Infusion Of AP5 Research Articles

RF01 | PMON45 Urocortin 3 Interaction with GABA and Glutamate in the Posterodorsal Subnucleus of The Medial Amygdala Mediates Suppression of GnRH Pulse Generator Frequency in Female Mice

Abstract Stress exerts a profound impact on reproductive function and disrupts pulsatile luteinizing hormone (LH) secretion. The posterodorsal subnucleus of the medial amygdala (MePD) is an upstream modulator of the hypothalamus pituitary gonadal axis. Stress alters neuronal activity within the MePD, increasing the expression of Urocortin3 (Ucn3) while enhancing the inhibitory output from the MePD to key hypothalamic reproductive centres. Designer receptor exclusively activated by designer drugs (DREADDs) inhibition of MePD Ucn3 neurons prevents psychological stress-induced suppression of LH pulses in female mice. The neurochemical pathways in the MePD responsible for regulating the GnRH pulse generator in the presence of stress remain unknown. We investigate the neurotransmission involved in the neural circuitry responsible for the suppression of GnRH pulse generator activity by MePD Ucn3 neuronal activation. Ucn3-Cre-tdTomato female ovariectomised (OVX) mice were unilaterally injected with AAV-ChR2 and implanted with optofluid cannulae targeting the MePD. Firstly, we optically stimulated MePD Ucn3 neurons with blue light at 10 Hz, 10 mW and monitored the effect on LH pulses. Next, we combined optogenetic activation of MePD Ucn3 neurons with pharmacological antagonism of GABAA or GABAB receptors with bicuculline or CGP, respectively, and observed the effect on LH pulsatility. Finally, during optical activation of MePD Ucn3 neurons we administered a combination of NMDA and AMPA receptor antagonists, AP5 and CNQX respectively, and monitored the effect on pulsatile LH secretion. Optogenetic stimulation of MePD Ucn3 neurons suppressed pulsatile LH secretion compared to controls and administration of aCSF had no effect on LH pulse interval (MePD Ucn3 activation: 36.04 ± 2.49 vs. pre-treatment control period: 17.50 ± 0.67 vs control virus: 21.67 ± 1.67 min; mean ± SEM; p<0.0001; n=12). Intra-MePD infusion of bicuculline (BIC) or CGP during optogenetic stimulation of MePD Ucn3 neurons completely blocked the suppressive effect of MePD Ucn3 neuronal activation on LH pulsatility (MePD Ucn3 activation and BIC: 20.50 ± 2.03 vs. pre-treatment control period: 17.58 ± 0.75 min; MePD Ucn3 activation and CGP: 16.30 ± 0.63 vs. pre-treatment control period: 17.50 ± 0.77 min; n=7-10). Intra-MePD infusion of AP5 + CNQX during optogenetic stimulation of MePD Ucn3 neurons similarly completely blocked the suppressive effect of MePD Ucn3 neuronal activation on LH pulsatility (MePD Ucn3 activation and AP5 + CNQX: 19.58 ± 1.42 min; n=6). Our findings show for the first time that optogenetic stimulation of MePD Ucn3 neurons inhibits GnRH pulse generator activity via GABA and glutamate signalling within the MePD. Presentation: Saturday, June 11, 2022 1:12 p.m. - 1:17 p.m., Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

The dorsal subiculum is not necessary for step-through inhibitory avoidance acquisition and consolidation in rats.

