Enhancement of excitability and inhibitory processes in hippocampal dentate gyrus by noradrenaline: a pharmacological study in awake, freely moving rats

A Feasibility Study to Investigate Chemogenetic Modulation of the Locus Coeruleus by Means of Single Unit Activity

Aim

Selective chemogenetic modulation of locus coeruleus (LC) neurons would allow dedicated investigation of the role of the LC-NA pathway in brain excitability and disorders such as epilepsy. This study investigated the feasibility of an experimental set-up where chemogenetic modification of the brainstem locus coeruleus NA neurons is aimed at and followed by LC unit activity recording in response to clozapine.

Methods

The LC of male Sprague-Dawley rats was injected with 10 nl of adeno-associated viral vector AAV2/7-PRSx8-hM3Dq-mCherry (n = 19, DREADD group) or AAV2/7-PRSx8-eGFP (n = 13, Controls). Three weeks later, LC unit recordings were performed in anesthetized rats. We investigated whether clozapine, a drug known to bind to modified neurons expressing hM3Dq receptors, was able to increase the LC firing rate. Baseline unit activity was recorded followed by subsequent administration of 0.01 and 0.1 mg/kg of clozapine in all rats. hM3Dq-mcherry expression levels were investigated using immunofluorescence staining of brainstem slices at the end of the experiment.

Results

Unit recordings could be performed in 12 rats and in a total of 12 neurons (DREADDs: n = 7, controls: n = 5). Clozapine 0.01 mg/kg did not affect the mean firing rate of recorded LC-neurons; 0.1 mg/kg induced an increased firing rate, irrespective whether neurons were recorded from DREADD or control rats (p = 0.006). Co-labeling of LC neurons and mCherry-tag showed that 20.6 ± 2.3% LC neurons expressed the hM3Dq receptor. Aspecific expression of hM3Dq-mCherry was also observed in non-LC neurons (26.0 ± 4.1%).

Conclusion

LC unit recording is feasible in an experimental set-up following manipulations for DREADD induction. A relatively low transduction efficiency of the used AAV was found. In view of this finding, the effect of injected clozapine on LC-NA could not be investigated as a reliable outcome parameter for activation of chemogenetically modified LC neurons. The use of AAV2/7, a vector previously applied successfully to target dopaminergic neurons in the substantia nigra, leads to insufficient chemogenetic modification of the LC compared to transduction with AAV2/9.

Effect of anxiety on behavioural pattern separation in humans

Behavioural pattern separation (BPS), the ability to distinguish among similar stimuli based on subtle physical differences, has been used to study the mechanism underlying stimulus generalisation. Fear overgeneralisation is often observed in individuals with posttraumatic stress disorder and other anxiety disorders. However, the relationship between anxiety and BPS remains unclear. The purpose of this study was to determine the effect of anxiety (threat of shock) on BPS, which was assessed across separate encoding and retrieval sessions. Images were encoded/retrieved during blocks of threat or safety in a 2 × 2 factorial design. During retrieval, participants indicated whether images were new, old, or altered. Better accuracy was observed for altered images encoded during periods of threat compared to safety, but only if those images were also retrieved during periods of safety. These results suggest that overgeneralisation in anxiety may be due to altered pattern separation.

Monoamines block kainate- and carbachol-induced gamma-oscillations but augment stimulus-induced gamma-oscillations in rat hippocampus in vitro

Monoamines are implicated in a cognitive processes in a variety of brain regions, including the hippocampal formation, where storage and retrieval of information are facilitated by synchronous network activities. We have investigated the effects of norepinephrine, serotonin, and dopamine on carbachol-, kainate-, and stimulus-induced hippocampal gamma-oscillations employing combined extra- and intracellular recordings. Monoamines dose-dependently and reversibly suppressed kainate- and carbachol-induced gamma-oscillations while increasing the frequency. The effect of serotonin was mimicked by fenfluramine, which releases serotonin from presynaptic terminals. Forskolin also suppressed kainate- and carbachol-induced gamma-oscillations. This effect was mimicked by 8-Br-cAMP and isoproterenol, an agonist of noradrenergic beta-receptor suggesting that the monoamines-mediated suppression of these oscillations could involve intracellular cyclic adenosine 3',5'-cyclic monophosphate (AMP). By contrast, stimulus-induced gamma-oscillations were dose-dependently augmented in power and duration after monoamines application. Intracellular recordings from pyramidal cells revealed that monoamines prolonged the stimulus-induced depolarization and membrane potential oscillations. Stimulus-induced gamma-oscillations were also suppressed by isoproterenol, the D1 agonist SKF-38393 forskolin, and 8-Br-cAMP. This suggests that the augmentation of stimulus-induced gamma-oscillations by monoamines involves--at least in part-different classes of cells than in case of carbachol- and kainate-induced gamma-oscillations.

