Oliver M. Schlüter

In Vivo Cocaine Experience Generates Silent Synapses

Studies over the past decade have enunciated silent synapses as prominent cellular substrates for synaptic plasticity in the developing brain. However, little is known about whether silent synapses can be generated postdevelopmentally. Here, we demonstrate that highly salient in vivo experience, such as exposure to cocaine, generates silent synapses in the nucleus accumbens (NAc) shell, a key brain region mediating addiction-related learning and memory. Furthermore, this cocaine-induced generation of silent synapses is mediated by membrane insertions of new, NR2B-containing N-methyl-D-aspartic acid receptors (NMDARs). These results provide evidence that silent synapses can be generated de novo by in vivo experience and thus may act as highly efficient neural substrates for the subsequent experience-dependent synaptic plasticity underlying extremely long-lasting memory.

Bidirectional Modulation of Incubation of Cocaine Craving by Silent Synapse-Based Remodeling of Prefrontal Cortex to Accumbens Projections

Glutamatergic projections from the medial prefrontal cortex (mPFC) to nucleus accumbens (NAc) contribute to cocaine relapse. Here we show that silent synapse-based remodeling of the two major mPFC-to-NAc projections differentially regulated the progressive increase in cue-induced cocaine seeking after withdrawal (incubation of cocaine craving). Specifically, cocaine self-administration in rats generated AMPA receptor-silent glutamatergic synapses within both infralimbic (IL) and prelimbic mPFC (PrL) to NAc projections, measured after 1 day of withdrawal. After 45 days of withdrawal, IL-to-NAc silent synapses became unsilenced/matured by recruiting calcium-permeable (CP) AMPARs, whereas PrL-to-NAc silent synapses matured by recruiting non-CP-AMPARs, resulting in differential remodeling of these projections. Optogenetic reversal of silent synapse-based remodeling of IL-to-NAc and PrL-to-NAc projections potentiated and inhibited, respectively, incubation of cocaine craving on withdrawal day 45. Thus, pro- and antirelapse circuitry remodeling is induced in parallel after cocaine self-administration. These results may provide substrates for utilizing endogenous antirelapse mechanisms to reduce cocaine relapse.

A Silent Synapse-Based Mechanism for Cocaine-Induced Locomotor Sensitization

Locomotor sensitization is a common and robust behavioral alteration in rodents whereby following exposure to abused drugs such as cocaine, the animal becomes significantly more hyperactive in response to an acute drug challenge. Here, we further analyzed the role of cocaine-induced silent synapses in the nucleus accumbens (NAc) shell and their contribution to the development of locomotor sensitization. Using a combination of viral vector-mediated genetic manipulations, biochemistry, and electrophysiology in a locomotor sensitization paradigm with repeated, daily, noncontingent cocaine (15 mg/kg) injections, we show that dominant-negative cAMP-element binding protein (CREB) prevents cocaine-induced generation of silent synapses of young (30 d old) rats, whereas constitutively active CREB is sufficient to increase the number of NR2B-containing NMDA receptors (NMDARs) at synapses and to generate silent synapses. We further show that occupancy of CREB at the NR2B promoter increases and is causally related to the increase in synaptic NR2B levels. Blockade of NR2B-containing NMDARs by administration of the NR2B-selective antagonist Ro256981 directly into the NAc, under conditions that inhibit cocaine-induced silent synapses, prevents the development of cocaine-elicited locomotor sensitization. Our data are consistent with a cellular cascade whereby cocaine-induced activation of CREB promotes CREB-dependent transcription of NR2B and synaptic incorporation of NR2B-containing NMDARs, which generates new silent synapses within the NAc. We propose that cocaine-induced activation of CREB and generation of new silent synapses may serve as key cellular events mediating cocaine-induced locomotor sensitization. These findings provide a novel cellular mechanism that may contribute to cocaine-induced behavioral alterations.

