Activation of prefrontal cortex and striatal regions in rats after shifting between rules in a T-maze

Prefrontal cortical and striatal areas have been identified by inactivation or lesion studies to be required for behavioral flexibility, including selecting and processing of different types of information. In order to identify these networks activated selectively during the acquisition of new reward contingency rules, rats were trained to discriminate orientations of bars presented in pseudorandom sequence on two video monitors positioned behind the goal sites on a T-maze with return arms. A second group already trained in the visual discrimination task learned to alternate left and right goal arm visits in the same maze while ignoring the visual cues still being presented. In each experimental group, once the rats reached criterion performance, the brains were prepared after a 90-min delay for later processing for c-fos immunohistochemistry. While both groups extinguished a prior strategy and acquired a new rule, they differed by the identity of the strategies and previous learning experience. Among the 28 forebrain areas examined, there were significant increases in the relative density of c-fos immunoreactive cell bodies after learning the second rule in the prefrontal cortex cingulate, the prelimbic and infralimbic areas, the dorsomedial striatum and the core of the nucleus accumbens, the ventral subiculum, and the central nucleus of the amygdala. These largely correspond to structures previously identified in inactivation studies, and their neurons fire synchronously during learning and strategy shifts. The data suggest that this dynamic network may underlie reward-based selection for action-a type of cognitive flexibility.

Locus coeruleus noradrenergic neurons phase-lock to prefrontal and hippocampal infra-slow rhythms that synchronize to behavioral events

The locus coeruleus (LC) is the primary source of noradrenergic projections to the forebrain, and, in prefrontal cortex, is implicated in decision-making and executive function. LC neurons phase-lock to cortical infra-slow wave oscillations during sleep. Such infra-slow rhythms are rarely reported in awake states, despite their interest, since they correspond to the time scale of behavior. Thus, we investigated LC neuronal synchrony with infra-slow rhythms in awake rats performing an attentional set-shifting task. Local field potential (LFP) oscillation cycles in prefrontal cortex and hippocampus on the order of 0.4 Hz phase-locked to task events at crucial maze locations. Indeed, successive cycles of the infra-slow rhythms showed different wavelengths, as if they are periodic oscillations that can reset phase relative to salient events. Simultaneously recorded infra-slow rhythms in prefrontal cortex and hippocampus could show different cycle durations as well, suggesting independent control. Most LC neurons (including optogenetically identified noradrenergic neurons) recorded here were phase-locked to these infra-slow rhythms, as were hippocampal and prefrontal units recorded on the LFP probes. The infra-slow oscillations also phase-modulated gamma amplitude, linking these rhythms at the time scale of behavior to those coordinating neuronal synchrony. This would provide a potential mechanism where noradrenaline, released by LC neurons in concert with the infra-slow rhythm, would facilitate synchronization or reset of these brain networks, underlying behavioral adaptation.

Abnormal Locus Coeruleus Sleep Activity Alters Sleep Signatures of Memory Consolidation and Impairs Place Cell Stability and Spatial Memory

Sleep is critical for proper memory consolidation. The locus coeruleus (LC) releases norepinephrine throughout the brain except when the LC falls silent throughout rapid eye movement (REM) sleep and prior to each non-REM (NREM) sleep spindle. We hypothesize that these transient LC silences allow the synaptic plasticity that is necessary to incorporate new information into pre-existing memory circuits. We found that spontaneous LC activity within sleep spindles triggers a decrease in spindle power. By optogenetically stimulating norepinephrine-containing LC neurons at 2 Hz during sleep, we reduced sleep spindle occurrence, as well as NREM delta power and REM theta power, without causing arousals or changing sleep amounts. Stimulating the LC during sleep following a hippocampus-dependent food location learning task interfered with consolidation of newly learned locations and reconsolidation of previous locations, disrupting next-day place cell activity. The LC stimulation-induced reduction in NREM sleep spindles, delta, and REM theta and reduced ripple-spindle coupling all correlated with decreased hippocampus-dependent performance on the task. Thus, periods of LC silence during sleep following learning are essential for normal spindle generation, delta and theta power, and consolidation of spatial memories.

Ripple-triggered stimulation of the locus coeruleus during post-learning sleep disrupts ripple/spindle coupling and impairs memory consolidation

Experience-induced replay of neuronal ensembles occurs during hippocampal high-frequency oscillations, or ripples. Post-learning increase in ripple rate is predictive of memory recall, while ripple disruption impairs learning. Ripples may thus present a fundamental component of a neurophysiological mechanism of memory consolidation. In addition to system-level local and cross-regional interactions, a consolidation mechanism involves stabilization of memory representations at the synaptic level. Synaptic plasticity within experience-activated neuronal networks is facilitated by noradrenaline release from the axon terminals of the locus coeruleus (LC). Here, to better understand interactions between the system and synaptic mechanisms underlying “off-line” consolidation, we examined the effects of ripple-associated LC activation on hippocampal and cortical activity and on spatial memory. Rats were trained on a radial maze; after each daily learning session neural activity was monitored for 1 h via implanted electrode arrays. Immediately following “on-line” detection of ripple, a brief train of electrical pulses (0.05 mA) was applied to LC. Low-frequency (20 Hz) stimulation had no effect on spatial learning, while higher-frequency (100 Hz) trains transiently blocked generation of ripple-associated cortical spindles and caused a reference memory deficit. Suppression of synchronous ripple/spindle events appears to interfere with hippocampal-cortical communication, thereby reducing the efficiency of “off-line” memory consolidation.

Reactivation, retrieval, replay and reconsolidation in and out of sleep: connecting the dots

The neurobiology of memory has taken on a new look over the past decade. Re-discovery of cue-dependent amnesia, wide availability of functional imaging tools and increased dialog among clinicians, cognitive psychologists, behavioral neuroscientists, and neurobiologists have provided impetus for the search for new paradigms for the study of memory. Memory is increasingly viewed as an open-ended process, with retrieval being recognized as an intricate part of the encoding process. New memories are always made on the background of past experience, so that every consolidation is, in fact reconsolidation, serving to update and strengthen memories after retrieval. Spontaneous reactivation of memory circuits occurs during sleep and there is converging evidence from rodent and human studies that this is an important part of the extended off-line memory processing. The noradrenergic neuromodulatory system is engaged at retrieval, facilitating recall. The noradrenergic system is also activated during sleep after learning and noradrenergic neurons fire in concert with cortical oscillations that are associated with reactivation of memory circuits. We suggest that the noradrenergic system and perhaps other neuromodulatory systems, may be a key to linking off-line memory reactivation, retrieval, and memory reconsolidation processes at both synaptic and systems levels, in and out of sleep.

