Several ways to wake you up by the thalamus.

An organism can be aroused in many different manners. Here, Wang el al.1 demonstrate that a multisensory thalamic region can mediate spontaneous, sensory, and defensive arousal via its widespread projection, which indicates a non-canonical function of this area.

Molecular characteristics and laminar distribution of prefrontal neurons projecting to the mesolimbic system.

Prefrontal cortical influence over the mesolimbic system - including the nucleus accumbens (NAc) and the ventral tegmental area (VTA) - is implicated in various cognitive processes and behavioral malfunctions. The functional versatility of this system could be explained by an underlying anatomical complexity; however, the detailed characterization of the medial prefrontal cortical (mPFC) innervation of the NAc and VTA is still lacking. Therefore, combining classical retrograde and conditional viral tracing techniques with multiple fluorescent immunohistochemistry, we sought to deliver a precise, cell- and layer-specific anatomical description of the cortico-mesolimbic pathways in mice. We demonstrated that NAc- (mPFCNAc) and VTA-projecting mPFC (mPFCVTA) populations show different laminar distribution (layers 2/3-5a and 5b-6, respectively) and express different molecular markers. Specifically, calbindin and Ntsr1 are specific to mPFCNAc neurons, while mPFCVTA neurons express high levels of Ctip2 and FoxP2, indicating that these populations are mostly separated at the cellular level. We directly tested this with double retrograde tracing and Canine adenovirus type 2-mediated viral labeling and found that there is indeed minimal overlap between the two populations. Furthermore, whole-brain analysis revealed that the projection pattern of these populations is also different throughout the brain. Taken together, we demonstrated that the NAc and the VTA are innervated by two, mostly nonoverlapping mPFC populations with different laminar distribution and molecular profile. These results can contribute to the advancement in our understanding of mesocorticolimbic functions and its disorders in future studies.

AgRP neurons control structure and function of the medial prefrontal cortex.

Hypothalamic agouti-related peptide and neuropeptide Y-expressing (AgRP) neurons have a critical role in both feeding and non-feeding behaviors of newborn, adolescent, and adult mice, suggesting their broad modulatory impact on brain functions. Here we show that constitutive impairment of AgRP neurons or their peripubertal chemogenetic inhibition resulted in both a numerical and functional reduction of neurons in the medial prefrontal cortex (mPFC) of mice. These changes were accompanied by alteration of oscillatory network activity in mPFC, impaired sensorimotor gating, and altered ambulatory behavior that could be reversed by the administration of clozapine, a non-selective dopamine receptor antagonist. The observed AgRP effects are transduced to mPFC in part via dopaminergic neurons in the ventral tegmental area and may also be conveyed by medial thalamic neurons. Our results unmasked a previously unsuspected role for hypothalamic AgRP neurons in control of neuronal pathways that regulate higher-order brain functions during development and in adulthood.

Associative and plastic thalamic signaling to the lateral amygdala controls fear behavior.

Decades of research support the idea that associations between a conditioned stimulus (CS) and an unconditioned stimulus (US) are encoded in the lateral amygdala (LA) during fear learning. However, direct proof for the sources of CS and US information is lacking. Definitive evidence of the LA as the primary site for cue association is also missing. Here, we show that calretinin (Calr)-expressing neurons of the lateral thalamus (Calr+LT neurons) convey the association of fast CS (tone) and US (foot shock) signals upstream from the LA in mice. Calr+LT input shapes a short-latency sensory-evoked activation pattern of the amygdala via both feedforward excitation and inhibition. Optogenetic silencing of Calr+LT input to the LA prevents auditory fear conditioning. Notably, fear conditioning drives plasticity in Calr+LT neurons, which is required for appropriate cue and contextual fear memory retrieval. Collectively, our results demonstrate that Calr+LT neurons provide integrated CS-US representations to the LA that support the formation of aversive memories.

Slow insertion of silicon probes improves the quality of acute neuronal recordings.

