A serotonergic discrimination favoring synaptic inputs that accompany robust spike firing in lateral amygdala neurons

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Structural, functional, and behavioral significance of sex and gonadal hormones in the basolateral amygdala: A review of preclinical literature

The basolateral amygdala (BLA) is intimately involved in the development of neuropsychiatric disorders such as anxiety and alcohol use disorder (AUD). These disorders have clear sex biases, with women more likely to develop an anxiety disorder and men more likely to develop AUD. Preclinical models have largely confirmed these sex-specific vulnerabilities and emphasize the effects of sex hormones on behaviors influenced by the BLA. This review will discuss sex differences in BLA-related behaviors and highlight potential mechanisms mediated by altered BLA structure and function, including the composition of GABAergic interneuron subpopulations, glutamatergic pyramidal neuron morphology, glutamate/GABA neurotransmission, and neuromodulators. Further, sex hormones differentially organize dimorphic circuits during sensitive developmental periods (organizational effects) and initiate more transient effects throughout adulthood (activational effects). Current literature indicates that estradiol and allopregnanolone, a neuroactive progestogen, generally reduce BLA-related behaviors through a variety of mechanisms, including activation of estrogen receptors or facilitation of GABA_A-mediated inhibition, respectively. This enhanced GABAergic inhibition may protect BLA pyramidal neurons from the excitability associated with anxiety and alcohol withdrawal. Understanding sex differences and the effects of sex hormones on BLA structure and function may help explain sex-specific vulnerabilities in BLA-related behaviors and ultimately improve treatments for anxiety and AUD.

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.

A Discrete Dorsal Raphe to Basal Amygdala 5-HT Circuit Calibrates Aversive Memory

Despite a wealth of clinical and preclinical data implicating the serotonin (5-HT) system in fear-related affective disorders, a precise definition of this neuromodulator's role in fear remains elusive. Using convergent anatomical and functional approaches, we interrogate the contribution to fear of basal amygdala (BA) 5-HT inputs from the dorsal raphe nucleus (DRN). We show the DRN→BA 5-HT pathway is engaged during fear memory formation and retrieval, and activity of these projections facilitates fear and impairs extinction. The DRN→BA 5-HT pathway amplifies fear-associated BA neuronal firing and theta power and phase-locking. Although fear recruits 5-HT and VGluT3 co-expressing DRN neurons, the fear-potentiating influence of the DRN→BA 5-HT pathway requires signaling at BA 5-HT1A/2A receptors. Input-output mapping illustrates how the DRN→BA 5-HT pathway is anatomically distinct and connected with other brain regions that mediate fear. These findings reveal how a discrete 5-HT circuit orchestrates a broader neural network to calibrate aversive memory.

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.

Post-training depletions of basolateral amygdala serotonin fail to disrupt discrimination, retention, or reversal learning

In goal-directed pursuits, the basolateral amygdala (BLA) is critical in learning about changes in the value of rewards. BLA-lesioned rats show enhanced reversal learning, a task employed to measure the flexibility of response to changes in reward. Similarly, there is a trend for enhanced discrimination learning, suggesting that BLA may modulate formation of stimulus-reward associations. There is a parallel literature on the importance of serotonin (5HT) in new stimulus-reward and reversal learning. Recent postulations implicate 5HT in learning from punishment. Whereas, dopaminergic involvement is critical in behavioral activation and reinforcement, 5HT may be most critical for aversive processing and behavioral inhibition, complementary cognitive processes. Given these findings, a 5HT-mediated mechanism in BLA may mediate the facilitated learning observed previously. The present study investigated the effects of selective 5HT lesions in BLA using 5,7-dihydroxytryptamine (5,7-DHT) vs. infusions of saline (Sham) on discrimination, retention, and deterministic reversal learning. Rats were required to reach an 85% correct pairwise discrimination and single reversal criterion prior to surgery. Postoperatively, rats were then tested on the (1) retention of the pretreatment discrimination pair, (2) discrimination of a novel pair, and (3) reversal learning performance. We found statistically comparable preoperative learning rates between groups, intact postoperative retention, and unaltered novel discrimination and reversal learning in 5,7-DHT rats. These findings suggest that 5HT in BLA is not required for formation and flexible adjustment of new stimulus-reward associations when the strategy to efficiently solve the task has already been learned. Given the complementary role of orbitofrontal cortex in reward learning and its interconnectivity with BLA, these findings add to the list of dissociable mechanisms for BLA and orbitofrontal cortex in reward learning.

