Dynamic patterns of colocalization of calbindin, parvalbumin and GABA in subpopulations of mouse basolateral amygdalar cells during development

Long-Lived Organotypic Slice Culture Model of the Rat Basolateral Amygdala

Organotypic slice cultures (OTCs) have been employed in the laboratory since the early 1980s and have proved to be useful for the study of a number of neural systems. Our recent work focuses on the development of behavioral stress resilience induced by repeated daily injections of neuropeptide Y into the basolateral amygdala (BLA). Resilience develops over weeks, persisting to 8 weeks. To unravel the cellular mechanisms underlying neuropeptide Y-induced stress resilience we developed in vitro OTCs of the BLA. Here, we provide an optimized protocol that consistently yields viable and healthy OTCs containing the BLA and surrounding tissue using the interface method, prepared with slices taken from postnatal (P) day 14 rats. We explain key points to optimizing tissue viability and discuss mitigation or avoidance of pitfalls that can arise to aid in successful implementation of this technique. We show that principal neurons in BLA OTCs (8 weeks in vitro = equivalent postnatal day 70) develop into networks that are electrophysiologically very similar to those from acute slices obtained from older rats (P70) and respond to pharmacological treatments in a comparable way. Furthermore, we highlight how these cultures be used to further understand the molecular, cellular, and circuit-level neuropathophysiological changes underlying stress disorders. BLA OTCs provide long-term physiological and pharmacological results whose predictions were borne out in vivo, supporting the validity of the BLA OTC as a model to unravel BLA neurocircuitry. Recent preliminary results also support the successful application of this approach to preparing long-lived OTCs of BLA and neocortex from mice. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Organotypic slice culture Support Protocol 1: Changing medium Support Protocol 2: Drug incubations Basic Protocol 2: Excision of OTC slices from inserts Support Protocol 3: Fixation of slices.

Effects of Early Life Stress on the Developing Basolateral Amygdala-Prefrontal Cortex Circuit: The Emerging Role of Local Inhibition and Perineuronal Nets

The links between early life stress (ELS) and the emergence of psychopathology such as increased anxiety and depression are now well established, although the specific neurobiological and developmental mechanisms that translate ELS into poor health outcomes are still unclear. The consequences of ELS are complex because they depend on the form and severity of early stress, duration, and age of exposure as well as co-occurrence with other forms of physical or psychological trauma. The long term effects of ELS on the corticolimbic circuit underlying emotional and social behavior are particularly salient because ELS occurs during critical developmental periods in the establishment of this circuit, its local balance of inhibition:excitation and its connections with other neuronal pathways. Using examples drawn from the human and rodent literature, we review some of the consequences of ELS on the development of the corticolimbic circuit and how it might impact fear regulation in a sex- and hemispheric-dependent manner in both humans and rodents. We explore the effects of ELS on local inhibitory neurons and the formation of perineuronal nets (PNNs) that terminate critical periods of plasticity and promote the formation of stable local networks. Overall, the bulk of ELS studies report transient and/or long lasting alterations in both glutamatergic circuits and local inhibitory interneurons (INs) and their associated PNNs. Since the activity of INs plays a key role in the maturation of cortical regions and the formation of local field potentials, alterations in these INs triggered by ELS might critically participate in the development of psychiatric disorders in adulthood, including impaired fear extinction and anxiety behavior.

Early life adversity decreases pre-adolescent fear expression by accelerating amygdala PV cell development

Early life adversity (ELA) is associated with increased risk for stress-related disorders later in life. The link between ELA and risk for psychopathology is well established but the developmental mechanisms remain unclear. Using a mouse model of resource insecurity, limited bedding (LB), we tested the effects of LB on the development of fear learning and neuronal structures involved in emotional regulation, the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA). LB delayed the ability of peri-weanling (21 days old) mice to express, but not form, an auditory conditioned fear memory. LB accelerated the developmental emergence of parvalbumin (PV)-positive cells in the BLA and increased anatomical connections between PL and BLA. Fear expression in LB mice was rescued through optogenetic inactivation of PV-positive cells in the BLA. The current results provide a model of transiently blunted emotional reactivity in early development, with latent fear-associated memories emerging later in adolescence.

