Noradrenergic responses in rat hippocampus: evidence for medication by alpha and beta receptors in the in vitro slice

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Noradrenergic Profile of Hippocampal Formation Theta Rhythm in Anaesthetized Rats

The present study was undertaken to identify the noradrenergic receptors underlying the production of hippocampal formation (HPC) type 2 theta rhythm. The experiments were performed on urethanized rats wherein type 2 theta is the only rhythm present. In three independent stages of experiments, the effects of noradrenaline (NE) and selective noradrenergic α and β agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and β1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC β2 and β3 noradrenergic receptors do not seem to be directly involved in the pharmacological mechanism responsible for the suppression of theta rhythm in anaesthetized rats. Obtained results provide evidence for the suppressant effect of exogenous NE on HPC type 2 theta rhythm and show the crucial role of α1, α2 and β1 noradrenergic receptors in the modulation of HPC mechanisms of oscillations and synchrony. This finding is in contrast to the effects of endogenous NE produced by electrical stimulation of the locus coeruleus (LC) and procaine injection into the LC (Broncel et al., 2020).

The role of α adrenergic receptors in mediating the inhibitory effect of electrical brain stimulation on epileptiform activity in rat hippocampal slices

The Inhibitory effect of electrical low-frequency stimulation (LFS) on neuronal excitability and seizure occurrence has been indicated in experimental models, but the precise mechanism has not established. This investigation was intended to figure out the role of α₁ and α₂ adrenergic receptors in LFS' inhibitory effect on neuronal excitability. Epileptiform activity induced in an in vitro rat hippocampal slice preparation by high K⁺ ACSF and LFS (900 square wave pulses at 1 Hz) was administered at the beginning of epileptiform activity to the Schaffer collaterals. In CA1 pyramidal neurons, the electrophysiological properties were measured at the baseline, before high K⁺ ACSF washout, and at 15 min after high K⁺ ACSF washout using whole-cell, patch-clamp recording. Results indicated that after high K⁺ ACSF washout, prazosine (10 µM; α₁ adrenergic receptor antagonist) and yohimbine (5 µM; α₂ adrenergic receptor antagonist) suppressed the LFS' effect of reducing rheobase current and utilization time following depolarizing ramp current, the latency to the first spike following a depolarizing current and latency to the first rebound action potential following hyperpolarizing current pulses. Thus, it may be proposed that LFS' inhibitory action on the neuronal hyperexcitability, in some way, is mediated by α₁ and α₂ adrenergic receptors.

Noradrenergic Modulation of Cognition in Health and Disease

Norepinephrine released by the locus coeruleus modulates cellular processes and synaptic transmission in the central nervous system through its actions at a number of pre- and postsynaptic receptors. This transmitter system facilitates sensory signal detection and promotes waking and arousal, processes which are necessary for navigating a complex and dynamic sensory environment. In addition to its effects on sensory processing and waking behavior, norepinephrine is now recognized as a contributor to various aspects of cognition, including attention, behavioral flexibility, working memory, and long-term mnemonic processes. Two areas of dense noradrenergic innervation, the prefrontal cortex and the hippocampus, are particularly important with regard to these functions. Due to its role in mediating normal cognitive function, it is reasonable to expect that noradrenergic transmission becomes dysfunctional in a number of neuropsychiatric and neurodegenerative diseases characterized by cognitive deficits. In this review, we summarize the unique role that norepinephrine plays in prefrontal cortical and hippocampal function and how its interaction with its various receptors contribute to cognitive behaviors. We further assess the changes that occur in the noradrenergic system in Alzheimer's disease, Parkinson's disease, attention-deficit/hyperactivity disorder, and schizophrenia and how these changes contribute to cognitive decline in these pathologies.

Effects of norepinephrine on spontaneous firing activity of cerebellar Purkinje cells in vivo in mice

Norepinephrine (NE), from the locus coeruleus (LC), has been supported to affect GABAergic system and parallel fiber (PF)-Purkinje cell (PC) synaptic transmission via adrenoceptor in cerebellum cortex. However, the effects of NE on the spontaneous spike activity of cerebellar PCs in living mouse have not yet been fully understood. We here examined the effects of NE on the spontaneous activity of PC in urethane-anesthetized mice by electrophysiological and pharmacological methods. Cerebellar surface application of NE (2.5-25μM) reduced the PC simple spike (SS) firing rate in a dose-dependent manner. The half-inhibitory concentration (IC50) was 5.97μM. In contrast, NE significantly increased the spontaneous firing rate of molecular layer interneuron (MLI). Application of GABAA receptor antagonist, gabazine (SR95531, 20μM) not only blocked the NE-induced inhibition of PC SS firing but also revealed NE-induced excitation of cerebellar PC. Blocking AMPA receptors activity enhanced NE-induced inhibition of PC spontaneous activity. Moreover, the effects of NE on PC spontaneous activity were abolished by simultaneously blocking GABAA and AMPA receptors activity. These results indicated that NE bidirectional modulated the spontaneous activity of PCs via enhancing both inhibitory inputs from MLIs and excitatory inputs of parallel fibers, but NE-induced enhance of inhibitory inputs overwhelmed the excitatory inputs under in vivo conditions.

