Reversible inactivation of the dorsal hippocampus by tetrodotoxin or lidocaine: A comparative study on cerebral functional activity and motor coordination in the rat

Ventral striatum regulates behavioral response to ethanol and MDMA combination

Our previous studies consistently showed that MDMA-induced locomotor hyperactivity is dramatically increased by coadministration of ethanol (EtOH) in rats, indicating possible potentiation of MDMA abuse liability. Thus, we aimed to identify the brain region(s) and neuropharmacological substrates involved in the pharmacodynamics of this potentiation. We first showed that potentiation of locomotor activity by the combination of ip administration of EtOH (1.5 g/kg) and MDMA (6.6 mg/kg) is delay sensitive and maximal when both drugs are injected simultaneously. Then, we used the 2-deoxyglucose quantitative autoradiography technique to assess the impact of EtOH, MDMA, or their combination on local cerebral metabolic rates for glucose (CMRglcs). We showed a specific metabolic activation in the ventral striatum (VS) under MDMA + EtOH versus MDMA or EtOH alone. We next tested if reversible (tetrodotoxin, TTX) or permanent (6-hydrodoxyopamine, 6-OHDA) lesion of the VS could affect locomotor response to MDMA and MDMA + EtOH. Finally, we blocked dopamine D1 or glutamate NMDA receptors in the VS and measured the effects of MDMA and MDMA + EtOH on locomotor activity. We showed that bilateral reversible inactivation (TTX) or permanent lesion (6-OHDA) of the VS prevented the potentiation by EtOH of MDMA-induced locomotor hyperactivity. Likewise, blockade of D1 or NMDA receptors in the VS also reduced the potentiation of MDMA locomotor activity by EtOH. These data indicate that dopamine D1 and glutamate NMDA receptor-driven mechanisms in the VS play a key role in the pharmacodynamics of EtOH-induced potentiation of the locomotor effects of MDMA.

Temporary inactivation of the infralimbic cortex impairs while the blockade of its dopamine D2 receptors enhances auditory fear extinction in rats

Fear extinction is defined as decline in conditioned fear responses that occurs with repeated and non-reinforced exposure to a feared conditioned stimulus. Experimental evidence suggests that the extinction of fear memory requires the integration of the medial prefrontal cortex (mPFC); nevertheless, the role of its sub-regions in regulating the expression and extinction of auditory fear has been rarely addressed in literature. The present study examined the roles of the infra-limbic (IL) and pre-limbic (PL) regions of the mPFC in the expression and extinction of auditory fear by temporally deactivating these regions using lidocaine (10 μg/0.5 μl) before training male Wistar rats in auditory fear-conditioning tasks. The results showed increased freezing levels and impaired extinction through deactivating the IL rather than the PL cortex. Given the role of the dopaminergic pathways in regulating fear memory, this study also investigated the role of D2 receptors located in the IL cortex in fear extinction. Fear extinction was improved in an inverted U-shape pattern through the intra-IL infusion of 15.125, 31.25, 62.5, 125, 250 and 500 ng/0.5 μl of the D2 receptor antagonist sulpiride. In other words, the moderate doses, i.e. 31.25, 62.5, 125, 250 ng/0.5 μl, enhanced auditory fear extinction, whereas the lowest and highest doses, i.e. 15.125 and 500 ng/0.5 μl, were ineffective. These findings demonstrated the key roles of the IL cortex and its dopamine D2 receptors in regulating auditory fear in rats.

Infralimbic Cortex Biases Preference Decision Making for Offspring over Competing Cocaine-Associated Stimuli in New Mother Rats

In the context of drug abuse, converging evidence suggests that cocaine use in new mothers is significantly reduced by the competing motivation related to child rearing. Given the demonstrated importance of the medial prefrontal cortex (mPFC) in decision-making processes, we investigated the contribution of distinct regions of the mPFC [anterior cingulate (Cg1), prelimbic (PrL), infralimbic (IL)] to decision making in new mother rats performing a concurrent pup/cocaine choice conditioned place preference (CPP) task. When given a choice, inactivation of IL cortex significantly biased decision making of mother rats toward cocaine-associated cues, highly contrasting the distribution of preferences by control groups. In contrast, inactivation of PrL cortex had the opposite effect, significantly increasing offspring bias in the decision making, such that none of the mothers chose the cocaine-associated alternative. Cg1 inactivation was without effect. Functional inactivation of these same mPFC subregions had no effect in a non-conflict CPP task in which context-induced cocaine or pup seeking were examined separately. Notably, inactivation of the IL cortex also interfered with maternal behavior. Taken together, we have identified a specific role of the IL cortex in the prioritization of offspring over drug competing alternatives, thus promoting resistance to drug use in new mothers.