Hippocampal sharp wave/ripples during sleep for consolidation of associative memory

The beneficial effect of sleep on memory has been well-established by extensive research on humans, but the neurophysiological mechanisms remain a matter of speculation. This study addresses the hypothesis that the fast oscillations known as ripples recorded in the CA1 region of the hippocampus during slow wave sleep (SWS) may provide a physiological substrate for long term memory consolidation. We trained rats in a spatial discrimination task to retrieve palatable reward in three fixed locations. Hippocampal local field potentials and cortical EEG were recorded for 2 h after each daily training session. There was an increase in ripple density during SWS after early training sessions, in both trained rats and in rats randomly rewarded for exploring the maze. In rats learning the place -reward association, there was a striking further significant increase in ripple density correlated with subsequent improvements in behavioral performance as the rat learned the spatial discrimination aspect of the task. The results corroborate others showing an experience-dependent increase in ripple activity and associated ensemble replay after exploratory activity, but in addition, for the first time, reveal a clear further increase in ripple activity related to associative learning based on spatial discrimination.

The influence of learning on sleep slow oscillations and associated spindles and ripples in humans and rats

The mechanisms underlying off-line consolidation of memory during sleep are elusive. Learning of hippocampus-dependent tasks increases neocortical slow oscillation synchrony, and thalamocortical spindle and hippocampal ripple activity during subsequent non-rapid eye movement sleep. Slow oscillations representing an oscillation between global neocortical states of increased (up-state) and decreased (down-state) neuronal firing temporally group thalamic spindle and hippocampal ripple activity, which both occur preferentially during slow oscillation up-states. Here we examined whether slow oscillations also group learning-induced increases in spindle and ripple activity, thereby providing time-frames of facilitated hippocampus-to-neocortical information transfer underlying the conversion of temporary into long-term memories. Learning (word-pairs in humans, odor-reward associations in rats) increased slow oscillation up-states and, in humans, shaped the timing of down-states. Slow oscillations grouped spindle and rat ripple activity into up-states under basal conditions. Prior learning produced in humans an increase in spindle activity focused on slow oscillation up-states. In rats, learning induced a distinct increase in spindle and ripple activity that was not synchronized to up-states. Event-correlation histograms indicated an increase in spindle activity with the occurrence of ripples. This increase was prolonged after learning, suggesting a direct temporal tuning between ripples and spindles. The lack of a grouping effect of slow oscillations on learning-induced spindles and ripples in rats, together with the less pronounced effects of learning on slow oscillations, presumably reflects a weaker dependence of odor learning on thalamo-neocortical circuitry. Slow oscillations might provide an effective temporal frame for hippocampus-to-neocortical information transfer only when thalamo-neocortical systems are already critically involved during learning.

Sustained increase in hippocampal sharp-wave ripple activity during slow-wave sleep after learning

High-frequency oscillations, known as sharp-wave/ripple (SPW-R) complexes occurring in hippocampus during slow-wave sleep (SWS), have been proposed to promote synaptic plasticity necessary for memory consolidation. We recorded sleep for 3 h after rats were trained on an odor-reward association task. Learning resulted in an increased number SPW-Rs during the first hour of post-learning SWS. The magnitude of ripple events and their duration were also elevated for up to 2 h after the newly formed memory. Rats that did not learn the discrimination during the training session did not show any change in SPW-Rs. Successful retrieval from remote memory was likewise accompanied by an increase in SPW-R density and magnitude, relative to the previously recorded baseline, but the effects were much shorter lasting and did not include increases in ripple duration and amplitude. A short-lasting increase of ripple activity was also observed when rats were rewarded for performing a motor component of the task only. There were no increases in ripple activity after habituation to the experimental environment. These experiments show that the characteristics of hippocampal high-frequency oscillations during SWS are affected by prior behavioral experience. Associative learning induces robust and sustained (up to 2 h) changes in several SPW-R characteristics, while after retrieval from remote memory or performance of a well-trained procedural aspect of the task, only transient changes in ripple density were induced.

Learning-dependent, transient increase of activity in noradrenergic neurons of locus coeruleus during slow wave sleep in the rat: brain stem-cortex interplay for memory consolidation?

Memory consolidation during sleep is regaining attention due to a wave of recent reports of memory improvements after sleep or deficits after sleep disturbance. Neuromodulators have been proposed as possible players in this putative off-line memory processing, without much experimental evidence. We recorded neuronal activity in the rat noradrenergic nucleus locus coeruleus (LC) using chronically implanted movable microelectrodes while monitoring the behavioral state via electrocorticogram and online video recording. Extracellular recordings of physiologically identified noradrenergic neurons of LC were made in freely behaving rats for 3 h before and after olfactory discrimination learning. On subsequent days, if LC recording remained stable, additional learning sessions were made within the olfactory discrimination protocol, including extinction, reversals, learning new odors. Contrary to the long-standing dogma about the quiescence of noradrenergic neurons of LC, we found a transient increase in LC activity in trained rats during slow wave sleep (SWS) 2 h after learning. The discovery of learning-dependent engagement of LC neurons during SWS encourages exploration of brain stem-cortical interaction during this delayed phase of memory consolidation and should bring new insights into mechanisms underlying memory formation.

Elevated sleep spindle density after learning or after retrieval in rats

Non-rapid eye movement sleep has been strongly implicated in consolidation of both declarative and procedural memory in humans. Elevated sleep-spindle density in slow-wave sleep after learning has been shown recently in humans. It has been proposed that sleep spindles, 12-15 Hz oscillations superimposed on slow waves (<1 Hz), in concert with high-frequency hippocampal sharp waves/ripples, promote neural plasticity underlying remote memory formation. The present study reports the first indication of learning-associated increase in spindle density in the rat, providing an animal model to study the role of brain oscillations in memory consolidation during sleep. An odor-reward association task, analogous in many respects to human paired-associate learning, is rapidly learned and leads to robust memory in rats. Rats learned the task over 10 massed trials within a single session, and EEG was monitored for 3 h after learning. Learning-induced increase in spindle density is reliably reproduced in rats in two different learning situations, differing primarily in the behavioral component of the task. This increase in spindle density is also present after reactivation of remote memory and in situations when memory update is required; it is not observed after noncontingent exposure to reward and training context. The latter results substantially extend findings in humans. The magnitude of increase (approximately 25%) and the time window of maximal effect (approximately 1 h after sleep onset) were remarkably similar to human data, making this a valid rodent model to study network interactions through the use of simultaneous unit recordings and local field potentials during postlearning sleep.