Neural probes designed for extracellular recording of brain electrical activity are traditionally implanted with an insertion speed between 1 µm/s and 1 mm/s into the brain tissue. Although the physical effects of insertion speed on the tissue are well studied, there is a lack of research investigating how the quality of the acquired electrophysiological signal depends on the speed of probe insertion. In this study, we used four different insertion speeds (0.002 mm/s, 0.02 mm/s, 0.1 mm/s, 1 mm/s) to implant high-density silicon probes into deep layers of the somatosensory cortex of ketamine/xylazine anesthetized rats. After implantation, various qualitative and quantitative properties of the recorded cortical activity were compared across different speeds in an acute manner. Our results demonstrate that after the slowest insertion both the signal-to-noise ratio and the number of separable single units were significantly higher compared with those measured after inserting probes at faster speeds. Furthermore, the amplitude of recorded spikes as well as the quality of single unit clusters showed similar speed-dependent differences. Post hoc quantification of the neuronal density around the probe track showed a significantly higher number of NeuN-labelled cells after the slowest insertion compared with the fastest insertion. Our findings suggest that advancing rigid probes slowly (~1 µm/s) into the brain tissue might result in less tissue damage, and thus in neuronal recordings of improved quality compared with measurements obtained after inserting probes with higher speeds.

A highly collateralized thalamic cell type with arousal-predicting activity serves as a key hub for graded state transitions in the forebrain.

Sleep cycles consist of rapid alterations between arousal states, including transient perturbation of sleep rhythms, microarousals, and full-blown awake states. Here we demonstrate that the calretinin (CR)-containing neurons in the dorsal medial thalamus (DMT) constitute a key diencephalic node that mediates distinct levels of forebrain arousal. Cell-type-specific activation of DMT/CR+ cells elicited active locomotion lasting for minutes, stereotyped microarousals, or transient disruption of sleep rhythms, depending on the parameters of the stimulation. State transitions could be induced in both slow-wave and rapid eye-movement sleep. The DMT/CR+ cells displayed elevated activity before arousal, received selective subcortical inputs, and innervated several forebrain sites via highly branched axons. Together, these features enable DMT/CR+ cells to summate subcortical arousal information and effectively transfer it as a rapid, synchronous signal to several forebrain regions to modulate the level of arousal.

Ongoing network state controls the length of sleep spindles via inhibitory activity.

Sleep spindles are major transient oscillations of the mammalian brain. Spindles are generated in the thalamus; however, what determines their duration is presently unclear. Here, we measured somatic activity of excitatory thalamocortical (TC) cells together with axonal activity of reciprocally coupled inhibitory reticular thalamic cells (nRTs) and quantified cycle-by-cycle alterations in their firing in vivo. We found that spindles with different durations were paralleled by distinct nRT activity, and nRT firing sharply dropped before the termination of all spindles. Both initial nRT and TC activity was correlated with spindle length, but nRT correlation was more robust. Analysis of spindles evoked by optogenetic activation of nRT showed that spindle probability, but not spindle length, was determined by the strength of the light stimulus. Our data indicate that during natural sleep a dynamically fluctuating thalamocortical network controls the duration of sleep spindles via the major inhibitory element of the circuits, the nRT.

The fear circuit of the mouse forebrain: connections between the mediodorsal thalamus, frontal cortices and basolateral amygdala.

A large forebrain circuit, including the thalamus, amygdala and frontal cortical regions, is responsible for the establishment and extinction of fear-related memories. Understanding interactions among these three regions is critical to deciphering the basic mechanisms of fear. With the advancement of molecular and optogenetics techniques, the mouse has become the main species used to study fear-related behaviours. However, the basic connectivity pattern of the forebrain circuits involved in processing fear has not been described in this species. In this study we mapped the connectivity between three key nodes of the circuit, i.e. the basolateral nucleus of the amygdala (BLA), the mediodorsal nucleus of the thalamus (MD) and the medial prefrontal cortex, which were shown to have closed triangular connectivity in rats. In contrast to rat, we found no evidence for this closed loop in mouse. There was no major input from the BLA to the MD and little overlap between medial prefrontal regions connected with both the BLA and MD. The common nodes in the frontal cortex, which displayed reciprocal connection with both the BLA and MD were the agranular insular cortex and the border zone of the cingulate and secondary motor cortex. In addition, the BLA can indirectly affect the MD via the orbital cortex. We attribute the difference between our results and earlier rat studies to methodological problems rather than to genuine species difference. Our data demonstrate that the BLA and MD communicate via cortical sectors, the roles in fear-related behaviour of which have not been extensively studied. In general, our study provides the morphological framework for studies of murine fear-related behaviours.

