Hippocampal noradrenergic responses in vivo and in vitro. Characterization of alpha and beta components

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).

Noradrenaline modulates neuronal and perceptual visual detectability via β-adrenergic receptor

RATIONALE

Noradrenaline (NA) is a neuromodulator secreted from noradrenergic neurons in the locus coeruleus to the whole brain depending on the physiological state and behavioral context. It regulates various brain functions including vision via three major adrenergic receptor (AR) subtypes. Previous studies investigating the noradrenergic modulations on vision reported different effects, including improvement and impairment of perceptual visual sensitivity in rodents via β-AR, an AR subtype. Therefore, it remains unknown how NA affects perceptual visual sensitivity via β-AR and what neuronal mechanisms underlie it.

OBJECTIVES

The current study investigated the noradrenergic modulation of perceptual and neuronal visual sensitivity via β-AR in the primary visual cortex (V1).

METHODS

We performed extracellular multi-point recordings from V1 of rats performing a go/no-go visual detection task under the head-fixed condition. A β-AR blocker, propranolol (10 mM), was topically administered onto the V1 surface, and the drug effect on behavioral and neuronal activities was quantified by comparing pre-and post-drug administration.

RESULTS

The topical administration of propranolol onto the V1 surface significantly improved the task performance. An analysis of the multi-unit activity in V1 showed that propranolol significantly suppressed spontaneous activity and facilitated the visual response of the recording sites in V1. We further calculated the signal-to-noise ratio (SNR), finding that the SNR was significantly improved after propranolol administration.

CONCLUSIONS

Pharmacological blockade of β-AR in V1 improves perceptual visual detectability by modifying the SNR of neuronal activity.

The effects of methylphenidate on cognitive performance of healthy male rats

WE AIMED TO INVESTIGATE THE EFFECTS OF METHYLPHENIDATE (MPH) IN HEALTHY RATS ON TWO DISTINCT RADIAL MAZE TASKS WHICH RELY ON BRAIN STRUCTURES AND NEUROTRANSMITTERS KNOWN TO BE AFFECTED BY MPH: the Random Foraging Non-Delay Task (RFNDT) and the Delayed Spatial Win Shift Task (DSWT). Hooded Lister rats were trained to complete either the RFNDT or the DSWT having received oral treatment of either a vehicle or MPH (3.0 mg/kg and 5.0 mg/kg for RFNDT, 3.0 mg/kg for DSWT). We found no effect of MPH on the RFNDT relative to the control group. However, those treated with 5.0 mg/kg MPH did take significantly longer to reach criterion performance than those treated with the 3.0 mg/kg MPH, suggesting some doses of MPH can have detrimental effects. For the DSWT, if MPH was present in both phases, performance did not differ from when it was absent in both phases. However, when present in only one phase there was an increase in errors made, although this only reached significance for when MPH was present only in the test-phase. These data suggest that MPH may have detrimental effects on task performance and can result in state-dependent effects in healthy individuals.

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.

Cross-talk between dopaminergic and noradrenergic systems in the rat ventral tegmental area, locus ceruleus, and dorsal hippocampus

A decreased central dopaminergic and/or noradrenergic transmission is believed to be involved in the pathophysiology of depression. It is known that dopamine (DA) neurons in the ventral tegmental area (VTA) and norepinephrine (NE) neurons in the locus ceruleus (LC) are autoregulated by somatodendritic D(2)-like and alpha(2)-adrenoceptors, respectively. Complementing these autoreceptor-mediated inhibitory feedbacks, anatomical and functional studies have established a role for noradrenergic inputs in regulating dopaminergic activity, and reciprocally. In the present study, a microiontophoretic approach was used to characterize the postsynaptic catecholamine heteroreceptors involved in such regulations. In the VTA, the application of DA and NE significantly reduced the firing activity of DA neurons. In addition to a role for D(2)-like receptors in the inhibitory effects of both catecholamines, it was demonstrated that the alpha(2)-adrenoceptor antagonist idazoxan dampened the DA- and NE-induced attenuations of DA neuronal activity, indicating that both of these receptors are involved in the responsiveness of VTA DA neurons to catecholamines. In the LC, the effectiveness of iontophoretically applied NE and DA to suppress NE neuronal firing was blocked by idazoxan but not by the D(2)-like receptor antagonist raclopride, which suggested that only alpha(2)-adrenoceptors were involved. In the dorsal hippocampus, a forebrain region having a sparse dopaminergic innervation but receiving a dense noradrenergic input, the suppressant effects of DA and NE on pyramidal neurons were attenuated by idazoxan but not by raclopride. The suppressant effect of DA was prolonged by administration of the selective NE reuptake inhibitor desipramine and, to lesser extent, of the selective DA reuptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)-piperazine (GBR12909), suggesting that both the NE and DA transporters were involved in DA uptake in the hippocampus. These findings might help in designing new antidepressant strategies aimed at enhancing DA and NE neurotransmission.

The mystery of sudden death: mechanisms for risks

This review addresses the possible overlapping mechanisms that may apply to the risk of sudden unexpected death occurring in epilepsy and in cardiac disease. It explores the interaction between the central and peripheral autonomic nervous systems and the cardiopulmonary systems. Included is a discussion of the potential interactive role of genetically determined subtle cardiac risk factors for arrhythmias with a predisposition for seizure-related cardiac arrhythmias. We address the possible mechanisms that are operant in producing both epileptogenic and cardiogenic arrhythmias. Finally, we speculate about potential preventive measures to minimize the risk of both sudden unexpected death in epilepsy and sudden cardiac death.