A neural circadian signal essential for ovulation is generated in the suprachiasmatic nucleus during each stage of the oestrous cycle

NEW FINDINGS

What is the central question of this study? Is the suprachiasmatic nucleus the structure that generates the neural circadian signals that occur during every stage of the oestrous cycle, not only pro-oestrus, and are these signals essential for proper regulation of ovulation? What is the main finding and its importance? Transient inhibition of Na⁺ -dependent action potentials in the suprachiasmatic nucleus by tetrodotoxin microinjection at 14.00 h inhibits ovulation irrespective of the stage of the oestrous cycle at which the procedure is performed. Microinjection of saline solution into the suprachiasmatic nucleus has a disruptive effect on ovulation that depends on the stage of the cycle at which it is administered.

ABSTRACT

Reproduction is a highly timed process that depends on both the reproductive and circadian systems. The core oscillator of the latter resides at the suprachiasmatic nuclei (SCN) and it is pivotal for the regulation of the pro-oestrus pre-ovulatory surge of gonadotropins in females. There is evidence to suggest that this system may be involved in the regulation of neuroendocrine events that are essential for ovulation and that occur prior to pro-oestrus. We explored this possibility by transiently inactivating the SCN. Female rats were implanted with guide cannulas aimed at the SCN. After recovery of the oestrous cycle, animals were injected with tetrodotoxin (TTX), artificial cerebrospinal fluid (ACSF) or saline solution while freely moving. Injections were performed at 14.00 h of each stage of the oestrous cycle. Animals were killed on the next predicted oestrus day, the number of ova shed was counted and intact rats at oestrus stage were used as absolute control. ACSF did not modify ovulation. Saline solution blocked ovulation in oestrus- and dioestrus-injected rats. Irrespectively of the stage of the oestrous cycle, TTX blocked ovulation. These results lead us to suggest that a neural circadian signal, pivotal for triggering the gonadotropin pre-ovulatory surge, arises from the SCN during the critical window of pro-oestrus. We also suggest that a similar signal, needed for the regulation of other events that are indispensable for proper regulation of ovulation, is also generated in this nucleus during the other stages of the cycle at a similar time.

Dopamine D1 receptor activation maintains motor coordination and balance in rats

Dopamine (DA) modulates motor coordination, and its depletion, as in Parkinson's disease, produces motor impairment. The basal ganglia, cerebellum and cerebral cortex are interconnected, have functional roles in motor coordination, and possess dopamine D₁ receptors (D₁Rs), which are expressed at a particularly high density in the basal ganglia. In this study, we examined whether the activation of D₁Rs modulates motor coordination and balance in the rat using a beam-walking test that has previously been used to detect motor coordination deficits. The systemic administration of the D₁R agonist SKF-38393 at 2, 3, or 4 mg/kg did not alter the beam-walking scores, but the subsequent administration of the D₁R antagonist SCH-23390 at 1 mg/kg did produce deficits in motor coordination, which were reversed by the full agonist SKF-82958. The co-administration of SKF-38393 and SCH-23390 did not alter the beam-walking scores compared with the control group, but significantly prevented the increase in beam-walking scores induced by SCH-23390. The effect of the D₁R agonist to prevent and reverse the effect of the D₁R antagonist in beam-walking scores is an indicator that the function of D₁Rs is necessary to maintain motor coordination and balance in rats. Our results support that D₁Rs mediate the SCH-23390-induced deficit in motor coordination.

Selective Pharmacological Augmentation of Hippocampal Activity Produces a Sustained Antidepressant-Like Response without Abuse-Related or Psychotomimetic Effects

BACKGROUND

Selective augmentation of hippocampal activity in ways similar to that caused by ketamine may have therapeutic advantages over ketamine, which has psychotomimetic and reinforcing effects likely due to effects outside the hippocampus (i.e., off-target effects).

