Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine D1 and NMDA receptor co-regulation of protein translation in cultured nucleus accumbens neurons

Protein translation is essential for some forms of synaptic plasticity. We used nucleus accumbens (NAc) medium spiny neurons (MSN), co-cultured with cortical neurons to restore excitatory synapses, to examine whether dopamine modulates protein translation in NAc MSN. FUNCAT was used to measure translation in MSNs under basal conditions and after disinhibiting excitatory transmission using the GABAA receptor antagonist bicuculline (2 hr). Under basal conditions, translation was not altered by the D1-class receptor (D1R) agonist SKF81297 or the D2-class receptor (D2R) agonist quinpirole. Bicuculline alone robustly increased translation. This was reversed by quinpirole but not SKF81297. It was also reversed by co-incubation with the D1R antagonist SCH23390, but not the D2R antagonist eticlopride, suggesting dopaminergic tone at D1Rs. This was surprising because no dopamine neurons are present. An alternative explanation is that bicuculline activates translation by increasing glutamate tone at NMDA receptors (NMDAR) within D1R/NMDAR heteromers, which have been described in other cell types. Supporting this, immunocytochemistry and proximity ligation assays revealed D1/NMDAR heteromers on NAc cells both in vitro and in vivo. Further, bicuculline's effect was reversed to the same extent by SCH23390 alone, the NMDAR antagonist APV alone, or SCH23390+APV. These results suggest that: 1) excitatory synaptic transmission stimulates translation in NAc MSNs, 2) this is opposed when glutamate activates D1R/NMDAR heteromers, even in the absence of dopamine, and 3) antagonist occupation of D1Rs within the heteromers prevents their activation. Our study is the first to suggest a role for D2 receptors and D1R/NMDAR heteromers in regulating protein translation.

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.

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.

Dopamine regulates intrinsic excitability thereby gating successful induction of spike timing-dependent plasticity in CA1 of the hippocampus

Long-term potentiation (LTP) and long-term depression (LTD) are generally assumed to be cellular correlates for learning and memory. Different types of LTP induction protocols differing in severity of stimulation can be distinguished in CA1 of the hippocampus. To better understand signaling mechanisms and involvement of neuromodulators such as dopamine (DA) in synaptic plasticity, less severe and more physiological low frequency induction protocols should be used. In the study which is reviewed here, critical determinants of spike timing-dependent plasticity (STDP) at hippocampal CA3-CA1 synapses were investigated. We found that DA via D1 receptor signaling, but not adrenergic signaling activated by the β-adrenergic agonist isoproterenol, is important for successful expression of STDP at CA3-CA1 synapses. The DA effect on STDP is paralleled by changes in spike firing properties, thereby changing intrinsic excitability of postsynaptic CA1 neurons, and gating STDP. Whereas β-adrenergic signaling also leads to a similar (but not identical) regulation of firing pattern, it does not enable STDP. In this focused review we will discuss the current literature on dopaminergic modulation of LTP in CA1, with a special focus on timing dependent (t-)LTP, and we will suggest possible reasons for the selective gating of STDP by DA [but not noradrenaline (NA)] in CA1.

Stress induced hippocampal mineralocorticoid and estrogen receptor β gene expression and long-term potentiation in male adult rats is sensitive to early-life stress experience

Glucocorticoid hormones and their receptors have been identified to be involved in emotional and cognitive disorders in early stressed subjects during adulthood. However, the impact of other steroid hormones and receptors has been considered less. Especially, functional roles of estrogen and estrogen receptors in male subjects are largely unknown. Therefore, we measured hippocampal concentrations of 17β-estradiol, corticosterone and testosterone, as well as the gene expression of estrogen receptor α and β (ERα, β), androgen receptor (AR), glucocorticoid (GR) and mineralocorticoid (MR) receptors after stress in adulthood in maternally separated (MS+; at postnatal days 14-16 for 6h each day) and control (MS-) male rats. In vivo hippocampal long-term potentiation (LTP) serves as a cellular model of learning and memory formation. Population spike- (PSA) and the fEPSP-LTP within the dentate gyrus (DG) were reinforced by elevated-platform-stress (EP-stress) in MS- but not in MS+ rats. MR- and ERβ-mRNA were upregulated 1h after EP-stress in MS- but not in MS+ rats as compared to non-stressed littermates. Infusion of an MR antagonist before LTP induction blocked early- and late-PSA- and -fEPSP-LTP, whereas blockade of ERβ impaired only the late PSA-LTP. Application of a DNA methyltransferase (DNMT) inhibitor partly restored the LTP-reinforcement in MS+ rats, accompanied by a retrieval of ERβ- but not MR-mRNA upregulation. Basal ERβ gene promoter methylation was similar between groups, whereas MS+ and MS- rats showed different methylation patterns across CpG sites after EP-stress. These findings indicate a key role of ERβ in early-stress mediated emotionality and emotion-induced late-LTP in adult male rats via DNA methylation mechanisms.

Dopaminergic signals in primary motor cortex

Brainstem monoamine areas such as the ventral tegmental area (VTA) send dopaminergic projections to the cerebral cortex that are widely distributed across different cortical regions. Whereas the projection to prefrontal areas (PFC) has been studied in detail, little is known about dopaminergic projections to primary motor cortex (M1). These projections have been anatomically characterized in rat and primate M1. Primates have even denser dopaminergic projections to M1 than rats. The physiological role, the effects of dopaminergic input on the activity of M1 circuits, and the behavioral function of this projection are unknown. This review explores the existing anatomical, electrophysiological and behavioral evidence on dopaminergic projections to M1 and speculates about its functional role. The projection may explain basic features of motor learning and memory phenomena. It is of clinical interest because of its potential for augmenting motor recovery after a brain lesion as well as for understanding the symptomatology of patients with Parkinson's disease. Therefore, targeted investigations are necessary.

Positive and negative modulation of word learning by reward anticipation

Recent evidence from neuroscience indicates that the anticipation of external rewards may enhance declarative memory consolidation by increasing dopaminergic-modulated plasticity in the hippocampus. A number of studies in psychology, however, have shown that external rewards may have null, or even negative, effects on learning. To shed light on this issue, we developed a novel task, in which native Japanese speakers were rewarded to learn unknown English words inside a functional MRI scanner. Rewards had no effect on recall performance unless we used a rating of reward-induced anxiety as a covariate. In this case, for highly rewarded words, we found a negative correlation between recall performance and anxiety ratings. For those words, high recall performance and low anxiety ratings were associated with enhanced activity in the midbrain dopaminergic centers, the hippocampus, and the amygdala. On the other hand, low recall performance and high anxiety ratings were associated with enhanced activity in the anterior cingulate and middle frontal gyrus, brain regions that have been shown to be involved with anxiety and divided attention, respectively. A connectivity analysis indicated positive functional connectivity between the midbrain dopaminergic centers and both the hippocampus and the amygdala, as well as negative connectivity between the anterior cingulate and the amygdala. Thus, both our behavioral and imaging results suggest that the anticipation of rewards can, depending on the individual level of reward-induced anxiety, have either a beneficial effect or a negative effect on word learning.