The dorsal subiculum (DSub) has reciprocal connections with the dorsal hippocampus, and these regions play a role in spatial representation in contextual fear conditioning (CFC). Recently, we used AP5 and muscimol infusions to show that the DSub is required for CFC consolidation. The CFC component can be present in other learning tasks, such as step-through inhibitory avoidance (ST IA), which requires the dorsal hippocampus for acquisition and consolidation. This suggests that the DSub may be also involved in ST IA if the CFC component of the protocol is strong enough. Therefore, this study tested whether the DSub participates in ST IA acquisition and consolidation in male Wistar rats. Our data showed that pre-or posttraining infusions of AP5 or muscimol into the DSub did not affect ST IA acquisition and consolidation. We discuss the present results in relation to our previous findings, which showed the involvement of the DSub in CFC consolidation, and highlight some reasons that may explain the divergent results between the tasks. First, we note the possibility to escape from the unconditioned stimulus that occurs in ST IA, but not in CFC. We also suggest that the instrumental component of ST IA seems to be more prominent than the CFC one. Finally, we consider the possible influence of aspects of anxiety present in the ST IA, but not in CFC. These possible interpretations provide a broad framework in respect of the present results and raise new questions that demand further studies exploring the DSub function in inhibitory avoidance. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

NMDA Receptors in Visual and Olfactory Sensory Integration in Male Long Evans Rats: A Role for the Orbitofrontal Cortex

Sensory integration (SI) is a cognitive process whereby the brain uses unimodal or multimodal sensory features to create a comprehensive representation of the environment. Integration of sensory input is necessary to achieve a coherent perception of the environment, and to subsequently plan and coordinate action. The neural mechanisms mediating SI are poorly understood; however, recent studies suggest that the regulation of SI involves N-methyl-d-aspartate receptors (NMDARs) in orbitofrontal cortex (OFC). Thus, we tested this hypothesis directly in two experiments using object oddity tests that require SI for visual and olfactory stimuli. First, we blocked NMDARs with acute CPP treatment (i.p., 10 mg/kg) and tested rats in unimodal visual and olfactory SI tests, and respective control unimodal oddity tests that do not require SI. Second, we used intra-OFC infusions of AP5 (30 mM) to examine the role of NMDARs in the OFC in the oddity tests requiring SI. Systemic blockade of NMDARs impaired performance on the visual tests regardless of whether SI was required for determining oddity. In the olfactory tests, systemic treatment with CPP impaired the test requiring SI while sparing olfactory oddity, demonstrating a selective impairment in the olfactory SI. Intra-OFC blockade of NMDARs impaired olfactory SI, without effect on visual SI, demonstrating that intra-OFC NMDARs are essential for olfactory, but not visual SI. The present results are discussed in the context of the function of the OFC and its associated circuitry.

The dorsal subiculum is required for contextual fear conditioning consolidation in rats

The hippocampal formation has a well-known role in contextual fear conditioning. The dorsal subiculum connects the hippocampus to the entorhinal cortex through pathways that seemingly rely on NMDA-dependent synaptic plasticity. The role of the dorsal subiculum in contextual fear conditioning retrieval, but not acquisition, has been previously reported. However, most of the critical biological phenomena involved in memory formation occur in the consolidation phase. The present study aimed to assess the effects of intra-dorsal subiculum muscimol or AP5 infusion on contextual fear conditioning consolidation. Our data show that dorsal subiculum integrity, as well as NMDA transmission in this region, seem to be necessary for contextual fear conditioning consolidation.

Methamphetamine-induced enhancement of hippocampal long-term potentiation is modulated by NMDA and GABA receptors in the shell-accumbens.

Addictive drugs modulate synaptic transmission in the meso-corticolimbic system by hijacking normal adaptive forms of experience-dependent synaptic plasticity. Psychostimulants such as METH have been shown to affect hippocampal synaptic plasticity, albeit with a less understood synaptic mechanism. METH is one of the most addictive drugs that elicit long-term alterations in the synaptic plasticity in brain areas involved in reinforcement learning and reward processing. Dopamine transporter (DAT) is one of the main targets of METH. As a substrate for DAT, METH decreases dopamine uptake and increases dopamine efflux via the transporter in the target brain regions such as nucleus accumbens (NAc) and hippocampus. Due to cross talk between NAc and hippocampus, stimulation of NAc has been shown to alter hippocampal plasticity. In this study, we tested the hypothesis that manipulation of glutamatergic and GABA-ergic systems in the shell-NAc modulates METH-induced enhancement of long term potentiation (LTP) in the hippocampus. Rats treated with METH (four injections of 5 mg/kg) exhibited enhanced LTP as compared to saline-treated animals. Intra-NAc infusion of muscimol (GABA receptor agonist) decreased METH-induced enhancement of dentate gyrus (DG)-LTP, while infusion of AP5 (NMDA receptor antagonist) prevented METH-induced enhancement of LTP. These data support the interpretation that reducing NAc activity can ameliorate METH-induced hippocampal LTP through a hippocampus-NAc-VTA circuit loop. Synapse 70:325-335, 2016. © 2016 Wiley Periodicals, Inc.