Profound context-dependent plasticity of mitral cell responses in olfactory bulb

On the basis of its primary circuit it has been postulated that the olfactory bulb (OB) is analogous to the retina in mammals. In retina, repeated exposure to the same visual stimulus results in a neural representation that remains relatively stable over time, even as the meaning of that stimulus to the animal changes. Stability of stimulus representation at early stages of processing allows for unbiased interpretation of incoming stimuli by higher order cortical centers. The alternative is that early stimulus representation is shaped by previously derived meaning, which could allow more efficient sampling of odor space providing a simplified yet biased interpretation of incoming stimuli. This study helps place the olfactory system on this continuum of subjective versus objective early sensory representation. Here we show that odor responses of the output cells of the OB, mitral cells, change transiently during a go-no-go odor discrimination task. The response changes occur in a manner that increases the ability of the circuit to convey information necessary to discriminate among closely related odors. Remarkably, a switch between which of the two odors is rewarded causes mitral cells to switch the polarity of their divergent responses. Taken together these results redefine the function of the OB as a transiently modifiable (active) filter, shaping early odor representations in behaviorally meaningful ways.

Idazoxan increases perforant path-evoked EPSP slope paired pulse inhibition and reduces perforant path-evoked population spike paired pulse facilitation in rat dentate gyrus

Norepinephrine, acting via beta-adrenoceptors, enhances the perforant path-evoked potential in dentate gyrus. Using systemic idazoxan to increase norepinephrine, and paired perforant path pulses to probe early inhibition, previous investigators reported that idazoxan increased initial spike amplitude and increased somatic feedback inhibition. Here, feedback inhibition was re-examined in idazoxan-treated (5 mg/kg) rats under urethane anesthesia. To control for initial increased spike amplitude after idazoxan, evoked potentials were matched, pre- and post-idazoxan, on initial population spike. Input-output current profiles were also compared pre- and post-idazoxan. Saline- and timolol-filled micropipettes permitted evaluation of a contribution of local beta-adrenoceptors. As previously observed, initial spike amplitude was potentiated by idazoxan. Comparable spike potentiation was not seen on the timolol micropipette. Paired pulse inhibition of spike amplitude apparently increased, but input-output curve comparisons revealed a loss of feedback facilitation rather than an increase in feedback inhibition. Initial EPSP slopes were depressed after idazoxan in input-output curve data. EPSP slope feedback ratios were significantly reduced following idazoxan. These data suggest idazoxan has multiple effects on perforant path input to the dentate gyrus. Spike potentiation following idazoxan has previously been shown to depend on intact norepinephrine input. Here, the reduction in spike potentiation on the timolol pipette is consistent with other evidence that norepinephrine-mediated potentiation of the perforant path-evoked potential is dependent on local beta-adrenoceptor activation. The input-output data suggest a decrease in feedback facilitation after idazoxan is likely to account for the apparent increase in feedback inhibition previously reported. Decreased EPSP slope ratios with similar paired pulse intervals have been reported in novel environments. Since exposure to novel environments activates locus coeruleus neurons, norepinephrine may mediate the change in EPSP slope inhibition reported in awake rats. In summary, these results are consistent with the hypothesis that idazoxan potentiates granule cell responses to perforant path input in the dentate gyrus via increases in norepinephrine that lead to beta-adrenoceptor activation, and, further, that idazoxan reduces paired pulse feedback spike facilitation and enhances EPSP slope, but not spike, feedback inhibition.