The ability of the mesocortical dopamine system to operate in distinct temporal modes

BACKGROUND

This review discusses evidence that cells in the mesocortical dopamine (DA) system influence information processing in target areas across three distinct temporal domains.

DISCUSSIONS

Phasic bursting of midbrain DA neurons may provide temporally precise information about the mismatch between expected and actual rewards (prediction errors) that has been hypothesized to serve as a learning signal in efferent regions. However, because DA acts as a relatively slow modulator of cortical neurotransmission, it is unclear whether DA can indeed act to precisely transmit prediction errors to prefrontal cortex (PFC). In light of recent physiological and anatomical evidence, we propose that corelease of glutamate from DA and/or non-DA neurons in the VTA could serve to transmit this temporally precise signal. In contrast, DA acts in a protracted manner to provide spatially and temporally diffuse modulation of PFC pyramidal neurons and interneurons. This modulation occurs first via a relatively rapid depolarization of fast-spiking interneurons that acts on the order of seconds. This is followed by a more protracted modulation of a variety of other ionic currents on timescales of minutes to hours, which may bias the manner in which cortical networks process information. However, the prolonged actions of DA may be curtailed by counteracting influences, which likely include opposing actions at D1 and D2-like receptors that have been shown to be time- and concentration-dependent. In this way, the mesocortical DA system optimizes the characteristics of glutamate, GABA, and DA neurotransmission both within the midbrain and cortex to communicate temporally precise information and to modulate network activity patterns on prolonged timescales.

The late maintenance of hippocampal LTP: Requirements, phases, ‘synaptic tagging’, ‘late-associativity’ and implications

Our review focuses on the mechanisms which enable the late maintenance of hippocampal long-term potentiation (LTP; >3h), a phenomenon which is thought to underlie prolonged memory. About 20 years ago we showed for the first time that the maintenance of LTP - like memory storage--depends on intact protein synthesis and thus, consists of at least two temporal phases. Here we concentrate on mechanisms required for the induction of the transient early-LTP and of the protein synthesis-dependent late-LTP. Our group has shown that the induction of late-LTP requires the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory, reinforcing inputs within specific, effective time windows. The induction of late-LTP is characterized by novel, late-associative properties such as 'synaptic tagging' and 'late-associative reinforcement'. Both phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity and thus the specific processing of afferent signals for the establishment of late-LTP. 'Late-associative reinforcement' describes a process where early-LTP by the co-activation of modulatory inputs can be transformed into late-LTP in activated synapses where a tag is set. Recent evidence from behavioral experiments, which studied processes of emotional and cognitive reinforcement of LTP, point to the physiological relevance of the above mechanisms during cellular and system's memory formation.

Dopamine has bidirectional effects on synaptic responses to cortical inputs in layer II of the lateral entorhinal cortex

Dopaminergic modulation of neuronal function has been extensively studied in the prefrontal cortex, but much less is known about its effects on glutamate-mediated synaptic transmission in the entorhinal cortex. The mesocortical dopamine system innervates the superficial layers of the lateral entorhinal cortex and may therefore modulate sensory inputs to this area. In awake rats, systemic administration of the dopamine reuptake inhibitor GBR12909 (10 mg/kg, ip) enhanced extracellular dopamine levels in the entorhinal cortex and significantly facilitated field excitatory postsynaptic potentials (fEPSPs) in layer II evoked by piriform cortex stimulation. An analysis of the receptor subtypes involved in the facilitation of evoked fEPSPs was conducted using horizontal slices of lateral entorhinal cortex in vitro. The effects of 15-min bath application of dopamine on synaptic responses were bidirectional and concentration dependent. Synaptic responses were enhanced by 10 microM dopamine and suppressed by concentrations of 50 and 100 microM. The D(1)-receptor antagonist SCH23390 (50 microM) blocked the significant facilitation of synaptic responses induced by 10 microM dopamine and the D(2)-receptor antagonist sulpiride (50 microM) prevented the suppression of fEPSPs observed with higher concentrations of dopamine. We propose here that dopamine release in the lateral entorhinal cortex, acting through D(1) receptors, can lead to an enhancement of the salience of sensory representations carried to this region from adjacent sensory cortices.

5‐HT4 receptor activation induces long‐lasting EPSP‐spike potentiation in CA1 pyramidal neurons

Recent studies implicated involvement of the 5-hydroxytryptamine4 (5-HT4) receptor in cognitive and emotional processes. The highest 5-HT4 receptor densities in the brain are found in the limbic system including the hippocampus. Here we used the selective 5-HT4 receptor full agonist, N-pentyl-N'-aminoguanidine carbazimidamide (SDZ-216454) to characterize effects of 5-HT4 receptor activation in whole-cell and field recordings in the area CA1 in hippocampal slices prepared from 3 to 4- and 6 to 9-week-old rats, respectively. Extracellular recordings showed that transient 5-HT4 receptor activation by 10-20 min application of SDZ-216454 induces field excitatory postsynaptic potential (fEPSP)-population spike potentiation (ESP(5-HT4)), which persisted for as long as we held the recordings (> 2 h). ESP(5-HT4) displayed characteristics different from EPSP-spike potentiation that accompanies long-term potentiation; it developed without an associated increase in synaptic transmission, was independent on afferent input, activity of postsynaptic neurons and N-methyl-d-aspartate receptor activation; and was expressed in the presence of GABA receptor antagonists. ESP(5-HT4) was also induced by transient application of the natural neurotransmitter, 5-HT. The increase in the evoked population spike (PS) induced by SDZ-216454 was not prevented by blockers of hyperpolarization-activated cation current (Ih), Cs+ and ZD-7288, but was mimicked and occluded by 150 microm Ba2+. Whole-cell voltage-clamp recordings from pyramidal neurons demonstrated that SDZ-216454 application increases membrane resistance with a concomitant decrease in a Ba2+-sensitive inwardly rectifying K+ current and the Ba2+-insensitive K+ current underlying slow afterhyperpolarization (I(sAHP)). We conclude that 5-HT4 receptor activation may cause a long-lasting excitability increase in CA1 pyramidal neurons by inhibition of a Ba2+-sensitive inwardly rectifying K+ current.

