Cortical sensory suppression during arousal is due to the activity-dependent depression of thalamocortical synapses

Input-specific effects of acetylcholine on sensory and intracortical evoked responses in the "barrel cortex" in vivo

The somatosensory neocortex processes extrinsic information from the thalamus and intrinsic information from local circuits. We compared the effects of acetylcholine (Ach) on neocortical field potential responses evoked by stimulation of the whiskers and by local electrical stimulation in the upper layers of the neocortex vibrissae representation ("barrel cortex") of adult rats anesthetized with urethane. In the barrel cortex, the cholinergic system was manipulated using microdialysis by exogenous application of Ach, by increasing the endogenous levels of Ach with physostigmine and by applying specific cholinergic agonists. The results revealed that Ach selectively enhances the sensory response relative to the intracortical response. Thus, pathways in the barrel cortex are differentially regulated by cholinergic inputs.

Gating of hippocampal-evoked activity in prefrontal cortical neurons by inputs from the mediodorsal thalamus and ventral tegmental area

Projections from the hippocampus, the mediodorsal thalamus (MD), and the ventral tegmental area (VTA) form interconnected neural circuits that converge in the prefrontal cortex (PFC) to participate in the regulation of executive functions. The present study assessed the roles that the MD and VTA play in regulating the hippocampal-PFC pathway using extracellular single-unit recordings in urethane-anesthetized rats. MD stimulation inhibited PFC neuron firing (approximately 100 msec duration) evoked by fimbria/fornix (FF) stimulation in a majority of neurons tested. However, this effect was reduced if activation of thalamocortical inputs occurred almost simultaneously (10 msec) with stimulation of the FF. In a separate population of neurons, burst stimulation of the MD produced a short-term (approximately 100 msec) inhibition or facilitation of FF-evoked firing in 66 and 33% of PFC neurons, respectively. Moreover, tetanic stimulation of the MD caused a longer-lasting (approximately 5 min) potentiation of FF-evoked firing. Burst stimulation of the VTA inhibited FF-evoked firing in a frequency-dependent manner: firing evoked by higher-frequency trains of pulses to the FF was less inhibited than firing evoked by single-pulse stimulation. The inhibitory actions of VTA stimulation were augmented by D1 receptor antagonism and attenuated by D2 and D4 antagonists. Moreover, stimulation of the MD 10 msec before stimulation of the FF attenuated the VTA-mediated inhibition of evoked firing. Thus, both the MD and VTA exert a complex gating action over PFC neural activity, either facilitating or inhibiting firing in the hippocampal-PFC pathway depending on the frequency and relative timing of the arrival of afferent input.

Thalamocortical control of feed-forward inhibition in awake somatosensory 'barrel' cortex

Intracortical inhibition plays a role in shaping sensory cortical receptive fields and is mediated by both feed-forward and feedback mechanisms. Feed-forward inhibition is the faster of the two processes, being generated by inhibitory interneurons driven by monosynaptic thalamocortical (TC) input. In principle, feed-forward inhibition can prevent targeted cortical neurons from ever reaching threshold when TC input is weak. To do so, however, inhibitory interneurons must respond to TC input at low thresholds and generate spikes very quickly. A powerful feed-forward inhibition would sharpen the tuning characteristics of targeted cortical neurons, and interneurons with sensitive and broadly tuned receptive fields could mediate this process. Suspected inhibitory interneurons (SINs) with precisely these properties are found in layer 4 of the somatosensory (S1) 'barrel' cortex of rodents and rabbits. These interneurons lack the directional selectivity seen in most cortical spiny neurons and in ventrobasal TC afferents, but are much more sensitive than cortical spiny neurons to low-amplitude whisker displacements. This paper is concerned with the activation of S1 SINs by TC impulses, and with the consequences of this activation. Multiple TC neurons and multiple S1 SINs were simultaneously studied in awake rabbits, and cross-correlation methods were used to examine functional connectivity. The results demonstrate a potent, temporally precise, dynamic and highly convergent/divergent functional input from ventrobasal TC neurons to SINs of the topographically aligned S1 barrel. Whereas the extensive pooling of convergent TC inputs onto SINs generates sensitive and broadly tuned inhibitory receptive fields, the potent TC divergence onto many SINs generates sharply synchronous activity among these elements. This TC feed-forward inhibitory network is well suited to provide a fast, potent, sensitive and broadly tuned inhibition of targeted spiny neurons that will suppress spike generation following all but the most optimal feed-forward excitatory inputs.

