Morphological evidence for a dopaminergic terminal field in the hippocampal formation of young and adult rat

In vivo structural connectivity of the reward system along the hippocampal long axis

Recent work has identified a critical role for the hippocampus in reward-sensitive behaviors, including motivated memory, reinforcement learning, and decision-making. Animal histology and human functional neuroimaging have shown that brain regions involved in reward processing and motivation are more interconnected with the ventral/anterior hippocampus. However, direct evidence examining gradients of structural connectivity between reward regions and the hippocampus in humans is lacking. The present study used diffusion MRI (dMRI) and probabilistic tractography to quantify the structural connectivity of the hippocampus with key reward processing regions in vivo. Using a large sample of subjects (N = 628) from the human connectome dMRI data release, we found that connectivity profiles with the hippocampus varied widely between different regions of the reward circuit. While the dopaminergic midbrain (ventral tegmental area) showed stronger connectivity with the anterior versus posterior hippocampus, the ventromedial prefrontal cortex showed stronger connectivity with the posterior hippocampus. The limbic (ventral) striatum demonstrated a more homogeneous connectivity profile along the hippocampal long axis. This is the first study to generate a probabilistic atlas of the hippocampal structural connectivity with reward-related networks, which is essential to investigating how these circuits contribute to normative adaptive behavior and maladaptive behaviors in psychiatric illness. These findings describe nuanced structural connectivity that sets the foundation to better understand how the hippocampus influences reward-guided behavior in humans.

Cognitive performance in aged rats is associated with differences in distinctive neuronal populations in the ventral tegmental area and altered synaptic plasticity in the hippocampus

Introduction

Deterioration of cognitive functions is commonly associated with aging, although there is wide variation in the onset and manifestation. Albeit heterogeneity in age-related cognitive decline has been studied at the cellular and molecular level, there is poor evidence for electrophysiological correlates. The aim of the current study was to address the electrophysiological basis of heterogeneity of cognitive functions in cognitively Inferior and Superior old (19-20 months) rats in the ventral tegmental area (VTA) and the hippocampus, having Young (12 weeks) rats as a control. The midbrain VTA operates as a hub amidst affective and cognitive facets, processing sensory inputs related to motivated behaviours and hippocampal memory. Increasing evidence shows direct dopaminergic and non-dopaminergic input from the VTA to the hippocampus.

Methods

Aged Superior and Inferior male rats were selected from a cohort of 88 animals based on their performance in a spatial learning and memory task. Using in vivo single-cell recording in the VTA, we examined the electrical activity of different neuronal populations (putative dopaminergic, glutamatergic and GABAergic neurons). In the same animals, basal synaptic transmission and synaptic plasticity were examined in hippocampal slices.

Results

Electrophysiological recordings from the VTA and hippocampus showed alterations associated with aging per se, together with differences specifically linked to the cognitive status of aged animals. In particular, the bursting activity of dopamine neurons was lower, while the firing frequency of glutamatergic neurons was higher in VTA of Inferior old rats. The response to high-frequency stimulation in hippocampal slices also discriminated between Superior and Inferior aged animals.

Discussion

This study provides new insight into electrophysiological information underlying compromised cerebral ageing. Further understanding of brain senescence, possibly related to neurocognitive decline, will help develop new strategies towards the preservation of a high quality of life.

Non-canonical interplay between glutamatergic NMDA and dopamine receptors shapes synaptogenesis

Direct interactions between receptors at the neuronal surface have long been proposed to tune signaling cascades and neuronal communication in health and disease. Yet, the lack of direct investigation methods to measure, in live neurons, the interaction between different membrane receptors at the single molecule level has raised unanswered questions on the biophysical properties and biological roles of such receptor interactome. Using a multidimensional spectral single molecule-localization microscopy (MS-SMLM) approach, we monitored the interaction between two membrane receptors, i.e. glutamatergic NMDA (NMDAR) and G protein-coupled dopamine D1 (D1R) receptors. The transient interaction was randomly observed along the dendritic tree of hippocampal neurons. It was higher early in development, promoting the formation of NMDAR-D1R complexes in an mGluR5- and CK1-dependent manner, favoring NMDAR clusters and synaptogenesis in a dopamine receptor signaling-independent manner. Preventing the interaction in the neonate, and not adult, brain alters in vivo spontaneous neuronal network activity pattern in male mice. Thus, a weak and transient interaction between NMDAR and D1R plays a structural and functional role in the developing brain.

Dopaminoceptive D1 and D2 neurons in ventral hippocampus arbitrate approach and avoidance in anxiety

The hippocampus 1-7, as well as dopamine circuits 8-11, coordinate decision-making in anxiety-eliciting situations. Yet, little is known about how dopamine modulates hippocampal representations of emotionally-salient stimuli to inform appropriate resolution of approach versus avoidance conflicts. We here study dopaminoceptive neurons in mouse ventral hippocampus (vHipp), molecularly distinguished by their expression of dopamine D1 or D2 receptors. We show that these neurons are transcriptionally distinct and topographically organized across vHipp subfields and cell types. In the ventral subiculum where they are enriched, both D1 and D2 neurons are recruited during anxiogenic exploration, yet with distinct profiles related to investigation and behavioral selection. In turn, they mediate opposite approach/avoidance responses, and are differentially modulated by dopaminergic transmission in that region. Together, these results suggest that vHipp dopamine dynamics gate exploratory behaviors under contextual uncertainty, implicating dopaminoception in the complex computation engaged in vHipp to govern emotional states.

Emotional- and cognitive-like responses induced by social defeat stress in male mice are modulated by the BNST, amygdala, and hippocampus

Introduction

Chronic exposure to social defeat stress (SDS) has been used to investigate the neurobiology of depressive- and anxiety-like responses and mnemonic processes. We hypothesized that these affective, emotional, and cognitive consequences induced by SDS are regulated via glutamatergic neurons located in the bed nucleus of the stria terminalis (BNST), amygdaloid complex, and hippocampus in mice.

Methods

Here, we investigated the influence of chronic SDS on (i) the avoidance behavior assessed in the social interaction test, (ii) the anxiety-like behavior (e.g., elevated plus-maze, and open field tests) (iii) depressive-like behaviors (e.g., coat state, sucrose splash, nesting building, and novel object exploration tests), (iv) the short-term memory (object recognition test), (v) ΔFosB, CaMKII as well as ΔFosB + CaMKII labeling in neurons located in the BNST, amygdaloid complex, dorsal (dHPC) and the ventral (vHPC) hippocampus.