Locus coeruleus activation by foot shock or electrical stimulation inhibits amygdala neurons

The noradrenergic nucleus locus coeruleus (LC) has a direct projection to the basal lateral amygdala (BLA). Behavioral, lesion and pharmacological studies suggest that this pathway has an important role in mediating responses to emotional stimuli and in the formation of long term memory. The effect of LC activation on the activity of BLA neurons in vivo is not known. Therefore, in the present experiments, simultaneous extracellular unit recordings were made in the two regions while the anesthetized rat received electrical stimulation of the paw to simulate a real-life acute stressor, commonly used as an aversive reinforcer in conditioning experiments. All LC neurons exhibited a multiphasic excitatory response followed by prolonged inhibition. Responses of BLA cells were more heterogeneous, but predominantly inhibitory, with a release from inhibition during the refractory phase of LC. Direct electrical stimulation of the LC with a single pulse also elicited an inhibitory response in BLA. BLA response to both footshock and LC stimulation was partially blocked by the beta adrenergic receptor antagonist, timolol, infused into the BLA. These experiments are the first to report in vivo effects of activation of the noradrenergic system on neuronal activity in the BLA.

Hippocampal Sharp Wave-Ripples Linked to Slow Oscillations in Rat Slow-Wave Sleep

Slow oscillations originating in the prefrontal neocortex during slow-wave sleep (SWS) group neuronal network activity and thereby presumably support the consolidation of memories. Here, we investigated whether the grouping influence of slow oscillations extends to hippocampal sharp wave-ripple (SPW) activity thought to underlie memory replay processes during SWS. The prefrontal surface EEG and multiunit activity (MUA), along with hippocampal local field potentials (LFP) from CA1, were recorded in rats during sleep. Average spindle and ripple activity and event correlation histograms of SPWs were calculated, time-locked to half-waves of slow oscillations. Results confirm decreased prefrontal MUA and spindle activity during EEG slow oscillation negativity and increases in this activity during subsequent positivity. A remarkably close temporal link was revealed between slow oscillations and hippocampal activity, with ripple activity and SPWs being also distinctly decreased during negative half-waves and increased during slow oscillation positivity. Fine-grained analyses of temporal dynamics revealed for the slow oscillation a phase delay of approximately 90 ms with reference to up and down states of prefrontal MUA, and of only approximately 60 ms with reference to changes in SPWs, indicating that up and down states in prefrontal MUA precede corresponding changes in hippocampal SPWs by approximately 30 ms. Results support the notion that the depolarizing surface-positive phase of the slow oscillation and the associated up state of prefrontal excitation promotes hippocampal SPWs via efferent pathways. The preceding disfacilitation of hippocampal events temporally coupled to the negative slow oscillation half-wave appears to serve a synchronizing role in this neocorticohippocampal interplay.

Network reset: a simplified overarching theory of locus coeruleus noradrenaline function

Unraveling the functional role of neuromodulatory systems has been a major challenge for cognitive neuroscience, giving rise to theories ranging from a simple role in vigilance to complex models concerning decision making, prediction errors or unexpected uncertainty. A new, simplified and overarching theory of noradrenaline function is inspired by an invertebrate model: neuromodulators in crustacea abruptly interrupt activity in neural networks and reorganize the elements into new functional networks determining the behavioral output. Analogously in mammals, phasic activation of noradrenergic neurons of the locus coeruleus in time with cognitive shifts could provoke or facilitate dynamic reorganization of target neural networks, permitting rapid behavioral adaptation to changing environmental imperatives. Detailed analysis and discussion of extensive electrophysiological data from the locus coeruleus of rats and monkeys in controlled behavioral situations is provided here to support this view. This simplified 'new look' at locus coeruleus noradrenaline function redirects the challenge of understanding neuromodulatory systems towards their target networks, particularly to the dynamics of their interactions and how they organize adaptive behavior.

Reconsolidation after remembering an odor-reward association requires NMDA receptors

A rapidly learned odor discrimination task based on spontaneous foraging behavior of the rat was used to evaluate the role of N-methyl-D-aspartate (NMDA) receptors (NMDARs) in ongoing memory consolidation. Rats were trained in a single session to discriminate among three odors, one of which was associated with palatable food reward. Previous experiments showed that the NMDAR antagonist DL-APV induced amnesia for this task when injected immediately after training. In the present study, memory was reactivated 24 h after training by exposure to the rewarded odor within the experimental context after which rats received an intracerebroventricular injection of APV. Combined reactivation-drug treatment induced profound amnesia when tested 48 h later. Animals receiving drug alone, in absence of reactivation, showed perfect retention. It is concluded that NMDARs support a consolidation process taking place after memory reactivation.

Reward expectation, orientation of attention and locus coeruleus-medial frontal cortex interplay during learning

Regulation of attention and promotion of behavioural flexibility are functions attributed to both the noradrenergic nucleus locus coeruleus (LC) and the prefrontal cortex (PFC). The PFC receives a large innervation from LC and small changes in catecholaminergic activity in PFC profoundly affect cognitive function. It is crucial to the understanding of learning-related plasticity, that the cognitive context driving LC neurons be determined and the relation to activity in PFC be elucidated. To this end simultaneous recordings were made from LC and prelimbic cortex (PL) during an odour-reward association task in the rat. Neuronal activity related to orientation of attention, reward predictability, reward itself, and changes in stimulus reinforcement contingencies, was measured. All LC neurons and a significant proportion of PL neurons were engaged during several aspects of a Go/NoGo task, especially after the signal for trial onset and CS+ presentation. LC activation was, however, more tightly aligned to the behavioural response than to the CS+ 22% of PL neurons were activated during the response-reward delay. This suggests that the activity of both these structures is related to reward anticipation. Finally, LC neurons exhibited rapid plasticity when the reward-contingency was modified. Within-trial response latencies were always shorter in LC than in PL and between-trial response adaptation in LC preceded that in PL by many trials. Identifying such temporal relationships is an essential step toward understanding how neuromodulatory inputs to forebrain networks might promote or permit experience-dependent plasticity in behavioural situations.

Noradrenergic action in prefrontal cortex in the late stage of memory consolidation

These experiments investigated the role of the noradrenergic system in the late stage of memory consolidation and in particular its action at beta receptors in the prelimbic region (PL) of the prefrontal cortex in the hours after training. Rats were trained in a rapidly acquired, appetitively motivated foraging task based on olfactory discrimination. They were injected with a beta adrenergic receptor antagonist into the PL 5 min or 2 h after training and tested 48 h later. Rats injected at 2 h showed amnesia, whereas those injected at 5 min had good retention, equivalent to saline-injected controls. Monitoring extracellular noradrenaline efflux in PL by in vivo microdialysis during the first hours after training revealed a significant increase shortly after training, with a rapid return to baseline, and then another increase around the 2-h posttraining time window. Pseudo-trained rats showed a smaller early efflux and did not show the second wave of efflux at 2 h. These results confirm earlier pharmacological and immunohistochemical studies suggesting a delayed role of noradrenaline in a late phase of long-term memory consolidation and the engagement of the PL during these consolidation processes.

