Long-term adaptation of prefrontal circuits in a mouse model of NMDAR hypofunction.

Pharmacological approaches to induce N-methyl-d-aspartate receptor (NMDAR) hypofunction have been intensively used to understand the aetiology and pathophysiology of schizophrenia. Yet, the precise cellular and molecular mechanisms that relate to brain network dysfunction remain largely unknown. Here, we used a set of complementary approaches to assess the functional network abnormalities present in male mice that underwent a 7-day subchronic phencyclidine (PCP 10 mg/kg, subcutaneously, once daily) treatment. Our data revealed that pharmacological intervention with PCP affected cognitive performance and auditory evoked gamma oscillations in the prefrontal cortex (PFC) mimicking endophenotypes of some schizophrenia patients. We further assessed PFC cellular function and identified altered neuronal intrinsic membrane properties, reduced parvalbumin (PV) immunostaining and diminished inhibition onto L5 PFC pyramidal cells. A decrease in the strength of optogenetically-evoked glutamatergic current at the ventral hippocampus to PFC synapse was also demonstrated, along with a weaker shunt of excitatory transmission by local PFC interneurons. On a macrocircuit level, functional ultrasound measurements indicated compromised functional connectivity within several brain regions particularly involving PFC and frontostriatal circuits. Herein, we reproduced a panel of schizophrenia endophenotypes induced by subchronic PCP application in mice. We further recapitulated electrophysiological signatures associated with schizophrenia and provided an anatomical reference to critical elements in the brain circuitry. Together, our findings contribute to a better understanding of the physiological underpinnings of deficits induced by subchronic NMDAR antagonist regimes and provide a test system for characterization of pharmacological compounds.

Social Play Behavior Is Critical for the Development of Prefrontal Inhibitory Synapses and Cognitive Flexibility in Rats.

Sensory driven activity during early life is critical for setting up the proper connectivity of the sensory cortices. We ask here whether social play behavior, a particular form of social interaction that is highly abundant during postweaning development, is equally important for setting up connections in the developing prefrontal cortex (PFC). Young male rats were deprived from social play with peers during the period in life when social play behavior normally peaks [postnatal day 21-42] (SPD rats), followed by resocialization until adulthood. We recorded synaptic currents in layer 5 cells in slices from medial PFC of adult SPD and control rats and observed that inhibitory synaptic currents were reduced in SPD slices, while excitatory synaptic currents were unaffected. This was associated with a decrease in perisomatic inhibitory synapses from parvalbumin-positive GABAergic cells. In parallel experiments, adult SPD rats achieved more reversals in a probabilistic reversal learning (PRL) task, which depends on the integrity of the PFC, by using a more simplified cognitive strategy than controls. Interestingly, we observed that one daily hour of play during SPD partially rescued the behavioral performance in the PRL, but did not prevent the decrease in PFC inhibitory synaptic inputs. Our data demonstrate the importance of unrestricted social play for the development of inhibitory synapses in the PFC and cognitive skills in adulthood and show that specific synaptic alterations in the PFC can result in a complex behavioral outcome.SIGNIFICANCE STATEMENT This study addressed the question whether social play behavior in juvenile rats contributes to functional development of the prefrontal cortex (PFC). We found that rats that had been deprived from juvenile social play (social play deprivation - SPD) showed a reduction in inhibitory synapses in the PFC and a simplified strategy to solve a complex behavioral task in adulthood. Providing one daily hour of play during SPD partially rescued the cognitive skills in these rats, but did not prevent the reduction in PFC inhibitory synapses. Our results demonstrate a key role for unrestricted juvenile social play in PFC development and emphasize the complex relation between PFC circuit connectivity and cognitive function.

Characterization of orexin input to dopamine neurons of the ventral tegmental area projecting to the medial prefrontal cortex and shell of nucleus accumbens.

