Differential distribution of 5-HT1A and 5-HT1B-like immunoreactivities in rat central nucleus of the amygdala neurones projecting to the caudal dorsomedial medulla oblongata

Focal pharmacological manipulation of serotonin signaling in the amygdala does not alter social behavior

Serotonin signaling plays critical roles in social and emotional behaviors. Likewise, decades of research demonstrate that the amygdala is a prime modulator of social behavior. Permanent excitotoxic lesions and transient amygdala inactivation consistently increase social behaviors in non-human primates. In rodents, acute systemic administration of drugs that increase serotonin signaling is associated with decreased social interactions. However, in primates, the direct involvement of serotonin signaling in the amygdala, particularly in affiliative social interaction, remains unexplored. Here, we examined the effects of serotonin manipulations within the amygdala on social behavior in eight pairs of familiar male macaques. We microinfused drugs targeting the serotonin system into either the basolateral (BLA) or central (CeA) amygdala and measured changes in social behavior. Surprisingly, the results demonstrated no significant differences in social behavior following the infusion of a selective serotonin reuptake inhibitor, 5-HT1A agonist or antagonist, 5-HT2A agonist or antagonist, or 5-HT3 agonist or antagonist into either the BLA or CeA. These findings suggest that serotonin signaling in the amygdala does not directly contribute to the regulation of social behavior between familiar conspecifics. Future research should explore alternative mechanisms and potential interactions with other brain regions to gain a comprehensive understanding of the complex neural circuitry governing social behavior.

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Serotonergic modulation of basolateral amygdala nucleus in the extinction of reward-driven learning: The role of 5-HT bioavailability and 5-HT1A receptor

Serotonin (5-HT) neurotransmission has been associated with reward-related behaviour. Moreover, the serotonergic system modulates the basolateral amygdala (BLA), a structure involved in reward encoding, and reward prediction error. However, the role played by 5-HT on BLA during a reward-driven task has not been fully elucidated. In this paper, we investigated whether serotonergic modulation of the BLA is involved in reward-driven learning. To this end, we trained Long Evans rats in an operant conditioning task, and examined the effects of fluoxetine treatment (a selective serotonin reuptake inhibitor, 10 mg/kg) in combination with BLA lesions with NMDA (20 mg/mL) on extinction learning. We also investigated whether intra-BLA injection of the serotonergic 5-HT_1A receptor agonist 8-OH DPAT, or antagonist WAY-100635, alters extinction performance. We found that fluoxetine treatment strongly accelerated extinction learning, while BLA lesions partially reverted this effect and slightly impaired consolidation of extinction. Stimulation and inhibition of 5-HT_1A receptors in BLA induced opposite effects to those of fluoxetine, impairing or accelerating extinction performance, respectively. Our findings suggest that 5-HT modulates reward-driven learning, and 5-HT_1A receptors located in the BLA are relevant for extinction.

A balancing act: the role of pro- and anti-stress peptides within the central amygdala in anxiety and alcohol use disorders

The central nucleus of the amygdala (CeA) is widely implicated as a structure that integrates both appetitive and aversive stimuli. While intrinsic CeA microcircuits primarily consist of GABAergic neurons that regulate amygdala output, a notable feature of the CeA is the heterogeneity of neuropeptides and neuropeptide/neuromodulator receptors that it expresses. There is growing interest in the role of the CeA in mediating psychopathologies, including stress and anxiety states and their interactions with alcohol use disorders. Within the CeA, neuropeptides and neuromodulators often exert pro- or anti- stress actions, which can influence anxiety and alcohol associated behaviours. In turn, alcohol use can cause adaptions within the CeA, which may render an individual more vulnerable to stress which is a major trigger of relapse to alcohol seeking. This review examines the neurocircuitry, neurochemical phenotypes and how pro- and anti-stress peptide systems act within the CeA to regulate anxiety and alcohol seeking, focusing on preclinical observations from animal models. Furthermore, literature exploring the targeting of genetically defined populations or neuronal ensembles and the role of the CeA in mediating sex differences in stress x alcohol interactions are explored.

Shape Deformation in the Brainstem of Medication-Naïve Female Patients with Major Depressive Disorder

OBJECTIVE

Although neuroimaging studies have shown volumetric reductions, such as the anterior cingulate, prefrontal cortices, and hippocampus in patients with major depressive disorder (MDD), few studies have investigated the volume of or shape alterations in the subcortical regions and the brainstem. We hypothesized that medication-naïve female adult patients with MDD might present with shape and volume alterations in the subcortical regions, including the brainstem, compared to healthy controls (HCs).

METHODS

A total of 20 medication-naïve female patients with MDD and 21 age-matched female HCs, underwent 3D T1-weighted structural magnetic resonance scanning. We analyzed the volumes of each subcortical region and each brainstem region, including the midbrain, pons, and medulla oblongata. We also performed surface-based vertex analyses on the subcortical areas and brainstem.