Results

The main results showed that the exposure of mice to SDS (a) increased defensive and anxiety-like behaviors and led to memory impairment without eliciting clear depressive-like or anhedonic effects; (b) increased ΔFosB + CaMKII labeling in BNST and amygdala, suggesting that both areas are strongly involved in the modulation of this type of stress; and produced opposite effects on neuronal activation in the vHPC and dHPC, i.e., increasing and decreasing, respectively, ΔFosB labeling. The effects of SDS on the hippocampus suggest that the vHPC is likely related to the increase of defensive- and anxiety-related behaviors, whereas the dHPC seems to modulate the memory impairment.

Discussion

Present findings add to a growing body of evidence indicating the involvement of glutamatergic neurotransmission in the circuits that modulate emotional and cognitive consequences induced by social defeat stress.

Restraint stress induced the antinociceptive responses via the dopamine receptors within the hippocampal CA1 area in animal model of persistent inflammatory pain

It has been declared that dopamine receptors within the hippocampal formation are involved in emotion, memory, and pain processing. Remarkably, both CA1 and dentate gyrus (DG) areas of the hippocampal formation are involved in persistent peripheral nociceptive perception. A prior study showed that dopamine receptors within the hippocampal DG have a critical role in antinociception induced by forced swim stress (FSS), as a physical stressor, in the presence of formalin irritation. The present experiments were designed to assess whether dopaminergic receptors within the CA1 have any role in antinociceptive responses induced by restraint stress (RS) as a psychological stressor after applying the formalin test as an animal model of persistent inflammatory pain. The D1- and D2-like dopamine receptor antagonists, SCH23390 and Sulpiride (0.25, 1, and 4 μg/0.5 μl), were injected into the CA1 areas of ninety-six male albino Wistar rats 5 min before a 3-h period of restraint stress. Ten min after stress termination, a 50-μl formalin 2.5 % was subcutaneously injected into the plantar surface of the rat's hind paw to induce persistent inflammatory pain. Nociceptive behaviors in both phases of the formalin test were analyzed in the 5-min blocks for a 60-min period. The obtained results demonstrate that although RS could induce an antinociceptive response in both phases of the formalin test, microinjection of D1- and D2-like dopamine receptors, antagonists attenuated RS-induced analgesia. These results support the hypothesis that acute restraint stress could induce analgesia via dopaminergic projection to the CA1 region of the hippocampal formation.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Behavioral and neurochemical interactions of the tricyclic antidepressant drug desipramine with L-DOPA in 6-OHDA-lesioned rats. Implications for motor and psychiatric functions in Parkinson's disease

RATIONALE

The pharmacological effects of antidepressants in modulating noradrenergic transmission as compared to serotonergic transmission in a rat model of Parkinson's disease under chronic L-DOPA therapy are insufficiently explored.

OBJECTIVES

The aim of the present study was to investigate the effect of the tricyclic antidepressant desipramine administered chronically alone or jointly with L-DOPA, on motor behavior and monoamine metabolism in selected brain structures of rats with the unilateral 6-OHDA lesion.

METHODS

The antiparkinsonian activities of L-DOPA and desipramine were assessed behaviorally using a rotation test and biochemically based on changes in the tissue concentrations of noradrenaline, dopamine and serotonin and their metabolites, evaluated separately for the ipsi- and contralateral motor (striatum, substantia nigra) and limbic (prefrontal cortex, hippocampus) structures of rat brain by HPLC method.

RESULTS

Desipramine administered alone did not induce rotational behavior, but in combination with L-DOPA, it increased the number of contralateral rotations more strongly than L-DOPA alone. Both L-DOPA and desipramine + L-DOPA significantly increased DA levels in the ipsilateral striatum, substantia nigra, prefrontal cortex and the ipsi- and contralateral hippocampus. The combined treatment also significantly increased noradrenaline content in the ipsi- and contralateral striatum, while L-DOPA alone decreased serotonin level on both sides of the hippocampus.

CONCLUSIONS

The performed analysis of the level of monoamines and their metabolites in the selected brain structures suggests that co-modulation of noradrenergic and dopaminergic transmission in Parkinson's disease by the combined therapy with desipramine + L-DOPA may have some positive implications for motor and psychiatric functions but further research is needed to exclude potential negative effects.

Blocking the dopaminergic receptors in the hippocampal dentate gyrus reduced the stress-induced analgesia in persistent inflammatory pain in the rat

Although the dentate gyrus (DG) as a component of the hippocampal formation has been well known for its role in memory, various studies showed a diverse population of unique cell types and various inputs and outputs in this region. Besides, brain dopamine is known for its roles in reward, motivation, pleasure, and being involved in the pain process. Further, previous studies demonstrated the participation of DG dopaminergic receptors in antinociception induced by lateral hypothalamus stimulation. This study aimed to investigate the role of DG dopaminergic receptors (D1- and D2-like dopamine receptors) in stress-induced analgesia (SIA) using the formalin test as a persistent inflammatory pain model. One hundred two male Wistar rats were unilaterally implanted with a cannula into the DG. Animals received an intra-DG infusion of SCH23390 (0.25, 1, and 4 μg/rat), or Sulpiride (0.25, 1, and 4 μg/rat) as D1- and D2-like dopamine receptor antagonists, respectively, five min before exposure to forced swim stress (FSS). Ten minutes after FSS termination, 2.5% formalin solution as an inflammatory agent was subcutaneously injected into the plantar surface of the hind paw, and the pain score was quantified for one hour. The findings revealed that exposure to FSS produced SIA, though this FSS-induced analgesia was attenuated in the early and late phase of the formalin test by intra-DG microinjection of SCH23390 or Sulpiride. These results suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in analgesia induced by FSS.

Withdrawal from cocaine conditioning progressively alters AMPA receptor-mediated transmission in the ventral hippocampus

Drugs of abuse, such as cocaine, produce aberrant changes in synaptic transmission and plasticity that emerge throughout withdrawal. One region of the brain that displays a high degree of synaptic plasticity, as well as connectivity with mesolimbic structures such as the nucleus accumbens, is the ventral hippocampus (vH). Here, we investigated the effects of an escalating cocaine dosing schedule on vH CA1 excitatory transmission by measuring place preference and recording excitatory postsynaptic currents (EPSCs) at three different withdrawal time points: withdrawal day (WD) 2, 9 or 28. Behaviourally, this escalating cocaine-conditioning protocol was capable of producing conditioned place preference that persisted through WD28. Physiologically, cocaine conditioning produced an increase in vH excitatory transmission on WD2 that appeared to be the result of an increase in calcium-impermeable (CI)-AMPA receptor density. Excitatory transmission was still enhanced in cocaine-treated animals on WD9; however, a significant increase in the contribution of calcium-permeable (CP)-AMPA receptors to EPSCs was detected as compared with WD2. By WD28, these CP-AMPA receptors provided a major contribution to vH CA1 excitatory transmission, resulting in synaptic responses distinct from WD2 and WD9. Taken together, these results highlight progressive changes in vH synaptic transmission during withdrawal that may enhance cocaine contextual associations.