METHODS

Here we evaluated the antidepressant-like response to a negative allosteric modulator of α5 subunit- containing gamma aminobutyric acid subtype A receptors, L-655,708, as these receptors are expressed to a much greater extent in the hippocampus than in other brain areas.

RESULTS

Systemic administration of L-655,708 produced a sustained antidepressant-like effect in the forced swim test that was comparable with that of ketamine and was blocked by hippocampal inactivation with lidocaine. However, in contrast to ketamine, L-655,708 did not affect prepulse inhibition of startle, nor did it maintain responding in rats trained to self-administer i.v. ketamine.

CONCLUSION

Taken together, these findings suggest that activation of the hippocampus by L-655,708 produces an antidepressant-like effect in the absence of any psychotomimetic or abuse-related effects.

Emergence of spatial behavioral function and associated mossy fiber connectivity and c-Fos labeling patterns in the hippocampus of rats

Improvement on spatial tasks is observed during a late, postnatal developmental period (PND18 – PND24).  The purpose of the current work was 1) to determine whether the emergence of spatial-behavioral function was based on the ability to generate appropriate behavioral output; 2) to assess whether mossy fiber connectivity patterns preceded the emergence of spatial-behavioral function; 3) to explore functional changes in the hippocampus to determine whether activity in hippocampal networks occurred in a training-dependent or developmentally-dependent fashion.  To these ends, male, Long Evans rats were trained on a spatial water or dry maze task for one day (PND16, PND18 or PND20) then euthanized.  Training on these 2 tasks with opposing behavioral demands (swimming versus exploration) was hypothesized to control for behavioral topology.  Only at PND20 was there evidence of spatial-behavioral function for both tasks.  Examination of synaptophysin staining in the CA3 region (i.e., mossy fiber projections) revealed enhanced connectivity patterns that preceded the emergence of spatial behavior.  Analysis of c-Fos labeling (functional changes) revealed developmentally-dependent increases in c-Fos positive cells in the dentate gyrus, CA3 and CA1 regions whereas training-dependent increases were noted in the CA3 and CA1 regions for the water-maze trained groups.  Results suggest that changes in mossy fiber connectivity in association with enhanced hippocampal functioning precede the emergence of spatial behavior observed at PND20.  The combination of neuroanatomical and behavioural results confirms the hypothesis that this time represents a sensitive period for hippocampal development and modification and the emergence of spatial/ cognitive function.

Dorsal hippocampus and medial prefrontal cortex each contribute to the retrieval of a recent spatial memory in rats

Systems-level consolidation models propose that recent memories are initially hippocampus-dependent. When remote, they are partially or completely dependent upon the medial prefrontal cortex (mPFC). An implication of the mPFC in recent memory, however, is still debated. Different amounts of muscimol (MSCI 0, 30, 50, 80 and 250 ng in 1 µL PBS) were used to assess the impact of inactivation of the dorsal hippocampus (dHip) or the mPFC (targeting the prelimbic cortex) on a 24-h delayed retrieval of a platform location that rats had learned drug-free in a water maze. The two smallest amounts of MSCI (30 and 50 ng) did not affect recall, whatever the region. 80 ng MSCI infused into the dHip disrupted spatial memory retrieval, as did the larger amount. Infusion of MSCI into the mPFC did not alter performance in the 0-80 ng range. At 250 ng, it induced an as dramatic memory impairment as after efficient dHip inactivation. Stereological quantifications showed that 80 ng MSCI in the dHip and 250 ng MSCI in the mPFC induced a more than 80% reduction of c-Fos expression, suggesting that, beyond the amounts infused, it is the magnitude of the neuronal activity decrease which is determinant as to the functional outcome of the inactivation. Because, based on the literature, even 250 ng MSCI is a small amount, our results point to a contribution of the mPFC to the recall of a recently acquired spatial memory and thereby extend our knowledge about the functions of this major actor of cognition.

