Metabotropic NMDA Receptor Signaling Contributes to Sex Differences in Synaptic Plasticity and Episodic Memory.

Men generally outperform women on encoding spatial components of episodic memory whereas the reverse holds for semantic elements. Here we show that female mice outperform males on tests for non-spatial aspects of episodic memory ("what", "when"), suggesting that the human findings are influenced by neurobiological factors common to mammals. Analysis of hippocampal synaptic plasticity mechanisms and encoding revealed unprecedented, sex-specific contributions of non-classical metabotropic NMDA receptor (NMDAR) functions. While both sexes used non-ionic NMDAR signaling to trigger actin polymerization needed to consolidate long-term potentiation (LTP), NMDAR GluN2B subunit antagonism blocked these effects in males only and had the corresponding sex-specific effect on episodic memory. Conversely, blocking estrogen receptor alpha eliminated metabotropic stabilization of LTP and episodic memory in females only. The results show that sex differences in metabotropic signaling critical for enduring synaptic plasticity in hippocampus have significant consequences for encoding episodic memories.

Novel types of frequency filtering in the lateral perforant path projections to dentate gyrus.

Despite its evident importance to learning theory and models, the manner in which the lateral perforant path (LPP) transforms signals from entorhinal cortex to hippocampus is not well understood. The present studies measured synaptic responses in the dentate gyrus (DG) of adult mouse hippocampal slices during different patterns of LPP stimulation. Theta (5 Hz) stimulation produced a modest within-train facilitation that was markedly enhanced at the level of DG output. Gamma (50 Hz) activation resulted in a singular pattern with initial synaptic facilitation being followed by a progressively greater depression. DG output was absent after only two pulses. Reducing release probability with low extracellular calcium instated frequency facilitation to gamma stimulation while long-term potentiation, which increases release by LPP terminals, enhanced within-train depression. Relatedly, per terminal concentrations of VGLUT2, a vesicular glutamate transporter associated with high release probability, were much greater in the LPP than in CA3-CA1 connections. Attempts to circumvent the potent gamma filter using a series of short (three-pulse) 50 Hz trains spaced by 200 ms were only partially successful: composite responses were substantially reduced after the first burst, an effect opposite to that recorded in field CA1. The interaction between bursts was surprisingly persistent (>1.0 s). Low calcium improved throughput during theta/gamma activation but buffering of postsynaptic calcium did not. In all, presynaptic specializations relating to release probability produce an unusual but potent type of frequency filtering in the LPP. Patterned burst input engages a different type of filter with substrates that are also likely to be located presynaptically. KEY POINTS: The lateral perforant path (LPP)-dentate gyrus (DG) synapse operates as a low-pass filter, where responses to a train of 50 Hz, γ frequency activation are greatly suppressed. Activation with brief bursts of γ frequency information engages a secondary filter that persists for prolonged periods (lasting seconds). Both forms of LPP frequency filtering are influenced by presynaptic, as opposed to postsynaptic, processes; this contrasts with other hippocampal synapses. LPP frequency filtering is modified by the unique presynaptic long-term potentiation at this synapse. Computational simulations indicate that presynaptic factors associated with release probability and vesicle recycling may underlie the potent LPP-DG frequency filtering.

Prepubescent female rodents have enhanced hippocampal LTP and learning relative to males, reversing in adulthood as inhibition increases.

Multiple studies indicate that adult male rodents perform better than females on spatial problems and have a lower threshold for long-term potentiation (LTP) of hippocampal CA3-to-CA1 synapses. We report here that, in rodents, prepubescent females rapidly encode spatial information and express low-threshold LTP, whereas age-matched males do not. The loss of low-threshold LTP across female puberty was associated with three inter-related changes: increased densities of α5 subunit-containing GABAARs at inhibitory synapses, greater shunting of burst responses used to induce LTP and a reduction of NMDAR-mediated synaptic responses. A negative allosteric modulator of α5-GABAARs increased burst responses to a greater degree in adult than in juvenile females and markedly enhanced both LTP and spatial memory in adults. The reasons for the gain of functions with male puberty do not involve these mechanisms. In all, puberty has opposite consequences for plasticity in the two sexes, albeit through different routes.

Epigenetic regulation of immediate-early gene Nr4a2/Nurr1 in the medial habenula during reinstatement of cocaine-associated behavior.