Serotonin induces depolarization in lateral amygdala neurons by activation of TRPC-like current and inhibition of GIRK current depending on 5-HT(2C) receptor

Regional differences are known in the serotonin-induced modulation of neuronal activity within the amygdala. This in vitro study in rats focuses on analyzing the ionic mechanism underlying serotonin-induced depolarization in the lateral amygdala. Serotonin depolarized membrane potential by 5 mV, which is underlain by a serotonin-induced inward current at rest with a characteristic reversal potential of -105 mV. From pharmacological experiments, the 5-HT2C subtype was singled out as the receptor subtype involved. Under blockade of K(+) channels by Ba(2+), 5-HT induced an inward current with no reversal at the range between -50 and -130 mV, which was identified as a TRPC-like current. This current was blocked by the specific phosphatidylinositol 3-kinse (PI3-kinase) inhibitor LY294002, pointing to its dependence on PI3-kinase. The Ba(2+)-sensitive component, obtained by subtraction, showed a strong outward rectification and the reversal potential of K(+), indicating that this component results from a serotonin-induced inhibition of G-protein coupled inwardly rectifying K(+) channel (GIRK) current. By wortmannin, an inhibitor of both PI3-kinase and PI4-kinase, a serotonin-induced phosphatidylinositol 4,5-bisphosphate (PIP2) depletion was revealed to underlie GIRK inhibition. Thus, the serotonin-induced current turned out to be caused by a combined occurrence of GIRK inhibition and PI3-kinase-dependent TRPC-like current. With serotonergic modulation, all these mechanisms should be recruited in lateral amygdala principal neurons and likely contribute to generation of region-specific neuronal activity patterns within the amygdala, which may at least partly implement its required role in fear and anxiety.

Long-term fluoxetine treatment induces input-specific LTP and LTD impairment and structural plasticity in the CA1 hippocampal subfield

Antidepressant drugs are usually administered for several weeks for the treatment of major depressive disorder. However, they are also prescribed in several additional psychiatric conditions as well as during long-term maintenance treatments. Antidepressants induce adaptive changes in several forebrain structures which include modifications at glutamatergic synapses. We recently found that repetitive administration of the selective serotonin reuptake inhibitor (SSRI) fluoxetine to naïve adult male rats induced an increase of mature, mushroom-type dendritic spines in several forebrain regions. This was associated with an increase of GluA2-containing α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA-Rs) in telencephalic postsynaptic densities. To unravel the functional significance of such a synaptic re-arrangement, we focused on glutamate neurotransmission in the hippocampus. We evaluated the effect of four weeks of 0.7 mg/kg fluoxetine on long-term potentiation (LTP) and long-term depression (LTD) in the CA1 hippocampal subfield. Recordings in hippocampal slices revealed profound deficits in LTP and LTD at Schaffer collateral-CA1 synapses associated to increased spine density and enhanced presence of mushroom-type spines, as revealed by Golgi staining. However, the same treatment had neither an effect on spine morphology, nor on LTP and LTD at perforant path-CA1 synapses. Cobalt staining and immunohistochemical experiments revealed decreased AMPA-R Ca(2+) permeability in the stratum radiatum (s.r.) together with increased GluA2-containing Ca(2+) impermeable AMPA-Rs. Therefore, 4 weeks of fluoxetine treatment promoted structural and functional adaptations in CA1 neurons in a pathway-specific manner that were selectively associated with impairment of activity-dependent plasticity at Schaffer collateral-CA1 synapses.