Orexin neurons are involved in homeostatic regulatory processes, including arousal and feeding, and provide a major input from the hypothalamus to the ventral tegmental area (VTA) of the midbrain. VTA neurons are a central hub processing reward and motivation and target the medial prefrontal cortex (mPFC) and the shell part of nucleus accumbens (NAcs). We investigated whether subpopulations of dopamine (DA) neurons in the VTA projecting either to the mPFC or the medial division of shell part of nucleus accumbens (mNAcs) receive differential input from orexin neurons and whether orexin exerts differential electrophysiological effects upon these cells. VTA neurons projecting to the mPFC or the mNAcs were traced retrogradely by Cav2-Cre virus and identified by expression of yellow fluorescent protein (YFP). Immunocytochemical analysis showed that a higher proportion of all orexin-innervated DA neurons projected to the mNAcs (34.5%) than to the mPFC (5.2%). Of all sampled VTA neurons projecting either to the mPFC or mNAcs, the dopaminergic (68.3 vs. 79.6%) and orexin-innervated DA neurons (68.9 vs. 64.4%) represented the major phenotype. Whole-cell current clamp recordings were obtained from fluorescently labeled neurons in slices during baseline periods and bath application of orexin A. Orexin similarly increased the firing rate of VTA dopamine neurons projecting to mNAcs (1.99 ± 0.61 Hz to 2.53 ± 0.72 Hz) and mPFC (0.40 ± 0.22 Hz to 1.45 ± 0.56 Hz). Thus, the hypothalamic orexin system targets mNAcs and to a lesser extent mPFC-projecting dopaminergic neurons of the VTA and exerts facilitatory effects on both clusters of dopamine neurons.

Leptin Receptor Expressing Neurons in the Substantia Nigra Regulate Locomotion, and in The Ventral Tegmental Area Motivation and Feeding.

Leptin is an anorexigenic hormone, important in the regulation of body weight. Leptin plays a role in food reward, feeding, locomotion and anxiety. Leptin receptors (LepR) are expressed in many brain areas, including the midbrain. In most studies that target the midbrain, either all LepR neurons of the midbrain or those of the ventral tegmental area (VTA) were targeted, but the role of substantia nigra (SN) LepR neurons has not been investigated. These studies have reported contradicting results regarding motivational behavior for food reward, feeding and locomotion. Since not all midbrain LepR mediated behaviors can be explained by LepR neurons in the VTA alone, we hypothesized that SN LepR neurons may provide further insight. We first characterized SN LepR and VTA LepR expression, which revealed LepR expression mainly on DA neurons. To further understand the role of midbrain LepR neurons in body weight regulation, we chemogenetically activated VTA LepR or SN LepR neurons in LepR-cre mice and tested for motivational behavior, feeding and locomotion. Activation of VTA LepR neurons in food restricted mice decreased motivation for food reward (p=0.032) and food intake (p=0.020), but not locomotion. In contrast, activation of SN LepR neurons in food restricted mice decreased locomotion (p=0.025), but not motivation for food reward or food intake. Our results provide evidence that VTA LepR and SN LepR neurons serve different functions, i.e. activation of VTA LepR neurons modulated motivation for food reward and feeding, while SN LepR neurons modulated locomotor activity.

Cue and Reward Evoked Dopamine Activity Is Necessary for Maintaining Learned Pavlovian Associations.

