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.

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.

Different output properties of perisomatic region-targeting interneurons in the basal amygdala.

The perisomatic region of principal neurons in cortical regions is innervated by three types of GABAergic interneuron, including parvalbumin-containing basket cells (PVBCs) and axo-axonic cells (AACs), as well as cholecystokinin and type 1 cannabinoid receptor-expressing basket cells (CCK/CB1BCs). These perisomatic inhibitory cell types can also be found in the basal nucleus of the amygdala, however, their output properties are largely unknown. Here, we performed whole-cell recordings in morphologically identified interneurons in slices prepared from transgenic mice, in which the GABAergic cells could be selectively targeted. Investigating the passive and active membrane properties of interneurons located within the basal amygdala revealed that the three interneuron types have distinct single-cell properties. For instance, the input resistance, spike rate, accommodation in discharge rate, or after-hyperpolarization width at the half maximal amplitude separated the three interneuron types. Furthermore, we performed paired recordings from interneurons and principal neurons to uncover the basic features of unitary inhibitory postsynaptic currents (uIPSCs). Although we found no difference in the magnitude of responses measured in the principal neurons, the uIPSCs originating from the distinct interneuron types differed in rise time, failure rate, latency, and short-term dynamics. Moreover, the asynchronous transmitter release induced by a train of action potentials was typical for the output synapses of CCK/CB1BCs. Our results suggest that, despite the similar uIPSC magnitudes originating from the three perisomatic inhibitory cell types, their distinct release properties together with the marked differences in their spiking characteristics may contribute to accomplish specific functions in amygdala network operation.

Synaptic Organization of Perisomatic GABAergic Inputs onto the Principal Cells of the Mouse Basolateral Amygdala.

Spike generation is most effectively controlled by inhibitory inputs that target the perisomatic region of neurons. Despite the critical importance of this functional domain, very little is known about the organization of the GABAergic inputs contacting the perisomatic region of principal cells (PCs) in the basolateral amygdala. Using immunocytochemistry combined with in vitro single-cell labeling we determined the number and sources of GABAergic inputs of PCs at light and electron microscopic levels in mice. We found that the soma and proximal dendrites of PCs were innervated primarily by two neurochemically distinct basket cell types expressing parvalbumin (PVBC) or cholecystokinin and CB1 cannabinoid receptors (CCK/CB1BC). The innervation of the initial segment of PC axons was found to be parceled out by PVBCs and axo-axonic cells (AAC), as the majority of GABAergic inputs onto the region nearest to the soma (between 0 and 10 µm) originated from PVBCs, while the largest portion of the axon initial segment was innervated by AACs. Detailed morphological investigations revealed that the three perisomatic region-targeting interneuron types significantly differed in dendritic and axonal arborization properties. We found that, although individual PVBCs targeted PCs via more terminals than CCK/CB1BCs, similar numbers (15-17) of the two BC types converge onto single PCs, whereas fewer (6-7) AACs innervate the axon initial segment of single PCs. Furthermore, we estimated that a PVBC and a CCK/CB1BC may target 800-900 and 700-800 PCs, respectively, while an AAC can innervate 600-650 PCs. Thus, BCs and AACs innervate ~10 and 20% of PC population, respectively, within their axonal cloud. Our results collectively suggest, that these interneuron types may be differently affiliated within the local amygdalar microcircuits in order to fulfill specific functions in network operation during various brain states.

The long-term impact of footshock stress on addiction-related behaviors in rats.

