Dissection of brain-wide spontaneous and functional somatosensory circuits by fMRI with optogenetic silencing

To further advance functional magnetic resonance imaging (fMRI)-based brain science, it is critical to dissect fMRI activities at a circuit level. To solve this issue, we propose to combine brain-wide fMRI with neuronal silencing in well-defined regions via temporally specific optogenetic stimulation. Since focal inactivation suppresses excitatory output to downstream pathways, intact input and downregulated output circuits can be separated. Highly specific cerebral blood volume-weighted fMRI was performed with optogenetic simulation of local GABAergic neurons in mouse somatosensory regions at 15.2 T. Brain-wide spontaneous somatosensory networks were found mostly in ipsilateral cortical and subcortical areas, which differ from the bilateral homotopic connections commonly observed in resting-state fMRI. Evoked fMRI response to somatosensory stimulation were dissected to spinothalamic, thalamocortical (TC), corticothalamic (CT), corticocortical (CC) inputs and local intracortical circuits. The primary somatosensory cortex (S1) receives TC inputs from the ventral posterior thalamic nucleus with spinothalamic inputs. The primary motor cortex (M1) has feedforward CC inputs from S1, and the posterior medial thalamic nucleus also receives CT inputs from S1. The secondary somatosensory cortex (S2) receives mostly direct CC inputs from S1 and a small amount of TC inputs. The TC and CC input layers in cortical regions were identified by laminar-specific fMRI responses. The long-range synaptic input in cortical areas is amplified approximately 2-fold by local intracortical circuits, which is consistent with electrophysiological recordings. Overall, whole-brain fMRI with optogenetic inactivation provides brain-wide, population-based long-range circuits, which will complement conventional microscopic functional circuit studies.

Serotonergic development of active sensing

Active sensing requires adaptive motor (positional) control of sensory organs based on contextual, sensory and task requirements, and develops postnatally after the maturation of intracortical circuits. Alterations in sensorimotor network connectivity during this period are likely to impact sensorimotor computation also in adulthood. Serotonin is among the cardinal developmental regulators of network formation, thus changing the serotonergic drive might have consequences for the emergence and maturation of sensorimotor control. Here we tested this hypothesis on an object localization task by quantifying the motor control dynamics of whiskers during tactile navigation. The results showed that sustained alterations in serotonergic signaling in serotonin transporter knockout rats, or the transient pharmacological inactivation of the transporter during early postnatal development, impairs the emergence of adaptive motor control of whisker position based on recent sensory information. A direct outcome of this altered motor control is that the mechanical force transmitted to whisker follicles upon contact is reduced, suggesting that increased excitability observed upon altered serotonergic signaling is not due to increased synaptic drive originating from the periphery upon whisker contact. These results argue that postnatal development of adaptive motor control requires intact serotonergic signaling and that even its transient dysregulation during early postnatal development causes lasting sensorimotor impairments in adulthood.

Determining the early corticospinal-motoneuronal responses to strength training: a systematic review and meta-analysis

Several studies have used transcranial magnetic stimulation to probe the corticospinal-motoneuronal responses to a single session of strength training; however, the findings are inconsistent. This systematic review and meta-analysis examined whether a single bout of strength training affects the excitability and inhibition of intracortical circuits of the primary motor cortex (M1) and the corticospinal-motoneuronal pathway. A systematic review was completed, tracking studies between January 1990 and May 2018. The methodological quality of studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nine studies (n=107) investigating the acute corticospinal-motoneuronal responses to strength training met the inclusion criteria. Meta-analyses detected that after strength training compared to control, corticospinal excitability [standardised mean difference (SMD), 1.26; 95% confidence interval (CI), 0.88, 1.63; p<0.0001] and intracortical facilitation (ICF) (SMD, 1.60; 95% CI, 0.18, 3.02; p=0.003) were increased. The duration of the corticospinal silent period was reduced (SMD, −17.57; 95% CI, −21.12, −14.01; p=0.00001), but strength training had no effect on the excitability of the intracortical inhibitory circuits [short-interval intracortical inhibition (SICI) SMD, 1.01; 95% CI, −1.67, 3.69; p=0.46; long-interval intracortical inhibition (LICI) SMD, 0.50; 95% CI, −1.13, 2.13; p=0.55]. Strength training increased the excitability of corticospinal axons (SMD, 4.47; 95% CI, 3.45, 5.49; p<0.0001). This systematic review and meta-analyses revealed that the acute neural changes to strength training involve subtle changes along the entire neuroaxis from the M1 to the spinal cord. These findings suggest that strength training is a clinically useful tool to modulate intracortical circuits involved in motor control.