RESULTS

Female patients with MDD showed non-significant volumetric differences in the subcortical regions, whole brainstem, and each brainstem region compared to the HCs. However, in the surface-based vertex analyses, significant shape contractions were observed in both cerebellar peduncles located on the lateral wall of the posterior brainstem [threshold-free cluster enhancement, corrected for family-wise error (FWE) at p<0.05] in patients with MDD.

CONCLUSION

We revealed shape alterations in the posterior brainstem in female patients with MDD.

Serotonergic control of GABAergic inhibition in the lateral amygdala

Much evidence implicates the serotonergic regulation of the amygdala in anxiety. Thus the present study was undertaken to characterize the influence of serotonin (5-HT) on principal neurons (PNs) of the rat lateral amygdala (LA), using whole cell recordings in vitro. Because inhibition is a major determinant of PN activity, we focused on the control of GABAergic transmission by 5-HT. IPSCs were elicited by local electrical stimulation of LA in the presence of glutamate receptor antagonists. We found that 5-HT reduces GABA_A inhibitory postsynaptic currents (IPSCs) via presynaptic 5-HT_1B receptors. While the presynaptic inhibition of GABA release also attenuated GABA_B currents, this effect was less pronounced than for GABA_A currents because 5-HT also induced a competing postsynaptic enhancement of GABA_B currents. That is, GABA_B currents elicited by pressure application of GABA or baclofen were enhanced by 5-HT. In addition, we obtained evidence suggesting that 5-HT differentially regulates distinct subsets of GABAergic synapses. Indeed, GABA_A IPSCs were comprised of two components: a relatively 5-HT-insensitive IPSC that had a fast time course and a 5-HT-sensitive component that had a slower time course. Because the relative contribution of these two components varied depending on whether neurons were recorded at proximity versus at a distance from the stimulating electrodes, we speculate that distinct subtypes of local-circuit cells contribute the two contingents of GABAergic synapses. Overall, our results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.NEW & NOTEWORTHY We report that 5-HT, acting via presynaptic 5-HT_1B receptors, attenuates GABA_A IPSCs by reducing GABA release in the lateral amygdala (LA). In parallel, 5-HT enhances GABA_B currents postsynaptically, such that GABA_B inhibitory postsynaptic currents (IPSCs) are relatively preserved from the presynaptic inhibition of GABA release. We also found that the time course of 5-HT-sensitive and -insensitive GABA_A IPSCs differ. Together, these results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.

Serotonin gating of cortical and thalamic glutamate inputs onto principal neurons of the basolateral amygdala

The basolateral amygdala (BLA) is a key site for crossmodal association of sensory stimuli and an important relay in the neural circuitry of emotion. Indeed, the BLA receives substantial glutamatergic inputs from multiple brain regions including the prefrontal cortex and thalamic nuclei. Modulation of glutamatergic transmission in the BLA regulates stress- and anxiety-related behaviors. Serotonin (5-HT) also plays an important role in regulating stress-related behavior through activation of both pre- and postsynaptic 5-HT receptors. Multiple 5-HT receptors are expressed in the BLA, where 5-HT has been reported to modulate glutamatergic transmission. However, the 5-HT receptor subtype mediating this effect is not yet clear. The aim of this study was to use patch-clamp recordings from BLA neurons in an ex vivo slice preparation to examine 1) the effect of 5-HT on extrinsic sensory inputs, and 2) to determine if any pathway specificity exists in 5-HT regulation of glutamatergic transmission. Two independent input pathways into the BLA were stimulated: the external capsule to mimic cortical input, and the internal capsule to mimic thalamic input. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) induced by stimulation of both pathways. The decrease was associated with an increase in the paired-pulse ratio and coefficient of variation of eEPSC amplitude, suggesting 5-HT acts presynaptically. Moreover, the effect of 5-HT in both pathways was mimicked by the selective 5-HT_1B receptor agonist CP93129, but not by the 5-HT_1A receptor agonist 8-OH DPAT. Similarly the effect of exogenous 5-HT was blocked by the 5-HT_1B receptor antagonist GR55562, but not affected by the 5-HT_1A receptor antagonist WAY 100635 or the 5-HT₂ receptor antagonists pirenperone and MDL 100907. Together these data suggest 5-HT gates cortical and thalamic glutamatergic inputs into the BLA by activating presynaptic 5-HT_1B receptors.