Consolidation of memory for odour-reward association requires transient polysialylation of the neural cell adhesion molecule in the rat hippocampal dentate gyrus

Cell adhesion molecule function is involved in hippocampal synaptic plasticity and is associated with memory consolidation. At the infragranular zone of the dentate gyrus, neurons expressing the polysialylated form of the neural cell adhesion molecule (NCAM PSA) transiently increase their frequency at the 12-hr posttraining time in behaviours elicited by stressful stimuli, such as those associated with conditioned avoidance, water maze, and fear conditioning paradigms. To determine whether learning-induced modulation of NCAM polysialylation is limited to stressful paradigms, we employed a reward-based odour discrimination task. Animals show a rapid acquisition and recall of this task in terms of latency to identify the food-associated odour and the number of choice errors. Immunohistochemical procedures were employed to determine the change in NCAM PSA expression following task acquisition. NCAM PSA immunoreactivity in the hippocampal formation was most intense on the granule-like neurons in the infragranular zone of the dentate gyrus, and their frequency transiently increased in the 12-hr posttraining period. The nature of the transient increase in NCAM PSA-immunoreactive neurons was indistinguishable from that observed following avoidance conditioning or spatial learning, in that it occurred at the same time. The transient increase in NCAM PSA expression is suggested to facilitate dendritic elaboration in response to the acquisition of novel behavioural repertoires.

Blockade of NMDA receptors in prelimbic cortex induces an enduring amnesia for odor-reward associative learning

The competitive antagonist 2-amino-5-phosphonoeptanoic acid (APV) was injected intracerebroventricularly to determine the involvement of NMDA receptors in different stages of memory consolidation. Subsequent experiments used local injections to determine possible sites of drug action. Rats were trained in a rapidly learned olfactory task to find palatable food in a hole in a sponge impregnated with the target odor in the presence of two other sponges with nonrewarded odors. APV injections were made intracerebroventricularly 5 min or 2 hr after the end of the training, and a retention test was given 48 hr later. The results showed that blockade of NMDA receptors immediately after training induces a profound and enduring amnesia with no effect when the treatment is delayed at 2 hr after training. To address the question of the effective sites of action of the intracerebroventricular treatment, APV injections into the hippocampus and into the prelimblic region of the frontal cortex (PLC) were made. Blockade of NMDA receptors into the PLC but not into the hippocampus impaired memory formation of the odor-reward association. The amnesia is not transient, because the retention tests were made 48 hr after training. These results underlie the role of NMDA receptors in the early stage of consolidation of a simple odor-reward associative memory and confirm the role of the PLC in the consolidation of long-term memory.

Phasic activation of locus ceruleus neurons by the central nucleus of the amygdala

The role of the central nucleus of the amygdala (CeN) in modulating output of noradrenaline in the forebrain was evaluated by recording extracellular, single-unit activity from the noradrenergic nucleus locus ceruleus (LC) during stimulation of the CeN. Short high-frequency trains (200 Hz) delivered at 800 microA in the CeN evoked phasic responses in 90% of the neurons recorded in LC. Single pulses were also effective but less reliably. The responses were complex, multiphasic with an initial latency of 10-20 msec. This early peak was diminished or, in some cases, completely blocked by local or intracerebroventricular application of the corticotrophin releasing factor antagonist alpha helical CRF (9-41). The later excitatory peak and subsequent inhibition were not effected by the drug treatment. The results underline the reciprocal functional relationship between the amygdaloid complex and the LC and suggest that the LC might be an important "effector" of CeN activation during learning.

Locus coeruleus activation modulates firing rate and temporal organization of odour-induced single-cell responses in rat piriform cortex

Piriform cortex (PCx) is the primary cortical projection region for olfactory information and has bidirectional monosynaptic connections with olfactory bulb and association cortices. PCx neurons display a complex receptive field, responding to odours rather than their molecular components, suggesting that these neurons are involved in higher order olfactory processing. Neuromodulators, especially noradrenaline (NA), have important influences on sensory processing in other cortical regions and might be responsible for the plasticity observed in PCx during learning. The present study is the first attempt to examine in vivo the actions of NA on sensory responses in the PCx. Stimulation of the noradrenergic nucleus locus coeruleus (LC) was used to induce release of NA in the forebrain in urethane-anaesthetized rats. Extracellular recording of single units was made simultaneously in anterior and posterior PCx. The responses to an odour stimulus were measured over 25 trials. Twenty-five subsequent odour presentations were preceded by stimulation of the ipsilateral LC through a bipolar electrode, previously placed in the LC under electrophysiological control. This priming stimulation modified the activity of 77 of the 135 recorded neurons. For most cells, LC stimulation enhanced cortical responses to odour in terms of both spike count and temporal organization, with some differential effects in anterior and posterior regions. These results are the first to show enhancement of sensory responses in the olfactory cortex by LC activation. Spontaneous activation of LC neurons such as occurs during learning could serve to enhance olfactory perception and promote learning.

Mapping of olfactory memory circuits: region-specific c-fos activation after odor-reward associative learning or after its retrieval

Although there is growing knowledge about intracellular mechanisms underlying neuronal plasticity and memory consolidation and reconsolidation after retrieval, information concerning the interaction among brain areas during formation and retrieval of memory is relatively sparse and fragmented. Addressing this question requires simultaneous monitoring of activity in multiple brain regions during learning, the post-acquisition consolidation period, and retrieval and subsequent reconsolidation. Immunoreaction to the immediate early gene c-fos is a powerful tool to mark neuronal activation of specific populations of neurons. Using this method, we are able to report, for the first time, post-training activation of a network of closely related brain regions, particularly in the frontal cortex and the basolateral amygdala (BLA), that is specific to the learning of an odor-reward association. On the other hand, retrieval of a well-established associative memory trace does not seem to differentially activate the same regions. The amygdala, in particular, is not engaged after retrieval, whereas the lateral habenula (LHab) shows strong activation that is restricted to animals having previously learned the association. Although intracellular mechanisms may be similar during consolidation and reconsolidation, this study indicates that different brain circuits are involved in the two processes, at least with respect to a rapidly learned olfactory task.