Phasic, nonsynaptic GABA-A receptor-mediated inhibition entrains thalamocortical oscillations.

GABA-A receptors (GABA-ARs) are typically expressed at synaptic or nonsynaptic sites mediating phasic and tonic inhibition, respectively. These two forms of inhibition conjointly control various network oscillations. To disentangle their roles in thalamocortical rhythms, we focally deleted synaptic, γ2 subunit-containing GABA-ARs in the thalamus using viral intervention in mice. After successful removal of γ2 subunit clusters, spontaneous and evoked GABAergic synaptic currents disappeared in thalamocortical cells when the presynaptic, reticular thalamic (nRT) neurons fired in tonic mode. However, when nRT cells fired in burst mode, slow phasic GABA-AR-mediated events persisted, indicating a dynamic, burst-specific recruitment of nonsynaptic GABA-ARs. In vivo, removal of synaptic GABA-ARs reduced the firing of individual thalamocortical cells but did not abolish slow oscillations or sleep spindles. We conclude that nonsynaptic GABA-ARs are recruited in a phasic manner specifically during burst firing of nRT cells and provide sufficient GABA-AR activation to control major thalamocortical oscillations.

Lateralization of observational fear learning at the cortical but not thalamic level in mice.

Major cognitive and emotional faculties are dominantly lateralized in the human cerebral cortex. The mechanism of this lateralization has remained elusive owing to the inaccessibility of human brains to many experimental manipulations. In this study we demonstrate the hemispheric lateralization of observational fear learning in mice. Using unilateral inactivation as well as electrical stimulation of the anterior cingulate cortex (ACC), we show that observational fear learning is controlled by the right but not the left ACC. In contrast to the cortex, inactivation of either left or right thalamic nuclei, both of which are in reciprocal connection to ACC, induced similar impairment of this behavior. The data suggest that lateralization of negative emotions is an evolutionarily conserved trait and mainly involves cortical operations. Lateralization of the observational fear learning behavior in a rodent model will allow detailed analysis of cortical asymmetry in cognitive functions.

Motor control by sensory cortex.

Classical studies of mammalian movement control define a prominent role for the primary motor cortex. Investigating the mouse whisker system, we found an additional and equally direct pathway for cortical motor control driven by the primary somatosensory cortex. Whereas activity in primary motor cortex directly evokes exploratory whisker protraction, primary somatosensory cortex directly drives whisker retraction, providing a rapid negative feedback signal for sensorimotor integration. Motor control by sensory cortex suggests the need to reevaluate the functional organization of cortical maps.

Long-range connectivity of mouse primary somatosensory barrel cortex.

The primary somatosensory barrel cortex processes tactile vibrissae information, allowing rodents to actively perceive spatial and textural features of their immediate surroundings. Each whisker on the snout is individually represented in the neocortex by an anatomically identifiable 'barrel' specified by the segregated termination zones of thalamocortical axons of the ventroposterior medial nucleus, which provide the primary sensory input to the neocortex. The sensory information is subsequently processed within local synaptically connected neocortical microcircuits, which have begun to be investigated in quantitative detail. In addition to these local synaptic microcircuits, the excitatory pyramidal neurons of the barrel cortex send and receive long-range glutamatergic axonal projections to and from a wide variety of specific brain regions. Much less is known about these long-range connections and their contribution to sensory processing. Here, we review current knowledge of the long-range axonal input and output of the mouse primary somatosensory barrel cortex. Prominent reciprocal projections are found between primary somatosensory cortex and secondary somatosensory cortex, motor cortex, perirhinal cortex and thalamus. Primary somatosensory barrel cortex also projects strongly to striatum, thalamic reticular nucleus, zona incerta, anterior pretectal nucleus, superior colliculus, pons, red nucleus and spinal trigeminal brain stem nuclei. These long-range connections of the barrel cortex with other specific cortical and subcortical brain regions are likely to play a crucial role in sensorimotor integration, sensory perception and associative learning.