Excitatory inputs to spiny cells in layers 4 and 6 of cat striate cortex

The principal target of lateral geniculate nucleus in the cat visual cortex is the stellate neurons of layer 4. In previously reported work with intracellular recording and extracellular stimulation in slices of visual cortex, three general classes of fast excitatory synaptic potentials (EPSPs) in layer 4a spiny stellate neurons were identified. One of these classes, characterized by large and relatively invariant amplitudes (mean 1.7 mV, average coefficient of variation (CV) 0.083) were attributed to the action of geniculate axons because, unlike the other two classes, they could not be matched by intracortical inputs, using paired recording. We have examined in detail the properties of this synaptic input in twelve examples, selecting for study those EPSPs where there was secure extracellular stimulation of the single fibre input to a pair of stimuli 50 ms apart. In our analysis, we conclude that the depression that these inputs show to the second stimulus is entirely postsynaptic, since the evidence strongly suggests that the probability of transmitter release at the synaptic site(s) remains 1.0 for both stimuli. We argue that the most plausible explanation for this postsynaptic depression is a reduction in the average probability of opening the synaptic channels. Using a simple biochemical analysis (c.f. Sigworth plot), it is then possible to calculate the number of synaptic channels and their probability of opening, for each of the 12 connections. The EPSPs had a mean amplitude of 1.91 mV (+/- 1.3 mV SD) and a mean CV of 0.067 (+/- 0.022). The calculated number of channels ranged from 20 to 158 (59.4 +/- 48.7) and their probability of opening to the first EPSP had an average of 0.83 (+/- 0.09), with an average depression of the probability to 0.60 for the second EPSP. Geniculate afferents also terminate in layer 6. Intracellular recordings were also made in the upper part of this layer and a total of 51 EPSPs were recorded from pyramidal cells of three principal types. Amongst this dataset we sought EPSPs with similar properties to those characterized in layer 4a. Three examples were found, which is a much lower percentage (6%) than the incidence of putative geniculate EPSPs found in layer 4a (42%).

Role of thalamocortical sensory suppression during arousal: focusing sensory inputs in neocortex

The thalamus serves as a gate that regulates the flow of sensory inputs to the neocortex, and this gate is controlled by neuromodulators from the brainstem reticular formation that are released during arousal. We found recently that sensory-evoked responses are suppressed in the neocortex during arousal. This sensory suppression results from the activity-dependent depression of the thalamocortical connection caused by increased tonic firing of thalamocortical cells during arousal. In the present study, the functional consequences of thalamocortical suppression during arousal were investigated using the vibrissae system of rodents. The results show that thalamocortical suppression is associated with a strong reduction in the spread of sensory inputs through the cortex, thus reducing the size of sensory representations. In addition, when the responses of single cells to principal and adjacent whiskers are compared, the response to the adjacent whiskers was found to be strongly suppressed, much more so than that of principal whiskers. Consequently, the receptive fields of cortical neurons become more focused to the principal whisker. The results indicate that thalamocortical suppression during arousal serves to focus sensory inputs to their appropriate representations in neocortex, which may be computationally helpful for the spatial processing of sensory information.

Role of thalamocortical sensory suppression during arousal: focusing sensory inputs in neocortex

The thalamus serves as a gate that regulates the flow of sensory inputs to the neocortex, and this gate is controlled by neuromodulators from the brainstem reticular formation that are released during arousal. We found recently that sensory-evoked responses are suppressed in the neocortex during arousal. This sensory suppression results from the activity-dependent depression of the thalamocortical connection caused by increased tonic firing of thalamocortical cells during arousal. In the present study, the functional consequences of thalamocortical suppression during arousal were investigated using the vibrissae system of rodents. The results show that thalamocortical suppression is associated with a strong reduction in the spread of sensory inputs through the cortex, thus reducing the size of sensory representations. In addition, when the responses of single cells to principal and adjacent whiskers are compared, the response to the adjacent whiskers was found to be strongly suppressed, much more so than that of principal whiskers. Consequently, the receptive fields of cortical neurons become more focused to the principal whisker. The results indicate that thalamocortical suppression during arousal serves to focus sensory inputs to their appropriate representations in neocortex, which may be computationally helpful for the spatial processing of sensory information.

Activation of a cortical column by a thalamocortical impulse

Thalamocortical (TC) impulses potently influence the sensory neocortex, but the functional impact of individual TC neurons throughout the layers of the cortex has proved difficult to assess. Here we examine, in awake rabbits, the vertical distribution of monosynaptic currents generated in a somatosensory cortical "barrel" column by spontaneous impulses of single, topographically aligned TC neurons. We show that closely neighboring TC neurons generate widely differing patterns of monosynaptic activation within layers 4 and 6 of their aligned column. Moreover, synaptic currents generated by TC impulses with long preceding interspike intervals are greatly enhanced in both of these layers. The degree of this enhancement differs reliably among neighboring TC neurons but, for a given neuron, is very similar in layers 4 and 6. Our results indicate that in the awake state, TC synapses throughout the depth of the cortex serve as powerful filters of sensory information that reflect individual characteristics of their parent TC neuron.

Activation of a cortical column by a thalamocortical impulse

Thalamocortical (TC) impulses potently influence the sensory neocortex, but the functional impact of individual TC neurons throughout the layers of the cortex has proved difficult to assess. Here we examine, in awake rabbits, the vertical distribution of monosynaptic currents generated in a somatosensory cortical "barrel" column by spontaneous impulses of single, topographically aligned TC neurons. We show that closely neighboring TC neurons generate widely differing patterns of monosynaptic activation within layers 4 and 6 of their aligned column. Moreover, synaptic currents generated by TC impulses with long preceding interspike intervals are greatly enhanced in both of these layers. The degree of this enhancement differs reliably among neighboring TC neurons but, for a given neuron, is very similar in layers 4 and 6. Our results indicate that in the awake state, TC synapses throughout the depth of the cortex serve as powerful filters of sensory information that reflect individual characteristics of their parent TC neuron.