Heterogeneity of neuronal firing type and morphology in retrosplenial cortex of male F344 rats

The rodent granular retrosplenial cortex (gRSC) has reciprocal connections to the hippocampus to support fear memories. Although activity-dependent plasticity occurs within the RSC during memory formation, the intrinsic and morphological properties of RSC neurons are poorly understood. The present study used whole-cell recordings to examine intrinsic neuronal firing and morphology of neurons in layer 2/3 (L2/3) and layer 5 (L5) of the gRSC in adult male rats. Five different classifications were observed: regular-spiking (RS), regular-spiking afterdepolarization (RS_ADP), late-spiking (LS), burst-spiking (BS), and fast-spiking (FS) neurons. RS_ADP neurons were the most commonly observed neuronal class, identified by their robust spike frequency adaptation and pronounced afterdepolarization (ADP) following an action potential (AP). They also had the most extensive dendritic branching compared with other cell types. LS neurons were predominantly found in L2/3 and exhibited a long delay before onset of their initial AP. They also had reduced dendritic branching compared with other cell types. BS neurons were limited to L5 and generated an initial burst of two or more APs. FS neurons demonstrated sustained firing and little frequency adaptation and were the only nonpyramidal firing type. Relative to adults, RS neurons from juvenile rats (PND 14-30) lacked an ADP and were less excitable. Bath application of group 1 mGluR blockers attenuated the ADP in adult neurons. In other fear-related brain structures, the ADP has been shown to enhance excitability and synaptic plasticity. Thus, understanding cellular mechanisms of the gRSC will provide insight regarding its precise role in memory-related processes across the lifespan.NEW & NOTEWORTHY This is the first study to demonstrate that granular retrosplenial cortical (gRSC) neurons exhibit five distinctive firing types: regular spiking (RS), regular spiking with an afterdepolarization (RS_ADP), late spiking (LS), burst spiking (BS), and fast spiking (FS). RS_ADP neurons were the most frequently observed cell type in adult gRSC neurons. Interestingly, RS neurons without an ADP were most common in gRSC neurons of juvenile rats (PND 14-30). Thus, the ADP property, which was previously shown to enhance neuronal excitability, emerges during development.

Circuits for social learning: A unified model and application to Autism Spectrum Disorder

Early life social experiences shape neural pathways in infants to develop lifelong social skills. This review presents the first unified circuit-based model of social learning that can be applied to early life social development, drawing together unique human developmental milestones, sensitive learning periods, and behavioral and neural scaffolds. Circuit domains for social learning are identified governing Activation, Integration, Discrimination, Response and Reward (AIDRR) to sculpt and drive human social learning. This unified model can be used to identify social delays earlier in development. We propose social impairments observed in Autism Spectrum Disorder are underpinned by early mistimed sensitive periods in brain development and alterations in amygdala development to disrupt the AIDRR circuits. This model directs how interventions can target neural circuits for social development and be applied early in life. To illustrate, the role of oxytocin and its use as an intervention is explored. The AIDRR model shifts focus away from delivering broad treatments based only on diagnostic classifications, to specifying and targeting the relevant circuits, at the right time of development, to optimize social learning.

Characterization of the relaxin family peptide receptor 3 system in the mouse bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a critical node involved in stress and reward-related behaviors. Relaxin family peptide receptor 3 (RXFP3) signaling in the BNST has been implicated in stress-induced alcohol seeking behavior. However, the neurochemical phenotype and connectivity of BNST RXFP3-expressing (RXFP3+) cells have yet to be elucidated. We interrogated the molecular signature and electrophysiological properties of BNST RXFP3+ neurons using a RXFP3-Cre reporter mouse line. BNST RXFP3+ cells are circumscribed to the dorsal BNST (dBNST) and are neurochemically heterogeneous, comprising a mix of inhibitory and excitatory neurons. Immunohistochemistry revealed that ~48% of BNST RXFP3+ neurons are GABAergic, and a quarter of these co-express the calcium-binding protein, calbindin. A subset of BNST RXFP3+ cells (~41%) co-express CaMKIIα, suggesting this subpopulation of BNST RXFP3+ neurons are excitatory. Corroborating this, RNAscope® revealed that ~35% of BNST RXFP3+ cells express vVGluT2 mRNA, indicating a subpopulation of RXFP3+ neurons are glutamatergic. RXFP3+ neurons show direct hyperpolarization to bath application of a selective RXFP3 agonist, RXFP3-A2, while around 50% of cells were depolarised by exogenous corticotrophin releasing factor. In behaviorally naive mice the majority of RXFP3+ neurons were Type II cells exhibiting I_h and T type calcium mediated currents. However, chronic swim stress caused persistent plasticity, decreasing the proportion of neurons that express these channels. These studies are the first to characterize the BNST RXFP3 system in mouse and lay the foundation for future functional studies appraising the role of the murine BNST RXFP3 system in more complex behaviors.