Modulation of theta rhythmicity in the medial septal neurons and the hippocampal electroencephalogram in the awake rabbit via actions at noradrenergic alpha2-receptors

The modulation of the firing discharge of medial septal neurons and of the hippocampal electroencephalogram (EEG) mediated by actions on alpha2-adrenoreceptors (ARs) was investigated in awake rabbits. Bilateral i.c.v. infusion of a relatively low dose (0.5 microg) of the alpha2-AR agonist clonidine produced a reduction in the theta rhythmicity of both medial septal neurons and the hippocampal EEG. In contrast, a high dose of clonidine (5 microg) increased the percentage and degree of rhythmicity of theta bursting medial septal neurons as well as the theta power of the hippocampal EEG. On the other hand, administration of alpha2-AR antagonist idazoxan produced the opposite dose-dependent effect. While a low dose of the antagonist (20 microg) produced an increase in both the theta rhythmicity of medial septal neurons and the theta power of the hippocampal EEG, a high dose (100 microg) caused a reduction of theta rhythmicity in both the medial septum and hippocampus. These results suggest that low doses of alpha2-ARs agents may act at autoreceptors regulating the synaptic release of noradrenaline, while high doses of alpha2-ARs drugs may have a predominant postsynaptic action. Similar results were observed after local injection of the alpha2-AR drugs into the medial septum suggesting that the effects induced by the i.c.v. infusion were primarily mediated at the medial septal level. We suggest that noradrenergic transmission via the postsynaptic alpha2-ARs produces fast and strong activation of the septohippocampal system in situations that require urgent selective attention to functionally significant information (alert, aware), whereas the action via the presynaptic alpha2-ARs allows a quick return of the activity to the initial level.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Systemic overexpression of the alpha 1B-adrenergic receptor in mice: an animal model of epilepsy

PURPOSE

A lack of selective alpha1-adrenergic receptor (alpha1-ARs) agonists and antagonists has made it difficult to clarify the precise function of these receptors in the CNS. We recently generated transgenic mice that overexpress either wild-type or a constitutively active mutant alpha 1B-AR in tissues that normally express the receptor. Both wild-type and mutant mice showed an age-progressive neurodegeneration with locomotor impairment and probable stress-induced motor events, which can be partially reversed by alpha 1-AR antagonists. We hypothesized that the wild-type and mutant mice may exhibit spontaneous epileptogenicity as compared with normal (nontransgenic) mice.

METHODS

Normal, wild-type, and mutant mice were studied. Twenty mice (1 year old) underwent prolonged video-EEG monitoring over a 4-week period. Raw EEG data were blindly analyzed by visual inspection for the presence of interictal and ictal epileptic activities.

RESULTS

During the acute postoperative period (< or = 3 days), both wild-type (26.1 +/- 8.07 spikes/day) and mutant mice (116.87 +/- 55.13) exhibited more frequent interictal spikes than did normal mice (2.17 +/- 0.75; p value, <0.05), but all three groups showed EEG and clinical seizures. During the later monitoring periods (>3 days), wild-type and mutant mice showed more frequent interictal spikes (15.44 +/- 4.07; p < 0.01; and 6.05 +/- 2.46; p < 0.05, respectively) as compared with normal mice (0.41 +/- 0.41), but only mutant mice had spontaneous clinical seizures (means +/- SEM).

CONCLUSIONS

The selective overexpression of the alpha 1B-AR is associated with increased in vivo spontaneous interictal epileptogenicity and EEG/behavioral seizures. These results suggest a possible role (direct or indirect) for the alpha 1B-ARs in the development and expression of epileptogenicity.

Inhibition by L-3,4-dihydroxyphenylalanine of hippocampal CA1 neurons with facilitation of noradrenaline and gamma-aminobutyric acid release

Electrophysiological studies were performed to elucidate whether L-3,4-dihydroxyphenylalanine (L-DOPA) acted on hippocampal CA1 neurons, since this drug has been reported to act as a neurotransmitter in the hypothalamus and striatum. Hippocampal slices (450 microM thick) obtained from male Wistar rats (4-7 weeks of age) were placed in a bath (maintained at 30+/-1 degrees C) continuously perfused with artificial cerebrospinal fluid. The population spikes elicited by electrical stimuli applied to the Schaffer collateral/commissural fibers were recorded in the hippocampal CA1 region, using a glass micropipette filled with 3 M NaCl. Drugs were applied in the bath through a perfusion system. The population spikes were inhibited by L-DOPA (1 nM-10 microM) with a bell-shaped concentration-response curve (n=7-15). Maximum inhibitory effects were obtained at 100 nM. L-DOPA cyclohexyl ester, a putative L-DOPA recognition site antagonist, antagonized the L-DOPA-induced inhibition of population spike. However, the inhibition remained unaffected in the presence of 3-hydroxybenzylhydrazine, an aromatic amino acid decarboxylase inhibitor. Furthermore, bath application of either phentolamine, an alpha-adrenoceptor antagonist, or bicuculline, a GABA(A) receptor antagonist, antagonized the inhibitory effects of L-DOPA on population spikes. In addition, bicuculline (1 microM) antagonized the inhibition of population spike induced by 6-fluoronorepinephrine (10 microM), an alpha-adrenoceptor agonist, while phentolamine (10 microM) did not affect the muscimol (1 microM)-induced inhibition. These results suggested that L-DOPA itself acted on L-DOPA recognition sites to release noradrenaline, and that the latter facilitates gamma-aminobutyric acid (GABA) release via alpha-adrenoceptors located on the GABA-containing cells and/or their nerve terminals, thereby inhibiting the population spikes in the hippocampal CA1 field.