The ventral midline thalamus (reuniens and rhomboid nuclei) contributes to the persistence of spatial memory in rats

The formation of enduring declarative-like memories engages a dialog between the hippocampus and the prefrontal cortex (PFC). Electrophysiological and neuroanatomical evidence for reciprocal connections with both of these structures makes the reuniens and rhomboid nuclei (ReRh) of the thalamus a major functional link between the PFC and hippocampus. Using immediate early gene imaging (c-Fos), fiber-sparing excitotoxic lesion, and reversible inactivation in rats, we provide evidence demonstrating a contribution of the ReRh to the persistence of a spatial memory. Intact rats trained in a Morris water maze showed increased c-Fos expression (vs home cage and visible platform groups: >500%) in the ReRh when tested in a probe trial at a 25 d delay, against no change at a 5 d delay; behavioral performance was comparable at both delays. In rats subjected to excitotoxic fiber-sparing NMDA lesions circumscribed to the ReRh, we found normal acquisition of the water-maze task (vs sham-operated controls) and normal probe trial performance at the 5 d delay, but there was no evidence for memory retrieval at the 25 d delay. In rats having learned the water-maze task, lidocaine-induced inactivation of the ReRh right before the probe trial did not alter memory retrieval tested at the 5 d or 25 d delay. Together, these data suggest an implication of the ReRh in the long-term consolidation of a spatial memory at the system level. These nuclei could then be a key structure contributing to the transformation of a new hippocampal-dependent spatial memory into a remote one also depending on cortical networks.

Lidocaine injections targeting CA3 hippocampus impair long-term spatial memory and prevent learning-induced mossy fiber remodeling

Learning a spatial location induces remodeling of the mossy fiber terminal field (MFTF) in the CA3 subfield of the dorsal hippocampus (Ramirez-Amaya et al. (2001) J Neurosci 21:7340-7348; Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007a) Learn Mem 14:416-421). These fibers appear to grow from the stratum lucidum into distal stratum oriens. Is this axonal growth dependent on “repeated and persistent” neural activity in the CA3 region during training? To address this issue, we targeted local inactivation of the MFTF region in a post-training, consolidation paradigm. Male Wistar rats, bilaterally implanted with chronic indwelling cannulae aimed at the MFTF CA3 region, were trained on a hidden platform water maze task (10 trials per day for 5 days). Immediately after the 10th trial on each training day, rats were injected with lidocaine (4% w/v; 171 mM; n=7) or phosphate-buffered saline (PBS; n=7). Behavioral measures of latency, path length, and thigmotaxis were recorded, as was directional heading. A retention test (probe trial) was given 7 days after the last training day, and brains were subsequently processed for MFTF distribution (Timm's stain) and cannula location. Lidocaine treatment was found to block the learning-associated structural remodeling of the MFTF that was reported previously and observed in the PBS-injected controls. During training, the lidocaine group showed elevated latencies and a misdirected heading to locate the platform on the first trial of each training day. On the 7-day retention probe trial, the lidocaine-injected group showed poor retention indicated by the absence of a search bias in the area where the platform had been located during training. These data suggest that the reduction of neuronal activity in the CA3 region impairs long-term storage of spatial information. As this was associated with reduced MFTF structural remodeling, it provides initial anatomical and behavioral evidence for an activity-dependent, presynaptic growth model of memory.

Reevaluating the role of the medial prefrontal cortex in delay eyeblink conditioning

It has been proposed that the medial prefrontal cortex (mPFC) is not necessary for delay eyeblink conditioning (DEC). Here, we investigated the involvement of the mPFC in DEC with a soft or loud tone as the conditioned stimulus (CS) by using electrolytic lesions or muscimol inactivation of guinea pig mPFC. Interestingly, when a soft tone was used as a CS, electrolytic lesions of the mPFC significantly retarded acquisition of the conditioned response (CR), and muscimol infusions into mPFC distinctly inhibited the acquisition and expression of CR, but had no significant effect on consolidation of well-learned CR. In contrast, both electrolytic lesions and muscimol inactivation of mPFC produced no significant deficits in the CR when a loud tone was used as the CS, or in the unconditioned response (UR) when a soft or loud tone was used as the CS. These results demonstrate that the mPFC is essential for the DEC with the soft tone CS but not for the DEC with the loud tone CS.

