptic activity, and that the gamma-band fluctuations contained intermixed rapid depolarizations and Cl–-mediated inhibition. The spread of evoked activity through auditory cortex was determined by extracellular mapping away from the excitatory focus (the site of the largest amplitude fast response). The short-latency potential traversed auditory cortex at 1.25 m/s and decreased over 1–2 mm, likely reflecting sequential activation of cells contacted by thalamocortical arbors. In contrast, polysynaptic activity did not decrease but propagated as a spatially restricted wave at a 57-fold slower velocity (0.022 m/s). Thus, stimulation of the auditory thalamocortical pathway in vitro elicited a fast glutamatergic potential in layer IV, followed by polysynaptic activity, including gamma-band fluctuations, that propagated through the cortex. Propagating activity may form transient neural assemblies that contribute to auditory information processing.

Evidence for the Hebbian hypothesis in experience-dependent physiological plasticity of neocortex: a critical review

Over the past decade, the number of experimental papers reporting physiological plasticity in primary neocortical regions, following certain types of controlled sensory experience, have increased greatly. These reports have been characterized by specific changes in receptive fields of individual neurons and/or the distributions of receptive fields across cortical maps. There is a widespread belief these types of plasticities have underlying Hebbian/covariance induction mechanisms. This belief appears to be based mainly on: (a) indirect evidence, largely from experiments on the kitten visual cortex, indicating that Hebbian induction mechanisms could be involved in neocortical plasticity; (b) the observation that some types of plasticity in systems other than neocortex follow Hebbian rules of induction; and (c) the adaptability of Hebbian induction mechanisms to models of neural plasticity. In addition, some experiments have directly tested the role of Hebbian induction mechanisms in experience-dependent neocortical plasticity. The present review critically analyzes these (and related) experiments, in order to evaluate the evidence for the Hebbian Hypothesis in experience-dependent physiological plasticity of neocortex. First, we present a set of criteria to show the involvement of a Hebbian process in any form of plasticity. Next, we compare evidence from each primary neocortical region to these criteria. Finally, we examine unresolved issues. While selected developmental studies are included, emphasis is placed on plasticity in the adult neocortex. It is concluded that there is some evidence meeting the criteria for the Hebbian hypothesis in neocortical plasticity. However, this evidence is quite limited considering the popular belief in the validity of the Hebbian hypothesis.

Thalamic Control of Layer 1 Circuits in Prefrontal Cortex

Knowledge of thalamocortical (TC) processing comes mainly from studying core thalamic systems that project to middle layers of primary sensory cortices. However, most thalamic relay neurons comprise a matrix of cells that are densest in the “nonspecific” thalamic nuclei and usually target layer 1 (L1) of multiple cortical areas. A longstanding hypothesis is that matrix TC systems are crucial for regulating neocortical excitability during changing behavioral states, yet we know almost nothing about the mechanisms of such regulation. It is also unclear whether synaptic and circuit mechanisms that are well established for core sensory TC systems apply to matrix TC systems. Here we describe studies of thalamic matrix influences on mouse prefrontal cortex using optogenetic and in vitro electrophysiology techniques. Channelrhodopsin-2 was expressed in midline and paralaminar (matrix) thalamic neurons, and their L1-projecting TC axons were activated optically. Contrary to conventional views, we found that matrix TC projections to L1 could transmit relatively strong, fast, high-fidelity synaptic signals. L1 TC projections preferentially drove inhibitory interneurons of L1, especially those of the late-spiking subtype, and often triggered feedforward inhibition in both L1 interneurons and pyramidal cells of L2/L3. Responses during repetitive stimulation were far more sustained for matrix than for core sensory TC pathways. Thus, matrix TC circuits appear to be specialized for robust transmission over relatively extended periods, consistent with the sort of persistent activation observed during working memory and potentially applicable to state-dependent regulation of excitability.