Norepinephrine exhibits two distinct profiles of action on sensory cortical neuron responses to excitatory synaptic stimuli

Located within the central gray of the caudal pons, the locus coeruleus (LC) is the sole source of norepinephrine (NE) projections to the forebrain. NE is released both tonically and phasically from axonal varicosities in LC efferent target circuits. NE has been shown to produce a diverse set of actions, including suppression of spontaneous and stimulus evoked discharge, augmentation of synaptically evoked excitation, and inhibition and gating of otherwise subthreshold synaptic inputs. Utilizing an extracellular in vitro tissue slice preparation and microiontophoretic techniques, the dose-dependent actions of NE on glutamate-evoked discharges of layer II/III and layer V sensory cortical neurons were investigated. Noradrenergic effects were further examined in terms of cell and adrenoceptor specificity. The results indicate two exclusive modulatory actions of NE: 1) ejection current-dependent suppression of glutamate evoked discharge, and 2) ejection current-dependent facilitation of glutamate-evoked discharge followed by suppression of the maximal facilitated response. These effects were observed in both normal and low Ca(2+) / high Mg(2+) bathing media, suggesting a postsynaptic site for NE's actions. The facilitation of glutamate evoked discharge was selectively mimicked by the alpha-1 agonist, phenylephrine, whereas the dose-dependent suppression was mimicked by the beta-agonist isoproterenol. These results suggest that the suppressant and facilitating actions of NE are mediated by beta and alpha-1 receptors, respectively. In general, these results are consistent with previous demonstrations of NE modulatory actions on central neurons, but indicate that in the cerebral cortex these effects are both cell- and receptor-specific.

Modulation of epileptiform discharges in the rat limbic system in vitro by noradrenergic agents

Application of bicuculline (20 microm) in a horizontal slice preparation of the rat limbic system induced epileptiform discharges ('bursts') that spread from the hippocampus to the entorhinal cortex (EC) and the basolateral amygdala (BLA). These bursts were time locked, with the CA3-CA2 regions discharging first at a 0.1+/-0. 008Hz (n=30 slices) frequency, followed after 27+/-3 ms by the superficial layers of the EC and after 44.3+/-3 ms by the BLA. Application of 50 microM noradrenaline (NA) for 4 min reduced the burst frequency to 40% of its initial value. Pharmacological study of NA action on burst frequency revealed that it consisted of a beta adrenoreceptor-mediated increase and an alpha(2) adrenoreceptor-mediated decrease of epileptiform bursting frequency.

Identification of the opioid receptors involved in passive-avoidance learning in the day-old chick during the second wave of neuronal activity

Long-term memory formation for passive-avoidance learning in the day-old chick is known to have two distinct time windows of protein synthesis (F.M. Freeman, S.P.R. Rose, A.B. Scholey, 1995. Two time windows of anisomycin-induced amnesia for passive-avoidance training in the day-old chick. Neurobiol. Learn. Mem. 63, 291-295). The lobus parolfactorius (LPO) is thought to be an important site for the second wave of protein synthesis which occurs 4-5 h after training. Birds received bilateral intracranial injections of agonists and antagonists for the mu-, delta-, kappa-opioid receptors and the opioid receptor-like (ORL(1)) receptor directly into the LPO at 5 h post-training and were tested for recall 24 h later. Also, 100 microM beta-funaltrexamine (beta-FAN), a mu-opioid receptor antagonist, significantly impaired memory formation (P<0.01). The delta-opioid receptor was also involved in memory formation at this time-point since antagonism of this receptor by 1 mM ICI-174,864 caused amnesia (P<0.01) which was reversed by the agonist, DPLPE. The kappa-opioid receptor appeared not to be involved during the second phase of neuronal activity since neither stimulation by dynorphin nor inhibition by nor-BIN caused amnesia for the task. The ORL(1) receptor agonist orphanin FQ also had no effect suggesting that this receptor was not involved at this 5-h time-point. Cytosolic and mitochondrial protein synthesis has been shown to be important in passive-avoidance learning in the day-old chick. Both chloramphenicol (CAP) and anisomycin (ANI), inhibitors of mitochondrial and cytosolic protein synthesis, respectively, caused disruption when injected 5 h post-training into the LPO (P<0.05). Endomorphin-2 (Endo-2), a mu-opioid receptor agonist, reversed both the ANI- and CAP-sensitivity. However, DPLPE, a delta-opioid receptor agonist, only reversed the effect due to CAP. Possible mechanisms for these effects are discussed.