Hippocampal NMDA receptors are involved in rats׳ spontaneous object recognition only under high memory load condition

The possible involvement of hippocampal N-methyl-d-aspartate (NMDA) receptors in spontaneous object recognition was investigated in rats under different memory load conditions. We first estimated rats׳ object memory span using 3–5 objects in “Different Objects Task (DOT)” in order to confirm the highest memory load condition in object recognition memory. Rats were allowed to explore a field in which 3 (3-DOT), 4 (4-DOT), or 5 (5-DOT) different objects were presented. After a delay period, they were placed again in the same field in which one of the sample objects was replaced by another object, and their object exploration behavior was analyzed. Rats could differentiate the novel object from the familiar ones in 3-DOT and 4-DOT but not in 5-DOT, suggesting that rats׳ object memory span was about 4. Then, we examined the effects of hippocampal AP5 infusion on performance in both 2-DOT (2 different objects were used) and 4-DOT. The drug treatment before the sample phase impaired performance only in 4-DOT. These results suggest that hippocampal NMDA receptors play a critical role in spontaneous object recognition only when the memory load is high.

Distinct effects of ventral tegmental area NMDA and acetylcholine receptor blockade on conditioned reinforcement produced by food-associated cues

Stimuli paired with rewards acquire reinforcing properties to promote reward-seeking behavior. Previous work supports the role of ventral tegmental area (VTA) nicotinic acetylcholine receptors (nAChRs) in mediating conditioned reinforcement elicited by drug-associated cues. However, it is not known whether these cholinergic mechanisms are specific to drug-associated cues or whether VTA cholinergic mechanisms also underlie the ability of cues paired with natural rewards to act as conditioned reinforcers. Burst firing of VTA dopamine (DA) neurons and the subsequent phasic DA release in the nucleus accumbens (NAc) plays an important role in cue-mediated behavior and in the ability of cues to acquire reinforcing properties. In the VTA, both AChRs and N-methyl-d-aspartate receptors (NMDARs) regulate DA burst firing and phasic DA release. Here, we tested the role of VTA nAChRs, muscarinic AChRs (mAChRs), and NMDARs in the conditioned reinforcement elicited by a food-associated, natural reward cue. Subjects received 10 consecutive days of Pavlovian conditioning training where lever extension served as a predictive cue for food availability. On day 11, rats received bilateral VTA infusion of saline, AP-5 (0.1 or 1μg), mecamylamine (MEC: 3 or 30μg) or scopolamine (SCOP: 3 or 66.7μg) immediately prior to the conditioned reinforcement test. During the test, nosepoking into the active (conditioned reinforced, CR) noseport produced a lever cue while nosepoking on the inactive (non-conditioned reinforced, NCR) noseport had no consequence. AP-5 robustly attenuated conditioned reinforcement and blocked discrimination between CR and NCR noseports at the 1-μg dose. MEC infusion decreased responding for both CR and NCR while 66.7-μg SCOP disrupted the subject’s ability to discriminate between CR and NCR. Together, our data suggest that VTA NMDARs and mAChRs, but not nAChRs, play a role in the ability of natural reward-associated cues to act as conditioned reinforcers.