Associating natural rewards with predictive environmental cues is crucial for survival. Dopamine (DA) neurons of the ventral tegmental area (VTA) are thought to play a crucial role in this process by encoding reward prediction errors (RPEs) that have been hypothesized to play a role in associative learning. However, it is unclear whether this signal is still necessary after animals have acquired a cue-reward association. In order to investigate this, we trained mice to learn a Pavlovian cue-reward association. After learning, mice show robust anticipatory and consummatory licking behavior. As expected, calcium activity of VTA DA neurons goes up for cue presentation as well as reward delivery. Optogenetic inhibition during the moment of reward delivery disrupts learned behavior, even in the continued presence of reward. This effect is more pronounced over trials and persists on the next training day. Moreover, outside of the task licking behavior and locomotion are unaffected. Similarly to inhibitions during the reward period, we find that inhibiting cue-induced dopamine (DA) signals robustly decreases learned licking behavior, indicating that cue-related DA signals are a potent driver for learned behavior. Overall, we show that inhibition of either of these DA signals directly impairs the expression of learned associative behavior. Thus, continued DA signaling in a learned state is necessary for consolidating Pavlovian associations.SIGNIFICANCE STATEMENT Dopamine (DA) neurons of the ventral tegmental area (VTA) have long been suggested to be necessary for animals to associate environmental cues with rewards that they predict. Here, we use time-locked optogenetic inhibition of these neurons to show that the activity of these neurons is directly necessary for performance on a Pavlovian conditioning task, without affecting locomotor per se These findings provide further support for the direct importance of second-by-second DA neuron activity in associative learning.

Identification of Novel Neurocircuitry Through Which Leptin Targets Multiple Inputs to the Dopamine System to Reduce Food Reward Seeking.

BACKGROUND: Leptin reduces the motivation to obtain food by modulating activity of the mesolimbic dopamine (DA) system upon presentation of cues that predict a food reward. Although leptin directly reduces the activity of ventral tegmental area (VTA) DA neurons, the majority of leptin receptor (LepR)-expressing DA neurons do not project to the nucleus accumbens, the projection implicated in driving food reward seeking. Therefore, the precise locus of leptin action to modulate motivation for a food reward is unresolved. METHODS: We used transgenic mice expressing Cre recombinase under the control of the LepR promoter, anatomical tracing, optogenetics-assisted patch-clamp electrophysiology, in vivo optogenetics with fiber photometric calcium measurements, and chemogenetics to unravel how leptin-targeted neurocircuitry inhibits food reward seeking. RESULTS: A large number of DA neurons projecting to the nucleus accumbens are innervated by local VTA LepR-expressing GABA (gamma-aminobutyric acid) neurons. Leptin enhances the activity of these GABA neurons and thereby inhibits nucleus accumbens-projecting DA neurons. In addition, we find that lateral hypothalamic LepR-expressing neurons projecting to the VTA are inhibited by leptin and that these neurons modulate DA neurons indirectly via inhibition of VTA GABA neurons. In accordance with such a disinhibitory function, optogenetically stimulating lateral hypothalamic LepR projections to the VTA potently activates DA neurons in vivo. Moreover, we found that chemogenetic activation of lateral hypothalamic LepR neurons increases the motivation to obtain a food reward only when mice are in a positive energy balance. CONCLUSIONS: We identify neurocircuitry through which leptin targets multiple inputs to the DA system to reduce food reward seeking.

Chemogenetic activation of dopamine neurons in the ventral tegmental area, but not substantia nigra, induces hyperactivity in rats.

Hyperactivity is a core symptom in various psychiatric disorders, including attention-deficit/hyperactivity disorder, schizophrenia, bipolar disorders, and anorexia nervosa. Although hyperactivity has been linked to dopaminergic signalling, the causal relationship between midbrain dopamine neuronal activity and locomotor hyperactivity remains unknown. In this study, we test whether increased dopamine neuronal activity is sufficient to induce locomotor hyperactivity. To do so, we used designer receptors exclusively activated by designer drugs (DREADD) to chemogenetically enhance neuronal activity in two main midbrain dopamine neuron populations, i.e. the ventral tegmental area (VTA) and substantia nigra pars compacta (SN), in TH:Cre rats. We found that activation of VTA dopamine neurons induced a pronounced and long-lasting hyperactive phenotype, whilst SN dopamine neuron activation only modestly increased home cage locomotion. Furthermore, this hyperactive phenotype was replicated by selective activation of the neuronal pathway from VTA to the nucleus accumbens (NAC). These results show a clear functional difference between neuronal subpopulations in the VTA and SN with regards to inducing locomotor hyperactivity, and suggest that the dopaminergic pathway from VTA to NAC may be a promising target for the treatment of hyperactivity disorders.