Locus ceruleus activation suppresses feedforward interneurons and reduces beta-gamma electroencephalogram frequencies while it enhances theta frequencies in rat dentate gyrus

The locus ceruleus is activated by novel stimuli, and its activation promotes learning and memory. Phasic activation of locus ceruleus neurons by glutamate enhances the dentate gyrus population spike amplitude and results in long-term potentiation of synaptic responses recorded after 24 h. Cholinergic activation of locus ceruleus neurons increases hippocampal . At the level of the cellular network, it is not clear how the potentiating effects of norepinephrine are mediated. Previous studies show that exogenous norepinephrine enhances inhibitory interneuron firing in the dentate gyrus. This finding appears at odds with evidence for potentiation. In this study, natural release of norepinephrine was induced by glutamate activation of locus ceruleus while we recorded EEGs and physiologically identified interneurons in the dentate gyrus of urethane-anesthetized rats. Feedforward neurons were inhibited (approximately 1-2 min) by locus ceruleus activation. Feedback interneurons showed both increased and decreased activity, whereas granule cells increased firing as predicted by evoked potential studies. EEG results replicated an increase in power (4-8 Hz) with locus ceruleus activation, but the effect with glutamatergic locus ceruleus activation was transient (approximately 1-2 min). Beta-gamma Frequencies were also transiently suppressed. Together, the data suggest that locus ceruleus activation enhances the throughput of concomitant sensory input by reducing feedforward inhibitory interneuron activity, which may reduce "binding" in existing cell assemblies, and enhances the conditions for synaptic plasticity through disinhibition, promotion of 4-8 Hz , and noradrenergic potentiation to facilitate the building of new representations.

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.

Increased extracellular release of hippocampal NE is associated with tetanization of the medial perforant pathway in the freely moving adult male rat

The induction of long-term potentiation (LTP) within the dentate gyrus of the hippocampal formation is modulated by many afferent influences from a number of subcortical structures known to be intimately involved in hippocampal-dependent learning and memory. It has been demonstrated in slice and anesthetized preparations that norepinephrine (NE) is one of these major neuromodulators involved in the induction of LTP. However, the majority of these studies have not been conducted in the freely moving animal. Recently, we developed surgical procedures and instrumentation techniques to simultaneously record electrophysiological and neurochemical data from the hippocampal formation. The present study uses these techniques to examine the underlying neurochemical changes in the hippocampus associated with the induction of hippocampal dentate LTP in the freely moving adult rat. These findings establish baseline levels of NE that can be used to evaluate the impact of various tetanization paradigms as well as the effect of a variety of insults on hippocampal plasticity.

Restoration of decaying long-term potentiation in the hippocampal formation by stimulation of neuromodulatory nuclei in freely moving rats

Induction of long-term potentiation within the hippocampal formation can be modulated by afferent influences from a number of subcortical structures known to be involved in hippocampal-dependent learning and memory. This study performed on freely moving rats investigated the effects of stimulation of the noradrenergic locus coeruleus nucleus and the serotonergic dorsal raphe nucleus on spontaneously decaying posttetanic long-term potentiation in the dentate gyrus and the hippocampal CA1 area, respectively. High-frequency electrical stimulation of the locus coeruleus or the dorsal raphe elicited a well-expressed behavioural reaction of exploratory or defensive type, respectively, but did not significantly alter transmission at perforant path-dentate gyrus or Schaffer collateral-CA synapses, when delivered either before tetanic stimulation of the perforant path or the Schaffer collaterals or long (hours and days) after previously induced long-term potentiation had completely decayed. However, when locus coeruleus or dorsal raphe stimulation was delivered with the same parameters during a limited time (minutes and hours) after marked or even complete decay of tetanus-induced long-term potentiation at perforant path-dentate gyrus or Schaffer collateral-CA1 synapses, the potentiation was partially or entirely restored but never increased beyond the initial level of potentiation. In CA1, stimulation of ipsilateral and contralateral Schaffer collaterals demonstrated that the restoration of previously existing long-term potentiation by dorsal raphe stimulation was input-specific, occurring, like tetanus-induced potentiation, only in the pathway which had previously been tetanized. These findings suggest that the noradrenergic locus coeruleus and the serotonergic dorsal raphe can influence not only induction, but also spontaneous decay of long-term potentiation in the hippocampal formation. Since hippocampal long-term potentiation is thought to play a role in certain kinds of learning and memory, and association of tetanic stimulation with activation of ascending neuromodulatory systems is required for full expression of long-term potentiation, the restoration of hippocampal long-term potentiation by activation of a neuromodulatory system alone may serve as a mechanism of associative reminder which may underlie facilitation of memory retrieval after a period of forgetting, as has been observed in trained rats under similar conditions.