N-methyl-D-aspartate receptor-mediated glutamate transmission in nucleus accumbens plays a more important role than that in dorsal striatum in cognitive flexibility.

Cognitive flexibility is a critical ability for adapting to an ever-changing environment in humans and animals. Deficits in cognitive flexibility are observed in most schizophrenia patients. Previous studies reported that the medial prefrontal cortex-to-ventral striatum and orbital frontal cortex-to-dorsal striatum circuits play important roles in extra- and intra-dimensional strategy switching, respectively. However, the precise function of striatal subregions in flexible behaviors is still unclear. N-methyl-D-aspartate receptors (NMDARs) are major glutamate receptors in the striatum that receive glutamatergic projections from the frontal cortex. The membrane insertion of Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs) depends on NMDAR activation and is required in learning and memory processes. In the present study, we measured set-shifting and reversal learning performance in operant chambers in rats and assessed the effects of blocking NMDARs and Ca2+-permeable AMPARs in striatal subregions on behavioral flexibility. The blockade of NMDARs in the nucleus accumbens (NAc) core by AP5 impaired set-shifting ability by causing a failure to modify prior learning. The suppression of NMDAR-mediated transmission in the NAc shell induced a deficit in set-shifting by disrupting the learning and maintenance of novel strategies. During reversal learning, infusions of AP5 into the NAc shell and core impaired the ability to learn and maintain new strategies. However, behavioral flexibility was not significantly affected by blocking NMDARs in the dorsal striatum. We also found that the blockade of Ca2+-permeable AMPARs by NASPM in any subregion of the striatum did not affect strategy switching. These findings suggest that NMDAR-mediated glutamate transmission in the NAc contributes more to cognitive execution compared with the dorsal striatum.

Object-in-Place Associative Recognition Memory Depends on Glutamate Receptor Neurotransmission Within Two Defined Hippocampal-Cortical Circuits: A Critical Role for AMPA and NMDA Receptors in the Hippocampus, Perirhinal, and Prefrontal Cortices

Object-in-place associative recognition memory depends on an interaction between the hippocampus (HPC), perirhinal (PRH), and medial prefrontal (mPFC) cortices, yet the contribution of glutamate receptor neurotransmission to these interactions is unknown. NMDA receptors (NMDAR) in the HPC were critical for encoding of object-in-place memory but not for single-item object recognition. Next, a disconnection procedure was used to examine the importance of “concurrent” glutamate neurotransmission in the HPC-mPFC and HPC-PRH. Contralateral unilateral infusions of NBQX (AMPAR antagonist), into the HPC-mPFC, or HPC-PRH, either before acquisition or test, impaired object-in-place performance. Thus, both circuits are necessary for encoding and retrieval. Crossed unilateral AP5 (NMDAR antagonist) infusions into the HPC-mPFC or HPC-PRH impaired encoding, but not retrieval. Specifically crossed HPC-mPFC infusions impaired both short-term (5 min) and longer term (1 h) memory while HPC-PRH infusions impaired longer term memory only. This delay-dependent effect of AP5 in the HPC-PRH on object-in-place memory, accords with its effects in the PRH, on single item object recognition memory, thereby suggesting that a single PRH synaptic plasticity mechanism underpins different recognition memory processes. Further, blocking excitatory neurotransmission in any pair of structures within the networks impaired “both” encoding and retrieval, thus object-in-place memory clearly requires network interdependency across multiple structures.