Ventral Tegmental Area Dopamine Cell Activation during Male Rat Sexual Behavior Regulates Neuroplasticity and d-Amphetamine Cross-Sensitization following Sex Abstinence.

UNLABELLED: Experience with sexual behavior causes cross-sensitization of amphetamine reward, an effect dependent on a period of sexual reward abstinence. We previously showed that ΔFosB in the nucleus accumbens (NAc) is a key mediator of this cross-sensitization, potentially via dopamine receptor activation. However, the role of mesolimbic dopamine for sexual behavior or cross-sensitization between natural and drug reward is unknown. This was tested using inhibitory designer receptors exclusively activated by designer drugs in ventral tegmental area (VTA) dopamine cells. rAAV5/hSvn-DIO-hm4D-mCherry was injected into the VTA of TH::Cre adult male rats. Males received clozapine N-oxide (CNO) or vehicle injections before each of 5 consecutive days of mating or handling. Following an abstinence period of 7 d, males were tested for amphetamine conditioned place preference (CPP). Next, males were injected with CNO or vehicle before mating or handling for analysis of mating-induced cFos, sex experience-induced ΔFosB, and reduction of VTA dopamine soma size. Results showed that CNO did not affect mating behavior. Instead, CNO prevented sexual experience-induced cross-sensitization of amphetamine CPP, ΔFosB in the NAc and medial prefrontal cortex, and decreases in VTA dopamine soma size. Expression of hm4D-mCherry was specific to VTA dopamine cells and CNO blocked excitation and mating-induced cFos expression in VTA dopamine cells. These findings provide direct evidence that VTA dopamine activation is not required for initiation or performance of sexual behavior. Instead, VTA dopamine directly contributes to increased vulnerability for drug use following loss of natural reward by causing neuroplasticity in the mesolimbic pathway during the natural reward experience. SIGNIFICANCE STATEMENT: Drugs of abuse act on the neural pathways that mediate natural reward learning and memory. Exposure to natural reward behaviors can alter subsequent drug-related reward. Specifically, experience with sexual behavior, followed by a period of abstinence from sexual behavior, causes increased reward for amphetamine in male rats. This study demonstrates that activation of ventral tegmental area dopamine neurons during sexual experience regulates cross-sensitization of amphetamine reward. Finally, ventral tegmental area dopamine cell activation is essential for experience-induced neural adaptations in the nucleus accumbens, prefrontal cortex, and ventral tegmental area. These findings demonstrate a role of mesolimbic dopamine in the interaction between natural and drug rewards, and identify mesolimbic dopamine as a key mediator of changes in vulnerability for drug use after loss of natural reward.

Melanocortin 3 Receptor Signaling in Midbrain Dopamine Neurons Increases the Motivation for Food Reward.

The central melanocortin (MC) system mediates its effects on food intake via MC3 (MC3R) and MC4 receptors (MC4R). Although the role of MC4R in meal size determination, satiation, food preference, and motivation is well established, the involvement of MC3R in the modulation of food intake has been less explored. Here, we investigated the role of MC3R on the incentive motivation for food, which is a crucial component of feeding behavior. Dopaminergic neurons within the ventral tegmental area (VTA) have a crucial role in the motivation for food. We here report that MC3Rs are expressed on VTA dopaminergic neurons and that pro-opiomelanocortinergic (POMC) neurons in the arcuate nucleus of the hypothalamus (Arc) innervate these VTA dopaminergic neurons. Our findings show that intracerebroventricular or intra-VTA infusion of the selective MC3R agonist γMSH increases responding for sucrose under a progressive ratio schedule of reinforcement, but not free sucrose consumption in rats. Furthermore, ex vivo electrophysiological recordings show increased VTA dopaminergic neuronal activity upon γMSH application. Consistent with a dopamine-mediated effect of γMSH, the increased motivation for sucrose after intra-VTA infusion of γMSH was blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. Taken together, we demonstrate an Arc POMC projection onto VTA dopaminergic neurons that modulates motivation for palatable food via activation of MC3R signaling.