New boundaries and dissociation of the mouse hippocampus along the dorsal-ventral axis based on glutamatergic, GABAergic and catecholaminergic receptor densities

In rodents, gene-expression, neuronal tuning, connectivity and neurogenesis studies have postulated that the dorsal, the intermediate and the ventral hippocampal formation (HF) are distinct entities. These findings are underpinned by behavioral studies showing a dissociable role of dorsal and ventral HF in learning, memory, stress and emotional processing. However, up to now, the molecular basis of such differences in relation to discrete boundaries is largely unknown. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA, kainate and mGluR_2/3 , GABAergic GABA_A (including benzodiazepine binding sites), GABA_B , dopaminergic D_1/5 and noradrenergic α₁ and α₂ receptors as key modulators for signal transmission in hippocampal functions, using quantitative in vitro receptor autoradiography along the dorsal-ventral axis of the mouse HF. Beside general different receptor profiles of the dentate gyrus (DG) and Cornu Ammonis fields (CA1, CA2, CA3, CA4/hilus), we detected substantial differences between dorsal, intermediate and ventral subdivisions and individual layers for all investigated receptor types, except GABA_B . For example, striking higher densities of α₂ receptors were detected in the ventral DG, while the dorsal DG possesses higher numbers of kainate, NMDA, GABA_A and D_1/5 receptors. CA1 dorsal and intermediate subdivisions showed higher AMPA, NMDA, mGluR_2/3 , GABA_A , D_1/5 receptors, while kainate receptors are higher expressed in ventral CA1, and noradrenergic α₁ and α₂ receptors in the intermediate region of CA1. CA2 dorsal was distinguished by higher kainate, α₁ and α₂ receptors in the intermediate region, while CA3 showed a more complex dissociation. Our findings resulted not only in a clear segmentation of the mouse hippocampus along the dorsal-ventral axis, but also provides insights into the neurochemical basis and likely associated physiological processes in hippocampal functions. Therein, the presented data has a high impact for future studies modeling and investigating dorsal, intermediate and ventral hippocampal dysfunction in relation to neurodegenerative diseases or psychiatric disorders.

Distribution of VTA Glutamate and Dopamine Terminals, and their Significance in CA1 Neural Network Activity

Reciprocal connection between the ventral tegmental area (VTA) and the hippocampus forms a loop that controls information entry into long-term memory. Compared with the widely studied VTA dopamine system, VTA glutamate terminals are anatomically dominant in the hippocampus and less understood. The current study employs anterograde and retrograde labeling of VTA dopamine and glutamate neurons to map the distribution of their terminals within the layers of the hippocampus. Also, functional tracing of VTA dopamine and glutamate projections to the hippocampus was performed by photostimulation of VTA cell bodies during CA1 extracellular voltage sampling in vivo. VTA dopamine terminals predominantly innervate the CA1 basal dendrite layer and modulate the firing rate of active putative neurons. In contrast, anatomical dominance of VTA glutamate terminals in the CA1 pyramidal cell and apical dendrite layers suggests the possible involvement of these terminals in excitability regulation. In support of these outcomes, photostimulation of VTA dopamine neurons increased the firing rate but not intrinsic excitability parameters for putative pyramidal units. Conversely, activation of VTA glutamate neurons increased CA1 network firing rate and burst rate. In addition, VTA glutamate inputs reduced the interspike and interburst intervals for putative CA1 neurons. Taken together, we deduced that layer-specific distribution of presynaptic dopamine and glutamate terminals in the hippocampus determinines VTA modulation (dopamine) or regulation (glutamate) of excitability in the CA1 neural network.

Dopaminergic Plasticity in the Bilateral Hippocampus Following Threat Reversal in Humans

When a cue no longer predicts a threat, a diminished ability to extinguish or reverse this association is thought to increase risk for stress-related disorders. Despite the clear clinical relevance, the mediating neurochemical mechanisms of threat reversal have received relatively little study. One neurotransmitter implicated in rodent research of changing associations with threat is dopamine. To study whether dopamine is involved in threat reversal in humans, we used high-resolution positron emission tomography (PET) coupled with 18F-fallypride. Twelve healthy volunteers (6 F/6 M) underwent three PET scans: (i) at baseline, (ii) following threat conditioning (the response to a cue associated with electric wrist shock), and (iii) following threat reversal (the response to the same cue now associated with safety). We observed moderate evidence of reduced dopamine D2/3 receptor availability, consistent with greater dopamine release, in the bilateral anterior hippocampus following threat reversal, in response to a safety cue that was previously associated with threat, as compared to both baseline and during exposure to the same cue prior to threat reversal. These findings offer the first preliminary evidence that the response to a previously threatening cue that has since become associated with safety involves dopaminergic neurotransmission within the hippocampus in healthy humans.

Noncanonical, Dopamine-Dependent Long-Term Potentiation at Hippocampal Output Synapses in a Rodent Model of First-Episode Psychosis

Cognitive deficits and positive symptoms in schizophrenia have both been linked to hippocampal dysfunction. Recently, subregion-specific aberrant and maladaptive hippocampal synaptic plasticity has been suggested as one of the mechanistic underpinnings. The subiculum is the final output hub of the hippocampus and orchestrates hippocampal information transfer to other brain regions. While most CA1 pyramidal neurons show regular-spiking behavior, subicular output neurons comprise bursting and regular-firing pyramidal cells. These two cell types target different brain regions and express unique forms of synaptic plasticity. Here, we used a single systemic application of the noncompetitive glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 to model first-episode psychosis in rats and studied long-term potentiation (LTP) in subicular regular-firing cells in acute hippocampal slices. Previously, we have reported a facilitation of a presynaptic, late-onset LTP in subicular bursting pyramidal cells after systemic NMDAR antagonism. Here, we show that single systemic NMDAR antagonist application also facilitates the induction of a noncanonical, but postsynaptic NMDAR-independent LTP in ventral subicular but not in CA1 regular-firing pyramidal cells. This form of LTP was dependent on D1/D5 dopamine receptor activation. Activation of D1/D5 dopamine receptors by a specific agonist mimicked and occluded LTP induced by electrical high-frequency stimulation (HFS). Furthermore, our results indicate that this form of LTP relies on postsynaptic Ca²⁺ signaling and requires the activation of protein kinase A. Considering the pivotal role of the subiculum as information gatekeeper between the hippocampus and other brain regions, this aberrant LTP in ventral subicular regular-firing neurons is expected to interfere with physiological hippocampal output processing and might thereby contribute to hippocampal dysfunction in psychotic events.