Differential role of ventral tegmental area acetylcholine and N-methyl-d-aspartate receptors in cocaine-seeking

Exposure to drug-associated cues evokes drug-seeking behavior and is regarded as a major cause of relapse. Cues evoke burst firing of ventral tegmental area (VTA) dopamine (DA) neurons and phasic DA release in the nucleus accumbens (NAc). Cholinergic and glutamatergic input to the VTA is suggested to gate phasic DA activity. However, the role of VTA cholinergic and glutamatergic receptors in regulating phasic dopamine release and cue-induced drug-seeking in cocaine experienced subjects is not known. In male Sprague–Dawley rats, we found that VTA inactivation strongly inhibited, while VTA stimulation promoted, cocaine-seeking behavior during early withdrawal. Blockade of phasic activated D1 receptors in the NAc core also strongly inhibited cue-induced cocaine-seeking – suggesting an important role of phasic DA activity in the VTA to NAc core circuit. Next, we examined the role of VTA acetylcholine receptors (AChRs) and N-methyl-d-aspartate receptors (NMDARs) in regulating both NAc core phasic DA release and cue-induced cocaine-seeking. In cocaine naïve subjects, VTA infusion of the nicotinic acetylcholine receptor (AChR) antagonist mecamylamine, the muscarinic AChR antagonist scopolamine, or the NMDAR antagonist AP-5, led to robust attenuation of phasic DA release in the NAc core. During early cocaine withdrawal, VTA infusion of AP-5 had limited effects on NAc phasic DA release and cue-induced cocaine-seeking while VTA infusion of mecamylamine or scopolamine robustly inhibited both phasic DA release and cocaine-seeking. The results demonstrate that VTA AChRs, but not NMDARs, strongly regulate cue-induced cocaine-seeking and phasic DA release during early cocaine withdrawal.

Blockade of the NMDA and AMPA/kainate receptors in the dorsal raphe nucleus prevents the 5-HT3 receptor agonist m-chlorophenylbiguanide-induced suppression of REM sleep in the rat

The effects of the selective 5-HT(3) receptor agonist m-chlorophenylbiguanide (m-CPBG), and of the NMDA (N-methyl-D-aspartate) and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate)/kainate antagonists AP-5 [(±)-2-amino-5-phosphono-pentanoic acid] and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), respectively, were studied in adult male Wistar rats implanted for chronic sleep recordings. The compounds were microinjected directly into the dorsal raphe nucleus (DRN) during the light period of the 12-h light/12-h dark cycle. Infusion of m-CPBG (2 and 4mM) into the DRN induced a significant reduction of rapid-eye-movement sleep (REMS) and of the number of REM periods. Local infusion of AP-5 (0.5-1 mM) and CNQX (2 mM) significantly increased slow wave sleep (SWS). Pretreatment with AP-5 (0.5 mM) or CNQX (0.5 mM) antagonized the m-CPBG-induced suppression of REMS. It is proposed that the reduction of REMS after microinjection of m-CPBG into de DRN is related to the activation of glutamatergic interneurons that express the 5-HT(3) receptor and make synaptic contacts with serotonergic cells. The resultant increase of serotonin release at postsynaptic sites involved in the induction of REMS would provoke the suppression of the behavioral state. Our findings provide, in addition, new details concerning the pharmacology of DRN serotonergic neurons in the rat that may become relevant to the development of drugs for enhancing cortical and subcortical serotonergic neurotransmission.

Effects of disconnection of amygdala dopamine and nucleus accumbensN-methyl-d-aspartate receptors on ethanol-seeking behavior in mice

There is a strong interest in harnessing the genetic manipulations that are possible in mice to investigate the functional neural mechanisms modulating the associative processes that control drug-seeking behavior. However, it is unknown whether intracranial techniques, such as the disconnection procedure commonly used in rats to examine serial connectivity between implicated areas, can be successfully applied to mice. We have previously demonstrated that the expression of ethanol-seeking behavior in mice is dependent upon amygdala (Amy) dopamine and nucleus accumbens (Acb) N-methyl-d-aspartate (NMDA) receptor activation (Gremel & Cunningham, 2009). Here, we used a neuropharmacological disconnection procedure to investigate whether dopamine activation of the Amy directly leading to increases in Acb glutamate release and binding of NMDA receptors modulates the expression of ethanol-seeking behavior. Immediately before testing the expression of an ethanol-induced conditioned place preference, mice were given an Amy infusion of flupenthixol and either an ipsilateral or contralateral Acb infusion of AP-5. Although both ipsilateral and contralateral manipulations reduced the expression of ethanol conditioned place preference, in a separate experiment we demonstrated that a unilateral Acb infusion of AP-5, but not Amy flupenthixol, is sufficient to disrupt preference. The finding of a significant blockade by unilateral AP-5 into the Acb precludes any conclusions about a unique role for the Amy/Acb neuroanatomical connection in this model of ethanol-seeking behavior. Further, the current results suggest potential limitations in transferring techniques from rats to mice in order to study serial interactions between neural areas underlying motivated behaviors. Nevertheless, these findings provide evidence showing that Acb NMDA receptors play an important role in the expression of ethanol-conditioned behavior.