Ocular dominance plasticity disrupts binocular inhibition-excitation matching in visual cortex.

BACKGROUND: To ensure that neuronal networks function in a stable fashion, neurons receive balanced inhibitory and excitatory inputs. In various brain regions, this balance has been found to change temporarily during plasticity. Whether changes in inhibition have an instructive or permissive role in plasticity remains unclear. Several studies have addressed this question using ocular dominance plasticity in the visual cortex as a model, but so far, it remains controversial whether changes in inhibition drive this form of plasticity by directly affecting eye-specific responses or through increasing the plasticity potential of excitatory connections. RESULTS: We tested how three major classes of interneurons affect eye-specific responses in normally reared or monocularly deprived mice by optogenetically suppressing their activity. We find that in contrast to somatostatin-expressing or vasoactive intestinal polypeptide-expressing interneurons, parvalbumin (PV)-expressing interneurons strongly inhibit visual responses. In individual neurons of normal mice, inhibition and excitation driven by either eye are balanced, and suppressing PV interneurons does not alter ocular preference. Monocular deprivation disrupts the binocular balance of inhibition and excitation in individual neurons, causing suppression of PV interneurons to change their ocular preference. Importantly, however, these changes do not consistently favor responses to one of the eyes at the population level. CONCLUSIONS: Monocular deprivation disrupts the binocular balance of inhibition and excitation of individual cells. This disbalance does not affect the overall expression of ocular dominance. Our data therefore support a permissive rather than an instructive role of inhibition in ocular dominance plasticity.

Accelerated age-related cognitive decline and neurodegeneration, caused by deficient DNA repair.

Age-related cognitive decline and neurodegenerative diseases are a growing challenge for our societies with their aging populations. Accumulation of DNA damage has been proposed to contribute to these impairments, but direct proof that DNA damage results in impaired neuronal plasticity and memory is lacking. Here we take advantage of Ercc1(Δ/-) mutant mice, which are impaired in DNA nucleotide excision repair, interstrand crosslink repair, and double-strand break repair. We show that these mice exhibit an age-dependent decrease in neuronal plasticity and progressive neuronal pathology, suggestive of neurodegenerative processes. A similar phenotype is observed in mice where the mutation is restricted to excitatory forebrain neurons. Moreover, these neuron-specific mutants develop a learning impairment. Together, these results suggest a causal relationship between unrepaired, accumulating DNA damage, and age-dependent cognitive decline and neurodegeneration. Hence, accumulated DNA damage could therefore be an important factor in the onset and progression of age-related cognitive decline and neurodegenerative diseases.

Up-regulation of GLT-1 severely impairs LTD at mossy fibre--CA3 synapses.

Glutamate transporters are responsible for clearing synaptically released glutamate from the extracellular space. By this action, they maintain low levels of ambient glutamate, thus preventing excitotoxic damage, and contribute to shaping synaptic currents. We show that up-regulation of the glutamate transporter GLT-1 by ceftriaxone severely impaired mGluR-dependent long-term depression (LTD), induced at rat mossy fibre (MF)-CA3 synapses by repetitive stimulation of afferent fibres. This effect involved GLT-1, since LTD was rescued by the selective GLT-1 antagonist dihydrokainate (DHK). DHK per se produced a modest decrease in fEPSP amplitude that rapidly regained control levels after DHK wash out. Moreover, the degree of fEPSP inhibition induced by the low-affinity glutamate receptor antagonist gamma-DGG was similar during basal synaptic transmission but not during LTD, indicating that in ceftriaxone-treated rats LTD induction did not alter synaptic glutamate transient concentration. Furthermore, ceftriaxone-induced GLT-1 up-regulation significantly reduced the magnitude of LTP at MF-CA3 synapses but not at Schaffer collateral-CA1 synapses. Postembedding immunogold studies in rats showed an increased density of gold particles coding for GLT-1a in astrocytic processes and in mossy fibre terminals; in the latter, gold particles were located near and within the active zones. In both CEF-treated and untreated GLT-1 KO mice used for verifying the specificity of immunostaining, the density of gold particles in MF terminals was comparable to background levels. The enhanced expression of GLT-1 at release sites may prevent activation of presynaptic receptors, thus revealing a novel mechanism by which GLT-1 regulates synaptic plasticity in the hippocampus.

