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Over the last two decades the combination of brain slice patch clamp electrophysiology with optogenetic stimulation has proven to be a powerful approach to analyze the architecture of neural circuits and (experience-dependent) synaptic plasticity in such networks. Using this combination of methods, originally termed channelrhodopsin-assisted circuit mapping (CRACM), a multitude of measures of synaptic functioning can be taken. The current review discusses their rationale, current applications in the field, and their associated caveats. Specifically, the review addresses: (1) How to assess the presence of synaptic connections, both in terms of ionotropic versus metabotropic receptor signaling, and in terms of mono- versus polysynaptic connectivity. (2) How to acquire and interpret measures for synaptic strength and function, like AMPAR/NMDAR, AMPAR rectification, paired-pulse ratio (PPR), coefficient of variance and input-specific quantal sizes. We also address how synaptic modulation by G protein-coupled receptors can be studied with pharmacological approaches and advanced technology. (3) Finally, we elaborate on advances on the use of dual color optogenetics in concurrent investigation of multiple synaptic pathways. Overall, with this review we seek to provide practical insights into the methods used to study neural circuits and synapses, by combining optogenetics and patch-clamp electrophysiology.

The extracellular space is occupied by a complex network of proteins creating a mesh-like assembly known as the extracellular matrix (ECM). ECM assembles into dense net-like structures, perineuronal nets (PNNs), that envelope cell somas and proximal neurites of predominantly parvalbumin+-(PV+) interneurons. ECM regulates cell-to-cell communication, thereby modulating neuronal network function. Accumulating evidence points to the importance of network dysfunction in the pathophysiology of psychiatric diseases, in which stress acts as a major predisposing factor. Here we review stress-induced changes in ECM/PNNs and PV+-interneurons in preclinical models of (or for) depression, with a special focus on social stress. We argue that the direction of these alterations largely depends on stress recency, as well as on stress timing and the brain region under investigation. A biphasic temporal regulation of ECM/PNNs and PV+-interneuron function is typically observed after stress. Understanding the complex mechanisms underlying ECM organization in relation to stress-induced molecular, cellular and network changes is crucial to further decipher the implications of ECM remodeling in the incubation of depressive symptoms.

Major depression and alcohol‐related disorders frequently co‐occur. Depression severity weighs on the magnitude and persistence of comorbid alcohol use disorder (AUD), with severe implications for disease prognosis. Here, we investigated whether depression vulnerability drives propensity to AUD at the preclinical level. We used the social defeat–induced persistent stress (SDPS) model of chronic depression in combination with operant alcohol self‐administration (SA). Male Wistar rats were subjected to social defeat (five episodes) and prolonged social isolation (~12 weeks) and subsequently classified as SDPS‐prone or SDPS‐resilient based on their affective and cognitive performance. Using an operant alcohol SA paradigm, acquisition, motivation, extinction, and cue‐induced reinstatement of alcohol seeking were examined in the two subpopulations. SDPS‐prone animals showed increased alcohol SA, heightened motivation to acquire alcohol, persistent alcohol seeking despite alcohol unavailability, signs of extinction resistance, and increased cue‐induced relapse; the latter could be blocked by the α2 adrenoreceptor agonist guanfacine. In SDPS‐resilient rats, prior exposure to social defeat increased alcohol SA without affecting any other measures of alcohol seeking and alcohol taking. Our data revealed that depression proneness confers vulnerability to alcohol, emulating patterns of alcohol dependence seen in human addicts, and that depression resilience to a large extent protects from the development of AUD‐like phenotypes. Furthermore, our data suggest that stress exposure alone, independently of depressive symptoms, alters alcohol intake in the long‐term., Stress contributes to depressive and addictive pathologies alike. We examined how individual differences in responsivity to stress affect patterns of alcohol seeking and alcohol taking in a preclinical model of chronic depression, the social defeat‐induced persistent stress (SDPS) paradigm. Our findings indicated that stress vulnerability, manifested in long‐lasting affective and cognitive deficits, triggers an alcohol‐prone phenotype that mirrors alcohol dependence as seen in humans. Accordingly, stress resilience, which inhibits the development of chronic depression, protects from the emergence of addiction‐like manifestations.

Stress can predispose to depressive episodes, yet the molecular mechanisms regulating the transition from the initial stress response to a persistent pathological depressive state remain poorly understood. To shed light on this stress-to-depression transition process, we profiled the development of an enduring depressive-like state in rat by assessing affective behavior and hippocampal function during the 2 months following social defeat stress. In addition, we measured remodeling of hippocampal extracellular matrix (ECM) during this period, as we recently identified ECM changes to mediate cognitive impairment during a sustained depressive-like state. We found affective disturbance and cognitive impairment to develop disparately after social stress. While affective deficits emerged gradually, spatial memory impairment was present both early after stress and during the late-emerging chronic depressive-like state. Surprisingly, these phases were separated by a period of normalized hippocampal function. Similarly, the SDPS paradigm induced a biphasic regulation of the hippocampal ECM coinciding with hippocampus-dependent memory deficits. Early after stress, synaptic ECM proteins and the number of perineuronal nets enwrapping parvalbumin-expressing interneurons were decreased. This was followed by a recovery period without ECM dysregulation, before subsequent decreased metalloproteinase activity and ECM build-up, previously shown to impair memory. This suggests that intact hippocampal function requires unaltered ECM levels. Together our data 1) reveal a dichotomy between affective and cognitive impairments similar to that observed in patients, 2) indicate different molecular processes taking place during early stress and the chronic depressive-like state, and 3) support a role of the ECM in mediating long-lasting memory-effects of social stress.