Protein Kinase A-Mediated Suppression of the Slow Afterhyperpolarizing KCa3.1 Current in Temporal Lobe Epilepsy

Brain insults, such as trauma, stroke, anoxia, and status epilepticus (SE), cause multiple changes in synaptic function and intrinsic properties of surviving neurons that may lead to the development of epilepsy. Experimentally, a single SE episode, induced by the convulsant pilocarpine, initiates the development of an epileptic condition resembling human temporal lobe epilepsy (TLE). Principal hippocampal neurons from such epileptic animals display enhanced spike output in response to excitatory stimuli compared with neurons from nonepileptic animals. This enhanced firing is negatively related to the size of the slow afterhyperpolarization (sAHP), which is reduced in the epileptic neurons. The sAHP is an intrinsic neuronal negative feedback mechanism consisting normally of two partially overlapping components produced by disparate mechanisms. One component is generated by activation of Ca²⁺-gated K⁺ (K_Ca) channels, likely KCa3.1, consequent to spike Ca²⁺ influx (the K_Ca-sAHP component). The second component is generated by enhancement of the electrogenic Na⁺/K⁺ ATPase (NKA) by spike Na⁺ influx (NKA-sAHP component). Here we show that the K_Ca-sAHP component is markedly reduced in male rat epileptic neurons, whereas the NKA-sAHP component is not altered. The K_Ca-sAHP reduction is due to the downregulation of KCa3.1 channels, mediated by cAMP-dependent protein kinase A (PKA). This sustained effect can be acutely reversed by applying PKA inhibitors, leading also to normalization of the spike output of epileptic neurons. We propose that the novel "acquired channelopathy" described here, namely, PKA-mediated downregulation of KCa3.1 activity, provides an innovative target for developing new treatments for TLE, hopefully overcoming the pharmacoresistance to traditional drugs.SIGNIFICANCE STATEMENT Epilepsy, a common neurological disorder, often develops following a brain insult. Identifying key molecular and cellular mechanisms underlying acquired epilepsy is critical for developing effective antiepileptic therapies. In an experimental model of acquired epilepsy, we show that principal hippocampal neurons become intrinsically hyperexcitable. This alteration is due predominantly to the downregulation of a ubiquitous class of potassium ion channels, KCa3.1, whose main function is to dampen neuronal excitability. KCa3.1 downregulation is mediated by the cAMP-dependent protein kinase A (PKA) signaling pathway. Most importantly, it can be acutely reversed by PKA inhibitors, leading to recovery of KCa3.1 function and normalization of neuronal excitability. The discovery of this novel epileptogenic mechanism hopefully will facilitate the development of more efficient pharmacotherapy for acquired epilepsy.

Gradient of Expression of Dopamine D2 Receptors Along the Dorso-Ventral Axis of the Hippocampus

Dopamine D2-like receptors (D2R) play an important role in the regulation of hippocampal neuronal excitability and contribute to the regulation of synaptic plasticity, the encoding of hippocampus-dependent memories and the regulation of affective state. In line with this, D2R are targeted in the treatment of psychosis and affective disorders. It has been proposed that the dorso-ventral axis of the hippocampus can be functionally delineated into the dorsal pole that predominantly processes spatial information and the ventral pole that mainly addresses hippocampal processing of emotional and affective state. Although dopaminergic control of hippocampal information processing has been the focus of a multitude of studies, very little is known about the precise distribution of D2R both within anatomically defined sublayers of the hippocampus and along its dorsoventral axis, that could in turn yield insights as to the functional significance of this receptor in supporting hippocampal processing of spatial and affective information. Here, we used an immunohistochemical approach to precisely scrutinize the protein expression of D2R both within the cellular and dendritic layers of the hippocampal subfields, and along the dorso-ventral hippocampal axis. In general, we detected significantly higher levels of protein expression of D2R in the ventral, compared to the dorsal poles with regard to the CA1, CA2, CA3 and dentate gyrus (DG) regions. Effects were very consistent: the molecular layer, granule cell layer and polymorphic layer of the DG exhibited higher D2R levels in the ventral compared to dorsal hippocampus. D2R levels were also significantly higher in the ventral Stratum oriens, Stratum radiatum, and Stratum lacunosum-moleculare layers of the CA1 and CA3 regions. The apical dendrites of the ventral CA2 region also exhibited higher D2R expression compared to the dorsal pole. Taken together, our study suggests that the higher D2R expression levels of the ventral hippocampus may contribute to reported gradients in the degree of expression of synaptic plasticity along the dorso-ventral hippocampal axis, and may support behavioral information processing by the ventral hippocampus.

Increased anxiety-like behavior following circuit-specific catecholamine denervation in mice

Parkinson's disease (PD) presents with a constellation of non-motor symptoms, notably increased anxiety, which are currently poorly treated and underrepresented in animal models of the disease. Human post-mortem studies report loss of catecholaminergic neurons in the pre-symptomatic phases of PD when anxiety symptoms emerge, and a large literature from rodent and human studies indicate that catecholamines are important mediators of anxiety via their modulatory effects on limbic regions such as the amygdala. On the basis of these observations, we hypothesized that anxiety in PD could result from an early loss of catecholaminergic inputs to the amygdala and/or other limbic structures. To interrogate this hypothesis, we bilaterally injected the neurotoxin 6-OHDA in the mouse basolateral amygdala (BL). This produced a restricted pattern of catecholaminergic (tyrosine-hydroxylase-labeled) denervation in the BL, intercalated cell masses and ventral hippocampus, but not the central amygdala or prefrontal cortex. We found that this circuit-specific lesion did not compromise performance on multiple measures of motor function (home cage, accelerating rotarod, beam balance, pole climbing), but did increase anxiety-like behavior in the elevated plus-maze and light-dark exploration tests. Fear behavior in the pavlovian cued conditioning and passive avoidance assays was, by contrast, unaffected; possibly due to preservation of catecholamine innervation of the central amygdala from the periaqueductal gray. These data provide some of the first evidence implicating loss of catecholaminergic neurotransmission in midbrain-amygdala circuits to increased anxiety-like behavior. Our findings offer an initial step towards identifying the neural substrates for pre-motor anxiety symptoms in PD.