A functional model of some Parkinson's disease symptoms using a Guided Propagation Network

This paper presents a computational model of Parkinson's Disease (PD) symptoms. Based on psychophysiological data, the underlying system (Guided Propagation Network) implements coincidence detection between internal flows and stimuli, and can be dynamically controlled for representing the action of neuromodulators such as dopamine (DA). By modelling the DA deficit involved in PD through a decrease of response thresholds in the production modules of a GPN, four symptoms are observed in experiments carried out on a computer simulation, and then attributed to a lack of synchrony between 'proprioceptive stimuli' and internal flows: reduced intensity, increased rate, saccades and spontaneous repetitions.

Systemic administration of atipamezole, a selective antagonist of alpha-2 adrenoceptors, facilitates behavioural activity but does not influence short-term or long-term memory in trimethyltin-intoxicated and control rats

The present study used trimethyltin (TMT)-intoxicated rats as a model for the behavioural syndrome seen after neuronal damage to the limbic system. Behavioural assessments indicated increased locomotor activity and reduced number of groomings in an open-arena task in TMT-intoxicated (6.6 mg/kg as a free base) rats, as has been found previously. A novel finding was the severe deficit in swimming to a visible platform in the water maze task, with reduced swimming speed at the beginning of the training period. During the reacquisition phase of a radial arm maze task, TMT-intoxicated rats made more short-term and long-term memory errors, and their behavioural activity was increased in comparison with controls. The administration of atipamezole (300 micrograms/kg), a selective antagonist of alpha 2-adrenoceptors, enhanced locomotor activity compared to saline-treated rats, but these effects did not differ between the TMT group and their controls. Atipamezole did not enhance short-term or long-term memory in either TMT or control groups. Taken together, the present data indicate that TMT intoxication is a model for global dementia rather than for a specific loss of relational memory. Previous studies on the neurochemical effects of TMT and the alleviation or prevention of neurotoxicity of TMT are reviewed.

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.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Modulation of vigilance and behavioral activation by alpha-1 adrenoceptors in the rat

This study investigated the role of alpha-1 adrenergic receptors in the modulation of attention and behavioral activity by assessing the effects of alpha-1 adrenergic receptor stimulation or blockade on the performance of rats in tasks involving vigilance (sustained attention) and selective attention [five-choice serial reaction time (5-CSRT)]. Pretesting subcutaneous administration of St-587 (a putative alpha-1 agonist) at 100 micrograms/kg, but not at 300 or 1000 micrograms/kg, significantly improved the choice accuracy of rats in the 5-CSRT task (monitoring of visual stimuli), whereas prazosin (a prototype alpha-1 antagonist) at 300 micrograms/kg administered subcutaneously slightly impaired choice accuracy of the rats in this task. Prazosin at 100 micrograms/kg blocked the ability of St-587 at 100 micrograms/kg to improve choice accuracy. Furthermore, St-587 at 100 micrograms/kg significantly increased the number of trials completed and reduced the probability of premature responses, whereas prazosin at 300 micrograms/kg decreased the number of trials completed and the latency of animals to make correct responses in the task. Prazosin at 100 micrograms/kg blocked the effect of St-587 at 100 micrograms/kg in increasing the number of trials completed. However, prazosin at 100 micrograms/kg did not abolish the effect of St-587 in reducing the probability of premature responses. Because the effect of St-587 at 100 micrograms/kg in improving choice accuracy is rather modest, it is possible that when the 100- and 300-microgram/kg doses of St-587 were administered in a counterbalanced order, this effect could have been overlooked due to day-to-day variation. Thus, the present results suggest that stimulation of alpha-1 adrenergic receptors can facilitate vigilance.

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.