Propensity to relapse following long periods of abstinence is a key feature of substance use disorder. Drugs of abuse, such as cocaine, cause long-term changes in the neural circuitry regulating reward, motivation, and memory processes through dysregulation of various molecular mechanisms, including epigenetic regulation of activity-dependent gene expression. Underlying drug-induced changes to neural circuit function are the molecular mechanisms regulating activity-dependent gene expression. Of note, histone acetyltransferases and histone deacetylases (HDACs), powerful epigenetic regulators of gene expression, are dysregulated following both acute and chronic cocaine exposure and are linked to cocaine-induced changes in neural circuit function. To better understand the effect of drug-induced changes on epigenetic function and behavior, we investigated HDAC3-mediated regulation of Nr4a2/Nurr1 in the medial habenula, an understudied pathway in cocaine-associated behaviors. Nr4a2, a transcription factor critical in cocaine-associated behaviors and necessary for MHb development, is enriched in the cholinergic cell-population of the MHb; yet, the role of NR4A2 within the MHb in the adult brain remains elusive. Here, we evaluated whether epigenetic regulation of Nr4a2 in the MHb has a role in reinstatement of cocaine-associated behaviors. We found that HDAC3 disengages from Nr4a2 in the MHb in response to cocaine-primed reinstatement. Whereas enhancing HDAC3 function in the MHb had no effect on reinstatement, we found, using a dominant-negative splice variant (NURR2C), that loss of NR4A2 function in the MHb blocked reinstatement behaviors. These results show for the first time that regulation of NR4A2 function in the MHb is critical in relapse-like behaviors.

Medial habenula cholinergic signaling regulates cocaine-associated relapse-like behavior.

Propensity to relapse, even following long periods of abstinence, is a key feature in substance use disorders. Relapse and relapse-like behaviors are known to be induced, in part, by re-exposure to drug-associated cues. Yet, while many critical nodes in the neural circuitry contributing to relapse have been identified and studied, a full description of the networks driving reinstatement of drug-seeking behaviors is lacking. One area that may provide further insight to the mechanisms of relapse is the habenula complex, an epithalamic region composed of lateral and medial (MHb) substructures, each with unique cell and target populations. Although well conserved across vertebrate species, the functions of the MHb are not well understood. Recent research has demonstrated that the MHb regulates nicotine aversion and withdrawal. However, it remains undetermined whether MHb function is limited to nicotine and aversive stimuli or if MHb circuit regulates responses to other drugs of abuse. Advances in circuit-level manipulations now allow for cell-type and temporally specific manipulations during behavior, specifically in spatially restrictive brain regions, such as the MHb. In this study, we focus on the response of the MHb to reinstatement of cocaine-associated behavior, demonstrating that cocaine-primed reinstatement of conditioned place preference engages habenula circuitry. Using chemogenetics, we demonstrate that MHb activity is sufficient to induce reinstatement behavior. Together, these data identify the MHb as a key hub in the circuitry underlying reinstatement and may serve as a target for regulating relapse-like behaviors.

UBE3A-mediated p18/LAMTOR1 ubiquitination and degradation regulate mTORC1 activity and synaptic plasticity.

Accumulating evidence indicates that the lysosomal Ragulator complex is essential for full activation of the mechanistic target of rapamycin complex 1 (mTORC1). Abnormal mTORC1 activation has been implicated in several developmental neurological disorders, including Angelman syndrome (AS), which is caused by maternal deficiency of the ubiquitin E3 ligase UBE3A. Here we report that Ube3a regulates mTORC1 signaling by targeting p18, a subunit of the Ragulator. Ube3a ubiquinates p18, resulting in its proteasomal degradation, and Ube3a deficiency in the hippocampus of AS mice induces increased lysosomal localization of p18 and other members of the Ragulator-Rag complex, and increased mTORC1 activity. p18 knockdown in hippocampal CA1 neurons of AS mice reduces elevated mTORC1 activity and improves dendritic spine maturation, long-term potentiation (LTP), as well as learning performance. Our results indicate that Ube3a-mediated regulation of p18 and subsequent mTORC1 signaling is critical for typical synaptic plasticity, dendritic spine development, and learning and memory.

Author Correction: Patch clamp-assisted single neuron lipidomics.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Atypical Endocannabinoid Signaling Initiates a New Form of Memory-Related Plasticity at a Cortical Input to Hippocampus.