Mesocortical dopamine neurons operate in distinct temporal domains using multimodal signaling

In vivo extracellular recording studies have traditionally shown that dopamine (DA) transiently inhibits prefrontal cortex (PFC) neurons, yet recent biophysical measurements in vitro indicate that DA enhances the evoked excitability of PFC neurons for prolonged periods. Moreover, although DA neurons apparently encode stimulus salience by transient alterations in firing, the temporal properties of the PFC DA signal associated with various behaviors is often extraordinarily prolonged. The present study used in vivo electrophysiological and electrochemical measures to show that the mesocortical system produces a fast non-DA-mediated postsynaptic response in the PFC that appears to be initiated by glutamate. In contrast, short burst stimulation of mesocortical DA neurons that produced transient (<4 s) DA release in the PFC caused a simultaneous reduction in spontaneous firing (consistent with extracellular in vivo recordings) and a form of DA-induced potentiation in which evoked firing was increased for tens of minutes (consistent with in vitro measurements). We suggest that the mesocortical system might transmit fast signals about reward or salience via corelease of glutamate, whereas the simultaneous prolonged DA-mediated modulation of firing biases the long-term processing dynamics of PFC networks.

Dopamine-Mediated Disinhibition in the CA1 Region of Rat Hippocampus via D3 Receptor Activation

The hippocampal formation is thought to contribute to both addictive behaviors and to psychotic disorders, and the actions of the neurotransmitter dopamine are intimately involved with these disease states. We have used both whole-cell and extracellular recording techniques in hippocampal slices to investigate the actions of both cocaine and dopamine receptor agonists in the CA1 region. In the presence of cocaine (10 microM), endogenously released dopamine decreased monosynaptic inhibitory postsynaptic currents (IPSCs) evoked from stratum radiatum but not from stratum oriens. This effect of cocaine was not blocked by the D(1/5) antagonist SCH 23390 ({R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine}) (3 microM), whereas several D(2)-like dopamine receptor antagonists prevented the cocaine-induced decrease in the IPSC. The most selective of the effective antagonists tested was the D(3) antagonist, U 99194 ({5,6-dimethoxy-indan-2-yl dipropylamine}) maleate (1 microM). An exogenously applied D(3)-selective dopamine receptor agonist, PD 128907 ({(+)-(4aR, 10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]-benzopyrano-[4,3-b]-1,4-oxazin-9-ol}) (1 microM), also significantly inhibited the IPSC, providing further evidence that the activation of the D(3) subtype of dopamine receptor by endogenously released dopamine can modulate inhibition in the CA1 region. This disinhibitory action on pyramidal cells also increased synaptic excitability following Schaffer collateral stimulation, as demonstrated by either a decrease in paired-pulse inhibition of the population spike response or by an increase in the excitatory component of the mixed synaptic response evoked from stratum radiatum. These actions indicate that the activation of D(3) receptors by endogenously released dopamine, especially under conditions of transporter blockade, may significantly impact the processing of synaptic information through the stratum radiatum layer of the hippocampus.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

Dopamine depresses cholinergic oscillatory network activity in rat hippocampus

The dopaminergic neuronal system is implicated in cognitive processes in a variety of brain regions including the mesolimbic system. We have investigated whether dopamine also affects synchronized network activity in the hippocampus, which has been ascribed to play a pivotal role in memory formation. Gamma frequency (20-80 Hz) oscillations were induced by the cholinergic agonist carbachol. Oscillatory activity was examined in area CA3 of Wistar rat hippocampal slices, employing field potential and intracellular recordings. Application of carbachol initiated synchronized population activity in the gamma band at 40 Hz. Induced gamma activity persisted over hours and required GABAA receptors. Dopamine reversibly decreased the integrated gamma band power of the carbachol rhythm by 62%, while its frequency was not changed. By contrast, individual pyramidal cells recorded during carbachol-induced field gamma activity exhibited theta frequency (5-15 Hz) membrane potential oscillations that were not altered by dopamine. The dopamine effect on the field gamma activity was mimicked by the D1 receptor agonist SKF-383393 and partially antagonized by the D1 antagonist SCH-23390. Conversely, the D2 receptor agonist quinpirole failed to depress the oscillations, and the D2 antagonist sulpiride did not prevent the suppressive dopamine effect. The data indicate that dopamine strongly depresses cholinergic gamma oscillations in area CA3 of rat hippocampus by activation of D1-like dopamine receptors and that this effect is most likely mediated via impairment of interneurons involved in generation and maintenance of the carbachol-induced network rhythm.

Dopamine: a potential substrate for synaptic plasticity and memory mechanisms

It is only recently that a number of studies on synaptic plasticity in the hippocampus and other brain areas have considered that a heterosynaptic modulatory input could be recruited as well as the coincident firing of pre- and post-synaptic neurons. So far, the strongest evidence for such a regulation has been attributed to dopaminergic (DA) systems but other modulatory pathways have also been considered to influence synaptic plasticity. This review will focus on dopamine contribution to synaptic plasticity in different brain areas (hippocampus, striatum and prefrontal cortex) with, for each region, a few lines on the distribution of DA projections and receptors. New insights into the possible mechanisms underlying these plastic changes will be considered. The contribution of various DA systems in certain forms of learning and memory will be reviewed with recent advances supporting the hypothesis of similar cellular mechanisms underlying DA regulation of synaptic plasticity and memory processes in which the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway has a potential role. To summarize, endogenous DA, which depends on the activity patterns of DA midbrain neurons in freely moving animals, appears as a key regulator in specific synaptic changes observed at certain stages of learning and memory and of synaptic plasticity.

Songorine, a diterpenoid alkaloid of the genus Aconitum, is a novel GABA(A) receptor antagonist in rat brain

Songorine, a diterpenoid alkaloid isolated from the genus Aconitum, was recently found to enhance the excitatory synaptic transmission in rat hippocampus. The mechanism underlying the effects was examined in the present study. The alkaloid at 0.1-300 microM inhibited the specific binding of [(3)H]muscimol to Triton-treated synaptic membranes of rat brain in a concentration-dependent manner (IC(50)=7.06 microM; 95% confidence limits: 3.28-10.84 microM). Scatchard analysis and Lineweaver-Burk double reciprocal plot of [(3)H]muscimol saturation binding data indicate a non-competitive inhibition of the alkaloid on the gamma-aminobutyric acid(A) (GABA(A)) receptor. In acutely dissociated rat hippocampal neurons the alkaloid did not elicit current response, but markedly inhibited the GABA-induced inward current (IC(50)=19.6 microM). The results suggest that songorine is a novel non-competitive antagonist at the GABA(A) receptor in rat brain.

Leitmotifs in the biochemistry of LTP induction: amplification, integration and coordination

Hippocampal long-term potentiation (LTP) is a robust and long-lasting form of synaptic plasticity that is the leading candidate for a cellular mechanism contributing to mammalian learning and memory. Investigations over the past decade have revealed that the biochemistry of LTP induction involves mechanisms of great subtlety and complexity. This review highlights themes that have emerged as a result of our increased knowledge of the signal transduction pathways involved in the induction of NMDA receptor-dependent LTP in area CA1 of the hippocampus. Among these themes are signal amplification, signal integration and signal coordination. Here we use these themes as an organizing context for reviewing the profusion of signaling mechanisms involved in the induction of LTP.