Effects of endogenous and exogenous D1/D5 dopamine receptor activation on LTP in ventral and dorsal CA1 hippocampal synapses

Hippocampus is importantly involved in dopamine-dependent behaviors and dopamine is a significant modulator of synaptic plasticity in the hippocampus. Moreover, the dopaminergic innervation appears to be disproportionally segregated along the hippocampal longitudinal (dorsoventral) axis with unknown consequences for synaptic plasticity. In this study we examined the actions of endogenously released dopamine and the effects of exogenous D1/D5 dopamine receptor agonists on theta-burst stimulation-induced long-term potentiation (LTP) of field excitatory synaptic potential (fEPSP) at Schaffer collateral-CA1 synapses in slices from dorsal (DH) and ventral hippocampus (VH). Furthermore, we quantified D1 receptor mRNA and protein expression levels in DH and VH. We found that blockade of D1/D5 receptors by SCH 23390 (20 μM) significantly reduced the magnitude of LTP in both DH and VH similarly suggesting that dopamine endogenously released during TBS, presumably mimicking low activity of DA neurons, exerts a homogeneous modulation of LTP along the hippocampal long axis. Moderate to high concentrations of the selective partial D1/D5 receptor agonist SKF 38393 (50-150 μM) did not significantly change LTP in either hippocampal segment. However, the full D1 receptor selective agonist SKF 82958 (10 μM) significantly enhanced LTP in VH but not DH. Furthermore, the expression of D1 receptor mRNA and protein was considerably higher in VH compared with DH. These results suggest that the dynamic range of D1/D5 receptor-mediated dopamine effects on LTP may be higher in VH than DH and that VH may be specialized to acquire information about behaviorally relevant strong stimuli signaled by the dopamine system.

Dopaminergic innervation and modulation of hippocampal networks

The catecholamine dopamine plays an important role in hippocampus-dependent plasticity and related learning and memory processes. Dopamine secretion in the hippocampus is activated by, e.g., salient or novel stimuli, thereby helping to establish and to stabilize hippocampus-dependent memories. Disturbed dopaminergic function in the hippocampus leads to severe pathophysiological conditions. While the role and importance of dopaminergic modulation of hippocampal networks have been unequivocally proven, there is still a lack of detailed molecular and cellular mechanistic understanding of how dopamine orchestrates these hippocampal processes. In this chapter of the special issue "Hippocampal structure and function," we will discuss the current understanding of dopaminergic modulation of basal synaptic transmission and long-lasting, activity-dependent potentiation or depression.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Striatal and Extrastriatal Dopamine Transporter Availability in Schizophrenia and Its Clinical Correlates: A Voxel-Based and High-Resolution PET Study

Neuroimaging studies investigating dopamine (DA) function widely support the hypothesis of presynaptic striatal DA hyperactivity in schizophrenia. However, published data on the striatal DA transporter (DAT) appear less consistent with this hypothesis, probably partly due to methodological limitations. Moreover, DAT in extrastriatal regions has been very poorly investigated in the context of schizophrenia. In order to address these issues, we used a high resolution positron emission tomograph and the selective DAT radioligand [11C]PE2I, coupled with a whole brain voxel-based analysis method to investigate DAT availability in striatal but also extra-striatal regions in 21 male chronic schizophrenia patients compared to 30 healthy male controls matched by age. We found higher DAT availability in schizophrenia patients in midbrain, striatal, and limbic regions. DAT availability in amygdala/hippocampus and putamen/pallidum was positively correlated with hallucinations and suspiciousness/persecution, respectively. These results are consistent with an increase of presynaptic DA function in patients with schizophrenia, and support the involvement of both striatal and extrastriatal DA dysfunction in positive psychotic symptoms. The study also highlights the whole brain voxel-based analysis method to explore DA dysfunction in schizophrenia.

Effect of Intrastriatal 6-OHDA Lesions on Extrastriatal Brain Structures in the Mouse

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor and non-motor symptoms. The underlying pathology of non-motor symptoms is poorly understood. Discussed are pathological changes of extrastriatal brain structures. In this study, we characterized histopathological alterations of extrastriatal brain structures in the 6-hydroxydopamine (6-OHDA) PD animal model. Lesions were induced by unilateral stereotactic injections of 6-OHDA into the striatum or medial forebrain bundle of adult male mice. Loss of tyrosine hydroxylase positive (TH⁺) fibers as well as glia activation was quantified following stereological principles. Loss of dopaminergic innervation was further investigated by western-blotting. As expected, 6-OHDA injection into the nigrostriatal route induced retrograde degeneration of dopaminergic neurons within the substantia nigra pars compacta (SNpc), less so within the ventral tegmental area. Furthermore, we observed a region-specific drop of TH⁺ projection fiber density in distinct cortical regions. This pathology was most pronounced in the cingulate- and motor cortex, whereas the piriform cortex was just modestly affected. Loss of cortical TH⁺ fibers was not paralleled by microglia or astrocyte activation. Our results demonstrate that the loss of dopaminergic neurons within the substantia nigra pars compacta is paralleled by a cortical dopaminergic denervation in the 6-OHDA model. This model serves as a valuable tool to investigate mechanisms operant during cortical pathology in PD patients. Further studies are needed to understand why cortical dopaminergic innervation is lost in this model, and what functional consequence is associated with the observed denervation.

Pramipexole restores depressed transmission in the ventral hippocampus following MPTP-lesion

The hippocampus has a significant association with memory, cognition and emotions. The dopaminergic projections from both the ventral tegmental area and substantia nigra are thought to be involved in hippocampal activity. To date, however, few studies have investigated dopaminergic innervation in the hippocampus or the functional consequences of reduced dopamine in disease models. Further complicating this, the hippocampus exhibits anatomical and functional differentiation along its dorso-ventral axis. In this work we investigated the role of dopamine on hippocampal long term potentiation using D-amphetamine, which stimulates dopamine release, and also examined how a dopaminergic lesion affects the synaptic transmission across the anatomic subdivisions of the hippocampus. Our findings indicate that a 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine induced dopaminergic lesion has time-dependent effects and impacts mainly on the ventral region of the hippocampus, consistent with the density of dopaminergic innervation. Treatment with a preferential D₃ receptor agonist pramipexole partly restored normal synaptic transmission and Long-Term Potentiation. These data suggest a new mechanism to explain some of the actions of pramipexole in Parkinson´s disease.

Cocaine self-administration induces changes in synaptic transmission and plasticity in ventral hippocampus

Allowing rats extended access to cocaine self-administration is thought to recapitulate several key aspects of cocaine addiction in humans. Understanding the mechanisms that underlie drug-induced neuroadaptations that persist in the brain after protracted periods of abstinence is crucial towards the goal of developing therapeutic interventions for this disease state. We have employed both whole-cell voltage clamp and extracellular recording technique to assess changes in neurotransmission and long-term potentiation (LTP) in stratum radiatum of the CA1 region using the rat ventral hippocampal slice preparation. Rats allowed to self-administer cocaine daily, including 'long access' (6 hours) sessions, exhibited an increase in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-d-aspartate current ratio and enhanced excitatory transmission following 3-5 weeks of abstinence. Inhibitory transmission was also significantly decreased in long-access animals, and the AMPA/N-methyl-d-aspartate ratio measured in the absence of GABAergic blockers was greatly enhanced. We also observed a significant reduction of LTP magnitude evoked in the long-access cocaine rats. These findings suggest the presence of synergistic effects of enhanced AMPA and diminished gamma-aminobutyric acid neurotransmission under physiological conditions in the CA1 region of cocaine-taking animals, supporting the conclusion that persisting enhancement of AMPA-mediated transmission and/or inhibition of gamma-aminobutyric acid-mediated transmission promoted a chronic state of potentiation that partially occluded further LTP. This increased output from the ventral hippocampus to other limbic areas would be among the drug-induced neuroadaptations that persist following abstinence from cocaine self-administration and therefore may contribute to the disease state of addiction.

