Phasic dopamine release in the medial prefrontal cortex enhances stimulus discrimination.

Phasic dopamine (DA) release is believed to guide associative learning. Most studies have focused on projections from the ventral tegmental area (VTA) to the striatum, and the action of DA in other VTA target regions remains unclear. Using optogenetic activation of VTA projections, we examined DA function in the medial prefrontal cortex (mPFC). We found that mice perceived optogenetically induced DA release in mPFC as neither rewarding nor aversive, and did not change their previously learned behavior in response to DA transients. However, repetitive temporal pairing of an auditory conditioned stimulus (CS) with mPFC DA release resulted in faster learning of a subsequent task involving discrimination of the same CS against unpaired stimuli. Similar results were obtained using both appetitive and aversive unconditioned stimuli, supporting the notion that DA transients in mPFC do not represent valence. Using extracellular recordings, we found that CS-DA pairings increased firing of mPFC neurons in response to CSs, and administration of D1 or D2 DA-receptor antagonists in mPFC during learning impaired stimulus discrimination. We conclude that DA transients tune mPFC neurons for the recognition of behaviorally relevant events during learning.

Coherent amygdalocortical theta promotes fear memory consolidation during paradoxical sleep.

Brain activity in sleep plays a crucial role in memory consolidation, an offline process that determines the long-term strength of memory traces. Consolidation efficacy differs across individuals, but the brain activity dynamics underlying these differences remain unknown. Here, we studied how interindividual variability in fear memory consolidation relates to neural activity in brain structures that participate in Pavlovian fear learning. From the end of training to testing 24 h later, some rats showed increased and others decreased conditioned fear responses. We found that overnight bidirectional changes in fear memory were selectively correlated with modifications in theta coherence between the amygdala, medial prefrontal cortex, and hippocampus during paradoxical sleep. Thus, our results suggest that theta coordination in the limbic system may influence interindividual differences in memory consolidation of aversive experiences.

Synaptic interactions underlying synchronized inhibition in the basal amygdala: evidence for existence of two types of projection cells.

The basal amygdala (BA) plays a key role in mediating the facilitating effects of emotions on memory. Recent findings indicate that this function depends on the ability of BA neurons to generate coherent oscillatory activity, facilitating synaptic plasticity in target neurons. However, the mechanisms allowing BA neurons to synchronize their activity remain poorly understood. Here, we aimed to shed light on this question, focusing on a slow periodic inhibitory oscillation previously observed in the BA in vitro. Paired patch recordings showed that these large inhibitory postsynaptic potentials (IPSPs) occur almost synchronously in BA projection neurons, that they were typically not preceded by excitatory postsynaptic potentials (EPSPs), and that they had little or no correlate in neighboring amygdala nuclei or cortical fields. The initial phase of the IPSPs was associated with an increase in membrane potential fluctuations at 50-100 Hz. In keeping with this, the IPSPs seen in projection cells were correlated with repetitive firing at 50-100 Hz in presumed interneurons and they could be abolished by picrotoxin. However, the IPSPs were also sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione, implying that they arose from the interplay between glutamatergic and GABAergic BA neurons. In support of this idea, we identified a small subset of projection cells (15%), positively identified as such by retrograde labeling from BA projection sites, that began firing shortly before the IPSP onset and presumably drove interneuronal firing. These results add to a rapidly growing body of data indicating that the BA contains at least two distinct types of projection cells that vary in their relation with interneurons and extra-amygdala targets.

Contrasting activity profile of two distributed cortical networks as a function of attentional demands.