Limited prefrontal cortical regulation over the basolateral amygdala in adolescent rats

Cognitive regulation of emotion develops from childhood into adulthood. This occurs in parallel with maturation of prefrontal cortical (PFC) regulation over the amygdala. The cellular substrates for this regulation may include PFC activation of inhibitory GABAergic elements in the amygdala. The purpose of this study was to determine whether PFC regulation over basolateral amygdala area (BLA) in vivo is immature in adolescence, and if this is due to immaturity of GABAergic elements or PFC excitatory inputs. Using in vivo extracellular electrophysiological recordings from anesthetized male rats we found that in vivo summation of PFC inputs to the BLA was less regulated by GABAergic inhibition in adolescents (postnatal day 39) than adults (postnatal day 72-75). In addition, stimulation of either prelimbic or infralimbic PFC evokes weaker inhibition over basal (BA) and lateral (LAT) nuclei of the BLA in adolescents. This was dictated by both weak recruitment of inhibition in LAT and weak excitatory effects of PFC in BA. The current results may contribute to differences in adolescent cognitive regulation of emotion. These findings identify specific elements that undergo adolescent maturation and may therefore be sensitive to environmental disruptions that increase risk for psychiatric disorders.

Development and physiology of GABAergic feedback excitation in parvalbumin expressing interneurons of the mouse basolateral amygdala

We have previously shown that in the basolateral amygdala (BLA), action potentials in one type of parvalbumin (PV)-expressing GABAergic interneuron can evoke a disynaptic feedback excitatory postsynaptic potential (fbEPSP) onto the same presynaptic interneuron. Here, using whole-cell recordings from PV-expressing interneurons in acute brain slices we expand on this finding to show that this response is first detectable at 2-week postnatal, and is most prevalent in animals beyond 3 weeks of age (>P21). This circuit has a very high fidelity, and single action potential evoked fbEPSPs display few failures. Reconstruction of filled neurons, and electron microscopy show that interneurons that receive feedback excitation make symmetrical synapses on both the axon initial segments (AIS), as well as the soma and proximal dendrites of local pyramidal neurons, suggesting fbEPSP interneurons are morphologically distinct from the highly specialized chandelier neurons that selectively target the axon initial segment of pyramidal neurons. Single PV interneurons could trigger very large (~ 1 nA) feedback excitatory postsynaptic currents (fbEPSCs) suggesting that these neurons are heavily reciprocally connected to local glutamatergic principal cells. We conclude that in the BLA, a subpopulation of PV interneurons forms a distinct neural circuit in which a single action potential can recruit multiple pyramidal neurons to discharge near simultaneously and feed back onto the presynaptic interneuron.

Effects of Repeated Stress on Age-Dependent GABAergic Regulation of the Lateral Nucleus of the Amygdala