Morphine dependence increases the response to a brief pentylenetetrazol administration in rat hippocampal CA1 in vitro.

PURPOSE: Herein we used electrophysiologic approaches in hippocampal area CA1 to estimate how morphine treatment alters the pentylenetetrazol (PTZ) effects. METHODS: Hippocampal slices taken from either control animals or animals made dependent via chronic morphine administration were examined. Changes in the population spike and epileptiform amplitudes were used as indices to quantify the effects of PTZ exposure in the control and morphine-dependent slices. Hippocampal slices taken either from control animals or from animals made dependent upon morphine were exposed to PTZ, either with or without morphine, naloxone, or morphine + naloxone. RESULTS: Morphine dependence increased a PTZ-induced long-term potentiation (LTP) of the population spike in CA1 in vitro. This LTP was not found in rats that had spontaneously withdrawn morphine or withdrawn with naloxone in vivo after chronic morphine intake. Morphine or naloxone in vitro blocked the PTZ-induced LTP changes in control and morphine-dependent slices. However, PTZ-induced multiple population spikes (epileptiform activity) in CA1 was not blocked by naloxone. DISCUSSION: It is concluded that dependence on morphine enhances PTZ-induced plastic and epileptic changes in CA1 excitability. We suggest that this model of neuronal activity in dependent slices could present an opportunity for studying the mechanisms underlying the increased likelihood of seizures in morphine users.

GAT-1 regulates both tonic and phasic GABA(A) receptor-mediated inhibition in the cerebral cortex.

gamma-Aminobutyric acid 1 (GAT-1) is the most copiously expressed GABA transporter; we studied its role in phasic and tonic inhibition in the neocortex using GAT-1 knockout (KO) mice. Immunoblotting and immunocytochemical studies showed that GAT-2 and GAT-3 levels in KOs were unchanged and that GAT-3 was not redistributed in KOs. Moreover, the expression of GAD65/67 was increased, whereas that of GABA or VGAT was unchanged. Microdialysis studies showed that in KOs spontaneous extracellular release of GABA and glutamate was comparable in WT and KO mice, whereas KCl-evoked output of GABA, but not of glutamate, was significantly increased in KOs. Recordings from layer II/III pyramids revealed a significant increase in GABA(A)R-mediated tonic conductance in KO mice. The frequency, amplitude and kinetics of spontaneous inhibitory post-synaptic currents (IPSCs) were unchanged, whereas the decay time of evoked IPSCs was significantly prolonged in KO mice. In KO mice, high frequency stimulation of GABAergic terminals induced large GABA(A)R-mediated inward currents associated with a reduction in amplitude and decay time of IPSCs evoked immediately after the train. The recovery process was slower in KO than in WT mice. These studies show that in the cerebral cortex of GAT-1 KO mice GAT-3 is not redistributed and GADs are adaptively changed and indicate that GAT-1 has a prominent role in both tonic and phasic GABA(A)R-mediated inhibition, in particular during sustained neuronal activity.

In the developing rat hippocampus a tonic GABAA-mediated conductance selectively enhances the glutamatergic drive of principal cells.