The ventral hippocampus is necessary for expressing a spatial memory

Current views posit the dorsal hippocampus (DHipp) as contributing to spatial memory processes. Conversely, the ventral hippocampus (VHipp) modulates stress, emotions and affects. Arguments supporting this segregation include differences in (i) connectivity: the DHipp is connected with the entorhinal cortex which receives visuospatial neocortical inputs; the VHipp is connected with both the amygdala and hypothalamus, (ii) electrophysiological characteristics: there is a larger proportion of place cells in the DHipp than in the VHipp, and an increasing dorsoventral gradient in the size of place fields, suggesting less refined spatial coding in the VHipp, and (iii) consequences of lesions: spatial memory is altered after DHipp lesions, less dramatically, sometimes not, after VHipp lesions. Using reversible inactivation, we report in rats, that lidocaine infusions into the DHipp or VHipp right before a probe trial impair retrieval performance in a water-maze task. This impairment was found at two post-acquisition delays compatible with recent memory (1 and 5 days). Pre-training blockade of the VHipp did not prevent task acquisition and drug-free retrieval, on the contrary to pre-training blockade of DHipp, which altered performance in a subsequent drug-free probe trial. Complementary experiments excluded possible locomotor, sensorimotor, motivational or anxiety-related biases from data interpretation. Our conclusion is that a spatial memory can be acquired with the DHipp, less efficiently with the VHipp, and that the retrieval of such a memory and/or the expression of its representation engages the dorsoventral axis of the hippocampus when the task has been learnt with an entirely functional hippocampus.

Spatial memory alterations by activation of septal 5HT 1A receptors: no implication of cholinergic septohippocampal neurons

INTRODUCTION

In rats, activation of medial septum (MS) 5-HT(1A) receptors with the 5-HT(1A)/5-HT(7) receptor agonist 8-OH-DPAT disrupts encoding and consolidation, but not retrieval of a spatial memory in the water maze task. These findings might be explained by an action of 8-OH-DPAT on 5-HT(1A) receptors located on cholinergic neurons which the drug could transiently hyperpolarise. If so, selective damage of these neurons should mimic the effects of 8-OH-DPAT, or, at least, synergistically interfere with them.

METHODS

To test this hypothesis, rats were subjected to intraseptal infusions of 8-OH-DPAT (or phosphate-buffered saline) during acquisition of a water maze task before and/or after 192 IgG-saporin-induced MS cholinergic lesion (vs. sham-operated).

RESULTS

We confirmed that only pre-acquisition intraseptal 8-OH-DPAT infusions prevented learning and subsequent drug-free retrieval of the platform location in intact rats and found that (1) the cholinergic lesion did not prevent recall of the platform location, and (2) the impairing effects of 8-OH-DPAT were similar in sham-operated and lesioned rats, whether naïve or not, to the task before lesion surgery.

CONCLUSIONS

An action of 8-OH-DPAT on only MS cholinergic neurons is not sufficient to account for the drug-induced memory impairments. A concomitant 8-OH-DPAT-induced hyperpolarisation of cholinergic and/or GABAergic and/or glutamatergic neurons (intact rats), or of only GABAergic and/or glutamatergic ones after cholinergic lesion, might be necessary to obliterate task acquisition, confirming that, in the MS, (1) the three neuronal populations could cooperate to process hippocampal-dependent information, and (2) non-cholinergic septohippocampal neurons might be more important than cholinergic ones in serotonin-induced modulation of hippocampus-dependent memory processing.

The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats

Converging evidence suggests that the motivation to seek cocaine during the postpartum period is significantly impacted by the competing incentives of offspring, a stimulus unique to this life stage. In the present study, the functional role of the medial preoptic area (mPOA), a critical site involved in maternal responsiveness, on processing incentive value of pup-associated cues and influencing response allocation for pup- over cocaine-associated environments was investigated using a concurrent pup/cocaine choice conditioned place preference (CPP) paradigm. Early postpartum females with bilateral guide cannulae aimed into the mPOA or into anatomical control sites were conditioned, from postpartum days (PPD) 4 to 7, to associate different uniquely featured environments with pups or cocaine. CPP was tested on PPD8 following intra-mPOA infusions of either 2% bupivacaine or saline vehicle. In two additional experiments, the effects of intra-mPOA infusions of bupivacaine on expression of conditioned responding induced by environments associated with either pups or cocaine were examined separately. Transient inactivation of the mPOA selectively blocked the conditioned preferences for pup-associated environments, significantly contrasting the robust pup-CPP found in non-surgical and intra-mPOA vehicle-treated females. In contrast, mPOA inactivation failed to alter cocaine-CPP in postpartum females. When given a choice between environments associated with pups or cocaine, transient functional inactivation of the mPOA altered choice behavior, biasing the preference of females toward cocaine-associated environments, such that almost all preferred cocaine- and none the pup-associated option. The anatomical specificity was revealed when inactivation of adjacent regions to the mPOA did not affect CPP responses for pups. The findings support a critical role for the mPOA in mediating pup-seeking behavior, and further suggest that the competing properties of pups over alternative incentives, including drugs of abuse, rely on mPOA integrity to provide relevant pup-related information to the circuitry underlying the choice behavior between pups and alternative stimuli.

