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 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.

Correlated species differences in the effects of cannabinoid ligands on anxiety and on GABAergic and glutamatergic synaptic transmission

Cannabinoid ligands show therapeutic potential in a variety of disorders including anxiety. However, the anxiety-related effects of cannabinoids remain controversial as agonists show opposite effects in mice and rats. Here we compared the effects of the cannabinoid agonist WIN-55,212 and the CB1 antagonist AM-251 in CD1 mice and Wistar rats. Special attention was paid to antagonist–agonist interactions, which had not yet been studied in rats. In mice, WIN-55,212 decreased whereas AM-251 increased anxiety. The antagonist abolished the effects of the agonist. In contrast, WIN-55,212 increased anxiety in rats. Surprisingly, the antagonist potentiated this effect. Cannabinoids affect both GABAergic and glutamatergic functions, which play opposite roles in anxiety. We hypothesized that discrepant findings resulted from species differences in the relative responsiveness of the two transmitter systems to cannabinoids. We investigated this hypothesis by studying the effects of WIN-55,212 on evoked hippocampal inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs). IPSCs were one order of magnitude more sensitive to WIN-55,212 in mice than in rats. In mice, IPSCs were more sensitive than EPSCs to WIN-55,212. This is the first study showing that the relative cannabinoid sensitivity of GABA and glutamate neurotransmission is species-dependent. Based on behavioural and electrophysiological findings, we hypothesize that WIN-55,212 reduced anxiety in mice by affecting GABA neurotransmission whereas it increased anxiety in rats via glutamatergic mechanisms. In rats, AM-251 potentiated this anxiogenic effect by inhibiting the anxiolytic GABAergic mechanism. We suggest that the anxiety-related effects of cannabinoids depend on the relative cannabinoid responsiveness of GABAergic and glutamatergic neurotransmission.

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.

Mechanisms underlying the long-term behavioral effects of traumatic experience in rats: the role of serotonin/noradrenaline balance and NMDA receptors

Traumatic stressors induce long-lasting changes in behavior. It is believed that all three glutamatergic, serotonergic and noradrenergic neurotransmission play a role in the development of such behavioral changes, but their relative importance and relationship is poorly understood. We have shown previously that a single exposure of rats to electric shocks induces social avoidance for about 10 days. Here we assessed social avoidance 24 h after shock exposure in rats with chemically lesioned serotonergic and noradrenergic neurons. The effects of the NMDA receptor blocker MK-801 were also studied. When the serotonin/noradrenaline balance was shifted towards serotonergic dominance via chemical lesions, the behavioral dysfunction was markedly attenuated. The disruption of serotonergic neurotransmission (that lead to noradrenergic dominance) significantly increased the behavioral deficit. Shock responding was not secondary to lesion-induced differences in social behavior. Noteworthy, the brain noradrenaline/serotonin ratio correlated negatively with shock-induced social avoidance, suggesting that the ratio rather than absolute levels are important in this respect. In line with this assumption, double lesions had minor effects on social avoidance, suggesting that these monoaminergic systems modulate, but do not mediate the behavioral deficit. The blockade of NMDA receptors abolished the development of stress-induced social avoidance both when applied before shocks and when applied before behavioral testing. We confirmed that the long-term behavioral effects of traumatic experience result from glutamatergic activation, the effects of which are mediated by NMDA receptors. The development of the behavioral deficit is modulated by the balance between serotonergic and noradrenergic neurotransmission, possibly via effects on shock-induced glutamatergic 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.