Endocannabinoids (ECBs) depress transmitter release at sites throughout the brain. Here, we describe another form of ECB signaling that triggers a novel form of long-term potentiation (LTP) localized to the lateral perforant path (LPP) which conveys semantic information from cortex to hippocampus. Two cannabinoid CB1 receptor (CB1R) signaling cascades were identified in hippocampus. The first is pregnenolone sensitive, targets vesicular protein Munc18-1 and depresses transmitter release; this cascade is engaged by CB1Rs in Schaffer-Commissural afferents to CA1 but not in the LPP, and it does not contribute to LTP. The second cascade is pregnenolone insensitive and LPP specific; it entails co-operative CB1R/ß1-integrin signaling to effect synaptic potentiation via stable enhancement of transmitter release. The latter cascade is engaged during LPP-dependent learning. These results link atypical ECB signaling to the encoding of a fundamental component of episodic memory and suggest a novel route whereby endogenous and exogenous cannabinoids affect cognition.

Patch clamp-assisted single neuron lipidomics.

Our understanding of the physiological and pathological functions of brain lipids is limited by the inability to analyze these molecules at cellular resolution. Here, we present a method that enables the detection of lipids in identified single neurons from live mammalian brains. Neuronal cell bodies are captured from perfused mouse brain slices by patch clamping, and lipids are analyzed using an optimized nanoflow liquid chromatography/mass spectrometry protocol. In a first application of the method, we identified more than 40 lipid species from dentate gyrus granule cells and CA1 pyramidal neurons of the hippocampus. This survey revealed substantial lipid profile differences between neurons and whole brain tissue, as well as between resting and physiologically stimulated neurons. The results suggest that patch clamp-assisted single neuron lipidomics could be broadly applied to investigate neuronal lipid homeostasis in healthy and diseased brains.

A Primary Cortical Input to Hippocampus Expresses a Pathway-Specific and Endocannabinoid-Dependent Form of Long-Term Potentiation.

The endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG), a key modulator of synaptic transmission in mammalian brain, is produced in dendritic spines and then crosses the synaptic junction to depress neurotransmitter release. Here we report that 2-AG-dependent retrograde signaling also mediates an enduring enhancement of glutamate release, as assessed with independent tests, in the lateral perforant path (LPP), one of two cortical inputs to the granule cells of the dentate gyrus. Induction of this form of long-term potentiation (LTP) involved two types of glutamate receptors, changes in postsynaptic calcium, and the postsynaptic enzyme that synthesizes 2-AG. Stochastic optical reconstruction microscopy confirmed that CB1 cannabinoid receptors are localized presynaptically to LPP terminals, while the inhibition or knockout of the receptors eliminated LPP-LTP. Suppressing the enzyme that degrades 2-AG dramatically enhanced LPP potentiation, while overexpressing it produced the opposite effect. Priming with a CB1 agonist markedly reduced the threshold for LTP. Latrunculin A, which prevents actin polymerization, blocked LPP-LTP when applied extracellularly but had no effect when infused postsynaptically into granule cells, indicating that critical actin remodeling resides in the presynaptic compartment. Importantly, there was no evidence for the LPP form of potentiation in the Schaffer-commissural innervation of field CA1 or in the medial perforant path. Peripheral injections of compounds that block or enhance LPP-LTP had corresponding effects on the formation of long-term memory for cues conveyed to the dentate gyrus by the LPP. Together, these results indicate that the encoding of information carried by a principal hippocampal afferent involves an unusual, regionally differentiated form of plasticity.

Chronic Ampakine Treatments Stimulate Dendritic Growth and Promote Learning in Middle-Aged Rats.