Stimulation of alpha-1 adrenergic receptors facilitates spatial learning in rats

The present experiments were designed to examine the effects of alpha-1 adrenergic stimulation and inhibition on memory encoding and to investigate whether the alpha-1 adrenergic and muscarinic cholinergic systems interact in the regulation of spatial navigation behavior in the Morris water maze test and we also studied the effects of D-cycloserine, a partial agonist at the glycine binding site on the N-methyl-D-aspartate (NMDA) receptor complex, on the performance of scopolamine-treated rats in this task. Pre-training subcutaneous administration of St-587 (a putative alpha-1 agonist) at 1000 micrograms kg-1 or 1500 micrograms kg-1 improved water maze navigation to a hidden platform. Prazosin (an alpha-1 antagonist), 300-2000 micrograms kg-1, did not significantly impair the spatial navigation performance. Pre-training administration of prazosin 1000 micrograms kg-1, but not 300 micrograms kg-1, slightly potentiated the deficit in water maze navigation seen after scopolamine (200 micrograms kg-1, pre-training intraperitoneal injection). Pre-training administration of St-587 at a dose 1500 micrograms kg-1, but not 500 micrograms kg-1, slightly ameliorated the scopolamine-induced (200 micrograms kg-1) impairment in performance of rats. Pre-training administration of prazosin at doses 300 or 1000 micrograms kg-1 or St-587 at doses 500 micrograms kg-1 or 1500 micrograms kg-1 did not have any significant influence on the scopolamine-induced (200 micrograms kg-1) increase of swimming speed. Furthermore, D-cycloserine at the dose of 300 micrograms kg-1 but not 1000 or 3000 micrograms kg-1 reversed the scopolamine (200 micrograms kg-1)-induced deficit in acquisition of the water maze task but not the increase in motor output (increased swimming speed). These results indicate that the stimulation of alpha-1 adrenoceptors may facilitate the encoding of new information. These findings suggest that alpha-1 adrenergic mechanisms do not participate or at least are not the most critical part of the noradrenergic system in the interaction between noradrenaline and muscarinic receptors in the modulation of learning and memory. In addition, these results suggest that D-cycloserine may be effective in alleviating states of central cholinergic hypofunction.

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.

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.

Effects of combined block of alpha 1-adrenoceptors and NMDA receptors on spatial and passive avoidance behavior in rats

The present study was designed to investigate the interactions between alpha 1-adrenoceptors and NMDA receptors in modulating spatial navigation and passive avoidance behavior in rats. Pretraining treatment with prazosin, and alpha 1-adrenoceptor antagonist, at 2 mg/kg i.p., impaired acquisition performance in a water maze navigation test and had no effect on passive avoidance behavior. Posttraining and pretest injections of prazosin had no effect on water maze or passive avoidance behavior. Pretraining treatment with ((+/-))-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), a competitive NMDA receptor antagonist, does dependently (3 and 10 mg/kg) impaired passive avoidance and water maze behavior. Posttraining treatment with CPP had no effect on water maze and passive avoidance behavior. A pretraining combination of subthreshold doses of CPP (1 mg/kg) and prazosin (1 mg/kg) impaired water maze behavior. A combination of subthreshold doses of CPP (3 mg/kg) and prazosin (1 mg/kg) injected posttraining or pretest had no marked effect on water maze or passive avoidance performance. A control experiment showed that CPP 3 mg/kg or CPP 1 mg/kg and prazosin 1 mg/kg injected pretraining had no effect on cue navigation to a clearly visible platform, but CPP 10 mg/kg markedly impaired performance. The present results indicate that alpha 1-adrenoceptors and NMDA receptors may synergistically regulate acquisition of spatial navigation performance. Therefore, it would be interesting to study the effects of combined stimulation of alpha 1-adrenoceptors and NMDA receptors on age-related memory defects.

Effect of alpha 2-adrenergic drugs dexmedetomidine and atipamezole on extracellular amino acid levels in vivo

alpha 2-Adrenoceptors are known to be involved in a variety of physiological functions and pathological conditions, including epilepsy and the extent of excitotoxin-induced cell death. In this study we evaluated whether selective alpha 2-adrenergic drugs can modulate the release of neurotransmitter amino acids. The effect of the alpha 2-adrenoceptor agonist dexmedetomidine (5 micrograms/kg, s.c.) and the alpha 2-adrenoceptor antagonist atipamezole (0.1 mg/kg and 1 mg/kg, s.c.) on the release of extracellular glutamate, aspartate and gamma-aminobutyric acid (GABA) was studied with microdialysis in the hippocampus of freely moving rats under basal and K(+)-evoked conditions. Atipamezole (1 mg/kg) decreased K(+)-evoked glutamate efflux by 30% compared to the control group (P < 0.05) but did not affect significantly the effluxes of aspartate and GABA. Dexmedetomidine and the lower dose of atipamezole (0.1 mg/kg) did not significantly alter the evoked overflow of amino acids. The results suggest that alpha 2-adrenergic drugs have only modest effects on the K(+)-stimulated overflow of extracellular neurotransmitter amino acids in rat hippocampus.