Homeostatic Synapse-Driven Membrane Plasticity in Nucleus Accumbens Neurons

Stable brain function relies on homeostatic maintenance of the functional output of individual neurons. In general, neurons function by converting synaptic input to output as action potential firing. To determine homeostatic mechanisms that balance this input–output/synapse–membrane interaction, we focused on nucleus accumbens (NAc) neurons and demonstrated a novel form of synapse-to-membrane homeostatic regulation, homeostatic synapse-driven membrane plasticity (hSMP). Through hSMP, NAc neurons adjusted their membrane excitability to functionally compensate for basal shifts in excitatory synaptic input. Furthermore, hSMP was triggered by synaptic NMDA receptors (NMDARs) and expressed by the modification of SK-type Ca2+-activated potassium channels. Moreover, hSMP in NAc neurons was abolished in rats during a short- (2 d) or long- (21 d) term withdrawal from repeated intraperitoneal injections of cocaine (15 mg/kg/d, 5 d). These results suggest that hSMP is a novel form of synapse-to-membrane homeostatic plasticity and dysregulation of hSMP may contribute to cocaine-induced cellular alterations in the NAc.

Environmental enrichment extends ocular dominance plasticity into adulthood and protects from stroke-induced impairments of plasticity

Significance Experimental animals are usually raised in small, so-called standard cages, depriving them of numerous natural stimuli. We show that raising mice in an enriched environment, allowing enhanced physical, social, and cognitive stimulation, preserved a juvenile brain into adulthood. Enrichment also rejuvenated the visual cortex after extended periods of standard cage rearing and protected adult mice from stroke-induced impairments of cortical plasticity. Because the local inhibitory tone in the visual cortex of adult enriched mice was not only significantly reduced compared with nonenriched animals but at juvenile levels, the plasticity-promoting effect of enrichment is most likely mediated by preserving low juvenile levels of inhibition into adulthood and thereby, extending sensitive phases of enhanced neuronal plasticity into an older age. Ocular dominance (OD) plasticity in mouse primary visual cortex (V1) declines during postnatal development and is absent beyond postnatal day 110 if mice are raised in standard cages (SCs). An enriched environment (EE) promotes OD plasticity in adult rats. Here, we explored cellular mechanisms of EE in mouse V1 and the therapeutic potential of EE to prevent impairments of plasticity after a cortical stroke. Using in vivo optical imaging, we observed that monocular deprivation in adult EE mice (i) caused a very strong OD plasticity previously only observed in 4-wk-old animals, (ii) restored already lost OD plasticity in adult SC-raised mice, and (iii) preserved OD plasticity after a stroke in the primary somatosensory cortex. Using patch-clamp electrophysiology in vitro, we also show that (iv) local inhibition was significantly reduced in V1 slices of adult EE mice and (v) the GABA/AMPA ratio was like that in 4-wk-old SC-raised animals. These observations were corroborated by in vivo analyses showing that diazepam treatment significantly reduced the OD shift of EE mice after monocular deprivation. Taken together, EE extended the sensitive phase for OD plasticity into late adulthood, rejuvenated V1 after 4 mo of SC-rearing, and protected adult mice from stroke-induced impairments of cortical plasticity. The EE effect was mediated most likely by preserving low juvenile levels of inhibition into adulthood, which potentially promoted adaptive changes in cortical circuits.

Selective presynaptic enhancement of the prefrontal cortex to nucleus accumbens pathway by cocaine.