β-Adrenergic Control of Hippocampal Function: Subserving the Choreography of Synaptic Information Storage and Memory

Noradrenaline (NA) is a key neuromodulator for the regulation of behavioral state and cognition. It supports learning by increasing arousal and vigilance, whereby new experiences are “earmarked” for encoding. Within the hippocampus, experience-dependent information storage occurs by means of synaptic plasticity. Furthermore, novel spatial, contextual, or associative learning drives changes in synaptic strength, reflected by the strengthening of long-term potentiation (LTP) or long-term depression (LTD). NA acting on β-adrenergic receptors (β-AR) is a key determinant as to whether new experiences result in persistent hippocampal synaptic plasticity. This can even dictate the direction of change of synaptic strength.

The different hippocampal subfields play different roles in encoding components of a spatial representation through LTP and LTD. Strikingly, the sensitivity of synaptic plasticity in these subfields to β-adrenergic control is very distinct (dentate gyrus > CA3 > CA1). Moreover, NA released from the locus coeruleus that acts on β-AR leads to hippocampal LTD and an enhancement of LTD-related memory processing. We propose that NA acting on hippocampal β-AR, that is graded according to the novelty or saliency of the experience, determines the content and persistency of synaptic information storage in the hippocampal subfields and therefore of spatial memories.

The role of REM sleep theta activity in emotional memory

While non-REM (NREM) sleep has been strongly implicated in the reactivation and consolidation of memory traces, the role of rapid-eye movement (REM) sleep remains unclear. A growing body of research on humans and animals provide behavioral evidence for a role of REM sleep in the strengthening and modulation of emotional memories. Theta activity-which describes low frequency oscillations in the local field potential within the hippocampus, amygdala and neocortex-is a prominent feature of both wake and REM sleep in humans and rodents. Theta coherence between the hippocampus and amygdala drives large-scale pontine-geniculo-occipital (PGO) waves, the density of which predicts increases in plasticity-related gene expression. This could potentially facilitate the processing of emotional memory traces within the hippocampus during REM sleep. Further, the timing of hippocampal activity in relation to theta phase is vital in determining subsequent potentiation of neuronal activity. This could allow the emotionally modulated strengthening of novel and gradual weakening of consolidated hippocampal memory traces during REM sleep. Hippocampal theta activity is also correlated with REM sleep levels of achetylcholine - which is thought to reduce hippocampal inputs in the neocortex. The additional low levels of noradrenaline during REM sleep, which facilitate feedback within the neocortex, could allow the integration of novel memory traces previously consolidated during NREM sleep. We therefore propose that REM sleep mediates the prioritized processing of emotional memories within the hippocampus, the integration of previously consolidated memory traces within the neocortex, as well as the disengagement of consolidated neocortical memory traces from the hippocampus.

The neuroenergetics of stress hormones in the hippocampus and implications for memory

Acute stress causes rapid release of norepinephrine (NE) and glucocorticoids (GCs), both of which bind to hippocampal receptors. This release continues, at varying concentrations, for several hours following the stressful event, and has powerful effects on hippocampally-dependent memory that generally promote acquisition and consolidation while impairing retrieval. Several studies have characterized the brain's energy usage both at baseline and during memory processing, but there are few data on energy requirements of memory processes under stressful conditions. Because memory is enhanced by emotional arousal such as during stress, it is likely that molecular memory processes under these conditions differ from those under non-stressful conditions that do not activate the hypothalamic-pituitary-adrenal (HPA) axis. Mobilization of peripheral and central energy stores during stress may increase hippocampal glucose metabolism that enhances salience and detail to facilitate memory enhancement. Several pathways activated by the HPA axis affect neural energy supply and metabolism, and may also prevent detrimental damage associated with chronic stress. We hypothesize that alterations in hippocampal metabolism during stress are key to understanding the effects of stress hormones on hippocampally-dependent memory formation. Second, we suggest that the effects of stress on hippocampal metabolism are bi-directional: within minutes, NE promotes glucose metabolism, while hours into the stress response GCs act to suppress metabolism. These bi-directional effects of NE and GCs on glucose metabolism may occur at least in part through direct modulation of glucose transporter-4. In contrast, chronic stress and prolonged elevation of hippocampal GCs cause chronically suppressed glucose metabolism, excitotoxicity and subsequent memory deficits.

Noradrenergic modulation of the parallel fiber-Purkinje cell synapse in mouse cerebellum

The signals arriving to Purkinje cells via parallel fibers are essential for all tasks in which the cerebellum is involved, including motor control, learning new motor skills and calibration of reflexes. Since learning also requires the activation of adrenergic receptors, we investigated the effects of adrenergic receptor agonists on the main plastic site of the cerebellar cortex, the parallel fiber-Purkinje cell synapse. Here we show that noradrenaline serves as an endogenous ligand for both α1-and α2-adrenergic receptors to produce synaptic depression between parallel fibers and Purkinje cells. On the contrary, PF-EPSCs were potentiated by the β-adrenergic receptor agonist isoproterenol. This short-term potentiation was postsynaptically expressed, required protein kinase A, and was mimicked by the β2-adrenoceptor agonist clenbuterol, suggesting that the β2-adrenoceptors mediate the noradrenergic facilitation of synaptic transmission between parallel fibers and Purkinje cells. Moreover, β-adrenoceptor activation lowered the threshold for cerebellar long-term potentiation induced by 1 Hz parallel fiber stimulation. The presence of both α and β adrenergic receptors on Purkinje cells suggests the existence of bidirectional mechanisms of regulation allowing the noradrenergic afferents to refine the signals arriving to Purkinje cells at particular arousal states or during learning.