Recent human functional MRI (fMRI) studies have revealed that two widely distributed groups of cortical areas display inverse changes in activity when attentional demands increase, with one group showing higher (task-on) and the second lower (task-off) blood oxygen level-dependent (BOLD) signals. Moreover, task-on and task-off regions also exhibit slow (<0.2 Hz) inversely correlated fluctuations in BOLD signal at rest. However, the neuronal correlates of these reciprocal BOLD signal fluctuations are unknown. Here, we addressed this question using simultaneous recordings of unit activity and local field potentials (LFPs) in the cat homologues of task-on and task-off regions. In all states of vigilance, LFP power was lower in task-off than task-on regions with no difference in firing rates. Both sets of regions displayed slow (0.5-0.15 Hz) cyclical modulations in LFP power in all frequency bands but with large and variable phase differences such that task-on and task-off regions were often anticorrelated. Inversely correlated LFP power fluctuations were state-dependent in that they were much more frequent in waking and paradoxical sleep than in slow-wave sleep. Moreover, consistent with fMRI findings, when attentional demands increased, LFP power in task-on and task-off regions changed in opposite directions, further augmenting and decreasing, respectively. At odds with previous fMRI studies, however, the decreased LFP power in task-off regions was associated with increased firing rates, suggesting that the engagement of task-off regions might not be reduced but in fact enhanced during attention.

Facilitation of corticostriatal plasticity by the amygdala requires Ca2+-induced Ca2+ release in the ventral striatum.

Motor learning and habit formation are thought to depend on corticostriatal synaptic plasticity. Moreover, basolateral amygdala (BLA) activity facilitates consolidation of striatal-dependent memories. Accordingly, BLA stimulation in vitro facilitates long-term potentiation (LTP) induction at corticostriatal synapses onto medium spiny neurons (MSNs). Although these effects were found to depend on N-methyl-d-aspartate (NMDA) receptor activation at BLA synapses and consequent Ca(2+) influx, it is unclear how this event can facilitate LTP at cortical synapses, even when the two inputs are not coactivated. Here, we aimed to shed light on this question, using whole cell recordings of MSNs in vitro. We first tested whether BLA inputs end at more proximal dendritic sites than cortical inputs. In this scenario, BLA synapses would experience stronger spike-related depolarizations and be in a strategic position to control the spread of second messengers. However, comparison of compound excitatory postsynaptic potentials and single-axon excitatory postsynaptic currents revealed that BLA and cortical synapses are intermingled. Next, we examined the sensitivity of cortical and BLA NMDA responses to ifenprodil because NR2A-containing NMDA receptors have faster kinetics than those containing NR2B subunits. However, the two inputs did not differ in this respect. Last, reasoning that propagating waves of Ca(2+)-induced Ca(2+) release (CICR) could bridge temporal gaps between the two inputs, we tested the effects of CICR inhibitors on the BLA facilitation of corticostriatal LTP induction. Pharmacological interference with CICR blocked corticostriatal LTP induction. Thus our results are consistent with the notion that NMDA-dependent Ca(2+) influx at BLA synapses initiates propagating waves of CICR, thereby biasing active corticostriatal inputs toward synaptic potentiation.

A Toolkit for Orthogonal and in vivo Optical Manipulation of Ionotropic Glutamate Receptors.

The ability to optically manipulate specific neuronal signaling proteins with genetic precision paves the way for the dissection of their roles in brain function, behavior, and disease. Chemical optogenetic control with photoswitchable tethered ligands (PTLs) enables rapid, reversible and reproducible activation or block of specific neurotransmitter-gated receptors and ion channels in specific cells. In this study, we further engineered and characterized the light-activated GluK2 kainate receptor, LiGluR, to develop a toolbox of LiGluR variants. Low-affinity LiGluRs allow for efficient optical control of GluK2 while removing activation by native glutamate, whereas variant RNA edited versions enable the synaptic role of receptors with high and low Ca(2+) permeability to be assessed and spectral variant photoswitches provide flexibility in illumination. Importantly, we establish that LiGluR works efficiently in the cortex of awake, adult mice using standard optogenetic techniques, thus opening the door to probing the role of specific synaptic receptors and cellular signals in the neural circuit operations of the mammalian brain in normal conditions and in disease. The principals developed in this study are widely relevant to the engineering and in vivo use of optically controllable proteins, including other neurotransmitter receptors.

Semaphorin3A regulates neuronal polarization by suppressing axon formation and promoting dendrite growth.