The nucleus accumbens (NAc) regulates motivated behavior by, in part, processing excitatory synaptic projections from several brain regions. Among these regions, the prefrontal cortex (PFC) and basolateral amygdala, convey executive control and affective states, respectively. Whereas glutamatergic synaptic transmission within the NAc has been recognized as a primary cellular target for cocaine and other drugs of abuse to induce addiction-related pathophysiological motivational states, the understanding has been thus far limited to drug-induced postsynaptic alterations. It remains elusive whether exposure to cocaine or other drugs of abuse influences presynaptic functions of these excitatory projections, and if so, in which projection pathways. Using optogenetic methods combined with biophysical assays, we demonstrate that the presynaptic release probability (Pr) of the PFC-to-NAc synapses was enhanced after short-term withdrawal (1 d) and long-term (45 d) withdrawal from either noncontingent (i.p. injection) or contingent (self-administration) exposure to cocaine. After long-term withdrawal of contingent drug exposure, the Pr was higher compared with i.p. injected rats. In contrast, within the basolateral amygdala afferents, presynaptic Pr was not significantly altered in any of these experimental conditions. Thus, cocaine-induced procedure- and pathway-specific presynaptic enhancement of excitatory synaptic transmission in the NAc. These results, together with previous findings of cocaine-induced postsynaptic enhancement, suggest an increased PFC-to-NAc shell glutamatergic synaptic transmission after withdrawal from exposure to cocaine. This presynaptic alteration may interact with other cocaine-induced cellular adaptations to shift the functional output of NAc neurons, contributing to the addictive emotional and motivational state.

Durable fear memories require PSD-95.

Traumatic fear memories are highly durable but also dynamic, undergoing repeated reactivation and rehearsal over time. Although overly persistent fear memories underlie anxiety disorders, such as posttraumatic stress disorder, the key neural and molecular mechanisms underlying fear memory durability remain unclear. Postsynaptic density 95 (PSD-95) is a synaptic protein regulating glutamate receptor anchoring, synaptic stability and certain types of memory. Using a loss-of-function mutant mouse lacking the guanylate kinase domain of PSD-95 (PSD-95GK), we analyzed the contribution of PSD-95 to fear memory formation and retrieval, and sought to identify the neural basis of PSD-95-mediated memory maintenance using ex vivo immediate-early gene mapping, in vivo neuronal recordings and viral-mediated knockdown (KD) approaches. We show that PSD-95 is dispensable for the formation and expression of recent fear memories, but essential for the formation of precise and flexible fear memories and for the maintenance of memories at remote time points. The failure of PSD-95GK mice to retrieve remote cued fear memory was associated with hypoactivation of the infralimbic (IL) cortex (but not the anterior cingulate cortex (ACC) or prelimbic cortex), reduced IL single-unit firing and bursting, and attenuated IL gamma and theta oscillations. Adeno-associated virus-mediated PSD-95 KD in the IL, but not the ACC, was sufficient to impair recent fear extinction and remote fear memory, and remodel IL dendritic spines. Collectively, these data identify PSD-95 in the IL as a critical mechanism supporting the durability of fear memories over time. These preclinical findings have implications for developing novel approaches to treating trauma-based anxiety disorders that target the weakening of overly persistent fear memories.

Progressive maturation of silent synapses governs the duration of a critical period

Significance During critical periods, cortical neural circuits are refined to optimize their functional properties. The prevailing notion is that the balance between excitation and inhibition determines the onset and closure of critical periods. Here, we show that postsynaptic density protein-95 (PSD-95)–dependent maturation of silent glutamatergic synapses onto principal neurons was sufficient to govern the duration of the critical period for ocular dominance plasticity (ODP) in the visual cortex of mice. Loss of PSD-95 before the onset of CPs resulted in lifelong ODP, loss after CP closure reinstated silent synapses, and ODP. Thus, PSD-95–dependent silent synapse maturation terminates the critical period of ODP, and in general, once silent synapses are consolidated in any neural circuit, critical periods may end. During critical periods, all cortical neural circuits are refined to optimize their functional properties. The prevailing notion is that the balance between excitation and inhibition determines the onset and closure of critical periods. In contrast, we show that maturation of silent glutamatergic synapses onto principal neurons was sufficient to govern the duration of the critical period for ocular dominance plasticity in the visual cortex of mice. Specifically, postsynaptic density protein-95 (PSD-95) was absolutely required for experience-dependent maturation of silent synapses, and its absence before the onset of critical periods resulted in lifelong juvenile ocular dominance plasticity. Loss of PSD-95 in the visual cortex after the closure of the critical period reinstated silent synapses, resulting in reopening of juvenile-like ocular dominance plasticity. Additionally, silent synapse-based ocular dominance plasticity was largely independent of the inhibitory tone, whose developmental maturation was independent of PSD-95. Moreover, glutamatergic synaptic transmission onto parvalbumin-positive interneurons was unaltered in PSD-95 KO mice. These findings reveal not only that PSD-95–dependent silent synapse maturation in visual cortical principal neurons terminates the critical period for ocular dominance plasticity but also indicate that, in general, once silent synapses are consolidated in any neural circuit, initial experience-dependent functional optimization and critical periods end.