Synaptic Overflow of Dopamine in the Nucleus Accumbens Arises from Neuronal Activity in the Ventral Tegmental Area

Dopamine concentrations fluctuate on a subsecond time scale in the nucleus accumbens (NAc) of awake rats. These transients occur in resting animals, are more frequent following administration of drugs of abuse, and become time-locked to cues predicting reward. Despite their importance in various behaviors, the origin of these signals has not been demonstrated. Here we show that dopamine transients are evoked by neural activity in the ventral tegmental area (VTA), a brain region containing dopaminergic cell bodies. The frequency of naturally occurring dopamine transients in a resting, awake animal was reduced by a local VTA microinfusion of either lidocaine or (+/-)2-amino,5-phosphopentanoic acid (AP-5), an NMDA receptor antagonist that attenuates phasic firing. When dopamine increases were pharmacologically evoked by noncontingent administration of cocaine, intra-VTA infusion of lidocaine or AP-5 significantly diminished this effect. Dopamine transients acquired in response to a cue during intracranial self-stimulation were also attenuated by intra-VTA microinfusion of AP-5, and this was accompanied by an increase in latency to lever press. The results from these three distinct experiments directly demonstrate, for the first time, how neuronal firing of dopamine neurons originating in the VTA translates into synaptic overflow in a key terminal region, the NAc shell.

Pretraining does not ameliorate spatial learning deficits induced by intrahippocampal infusion of AP5.

Previous studies show discrepancies concerning the effects of pretraining on spatial learning deficits induced by blockade of the N-methyl-D-aspartate (NMDA) receptor. These inconsistencies might be attributed to the differences in the nature of the pretraining tasks and the method of blocking NMDA receptors. In the present study, the authors pretrained rats in a spatial water maze task. The authors then trained them with a novel spatial task in a novel environment under chronic blockade of hippocampal NMDA receptors by intrahippocampal infusion of 2-amino-5-phosphonopentanoic acid (AP5) using osmotic pumps. Although the rats had acquired the basic techniques needed to solve a water-maze spatial task during pretraining, those given high or low doses of AP5 showed acquisition deficits. As the spatial pretraining failed to ameliorate the acquisition deficits of a new task in a novel environment, it was suggested that NMDA receptors were necessary in forming spatial representations. Because neither dose of AP5 affected the performance of a spatial task in the retention phase, sensory motor disturbances could not have caused these deficits.

Medial prefrontal cortex infusions of bupivacaine or AP-5 block extinction of amphetamine conditioned place preference

The present experiments used reversible lesion techniques and intra-mPFC infusions of the n-methyl d-aspartate (NMDA) receptor antagonist d, l-2-amino-5-phosphonovaleric acid (AP-5) to examine the role of the mPFC in extinction of an amphetamine conditioned place preference (CPP). Following initial training and testing for an amphetamine (2 mg/kg) CPP, adult male Long–Evans rats were given extinction trials that were identical to training, except in the absence of peripheral amphetamine injections. Immediately prior to each extinction trial, rats received intra-mPFC infusions of the anesthetic drug bupivacaine (0.75% solution/0.5 μl), AP-5 (1.25, 2.5, 5.0 μg/0.5 μl), or saline. Following extinction training, rats were given a second CPP test session. Rats receiving intra-mPFC infusions of saline displayed extinction of CPP behavior. In contrast, intra-mPFC infusions of bupivacaine or AP-5 (2.5, 5.0 μg) blocked CPP extinction. The findings indicate (1) the mPFC mediates extinction of approach behavior to drug-associated environmental contexts, and (2) NMDA receptor blockade within the mPFC is sufficient to block extinction of amphetamine CPP behavior.