Hippocampo-prefrontal cortex pathway: anatomical and electrophysiological characteristics

The hippocampus, the prefrontal cortex, and interconnected neural circuits are implicated in several aspects of cognitive and memory processes. The present review is dedicated to the description of the anatomo-functional characteristics of the hippocampo-prefrontal pathway and related neuronal circuits in the rat. This pathway, which originates from the hippocampal CA1/subiculum fields, innervates the prelimbic/medial orbital areas of the prefrontal cortex (PL/MO). Its synaptic influence on cortical pyramidal neurons consists in an early monosynaptic excitation followed by an inhibition and, in some cases, a late excitation. These later effects are likely due to the subsequent activation of the local cortical network. PL/MO areas and the CA1/subiculum both send projections to the nucleus accumbens, a region of the ventral striatum which is particularly implicated in goal-directed behavior. Therefore, emphasis is placed on respective projections from PL/MO areas and from the CA1/subiculum on the "core" and the "shell" regions of the nucleus accumbens, as well as on their interconnected circuits. Signals which are directed to the prefrontal cortex through these circuits might modulate hippocampo-prefrontal inputs. Finally, the direct and/or indirect relationships of the hippocampus, prefrontal cortex, and nucleus accumbens with the ventral tegmental area/substantia nigra pars compacta complex (VTA/SNC) (where dopamine neurons are located) will also be described, because these neurons are known to modulate synaptic transmission and plasticity in their target structures and to play a fundamental role in motivational processes.

Dopamine depresses excitatory synaptic transmission onto rat subicular neurons via presynaptic D1-like dopamine receptors

Schizophrenia is considered to be associated with an abnormal functioning of the hippocampal output. The high clinical potency of antipsychotics that act as antagonists at dopamine (DA) receptors indicate a hyperfunction of the dopaminergic system. The subiculum obtains information from area CA1 and the entorhinal cortex and represents the major output region of the hippocampal complex. To clarify whether an enhanced dopaminergic activity alters the hippocampal output, the effect of DA on alveus- and perforant path-evoked excitatory postsynaptic currents (EPSCs) in subicular neurons was examined using conventional intracellular and whole cell voltage-clamp recordings. Dopamine (100 microM) depressed alveus-elicited (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated EPSCs to 56 +/- 8% of control while perforant path-evoked EPSCs were attenuated to only 76 +/- 7% of control. Dopamine had no effect on the EPSC kinetics. Dopamine reduced the frequency of spontaneous miniature EPSCs without affecting their amplitudes. The sensitivity of subicular neurons to the glutamate receptor agonist (S)-alpha-amino-3-hydoxy-5-methyl-4-isoxazolepropionic acid was unchanged by DA pretreatment, excluding a postsynaptic mechanism for the observed reduction of excitatory synaptic transmission. The effect of DA on evoked EPSCs was mimicked by the D1 receptor agonist SFK 38393 and partially antagonized by the D1 receptor antagonist SCH 23390. While the D2 receptor agonist quinelorane failed to reduce the EPSCs, the D2 receptor antagonist sulpiride did not block the action of DA. The results indicate that DA strongly depresses the hippocampal and the entorhinal excitatory input onto subicular neurons by decreasing the glutamate release following activation of presynaptic D1-like DA receptors.

Dopamine depresses polysynaptic inhibition in rat subicular neurons

Schizophrenia is considered to be associated with a hyperfunction of the dopaminergic system and with abnormalities in hippocampal information processing. To clarify whether an enhanced dopaminergic activity alters the hippocampal output, the effect of dopamine (DA) on inhibitory postsynaptic responses (IPSPs) in subicular neurons was examined. DA (200 microM) induced a small and inconsistent hyperpolarization that was accompanied by a reduction of membrane resistance. DA decreased polysynaptic IPSPs which was paralleled by a depression of isolated AMPA/kainate and NMDA receptor-mediated excitatory postsynaptic responses (EPSPs). In contrast, DA had no effect on isolated monosynaptic GABA(A) and GABA(B) receptor-mediated IPSP/Cs. We conclude that in addition to membrane effects, DA decreases polysynaptic IPSPs by attenuating the glutamatergic drive onto subicular interneurons.

Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus

The mechanisms underlying the facilitation (priming) of long-term potentiation (LTP) by prior activation of metabotropic glutamate receptors (mGluRs) were investigated in area CA1 of rat hippocampal slices. In particular, we focused on whether a long-lasting increase in postsynaptic excitability could account for the facilitated LTP. Administration of the mGluR agonist 1S, 3R-aminocyclopentanedicarboxylic acid (ACPD) produced rapid decreases in the amplitude of both the slow spike afterhyperpolarization (AHP(slow)) and spike frequency adaptation recorded intracellularly from CA1 pyramidal cells. These changes persisted after drug washout, showing only a slow decay over 20 min. ACPD also caused a leftward shift of the field EPSP-population spike relation and an overall increase in population spike amplitude, but this effect was not as persistent as the intracellularly measured alterations in cell excitability. ACPD-treated cells showed increased spike discharges during LTP-inducing tetanic stimulation, and the amplitude of the AHP(slow) was negatively correlated with the degree of initial LTP induction. The beta-adrenergic agonist isoproterenol also caused excitability changes as recorded intracellularly, whereas in extracellular experiments it weakly primed the induction but not the persistence of LTP. ACPD primed both LTP measures. Isoproterenol administration during the tetanus occluded the priming effect of ACPD on initial LTP induction but not its effect on LTP persistence. We conclude that the persistent excitability changes elicited by ACPD contributes to the priming of LTP induction but that other ACPD-triggered mechanisms must account for the facilitated persistence of LTP in the priming paradigm.

Plasticity of first-order sensory synapses: interactions between homosynaptic long-term potentiation and heterosynaptically evoked dopaminergic potentiation

Persistent potentiations of the chemical and electrotonic components of the eighth nerve (NVIII) EPSP recorded in vivo in the goldfish reticulospinal neuron, the Mauthner cell, can be evoked by afferent tetanization or local dendritic application of an endogenous transmitter, dopamine (3-hydroxytyramine). These modifications are attributable to the activation of distinct intracellular kinase cascades. Although dopamine-evoked potentiation (DEP) is mediated by the cAMP-dependent protein kinase (PKA), tetanization most likely activates a Ca2+-dependent protein kinase via an increased intracellular Ca2+ concentration. We present evidence that the eighth nerve tetanus that induces LTP does not act by triggering dopamine release, because it is evoked in the presence of a broad spectrum of dopamine antagonists. To test for interactions between these pathways, we applied the potentiating paradigms sequentially. When dopamine was applied first, tetanization produced additional potentiation of the mixed synaptic response, but when the sequence was reversed, DEP was occluded, indicating that the synapses potentiated by the two procedures belong to the same or overlapping populations. Experiments were conducted to determine interactions between the underlying regulatory mechanisms and the level of their convergence. Inhibiting PKA does not impede tetanus-induced LTP, and chelating postsynaptic Ca2+ with BAPTA does not block DEP, indicating that the initial steps of the induction processes are independent. Pharmacological and voltage-clamp analyses indicate that the two pathways converge on functional AMPA/kainate receptors for the chemically mediated EPSP and gap junctions for the electrotonic component or at intermediaries common to both pathways. A cellular model incorporating these interactions is proposed on the basis of differential modulation of synaptic responses via receptor-protein phosphorylation.