The changing role of the medial preoptic area in the regulation of maternal behavior across the postpartum period: facilitation followed by inhibition

Maternal behavior in rats undergoes considerable plasticity in parallel to the developmental stage of the pups, resulting in distinct patterns of maternal behavior and care at different postpartum time points. The medial preoptic area (mPOA) of the hypothalamus is one critical neural substrate underlying the onset and early expression of maternal behavior in rats but little is known about its specific functional role in the evolving expression of maternal behavior across the postpartum period. The present study uses a reversible local neural inactivation method to examine the role of the mPOA in the regulation of maternal behavior throughout the postpartum period, particularly extending into the late postpartum, a little examined period. This approach avoids the compensatory plasticity in CNS that occurs after permanent lesions, and allows the repeated testing of same individuals. Early (PPD7-8) and late (PPD13-14) postpartum maternal behavior was evaluated in female rats following infusions of bupivacaine or vehicle into the mPOA or into control areas. As expected, mPOA inactivation severely but transiently disrupted early postpartum maternal behavior whereas infusion of vehicle or inactivation of adjacent control sites did not. Later in the postpartum period, however, transient mPOA inactivation facilitated the expression of maternal behaviors, highly contrasting the behavioral expression levels characteristic of late postpartum. Results strongly demonstrate that the mPOA is differentially engaged throughout postpartum in orchestrating appropriate maternal responses with the developmental stage of the pups.

Transient inactivation of the ventral tegmental area selectively disrupts the expression of conditioned place preference for pup- but not cocaine-paired contexts

The ventral tegmental area (VTA) plays a critical role in motivated behavior. However, it remains unclear whether intact VTA function is necessary for motivated behavior to seek contexts repeatedly paired with natural stimuli and/or pharmacological stimuli. In the present study, conditioned place preference (CPP) was induced with highly salient natural or drug stimuli attributed with strong incentive-motivational value in each of 2 female models: Postpartum females were conditioned to associate one unique context in the CPP apparatus with young offspring (pups) and a second context with a neutral stimulus, and virgin females were conditioned to associate unique contexts with cocaine (5 mg/kg ip) and saline injections. Immediately before CPP testing, each female received a microinfusion of bupivacaine bilaterally into the VTA to transiently inactivate the region; subjects were also tested after saline microinfusion into the VTA. Postpartum females' preference for the pup-paired context was abolished by VTA inactivation but was restored to high control levels after saline microinfusion. In separate tests, VTA inactivation also reduced motivated pup licking and pup retrieval in postpartum females, suggesting that intact VTA function is required for the expression of both pup CPP and motivated pup-directed behaviors. Cocaine CPP remained unaffected by VTA inactivation. Locomotion was not affected by VTA microinfusions but was severely impaired by bupivacaine microinfusions into the substantia nigra. We concluded that the VTA is differentially involved in the expression of conditioned preference for contexts paired with pups, a salient natural stimulus, and contexts paired with cocaine.

The intralaminar thalamic nuclei contribute to remote spatial memory

Recent studies have shown that the anterior (ATN) and lateral thalamic nuclei (including the intralaminar nuclei; ILN/LT) play different roles in memory processes. These nuclei have prominent direct and indirect connections with the hippocampal system and/or the prefrontal cortex and may thus participate in the time-dependent reorganization of memory traces during systems-level consolidation. We investigated whether ATN or ILN/LT lesions in rats influenced acquisition and subsequent retrieval of spatial memory in a Morris water maze. Retrieval was assessed with a probe trial after a short (5 d, recent memory) or a long (25 d, remote memory) postacquisition delay. The ATN group showed impaired acquisition compared with the Sham controls and ILN/LT groups, which did not differ during acquisition, and exhibited no preference for the target quadrant during the recent or remote memory probe trials. In contrast, probe trial performance in rats with ILN/LT lesions differed according to the age of the memory, with accurate spatial retrieval for the recent memory probe trial but impaired retrieval during the remote memory one. These findings confirm that ATN but not ILN/LT lesions disrupt the acquisition of spatial memory and provide new evidence that the ILN/LT region contributes to remote memory processing. Thus, the lateral thalamus may modulate some aspects of remote memory formation and/or retrieval during the course of systems-level consolidation.