Muscarinic modulation of spike-timing dependent plasticity at recurrent layer 2/3 synapses in mouse auditory cortex

Cholinergic systems contribute to the refinement of auditory cortical receptive fields by activating muscarinic acetylcholine receptors (mAChRs). However, the specific cellular and synaptic mechanisms underlying acetylcholine’s effects on cortical circuits are not fully understood. In this study, we investigate the effects of muscarinic receptor modulation on spike-timing dependent plasticity (STDP) at synapses onto layer 2/3 pyramidal neurons in mouse auditory cortex (AC). Synapses onto layer 2/3 pyramidal neurons exhibit a STDP rule for pairing of postsynaptic spike bursts with single presynaptic stimuli. Pre-before-post pairing at +10 ms results in a timing-dependent long-term potentiation (tLTP), whereas pre-before-post pairing at +50 ms intervals, and post-before-pre pairing at -10 to -20 ms produce a timing-dependent long-term depression. We also characterize how mAChR activation affects plasticity at these synapses, focusing on the induction of tLTP. During pre-before-post pairing at +10 ms, mAChR activation by either carbachol or oxotremorine-M suppresses tLTP. mAChR activation also reduces the NMDA-receptor dependent synaptically evoked increase in calcium in dendrites, apparently without affecting presynaptic transmitter release. Pharmacological experiments suggest that M1 and M3 receptors are not involved in the mAChR-mediated suppression of tLTP. Taken together, these results suggest activating mAChRs in layer 2/3 intracortical circuits can modify the circuit dynamics of AC by depressing tLTP mediated by NMDA receptors, and depressing calcium influx at excitatory synapses onto layer 2/3 pyramidal cells.

Excitatory and inhibitory intracortical circuits for orientation and direction selectivity

The computations performed by a neuron arise from the functional properties of the circuits providing its synaptic inputs. A prime example of these computations is the selectivity of primary visual cortex (V1) for orientation and motion direction. V1 neurons in layer 2/3 (L2/3) receive input mostly from intracortical circuits1, which involve excitation2-9 and inhibition10-12. To understand how an L2/3 neuron achieves its selectivity, therefore, one must characterize the functional organization of both its excitatory and inhibitory presynaptic ensembles. Here we establish this organization, and show how it predicts orientation selectivity and reveals a new cortical circuit for direction selectivity. We identified the presynaptic partners of pyramidal neurons in mouse V1 through rabies monosynaptic tracing1,13, and imaged the functional properties of the postsynaptic neuron and of its presynaptic ensemble. Excitatory presynaptic neurons were predominantly tuned to the postsynaptic neuron’s preferred orientation. Excitation and inhibition described an inverted Mexican hat, with inhibitory presynaptic neurons densest near the postsynaptic neuron and excitatory ones distributed more distally. Excitation and inhibition also differed in laminar origin: inhibitory presynaptic neurons concentrated in L2/3 while excitatory ones dominated in L4. The distribution of excitatory neurons in visual space was coaxial with the postsynaptic neuron’s preferred orientation and lay upstream of the neuron’s preferred direction. Inhibitory presynaptic neurons, instead, clustered more symmetrically around the postsynaptic neuron and favoured locations downstream of its preferred direction. These results demonstrate that L2/3 neurons obtain orientation selectivity from co-tuned neurons in L4 and beyond, and enhance it by contrasting an elongated excitatory input with a concentric inhibitory input. Moreover, L2/3 neurons can obtain direction selectivity through visually offset14 excitation and inhibition. These circuit motifs resemble those seen in the thalamocortical pathway15-20 and in direction selective cells in the retina21,22, suggesting that they are canonical across brain regions.

Imbalance of cortical facilitatory and inhibitory circuits underlies hyperexcitability in ALS

ObjectiveTo determine the relative contribution of inhibitory and facilitatory circuits in the development of cortical hyperexcitability in amyotrophic lateral sclerosis (ALS).MethodsIn this cross-sectional study, cortical excitability was assessed in 27 patients with ALS, and results compared to 25 healthy controls. In addition, a novel neurophysiologic measure of cortical function, short-interval intracortical facilitation (SICF), was assessed reflecting activity of the facilitatory circuits.ResultsThere was a significant increase in SICF (ALS −18.51 ± 1.56%, controls −8.52 ± 1.21%, p < 0.001) in patients with ALS that was accompanied by a reduction of short-interval intracortical inhibition (ALS 3.94 ± 1.29%, controls 14.23 ± 1.18%, p < 0.001) and cortical silent period duration (p = 0.034). The index of excitation, a biomarker reflecting the contribution of inhibitory and facilitatory circuit activity, was significantly increased in patients with ALS (82.79 ± 6.01%) compared to controls (36.15 ± 3.44, p < 0.001), suggesting a shift toward cortical excitation. Increased excitation correlated with upper motor neuron signs (R2 = 0.235, p = 0.016) and greater functional disability as reflected by a correlation with the Amyotrophic Lateral Sclerosis Functional Rating Scale–Revised score (R2 = 0.335, p = 0.002).ConclusionsThe present study established that cortical hyperexcitability is a key contributor to ALS pathophysiology, mediated through dysfunction of inhibitory and facilitatory intracortical circuits. Therapies aimed at restoring the cortical inhibitory imbalance provide novel avenues for future therapeutic targets.