Positive allosteric modulators of AMPA-type glutamate receptors (ampakines) have been shown to rescue synaptic plasticity and reduce neuropathology in rodent models of cognitive disorders. Here we tested whether chronic ampakine treatment offsets age-related dendritic retraction in middle-aged (MA) rats. Starting at 10 months of age, rats were housed in an enriched environment and given daily treatment with a short half-life ampakine or vehicle for 3 months. Dendritic branching and spine measures were collected from 3D reconstructions of Lucifer yellow-filled CA1 pyramidal cells. There was a substantial loss of secondary branches, relative to enriched 2.5-month-old rats, in apical and basal dendritic fields of vehicle-treated, but not ampakine-treated, 13-month-old rats. Baseline synaptic responses in CA1 were only subtly different between the two MA groups, but long-term potentiation was greater in ampakine-treated rats. Unsupervised learning of a complex environment was used to assess treatment effects on behavior. Vehicle- and drug-treated rats behaved similarly during a first 30 min session in the novel environment but differed markedly on subsequent measures of long-term memory. Markov sequence analysis uncovered a clear increase in the predictability of serial movements between behavioral sessions 2 and 3 in the ampakine, but not vehicle, group. These results show that a surprising degree of dendritic retraction occurs by middle age and that this can be mostly offset by pharmacological treatments without evidence for unwanted side effects. The functional consequences of rescue were prominent with regard to memory but also extended to self-organization of behavior. SIGNIFICANCE STATEMENT: Brain aging is characterized by a progressive loss of dendritic arbors and the emergence of impairments to learning-related synaptic plasticity. The present studies show that dendritic losses are evident by middle age despite housing in an enriched environment and can be mostly reversed by long-term, oral administration of a positive allosteric modulator of AMPA-type glutamate receptors. Dendritic recovery was accompanied by improvements to both synaptic plasticity and the encoding of long-term memory of a novel, complex environment. Because the short half-life compound had no evident negative effects, the results suggest a plausible strategy for treating age-related neuronal deterioration.

Pronounced differences in signal processing and synaptic plasticity between piriform-hippocampal network stages: a prominent role for adenosine.

KEY POINTS: Extended trains of theta rhythm afferent activity lead to a biphasic response facilitation in field CA1 but not in the lateral perforant path input to the dentate gyrus. Processes that reverse long-term potentiation in field CA1 are not operative in the lateral perforant path: multiple lines of evidence indicate that this reflects differences in adenosine signalling. Adenosine A1 receptors modulate baseline synaptic transmission in the lateral olfactory tract but not the associational afferents of the piriform cortex. Levels of ecto-5'-nucleotidase (CD73), an enzyme that converts extracellular ATP into adenosine, are markedly different between regions and correlate with adenosine signalling and the efficacy of theta pulse stimulation in reversing long-term potentiation. Variations in transmitter mobilization, CD73 levels, and afferent divergence result in multivariate differences in signal processing through nodes in the cortico-hippocampal network. ABSTRACT: The present study evaluated learning-related synaptic operations across the serial stages of the olfactory cortex-hippocampus network. Theta frequency stimulation produced very different time-varying responses in the Schaffer-commissural projections than in the lateral perforant path (LPP), an effect associated with distinctions in transmitter mobilization. Long-term potentiation (LTP) had a higher threshold in LPP field potential studies but not in voltage clamped neurons; coupled with input/output relationships, these results suggest that LTP threshold differences reflect the degree of input divergence. Theta pulse stimulation erased LTP in CA1 but not in the dentate gyrus (DG), although adenosine eliminated potentiation in both areas, suggesting that theta increases extracellular adenosine to a greater degree in CA1. Moreover, adenosine A1 receptor antagonism had larger effects on theta responses in CA1 than in the DG, and concentrations of ecto-5'-nucleotidase (CD73) were much higher in CA1. Input/output curves for two connections in the piriform cortex were similar to those for the LPP, whereas adenosine modulation again correlated with levels of CD73. In sum, multiple relays in a network extending from the piriform cortex through the hippocampus can be differentiated along three dimensions (input divergence, transmitter mobilization, adenosine modulation) that potently influence throughput and plasticity. A model that incorporates the regional differences, supplemented with data for three additional links, suggests that network output goes through three transitions during the processing of theta input. It is proposed that individuated relays allow the circuit to deal with different types of behavioural problems.

Chemically induced specification of retinal ganglion cells from human embryonic and induced pluripotent stem cells.

The loss of retinal ganglion cells (RGCs) is the primary pathological change for many retinal degenerative diseases. Although there is currently no effective treatment for this group of diseases, cell transplantation to replace lost RGCs holds great potential. However, for the development of cell replacement therapy, better understanding of the molecular details involved in differentiating stem cells into RGCs is essential. In this study, a novel, stepwise chemical protocol is described for the differentiation of human embryonic stem cells and induced pluripotent stem cells into functional RGCs. Briefly, stem cells were differentiated into neural rosettes, which were then cultured with the Notch inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT). The expression of neural and RGC markers (BRN3A, BRN3B, ATOH7/Math5, γ-synuclein, Islet-1, and THY-1) was examined. Approximately 30% of the cell population obtained expressed the neuronal marker TUJ1 as well the RGC markers. Moreover, the differentiated RGCs generated action potentials and exhibited both spontaneous and evoked excitatory postsynaptic currents, indicating that functional and mature RGCs were generated. In combination, these data demonstrate that a single chemical (DAPT) can induce PAX6/RX-positive stem cells to undergo differentiation into functional RGCs.