Effect of reversible inactivation of the reuniens nucleus on spatial learning and memory in rats using Morris water maze task

The involvement of thalamic midline nuclei (MLN) in early stage of Alzheimer's disease and in diencephalic amnesia has drawn attention to the connectivity between the nucleus reuniens (RE) and structures of medial temporal lobe. RE is major source of thalamic afferents to the hippocampus and has been shown to exert powerful excitatory action on CA1 of hippocampus, which is supposed to be involved in learning and memory processes. However, the role of the RE on spatial memory is a controversial issue. The present study was designed to evaluate the role of the RE in acquisition, consolidation and retrieval of spatial reference memory (RM) and working memory (WM). We assessed the effect of reversible inactivation of RE with tetracaine (0.5 microl, 2%) on different stages of memory. Rats were trained on RM and WM versions of the Morris water maze (MWM) task. RE was inactivated before or immediately after training or before the probe trial of retrieval tests. The data showed that reversible inactivation of the RE significantly impaired both RM and WM versions of MWM. Therefore, it seems that nucleus reuniens of thalamus plays a role in spatial RM and WM version of the MWM task in rats.

The dorsal subiculum mediates the acquisition of conditioned reinstatement of cocaine-seeking

Contextual stimuli present during a single lifetime cocaine self-administration experience acquire occasion-setting actions sufficient to persistently elicit cocaine-seeking behavior in rats, with effects lasting nearly 1 year. The goal of this study was to identify neural substrates mediating the acquisition of drug-related conditioning taking place during a single cocaine self-administration experience with focus on the subicular formation, a brain site that has been implicated in associative learning relevant for conditioned reward-seeking including conditioned reinstatement. Male Wistar rats were given 2 h of response-contingent access to intravenous cocaine or saline in the presence of distinct stimuli that served as contextual stimuli associated with the availability and subjective effects of cocaine (S(+)) vs saline (S(-)). Before onset of the sessions, rats received bilateral microinjections of tetrodotoxin (TTX) into the ventral subiculum (VSUB) or dorsal subiculum (DSUB). Following extinction of responding by withholding cocaine, rats were subjected to reinstatement tests in which exposure to the cocaine- (but not saline) associated stimulus produced strong recovery of responding. This effect was completely abolished in rats with transient TTX inactivation of the DSUB during the conditioning session. TTX inactivation of the VSUB during conditioning did not alter the response-reinstating effects of the cocaine cue. The results suggest that functional integrity of the DSUB, but not VSUB, is critical for the acquisition of conditioned cocaine-seeking controlled by contextual stimuli under conditions where such learning occurs during a single conditioning trial.

Hippocampal regulation of contextual cue-induced reinstatement of cocaine-seeking behavior

Associations between cocaine and cues facilitate development and maintenance of addiction. We hypothesized that the ventral hippocampus is important for acquisition of these associations. Rats were trained to self-administer cocaine, with or without pre-exposure to distinct sets of cocaine- and saline-paired contextual cues. Next, rats were conditioned for 3 days with the distinct sets of contextual cues paired with cocaine and saline along with distinct discrete cues. Vehicle or lidocaine was infused into the ventral hippocampus prior to conditioning sessions. Following extinction, reinstatement of cocaine-seeking behavior was examined following exposure to contextual cues, discrete cues, or their combination. Inactivation of the ventral hippocampus during conditioning blocked acquisition of the association between cocaine and cocaine-paired contextual cues in that only lidocaine-treated rats with short-term cue exposure failed to reinstate responding in the presence of cocaine-paired contextual cues. Lidocaine also prevented rats in both cue exposure groups from discriminating between cocaine- and saline-paired contextual cues during reinstatement tests. Reinstatement induced by cocaine-paired discrete cues or by contextual and discrete cues together was not impaired for either cue exposure condition. The hippocampus is important for acquisition of the association between cocaine and context and in maintaining discrimination between cocaine-relevant and -irrelevant contextual cues.