Effects of dorsal hippocampus catecholamine depletion on paired-associates learning and place learning in rats

Growing evidence suggests that the catecholamine (CA) neurotransmitters dopamine and noradrenaline support hippocampus-mediated learning and memory. However, little is known to date about which forms of hippocampus-mediated spatial learning are modulated by CA signaling in the hippocampus. Therefore, in the current study we examined the effects of 6-hydroxydopamine-induced CA depletion in the dorsal hippocampus on two prominent forms of hippocampus-based spatial learning, that is learning of object-location associations (paired-associates learning) as well as learning and choosing actions based on a representation of the context (place learning). Results show that rats with CA depletion of the dorsal hippocampus were able to learn object-location associations in an automated touch screen paired-associates learning (PAL) task. One possibility to explain this negative result is that object-location learning as tested in the touchscreen PAL task seems to require relatively little hippocampal processing. Results further show that in rats with CA depletion of the dorsal hippocampus the use of a response strategy was facilitated in a T-maze spatial learning task. We suspect that impaired hippocampus CA signaling may attenuate hippocampus-based place learning and favor dorsolateral striatum-based response learning.

Modulation of Postnatal Neurogenesis by Perinatal Asphyxia: Effect of D1 and D2 Dopamine Receptor Agonists

Perinatal asphyxia (PA) is associated to delayed cell death, affecting neurocircuitries of basal ganglia and hippocampus, and long-term neuropsychiatric disabilities. Several compensatory mechanisms have been suggested to take place, including cell proliferation and neurogenesis. There is evidence that PA can increase postnatal neurogenesis in hippocampus and subventricular zone (SVZ), modulated by dopamine, by still unclear mechanisms. We have studied here the effect of selective dopamine receptor agonists on cell death, cell proliferation and neurogenesis in organotypic cultures from control and asphyxia-exposed rats. Hippocampus and SVZ sampled at 1-3 postnatal days were cultured for 20-21 days. At day in vitro (DIV) 19, cultures were treated either with SKF38393 (10 and 100 µM, a D₁ agonist), quinpirole (10 µM, a D₂ agonist) or sulpiride (10 μM, a D₂ antagonist) + quinpirole (10 μM) and BrdU (10 μM, a mitosis marker) for 24 h. At DIV 20-21, cultures were processed for immunocytochemistry for microtubule-associated protein-2 (MAP-2, a neuronal marker), and BrdU, evaluated by confocal microscopy. Some cultures were analysed for cell viability at DIV 20-21 (LIVE/DEAD kit). PA increased cell death, cell proliferation and neurogenesis in hippocampus and SVZ cultures. The increase in cell death, but not in cell proliferation, was inhibited by both SKF38393 and quinpirole treatment. Neurogenesis was increased by quinpirole, but only in hippocampus, in cultures from both asphyxia-exposed and control-animals, effect that was antagonised by sulpiride, leading to the conclusion that dopamine modulates neurogenesis in hippocampus, mainly via D₂ receptors.

Toward an Integration of Deep Learning and Neuroscience

Neuroscience has focused on the detailed implementation of computation, studying neural codes, dynamics and circuits. In machine learning, however, artificial neural networks tend to eschew precisely designed codes, dynamics or circuits in favor of brute force optimization of a cost function, often using simple and relatively uniform initial architectures. Two recent developments have emerged within machine learning that create an opportunity to connect these seemingly divergent perspectives. First, structured architectures are used, including dedicated systems for attention, recursion and various forms of short- and long-term memory storage. Second, cost functions and training procedures have become more complex and are varied across layers and over time. Here we think about the brain in terms of these ideas. We hypothesize that (1) the brain optimizes cost functions, (2) the cost functions are diverse and differ across brain locations and over development, and (3) optimization operates within a pre-structured architecture matched to the computational problems posed by behavior. In support of these hypotheses, we argue that a range of implementations of credit assignment through multiple layers of neurons are compatible with our current knowledge of neural circuitry, and that the brain's specialized systems can be interpreted as enabling efficient optimization for specific problem classes. Such a heterogeneously optimized system, enabled by a series of interacting cost functions, serves to make learning data-efficient and precisely targeted to the needs of the organism. We suggest directions by which neuroscience could seek to refine and test these hypotheses.

Behavioral Responses to Amphetamine Challenge following 6-Ohda Hippocampal Lesions in Rats

The effects of 6-hydroxydopamine (6-OHDA, 8 μ g) retrograde selective lesions of bilateral dopaminergic pathways, projecting from the ventral tegmental area of Tsai (VTA-A10) to the hippocampal formation (HF), were studied through assessment of male Sprague-Dawley rats behavior, before and following amphetamine challenge (day 15 and day 30, respectively after 6-OHDA injections). Significant differences in locomotor and exploratory activities and motor coordination were evidenced between and within groups by one-way analysis of variance (ANOVA). These VTA-hippocampal dopaminergic pathway lesions may provide a morphological basis for the interpretation of the psychomotor deficits shown in this study.

Functional Connectome Analysis of Dopamine Neuron Glutamatergic Connections in Forebrain Regions

In the ventral tegmental area (VTA), a subpopulation of dopamine neurons express vesicular glutamate transporter 2 and make glutamatergic connections to nucleus accumbens (NAc) and olfactory tubercle (OT) neurons. However, their glutamatergic connections across the forebrain have not been explored systematically. To visualize dopamine neuron forebrain projections and to enable photostimulation of their axons independent of transmitter status, we virally transfected VTA neurons with channelrhodopsin-2 fused to enhanced yellow fluorescent protein (ChR2-EYFP) and used DAT(IREScre) mice to restrict expression to dopamine neurons. ChR2-EYFP-expressing neurons almost invariably stained for tyrosine hydroxylase, identifying them as dopaminergic. Dopamine neuron axons visualized by ChR2-EYFP fluorescence projected most densely to the striatum, moderately to the amygdala and entorhinal cortex (ERC), sparsely to prefrontal and cingulate cortices, and rarely to the hippocampus. Guided by ChR2-EYFP fluorescence, we recorded systematically from putative principal neurons in target areas and determined the incidence and strength of glutamatergic connections by activating all dopamine neuron terminals impinging on recorded neurons with wide-field photostimulation. This revealed strong glutamatergic connections in the NAc, OT, and ERC; moderate strength connections in the central amygdala; and weak connections in the cingulate cortex. No glutamatergic connections were found in the dorsal striatum, hippocampus, basolateral amygdala, or prefrontal cortex. These results indicate that VTA dopamine neurons elicit widespread, but regionally distinct, glutamatergic signals in the forebrain and begin to define the dopamine neuron excitatory functional connectome.