β-Adrenergic modulation of spontaneous spatiotemporal activity patterns and synchrony in hyperexcitable hippocampal circuits

A description of healthy and pathological brain dynamics requires an understanding of spatiotemporal patterns of neural activity and characteristics of its propagation between interconnected circuits. However, the structure and modulation of the neural activation maps underlying these patterns and their propagation remain elusive. We investigated effects of β-adrenergic receptor (β-AR) stimulation on the spatiotemporal characteristics of emergent activity in rat hippocampal circuits. Synchronized epileptiform-like activity, such as interictal bursts (IBs) and ictal-like events (ILEs), were evoked by 4-aminopyridine (4-AP), and their dynamics were studied using a combination of electrophysiology and fast voltage-sensitive dye imaging. Dynamic characterization of the spontaneous IBs showed that they originated in dentate gyrus/CA3 border and propagated toward CA1. To determine how β-AR modulates spatiotemporal characteristics of the emergent IBs, we used the β-AR agonist isoproterenol (ISO). ISO significantly reduced the spatiotemporal extent and propagation velocity of the IBs and significantly altered network activity in the 1- to 20-Hz range. Dual whole cell recordings of the IBs in CA3/CA1 pyramidal cells and optical analysis of those regions showed that ISO application reduced interpyramidal and interregional synchrony during the IBs. In addition, ISO significantly reduced duration not only of the shorter duration IBs but also the prolonged ILEs in 4-AP. To test whether the decrease in ILE duration was model dependent, we used a different hyperexcitability model, zero magnesium (0 Mg(2+)). Prolonged ILEs were readily formed in 0 Mg(2+), and addition of ISO significantly reduced their durations. Taken together, these novel results provide evidence that β-AR activation dynamically reshapes the spatiotemporal activity patterns in hyperexcitable circuits by altering network rhythmogenesis, propagation velocity, and intercellular/regional synchronization.

Noradrenaline increases neural precursor cells derived from adult rat dentate gyrus through β2 receptor

Several preclinical researches indicate that increased neurogenesis in the adult hippocampus might underlie the therapeutic effect of antidepressant treatment. Most antidepressant drugs have ability to increase serotonin (5-HT) and/or noradrenaline (NA) in brain, and chronic treatment with antidepressant drugs increases the number of proliferating neural precursor cells and neurogenesis in hippocampus. However, the direct effects of antidepressant drugs, 5-HT and NA on the neural precursor cells remain largely unknown. Neural precursor cells in adult hippocampus are divided into stem/progenitor cells of four types based on stages of neural development. We recently established a culture system of adult rat dentate gyrus-derived neural precursor cells (ADPs), which correspond to be type 2a early progenitor cells. Here the direct effects of antidepressant drugs of four types (fluoxetine, imipramine, reboxetine, and tranylcypromine) and two neurotransmitters (5-HT and NA) on the proliferation of ADPs were investigated. Neither antidepressant drugs of all types nor 5-HT increased the number of ADPs. On the other hand, NA increased the number and the DNA synthesis of ADPs. The effect of NA on ADP proliferation was antagonized by propranolol and timolol (β-adrenergic receptor (AR) antagonists), but not by phentolamine (α-AR antagonist), prazosin (α1-AR antagonist), or yohimbine (α2-AR antagonist). Moreover, it was antagonized by ICI 118, 551 (β2-AR selective antagonist) and salmeterol (β2-AR selective agonist) promoted ADP proliferation. These results suggest that NA might increase the proliferation of early progenitor cells in adult hippocampus via β2-AR directly, but antidepressant drugs and 5-HT do not.

Neuroplasticity regulation by noradrenaline in mammalian brain

The neuromodulator noradrenaline (NA) is released in almost all brain areas in a highly diffused manner. Its action is slow, as it acts through G protein-coupled receptors, but its wide release in the brain makes NA a crucial regulator for various fundamental brain functions such as arousal, attention and memory processes [102]. To understand how NA acts in the brain to promote such diverse actions, it is necessary to dissect the cellular actions of NA at the level of single neurons as well as at the level of neuronal networks. In the present article, we will provide a compact review of the main literatures concerning the NA actions on neuroplasticity processes. Depending on which subtype of adrenoceptor is activated, NA differently affects intrinsic membrane properties of postsynaptic neurons and synaptic plasticity. For example, beta-adrenoceptor activation is mainly related to the potentiation of synaptic responses and learning and memory processes. alpha2-adrenoceptor activation may contribute to a high-order information processing such as executive function, but currently the direction of synaptic plasticity modification by alpha2-adrenoceptors has not been clearly determined. The activation of alpha1-adrenoceptors appears to mainly induce synaptic depression in the brain. But its physiological roles are still unclear: while its activation has been described as beneficial for cognitive functions, it may also exert detrimental effects in some brain structures such as the prefrontal cortex.

Viagra for your synapses: Enhancement of hippocampal long-term potentiation by activation of beta-adrenergic receptors

Beta-adrenergic receptors (beta-ARs) critically modulate long-lasting synaptic plasticity and long-term memory storage in the mammalian brain. Synaptic plasticity is widely believed to mediate memory storage at the cellular level. Long-term potentiation (LTP) is one type of synaptic plasticity that has been linked to memory storage. Activation of beta-ARs can enhance LTP and facilitate long-term memory storage. Interestingly, many of the molecular signaling pathways that are critical for beta-adrenergic modulation of LTP mirror those required for the persistence of memory. In this article, we review the roles of signaling cascades and translation regulation in enabling beta-ARs to control expression of long-lasting LTP in the rodent hippocampus. These include the cyclic-AMP/protein kinase-A (cAMP-PKA) and extracellular signal-regulated protein kinase cascades, two key pathways known to link transmitter receptors with translation regulation. Future research directions are discussed, with emphasis on defining the roles of signaling complexes (e.g. PSD-95) and glutamatergic receptors in controlling the efficacy of beta-AR modulation of LTP.