Spontaneous and auditory-evoked activity of medial agranular cortex as a function of arousal state in the freely moving rat: interaction with locus coeruleus activity

To characterize the electrophysiological properties of neurons in the medial agranular frontal cortex (Fr2) with respect to arousal level and locus coeruleus (LC) activity, we recorded spontaneous and auditory-evoked single unit activity in these areas simultaneously during different states of arousal in the rat. In the low-arousal state, as determined by EEG, 14/56 Fr2 neurons showed a tonic increase in spontaneous firing rate and 9/56 presented a clear inhibitory response to tone (onset latency 37 ms, duration 200 ms). The inhibitory response was not clear during the high-arousal state. Cross-correlation analysis of pairs of Fr2 and LC units, excluding activity during the actual tone, also showed a negative correlation from 120 ms before, to 170 ms after, Fr2 discharge in 5/63 pairs, only during low arousal. Significantly, 4/5 of the Fr2 neurons having this negative correlation with LC were included in that population which showed a tonic increase in spontaneous firing rate and inhibited to tone during low arousal. LC neurons, on the other hand, all showed excitation to tone stimulation (peak latency 30 ms), and response amplitude was not affected by changes in arousal level. The negative correlation in spontaneous activity, as well as their differential responses to tone, suggests an interaction between a select population of Fr2 neurons and the LC during the low-arousal state. Future studies with lesion or pharmacological manipulations would be necessary to confirm the presence of this interaction.

Attenuation of emotional and nonemotional memories after their reactivation: role of beta adrenergic receptors

A memory trace in its active state is susceptible to interference by amnesic agents, such as hypothermia and electroconvulsive shock, and by NMDA receptor antagonists, suggesting that a time-dependent consolidation process occurs each time a memory is reactivated. The role of beta noradrenergic receptors in reconsolidation in rats was examined in both a positively reinforced radial maze task and a footshock-reinforced conditioned emotional response task. For the former, rats were trained over several days in a spatial reference memory task and received a single reactivation trial followed by propranolol. A temporally graded impairment was observed when propranolol treatment occurred after the memory reactivation trial. In the emotional task, memory impairing effects of propranolol were greater when the drug was administered after a reactivation trial than when administered immediately after the initial training. These results suggest that reactivation of memory triggers a beta receptor-dependent cascade of intracellular events, recapitulating that which occurs during initial postacquisition consolidation, thus permitting reorganization of the existing memory as a function of new information in the retrieval environment. This remarkable lability of an active memory trace provides a new basis for pharmacotherapeutic intervention in such syndromes as Posttraumatic Stress Disorder. beta adrenoreceptor antagonists may be promising pharmacological agents for attenuating debilitating memories at the time of their controlled reactivation.

Learning-induced plasticity of N-methyl-D-aspartate receptors is task and region specific

Changes in binding of [3H]dizocilpine maleate to N-methyl-D-aspartate-sensitive ion channel receptors were evaluated after learning in order to specify brain regions which might be involved in memory formation. Rats were trained in a five-trial session of 40 min, to discriminate among three odours to obtain food reinforcement. Another group was trained in an eight-arm maze to choose always the same three arms to obtain food reinforcement (nine trials over 150 min). In rats killed 30 min after odour discrimination learning, dizocilpine maleate binding was significantly reduced in hippocampal sub-regions CA3, CA1 and fascia dentata and in frontal cortex. After spatial learning, changes in binding were limited to the amygdala, where a decrease was also observed. These results indicate that functional changes occur in specific brain regions after learning and suggest anatomical loci for further study of synaptic changes at a morphological level, after spatial learning or odour discrimination.

Consolidation of Memory for Odor–Reward Association: β-Adrenergic Receptor Involvement in the Late Phase

Experimentally naive rats can learn rapidly to discriminate among three odors to obtain food reinforcement. After three massed trials, they show almost errorless performance. This task has proved to be useful in studying time-dependent postacquisition intracellular processes necessary for long-term memory. The present experiments evaluated the temporal dynamics of the role of β-noradrenergic receptors in long-term consolidation. Rats were implanted with intracerebroventricular cannulae and trained in a single session to find reinforcement in a hole in a sponge impregnated with a particular odor. Injections of the β-receptor antagonist timolol were made at 5 min, 1, 2, or 5 hr after training. Memory and relearning ability were evaluated 48 hr later. Rats treated with timolol 2 hr after training showed a memory deficit at the retention test, but were able to relearn the task normally. Injections at the earlier or later time points were ineffective. The results reinforce previous observations with systemic injections that β-noradrenergic receptors are involved in the late phase of memory consolidation and suggest a critical time window during which they are necessary. The time window is compatible with the current view that long-term memory depends on late involvement of the cAMP cascade leading to new protein synthesis necessary for synaptic reorganization.

Consolidation of memory for odor-reward association: beta-adrenergic receptor involvement in the late phase

Experimentally naive rats can learn rapidly to discriminate among three odors to obtain food reinforcement. After three massed trials, they show almost errorless performance. This task has proved to be useful in studying time-dependent postacquisition intracellular processes necessary for long-term memory. The present experiments evaluated the temporal dynamics of the role of beta-noradrenergic receptors in long-term consolidation. Rats were implanted with intracerebroventricular cannulae and trained in a single session to find reinforcement in a hole in a sponge impregnated with a particular odor. Injections of the beta-receptor antagonist timolol were made at 5 min, 1, 2, or 5 hr after training. Memory and relearning ability were evaluated 48 hr later. Rats treated with timolol 2 hr after training showed a memory deficit at the retention test, but were able to relearn the task normally. Injections at the earlier or later time points were ineffective. The results reinforce previous observations with systemic injections that beta-noradrenergic receptors are involved in the late phase of memory consolidation and suggest a critical time window during which they are necessary. The time window is compatible with the current view that long-term memory depends on late involvement of the cAMP cascade leading to new protein synthesis necessary for synaptic reorganization.

Learning by neurones: role of attention, reinforcement and behaviour

The importance of the behavioural situation, attentional demands of the task, and stimulus-reinforcement contingencies in promoting or permitting experience-dependent neuronal plasticity is argued. Evidence is provided for the specific activation of the locus coeruleus noradrenergic system of the rat by novel stimuli encountered while investigating the environment, as well as during a formal learning situation. Noradrenergic neurons are particularly concerned with changes in the predictive value of the stimulus, when new learning should occur. Noradrenaline, released at LC terminals in target sensory systems, could facilitate shifts in attention, information processing and memory through its well-documented gating and tuning effects and its permissive role in long-term potentiation. Dopamine neurons, which fire persistently to reward during learning, could be involved in maintaining the behavioural response.