Major depression and alcohol-related disorders frequently co-occur. Depression severity weighs on the magnitude and persistence of comorbid alcohol use disorder (AUD), with severe implications for disease prognosis. Here, we investigated whether depression vulnerability drives propensity to AUD at the preclinical level. We used the social defeat-induced persistent stress (SDPS) model of chronic depression in combination with operant alcohol self-administration (SA). Male Wistar rats were subjected to social defeat (5 episodes) and prolonged social isolation (~12 weeks) and subsequently classified as SDPS-prone or SDPS-resilient based on their affective and cognitive performance. Using an operant alcohol SA paradigm, acquisition, motivation, extinction and cue-induced reinstatement of alcohol-seeking were examined in the two subpopulations. SDPS-prone animals showed increased alcohol SA, excessive motivation to acquire alcohol, persistent alcohol-seeking despite alcohol unavailability, extinction resistance and increased cue-induced relapse; the latter could be blocked by the α2 adrenoreceptor agonist guanfacine. In SDPS-resilient rats, prior exposure to social defeat increased alcohol SA without affecting any other measures of alcohol-seeking and -taking. Our data revealed that depression proneness confers vulnerability to alcohol, emulating patterns of alcohol dependence seen in human addicts, and that depression resilience to a large extent protects from the development of AUD-like phenotypes. Furthermore, our data suggest that stress exposure alone, independently of depressive symptoms, alters alcohol intake in the long-term.

Major Depression is a prevalent mental disorder that is characterized by negative mood and reduced motivation, and frequently results in social withdrawal and memory-related deficits. Repeated stressors, such as adverse life events, increase the risk for development of the disorder. Consequently, individual variability in stress response greatly weighs on depression-vulnerability and -resilience. Here, we employed the social defeat-induced persistent stress (SDPS) paradigm to identify depression-prone individuals and to examine the temporal development of depression in the months following exposure to brief defeat stress. Male Wistar rats were socially defeated (5 defeat episodes) and single-housed for a prolonged period of time (~24 weeks). We assessed the emergence of a sustained depressive-like state by repeatedly evaluating social motivation (social approach avoidance) and spatial memory (object place recognition) in SDPS rats during the isolation period. Individual variability in the effects of SDPS yielded two extreme subpopulations: an SDPS-prone group that showed gradual affective and cognitive deterioration in terms of social approach and memory retention, and a SDPS-resilient group that did not develop this phenotype. Notably, in SDPS-prone individuals, the affective deficits preceded later cognitive impairments, providing a novel temporal profile of the development of pathology in this preclinical model of sustained depression.

Rationale: Treatment of the most widely abused drugs, nicotine and alcohol, is hampered by high rates of relapse. Varenicline tartrate, an α4β2 nicotinic receptor partial agonist, is currently prescribed as a smoking cessation aid. However, there is emerging evidence that it may also modulate alcohol seeking and cognitive functioning in rats. Objectives: As preclinical data on alcohol taking and relapse are limited, we used a self-administration- reinstatement model to evaluate the effects of varenicline on operant responding for alcohol (12%, v/v), intravenous nicotine (40 μg/kg/inf.), sucrose (10%, w/v) and on cue-induced relapse to alcohol and nicotine seeking in rats. At the cognitive level, we assed varenicline's effects on 5-choice serial reaction time task (5-CSRTT) performance with a focus on correct responses (attention) and premature responding (impulsivity), modalities that have previously been associated with addictive behaviour. Results: Varenicline, at doses of 1.5 and 2.5 mg/kg, reduced alcohol and nicotine self-administration and enhanced operant responding for sucrose. At these doses, varenicline reduced cue-induced relapse to alcohol, but not nicotine seeking. In contrast, at 0.5 mg/kg, varenicline facilitated cue-induced nicotine seeking. Similar to nicotine, varenicline increased premature responding at low doses, but had no effect on any of the other behavioural parameters in the 5-CSRTT. Conclusions: Our data indicate that varenicline specifically reduced responding for nicotine and alcohol, but not for natural reinforcers such as sucrose. Interestingly, varenicline strongly attenuated cue-induced relapse to alcohol seeking, but not nicotine seeking. Varenicline may therefore be a promising aid in the treatment of alcohol addiction. © 2011 The Author(s).