Membrane potential dynamics of GABAergic neurons in the barrel cortex of behaving mice.

Computations in cortical circuits are mediated by synaptic interactions between excitatory and inhibitory neurons, and yet we know little about their activity in awake animals. Here, through single and dual whole-cell recordings combined with two-photon microscopy in the barrel cortex of behaving mice, we directly compare the synaptically driven membrane potential dynamics of inhibitory and excitatory layer 2/3 neurons. We find that inhibitory neurons depolarize synchronously with excitatory neurons, but they are much more active with differential contributions of two classes of inhibitory neurons during different brain states. Fast-spiking GABAergic neurons dominate during quiet wakefulness, but during active wakefulness Non-fast-spiking GABAergic neurons depolarize, firing action potentials at increased rates. Sparse uncorrelated action potential firing in excitatory neurons is driven by fast, large, and cell-specific depolarization. In contrast, inhibitory neurons fire correlated action potentials at much higher frequencies driven by slower, smaller, and broadly synchronized depolarization.

Identification of the sites of 2-arachidonoylglycerol synthesis and action imply retrograde endocannabinoid signaling at both GABAergic and glutamatergic synapses in the ventral tegmental area.

Intact endogenous cannabinoid signaling is involved in several aspects of drug addiction. Most importantly, endocannabinoids exert pronounced influence on primary rewarding effects of abused drugs, including exogenous cannabis itself, through the regulation of drug-induced increase in bursting activity of dopaminergic neurons in the ventral tegmental area (VTA). Previous electrophysiological studies have proposed that these dopaminergic neurons may release endocannabinoids in an activity-dependent manner to regulate their various synaptic inputs; however, the underlying molecular and anatomical substrates have so far been elusive. To facilitate understanding of the neurobiological mechanisms involving endocannabinoid signaling in drug addiction, we carried out detailed analysis of the molecular architecture of the endocannabinoid system in the VTA. In situ hybridization for sn-1-diacylglycerol lipase-alpha (DGL-alpha), the biosynthetic enzyme of the most abundant endocannabinoid, 2-arachidonoylglycerol (2-AG), revealed that DGL-alpha was expressed at moderate to high levels by most neurons of the VTA. Immunostaining for DGL-alpha resulted in a widespread punctate pattern at the light microscopic level, whereas high-resolution electron microscopic analysis demonstrated that this pattern is due to accumulation of the enzyme adjacent to postsynaptic specializations of several distinct morphological types of glutamatergic and GABAergic synapses. These axon terminal types carried presynaptic CB(1) cannabinoid receptors on the opposite side of DGL-alpha-containing synapses and double immunostaining confirmed that DGL-alpha is present on the plasma membrane of both tyrosine hydroxylase (TH)-positive (dopaminergic) and TH-negative dendrites. These findings indicate that retrograde synaptic signaling mediated by 2-AG via CB(1) may influence the drug-reward circuitry at multiple types of synapses in the VTA.

Molecular architecture of the cannabinoid signaling system in the core of the nucleus accumbens.