Ventral hippocampal dopamine D1 and D2 systems and spatial working memory in rats

The hippocampus has long been known to be important for memory function. However, the involvement of hippocampal dopamine systems with memory has received little attention. In the current study, dopamine D1 and D2 hippocampal receptor system involvement with memory was assessed in female Sprague-Dawley rats by local infusion of D1 and D2 agonists and antagonists into the ventral hippocampus. Working memory performance was assessed on the radial-arm maze. Neither the D1 agonist dihydrexidine (1.1-10 microg/side) nor the D1 antagonist SCH 23390 (0.19-1.67 microg/side) was effective in significantly altering radial-arm maze choice accuracy. In contrast, there were significant and opposite effects of D2 agonist and antagonist treatments. The D2 agonist quinpirole caused a significant (P<0.05) dose-related improvement in choice accuracy over a dose range of 1.1-10 microg/side. In a complementary fashion, the D2 antagonist raclopride caused a significant (P<0.05) dose-related choice accuracy deficit over a range of 0.19-1.67 microg/side. This study provides clear evidence that hippocampal D2 activity is positively related to working memory performance, while evidence for D1 systems is less compelling. Dopamine D2 receptors in the ventral hippocampus were shown to have important influences on spatial working memory. In a consistent pattern of effects ventral hippocampal infusion of the D2 agonist quinpirole improved working memory performance in the radial-arm maze, while ventral hippocampal infusion of the D2 antagonist raclopride impaired performance.

Dopamine selectively inhibits the direct cortical pathway to the CA1 hippocampal region

The perforant path input (pp) is a major direct source of specific sensory information for the CA1 hippocampal region. The termination area of this pathway, the stratum lacunosum-moleculare, has the highest concentration of dopamine receptors in the hippocampus. We have examined the properties of the pp input and its modulation by dopamine. The input is glutamatergic and has a larger NMDA component than the Schaffer collateral (sc) input. Dopamine strongly inhibits the response to pp stimulation (IC50 approximately 3 microM) but not the response to sc stimulation. Dopamine reduces both the NMDA and AMPA components of transmission at the pp and increases paired-pulse facilitation. In the sc, the NMDA component but not the AMPA component is decreased, and paired-pulse facilitation is not affected. The effect of dopamine on the pp does not depend on GABAA inhibition but is reduced by the antagonists of both D1 and D2 families of dopamine receptors. The effect is not completely blocked by the combination of D1 and D2 antagonists, but is completely blocked by the atypical neuroleptic clozapine. Our results provide the first evidence for strong dopaminergic control of transmission in the perforant path. By inhibiting this pathway, dopamine hyperfunction and/or NMDA hypofunction abnormalities implicated in schizophrenia may isolate CA1 from its main source of sensory information.

Effects of the Aconitum alkaloid songorine on synaptic transmission and paired-pulse facilitation of CA1 pyramidal cells in rat hippocampal slices

1. The present study investigated the electrophysiological effects of songorine (1 100 microM), an alkaloid occurring in plants of the Aconitum genus, in rat hippocampal slices. 2. Songorine (10-100 microM) evoked a concentration-dependent increase in the amplitude of the orthodromic population spike and in the slope of the field e.p.s.p. The enhancement was long-lasting and was not reversed by up to 90 min of washout. Songorine failed to affect size and shape of the presynaptic fiber spike which represents the compound action potential of the Schaffer collaterals. This indicates that enhancement of the synaptic response is no consequence of an increased afferent excitability. 3. The antidromically evoked population spike was not affected by songorine at concentrations up to 100 microM suggesting that the enhancement of the orthodromic population spike and of the field e.p.s.p. was not due to an increase in pyramidal cell excitability. 4 The input-output curve for the postsynaptic population spike was shifted to the left implying that a presynaptic fiber spike of the same size elicited a larger postsynaptic response, indicating a decrease in threshold for generation of the population spike. 5. The songorine-evoked increase in excitability was not affected by the NMDA receptor antagonist, D-AP5. However, the effect of songorine was completely abolished by the selective dopamine D2 receptor antagonist sulpiride (0.1 microM) as well as by haloperidol (10 microM) and was mimicked by application of the dopamine releaser, amantadine (100 mM). In contrast, the selective D1 receptor antagonist, SCH23390, did not block the action of songorine. 6. The results indicate that the plant alkaloid songorine enhances excitatory synaptic transmission which may be due to an agonistic action at D2 receptors.

Synaptic tagging: implications for late maintenance of hippocampal long-term potentiation

A novel property of hippocampal LTP, 'variable persistence', has recently been described that is, we argue, relevant to the role of LTP in information storage. Specifically, new results indicate that a particular pattern of synaptic activation can give rise, either to a relatively short-lasting LTP, or to a longer-lasting LTP as a function of the history of activation of the neuron. This has led to the idea that the induction of LTP is associated with the setting of a'synaptic tag' at activated synapses, whose role is to sequester plasticity-related proteins that then serve to stabilize temporary synaptic changes and so extend their persistence. In this article, we outline the synaptic tag hypothesis, compare predictions it makes with those of other theories about the persistence of LTP, and speculate about the cellular identity of the tag. In addition, we outline the requirement for aminergic activation to induce late LTP and consider the functional implications of the synaptic tag hypothesis with respect to long-term memory.

Dopamine D1-deficient mutant mice do not express the late phase of hippocampal long-term potentiation

The possible involvement of the dopamine D1 receptor subtype in mechanisms of long-term potentiation (LTP) of the Schaffer collateral-commissural input of CA1 neurones was investigated using D1-deficient mutant mice. In transversal hippocampus slices from mice lacking the D1 receptor a normal post-tetanic and short-term potentiation could be induced after applying a triple 100 Hz tetanization. However, the potentiated fEPSP in the mutant mice declined to control value about 140 min following tetanization, whereas in the wild type mice a normal, non-decremental LTP was observed. These data support the idea that besides the glutamatergic system, the synergistic activation of dopaminergic synapses is necessary for LTP maintenance.