SIGNIFICANCE STATEMENT

Dopamine neurons are important for the control of motivated behavior and are involved in the pathophysiology of several major neuropsychiatric disorders. Recent studies have shown that some ventral midbrain dopamine neurons are capable of glutamate cotransmission. With conditional expression of channelrhodopsin in dopamine neurons, we systematically explored dopamine neuron connections in the forebrain and identified regionally specific dopamine neuron excitatory connections. Establishing that only a subset of forebrain regions receive excitatory connections from dopamine neurons will help to determine the function of dopamine neuron glutamate cotransmission, which likely involves transmission of precise temporal signals and enhancement of the dynamic range of dopamine neuron signals.

Priming Spatial Activity by Single-Cell Stimulation in the Dentate Gyrus of Freely Moving Rats

An essential requirement for hippocampal circuits to function in episodic memory is the ability to rapidly disambiguate and store incoming sensory information. This "pattern separation" function has been classically associated to the dentate gyrus, where spatial learning is accompanied by rapid and persistent modifications of place-cell representation. How these rapid modifications are implemented at the cellular level has remained largely unresolved. Here, we tested whether plasticity-inducing stimuli--spike trains--evoked in postsynaptic neurons are sufficient for the rapid induction of place-field activity in the dentate gyrus. We juxtacellularly stimulated 67 silent granule cells while rats explored a maze for the first time. Spike trains with different characteristics (e.g., number of spikes, frequency, and theta-rhythmicity) were evoked at randomly selected spatial locations. We found that, under novelty, ∼30% (10/33) of the stimulated neurons fired selectively at the "primed" spatial location on subsequent laps. Induced place fields were either transient or persisted for multiple laps. The "priming" effect was experience dependent, as it was less frequently observed in habituated animals (3/34 neurons), and it correlated with the number of spikes and theta-rhythmicity of the stimulus trains. These data indicate that, albeit with low efficiency, evoked theta-rhythmic spike trains can be sufficient for priming spatial activity in the dentate gyrus and thus recruiting silent granule cells into the coding population.

Lasting Differential Effects on Plasticity Induced by Prenatal Stress in Dorsal and Ventral Hippocampus

Early life adversaries have a profound impact on the developing brain structure and functions that persist long after the original traumatic experience has vanished. One of the extensively studied brain structures in relation to early life stress has been the hippocampus because of its unique association with cognitive processes of the brain. While the entire hippocampus shares the same intrinsic organization, it assumes different functions in its dorsal and ventral sectors (DH and VH, resp.), based on different connectivity with other brain structures. In the present review, we summarize the differences between DH and VH and discuss functional and structural effects of prenatal stress in the two sectors, with the realization that much is yet to be explored in understanding the opposite reactivity of the DH and VH to stressful stimulation.

Major dorsoventral differences in the modulation of the local CA1 hippocampal network by NMDA, mGlu5, adenosine A2A and cannabinoid CB1 receptors

Recent research points to diversification in the local neuronal circuitry between dorsal (DH) and ventral (VH) hippocampus that may be involved in the large-scale functional segregation along the long axis of the hippocampus. Here, using CA1 field recordings from rat hippocampal slices, we show that activation of N-methyl-d-aspartate receptors (NMDARs) reduced excitatory transmission more in VH than in DH, with an adenosine A1 receptor-independent mechanism, and reduced inhibition and enhanced postsynaptic excitability only in DH. Strikingly, co-activation of metabotropic glutamate receptor-5 (mGluR5) with NMDAR, by CHPG and NMDA respectively, strongly potentiated the effects of NMDAR in DH but had not any potentiating effect in VH. Furthermore, the synergistic actions in DH were occluded by blockade of adenosine A2A receptors (A2ARs) by their antagonist ZM 241385 demonstrating a tonic action of these receptors in DH. Exogenous activation of A2ARs by 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680) did not change the effects of mGluR5-NMDAR co-activation in either hippocampal pole. Importantly, blockade of cannabinoid CB1 receptors (CB1Rs) by their antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281) restricted the synergistic actions of mGluR5-NMDARs on excitatory synaptic transmission and postsynaptic excitability and abolished their effect on inhibition. Furthermore, AM 281 increased the excitatory transmission only in DH indicating that CB1Rs were tonically active in DH but not VH. Removing the magnesium ions from the perfusion medium neither stimulated the interaction between mGluR5 and NMDAR in VH nor augmented the synergy of the two receptors in DH. These findings show that the NMDAR-dependent modulation of fundamental parameters of the local neuronal network, by mGluR5, A2AR and CB1R, markedly differs between DH and VH. We propose that the higher modulatory role of A2AR and mGluR5, in combination with the role of CB1Rs, provide DH with higher functional flexibility of its NMDARs, compared with VH.

Higher intrinsic network excitability in ventral compared with the dorsal hippocampus is controlled less effectively by GABAB receptors

BACKGROUND

Elucidating specializations of the intrinsic neuronal network between the dorsal and the ventral hippocampus is a recently emerging area of research that is expected to help us understand the mechanisms underlying large scale functional diversification along the hippocampus. The aim of this study was to characterize spontaneous network activity between the dorsal and the ventral hippocampus induced under conditions of partial or complete blockade of GABAergic inhibition (i.e. disinhibition).

RESULTS

Using field recordings from the CA3 and CA1 fields of hippocampal slices from adult rats I found that ventral compared with dorsal hippocampus slices displayed higher propensity for and higher frequency of occurrence of spontaneous field potentials (spfps) at every level of disinhibition. Also NMDA receptor-depended spfps (spfps(-nmda)) occurred with higher probability more frequently and were larger in the ventral compared with the dorsal hippocampus. Importantly, blockade of GABA(B) receptors produced a stronger effect in enhancing the probability of generation of spfps and spfps(-nmda) in the dorsal compared with the ventral hippocampal slices and increased spfps(-nmda) only in dorsal slices.

CONCLUSION

These results demonstrate a higher intrinsic neuronal excitability of the ventral compared with the dorsal local circuitry with the considerable contribution of NMDA receptors. Furthermore, the GABA(B) receptors control the total and the NMDA receptor-dependent excitation much less effectively in the ventral part of the hippocampus. It is proposed that NMDA and GABA(B) receptors significantly contribute to differentiate local network dynamics between the dorsal and the ventral hippocampus with important implications in the information processing performed along the long hippocampal axis.