The adolescent age is associated with lability of mood and emotion. The onset of depression and anxiety disorders peaks during adolescence and there are differences in symptomology during adolescence. This points to differences in the adolescent neural circuitry that underlies mood and emotion, such as the amygdala. The human adolescent amygdala is more responsive to evocative stimuli, hinting to less local inhibitory regulation of the amygdala, but this has not been explored in adolescents. The amygdala, including the lateral nucleus (LAT) of the basolateral amygdala complex, is sensitive to stress. The amygdala undergoes maturational processes during adolescence, and therefore may be more vulnerable to harmful effects of stress during this time period. However, little is known about the effects of stress on the LAT during adolescence. GABAergic inhibition is a key regulator of LAT activity. Therefore, the purpose of this study was to test whether there are differences in the local GABAergic regulation of the rat adolescent LAT, and differences in its sensitivity to repeated stress. We found that LAT projection neurons are subjected to weaker GABAergic inhibition during adolescence. Repeated stress reduced in vivo endogenous and exogenous GABAergic inhibition of LAT projection neurons in adolescent rats. Furthermore, repeated stress decreased measures of presynaptic GABA function and interneuron activity in adolescent rats. In contrast, repeated stress enhanced glutamatergic drive of LAT projection neurons in adult rats. These results demonstrate age differences in GABAergic regulation of the LAT, and age differences in the mechanism for the effects of repeated stress on LAT neuron activity. These findings provide a substrate for increased mood lability in adolescents, and provide a substrate by which adolescent repeated stress can induce distinct behavioral outcomes and psychiatric symptoms.

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

Altered activity of the medial prefrontal cortex and amygdala during acquisition and extinction of an active avoidance task

Altered medial prefrontal cortex (mPFC) and amygdala function is associated with anxiety-related disorders. While the mPFC-amygdala pathway has a clear role in fear conditioning, these structures are also involved in active avoidance. Given that avoidance perseveration represents a core symptom of anxiety disorders, the neural substrate of avoidance, especially its extinction, requires better understanding. The present study was designed to investigate the activity, particularly, inhibitory neuronal activity in mPFC and amygdala during acquisition and extinction of lever-press avoidance in rats. Neural activity was examined in the mPFC, intercalated cell clusters (ITCs) lateral (LA), basal (BA) and central (CeA) amygdala, at various time points during acquisition and extinction, using induction of the immediate early gene product, c-Fos. Neural activity was greater in the mPFC, LA, BA, and ITC during the extinction phase as compared to the acquisition phase. In contrast, the CeA was the only region that was more activated during acquisition than during extinction. Our results indicate inhibitory neurons are more activated during late phase of acquisition and extinction in the mPFC and LA, suggesting the dynamic involvement of inhibitory circuits in the development and extinction of avoidance response. Together, these data start to identify the key brain regions important in active avoidance behavior, areas that could be associated with avoidance perseveration in anxiety disorders.

Postnatal maturation of GABAergic modulation of sensory inputs onto lateral amygdala principal neurons

KEY POINTS

Throughout life, fear learning is indispensable for survival and neural plasticity in the lateral amygdala underlies this learning and storage of fear memories. During development, properties of fear learning continue to change into adulthood, but currently little is known about changes in amygdala circuits that enable these behavioural transitions. In recordings from neurons in lateral amygdala brain slices from infant up to adult mice, we show that spontaneous and evoked excitatory and inhibitory synaptic transmissions mature into adolescence. At this time, increased inhibitory activity and signalling has the ability to restrict the function of excitation by presynaptic modulation, and may thus enable precise stimulus associations to limit fear generalization from adolescence onward. Our results provide a basis for addressing plasticity mechanisms that underlie altered fear behaviour in young animals.

ABSTRACT

Convergent evidence suggests that plasticity in the lateral amygdala (LA) participates in acquisition and storage of fear memory. Sensory inputs from thalamic and cortical areas activate principal neurons and local GABAergic interneurons, which provide feed-forward inhibition that tightly controls LA activity and plasticity via pre- and postsynaptic GABAA and GABAB receptors. GABAergic control is also critical during fear expression, generalization and extinction in adult animals. During rodent development, properties of fear and extinction learning continue to change into early adulthood. Currently, few studies have assessed physiological changes in amygdala circuits that may enable these behavioural transitions. To obtain first insights, we investigated changes in spontaneous and sensory input-evoked inhibition onto LA principal neurons and then focused on GABAB receptor-mediated modulation of excitatory sensory inputs in infant, juvenile, adolescent and young adult mice. We found that spontaneous and sensory-evoked inhibition increased during development. Physiological changes were accompanied by changes in dendritic morphology. While GABAB heteroreceptors were functionally expressed on sensory afferents already early in development, they could only be physiologically recruited by sensory-evoked GABA release to mediate heterosynaptic inhibition from adolescence onward. Furthermore, we found GABAB -mediated tonic inhibition of sensory inputs by ambient GABA that also emerged in adolescence. The observed increase in GABAergic drive may be a substrate for providing modulatory GABA. Our data suggest that GABAB -mediated tonic and evoked presynaptic inhibition can suppress sensory input-driven excitation in the LA to enable precise stimulus associations and limit generalization of conditioned fear from adolescence onward.