MDMA induces EPSP-Spike potentiation in rat ventral hippocampus in vitro via serotonin and noradrenaline release and coactivation of 5-HT4 and beta1 receptors

It is well documented that N-methyl-3,4-methylenedioxyamphetamine (MDMA, ecstasy) releases brain serotonin (5-HT; 5-hydroxytryptamine), noradrenaline (NE; norepinephrine), and dopamine, but the consequent effect on brain functioning remains elusive. In this study, we characterized the effects of MDMA on electrically evoked responses in the ventral CA1 region of a rat hippocampal slice preparation. Superfusion with MDMA (10 microM, 30 min) increased the population spike amplitude (PSA) by 48.9+/-31.2% and decreased population spike latency (PSL) by 103+/-139 mus (both: mean+/-SD, n=123; p<0.0001, Wilcoxon test), without affecting field excitatory postsynaptic potential (fEPSP). This effect persisted for at least 1 h after MDMA washout; we have called this EPSP-spike potentiation (ESP) by MDMA, ESP MDMA. Antagonism of GABAergic transmission did not prevent ESP MDMA, suggesting that an increase in excitability of pyramidal cells underlies this MDMA action. Block of serotonin transporter (SERT) with citalopram or 5-HT depletion with (+/-)-p-chlorophenylalanine pretreatment partially inhibited the ESP MDMA. Block of both SERT and NE transporter prevented ESP MDMA, indicating its dependence on release of both 5-HT and NE. ESP MDMA is produced by simultaneous activation of 5-HT4 and beta1 receptors, with a predominant role of 5-HT4 receptors. Block of both 5-HT4 and beta1 receptors revealed an inhibitory component of the MDMA action mediated by 5-HT1A receptor. The concentration range of MDMA which produced ESP MDMA (1-30 microM) corresponds to that commonly reached in human plasma following the ingestion of psychoactive MDMA doses, suggesting that release of both 5-HT and NE, and consequent ESP MDMA may underlie some of the psychoactive effects of MDMA in humans.

Impaired hippocampal LTP in inbred mouse strains can be rescued by β-adrenergic receptor activation

Long-term potentiation (LTP), an activity-dependent enhancement of synaptic strength, and memory can be influenced by neuromodulatory transmitters such as norepinephrine (NE) and also by genetic background. beta-Adrenergic receptor activation can facilitate the expression of hippocampal CA1 LTP induced by weak stimulus patterns, but its influence on LTP induced by stronger stimulus patterns is unclear. We examined neural NE and dopamine (DA) levels, beta-adrenergic receptor expression and hippocampal LTP in genetically diverse inbred mouse strains. Brain tissue levels of NE were significantly lower in strains 129S1/SvImJ (129), BALB/cByJ (BALB) and C3H/HeJ (C3H) than in C57BL/6NCrlBR (B6). Western blot analysis showed that hippocampal beta(1)-adrenergic receptor expression was similar in strains B6, 129 and C3H, but was increased in BALB. LTP was induced in area CA1 of hippocampal slices by four trains of high-frequency stimulation (HFS) of the Schaeffer collaterals in the four inbred strains. Two hours after induction, LTP was significantly reduced in strains 129, BALB and C3H compared to B6, correlating with neural NE levels. We rescued hippocampal LTP in strains 129, BALB and C3H to levels seen in B6 by bath application of 1 microm isoproterenol, a beta-adrenergic receptor agonist, during HFS. Propranolol, a beta-adrenergic receptor antagonist, blocked this rescue in 129, BALB and C3H but did not affect LTP in strain B6. Thus, although this form of multitrain LTP does not rely on beta-adrenergic receptor activation, our data show that pharmacological activation of beta-adrenergic receptors during multiple trains of HFS can rescue CA1 LTP in genetically diverse strains with impaired LTP.

Vagus nerve stimulation potentiates hippocampal LTP in freely-moving rats

Previous studies have demonstrated that electrical stimulation of the vagus nerve (VNS) delivered at a moderate intensity following a learning experience enhances memory in laboratory rats and human subjects, while VNS at lower or higher intensities has little or no effect. This finding suggests that VNS may affect memory processes by modulating neural plasticity in brain structures associated with memory storage such as the hippocampus. To test this hypothesis, the present study investigated the modulatory effect of VNS on the development of long-term potentiation (LTP) in the dentate gyrus of freely-moving rats. Rats receiving 0.4 mA VNS showed enhanced potentiation of the population spike amplitude for at least 24 h after tetanus relative to the sham-stimulation group. In contrast, no such effect was observed with 0.2 mA VNS. Stimulation at 0.8 mA had a short-term effect and tended to enhance early LTP, but to a lesser extent than did 0.4 mA. The 0.4 mA stimulation was the same intensity that was previously shown to enhance retention performance in an inhibitory avoidance task. These findings suggest that the neural mechanisms underlying the mnemonic effect of VNS may involve modulating synaptic plasticity in the hippocampus. These data also suggest that neural activity in the vagus nerve, occurring as a result of changes in peripheral state, is an important mechanism by which emotional experiences and arousal can enhance the storage of memories of those experiences.