In the adult hippocampus, two different forms of GABA(A) receptor-mediated inhibition have been identified: phasic and tonic. The first is due to the activation of GABA(A) receptors facing the presynaptic releasing sites, whereas the second is due to the activation of receptors localized away from the synapses. Because of their high affinity and low desensitization rate, extrasynaptic receptors are persistently able to sense low concentrations of GABA. Here we show that, early in postnatal life, between postnatal day (P) 2 and P6, CA1 and CA3 pyramidal cells but not stratum radiatum interneurons, express a tonic GABA(A)-mediated conductance. Block of the neuronal GABA transporter GAT-1 slightly enhanced the persistent GABA conductance in principal cells but not in GABAergic interneurons. However, in adulthood, a tonic GABA(A)-mediated conductance could be revealed in stratum radiatum interneurons, indicating that the ability of these cells to sense ambient GABA levels is developmentally regulated. Pharmacological analysis of the tonic conductance in principal cells demonstrated the involvement of beta2/beta 3, alpha 5 and gamma 2 GABA(A) receptor subunits. Removal of the tonic depolarizing action of GABA with picrotoxin, reduced the excitability and the glutamatergic drive of principal cells but did not modify the excitability of stratum radiatum interneurons. The increased cell excitability and synaptic activity following the activation of extrasynaptic GABA(A) receptors by ambient GABA would facilitate the induction of giant depolarizing potentials.

Reversal of pentylenetetrazol-induced potentiation phenomenon by theta pulse stimulation in the CA1 region of rat hippocampal slices.

The effect of theta pulse stimulation (TPS) on pentylenetetrazol (PTZ)-induced long-term potentiation of population spikes was studied in the CA1 region of rat hippocampal slices. The field excitatory postsynaptic potential (fEPSP) and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, following stimulation of Schaffer collaterals. A transient PTZ application produced a long-lasting enhancement of PS amplitude. A 3-min episode of TPS delivered at test-pulse intensity failed to reverse the PTZ potentiation. However, the same stimulation at a higher intensity produced complete reversal of the PTZ potentiation when delivered during the last minutes of PTZ application. Prior application of high-intensity TPS also decreased the amount of PTZ potentiation, whereas it had no long-lasting effect on baseline synaptic responses. High-intensity TPS induced reversal was blocked by adenosine A1 receptor antagonist and, furthermore, was reduced by protein phosphatase 1 inhibitor. The results suggest that mechanism of PTZ-induced LTP reversal involves activation of adenosine receptors and protein phosphatases.

Contribution of ionotropic glutamate receptors and voltage-dependent calcium channels to the potentiation phenomenon induced by transient pentylenetetrazol in the CA1 region of rat hippocampal slices.

The role of ionotropic glutamate receptors and voltage-dependent calcium channels (VDCCs) in potentiation phenomenon and epileptic activity induced by a transient pentylenetetrazol (PTZ) application in the CA1 region of rat hippocampal slices was investigated. Also we examined whether adenosine as an inhibitory neuromodulator would interact with expression of the long-lasting effect of transient PTZ. Population spikes (PS) were recorded in the CA1 cell body layer of the hippocampal slices following stratum radiatum stimulation. Changes in the PS amplitude potentiation and number of extra PS, which induced by transient PTZ were used as indices to quantify the effects of drugs. PS input-output curve was significantly increased 10 min after PTZ application and persisted at least for 60 min after PTZ washout. Polyspikes also appeared, but did not persist. Both ketamine and APV reduced the extent of potentiation of PS amplitude but had no effect on number of extra PS. The selective non-NMDA receptor antagonist CNQX prevented the amplitude potentiation and the generation of extra PS. The blocker of VDCCs, verapamil, prevented the amplitude potentiation and inhibited polyspike activity. Co-application of adenosine and PTZ produced a rapid and reversible decrease in the PS amplitude, but PTZ-induced potentiation phenomenon was observed after washout. It is concluded that ionotropic glutamate receptors as well as VDCCs involve in the PTZ-induced LTP of PS amplitude. PTZ-induced LTP is also insensitive to adenosine. The epileptiform activity induced by a transient PTZ application could be attributed to VDCCs. The polyspikes mediated by VDCCs are dependent on prior activation of AMPA receptors.