Slow oscillations as a probe of the dynamics of the locus coeruleus-frontal cortex interaction in anesthetized rats

Multiunit or single unit activity recorded simultaneously from frontal cortex (FC) and locus coeruleus (LC) under ketamine anesthesia revealed that both regions show slow oscillatory activity, together or separately. If, however, both regions are engaged in this oscillatory activity, there is a systematic relationship between their phases with peak LC firing always following FC firing by 200-400 ms. This was confirmed by cross-correlational analyses, which indicated that the two structures temporarily form a resonant system. The FC-LC resonant state is, however, loose enough to remain open to other intrinsic or extrinsic influences, keeping the measured frequencies of oscillations at each site slightly different, as demonstrated by a detailed analysis of the autocorrelograms. An injection of lidocaine at the frontal cortex site, while sharply reducing the prefrontal activity to essentially zero, leads to an increase of the LC activity and to a modification of the shape of the LC autocorrelogram, but does not change appreciably the phase relationship between the activity in the two structures during the diminishing activity in FC.

Neural cell adhesion molecules play a role in rat memory formation in appetitive as well as aversive tasks

A polyclonal antibody (R1), raised against chick synaptic membrane glycoproteins and recognizing the neural cell adhesion molecule (NCAM) caused amnesia for avoidance tasks when injected into day-old chicks and adult rats 5.5 h post-training. We investigated the effects of R1 antibody on memory formation in a non-aversive task, where stress is minimal: a massed trial odour discrimination task in rats. Preimmune serum or R1 antibody was injected i.c.v. 5.5 h after the last training session. Forty-eight hours after the training session, control rats showed very good retention whereas R1 antibody injection significantly disrupted retention. The results suggest that glycoproteins recognized by R1 in the rat play a specific role in memory formation for appetitive events as well as in memory formation for aversive situations.

Reconsolidation of memory after its reactivation

We report here data suggesting that reactivation of a well-established memory by a retention test triggers cellular events which depend upon N-methyl-D-aspartate (NMDA) receptors for up to 2 h after reactivation. Rats were overtrained on a maze task requiring integration of distal spatial information contained in cues strategically placed around the maze. Previous experiments showed that pretrial injection of the noncompetitive NMDA receptor antagonist, MK-801, at a dose which had no effect on overt behavior (0.05 mg/kg), markedly disrupted the well-trained performance of the task. Surprisingly, the behavioral deficit persisted on subsequent, drug-free trials, 24 h later. The present experiments showed that post-trial injections produced the same effects on performance on one or two subsequent daily trials. A temporal gradient for this amnestic effect of the drug treatment was established by injecting rats at 5, 30, 60, 90, 120 or 180 min after the performance trial. Only those rats whose MK-801 treatment was delayed for 120 min or more after the trial were able to perform the task normally 24 h later. All other treatment times induced significant amnesia for the task, when the rats were tested 24 h later. A subsequent experiment, using a more difficult version of the task, showed a longer amnesia gradient, but the predrug performance level could be reinstated within one multiple trial test session. Thus, it appears that activation of a well-established memory circuit renders the trace labile, requiring its reconsolidation. To what extent the entire post-acquisition cascade of NMDA receptor-dependent intracellular events is recapitulated each time a memory is activated and reorganised is probably a function of the age and complexity of the memory and the amount of new information to be integrated into the circuit. These results provide physiological evidence for the notion that memory is a dynamic process undergoing continual reorganization as a function of the ongoing experience of the organism.

Antibody to day-old chick brain glycoprotein produces amnesia in adult rats

Polyclonal antibody R-1, raised against a chick synaptic membrane glycoprotein fraction whose synthesis is enhanced following training on a passive avoidance task, produces amnesia when injected into chick forebrain 5.5 h posttraining. The amnestic IgG fraction specifically recognizes a low sialylated isoform of NCAM (Mileusnic Rose, Lancashire, & Bullock, 1995). We have now investigated the effects of this antibody on memory formation in adult rats. R-1, preimmune serum, or saline was injected intracerebroventricularly 5.5 h posttraining through bilaterally implanted cannulae. Rats injected with R-1 and tested 48 h later showed a significant amnesia for avoidance compared with the controls. Amnesia was not apparent at 24 h posttraining. R-1 injections were without effect on spontaneous locomotor or exploratory activity in a holeboard test. The results contribute to the argument that the role of cell adhesion molecules in neuronal plasticity is not limited to the developing nervous system, but they play a more general role in the experience-dependent synaptic remodeling underlying long-term memory.

Novelty-elicited, noradrenaline-dependent enhancement of excitability in the dentate gyrus

In order to relate noradrenaline-dependent potentiation in the dentate gyrus to behavioural events, rats were made to explore an environment in which their encounters with novel stimuli could be strictly controlled and monitored. Previous experiments have shown that an encounter with novel objects in a holeboard elicits a burst response in a large population of noradrenergic neurons of the locus coeruleus. Such a burst response has been demonstrated to produce a large and transient potentiation of the population spike in the dentate gyrus. In the present series of experiments, rats were chronically implanted with stimulating electrodes in the perforant pathway and recording electrodes in the dentate gyrus. Evoked potentials were monitored in the awake rat, first while it was resting quietly in a familiar environment and then while it was exploring the holeboard containing a novel object in a specific hole. There was a tonic increase in population spike amplitude when the rat was placed in the novel holeboard environment, but this effect gradually dissipated. This increase was partly blocked by the beta-noradrenergic antagonist propranolol. In addition there was a robust phasic increase in spike amplitude when the rat encountered a novel stimulus. This phasic response lasted approximately 50-75 s and was absent in animals treated with propranolol. These results show that a behavioural encounter with a novel stimulus can transiently enhance information transmission through the hippocampus, and suggest that activation of the noradrenergic system by the novel stimulus mediates this behavior-dependent gating.

Rapid habituation of auditory responses of locus coeruleus cells in anaesthetized and awake rats

The auditory response of locus coeruleus (LC) neurones evoked by novel tones was investigated in anaesthetized and awake rats. Recording the single unit activity of LC neurones, responses to auditory stimuli are found under anaesthesia as well as in the awake animal. There are three types of LC responses to tone: first and by far the most frequent, a burst of several spikes at onset of the tone; second, a burst at tone offset and lastly, a total inhibition to the tone. All responses present a rapid habituation after the first few presentations of the stimulus. The results support the view that the LC plays a role in mediating responses to environmental changes.

Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

These experiments were designed to investigate the role of the noradrenergic system in promoting investigation of novelty in rats. Behavior was monitored in a hole board equipped with photoelectric cells strategically placed so that locomotor activity, rearing and investigation of each of the holes could be quantified independently. Specially designed computer software permitted recording of the sequence and cumulative duration of the visits to specific holes throughout the session. Dose-response curves of the sedative effect of the alpha 2 adrenergic receptor agonist clonidine were established, a sedative effect being defined as a decrease in overall horizontal displacements, rearings and hole visits. After a one week interval, the rats were rerun in the holeboard, with novel objects placed in four of the nine holes. Previous experiments had shown that rats spend significantly more time investigating holes containing objects than empty holes in this apparatus and this was replicated here. Doses of clonidine which were below threshold for inducing any sedative effect (10 micrograms/kg) totally eliminated preference for holes with objects while having no effect on total time investigating the holes. A subsequent experiment showed that the beta receptor antagonist propranolol (10 mg/kg) produced a similar effect. These results suggest that the noradrenergic system is implicated in stimulus seeking behavior and the post-synaptic beta receptors are involved in mediating the behavior.

Inhibitory influence of frontal cortex on locus coeruleus neurons

The functional influence of the frontal cortex (FC) on the noradrenergic nucleus locus coeruleus (LC) was studied in the rat under ketamine anesthesia. The FC was inactivated by local infusion of lidocaine or ice-cold Ringer's solution while recording neuronal activity simultaneously in FC and LC. Lidocaine produced a transient increase in activity in FC, accompanied by a decrease in LC unit and multiunit activity. This was followed by a total inactivation of FC and a sustained increase in firing rate of LC neurons. Subsequent experiments revealed antidromic responses in the FC when stimulation was applied to the LC region. The antidromic responses in FC were found in a population of neurons (about 8%) restricted to the dorsomedial area, FR2. The results indicate that there is a strong inhibitory influence of FC on the tonic activity of LC neurons. The antidromic responses in FC to stimulation of the LC region suggest that this influence is locally mediated, perhaps through interneurons within the nucleus or neighboring the LC.

Response to novelty and its rapid habituation in locus coeruleus neurons of the freely exploring rat

Activity of single units of the noradrenergic nucleus locus coeruleus was recorded in rats during active exploration of a novel environment. Novelty was controlled by the placement of objects in given holes in a hole board. The basic protocol included a habituation session in which the holes were empty and an object session in which a novel object was placed in one of the two holes. During the habituation session, when the whole environment was unfamiliar, there was a phasic response the first time the rat visited any hole, which habituated after one visit. During the second session, when one of the holes contained an object, the cell fired when the rat encountered the novel object. There was no response to empty holes in this session. The neuronal response was markedly diminished or entirely absent on the second and subsequent visits to object-containing holes, indicative of rapid habituation. In some rats it was possible to run a second object session, when a new object was introduced into a previously empty hole. Visits to this hole elicited a robust response, which again habituated after one single visit. The results show that the responses of locus coeruleus to novelty or change, which has been demonstrated in formal learning situations, occurs in freely behaving rats while they are learning about a new environment. Moreover, the response to novelty and change in the environment is short-lived, rapidly habituating after one or two encounters with the stimulus.

Noradrenergic hyperactivity in hippocampus after partial denervation: pharmacological, behavioral, and electrophysiological studies

Previous studies have shown that, in addition to partial damage to the cholinergic system, partial fornix section causes changes in the noradrenergic (NA) system and an increase in NA activity in the dorsal hippocampus. Behaviorally, this NA hyperactivity contributes to the deficits observed in the radial arm maze, since a reduction of NA activity restores the performance of rats with a partial fornix lesion. The reorganization of the NA system after partial fornix section should modify its responsiveness, and the present series of experiments examines these changes. In the first experiment, sensitivity to the sedative effects of the alpha 2 agonist clonidine was evaluated by determining a dose-response curve to clonidine in a hole board. Rats with partial fornix lesion were resistant to the sedative effects of clonidine, suggesting differences in alpha 2-receptor sensitivity. In the second experiments, rats were submitted to a test for novelty-seeking behavior in the hole board with objects placed in some holes. Rats with fornix sections spent more time in contact with novel objects than the control rats, a behavior which has previously been observed in hyper-noradrenergic animals. Finally, single-unit recording of locus coeruleus (LC) cells in anesthetized rats showed there were no effects of the partial fornix lesion either on spontaneous firing rate of LC cells or on their responsiveness to clonidine. These last results suggest that the behavioral differences and differences in NA activity observed after partial denervation are a result of local regulation of release at the NA terminals and are not due to changes in the cell bodies within the LC.

Central noradrenergic reactivity to stress in Maudsley rat strains

Maudsley reactive (MR) and Maudsley nonreactive (MNRA) rats were submitted to a single session of acute 5-min immobilization stress and immediately sacrificed by decapitation. Subsequent neurochemical analysis revealed an elevation of 3,4-dihydroxyphenylacetic acid levels in the locus coeruleus and in the ventrolateral medulla, but not in the dorsomedial medulla, of rats of the two strains compared with nonstressed controls. This response was greater in the MR than in the MNRA group, suggesting a strain difference in the reactivity of the central noradrenergic cells to acute stress.

The Maudsley rat strains as a probe to investigate noradrenergic-cholinergic interaction in cognitive function

Central noradrenergic function in relation to cognitive performance was studied in the Maudsley rat strains. Neurochemical studies revealed a higher response to acute stress in the locus coeruleus (LC) in the Maudsley reactives (MR) than in the Maudsley non-reactives (MNRA). Autoradiographic studies showed that MNRAs had greater 125I clonidine binding to alpha 2 receptors in LC, which was accompanied by a higher behavioral sensitivity to clonidine. MRs had a deficit in working memory, but were superior to MNRAs in two reference memory tasks. MRs displayed a stronger preference for novel objects, with no strain differences in general exploratory activity. The behavioral profile of the MRs is similar to rats treated with drugs which enhance noradrenergic function. Furthermore, MNRA rats had greater availability of muscarinic receptors, which correlated with behavioral performance in the spatial working memory task. The differences in noradrenergic and cholinergic systems and their relationship to the behavioral profile make the Maudsley strains a useful tool to probe the interaction between two neurotransmitter systems in cognitive function.

Locus coeruleus-evoked responses in behaving rats: a clue to the role of noradrenaline in memory

Neuromodulatory properties of noradrenaline (NA) suggest that the coreruleo-cortical NA projection should play an important role in attention and memory processes. Our research is aimed at providing some behavioral evidence. Single units of the locus coeruleus (LC) are recorded during controlled behavioral situations, in order to relate LC activation to specific behavioral contexts. LC cells respond in burst to imposed novel sensory stimuli or to novel objects encountered during free exploration. When there is no predictive value of the stimulus or no behavioral response required, there is rapid habituation of the LC response. When a stimulus is then associated with reinforcement, there is a renewed response, which is transient. During extinction, LC neuronal responses reappear. Thus, LC cells respond to novelty or change in incoming information, but do not have a sustained response to stimuli, even when they have a high level of biological significance. The gating and tuning action of NA released in target sensory systems would promote selective attention to relevant stimuli at the critical moment of change. The adaptive behavioral outcome would result from the integration of retrieved memory with the sensory information selected from the environment.