The medial prefrontal cortex (mPFC) is critically involved in numerous cognitive functions, including attention, inhibitory control, habit formation, working memory and long-term memory. Moreover, through its dense interconnectivity with subcortical regions (e.g., thalamus, striatum, amygdala and hippocampus), the mPFC is thought to exert top-down executive control over the processing of aversive and appetitive stimuli. Because the mPFC has been implicated in the processing of a wide range of cognitive and emotional stimuli, it is thought to function as a central hub in the brain circuitry mediating symptoms of psychiatric disorders. New optogenetics technology enables anatomical and functional dissection of mPFC circuitry with unprecedented spatial and temporal resolution. This provides important novel insights in the contribution of specific neuronal subpopulations and their connectivity to mPFC function in health and disease states. In this review, we present the current knowledge obtained with optogenetic methods concerning mPFC function and dysfunction and integrate this with findings from traditional intervention approaches used to investigate the mPFC circuitry in animal models of cognitive processing and psychiatric disorders.

High rates of comorbidity between alcohol use disorder (AUD) and major depressive disorder (MDD) are reported. Preclinical models examining effects of primary depression on secondary AUD are currently absent, preventing adequate testing of drug treatment. Here, we combined social defeat-induced persistent stress (SDPS) and operant alcohol self-administration (SA) paradigms to assess causality between these two neuropsychiatric disorders. We then exploited guanfacine, an FDA-approved adrenergic agent reported to reduce drug craving in humans, against SDPS-induced modulation of operant alcohol SA. Wistar rats were socially defeated and isolated for a period of ≥9 weeks, during which depression-like symptomatology (cognitive and social behavioral symptoms) was assessed. Subsequently, animals were subjected to a 5-month operant alcohol SA paradigm, examining acquisition, motivation, extinction, and cue-induced reinstatement of alcohol seeking. The effects of guanfacine on motivation and relapse were measured at >6 months following defeat. SDPS rats exhibited significant disruption of social and cognitive behavior, including short-term spatial and long-term social memory, several months following defeat. Notably, SDPS increased motivation to obtain alcohol, and cue-induced relapse vulnerability. Guanfacine reversed the SDPS-induced effects on motivation and relapse. Together, our model mimics core symptomatology of a sustained depressive-like state and a subsequent vulnerability to alcohol abuse. We show that SDPS is strongly associated with an enhanced motivation for alcohol intake and relapse. Finally, we show that the clinically employed drug guanfacine has potential as a novel treatment option in comorbid patients, as it effectively reduced the enhanced sensitivity to alcohol and alcohol-associated stimuli. © 2014 American College of Neuropsychopharmacology.

Patients with depression often suffer from cognitive impairments that contribute to disease burden. We used social defeat-induced persistent stress (SDPS) to induce a depressive-like state in rats and then studied long-lasting memory deficits in the absence of acute stressors in these animals. The SDPS rat model showed reduced short-term object location memory and maintenance of long-term potentiation (LTP) in CA1 pyramidal neurons of the dorsal hippocampus. SDPS animals displayed increased expression of synaptic chondroitin sulfate proteoglycans in the dorsal hippocampus. These effects were abrogated by a 3-week treatment with the antidepressant imipramine starting 8 weeks after the last defeat encounter. Next, we observed an increase in the number of perineuronal nets (PNNs) surrounding parvalbumin-expressing interneurons and a decrease in the frequency of inhibitory postsynaptic currents (IPSCs) in the hippocampal CA1 region in SDPS animals. In vivo breakdown of the hippocampus CA1 extracellular matrix by the enzyme chondroitinase ABC administered intracranially restored the number of PNNs, LTP maintenance, hippocampal inhibitory tone, and memory performance on the object place recognition test. Our data reveal a causal link between increased hippocampal extracellular matrix and the cognitive deficits associated with a chronic depressive-like state in rats exposed to SDPS.

Successful treatment of drug addiction is hampered by high relapse rates during periods of abstinence. Neuroadaptation in the medial prefrontal cortex (mPFC) is thought to have a crucial role in vulnerability to relapse to drug seeking, but the molecular and cellular mechanisms remain largely unknown. To identify protein changes that contribute to relapse susceptibility, we investigated synaptic membrane fractions from the mPFC of rats that underwent 21 days of forced abstinence following heroin self-administration. Quantitative proteomics revealed that long-term abstinence from heroin self-administration was associated with reduced levels of extracellular matrix (ECM) proteins. After extinction of heroin self-administration, downregulation of ECM proteins was also present in the mPFC, as well as nucleus accumbens (NAc), and these adaptations were partially restored following cue-induced reinstatement of heroin seeking. In the mPFC, these ECM proteins are condensed in the perineuronal nets that exclusively surround GABAergic interneurons, indicating that ECM adaptation might alter the activity of GABAergic interneurons. In support of this, we observed an increase in the inhibitory GABAergic synaptic inputs received by the mPFC pyramidal cells after the re-exposure to heroin-conditioned cues. Recovering levels of ECM constituents by metalloproteinase inhibitor treatment (FN-439; i.c.v.) prior to a reinstatement test attenuated subsequent heroin seeking, suggesting that the reduced synaptic ECM levels during heroin abstinence enhanced sensitivity to respond to heroin-conditioned cues. We provide evidence for a novel neuroadaptive mechanism, in which heroin self-administration-induced adaptation of the ECM increased relapse vulnerability, potentially by augmenting the responsivity of mPFC GABAergic interneurons to heroin-associated stimuli. © 2010 Nature Publishing Group All rights reserved.