Postnatal maturation of GABAergic transmission in the rat basolateral amygdala

Many psychiatric disorders, including anxiety and autism spectrum disorders, have early ages of onset and high incidence in juveniles. To better treat and prevent these disorders, it is important to first understand normal development of brain circuits that process emotion. Healthy and maladaptive emotional processing involve the basolateral amygdala (BLA), dysfunction of which has been implicated in numerous psychiatric disorders. Normal function of the adult BLA relies on a fine balance of glutamatergic excitation and GABAergic inhibition. Elsewhere in the brain GABAergic transmission changes throughout development, but little is known about the maturation of GABAergic transmission in the BLA. Here we used whole cell patch-clamp recording and single-cell RT-PCR to study GABAergic transmission in rat BLA principal neurons at postnatal day (P)7, P14, P21, P28, and P35. GABAA currents exhibited a significant twofold reduction in rise time and nearly 25% reduction in decay time constant between P7 and P28. This corresponded with a shift in expression of GABAA receptor subunit mRNA from the α2- to the α1-subunit. The reversal potential for GABAA receptors transitioned from depolarizing to hyperpolarizing with age, from around -55 mV at P7 to -70 mV by P21. There was a corresponding shift in expression of opposing chloride pumps that influence the reversal, from NKCC1 to KCC2. Finally, we observed short-term depression of GABAA postsynaptic currents in immature neurons that was significantly and gradually abolished by P28. These findings reveal that in the developing BLA GABAergic transmission is highly dynamic, reaching maturity at the end of the first postnatal month.

Repeated restraint stress enhances cue-elicited conditioned freezing and impairs acquisition of extinction in an age-dependent manner

Affective disorders are believed to involve dysfunction within the amygdala, a key structure for processing emotional information. Chronic stress may contribute to affective disorders such as depression and anxiety via its effects on the amygdala. Previous research has shown that chronic stress increases amygdala neuronal activity in an age-dependent manner. However, whether these distinct changes in amgydala neuronal activity are accompanied by age-dependent changes in amygdala-dependent affective behavior is unclear. In this study, we investigated how chronic stress impacts amgydala-dependent auditory fear conditioning in adolescent and adult rats in a repeated restraint model. We found that repeated restraint enhanced conditioned freezing in both adolescent and adult rats. But repeated restraint led to impaired acquisition of fear extinction only in adolescent rats. Along with previous findings, these results suggest that chronic stress may precipitate affective disorders via differential mechanisms, with different outcomes at different ages.

Stuttering interneurons generate fast and robust inhibition onto projection neurons with low capacity of short term modulation in mouse lateral amygdala

The stuttering interneurons (STi) represent one minor subset of interneuron population and exhibit characteristic stuttering firing upon depolarization current injection. While it has been long held that the GABAergic inhibitory transmission largely varies with the subtype identity of presynaptic interneurons, whether such a rule also applies to STi is largely unknown. Here, by paired recording of interneuron and their neighboring projection neuron in lateral amygdala, we found that relative to the fast spiking and late spiking interneurons, the STi-evoked unitary postsynaptic currents onto the projection neurons had markedly larger amplitude, shorter onset latency and faster rising and decay kinetics. The quantal content and the number of vesicles in the readily releasable pool were also larger in synapses made by STi versus other interneurons. Moreover, the short-term plasticity, as reflected by the paired pulse depression and depolarization-induced suppression of inhibition, was the least prominent in the output synapses of STi. Thus, the fast and robust inhibition together with its low capacity of short term modulation may suggest an important role for STi in preventing the overexcitation of the projection neurons and thus gating the information traffic in amygdala.