The neuron-specific chromatin regulatory subunit BAF53b is necessary for synaptic plasticity and memory.

Recent exome sequencing studies have implicated polymorphic Brg1-associated factor (BAF) complexes (mammalian SWI/SNF chromatin remodeling complexes) in several human intellectual disabilities and cognitive disorders. However, it is currently unknown how mutations in BAF complexes result in impaired cognitive function. Postmitotic neurons express a neuron-specific assembly, nBAF, characterized by the neuron-specific subunit BAF53b. Mice harboring selective genetic manipulations of BAF53b have severe defects in long-term memory and long-lasting forms of hippocampal synaptic plasticity. We rescued memory impairments in BAF53b mutant mice by reintroducing BAF53b in the adult hippocampus, which suggests a role for BAF53b beyond neuronal development. The defects in BAF53b mutant mice appeared to derive from alterations in gene expression that produce abnormal postsynaptic components, such as spine structure and function, and ultimately lead to deficits in synaptic plasticity. Our results provide new insight into the role of dominant mutations in subunits of BAF complexes in human intellectual and cognitive disorders.

Presynaptic BDNF promotes postsynaptic long-term potentiation in the dorsal striatum.

Brain-derived neurotrophic factor (BDNF) facilitates the formation of long-term potentiation (LTP) in hippocampus, but whether this involves release from presynaptic versus postsynaptic pools is unclear. We therefore tested whether BDNF is essential for LTP in dorsal striatum, a structure in which the neurotrophin is present only in afferent terminals. Whole-cell recordings were collected from medium spiny neurons in striatal slices prepared from adult mice. High-frequency stimulation (HFS) of neocortical afferents produced a rapid and stable NMDA receptor-dependent potentiation. The ratio of AMPA to NMDA receptor-mediated components of the EPSPs was substantially increased after inducing potentiation, suggesting that the response enhancement involved postsynaptic changes. In accord with this, paired-pulse response ratios, a measure of transmitter release kinetics, were reduced by elevated calcium but not by LTP. Infusion of the BDNF scavenger TrkB-Fc blocked the formation of potentiation, beginning with the second minute after HFS, without reducing responses to HFS. These results suggest that presynaptic pools of BDNF can act within 2 min of HFS to support the formation of a postsynaptic form of LTP in striatum.

Myosin IIb regulates actin dynamics during synaptic plasticity and memory formation.

Reorganization of the actin cytoskeleton is essential for synaptic plasticity and memory formation. Presently, the mechanisms that trigger actin dynamics during these brain processes are poorly understood. In this study, we show that myosin II motor activity is downstream of LTP induction and is necessary for the emergence of specialized actin structures that stabilize an early phase of LTP. We also demonstrate that myosin II activity contributes importantly to an actin-dependent process that underlies memory consolidation. Pharmacological treatments that promote actin polymerization reversed the effects of a myosin II inhibitor on LTP and memory. We conclude that myosin II motors regulate plasticity by imparting mechanical forces onto the spine actin cytoskeleton in response to synaptic stimulation. These cytoskeletal forces trigger the emergence of actin structures that stabilize synaptic plasticity. Our studies provide a mechanical framework for understanding cytoskeletal dynamics associated with synaptic plasticity and memory formation.

Serotonin-induced region-specific responses of the arcuate and ventromedial hypothalamic nuclei.

The arcuate nucleus (ARC) modulates both satiety and hunger signals at the lateral and medial sites, respectively, though these competing responses may be both mediated by serotonin (5-HT). We sought to determine region-specific effects of 5-HT on ARC neurons. Electrical activities in rat hypothalamic slices were simultaneously recorded with an extracellular multielectrode array. 5-HT effects on the ARC were region-specific: primary inhibition of medial ARC and stimulation of lateral ARC neurons. 5-HT primarily inhibited ventromedial nucleus neurons. These results suggest ARC region-specific responses to 5-HT consistent with the anatomical location of lateral ARC anorexigenic proopiomelanocortin neurons and medial ARC orexigenic neuropeptide Y neurons.

Nicotine facilitates long-term potentiation induction in oriens-lacunosum moleculare cells via Ca2+ entry through non-alpha7 nicotinic acetylcholine receptors.