Semaphorin 3A (Sema3A) is a secreted factor known to guide axon/dendrite growth and neuronal migration. We found that it also acts as a polarizing factor for axon/dendrite development in cultured hippocampal neurons. Exposure of the undifferentiated neurite to localized Sema3A suppressed its differentiation into axon and promoted dendrite formation, resulting in axon formation away from the Sema3A source, and bath application of Sema3A to polarized neurons promoted dendrite growth but suppressed axon growth. Fluorescence resonance energy transfer (FRET) imaging showed that Sema3A elevated the cGMP but reduced cAMP and protein kinase A (PKA) activity, and its axon suppression is attributed to the downregulation of PKA-dependent phosphorylation of axon determinants LKB1 and GSK-3ß. Downregulating Sema3A signaling in rat embryonic cortical progenitors via in utero electroporation of siRNAs against the Sema3A receptor neuropilin-1 also resulted in polarization defects in vivo. Thus, Sema3A regulates the earliest step of neuronal morphogenesis by polarizing axon/dendrite formation.

Coherent gamma oscillations couple the amygdala and striatum during learning.

The basolateral amygdala (BLA) mediates the facilitating effects of emotions on memory. The BLA's enhancing influence extends to various types of memories, including striatal-dependent habit formation. To shed light on the underlying mechanisms, we carried out unit and local field potential (LFP) recordings in BLA, striatum, auditory cortex and intralaminar thalamus in cats trained on a stimulus-response task in which the presentation of one of two tones predicted reward delivery. The coherence of BLA, but not of cortical or thalamic, LFPs was highest with striatal gamma activity, and intra-BLA muscimol infusions selectively reduced striatal gamma power. Moreover, coupling of BLA-striatal unit activity increased when LFP gamma power was augmented. Early in training, the rewarded and unrewarded tones elicited a modest increase in coherent BLA-striatal gamma. As learning progressed, this gamma coupling selectively increased in relation to the rewarded tone. Thus, coherent gamma oscillations coordinate amygdalostriatal interactions during learning and might facilitate synaptic plasticity.

NMDA-dependent facilitation of corticostriatal plasticity by the amygdala.

Emotions generally improve memory, and the basolateral amygdala (BLA) is believed to mediate this effect. After emotional arousal, BLA neurons increase their firing rate, facilitating memory consolidation in BLA targets. The enhancing effects of BLA activity extend to various types of memories, including motor learning, which is thought to involve activity-dependent plasticity at corticostriatal synapses. However, the underlying mechanisms are unknown. Here we show that the NMDA-to-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ratio is nearly twice as high at BLA as compared with cortical synapses onto principal striatal neurons and that activation of BLA inputs greatly facilitates long-term potentiation induction at corticostriatal synapses. This facilitation was NMDA-dependent, but it occurred even when BLA and cortical stimuli were 0.5 s apart during long-term potentiation induction. Overall, these results suggest that BLA activity opens long time windows during which the induction of corticostriatal plasticity is facilitated.

Identification of basolateral amygdala projection cells and interneurons using extracellular recordings.

This study tested whether firing rate and spike shape could be used to distinguish projection cells from interneurons in extracellular recordings of basolateral amygdala (BLA) neurons. To this end, we recorded BLA neurons in isoflurane-anesthetized animals with tungsten microelectrodes. Projection cells were identified by antidromic activation from cortical projection sites of the BLA. Although most projection cells fired spontaneously at low rates (<1 Hz), an important subset fired at higher rates (up to 6.8 Hz). In fact, the distribution of firing rates in projection cells and unidentified BLA neurons overlapped extensively, even though the latter cell group presumably contains a higher proportion of interneurons. The only difference between the two distributions was a small subset (5.1%) of unidentified neurons with unusually high firing rates (9-16 Hz). Similarly, distributions of spike durations in both cell groups were indistinguishable, although most of the fast-firing neurons had spike durations at the low end of the distribution. However, we observed that spike durations depended on the exact position of the electrode with respect to the recorded cell, varying by as much as 0.7 ms. Thus neither firing rate nor spike waveform allowed for unequivocal separation of projection cells from interneurons. Nevertheless, we propose the use of two firing rate cutoffs to obtain relatively pure samples of projection cells and interneurons: < or =1 Hz for projection cells and > or =7 Hz for fast-spiking interneurons. Supplemented with spike-duration cutoffs of > or =0.7 ms for projection cells and < or =0.5 ms for interneurons, this approach should keep instances of misclassifications to a minimum.