Several abused drugs are known to alter glutamatergic signaling in reward pathways of the brain, and these plastic changes may contribute to the establishment of addiction-related behaviour. Glutamatergic synapses of the prefrontal cortical projections to the nucleus accumbens (nAcb)--which are suggested to be under endocannabinoid (eCB) control - play a central role in the addiction process. The most abundant eCB in the brain is 2-arachi-donoyl-glycerol (2-AG). It is synthesized by diacylglycerol lipase alpha (DGL-alpha), and exerts its action via type 1 cannabinoid receptors (CB1). However, the precise localization of DGL-alpha and CB1 - i.e. the sites of synthesis and action of 2AG - is still unknown. At the light microscopic level, immunocytochemistry revealed a granular pattern of DGL-alpha distribution in the core of the nAcb. Electron microscopic analysis confirmed that these granules corresponded to the heads of dendritic spines. On the other hand, presynaptic axon terminals forming excitatory synapses on these spineheads were found to express CB1 receptors. Our results demonstrate that the molecular constituents for a retrograde endocannabinoid control of glutamatergic transmission are available in the core of the nAcb, and their relative subcellular location is consistent with a role of 2-AG in addiction-related plasticity of cortical excitatory synapses in this reward area.

Immunocytochemically defined interneuron populations in the hippocampus of mouse strains used in transgenic technology.

Transgenic mice are overtaking the role of model animals in neuroscience. They are used in developmental, anatomical, and physiological as well as experimental neurology. However, most results on the organization of the nervous system derive from the rat. The rat hippocampus and its neuronal elements have been thoroughly investigated, revealing remarkable functional and morphological diversity and specificity among hippocampal interneurons. Our aim was to examine the properties of distinct hippocampal interneuron populations, i.e., those immunoreactive for calcium-binding proteins (parvalbumin, calbindin, and calretinin), neuropeptides (cholecystokinin, neuropeptide Y, somatostatin, vasoactive intestinal polypeptide), and certain receptors (metabotropic glutamate receptor 1alpha, cannabinoid receptor type 1) in four strains of mice widely used in transgenic technology, and to compare their properties to those in the rat. Our data indicate that the distribution as well as the dendritic and axonal arborization of mouse interneurons immunoreactive for the different markers was identical in the examined mouse strains, and in most respects are similar to the features found in the rat. The postsynaptic targets of neurons terminating in the perisomatic (parvalbumin), proximal (calbindin), and distal (somatostatin) dendritic region, as well as on other interneurons (calretinin), also matched those found in the rat. However, a few significant differences could also be observed between the two species in addition to the already described immunoreactivity of mossy cells for calretinin: the absence of spiny calretinin-immunoreactive interneurons in the CA3 region, sparse contacts between calretinin-immunoreactive interneurons, and the axon staining for somatostatin and neuropil labeling for cholecystokinin. We can conclude that the morphofunctional classification of interneurons established in the rat is largely valid for mouse strains used in transgenic procedures.

Convergence of excitatory and inhibitory inputs onto CCK-containing basket cells in the CA1 area of the rat hippocampus.

The number and distribution of excitatory and inhibitory inputs affect the integrative properties of neurons. These parameters have been studied recently for several hippocampal neuron populations. Besides parvalbumin- (PV) containing cells that include basket and axo-axonic cells, cholecystokinin (CCK)-containing interneurons also form a basket cell population with several properties distinct from PV cells. Here, at the light microscopic level, we reconstructed the entire dendritic tree of CCK-immunoreactive (IR) basket cells to describe their geometry, the total length and laminar distribution of their dendrites. This was followed by an electron microscopic analysis of serial ultrathin sections immunostained against gamma-aminobutyric acid, to estimate the density of excitatory and inhibitory synapses on their somata, axon initial segments and different subclasses of dendrites. The dendritic tree of CCK-IR basket cells has an average length of 6300 microm and penetrates all layers. At the electron microscopic level, CCK basket cells receive dendritic inputs with a density of 80-230 per 100 microm. The ratio of inhibitory inputs is relatively high (35%) and increases towards the soma (83%). The total numbers of excitatory and inhibitory synapses converging onto CCK-IR cells are approximately 8200. Comparison of the two, neurochemically distinct basket cells reveals that CCK-containing basket cells receive much less synaptic input than PV cells; however, the relative weight of inhibition is higher on CCK cells. Additional differences in their anatomical and physiological properties predict that CCK basket cells are under a more diverse, elaborate control than PV basket cells, and thus the function of the two populations must be different.