Noradrenergic enhancement of GABA-induced input resistance changes in layer V regular spiking pyramidal neurons of rat somatosensory cortex

Previous in vivo studies have shown that microiontophoretic application of norepinephrine (NE) and isoproterenol (ISO) can enhance gamma-aminobutyric acid (GABA)-induced depressant responses of rat somatosensory cortical neurons. In the present investigation we have examined the transmembrane electrophysiological events which are associated with interactions between NE and GABA in layer V pyramidal neurons of rat barrel field cortex. Intracellular recordings were made from electrophysiologically identified cells in a superfused cortical tissue slice preparation before, during and after bath or microdrop application of GABA, NE and ISO, alone or in combination. GABA application produced a small depolarization from resting membrane potential associated with a reduction (22%) in input resistance. NE and ISO (10-100 microM) also produced in some cases small membrane depolarizations (1-4 mV) but little concomitant changes in input resistance. Simultaneous application of NE with GABA potentiated amino acid-induced changes in input resistance in 4 cases and antagonized (n = 4) or had no effect (n = 4) on GABA-associated membrane events in 8 other cases. When the alpha-blocker, phentolamine (20 microM), was added to the medium, NE-induced enhancement of the GABA response was observed in 3 of 5 cases (60%), suggesting both, a beta-adrenergic mediation and a possible alpha-receptor masking of this noradrenergic-potentiating action. Consistent with this interpretation was the finding that the beta-agonist, ISO (10-100 microM), produced net increases in GABA-induced input resistance changes in 64% of cases tested (9 of 14). The potentiating effect of NE and ISO was mimicked by the adenyl cyclase activator, forskolin (n = 2), and a membrane permeant analog of cyclic-AMP, 8-bromo-cyclic AMP (n = 3); and could also be demonstrated when the GABAA agonist muscimol (0.5-1 microM) was substituted for GABA. The reversal potential for GABA and GABA + NE remained the same. These findings suggest that previous demonstrations of NE-potentiating effects on GABA inhibition may be mediated by beta-receptor/cyclic-AMP-linked actions on mechanisms which regulate GABAA receptor-induced membrane conductance changes.

Norepinephrine modulates excitatory amino acid-induced responses in developing human and adult rat cerebral cortex

These experiments were designed to assess the ability of norepinephrine and its beta-receptor agonist, isoproterenol, to modulate responses induced by activation of excitatory amino acid receptors in brain slices obtained from developing human cortex or adult rat cortex. Human cortical slices were obtained from children undergoing surgery for intractable epilepsy (9 months to 10 yr of age). For comparison, slices were also obtained from rats (2-3 months of age). Iontophoretic application of glutamate, N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) produced excitatory responses consisting of membrane depolarizations accompanied by action potentials. Iontophoretic or bath application of norepinephrine or isoproterenol enhanced responses evoked by glutamate or N-methyl-D-aspartate. Depolarizations occurred with shorter latencies and their amplitudes increased. Action potential frequency was also increased and responses were of longer duration. In contrast, norepinephrine or isoproterenol had no effect on responses induced by AMPA. The enhancement of responses induced by N-methyl-D-aspartate or glutamate was antagonized by the beta-adrenergic receptor antagonist propranolol. Similar findings were obtained from neurons in humans or rats. These results suggest that norepinephrine, possibly via beta-receptors, potentiates responses mediated by glutamate and N-methyl-D-aspartate receptors without affecting those mediated by AMPA receptors. These effects were observed at all ages studied, indicating that the ability of norepinephrine to modulate excitatory neuronal transmission is well developed in human cortex by 9 months of age.

Pyramidal neurons in immature rat hippocampus are sensitive to beta-adrenergic agents

The development of hippocampal neuronal sensitivities to the beta-noradrenergic agent, isoproterenol, was examined in tissue from immature rats. The in vitro hippocampal slice preparation was used to assess intracellularly recorded responses from hippocampal neurons to pressure-pulse and bath application of noradrenergic drugs. Effects of the drug on individual hippocampal CA3 pyramidal neurons were compared across several stages of development, ranging from postnatal day 4-5 (P4-5) to maturity. Isoproterenol, pressure-pulse applied to CA3c pyramidal cells, produced a depolarization of membrane potential and an increase in cell input resistance in tissue as young as P7. Spike frequency adaptation (in trains of action potentials triggered by depolarizing pulses) was reduced, as were the slow after-hyperpolarizations following the spike trains. All agonist effects were blocked by timolol, a beta-antagonist. Drug-induced changes in cell membrane and firing properties in immature tissue were qualitatively similar to beta-receptor-mediated noradrenergic effects in adult tissue. These results indicate that the beta-receptor-mediated component of the noradrenergic effect in rat hippocampus is physiologically functional by the seventh day of postnatal life; at earlier times (P4-5) these beta-receptor-mediated noradrenergic actions are, at best, equivocal.

Vitamin B-6 supplementation in elderly men: effects on mood, memory, performance and mental effort

This study evaluates the effects of vitamin B-6 supplementation (20 mg pyridoxine HCL daily for 3 months) on mood and performance in 38 self-supporting healthy men, aged between 70-79 years. Effects were compared with 38 controls who received placebo and were matched for age, plasma pyridoxal-5'-phosphate (PLP) concentration and intelligence score. Before and after drug intervention vitamin B-6 status was determined, and mood and performance were measured by means of a computerized testing system. In addition, the phasic pupil response was measured in order to assess mental effort. Positive effects of vitamin B-6 supplementation were only found with respect to memory, especially concerning long-term memory. In view of the finding that mental performance improvement and delta PLP values were most strongly correlated within an intermediate range of delta PLP, it is suggested that cognitive effects are primarily associated with a certain range of vitamin B-6 status increment. The general conclusion is that vitamin B-6 supplementation improves storage of information modestly but significantly.