Adrenergic receptor modulation of hippocampal CA3 network activity

Norepinephrine (NE) has demonstrated proconvulsant and antiepileptic properties; however, the specific pharmacology of these actions has not been clearly established. To address this, we studied the effect of NE on hippocampal CA3 epileptiform activity. Frequency changes of burst discharges in response to NE were biphasic; low concentrations increased the number of bursts, while higher concentrations reduced their frequency, suggesting the involvement of multiple adrenergic receptor (AR) types. This hypothesis was confirmed when, in the presence of betaAR blockade, increasing concentrations of NE caused a monophasic decrease in epileptiform activity. Antagonists selective for alpha1 or alpha2ARs were then used to determine which alphaAR type was involved. While discriminating concentrations of the alpha1AR antagonists prazosin and terazosin had no effect, selective amounts of the alpha2AR antagonists RS79948 and RX821002 significantly reduced the potency of NE in decreasing epileptiform activity. Furthermore, this antiepileptic action of NE persisted when all GABA-mediated inhibition was blocked. This data suggests that, under conditions of impaired GABAergic inhibition, the excitatory and inhibitory effects of NE on hippocampal CA3 epileptiform activity are mediated primarily via beta and alpha2ARs, respectively. Moreover, our results imply that the antiepileptic effect of alpha2AR activation in CA3 is not dependent on the GABAergic system.

Adenylate Cyclase-Mediated Forms of Neuronal Plasticity in Hippocampal Area CA1 Are Reduced With Aging

Beta-adrenergic receptors and the cyclic AMP signaling pathway play an important role in neuronal plasticity and in learning and memory and are known to change with aging. We examined the effects of β-adrenergic stimulation paired with 5-Hz low frequency stimulation (LFS) of Schaffer collateral-commissural afferents on population spike amplitude in area CA1 of hippocampal slices from young (3 mo) and aged (22 mo) Fischer 344 rats. Application of the β-adrenergic agonist isoproterenol (1 μM) for 10 min followed immediately by 3 min LFS produced long-lasting potentiation in young hippocampi, but the magnitude of potentiation in aged rats was significantly attenuated and was not long-lasting. In slices prepared from young rats, long-term potentiation (LTP) induced by this protocol occludes subsequent attempts to produce conventional high frequency stimulation-induced LTP, and vice versa, suggesting that these two forms of potentiation share one or more molecular mechanisms. Age-related differences in response to LFS alone were not observed, but significant differences in response to β-adrenergic stimulation were apparent. Similarly, significant age-related differences in response to direct activation of adenylate cyclase with forskolin (10 μM) were observed. In both age groups, this enhancement produced by isoproterenol or forskolin is only transient, returning to baseline within 60 or 90 min, respectively. Taken together, these studies of adenylate cyclase-mediated forms of potentiation in area CA1 suggest that there is an age-related defect, either upstream or downstream of adenylate cyclase activation, in this important signaling system. Such changes may contribute to the compromised performance on memory tasks that is often observed with normal aging.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

ERK activation by G-protein-coupled receptors in mouse brain is receptor identity-specific

In transfected cells and non-neuronal tissues many G-protein-coupled receptors activate p44/42 MAP kinase (ERK), a kinase involved in both hippocampal synaptic plasticity and learning and memory. However, it is not clear to what degree these receptors couple to ERK in brain. G(s)-coupled beta-adrenergic receptor activation of ERK in neurons is critical in the regulation of synaptic plasticity in area CA1 of the hippocampus. In addition, alpha(1)- and alpha(2)-adrenergic receptors, present in CA1, could potentially activate ERK. We find that, like the beta-adrenergic receptor, the G(q)-coupled alpha(1)AR activates ERK in adult mouse CA1. However, activation of the G(i/o)-coupled alpha(2)AR does not activate ERK, nor does activation of a homologous G(i/o)-coupled receptor enriched in adult mouse CA1, the 5HT(1A) receptor. In contrast, the nonhomologous G(i/o)-coupled gamma-aminobutyric acid type B receptor does activate ERK in adult mouse CA1. Surprisingly, activation of alpha(2)ARs in CA1 from immature animals where basal phospho-ERK is low induces ERK phosphorylation. These data suggest that although most G-protein-coupled receptor subtypes activate ERK in non-neuronal cells, the coupling of G(i/o) to ERK is tightly regulated in brain.

Prenatal morphine exposure induces age- and sex-dependent changes in seizure susceptibility

1. Prenatal exposure to morphine induces long-term alterations in seizure susceptibility, which are age-, sex-, and seizure model-specific. 2. Adult male and female rats exposed prenatally to morphine show decreased susceptibility to GABA-regulated seizures. 3. Prenatally morphine-exposed, adult male rats are more sensitive to excitatory amino acid receptor-mediated seizures than control males, control females, or morphine-exposed females.

5-HT(1A) agonist potential of pindolol: electrophysiologic studies in the dorsal raphe nucleus and hippocampus

BACKGROUND

The ability of pindolol to block 5-HT(1A) autoreceptors on serotonin-containing neurons in the raphe nuclei is thought to underlie the clinical reports of enhanced efficacy and rate of improvement in depressed patients treated with pindolol/selective serotonin reuptake inhibitor (SSRI) combinations. Selectivity for somatodendritic 5-HT(1A) autoreceptors is a crucial requirement, as blockade of postsynaptic 5-HT(1A) sites may jeopardize the therapeutic response. Previous investigators have probed the effects of pindolol on serotonergic dorsal raphe cell firing in animal species; here we confirm their findings and extend them to include observations on postsynaptic 5-HT(1A) receptors in the hippocampus.