Hippocampal inhibitory interneurons have a central role in the control of network activity, and excitatory synapses that they receive express Hebbian and anti-Hebbian long-term potentiation (LTP). Because many interneurons in the hippocampus express nicotinic acetylcholine receptors (nAChRs), we explored whether exposure to nicotine promotes LTP induction in these interneurons. We focussed on a subset of interneurons in the stratum oriens/alveus that were continuously activated in the presence of nicotine due to the expression of non-desensitizing non-alpha7 nAChRs. We found that, in addition to alpha2 subunit mRNAs, these interneurons were consistently positive for somatostatin and neuropeptide Y mRNAs, and showed morphological characteristics of oriens-lacunosum moleculare cells. Activation of non-alpha7 nAChRs increased intracellular Ca(2+) levels at least in part via Ca(2+) entry through their channels. Presynaptic tetanic stimulation induced N-methyl-D-aspartate receptor-independent LTP in voltage-clamped interneurons at -70 mV when in the presence, but not absence, of nicotine. Intracellular application of a Ca(2+) chelator blocked LTP induction, suggesting the requirement of Ca(2+) signal for LTP induction. The induction of LTP was still observed in the presence of ryanodine, which inhibits Ca(2+) -induced Ca(2+) release from ryanodine-sensitive intracellular stores, and the L-type Ca(2+) channel blocker nifedipine. These results suggest that Ca(2+) entry through non-alpha7 nAChR channels is critical for LTP induction. Thus, nicotine affects hippocampal network activity by promoting LTP induction in oriens-lacunosum moleculare cells via continuous activation of non-alpha7 nAChRs.

Origins of an intrinsic hippocampal EEG pattern.

Sharp waves (SPWs) are irregular waves that originate in field CA3 and spread throughout the hippocampus when animals are alert but immobile or as a component of the sleep EEG. The work described here used rat hippocampal slices to investigate the factors that initiate SPWs and govern their frequency. Acute transection of the mossy fibers reduced the amplitude but not the frequency of SPWs, suggesting that activity in the dentate gyrus may enhance, but is not essential for, the CA3 waves. However, selective destruction of the granule cells and mossy fibers by in vivo colchicine injections profoundly depressed SPW frequency. Reducing mossy fiber release with an mGluR2 receptor agonist or enhancing it with forskolin respectively depressed or increased the incidence of SPWs. Collectively, these results indicate that SPWs can be triggered by constitutive release from the mossy fibers. The waves were not followed by large after-hyperpolarizing potentials and their frequency was not strongly affected by blockers of various slow potassium channels. Antagonists of GABA-B mediated IPSCs also had little effect on incidence. It appears from these results that the spacing of SPWs is not dictated by slow potentials. However, modeling work suggests that the frequency and variance of large mEPSCs from the mossy boutons can account for the temporal distribution of the waves. Together, these results indicate that constitutive release from the mossy fiber terminal boutons regulates the incidence of SPWs and their contribution to information processing in hippocampus.

Alpha2 nicotine receptors function as a molecular switch to continuously excite a subset of interneurons in rat hippocampal circuits.

Rapid activation of nicotinic acetylcholine receptors (nAChRs) at various anatomical and cellular locations in the hippocampus differentially modulates the operation of hippocampal circuits. However, it is largely unknown how the continued presence of nicotine affects the normal operation of hippocampal circuits. Here, we used single and dual whole-cell recordings to address this question. We found that horizontally oriented interneurons in the stratum oriens/alveus continuously discharged action potentials in the presence of nicotine. In these interneurons, bath application of nicotine produced slow inward currents that were well maintained and inhibited by the non-alpha 7 antagonist dihydro-beta-erythroidine. Single-cell reverse transcription-polymerase chain reaction analysis showed that nicotine-responding interneurons were consistently positive for the alpha2 subunit mRNA. These observations suggest that in the presence of nicotine, a subset of interneurons in the stratum oriens/alveus are continuously excited due to the sustained activation of alpha2* nAChRs. These interneurons were synaptically connected to pyramidal cells, and nicotine increased inhibitory baseline currents at the synapses and suppressed phasic inhibition at the same synapses. Nicotine-induced inhibitory activity increased background noise and masked small phasic inhibition in pyramidal cells, originating from other interneurons in the stratum radiatum. Thus, the continued presence of nicotine alters the normal operation of hippocampal circuits by gating inhibitory circuits through activating a non-desensitizing alpha2 nAChR subtype on a distinct population of interneurons.