Serotonin, Amygdala and Fear: Assembling the Puzzle

The fear circuitry orchestrates defense mechanisms in response to environmental threats. This circuitry is evolutionarily crucial for survival, but its dysregulation is thought to play a major role in the pathophysiology of psychiatric conditions in humans. The amygdala is a key player in the processing of fear. This brain area is prominently modulated by the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). The 5-HT input to the amygdala has drawn particular interest because genetic and pharmacological alterations of the 5-HT transporter (5-HTT) affect amygdala activation in response to emotional stimuli. Nonetheless, the impact of 5-HT on fear processing remains poorly understood.The aim of this review is to elucidate the physiological role of 5-HT in fear learning via its action on the neuronal circuits of the amygdala. Since 5-HT release increases in the basolateral amygdala (BLA) during both fear memory acquisition and expression, we examine whether and how 5-HT neurons encode aversive stimuli and aversive cues. Next, we describe pharmacological and genetic alterations of 5-HT neurotransmission that, in both rodents and humans, lead to altered fear learning. To explore the mechanisms through which 5-HT could modulate conditioned fear, we focus on the rodent BLA. We propose that a circuit-based approach taking into account the localization of specific 5-HT receptors on neurochemically-defined neurons in the BLA may be essential to decipher the role of 5-HT in emotional behavior. In keeping with a 5-HT control of fear learning, we review electrophysiological data suggesting that 5-HT regulates synaptic plasticity, spike synchrony and theta oscillations in the BLA via actions on different subcellular compartments of principal neurons and distinct GABAergic interneuron populations. Finally, we discuss how recently developed optogenetic tools combined with electrophysiological recordings and behavior could progress the knowledge of the mechanisms underlying 5-HT modulation of fear learning via action on amygdala circuits. Such advancement could pave the way for a deeper understanding of 5-HT in emotional behavior in both health and disease.

Developmental exposure to paracetamol causes biochemical alterations in medulla oblongata

The effect and safety of prenatal and early life administration of paracetamol - routinely used over-the-counter antipyretic and analgesic medication on monoamines content and balance of amino acids in the medulla oblongata is still unknown. In this study we have determined the level of neurotransmitters in this structure in two-month old Wistar male rats exposed to paracetamol in the dose of 5 (P5, n=10) or 15mg/kg b.w. (P15, n=10) during prenatal period, lactation and till the end of the second month of life. Control group received drinking water (Con, n=10). Monoamines, their metabolites and amino acids concentration in medulla oblongata of rats were determined using high performance liquid chromatography (HPLC) in 60 postnatal day (PND60). This experiment shows that prenatal and early life paracetamol exposure modulates neurotransmission associated with serotonergic, noradrenergic and dopaminergic system in medulla oblongata. Reduction of alanine and taurine levels has also been established.

Anxiety as a neurodevelopmental disorder in a neuronal subpopulation: Evidence from gene expression data

The relationship between genes and anxious behavior, is nor linear nor monotonic. To address this problem, we analyzed with a meta-analytic method the literature data of the behavior of knockout mice, retrieving 33 genes whose deletion was accompanied by increased anxious behavior, 34 genes related to decreased anxious behavior and 48 genes not involved in anxiety. We correlated the anxious behavior resulting from the deletion of these genes to their brain expression, using the Allen Brain Atlas and Gene Expression Omnibus (GEO) database. The main finding is that the genes accompanied, after deletion, by a modification of the anxious behavior, have lower expression in the cerebral cortex, the amygdala and the ventral striatum. The lower expression level was putatively due to their selective presence in a neuronal subpopulation. This difference was replicated also using a database of human gene expression, further showing that the differential expression pertained, in humans, a temporal window of young postnatal age (4 months up to 4 years) but was not evident at fetal or adult human stages. Finally, using gene enrichment analysis we also show that presynaptic genes are involved in the emergence of anxiety and postsynaptic genes in the reduction of anxiety after gene deletion.

Co-expression of serotonin 5-HT(1B) and 5-HT(4) receptors in p11 containing cells in cerebral cortex, hippocampus, caudate-putamen and cerebellum

p11 is an adaptor protein which binds to serotonin 5-HT(1B) receptors and 5-HT(4) receptors and regulates their localization at the cell surface. In the present study, we examined to what extent p11 containing neurons co-expressed 5-HT(1B)R and/or 5-HT(4)R in cerebral cortex, hippocampus, cerebellum and caudate-putamen. A triple-labeling immunohistochemical approach was taken using antibodies to detect native p11 and 5-HT(1B)R combined with visualization of EGFP driven under the 5-HT(4)R promoter in BAC-transgenic mice. In the caudate-putamen, the hippocampal pyramidal cell layer of CA1 and the hippocampal granule cell layer of dentate gyrus, most p11 containing cells co-expressed both 5-HT(1B)R and 5-HT(4)R. In the cingulate cortex, stratum radiatum/oriens of CA1, hilus of the dentate gyrus and cerebellar cortex, many cells co-expressed p11 and 5-HT(1B)R, but not 5-HT(4)R. In the studied brain regions, few cells solely expressed p11 without any significant expression of 5-HT(1B)R or 5-HT(4)R. It can be concluded that p11 is anatomically positioned to modulate serotonin neurotransmission, via 5-HT(1B)R and 5-HT(4)R, in brain regions important for emotionality, cognition and locomotion.