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

Enhanced dopamine-dependent hippocampal plasticity after single MK-801 application

Dopaminergic hyperfunction and N-methyl-D-aspartate receptor (NMDAR) hypofunction have both been implicated in psychosis. Dopamine-releasing drugs and NMDAR antagonists replicate symptoms associated with psychosis in healthy humans and exacerbate symptoms in patients with schizophrenia. Though hippocampal dysfunction contributes to psychosis, the impact of NMDAR hypofunction on hippocampal plasticity remains poorly understood. Here, we used an NMDAR antagonist rodent model of psychosis to investigate hippocampal long-term potentiation (LTP). We found that single systemic NMDAR antagonism results in a region-specific, presynaptic LTP at hippocampal CA1-subiculum synapses that is induced by activation of D1/D5 dopamine receptors and modulated by L-type voltage-gated Ca(2+) channels. Thereby, our findings may provide a cellular mechanism how NMDAR antagonism can lead to an enhanced hippocampal output causing activation of the hippocampus-ventral tegmental area-loop and overdrive of the dopamine system.

MDMA administration during adolescence exacerbates MPTP-induced cognitive impairment and neuroinflammation in the hippocampus and prefrontal cortex

RATIONALE

We have recently shown that chronic exposure to 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") of adolescent mice exacerbates dopamine neurotoxicity and neuroinflammatory effects elicited by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the substantia nigra and striatum at adulthood.

OBJECTIVES

The present study investigated whether the amplification of MPTP effects by previous treatment with MDMA extends to the limbic and cortical regions and consequently affects cognitive performance.

METHODS

Mice received MDMA (10 mg/kg, twice a day/twice a week) for 9 weeks, followed by MPTP (20 mg/kg × 4 administrations), starting 2 weeks after MDMA discontinuation. Complement type 3 receptor (CD11b) and glial fibrillary acidic protein (GFAP) were evaluated by immunohistochemistry in both the hippocampus and the medial prefrontal cortex (mPFC) to measure microglia and astroglia activation. These neurochemical evaluations were paired with an assessment of cognitive performance by means of the novel object recognition (NOR) and spontaneous alternation tasks.

RESULTS

MPTP administration to MDMA-pretreated mice elicited a stronger activation of CD11b and GFAP in both the hippocampus and the mPFC compared with either substance administered alone. Furthermore, NOR performance was lower in MDMA-pretreated mice administered MPTP compared with mice that received either substance alone.

CONCLUSIONS

These results demonstrate that MDMA-MPTP negative interactions extend to the limbic and cortical regions and may result in cognitive impairment, providing further evidence that exposure to MDMA may amplify the effects of later neurotoxic insults.

Stress in Neonatal Rats with Different Maternal Care Backgrounds: Monoaminergic and Hormonal Responses

The first 2 weeks of life in rats are known as the stress hyporesponsive period because stress responses in pups are diminished as compared to adult animals. However, it is considered a critical period in development in which infant rats are susceptible to environmental events, such as stressful stimuli and quality of maternal care received. These early life events have long-lasting effects, shaping a variety of outcomes, such as stress responsivity. This study investigated the effects of maternal care and sex differences on the response to an aversive stimulus in rat pups from high (HL) and low licking (LL) mothers. Plasma corticosterone, oxytocin, and central monoaminergic activity in 13-day-old rats submitted to cold stress were analyzed. Stress increased plasma corticosterone and marginally decreased hypothalamic dihydroxyphenylacetic acid/dopamine (DOPAC/DA) ratio. HL pups showed higher levels of plasma oxytocin than LL pups. The maternal effect was also detected in the hippocampus, in which 5-hydroxyindole-3-acetic acid/serotonin (5-HIAA/5-HT) ratio was increased in HL pups, independently of the sex and stress. Investigating the early life events is useful not only into understand the neurobiological and hormonal mechanisms underlying maternal and stressful influences on infant development into a healthy or psychopathological adult phenotype, but also to unveil the immediate outcomes on infancy.

Precise spatial coding is preserved along the longitudinal hippocampal axis

Compared with the dorsal hippocampus, relatively few studies have characterized neuronal responses in the ventral hippocampus. In particular, it is unclear whether and how cells in the ventral region represent space and/or respond to contextual changes. We recorded from dorsal and ventral CA1 neurons in freely moving mice exposed to manipulations of visuospatial and olfactory contexts. We found that ventral cells respond to alterations of the visuospatial environment such as exposure to novel local cues, cue rotations, and contextual expansion in similar ways to dorsal cells, with the exception of cue rotations. Furthermore, we found that ventral cells responded to odors much more strongly than dorsal cells, particularly to odors of high valence. Similar to earlier studies recording from the ventral hippocampus in CA3, we also found increased scaling of place cell field size along the longitudinal hippocampal axis. Although the increase in place field size observed toward the ventral pole has previously been taken to suggest a decrease in spatial information coded by ventral place cells, we hypothesized that a change in spatial scaling could instead signal a shift in representational coding that preserves the resolution of spatial information. To explore this possibility, we examined population activity using principal component analysis (PCA) and neural location reconstruction techniques. Our results suggest that ventral populations encode a distributed representation of space, and that the resolution of spatial information at the population level is comparable to that of dorsal populations of similar size. Finally, through the use of neural network modeling, we suggest that the redundancy in spatial representation along the longitudinal hippocampal axis may allow the hippocampus to overcome the conflict between memory interference and generalization inherent in neural network memory. Our results indicate that ventral population activity is well suited for generalization across locations and contexts.

The indusium griseum and the longitudinal striae of the corpus callosum

In the eighteenth century, Lancisi described the indusium griseum (IG) and the longitudinal striae (LS) of the corpus callosum. The IG is a thin neuronal lamina above the corpus callosum, covered on each side of the midline by the medial and lateral LS. The medial LS (nerves of Lancisi) and lateral LS are two pairs of myelinated fiber bands found in the gray matter of the IG on the dorsal aspect of the corpus callosum. Embryologically, the IG and LS are dorsal remnants of the archicortex of the hippocampus and fornix and thus they are considered components of the limbic system. Recent studies using immunohistochemistry reported that acetylcholine, dopamine, noradrenaline, 5-hydroxytryptamine and GABA neurons innervate the IG. Newer imaging techniques, such as high field MRI and diffusion tensor imaging, provide new tools for studying these structures, whose true function remains still unclear. The present paper reviews the history of the discovery of the IG and LS of the corpus callosum, with a holistic overview on these interesting structures from the anatomical, embryological, neurochemical, radiological and clinical perspective.