Exposure to Cocaine Alters Dynorphin-Mediated Regulation of Excitatory Synaptic Transmission in Nucleus Accumbens Neurons

BACKGROUND Dysregulation of excitatory synaptic input to nucleus accumbens (NAc) medium spiny neurons (MSNs) underlies a key pathophysiology of drug addiction and addiction-associated emotional and motivational alterations. Dynorphin peptides, which exhibit higher affinity to κ type opioid receptors, are upregulated within the NAc upon exposure to cocaine administration, and the increased dynorphin-signaling in the NAc has been critically implicated in negative mood observed in cocaine- or stress-exposed animals. Despite such apparent behavioral significance of the NAc dynorphins, the understanding of how dynorphins regulate excitatory synaptic transmission in the NAc remains incomplete. METHODS We used electrophysiological recording in brain slices to examine the effects of dynorphins on excitatory synaptic transmission in the NAc. RESULTS We focused on two key dynorphins, dynorphin A and B. Our current results show that dynorphin A and B differentially regulated excitatory postsynaptic currents (EPSCs) in NAc MSNs. Whereas perfusions of both dynorphin A and B to NAc slices decreased EPSCs in MSNs, the effect of dynorphin A but not dynorphin B was completely reversed by the κ receptor-selective antagonist nor-binaltorphimine. These results implicate κ receptor-independent mechanisms in dynorphin B-mediated synaptic effects in the NAc. Furthermore, repeated exposure to cocaine (15 mg/kg/day via intraperitoneal injection for 5 days, with 1, 2, or 14 days withdrawal) completely abolished dynorphin A-mediated modulation of EPSCs in NAc MSNs, whereas the effect of dynorphin B remained largely unchanged. CONCLUSIONS Given the quantitatively higher abundance of dynorphin B in the NAc, our present results suggest that the dynorphin B-mediated, κ receptor-independent pathways predominate in the overall effect of dynorphins in cocaine-pretreated animals and potentially in cocaine-induced alterations in mood.

Dopamine Triggers Heterosynaptic Plasticity

As a classic neuromodulator, dopamine has long been thought to modulate, rather than trigger, synaptic plasticity. In contrast, our present results demonstrate that within the parallel projections of dopaminergic and GABAergic terminals from the ventral tegmental area to the nucleus accumbens core (NAcCo), action-potential-activated release of dopamine heterosynaptically triggers LTD at GABAergic synapses, which is likely mediated by activating presynaptically located dopamine D1 class receptors and expressed by inhibiting presynaptic release of GABA. Moreover, this dopamine-mediated heterosynaptic LTD is abolished after withdrawal from cocaine exposure. These results suggest that action-potential-dependent dopamine release triggers very different cellular consequences from those induced by volume release or pharmacological manipulation. Activation of the ventral tegmental area to NAcCo projections is essential for emotional and motivational responses. This dopamine-mediated LTD allows a flexible output of NAcCo neurons, whereas disruption of this LTD may contribute to the rigid emotional and motivational state observed in addicts during cocaine withdrawal.