Pattern of calbindin-D28k and calretinin immunoreactivity in the brain of Xenopus laevis during embryonic and larval development

The present study represents a detailed spatiotemporal analysis of the localization of calbindin-D28k (CB) and calretinin (CR) immunoreactive structures in the brain of Xenopus laevis throughout development, conducted with the aim to correlate the onset of the immunoreactivity with the development of compartmentalization of distinct subdivisions recently identified in the brain of adult amphibians and primarily highlighted when analyzed within a segmental paradigm. CR and CB are expressed early in the brain and showed a progressively increasing expression throughout development, although transient expression in some neuronal subpopulations was also noted. Common and distinct characteristics in Xenopus, as compared with reported features during development in the brain of mammals, were observed. The development of specific regions in the forebrain such as the olfactory bulbs, the components of the basal ganglia and the amygdaloid complex, the alar and basal hypothalamic regions, and the distinct diencephalic neuromeres could be analyzed on the basis of the distinct expression of CB and CR in subregions. Similarly, the compartments of the mesencephalon and the main rhombencephalic regions, including the cerebellum, were differently highlighted by their specific content in CB and CR throughout development. Our results show the usefulness of the analysis of the distribution of these proteins as a tool in neuroanatomy to interpret developmental aspects of many brain regions.

GABA concentration and GABAergic neuron populations in limbic areas are differentially altered by brain serotonin deficiency in Tph2 knockout mice

While tryptophan hydroxylase-2 (Tph2) null mutant (Tph2(-/-)) mice are completely deficient in brain serotonin (5-HT) synthesis, the formation of serotonergic neurons and pathfinding of their projections are not impaired. However, 5-HT deficiency, during development and in the adult, might affect morphological and functional parameters of other neural systems. To assess the influence of 5-HT deficiency on γ-amino butyric acid (GABA) systems, we carried out measurements of GABA concentrations in limbic brain regions of adult male wildtype (wt), heterozygous (Tph2(+/-)) and Tph2(-/-) mice. In addition, unbiased stereological estimation of GABAergic interneuron numbers and density was performed in subregions of amygdala and hippocampus. Amygdala and prefrontal cortex displayed significantly increased and decreased GABA concentrations, respectively, exclusively in Tph2(+/-) mice while no changes were detected between Tph2(-/-) and wt mice. In contrast, in the hippocampus, increased GABA concentrations were found in Tph2(-/-) mice. While total cell density in the anterior basolateral amygdala did not differ between genotypes, the number and density of the GABAergic interneurons were significantly decreased in Tph2(-/-) mice, with the group of parvalbumin (PV)-immunoreactive (ir) interneurons contributing somewhat less to the decrease than that of non-PV-ir GABAergic interneurons. Major morphological changes were also absent in the dorsal hippocampus, and only a trend toward reduced density of PV-ir cells was observed in the CA3 region of Tph2(-/-) mice. Our findings are the first to document that life-long reduction or complete lack of brain 5-HT transmission causes differential changes of GABA systems in limbic regions which are key players in emotional learning and memory processes. The changes likely reflect a combination of developmental alterations and functional adaptations of emotion circuits to balance the lack of 5-HT, and may underlie altered emotional behavior in 5-HT-deficient mice. Taken together, our findings provide further insight into the mechanisms how life-long 5-HT deficiency impacts the pathogenesis of anxiety- and fear-related disorders.

Postnatal development of electrophysiological properties of principal neurons in the rat basolateral amygdala