Metaplasticity: a new vista across the field of synaptic plasticity

Over the past 20 years there has been an increasing understanding of the properties and mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy, putative learning and memory mechanisms in the mammalian brain. More recently, however, it has become apparent that synaptic activity can also elicit persistent neuronal responses not manifest as changes in synaptic strength. Some of these changes may nonetheless modify the ability of synapses to undergo strength changes in response to subsequent episodes of synaptic activity. This kind of activity-dependent modulatory plasticity we have termed "metaplasticity". Metaplasticity has been observed physiologically as an inhibition of LTP and concomitant facilitation of LTD by prior N-methyl-D-aspartate receptor activation or, conversely, a facilitation of LTP induction by prior metabotropic glutamate receptor activation. The examples of metaplasticity described to date are input specific, and last as long as several hours. The mechanisms underlying such phenomena remain to be fully characterized, although some likely possibilities are an altered N-methyl-D-aspartate receptor function, altered calcium buffering, altered states of kinases or phosphatases, and a priming of protein synthesis machinery. While some details vary, experimentally observed metaplasticity bears some similarity to the "sliding threshold" feature of the Bienenstock, Cooper and Munro model of experience-dependent synaptic plasticity. Metaplasticity may serve several functions including (1) providing a way for synapses to integrate a response across temporally spaced episodes of synaptic activity and (2) keeping synapses within a dynamic functional range, and thus preventing them from entering states of saturated LTP or LTD.

D1/D5 dopamine receptor activation increases the magnitude of early long-term potentiation at CA1 hippocampal synapses

The role of the mesolimbic dopaminergic system in the reinforcement of learning suggests that dopamine should be able to modulate activity-dependent synaptic plasticity. We have examined the effect of D1/D5 agonists on early long-term potentiation (LTP) (40 min) in the CA1 region of hippocampal slices. D1/D5 agonists (+)bromo-APB, 6-chloro-PB, and dihydrexidine increased the magnitude of LTP in a synapse-specific manner (by approximately 10, 15, and 20%, respectively). This D1/D5 effect was mimicked by a low dose (10 microM) of the adenylyl cyclase activator forskolin. The D1/D5 antagonist (+)SCH 23390 reduced early LTP. In catecholamine-depleted slices, LTP was smaller by approximately 20-25% and could not be decreased further by D1/D5 antagonist. Under these conditions, D1/D5 agonist 6-chloro-PB and forskolin produced a larger enhancement of LTP (20-25%), restoring it to the control level. At the same dose, dideoxyforskolin did not affect early LTP. The D1/D5 agonist effect was completely blocked by the D1/D5 antagonist (+)SCH 23390. These results indicate that dopamine produces a synapse-specific enhancement of early LTP through D1/D5 receptors and cAMP.

The dopaminergic mesencephalic projections to the hippocampal formation in the rat

1. The dopaminergic mesencephalic neurons projecting to the hippocampal formation are distributed in three cell groups: A8 region in the retrorubral field, A9 region in the substantia nigra and A10 region in the ventral tegmental area. 2. Anterograde and retrograde tract-tracing techniques combined with immunohistochemical procedures indicate a topographical organization of mesencephalic dopaminergic projections towards the hippocampal formation. 3. Electrophysiological evidence suggest that dopaminergic mesencephalic neurons could have a regulatory role in suppressing hippocampal excitability. 4. The functional significance of the mesohippocampal dopaminergic system is largely unknown, although it was suggested that this projection could have a role in methamphetamine-produced hypermotility and in modulation of memory processes.

Spatial memory impairment induced by lesion of the mesohippocampal dopaminergic system in the rat

The hippocampal formation has long been thought to play a role in learning and memory. Previous studies from our laboratory examined the organization of mesencephalic projections to the hippocampal formation in the rat. In order to evaluate the effects on learning and memory of retrograde selective lesions of mesencephalic dopaminergic neurons, following bilateral injection of 6-hydroxydopamine in the dorsal and ventral subiculum and adjacent CA1 field of the hippocampal formation, young adult Sprague-Dawley rats were trained in classical inhibitory avoidance, inhibitory avoidance using a multiple trial (training to criterion) and the standard Morris water maze task (cued and spatial versions). With regard to inhibitory avoidance, retention was examined one, three and 10 days after training. Concerning the Morris water maze task, 6-hydroxydopamine-lesioned and sham-operated rats received four training trials on each of four days. After training sessions, the rats were tested during a 60-s probe trial (free-swim trial) in which the platform was removed from the maze. The loss of mesencephalic dopaminergic neurons in the 6-hydroxydopamine-lesioned rats, compared to sham-operated rats, was verified by tyrosine hydroxylase immunohistochemistry. Although the 6-hydroxydopamine-lesioned rats were indistinguishable from sham-operated rats in performing the inhibitory avoidance and the cued version of the Morris water maze task, in the spatial version of the Morris water maze, lesioned rats, compared to controls, exhibited significant differences in the latency (P < 0.05), quadrant time (P < 0.01) and number of platform crossings (P < 0.05). These results suggest that the rat's ability to acquire spatial learning and memory for place navigation in the Morris water maze is likely to be dependent also on the integrity of mesohippocampal dopaminergic connections.

In utero cocaine exposure decreases dopamine D1 receptor modulation of hippocampal long-term potentiation in the rabbit

Cocaine increases the synaptic concentration of neurotransmitters by inhibiting catecholamine transporters. Disturbances of behavior and cellular physiology have been associated with prenatal cocaine exposure and are related to changes in dopamine transmission. Recently we found the magnitude of long-term potentiation (LTP) was greater in hippocampal slices from cocaine exposed offspring. In the hippocampus, D1 dopamine receptor antagonists inhibit the expression of LTP while agonists facilitate it. To test the functionality of the D1 receptor we examined the effect of the D1 antagonist SCH-23390 on LTP using a rabbit model of gestational cocaine exposure. Tetanization during exposure to the D1 antagonist SCH-23390 resulted in a long lasting potentiation in animals prenatally exposed to cocaine while the potentiation of control slices returned to baseline.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Neuropeptide Y receptor agonists: multiple effects on spontaneous activity in the paraventricular hypothalamus

In vitro rat hypothalamic slices were used to examine the ability of neuropeptide Y (NPY), and the putative Y1 and Y2 receptor agonists [Pro34]NPY and [C2]NPY, to modify spontaneous single-neuron discharge in the paraventricular nucleus (PVN). NPY and [Pro34]NPY, at high concentrations (1500 nM), decreased discharge rates. At intermediate concentrations (150 nM) these peptides produced multiple effects, including increases, decreases, and biphasic changes. At lower concentrations (0.15-15 nM), they typically increased discharge rates. In contrast, [C2]NPY, at all concentrations (1.5-1500 nM), predominantly increased discharge rates. Thus, these NPY subtype agonists have multiple effects on discharge rate, which may be due to actions on multiple NPY receptor subtypes.