METHODS

Extracellular single-unit recordings were made in rats using standard electrophysiologic techniques. Firing rates of serotonin-containing neurons in the dorsal raphe nucleus and CA3 hippocampal pyramidal neurons were monitored and the effects of pindolol given alone or in combination with an SSRI (fluoxetine) or a 5-HT(1A) antagonist (WAY-100,635) were determined.

RESULTS

Pindolol inhibited the firing rates of serotonergic dorsal raphe neurons in a dose-dependent manner. Recovery to baseline firing rates was gradual, but this inhibition could be acutely reversed by WAY-100,635. A range of pindolol doses failed to block the inhibitory effects of fluoxetine on dorsal raphe cell firing. In the hippocampus, pindolol also inhibited cell firing as a function of dose, although these effects were insensitive to WAY-100,635 treatment.

CONCLUSIONS

The ability of pindolol to inhibit serotonergic dorsal raphe cell firing is indicative of its agonist potential and is consistent with previous studies. The lack of observable antagonism of the SSRI-induced slowing of raphe unit activity casts doubt on the suitability of this mechanism of action to account for the positive findings in clinical studies utilizing pindolol/SSRI combinations. The 5-HT(1A)-independent inhibition of hippocampal CA3 cell firing by pindolol suggests that this compound invokes multiple pharmacologic actions, all of which need to be assimilated into any proposed mechanism of action.

Neuromodulation: acetylcholine and memory consolidation

Clinical and experimental evidence suggests that hippocampal damage causes more severe disruption of episodic memories if those memories were encoded in the recent rather than the more distant past. This decrease in sensitivity to damage over time might reflect the formation of multiple traces within the hippocampus itself, or the formation of additional associative links in entorhinal and association cortices. Physiological evidence also supports a two-stage model of the encoding process in which the initial encoding occurs during active waking and deeper consolidation occurs via the formation of additional memory traces during quiet waking or slow-wave sleep. In this article I will describe the changes in cholinergic tone within the hippocampus in different stages of the sleep-wake cycle and will propose that these changes modulate different stages of memory formation. In particular, I will suggest that the high levels of acetylcholine that are present during active waking might set the appropriate dynamics for encoding new information in the hippocampus, by partially suppressing excitatory feedback connections and so facilitating encoding without interference from previously stored information. By contrast, the lower levels of acetylcholine that are present during quiet waking and slow-wave sleep might release this suppression and thereby allow a stronger spread of activity within the hippocampus itself and from the hippocampus to the entorhinal cortex, thus facilitating the process of consolidation of separate memory traces.

Central alpha1-adrenoceptors: their role in the modulation of attention and memory formation

Adrenoceptors presently are classified into three main subclasses: alpha1-, alpha2-, and beta-receptors, each with three (perhaps more) subtypes. All three alpha1-adrenoceptor subtypes are present in rat brain. The purpose of this review is to assess the role of alpha1-adrenoceptors in the modulation of synaptic transmission and plasticity, as well as their ability to modulate higher cerebral functions, such as attentional and memory processes. However, since there are no truly subtype-specific agonists or antagonists available at present, it is virtually impossible to allocate a particular central effect to one or other of the subtypes. The activation of alpha1-adrenoceptors reduces the firing probability and glutamate release in the cornu ammonis of the hippocampus. Alpha1-Adrenoceptors may flexibly modulate weak and strong activation of the pyramidal neurones in the neocortex. Alpha1-Adrenoceptors play only a minor role in the modulation of long-term potentiation in the hippocampus, and may influence many brain functions also via non-neuronal mechanisms. since glial cells can express alpha1-adrenoceptors. At the behavioural level, the activation of alpha1-adrenoceptors promotes vigilance and influences working memory and behavioural activation, while having only a minor role in the modulation of long-term memory.

Effect of noradrenergic inputs on the cardiovascular depressor responses to stimulation of central nucleus of the amygdala

Experiments were done in chloralose anesthetized, paralyzed and artificially ventilated male Wistar rats to investigate the effects of microinjections of either norepinephrine (NE) or tyramine into the central nucleus of the amygdala (ACe) on the arterial pressure (AP) and heart rate (HR) responses elicited by glutamate (Glu) stimulation of the ACe. Microinjections of Glu into the ACe elicited decreases in mean AP (-23+/-3 mmHg) and HR (-11+/-3 bpm). Microinjections of NE or tyramine into these sites did not elicit cardiovascular responses. However, Glu into the ACe in the presence of NE or tyramine elicited depressor or bradycardic response that were significantly smaller (70-100%) in magnitude than to Glu alone. These data suggest that noradrenergic mechanisms in the ACe alter the excitability of ACe neurons involved in mediating changes in systemic AP and HR.