Distinct ventral and dorsal hippocampus AP5 anxiolytic effects revealed in the elevated plus-maze task in rats

The hippocampal formation (HPC) mediates processes associated with learning, memory, anxiety and fear. The glutamate N-methyl- d-aspartate (NMDA)-receptor subtype is involved in many HPC functional processes related to learning and memory. Although not tested for the HPC, NMDA-receptor antagonists reduced fear and anxiety related responses when applied to other brain regions mediating defensive behaviour. Consequently, this study evaluated the effects of ventral or dorsal HPC application of the NMDA-receptor antagonist, AP5, in rats submitted to the Trial 1/Trial 2 elevated plus-maze (EPM) task. Ventral, but not dorsal, infusions of AP5 (6 and 24 nmol) before EPM Trial 1 increased open arms exploration and reduced risk assessment behavior, suggesting an anxiolytic-like effect. Furthermore, no interference in the avoidance responses was detected during EPM Trial 2 after AP5 infusion into the ventral or dorsal HPC before Trial 1, post-trial 1, or before Trial 2. These data support the notion of differential involvement of ventral HPC, but not dorsal, in mechanisms associated with anxiety and suggest the participation of the glutamatergic transmission, through NMDA receptor, into the ventral HPC in the mediation of defensive behavior.

Septohippocampal properties of N‐methyl‐D‐aspartate‐induced theta‐band oscillation and synchrony

Microinfusion of N-methyl-D-aspartate (NMDA) into apical dendrites of hippocampal CA1 pyramidal cells of urethane-anesthetized rats resulted in long lasting (20-30 min) induction of hippocampal synchrony at the field and cellular level. Power but not frequency of NMDA-induced theta was significantly greater than tail pinch-induced theta activity. This effect was antagonized by intrahippocampal infusion of AP5, but unaffected by i.v. atropine sulfate. During AP5 blockade tail pinch theta frequency and power were significantly reduced. Microinfusion of NMDA into the medial septum also resulted in long lasting induction of hippocampal theta field activity. Contrary to the results of hippocampal NMDA microinfusions, frequency but not power of NMDA-induced theta was significantly greater than tail pinch- induced theta activity. Microinfusion of AP5 into the medial septum significantly lowered power of tail pinch-induced theta but did not affect frequency. Wheel running behavior of rats induced by low levels of electrical stimulation of the posterior hypothalamic nucleus (PH) was completely abolished by microinfusion of AP5 into the medial septum, accompanied by a significant reduction in theta power and frequency. Wheel running and theta were maintained at control levels with high intensity PH stimulation. We propose that: (1) the glutamatergic septohippocampal projection represents a third pathway capable of generating hippocampal field and cellular synchrony, independent of that generated by the septohippocampal cholinergic and GABAergic projections, and (2) the septohippocampal glutamatergic projection serves to function as an interface between cholinergic and GABAergic modulated sensory processing Type 2 theta and movement related Type 1 theta.