Synaptic localization of adrenergic disinhibition in the rat hippocampus

Norepinephrine is an endogenous neurotransmitter that reduces synaptic inhibition onto pyramidal neurons in the hippocampus by an action at an alpha-adrenergic receptor. The physiological mechanism of this disinhibition was previously not known, except that it occurred at a site presynaptic to the inhibited pyramidal cell. In this paper we present evidence that adrenergic disinhibition is restricted to the early phase of the evoked inhibitory postsynaptic potential in area CA1 of the hippocampus. The locus of disinhibition does not appear to reside in the interneuronal terminal, axon, or cell body. Instead, adrenergic agonists appear to reduce evoked synaptic inhibition by depressing excitatory synapses that activate the interneuron.

Actions of noradrenaline on neocortical neurons in vitro

The effects of noradrenaline (NA) and alpha- and beta-adrenergic agonists on neocortical neurons in rat brain slices were studied by intracellular recording. NA added to the perfusion medium decreased the orthodromically evoked excitatory postsynaptic potential (EPSP) and increased the neuronal excitability on current injection. The decrease of the EPSP by NA was mimicked by alpha-adrenoceptor activation. The beta-adrenergic agonist, isoprenaline, increased the neuronal excitability to current injections similarly to NA. In addition, isoprenaline markedly increased the amplitude of a late component of the EPSP (I-EPSP). The increase of the I-EPSP was long-lasting (up to 90 min). Since this late component of the EPSP is blocked by the NMDA (N-methyl-D-aspartate)-antagonist APV (D-2-amino-5-phosphonovalerate), an enhancement of this excitatory synaptic transient via beta-adrenoceptors may be relevant for the facilitatory effect of NA on neuronal plasticity.

Second messenger-mediated actions of norepinephrine on target neurons in central circuits: a new perspective on intracellular mechanisms and functional consequences

Ever since the initial demonstration of a widespread distribution of noradrenergic fibers to functionally diverse regions of the mammalian forebrain, there has been considerable interest in determining the electrophysiological effects of norepinephrine (NE) on individual neurons within these target areas. While early studies showed that NE could directly inhibit cell firing via increased intracellular levels of cyclic AMP, more recent work has revealed a spectrum of noradrenergic actions, which are more accurately characterized as neuromodulatory. More specifically, numerous experimental conditions have been described where NE at levels subthreshold for producing direct depressant effects on spontaneous firing can facilitate neuronal responses to both excitatory and inhibitory synaptic stimuli. The goal of this report is to review recent evidence which suggests that the various modulatory actions of NE on central neurons result from the activation of different adrenoceptor-linked second messenger systems. In particular, we have focused on the candidate signal transduction mechanisms that may underlie NE's ability to augment cerebellar and cortical neuronal responsiveness to GABAergic synaptic inputs. The consequences of such NE-induced changes in synaptic efficacy are considered not only with respect to their influences on feature extraction properties of individual sensory cortical neurons but also with regard to the potential impact such actions would have on the signal processing capabilities of a network of noradrenergically innervated cortical cells.

Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In vivo electrochemical measurements of chronoamperometric recordings from Nafion-coated electrodes were used to investigate monoamine overflow from selected regions of the rat hippocampal slice. Concurrent electrophysiological measurements of evoked CA1 pyramidal cell population spike responses were used to characterize changes in the electrical activity in the slices that occur during potassium-induced neurotransmitter overflow. Superfusion with elevated K+ (10-50 mM, 5 min) elicited consistent concentration-dependent increases in the electrochemical responses recorded from the dentate gyrus. At the onset of K+ perfusion, there was an initial increase in the population spike response, followed by electrical silence, which usually lasted 5-10 min following the return to normal medium, and required 20-30 min for complete recovery of the response. The potassium-induced electrochemical signal always increased following the decline in the electrophysiological response. Although the electrochemical signal usually returned to baseline much before the electrophysiological response (usually within 5 min), both signals remained refractory for some time. Cocaine pretreatment (10-50 microM) caused a dose-dependent augmentation of the electrochemical responses. Local pressure ejection of K+ via a micropipette elicited dose-dependent increases in the electrochemical signals that were of relativity brief duration as compared to superfusion with K+. Such potassium-evoked responses were highly localized, and were attenuated in amplitude in animals that had been previously treated with the selective noradrenergic neurotoxin, DSP-4. In addition to K+, local applications of methyl-amphetamine, tyramine and veratridine also elicited electrochemical signals, and the time courses of these responses were specific to the releasing agent that was used. Preliminary data obtained using high-speed electrochemical recordings of both oxidation and reduction current suggested that tyramine ejections evoked primarily norepinephrine overflow, while K+ evoked the overflow of both norepinephrine and serotonin. The present experiments demonstrate that simultaneous electrophysiological and electrochemical experiments can be used in an isolated preparation of brain such as the hippocampal slice to characterize the electrophysiological events that occur during stimulated transmitter release.