Masking of forskolin-induced long-term potentiation by adenosine accumulation in area CA1 of the rat hippocampus

At hippocampal Schaffer collateral-CA1 synapses, activation of beta-adrenergic receptors and adenylyl cyclase increases transmitter release. However, this effect is transient, which is in contrast to that seen at mossy fiber-CA3 synapses, where activation of cyclic-AMP-dependent protein kinase results in long-lasting facilitation of transmitter release, a phenomenon known as a presynaptic form of long-term potentiation. The present study was aimed at investigating whether forskolin, an adenylyl cyclase activator, could produce long-term effects at the Schaffer collateral-CA1 synapses using extracellular recording techniques. As has been reported previously, forskolin persistently increased the amplitude of evoked population spikes without having a long-term effect on the field excitatory postsynaptic potentials. However, under the conditions where adenosine A1 receptors are inhibited, cyclic-AMP metabolism is disrupted or the transport of cyclic-AMP is blocked, forskolin induces long-term potentiation. Forskolin-induced potentiation is associated with a decrease in paired-pulse facilitation and is blocked by the cyclic-AMP-dependent protein kinase inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate. Activation of N-methyl-D-aspartate receptors is not required for forskolin-induced long-term potentiation, because pretreatment of slices with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate did not prevent forskolin-induced potentiation. These results suggest that blockade of adenosine A1 receptors unmasks forskolin-induced long-term potentiation, and activation of cyclic-AMP-dependent protein kinase induces a form of long-term potentiation which is different from that induced by tetanic stimulation.

Calcium waves precede electrophysiological changes of spreading depression in hippocampal organ cultures

Although intercellular Ca2+ waves resemble spreading depression (SD) and occur in hippocampal organ cultures (HOTCs), SD has not been reported in these cultures. Accordingly, electrophysiological and Ca2+ imaging techniques were used to examine potential interrelations between Ca2+ waves and electrophysiological changes of SD. Our results show, for the first time, that HOTCs can support SD. Furthermore, two distinct Ca2+ waves were found to precede SD. The first traveled >100 micron/sec along the pyramidal cell dendritic layer. The second subsequently traveled mostly perpendicular to the pyramidal cell layer from CA3 (or CA1) but also in all directions from its area of initiation. This second, slower wave spread with the interstitial DC change of SD at millimeters per minute but always ahead of it by 6-16 sec. Heptanol, which uncouples gap junctions, blocked both of these Ca2+ waves and SD. Thus, two types of Ca2+ waves occur with the initiation and propagation of SD. The first might reflect interneuronal changes linked by gap junctions, whereas the second might stem from interastrocyte changes linked via similar connections. Because individual cells can be followed in space and time for protracted periods in HOTCs, this preparation may be ideal for studies designed to explore not only the mechanisms of SD but also the long-term consequences of SD, such as ischemic tolerance.

Intracerebroventricular norepinephrine potentiation of the perforant path-evoked potential in dentate gyrus of anesthetized and awake rats: A role for both alpha- and beta-adrenoceptor activation

Norepinephrine (NE) applied iontophoretically to the dentate gyrus in vivo, and bath applied to hippocampal slices in vitro, produces potentiation of the perforant path-evoked potential. beta-receptors mediate exogenous NE potentiation in vitro, while alpha-receptors are implicated in exogenous effects in vivo. The present study uses intracerebroventricular (i.c.v.) NE to mimic in vitro bath conditions in vivo. Short-term NE potentiation was reliably seen with 10 microg [+/-] NE in 2 microl of 0.9% saline i.c.v. Long-term potentiation occurred with higher doses of NE. The beta-agonist isoproterenol and the alpha-agonist phenylephrine also produced potentiation. Long-term effects were common with isoproterenol. The beta-antagonist metoprolol and the alpha-antagonist phentolamine attenuated NE potentiation. The results suggest that both alpha- and beta-receptors could play a role in NE potentiation in dentate gyrus in vivo. In awake animals, 10 microg NE i.c.v. reproduced the potentiation pattern seen in anesthetized rats. NE potentiation in awake rats was independent of behavioral variation.

Minor role for alpha1-adrenoceptors in the facilitation of induction and early maintenance of long-term potentiation in the CA1 field of the hippocampus

The influences of noradrenaline on the modulation of learning and memory functions, as well as synaptic plasticity, e.g., long-term potentiation (LTP), via beta-adrenoceptors are well documented, whereas the role of alpha1-adrenoceptors has not been studied extensively. Therefore, the effects of alpha1-agonists (ST 587 and methoxamine) on the induction of LTP were examined in the CA1 area of the hippocampus in vitro. Submaximal LTP in extracellular excitatory postsynaptic potentials (EPSP) was induced with theta burst stimulation using 4 bursts. The effects of a beta-agonist, isoproterenol, on synaptic potentiation were studied as a comparison in this preparation. At a concentration of 1 microM, ST 587 slightly increased the magnitude of potentiation in EPSPs (measured 30 min after stimulation) compared to a control pathway potentiated 30 min before drug infusion, whereas a lower concentration (0.3 microM) was not effective. Methoxamine did not induce any increase in the amount of submaximal LTP at concentrations of 0.3, 1.0, or 3.0 microM. Isoproterenol (1.5 microM) increased the amount of LTP when measured 30 min after stimulation, and also transiently increased synaptic transmission, measured both in the slope and amplitude of the field EPSP in the prepotentiated control pathway. Thus, the present results indicate that (1) alpha1-adrenoceptors have only a minor role in hippocampal synaptic plasticity in the CA1 area, but (2) the synaptic plasticity in the CA1 area of the hippocampus assessed by induction and early maintenance of LTP in vitro can be modulated through beta-adrenoceptors.