The effect of N-methyl- d-aspartate receptor blockade on acetylcholine efflux in the dorsomedial striatum during response reversal learning

Separate experiments found that activation of N-methyl- d-aspartate (NMDA) receptors or increased acetylcholine (ACh) efflux in the rat dorsomedial striatum is critical for learning when conditions require a shift in strategies. Increasing evidence indicates that NMDA receptor activity affects cholinergic efflux in the basal ganglia. The present studies determined whether NMDA receptor blockade in the dorsomedial striatum with dl-2-amino-5-phosphonopentanoic acid (AP-5) affects dorsomedial striatal ACh output in a resting condition, as well as during response reversal learning. Experiment 1 investigated the effects of AP-5 (12.5, 25 or 50 μM) infused into the dorsomedial striatum on ACh output in a resting condition. AP-5 infusion at 25 and 50 μM led to a 20% and 40% decrease in dorsomedial striatal ACh output, respectively. AP-5 (12.5 μM) infusion did not change dorsomedial striatal ACh output from basal levels. Experiment 2 determined whether dorsomedial striatal ACh efflux increases during response reversal learning and whether AP-5, at a dose that does not affect basal levels, modifies response reversal learning and ACh efflux. Following acquisition of a response discrimination, rats had microdialysis probes bilaterally inserted into the dorsomedial striatum prior to the reversal learning test. After baseline samples, rats received a response reversal learning test for 30 min. Control rats rapidly improved in the reversal learning session while simultaneously exhibiting an approximately 40% increase in ACh output compared with baseline levels. AP-5 (12.5 μM) treatment during testing significantly impaired response reversal learning while concomitantly blocking an increase in ACh output. These findings suggest that NMDA receptor activation in the dorsomedial striatum may facilitate a shift in response patterns, in part, by increasing ACh efflux.

NMDA receptor-mediated activation of medullary pro-nociceptive neurons is required for secondary thermal hyperalgesia

There is now direct evidence that a class of neurons in the rostral ventromedial medulla (RVM) exerts a net facilitatory influence on spinal nociception. The present experiments were designed to test whether activation of these neurons, referred to as “on-cells”, is required as part of a positive feedback loop leading to secondary hyperalgesia in acute inflammation produced by topical application of mustard oil. Activity of a characterized RVM neuron and paw withdrawals to heat (plantar surface) were recorded in barbiturate-anesthetized rats. Following three baseline trials, mustard oil was applied to the skin above the knee. Cell activity and paw withdrawal latencies were monitored for an additional 45 min. Application of mustard oil produced an increase in on-cell discharge that was associated with a substantial decrease in withdrawal latency of the ipsilateral paw. Blocking on-cell activation using local infusion of the NMDA-receptor antagonist AP5 into the RVM prevented hyperalgesia. Secondary thermal hyperalgesia following mustard oil was also associated with a significant decrease in the firing of “off-cells”, a cell population thought to exert a net inhibitory influence on nociception. Depression of off-cell firing was unaffected by AP5 microinjection. The firing of “neutral cells”, which have no documented role in nociceptive modulation, was unchanged following mustard oil and also unaffected by AP5 infusion in the RVM. Brainstem descending controls are receiving increasing attention in efforts to understand hyperalgesia and persistent pain states. The present experiments demonstrate that a novel, NMDA-mediated activation of on-cells is required for secondary thermal hyperalgesia in acute inflammation.

Subplate Neurons Regulate Maturation of Cortical Inhibition and Outcome of Ocular Dominance Plasticity

Synaptic plasticity during critical periods of development requires intact inhibitory circuitry. We report that subplate neurons are needed both for maturation of inhibition and for the proper sign of ocular dominance (OD) plasticity. Removal of subplate neurons prevents the developmental upregulation of genes involved in mature, fast GABAergic transmission in cortical layer 4, including GABA receptor subunits and KCC2, and thus prevents the switch to a hyperpolarizing effect of GABA. To understand the implications of these changes, a realistic circuit model was formulated. Simulations predicted that without subplate neurons, monocular deprivation (MD) paradoxically favors LGN axons representing the deprived (less active) eye, exactly what was then observed experimentally. Simulations also account for published results showing that OD plasticity requires mature inhibition. Thus, subplate neurons regulate molecular machinery required to establish an adult balance of excitation and inhibition in layer 4, and thereby influence the outcome of OD plasticity.