Dexmedetomidine reduces response tendency, but not accuracy of rats in attention and short-term memory tasks

The present study investigated the role of alpha 2-adrenergic mechanisms in the performance of motor responses, attention and short-term memory in rats. A low dose (3.0 micrograms/kg, s.c.) of dexmedetomidine, an alpha 2-adrenoceptor agonist, reduced response tendency in an attentional task and a working memory task, but it did not affect the choice accuracy of rats. Atipamezole (300 micrograms/kg), an alpha 2-adrenoceptor antagonist, increased anticipatory responding. Although atipamezole did not affect the number of omissions, it reversed the effects of dexmedetomidine on that parameter. We also investigated the effects of dexmedetomidine in rats with partial destruction of noradrenergic nerves induced by the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). On its own, DSP-4 treatment did not affect choice accuracy or behavioural activity of rats in the attentional task. The effects of dexmedetomidine (0.3-3.0 micrograms/kg) on anticipatory responses did not differ between controls and DSP-4 group. Furthermore, the effect on omissions was not consistently diminished in DSP-4 treated rats. These results suggest that the activation of postsynaptic alpha 2-adrenoreceptors may be responsible for dexmedetomidine-induced reduction of response tendency while attention and short-term memory are not markedly affected.

The azapirone metabolite 1-(2-pyrimidinyl)piperazine depresses excitatory synaptic transmission in the hippocampus of the alert rat via 5-HT1A receptors

The effects of acute and repeated treatment with 1-(2-pyrimidinyl)piperazine (1-PP), a metabolite of the 5-HT1A receptor ligand azapirones, were investigated on hippocampal excitatory synaptic transmission. Recordings of the electrically evoked field population excitatory post-synaptic potentials (e.p.s.p.s.) were carried out in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert rats. Acute i.p. administration of 1-PP transiently reduced the e.p.s.p. amplitude in a dose-dependent (0.25-1 mg/kg) manner. This effect was blocked by the 5-HT1A receptor antagonists spiroxatrine (1 mg/kg) and MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl methylaminoethyl]-8-azaspirol[4,5]decane-7,9-dione methyl sulphonate, 2 mg/kg). Intrahippocampal administration of 1-PP (5 microg) evoked a transient reduction of the e.p.s.p. amplitude which was similar to that obtained with 5-HT (10 microg). 1-PP (0.25 mg/kg per day) administered for 9 days produced a gradual reduction in the daily pre-injection baseline e.p.s.p. amplitude coupled with a decrease in the acute response to the drug. The chronic baseline reduction was transiently reversed by spiroxatrine and full recovery to pretreatment levels was observed 4 days after the last 1-PP dose. These findings indicate that the previously reported reduction in the e.p.s.p. produced by the azapirone group of 5-HT1A receptor ligands may be mediated in part by their metabolite 1-PP through activation of 5-HT1A receptors.

Decreased hippocampal noradrenaline does not affect corticosterone release following electrical stimulation of CA1 pyramidal cells

Bipolar electrodes were implanted into the CA1 pyramidal cells of the dorsal hippocampus and the effect of electrical stimulation of these cells on corticosterone secretion was investigated in freely moving rats. Histology showed that the electrodes were positioned in close proximity to the CA1 pyramidal cells. Rats that were subjected to high intensity electrical stimulation (1, 10, and 100 microA) behaved differently when compared to their sham stimulated controls. They were more active and displayed wet dog shakes. Plasma corticosterone levels increased dose-dependently in rats subjected to different electrical stimulation intensities. Although prior treatment (24 hours) of rats with DSP4 (60 mg/kg, i.p.) significantly reduced hippocampal noradrenaline content by 46%, it did not bring about any behavioural changes. DSP4 treatment also had no effect on electrically stimulated corticosterone release. These data suggested that stimulation of CA1 pyramidal cells may lead to increased corticosterone release and that a decrease in hippocampal noradrenaline concentration was unable to alter this corticosterone response.