Inhibition of stimulus-triggered and spontaneous epileptiform activity in rat hippocampal slices by the Aconitum alkaloid mesaconitine

The aim of the present study was to investigate if the plant alkaloid, mesaconitine, which has been reported to have antinociceptive effects via stimulation of the noradrenergic system, inhibits epileptiform field potentials. The experiments were performed as extracellular recordings on rat hippocampal slices. Epileptiform activity was induced by omission of Mg2+ from the bathing medium or by addition of bicuculline and stimulus-evoked population bursts were recorded in the CA1 region. Spontaneous epileptiform activity was elicited by perfusing a nominally Mg2+-free bathing medium with high K+ concentration (5 mM). Both stimulus-triggered and spontaneous epileptiform activity was attenuated in a concentration-dependent manner by mesaconitine (30 nM-1 microM). The inhibitory effect was rather variable in appearance when lower concentrations (30 and 100 nM) of mesaconitine were applied. Pretreatment of the slices with the alpha-adrenoceptor antagonist yohimbine (1 microM) prevented the effect of mesaconitine. It is concluded that the inhibitory action of mesaconitine at low concentration is mediated via alpha-adrenoceptors.

Effects of the Aconitum alkaloid mesaconitine in rat hippocampal slices and the involvement of alpha- and beta-adrenoceptors

1 The effects of mesaconitine, the main alkaloid contained in Aconiti tuber, were investigated by use of extracellular recordings of stimulus-evoked population spikes and field excitatory postsynaptic potentials (e.p.s.ps) in the CA1 region of rat hippocampal slices. 2 At a concentration of 10 nM, mesaconitine evoked excitations, which were manifested as an increase in the amplitude of the orthodromic spike and the appearance of multiple spikes following the first postsynaptic spike, without affecting the magnitude of paired-pulse facilitation. The increase in spike amplitude was persistent and was not reversed by up to 90 min of washout. At concentrations of 30 and 100 nM, the alkaloid produced a biphasic effect, that is an excitation followed by an inhibition without having any effect upon the field e.p.s.p. At concentrations above 100 nM, mesaconitine suppressed the orthodromic population spike and the field e.p.s.p. 3 The excitatory effect was also observed when electrical stimulation was stopped completely during the application of mesaconitine (10 nM) and during the first 15 min of washout. 4 The enhancement of the population spike and the appearance of multiple spikes induced by mesaconitine (10-100 nM) were blocked by pretreatment with the beta-adrenoceptor antagonists propranolol (1 microM) and timolol (1 microM), whereas the inhibitory effect was blocked by the alpha-adrenoceptor antagonists yohimbine (1 microM) and phentolamine (10 microM). However, when the beta-adrenoceptor antagonist timolol was added 10 min after the application of mesaconitine, it failed to block the long-lasting enhancement of the spike amplitude and the appearance of multiple population spikes. 5 Application of the selective beta-adrenoceptor agonist isoprenaline (500 nM) to the hippocampal slices induced an increase in the amplitude of the orthodromic population spike and elicited 2-3 additional spikes. Mesaconitine (10 nM) did not further potentiate this enhancement of the spike amplitude when added after a 15 min pretreatment with isoprenaline. 6 Perfusion of forskolin, which directly activates adenylate cyclase, enhanced the population spike. Mesaconitine had no additional effect when applied after pretreatment with forskolin. 7 It is concluded that the excitatory effects evoked by lower concentrations of the plant alkaloid mesaconitine are mediated by stimulation of beta-adrenoceptors and the consequent activation of intracellular processes which lead to the long-lasting changes in excitability.

Noradrenergic regulation of synaptic plasticity in the hippocampal CA1 region

The effects of norepinephrine (NE) and related agents on long-lasting changes in synaptic efficacy induced by several patterns of afferent stimuli were investigated in the CA1 region of rat hippocampal slices. NE (10 microM) showed little effect on the induction of long-term potentiation (LTP) triggered by theta-burst-patterned stimulation, whereas it inhibited the induction of long-term depression (LTD) triggered by 900 pulses of 1-Hz stimulation. In nontreated slices, 900 pulses of stimuli induced LTD when applied at lower frequencies (1-3 Hz), and induced LTP when applied at a higher frequency (30 Hz). NE (10 microM) caused a shift of the frequency-response relationship in the direction preferring potentiation. The effect of NE was most prominent at a stimulus frequency of 10 Hz, which induced no changes in control slices but clearly induced LTP in the presence of NE. The facilitating effect of NE on the induction of LTP by 10-Hz stimulation was blocked by the beta-adrenergic receptor antagonist timolol (50 microM), but not by the alpha receptor antagonist phentolamine (50 microM), and was mimicked by the beta-agonist isoproterenol (0.3 microM), but not by the alpha1 agonist phenylephrine (10 microM). The induction of LTD by 1-Hz stimulation was prevented by isoproterenol but not by phenylephrine, indicating that the activation of beta-receptors is responsible for these effects of NE. NE (10 microM) also prevented the reversal of LTP (depotentiation) by 900 pulses of 1-Hz stimulation delivered 30 min after LTP induction. In contrast to effects on naive (nonpotentiated) synapses, the effect of NE on previously potentiated synapses was only partially mimicked by isoproterenol, but fully mimicked by coapplication of phenylephrine and isoproterenol. In addition, the effect of NE was attenuated either by phentolamine or by timolol, indicating that activation of both alpha1 and beta-receptors is required. These results show that NE plays a modulatory role in the induction of hippocampal synaptic plasticity. Although beta-receptor activation is essential, alpha1 receptor activation is also necessary in determining effects on previously potentiated synapses.