Maudsley rat strains, selected for differences in emotional responses, differ in behavioral response to clonidine and in [125I]clonidine binding in the locus coeruleus

Maudsley rats, selectively inbred for emotionality for over sixty generations, differ in reactivity to stress, both at the peripheral level and within the central noradrenergic system. The present experiments examine to what extent these central differences might be due to differences in the inhibitory processes mediated by alpha 2 autoreceptors within the locus coeruleus. Maudsley reactive rats (MRs), the strain which showed a much higher central noradrenergic response to immobilisation stress, required higher doses of the alpha 2 receptor agonist, clonidine, to induce behavioral sedation than the Maudsley non-reactive rats (MNRA). Autoradiographic studies showed a significantly higher level of binding of 125iodeclonidine in the locus coeruleus of the MNRAs compared to the MRs, indicating that the former had more alpha 2 receptors and/or these receptors had a greater affinity for the agonist. Thus autoinhibitory processes within the locus coeruleus are different in the two strains, which could account for the differences in reactivity to stress seen in the biochemical and behavioral studies.

Limbic forebrain toxin trimethyltin reduces behavioral suppression by clonidine

Trimethyltin (TMT) at moderate doses selectively damages hippocampus and related olfactory cortex and produces learning and memory impairments. TMT also increases forebrain beta-adrenergic ligand binding; this could be ancillary to reduced noradrenergic neurotransmission, which in turn could be involved in the cognitive deficit caused by TMT. If this hypothesis is correct, then the alpha 1-adrenergic agonist clonidine, which inhibits noradrenergic neurotransmission in normal subjects, should be less behaviourally effective after TMT poisoning. Thus, rats treated with water vehicle or TMT (6 mg/kg, PO) were given saline or clonidine IP (5, 10, or 20 micrograms/kg) 30 min before placement in a hole-board apparatus. Exploratory activity was reduced in controls by 10 or 20 micrograms/kg. Clonidine at 10 micrograms/kg was ineffective in rats given TMT. At 20 micrograms/kg, an apparent reduction in exploratory activity was not significant because variability of responding was higher after TMT treatment. The results suggest an impairment in noradrenergic neurotransmission following TMT poisoning.

Locus coeruleus bursts induced by glutamate trigger delayed perforant path spike amplitude potentiation in the dentate gyrus

Glutamate pressure ejections in the vicinity of locus coeruleus (LC) neurons have been shown to produce both short and long-lasting potentiation of perforant path (PP) evoked population spike amplitude in the dentate gyrus (DG). These effects of LC-glutamate activation resemble those produced by direct application of NE in vitro or in vivo. The present study monitored the cellular response of LC neurons to local glutamate ejections concomitant with stimulation of the PP evoked potential. Double barrel micropipettes or 33 ga cannula-electrode assemblies permitted LC unit recording and glutamate ejection at or near the same site in urethane anesthetized rats. Glutamate ejections produced a burst of LC activity lasting 250-400 ms and followed by a depression of unit activity lasting 4.6 min. The maximal spike potentiation produced by LC activation was 158%. The first spike to exceed the control range occurred 34 s after the LC burst. Comparable silent intervals in LC unit activity induced by systemic clonidine were not accompanied by population spike amplitude potentiation. The mean duration of potentiation was 4.4 min except in four cases where responses remained potentiated for the duration of the experiment. The duration of potentiation was not correlated with the termination of LC depression. LC units recovered to baseline rates following glutamate induced depression of activity. The occurrence of potentiation appeared to require that glutamate activation reach a critical number of LC neurons since small glutamate ejections could produce a local burst without eliciting potentiation. Long-lasting changes were also related to larger glutamate volumes (100 nl).(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic hyperactivity after partial fornix section: role in cholinergic dependent memory performance

Rats with unilateral or bilateral partial section of the fornix were impaired on an eight arm radial maze task. Neurochemical analysis of hippocampal tissue four weeks after the lesions revealed a 50% reduction of choline acetyltransferase (ChAT) activity. The cholinergic marker was correlated negatively with the number of errors in the maze; the lower the ChAT activity, the higher the error score. The fornix lesion also induced a 50% reduction in norepinephrine (NE), but no change in the noradrenergic metabolite methylhydroxyphenylglycol (MHPG), suggesting a net increase in turnover of NE in these animals. Additional lesion of the noradrenergic system with the neurotoxin DSP4 reduced both MHPG and NE levels by more than 90%, compared to nonlesioned controls, and reversed the behavioral deficit. This treatment had no further effect on cholinergic markers. There was a significant negative correlation between ChAT activity and the index of NE turnover, suggesting that hyperactivity in the noradrenergic system after fornix section inhibits the spared cholinergic function and thus exacerbates the cognitive deficit. The pattern of neurochemical results bear a striking resemblance to those seen in some Alzheimer's patients and suggest that an equilibrium among neurotransmitters is important to cognitive function.

Clonidine reverses spatial learning deficits and reinstates theta frequencies in rats with partial fornix section

Rats received knife-cuts to the dorsal fornix or sham-operations. Half of the animals from each group were injected with clonidine (0.01 mg/kg) and the others with saline before each daily trail of a 10-trial radial 8-arm maze task. The number of choices before the first repetition and the run time were used as performance indices. Lesioned rats were significantly impaired in the acquisition of this task. Clonidine-treated rats, lesioned or not, had an acquisition profile indistinguishable from that of sham-operated saline-injected rats, in spite of their increased run time. When tested one week after the last learning trial in a no-drug condition, lesioned rats treated with clonidine throughout learning maintained a high level of performance during the 5-day retraining phase. A parallel analysis of theta rhythms recorded in an independent group of rats placed in equivalent treatment and/or lesion conditions was then performed. Preoperatively, clonidine injections decreased theta frequency during both alert immobility and movement. Partial fornix lesions produced an increase in theta frequency. Finally, clonidine in fornix-damaged rats decreased theta frequency, thus reinstating the postoperative values at a level statistically no different from that recorded preoperatively. The role of clonidine in restoring the function of the septo-hippocampal input in partially fornix-damaged rats through a noradrenergic modulation of hippocampal acetylcholine release is discussed.