Effect of naloxone pretreatment on habituation in the crab Chasmagnathus granulatus

On sudden presentation of a passing shadow, the crab Chasmagnathus granulatus reacts with an escape response that habituates after repeating the same stimulus. In a first series of experiments, a range of naloxone (NX) doses (0.8, 2.4, 3.2, and 6.2 micrograms/g) was injected into crabs 15 min before one 15-trial habituation session. An enhancing effect of 3.2 micrograms NX/g on responsiveness appeared over trials, that cannot be explained either by a ceiling effect or by a delay in peak drug action. Two doses below 3.2 microgram/g and one dose above had no significant effect. Results from a second series of experiments showed that the 3.2 microgram NX/g effect vanishes after 15 trials (1 h after injection). The hypothesis that crab's habituation involves the action of an endogenous opioid mechanism is put forward to account for the naloxone pretreatment effect.

Imipramine alters beta-adrenergic, but not serotonergic, mediated responses in rat hippocampal pyramidal cells

Imipramine, a tricyclic antidepressant, acts acutely to block the reuptake of serotonin (5-HT) and norepinephrine (NE). However, imipramine's action as an antidepressant takes several weeks to develop. This study investigated acute and chronic effects of imipramine on intracellularly-recorded responses mediated by 5-HT and beta-adrenergic receptors on pyramidal cells from area CA1 of rat hippocampal slices maintained in vitro. Addition of 10 microM imipramine in the perfusion medium sinistrally shifted the 5-HT1A concentration-response curve for membrane hyperpolarization and the 5-HT concentration-response curve for the reduction in the amplitude of the slow afterhyperpolarization (AHP) elicited by a train of action potentials. After two weeks of treatment with imipramine (10 mg/kg daily i.p. injections or s.c. osmotic mini-pumps) the responses to 5-HT were not altered. In contrast the concentration-response curve for the beta-adrenergic mediated reduction in AHP amplitude was significantly altered; there was a reduction in Emax and a log unit dextral shift in EC50. There was no change in the concentration-response curve for the beta-adrenergic mediated depolarization. These data are in agreement with previous biochemical results reporting a decrease in beta-adrenergic receptor mediated stimulation in adenylyl cyclase and down-regulation of beta-receptor in cortex and hippocampus. These findings suggest that a consequence of long-term imipramine treatment is a decrease in the augmentation of cell excitation normally produced by beta-adrenergic receptor stimulation.

Antidepressant drugs potentiate the alpha 1-adrenoceptor effect in hippocampal slices

The effect of prolonged treatment with antidepressant drugs on the phenylephrine- and norepinephrine (NE)-evoked reaction in hippocampal slices was examined by extracellular recording of the spontaneous activity of CA1 layer neurons. The alpha 1-adrenoceptor agonists, phenylephrine and methoxamine, depressed the neuronal discharges of most of the units tested, while NE evoked both excitatory and inhibitory effects which were blocked by propranolol and phentolamine or prazosin, respectively. Imipramine, mianserin, (+)- and (-)-oxaprotiline administered subchronically (10 mg/kg p.o., twice daily for 14 days, withdrawal 48 h), potentiated the inhibitory reaction to phenylephrine. Mianserin was the only drug tested in the acute dose to effectively augment the reaction to alpha 1-adrenoceptor stimulation. Prolonged administration of mianserin and imipramine attenuated the excitatory effect to NE, which probably reflects beta-receptor down-regulation; however, only mianserin, but not imipramine, enhanced the NE-induced inhibition. The observed potentiation of the alpha 1-adrenoceptor-related inhibitory reaction to phenylephrine produced by antidepressant drugs may reflect the development of the alpha 1-adrenergic system supersensitivity in the hippocampus.

Participation of cyclic adenosine monophosphate and beta-adrenergic receptors in the facilitatory effect of noradrenaline on the response of cultured cerebellar neurons to glutamate

For the purpose of examining possible involvement of the cyclic adenosine monophosphate (cAMP) system and adrenergic receptors in the modulatory effect of noradrenaline (NA) on the glutamate-induced depolarizing response, the effects of dibutyryl cAMP (DBcAMP), forskolin, theophylline, clonidine, isoproterenol and propranolol were intracellularly investigated in the cerebellar neurons cultured from chick embryos. Not only NA-induced hyperpolarization and increase in input resistance but also the facilitatory effect of NA on the glutamate response were mimicked by DBcAMP and isoproterenol. This facilitatory effect of DBcAMP was enhanced by theophylline or forskolin, while that of isoproterenol was antagonized by propranolol. Clonidine suppressed glutamate-induced depolarization. These results that the enhancing action of NA on the responsiveness of cultured cerebellar neurons to excitatory amino acids is mediated by beta-adrenergic receptors and the intracellular cAMP system.

Ultrastructural localization of tyrosine hydroxylase-like immunoreactivity in the rat hippocampal formation