Intraventricular administration of galanin does not affect behaviors associated with locus coeruleus activation in rats

The 29 amino acid peptide galanin (GAL) coexists with norepinephrine in rat locus coeruleus (LC) neurons to a remarkably high degree. The effects of central administration of GAL were examined in three behavioral paradigms that putatively involve increases in the activity of LC neurons. GAL did not affect behavioral signs associated with naloxone-precipitated withdrawal in rats treated chronically with morphine, a condition in which the firing rate of LC neurons is dramatically increased, although the behavioral signs of withdrawal were abolished by clonidine. Foot shock induced freezing behavior was similarly unaffected by either dose of GAL but was significantly diminished by clonidine and the corticotropin-releasing factor (CRF) antagonist alpha-helical CRF. GAL did not influence the decrease in exploratory activity in a novel open field induced by idazoxan. The behavioral activity of the peptide and route of administration were confirmed in a feeding paradigm. Doses of GAL that were inactive in the three paradigms were active in stimulating intake of a palatable food to a similar degree as clonidine-stimulated intake. These results suggest that intraventricularly administered GAL may not influence behaviors thought to be mediated by activation of neurons in the LC.

Locus coeruleus activation induces perforant path-evoked population spike potentiation in the dentate gyrus of awake rat

In vitro norepinephrine (NE) induces both short- and long-term beta-receptor-mediated potentiation of the perforant path-evoked population spike in the dentate gyrus. NE or locus coeruleus (LC) activation in vivo also produces a beta-receptor dependent potentiation of population spike amplitude in anesthetized rat. Studies of behavioral state modulation of population spike amplitude in awake rats, and in rats depleted of NE, however, have led to the hypothesis that LC-NE activation should act to suppress or reduce population spike amplitude in the dentate gyrus of unanesthetized rat. Using glutamate activation of LC in awake unrestrained rats (n = 12), the present study provides evidence that LC activation in the awake rat does not reduce, but potentiates, population spike amplitude. The potentiation effect was long-lasting (> 25 min) in 50% of the experiments. In addition glutamate ejections in the third lobe of cerebellar rostral vermis produced potentiation of population spike amplitude (n = 3) and population excitatory postsynaptic potential slope. Ejections at sites outside the LC and rostral vermis were ineffective (n = 5). Behavioral effects of glutamate ejection did not predict the occurrence of potentiation. These data support the hypothesis that phasic activation of LC cells is likely to induce short-term, and possibly long-term, potentiation of dentate gyrus throughput in alert animals.

The effects of selective alpha-2 adrenergic agents on the performance of rats in a 5-choice serial reaction time task

The present study investigated how the systemic administration of alpha-2 adrenergic agents that modulate the function of the noradrenergic system in brain, affect rousal and sustained attention. Food-deprivated rats were trained to detect and respond to brief flashes of light presented randomly in one of five spatially diverse locations. The effects of single-dose administrations of dexmedetomidine, an alpha-2 agonist, and atipamezole, an alpha-2 antagonists, on the choice accuracy, errors of omissions, speed of responding, and collection of the reward could be assessed in this task. Dexmedetomidine increased the amount of omissions, speed of response, and decreased the number of premature responses, although it did not markedly lengthen response latencies and food collection latency. Atipamezole increased the number of premature responses. Neither dexmedetomidine nor atipamezole had any effect on choice accuracy of rats in this task. The results suggest that dexmedetomidine decreased behavioral activity and arousal of rats, whereas atipamezole had mild stimulant effect on behavior.

Use-dependent effects of acute and chronic treatment with imipramine and buspirone on excitatory synaptic transmission in the rat hippocampus in vivo

The effects of acute and long-term treatment with imipramine and buspirone on the responses of rat hippocampal neurones to low and high frequency electrical stimulation were compared. Whereas acute treatment with imipramine (10 mg/kg, i.p.) had no effect on synaptic responses to low frequency stimulation, chronic treatment for 14 days significantly reduced the amplitude of the field excitatory postsynaptic potential. Both acute and chronic imipramine treatment markedly reduced the amplitude of the nerve volley and excitatory postsynaptic potential evoked at high frequency stimulation rates in a use-dependent manner. Buspirone (0.5-3 mg/kg, i.p.) produced a significant reduction of the excitatory postsynaptic potential at high frequencies. This was enhanced after repeated administration of a dose of 0.5 mg/kg for 14 days. We previously reported a similar effect of buspirone at low frequency stimulation. Both compounds therefore share the ability to exert strong depressant effects on transmission in the hippocampus especially after chronic treatment.