We investigated the impact of electric shocks--frequently used to model post-traumatic stress disorder in rodents--on behaviors relevant to drug abuse in rats. Rats exposed to 10 shocks of 3 mA over 5 min showed a robust conditioned fear 28 days later, which confirms the traumatic nature of shock exposure. A different set of rats was studied in the conditioned place preference paradigm beginning with the 27th post-shock day. 10mg/kg morphine induced a marked place preference in both shocked and non-shocked rats. Although the magnitude of place preference was not affected, extinction was markedly delayed in shocked rats. We also investigated tolerance to the hyperthermic effects of morphine. A low dose (5mg/kg) that was administered 4 weeks after shock exposure robustly increased body temperature in both shocked and non-shocked rats. Repeated injections resulted in a mild tolerance in non-shocked controls; yet, morphine readily increased body temperature in these rats on the 5th day of injections. In contrast, the temperature-heightening effect of morphine was abolished in shocked rats after 2 days. Thus, shock exposure considerably delayed the extinction of place preference induced by, and dramatically accelerated the tolerance to the effects of, morphine. Our study shows that electric shocks durably affect behavior in tests relevant to drug abuse in conjunction with the development of post-traumatic stress disorder-like behavioral dysfunctions.

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.

The context specificity of anxiety responses induced by chronic psychosocial stress in rats: a shift from anxiety to social phobia?

The aim of the present study was to evaluate whether the anxiety-increasing effects of chronic psychosocial stress generalize to non-social (i.e. heterotypic) stressful situations. To investigate this issue, we repeatedly exposed rats to predictable or unpredictable psychosocial stress for 5 or 12 days and examined their anxiety in two markedly different contexts: the elevated plus maze and social interaction tests. Psychosocial stress and the social interaction test were administered under highly similar conditions, i.e. the two situations were homotypic. Psychosocial stress did not affect anxiety in the elevated plus-maze under any condition, but markedly increased anxiety in the social interaction test. In contrast, repeated restraint-a non-social stressor heterotypic to both the elevated plus maze and social interaction tests-increased plus-maze anxiety, demonstrating that anxiety in this test was sensitive to repeated restraint, and the effects were manifested in heterotypic situations. Thus, the anxiety-related effects of chronic psychosocial stress-unlike those of the chronic non-social stressor-were context-dependent. This is reminiscent of phobic anxiety, which manifests in specific situations only. In addition, behavior in the social interaction test showed changes that went beyond simple anxiogenesis. Socially stressed rats spent nearly 40% of total time in aggressive interactions. Based on recent data showing that social phobics are prone to violence under social pressure, and also based on the situation-dependent effects of the social stressor, we suggest that chronic psychosocial stress leads to a behavioral profile akin to social phobia.

Early social deprivation induces disturbed social communication and violent aggression in adulthood.

Disturbed social relations during childhood (e.g., social neglect) often lead to aggression-related psychopathologies in adulthood. Social isolation also increased aggressiveness in laboratory animals. Here the authors show in rats, that social isolation from weaning not only increases the level of aggressiveness, but results in abnormal attack patterns and deficits in social communication. In socially deprived rats, the share of attacks aimed at vulnerable body parts of opponents (head, throat, and belly) dramatically increased and the attack/threat ratio was shifted toward attacks, suggesting a decrease in intention signaling. Moreover, a Multiple Regression Analysis showed that the nonassociation of attacks with offensive threats predicted the occurrence of vulnerable attacks with 81.1% accuracy. The authors suggest that the social deprivation-induced abnormal aggression models the aggression-related problems resulting from early social neglect in humans, and studies on its brain mechanisms may increase our understanding of the mechanisms underlying psychopathologies resulting from early social problems.

The effect of neurokinin1 receptor blockade on territorial aggression and in a model of violent aggression.