Stimulation at a site of auditory-somatosensory convergence in the medial geniculate nucleus is an effective unconditioned stimulus for fear conditioning

The medial division of the medial geniculate nucleus (MGm) and the posterior intralaminar nucleus (PIN) are necessary for fear conditioning to an auditory conditioned stimulus (CS), receive both auditory and somatosensory input, and project to the amygdala, which is involved in production of fear conditioned responses. If CS-unconditioned stimulus (US) convergence in the MGm-PIN is critical for fear conditioning, then microstimulation of this area should serve as an effective US during classical conditioning, in place of standard footshock. Guinea pigs underwent conditioning (40-60 trials) using a tone as the CS and medial geniculate complex microstimulation as the US. Conditioned bradycardia developed when the US electrodes were in the PIN. However, microstimulation was not an effective US for conditioning in other parts of the medial geniculate or for sensitization training in the PIN or elsewhere. Learning curves were similar to those found previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for fear conditioning to acoustic stimuli.

A Corticothalamic Switch: Controlling the Thalamus with Dynamic Synapses

Corticothalamic neurons provide massive input to the thalamus. This top-down projection may allow the cortex to regulate sensory processing by modulating the excitability of thalamic cells. Layer 6 corticothalamic neurons monosynaptically excite thalamocortical cells, but also indirectly inhibit them by driving inhibitory cells of the thalamic reticular nucleus. Whether corticothalamic activity generally suppresses or excites the thalamus remains unclear. Here we show that the corticothalamic influence is dynamic, with the excitatory-inhibitory balance shifting in an activity-dependent fashion. During low-frequency activity, corticothalamic effects are mainly suppressive, whereas higher-frequency activity (even a short bout of gamma frequency oscillations) converts the corticothalamic influence to enhancement. The mechanism of this switching depends on distinct forms of short-term synaptic plasticity across multiple corticothalamic circuit components. Our results reveal an activity-dependent mechanism by which corticothalamic neurons can bidirectionally switch the excitability and sensory throughput of the thalamus, possibly to meet changing behavioral demands.

Abrupt Maturation of a Spike-Synchronizing Mechanism in Neocortex

Synchronous activity is common in the neocortex, although its significance, mechanisms, and development are poorly understood. Previous work showed that networks of electrically coupled inhibitory interneurons called low-threshold spiking (LTS) cells can fire synchronously when stimulated by metabotropic glutamate receptors. Here we found that the coordinated inhibition emerging from an activated LTS network could induce correlated spiking patterns among neighboring excitatory cells. Synchronous activity among LTS cells was absent at postnatal day 12 (P12) but appeared abruptly over the next few days. The rapid development of the LTS-synchronizing system coincided with the maturation of the inhibitory outputs and intrinsic membrane properties of the neurons. In contrast, the incidence and magnitude of electrical synapses remained constant between P8 and P15. The developmental transformation of LTS interneurons into a synchronous, oscillatory network overlaps with the onset of active somatosensory exploration, suggesting a potential role for this synchronizing system in sensory processing.

Stability of Electrical Coupling despite Massive Developmental Changes of Intrinsic Neuronal Physiology

Gap junctions mediate metabolic and electrical interactions between some cells of the CNS. For many types of neurons, gap junction-mediated electrical coupling is most prevalent during early development, then decreases sharply with maturation. However, neurons in the thalamic reticular nucleus (TRN), which exert powerful inhibitory control over thalamic relay cells, are electrically coupled in relatively mature animals. It is not known whether TRN cells or any neurons that are electrically coupled when mature are also coupled during early development. We used dual whole-cell recordings in mouse brain slices to study the postnatal development of electrical and chemical synapses that interconnect TRN neurons. Inhibitory chemical synapses were seen as early as postnatal day 4 but were infrequent at all ages, whereas TRN cells were extensively connected by electrical synapses from birth onward. Surprisingly, the functional strength of electrical coupling, assayed under steady-state conditions or during spiking, remained relatively constant as the brain matured despite dramatic concurrent changes of intrinsic membrane properties. Most notably, neuronal input resistances declined almost eightfold during the first two postnatal weeks, but there were offsetting increases in gap junctional conductances. This suggests that the size or number of gap junctions increase homeostatically to compensate for leakier nonjunctional membranes. Additionally, we found that the ability of electrical synapses to synchronize high frequency subthreshold signals improved as TRN cells matured. Our results demonstrate that certain central neurons may maintain or even increase their gap junctional communication as they mature.