The basolateral amygdala (BLA) is critically involved in the pathophysiology of psychiatric disorders, which often emerge during brain development. Several studies have characterized postnatal changes to the morphology and biochemistry of BLA neurons, and many more have identified sensitive periods of emotional maturation. However, it is impossible to determine how BLA development contributes to emotional development or the aetiology of psychiatric disorders because no study has characterized the physiological maturation of BLA neurons. We addressed this critical knowledge gap for the first time using whole-cell patch clamp recording in rat BLA principal neurons to measure electrophysiological properties at postnatal day (P)7, P10, P14, P21, P28 and after P35. We show that intrinsic properties of these neurons undergo significant transitions before P21 and reach maturity around P28. Specifically, we observed significant reductions in input resistance and membrane time constant of nearly 10-and 4-fold, respectively, from P7 to P28. The frequency selectivity of these neurons to input also changed significantly, with peak resonance frequency increasing from 1.0 Hz at P7 to 5.7 Hz at P28. In the same period, maximal firing frequency significantly increased and doublets and triplets of action potentials emerged. Concomitantly, individual action potentials became significantly faster, firing threshold hyperpolarized 6.7 mV, the medium AHP became faster and shallower, and a fast AHP emerged. These results demonstrate neurons of the BLA undergo vast change throughout postnatal development, and studies of emotional development and treatments for juvenile psychiatric disorders should consider the dynamic physiology of the immature BLA.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.

Interneurons in the basolateral amygdala

The amygdala is a temporal lobe structure that is the center of emotion processing in the mammalian brain. Recent interest in the amygdala arises from its role in processing fear and the relationship of fear to human anxiety. The amygdaloid complex is divided into a number of subnuclei that have extensive intra and extra nuclear connections. In this review we discuss recent findings on the physiology and plasticity of inputs to interneurons in the basolateral amygdala, the primary input station. These interneurons are a heterogeneous group of cells that can be separated on immunohistochemical and electrophysiological grounds. Glutamatergic inputs to these interneurons form diverse types of excitatory synapses. This diversity is manifest in both the subunit composition of the underlying NMDA receptors as well as their ability to show plasticity. We discuss these differences and their relationship to fear learning. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Alcohol withdrawal and brain injuries: beyond classical mechanisms

Unmanaged sudden withdrawal from the excessive consumption of alcohol (ethanol) adversely alters neuronal integrity in vulnerable brain regions such as the cerebellum, hippocampus, or cortex. In addition to well known hyperexcitatory neurotransmissions, ethanol withdrawal (EW) provokes the intense generation of reactive oxygen species (ROS) and the activation of stress-responding protein kinases, which are the focus of this review article. EW also inflicts mitochondrial membranes/membrane potential, perturbs redox balance, and suppresses mitochondrial enzymes, all of which impair a fundamental function of mitochondria. Moreover, EW acts as an age-provoking stressor. The vulnerable age to EW stress is not necessarily the oldest age and varies depending upon the target molecule of EW. A major female sex steroid, 17β-estradiol (E2), interferes with the EW-induced alteration of oxidative signaling pathways and thereby protects neurons, mitochondria, and behaviors. The current review attempts to provide integrated information at the levels of oxidative signaling mechanisms by which EW provokes brain injuries and E2 protects against it.

GABAergic interneurons in the mouse lateral amygdala: a classification study

Whole cell patch-clamp recordings were performed in GABAergic interneurons labeled by green fluorescent protein (GFP) in the lateral amygdala (LA) in vitro from glutamic acid decarboxylase 67 (GAD67)-GFP mice. Neurons were characterized by electrotonic and electrogenic parameters. Cytoplasm was collected from individual neurons, and single-cell RT-PCR was used for detection of molecular markers typifying LA interneurons. Hierarchical cluster and multiple discriminant analysis demonstrated the existence of five types of GABAergic interneurons, which can be reliably identified through electrophysiological criteria. Action potentials were of a short duration followed by pronounced fast afterhyperpolarization (AHP) in interneurons of all types, except for type V, which generated broad action potentials and displayed typical spike bursts at the beginning of depolarizing stimuli and prominent anomalous inward rectification. Interneurons of type I and II generated series of action potentials with frequency adaptation on maintained depolarizing current stimulation with overall frequencies at high levels and presented delayed firing, stuttering or fast-spiking behavior. Further distinguishing features of type II interneurons were a medium AHP following spike trains and pronounced anomalous inward rectification. Types III and IV of neurons fired regularly, whereas type IV displayed no prominent spike frequency adaptation. Additionally, interneurons of all five types contained mRNA of glutamic acid decarboxylase 65 and cholecystokinin, whereas only type I interneurons were somatostatin-positive. Overall, these data represent a detailed and reliable classification scheme of LA GABAergic interneurons and will provide a feasible basis for subsequent functional studies.