BACKGROUND: Neurokinin1 (NK1) receptor blockers were recently proposed for the treatment of anxiety and depression. Disparate data suggest that NK1 receptors are also involved in the control of aggressiveness, but their role is poorly known. METHODS: We evaluated the aggression-induced activation of NK1 neurons by double-labeling brain sections for NK1 receptors and c-Fos in two laboratory models of aggression. We also studied the effects of the NK1 antagonist L-703,606 in these models. RESULTS: Aggressive encounters activated a large number of NK1 receptor-expressing neurons in areas relevant for aggression control. The activation was aggression-specific, because the effects of psychosocial encounters (that allowed sensory but not physical contacts) were markedly weaker. In the medial amygdala, the activation of neurons expressing NK1 receptors showed a marked positive correlation with the occurrence of violent attacks. In resident/intruder conflicts, NK1 blockade lowered the number of hard bites, without affecting milder forms of attack. In the model of violent aggression, attacks on vulnerable body parts of opponents (the main indicators of violence in this model) were decreased to the levels seen in control subjects. Autonomic deficits seen in the model of violent aggression were also ameliorated. The effects of the compound were not secondary to changes in locomotion or in the behavior of intruders. CONCLUSIONS: Our data show that neurons expressing NK1 receptors are involved in the control of aggressiveness, especially in the expression of violent attacks. This suggests that NK1 antagonists-beyond anxiety and depression-might also be useful in the treatment of aggressiveness and violence.

The effect glucocorticoids on aggressiveness in established colonies of rats.

It was repeatedly shown that glucocorticoids increase aggressiveness when subjects are socially challenged. However, the interaction between challenge exposure and glucocorticoid effects was not investigated yet. We studied this interaction by assessing the effects of glucocorticoids in established colonies of rats, i.e. in rats that were not exposed to an acute social challenge. Aggressiveness was high immediately after colony formation but decreased sharply within 4 days and remained stable thereafter. Mild dominance relations were observed in 11 colonies (65%). Approximately three weeks after colony formation, rats remained undisturbed or were injected with vehicle or corticosterone. Routine colony life was followed for 1h after treatments. Injections per se induced a mild and transient behavioral activation: resting was reduced, whereas exploration, social and agonistic interactions were increased. The change lasted about 15min. Corticosterone--although plasma corticosterone levels were increased--had no specific effect, as the behavior of vehicle- and corticosterone-treated rats was similar. Social rank had a minor impact on the results. In contrast, the pro-aggressive effects of corticosterone were robust under conditions of social challenge and were maintained after repeated exposure to aggressive encounters. It occurs that an acute increase in glucocorticoids promotes social challenge-induced aggressiveness, but does not increase aggressiveness under routine conditions. We hypothesize that the pro-aggressive effects of glucocorticoids develop in conjunction with challenge-induced neuronal (e.g. monoaminergic) activation.

Behavioral specificity of non-genomic glucocorticoid effects in rats: effects on risk assessment in the elevated plus-maze and the open-field.

The rapid effects of glucocorticoids on various behaviors suggest that these hormones play a role in rapidly coping with challenging situations. The variety of behaviors affected in different situations raise, however, questions regarding the specificity and roles of glucocorticoids in controlling behavior. To clarify this issue, we assessed the rapid behavioral effects of glucocorticoids in the elevated plus-maze (EPM) and the open-field (OF) tests in male rats. Both tests measure three different kinds of behavioral responses: locomotion, anxiety-like behaviors (central area and open arm exploration in the OF and EPM tests, respectively), and risk assessment (investigating aversive areas in a stretched attend posture). The acute inhibition of glucocorticoid synthesis by metyrapone decreased risk assessment but did not affect locomotion and anxiety-like behaviors. Corticosterone administration increased risk assessment, without affecting locomotion and anxiety-like behaviors. Moreover, plasma corticosterone levels measured immediately after testing strongly correlated with the intensity of risk assessment. The effects of corticosterone were rapid, as occurred even when the hormone was injected 2 min before behavioral testing. In addition, the effect was resistant to protein synthesis inhibition. These data demonstrate that glucocorticoids are able to increase specifically risk assessment behaviors by non-genomic mechanisms in two different, novelty-related, non-social challenging situations. Thus, glucocorticoids appear to rapidly induce specific behavioral adjustments to meet immediate requirements set by the challenge. These data support earlier assumptions on the role of glucocorticoids in coping, and it can be hypothesized that the rapid activation of the HPA-axis may play a role in forming coping responses.