Activation of septal 5-HT1A receptors alters spatial memory encoding, interferes with consolidation, but does not affect retrieval in rats subjected to a water-maze task

Using Long-Evans rats tested in a water maze, this study assessed the role of 5-HT1A/5-HT7 receptors of the medial septum in encoding, consolidation, and retrieval of spatial information. The testing protocol (acquisition: daily four-trial sessions over three consecutive days; retention: probe trial on day 4) was first validated by showing that intraseptal infusions of lidocaine (LIDO; 40 microg/0.5 microL) disrupted acquisition and retrieval of the task. 8-OH-DPAT (4 microg/0.5 microL) infused before each acquisition session prevented learning/retention of the platform location, an effect attenuated by pretreatment with the 5-HT1A receptor antagonist WAY 100635. With the 5-HT7 antagonist SB 269970, the 8-OH-DPAT-induced acquisition deficit seemed attenuated, but there was no subsequent retention. When infused immediately, 1, 4, or 6 h after each acquisition session, 8-OH-DPAT did not hinder consolidation. When the infusions were performed 2 h postacquisition, however, consolidation was disrupted. Finally, when infused before a probe trial after drug-free acquisition, 8-OH-DPAT had no effect, suggesting no interference with retrieval processes. We also established that 8-OH-DPAT had no effects when the platform was visible, and altered neither home-cage activity nor anxiety-related behavior (elevated plus-maze). Altogether, these results show that 5-HT1A receptors in the septal region contribute both to declarative-like information encoding and subsequently, within a given postacquisition time window, to its consolidation. They do not participate in the retrieval of recently learned declarative-like information. These observations suggest that 5-HT1A receptors of the medial septum contribute to a serotonin-mediated mechanism involved in the encoding and consolidation, not the retrieval of spatial hippocampal-dependent knowledge. These results might have some relevance to approaches aimed at modifying serotonergic functions in the brain for the treatment of disorders such as depression, anxiety, post-traumatic stress, and amnesia.

From synapse to behavior: rapid modulation of defined neuronal types with engineered GABAA receptors

In mammals, identifying the contribution of specific neurons or networks to behavior is a key challenge. Here we describe an approach that facilitates this process by enabling the rapid modulation of synaptic inhibition in defined cell populations. Binding of zolpidem, a systemically active allosteric modulator that enhances the function of the GABAA receptor, requires a phenylalanine residue (Phe77) in the gamma2 subunit. Mice in which this residue is changed to isoleucine are insensitive to zolpidem. By Cre recombinase-induced swapping of the gamma2 subunit (that is, exchanging Ile77 for Phe77), zolpidem sensitivity can be restored to GABAA receptors in chosen cell types. We demonstrate the power of this method in the cerebellum, where zolpidem rapidly induces significant motor deficits when Purkinje cells are made uniquely sensitive to its action. This combined molecular and pharmacological technique has demonstrable advantages over targeted cell ablation and will be invaluable for investigating many neuronal circuits.

The hippocampus plays a critical role at encoding discontiguous events for subsequent declarative memory expression in mice

The hypothesis that hippocampal activity at encoding is causally related to subsequent declarative memory expression is tested in the mouse, by using lidocaine inactivation of the hippocampus in combination with c-fos neuroimaging analysis. We employed a two-stage radial maze paradigm of spatial discrimination, which was previously shown to dissociate between declarative and nondeclarative expression of memory related to the same acquired material. In Stage 1 (encoding), mice learnt the constant location of food among a set of six arms (three baited, three unbaited) by being submitted repeatedly to discontiguous experiences with each arm separately ("go/no-go" discrimination). In Stage 2 (test-session), they are challenged with novel presentations of the arms, which are either combined into pairs of opposite valence ("two-choice" discrimination), or opened all six together ("six-choice" discrimination). Previous experiments have demonstrated that the "two-choice" situation is a critical test for declarative memory while "six-choice" discrimination may rely on procedural memory. We observed that (i) hippocampal activity measured by c-fos mRNA expression was increased by "go/no-go" learning, and this activation was blocked by pre-training local infusions of lidocaine; (ii) when performed just before each session of Stage 1, such inactivation spared the acquisition of "go/no-go" discrimination but produced, subsequently, a selective deficit in the "two-choice" test (not in the "six-choice" test). This study indicates that the hippocampus is "spontaneously" engaged in encoding processes necessary for long-term storage of discontiguous experiences under a form enabling flexible declarative memory expression.