An integrative analysis to sleep functions

Various studies suggest that some sleep functions, especially some slow wave sleep functions, are indispensable in mammals and related to brain regulation. It has been proposed that two of these functions are the adjustment of emotional balance and the processing of acquired emotional memories. During waking, the gradual accumulation of various randomly learned emotional memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors. Although the emotional balance can be restored during waking by the ascending NA, DA, ACh and 5-HT systems, their roles in memory retention and emotional regulation may sometimes be dissociated and their adjustment of the emotional balance can only be a transient effect. On the other hand, the function of slow wave sleep for emotional adjustment can be long-lasting and is in agreement with its function on the processing of emotional memories. As a result, these sleep functions become indispensable in preventing the emotional imbalance inevitably caused by the accumulation of emotional memories. The effects of rapid eye movement sleep on memory and emotional regulation are just opposite to those of slow wave sleep. Low vigilance is required as premise for sleep to accomplish these indispensable functions.

Dopamine D2-like receptors labeled by [3H]YM-09151-2 in the rat hippocampus: characterization and autoradiographic distribution

Dopamine D2-like receptor labeled by [3H]YM-09151-2 in the rat hippocampus proper was examined by in vitro receptor autoradiography. In the dorsal hippocampus, [3H]YM-09151-2 bindings were high in the whole layers of CA1, the stratum pyramidale of CA4 and the stratum molecular of gyrus dentatus, moderate in the stratum oriens of CA3 and hilus of the gyrus dentatus, and low in remaining CA3 and the subiculum. In the ventral hippocampus, the binding densities were high in the stratum oriens and the stratum radiatum of CA1, the stratum pyramidale of CA4, and the stratum moleculare of gyrus dentatus, moderate in the stratum lacnosum moleculare of CA1 and the hilus of the gyrus dentatus. Saturation analysis using hippocampal sections demonstrated that the Kd value was about five times higher than that using striatal sections. The rank order potency of competition on [3H]YM-09151-2 binding by dopaminergic ligands in the hippocampus was YM-09151-2 > (+)-butaclamol > dopamine > sulpiride > SCH-23390; which shows the appropriate dopamine D2-like receptor profile. The hippocampal [3H]YM-09151-2 binding did not represent serotonergic receptors (5-HT1A and 5-HT2) and sigma receptor, since Ki values of ketanserine, serotonin, 8-OH-DPAT and DTG were much lower than D2-like receptor antagonists. These findings suggest tha [3H]YM-09151-2 binds hippocampal D2-like receptor site with different association kinetics of striatal D2-like receptor site, and demonstrates widespread distribution of D2-like receptor in the hippocampus with distinct region-specific profile.

Activation of dopamine D1 receptors enhances long-term depression of synaptic transmission induced by low frequency stimulation in rat hippocampal CA1 neurons

The effects of both activation and blockade of dopamine (DA) D1 receptors on long-term depression (LTD) of synaptic transmission were examined in CA1 neurons of rat hippocampal slices. Low frequency stimulation (LFS) consisting of 450 pulses at 1 Hz induced LTD (-14.3%, mean, n = 10) in the slope of the field excitatory postsynaptic potential. SKF-38393 (3-10 microM), an agonist of DA D1 receptors, significantly enhanced LFS-induced LTD (-31.1%, n = 11). SCH-23390 (2 microM), an antagonist of DA D1 receptors, blocked the induction of LTD by LFS (2.5%, n = 6). These results indicate that DA D1 receptors play an important role in the modulation of LFS-induced LTD in rat hippocampal CA1 neurons.

D1/D5 receptor agonists induce a protein synthesis-dependent late potentiation in the CA1 region of the hippocampus

Agonists of the dopamine D1/D5 receptors that are positively coupled to adenylyl cyclase specifically induce a slowly developing long-lasting potentiation of the field excitatory postsynaptic potential in the CA1 region of the hippocampus that lasts for > 6 hr. This potentiation is blocked by the specific D1/D5 receptor antagonist SCH 23390 and is occluded by the potentiation induced by cAMP agonists. An agonist of the D2 receptor, which is negatively coupled to adenylyl cyclase through G alpha i, did not induce potentiation. Although this slow D1/D5 agonist-induced potentiation is partially independent of N-methyl-D-aspartate receptors, it seems to share some steps with and is occluded by the late phase of long-term potentiation (LTP) produced by three repeated trains of nerve stimuli applied to the Schaffer collateral pathway. Similarly, the D1/D5 antagonist SCH 23390 attenuates the late phase of the LTP induced by repeated trains, and the D1/D5 agonist-induced potentiation is blocked by the protein synthesis inhibitor anisomycin. These results suggest that the D1/D5 receptor may be involved in the late, protein synthesis-dependent component of LTP in the hippocampal CA1 region, either as an ancillary component or as a mediator directly contributing to the late phase.

Autoradiographic distribution of [3H]YM-09151-2, a high-affinity and selective antagonist ligand for the dopamine D2 receptor group, in the rat brain and spinal cord

We determined the regional distribution of the dopamine D2 receptor group in the rat central nervous system by quantitative receptor autoradiography with a high-affinity and selective antagonist, [3H]YM-09151-2. Saturation and competition experiments demonstrated that the binding of [3H]YM-09151-2 to striatal sections was saturable (Bmax = 37.3 fmol/section), of high affinity (Kd = 0.315 nM), and was inhibited selectively by prototypic D2 ligands. The anatomical localization of binding sites was determined by comparison of autoradiograms and the original 3H-ligand-exposed sections stained with cresyl violet. Very high levels of [3H]YM-09151-2 binding were found in the caudate-putamen, nucleus accumbens, tuberculum olfactorium and the insula of Calleja, to each of which midbrain dopaminergic neurons project densely. High levels of binding were also observed in other regions rich in dopaminergic neurons and fibers including the glomerular layer of the olfactory bulb, the intermediate lobe of the pituitary, lateral septum, substantia nigra pars compacta, interfascicular nucleus, dorsal raphe nucleus, locus coeruleus, and nucleus of the solitary tract. Some regions poor in dopaminergic innervation, however, had high levels of [3H]YM-09151-2 binding including the molecular layer of gyrus dentatus, all layers of CA1 and the nonpyramidal layer of CA4 of hippocampus, and the deeper layer of medial entorhinal cortex. Motor neurons present in brainstem motor nuclei and spinal ventral horn were also strongly labeled. Neocortical, cerebellar, and thalamic regions had low levels of binding, except lobules 9-10 of the cerebellum, the olivary pretectal nucleus, zona incerta and lateral mammillary nucleus, in which moderate to high levels of binding were detected. Our findings concerning the widespread but region-specific localization of [3H]YM-09151-2 binding sites in the brain and spinal cord may prove useful for analyzing various dopaminergic functions in the central nervous system.