Dose- and parameter-dependent effects of atipamezole, an alpha 2-antagonist, on the performance of rats in a five-choice serial reaction time task

The present study investigated whether atipamezole (ATI), a potent alpha 2-adrenoceptor antagonist that increases the release of noradrenaline in brain, improves attention in rats. Thus, the effects of ATI on the performance of adult male rats in the five-choice serial reaction time task were studied. Food-deprived rats were trained to detect and respond to brief flashes of light presented randomly in one of five spatially diverse locations. The effects of single-dose administration of ATI (0.03-3.0 mg/kg) on the performance of rats under different parametric manipulations of the task were tested: 1) the visual stimuli were presented at unpredictable intertrial intervals (ITIs) or b) the intensity (brightness) of visual stimuli was reduced, thus placing an additional load on attentional processing for animals. Presenting the stimuli earlier than normally or reducing its intensity markedly impaired the choice accuracy of rats. At doses of 0.03, 0.3, and 1.0 mg/kg, ATI improved the choice accuracy of rats when tested using reduced stimulus intensity. ATI 3.0 mg/kg did not affect accuracy performance when tested using reduced stimulus intensity but impaired it when tested using unpredictable ITIs. The other doses of ATI (0.03, 0.3, and 1.0 mg/kg) did not markedly affect choice accuracy of rats tested using unpredictable ITI. Our results could be explained by the assumption that an acute, systemic administration of ATI affects arousal mechanisms and facilitates the processing of visual stimuli related to reward.

Biochemical correlates of long-term potentiation in hippocampal synapses

Figure 2 summarizes biochemical events which are currently known or hypothesized to participate in LTP induction/maintenance. Current evidence strongly suggests that postsynaptic Ca2+, both entered from the outside of cells and released from intracellular stores, is the initial key substance for the induction of LTP. A rise of [Ca2+]i triggers a variety of enzymatic reactions and initiates the enhancement of synaptic transmission. This first step may be achieved by direct/indirect phosphorylations of protein molecules in postsynaptic receptors/ion channels. This would result in an increase in receptor sensitivity. An immediate increase in the number of available postsynaptic receptors by modifications of spine morphology is another candidate. Such modifications may be accomplished by cytoskeleton rearrangements or changes in extracellular environments. A change in spine structure may also cause an increase in spine neck conductance. Although it is unknown to what extent the increase in [Ca2+]i affects cellular chemistry, Ca2+ probably also directly/indirectly stimulates cascades which exert effects more slowly. A delayed increase in metabotropic receptor sensitivity may occur. New synthesis of protein molecules may be involved in late periods of LTP by replacing turnovered molecules and/or by supplying new materials. Some of these chains of biochemical events may also apply to presynaptic terminals, although the existence of retrograde messenger substances must still be confirmed. In addition, interactions between different protein kinases and second messengers appear to occur to bring about final effects.

Electrolytic lesions of the locus coeruleus or 6-hydroxydopamine lesions of the medial forebrain bundle protect against excitotoxic damage in rat hippocampus

Unilateral lesioning of the rat locus coeruleus by an electric current (0.3 mA/10 s) reduces the ipsilateral hippocampal tissue content of dopamine (DA) and noradrenaline (NA) by 80% and 60% respectively, and protects against the excitotoxic action of bilateral kainate injections (1.1 nmol/microliters) in the hippocampus. On the ipsilateral side hippocampal CA1, CA2 and DG cells were protected; lesser protection of CA1 cells was seen on the contralateral side. The unilateral injection of 6-hydroxydopamine (8.8 micrograms/2 microliters/5 min) into the medial forebrain bundle abolishes the increase in extracellular NA concentration induced by kainate in the ipsilateral hippocampus without altering the low extracellular DA levels. This medial forebrain bundle lesion protects (on the ipsilateral side) against kainate toxicity (500 microM through the probe for 20 min) in the hippocampus (CA1, CA2 and DG cells). The noradrenergic innervation of the hippocampus apparently potentiates the excitotoxic effect of focal injection of kainate.