Carbamazepine-induced blockade of induction of long-term potentiation in the perforant path-dentate gyrus pathway in chronically prepared rabbits

We investigated the effects of a representative anti-epileptic or anti-psychotic drug, carbamazepine (CBZ), on the induction of long-term potentiation (LTP) in the perforant path-dentate gyrus pathway in 15 chronically prepared rabbits. Pharmacokinetically injected low-dose CBZ, which produced steady serum levels of 2.2-3.3 micrograms/ml (mean +/- S.D. 2.85 +/- 0.40), variably affected the induction of LTP, blocking it or showing almost no effect. The high-dose CBZ, which produced steady serum levels of 6.5-8.6 micrograms/ml (8.01 +/- 0.87), always blocked the induction of LTP. These results indicate that CBZ dose-dependently blocks the induction of LTP. The possible mechanisms underlying this blockade are discussed, especially in association with the possible inhibitory action of CBZ on N-methyl-D-aspartate (NMDA) receptors.

Haloperidol-induced blockade of induction of long-term potentiation in perforant path-dentate gyrus pathway in chronically prepared rabbits

We investigated the effects of the representative neuroleptic and dopamine receptor antagonist haloperidol (HPD) on the induction of long-term potentiation (LTP) or on the previously induced LTP in the perforant path-dentate gyrus pathway in chronically prepared rabbits. The IP HPD injection of 0.8 mg/kg blocked the induction of LTP when it was given before LTP-inducing tetanic stimulations, although this dose showed virtually no effect on the baseline control responses in the perforant path-dentate gyrus pathway to single shocks. However, neither 0.8-mg/kg nor 1.6-mg/kg HPD doses affected the previously induced LTP. The possible mechanisms underlying these results, notably the HPD-induced blockade of LTP induction, are discussed, especially in association with the inhibitory action of HPD on calmodulin-mediated events rather than dopaminergic function.

A cellular analogue of operant conditioning

Using the hippocampal-slice preparation, we attempted to model operant conditioning in vitro by reinforcing pyramidal cell bursting responses with local micropressure applications of transmitters and drugs. The same injections were administered independently of bursting to provide a "noncontingent" control for direct pharmacological stimulation or facilitation of firing. The results suggested that the bursting responses of individual CA1 pyramidal neurons may be reinforced in a dose-related manner by response-contingent (but not noncontingent) injections of dopamine and the selective dopamine D2 agonist, N-0923. N-0924, a stereoisomer of N-0923 that is largely devoid of D2-agonist activity, failed to reinforce CA1 bursting. Burst-contingent injections of the excitatory neurotransmitter glutamate also failed to reinforce CA1 bursting; indeed, the glutamate applications (whether contingent or random) reduced the likelihood of bursts while increasing the frequency of solitary spikes. Reinforcement delays exceeding 200 ms largely eliminated the reinforcing efficacy of the D2 agonist N-0437 in CA1 operant conditioning. The results are consistent with the suggestion that the behaviorally reinforcing effects of dopaminergic agents can be modeled in vitro in the hippocampal-slice preparation.

Cocaine inhibits hippocampal long-term potentiation

Long-term potentiation (LTP) is a form of synaptic plasticity that may underlie learning and memory. The experiments reported here demonstrate that cocaine blocks the induction of LTP at the excitatory synapses in the CA1 region of the hippocampus, but does not appear to do so by blocking NMDA receptors or channels. Once LTP had been established, however, cocaine had no effect on the potentiated response. Cocaine was also able to block LTP initiated by superfusing slices with 25 mM TEA. The ability to block LTP was shared by the local anesthetics lidocaine and procaine, but not by tetrodotoxin, suggesting that the blockade of sodium channels alone did not disrupt LTP. Biochemical experiments demonstrated that cocaine can inhibit phosphorylation of purified Synapsin I by Ca2+/calmodulin-dependent protein kinase II. This effect, presumably mediated by effects on calmodulin, is a previously unreported action of cocaine, and suggests that cocaine at high dose levels might disrupt types of learning that are mediated by an LTP-like mechanism.

Effects of trifluoperazine on synaptically evoked potentials and membrane properties of CA1 pyramidal neurons of rat hippocampus in situ and in vitro

The effects of trifluoperazine (TFP), a phenothiazine antipsychotic, on hippocampal activity were studied in the CA1 subfield, both in situ and in slices. In the extracellular studies in situ and in vitro, both somatic population spikes and dendritic excitatory postsynaptic potentials (EPSP) fields were depressed reversibly by TFP, applied by microiontophoresis or in the bath (50-100 microM). Similar effects were also seen during iontophoretic applications of sphingosine in situ. Like TFP (at micromolar concentrations) sphingosine is a dual Ca2+/calmodulin-dependent kinase and protein kinase C (PKC) inhibitor. In intracellular recordings from slices, 50-100 microM TFP induced a slow depolarization and a decrease in input resistance (RN), probably through a gamma-aminobutyric acid (GABA)-mediated increase in Cl- conductance (GCl). TFP also reduced the slow afterhyperpolarization (AHP) as well as electrically evoked inhibitory postsynaptic potentials (IPSPs), but EPSPs were augmented in both amplitude and duration. When CA1 neurons were voltage clamped, TFP elicited a corresponding inward current (consistent with depolarization), increased the leak conductance, and enhanced excitatory synaptic currents; whereas inhibitory synaptic currents and high-threshold Ca2+ currents were reduced. In conclusion, these effects of TFP--which cannot be readily explained by its potent antidopamine action--are in keeping with other evidence that both Ca2+/calmodulin-dependent kinase and PKC can modulate GCl-conductance and high-threshold Ca(2+)-conductance, as well as inhibitory and excitatory postsynaptic currents.

Synaptic potentials and effects of amino acid antagonists in the auditory cortex

Neurons of in vitro guinea pig and rat auditory cortex receive a complex synaptic pattern of afferent information. As many as four synaptic responses to a single-stimulus pulse to the gray or white matter can occur; an early-EPSP followed, sequentially, by an early-IPSP, late-EPSP, and late-IPSP. Paired pulse stimulation and pharmacological studies show that the early-IPSP can modify information transmission that occurs by way of the early-EPSP. Each of these four synaptic responses differed in estimated reversal potential, and each was differentially sensitive to antagonism by pharmacological agents. DNQX (6,7-dinitroquinoxaline-2,3-dione), a quisqualate/kainate receptor antagonist, blocked the early-EPSP, and the late-EPSP was blocked by the NMDA receptor antagonist APV (D-2-amino-5-phosphonovalerate). The early-IPSP was blocked by the GABA-a receptor antagonist bicuculline, and the late-IPSP by the GABA-b receptor antagonists 2-OH saclofen or phaclofen. Presentation of stimulus trains, even at relatively low intensities, could produce a long-lasting APV-sensitive membrane depolarization. Also discussed is the possible role of these synaptic potentials in auditory cortical function and plasticity.