The light and electron microscopic localization of antigenic sites for a polyclonal antiserum directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), was examined in the hippocampal formation of the rat brain with a double-bridged peroxidase-antiperoxidase method. By light microscopy, the majority of varicose processes with intense TH-like immunoreactivity (LI) were contained in the hilus of the dentate gyrus (DG) and strata radiatum and lacunosum-moleculare of the CA3 region of the hippocampus. Only a few immunoreactive fibers were observed in the molecular and granule cell layers of the DG, in strata oriens and pyramidale of CA3, and in all layers of CA1. Electron microscopy confirmed that these labeled processes were primarily axons and axon terminals. Terminals with TH-LI were 0.4-1.1 micron in diameter and contained many small clear vesicles and from 0 to 3 larger dense-core vesicles. The number and types of associations formed by terminals with TH-LI were remarkably similar in the DG and hippocampus proper despite known differences in intrinsic cells and function. In both regions, the majority of terminals with TH-LI formed junctions on small (distal dendrites (52% of 112 in the DG; 67% of 116 in CA3) and dendritic spines (30% in the DG; 18% in CA3) that were both asymmetric and symmetric. In the DG, axosomatic junctions (2% of 112) were symmetric and occurred exclusively on the perikarya of granule cells, whereas junctions on large (proximal) dendrites were more numerous (16%), exhibited symmetric as well as asymmetric membrane specializations, and were of both granule (molecular layer) and nongranule (hilus) cell origin. In CA3, synaptic contacts on perikarya (5% of 116) and large (proximal) dendrites (10%) of both pyramidal cell and nonpyramidal cell origin were few and all symmetric. The distribution and types of synaptic associations formed by terminals with TH-LI in the CA1 region paralleled that seen in the CA3 region. In both the dentate and hippocampus proper, 10% of the terminals with TH-LI were observed closely apposed to unlabeled terminals that formed asymmetric synapses with dendrites and dendritic spines. In rare instances, TH-immunoreactive terminals were found in close association with the basement membrane of blood vessels, astrocytic processes, or with other unlabeled terminals not forming recognizable junctions. In addition TH-LI was occasionally detected within the cytoplasm of a minority of astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple changes in noradrenergic mechanisms in the coeruleo-hippocampal pathway during aging. Structural and functional correlates in intraocular double grafts

Age-related changes of the coeruleo-hippocampal noradrenergic system were investigated using intraocular double transplants. Pieces of fetal hippocampus were grafted into the anterior chamber of the eye and placed into contact with previously inserted locus coeruleus grafts. Ages of both transplants and hosts were varied to enable studies of intrinsic versus extrinsic determinants of aging in an isolated neuronal circuit. Four different experimental groups, with the approximate age in months of grafts/hosts at the time of recording given in parentheses, were studied; young grafts in the eyes of young hosts (3/7), young grafts in the eyes of old hosts (3/23), mature transplants in adult host rats (8/12) and aged transplants in the eyes of aged rats (21/25). Extracellular recordings from the hippocampal part of the double grafts were performed. Superfusion with alpha-adrenergic antagonists and the alpha 2-agonist clonidine elicited significant increases in the discharge rate of the grafted hippocampal neurons in all groups except the aged transplants in the aged hosts (21/25), where a small excitation was elicited with clonidine and no effect at all was seen with alpha-adrenergic antagonists. The host age did not seem to be important since young transplants in the old hosts (3/23) showed a similar increase in discharge rate as transplants in the young and adult hosts. Tyrosine hydroxylase immunohistochemistry and high-performance liquid chromatography revealed that hippocampal transplants remaining in oculo for a minimum of 6-10 months became permanently hyperinnervated by noradrenergic fibers from the locus coeruleus grafts. The density of noradrenergic fibers was significantly lower in young transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related alterations in noradrenergic input to the hippocampal formation: structural and functional studies in intraocular transplants

Intrinsic versus extrinsic determinants of age-related alterations in hippocampal noradrenergic transmission were investigated using intraocular allografts in rats. Three groups of animals were examined: young hippocampal transplants in young hosts, old transplants in old hosts and young transplants in old hosts. Postsynaptic sensitivity to noradrenaline (NA) was measured by extracellular recordings of spontaneous activity and superfusion with known concentrations of catecholamines in the anterior chamber of the eye. Hill plots demonstrated that the dose-response relationships of NA-induced depressions were linear and parallel in the 3 groups. Aged hippocampal grafts displayed a highly significant subsensitivity to NA of one order of magnitude. The EC50 for this group was 203.1 microM as compared to 29.2 in young grafts. Young intraocular grafts in old hosts responded similarly to transplants in young hosts, with an EC50 of 32.4 microM for the depressant actions of NA. Collaterals of the host iris sympathetic ground plexus invaded the hippocampal grafts. The density of this noradrenergic innervation was estimated by immunohistochemistry for tyrosine hydroxylase. A slightly increased density and fluorescence intensity of the noradrenergic fibers were observed in the old transplants as compared to the young transplants in young and old hosts. This was correlated with a significantly (P less than 0.01) increased content of NA in old transplants, as measured with high performance liquid chromatography. The old transplants also contained a large number of autofluorescent lipofuchsin granules, which were absent in the young transplants, regardless of the recipient age. Taken together, these results suggest the existence of alterations in pre- as well as postsynaptic noradrenergic mechanisms in the aging hippocampus. These changes were dependent on transplant age rather than host age, thus suggesting an involvement of intrinsic rather than extrinsic determinants in this model system.

Increases in diversive exploration in rats during hippocampal microinfusions of isoproterenol but not methoxamine

To test the hypothesis that the increase in locomotor activity and the diversification of stimulus sampling observed during intrahippocampal microinfusions of norepinephrine are mediated by beta adrenergic receptors, rats were infused with either the beta agonist isoproterenol or the alpha-1 agonist methoxamine while being tested in a Behavioral Pattern Monitor. The behavioral profile induced by isoproterenol was qualitatively similar to that observed previously with norepinephrine. Methoxamine had no significant effects. The ability of isoproterenol to mimic the norepinephrine-induced blockade of the hyperactivity associated with intrahippocampal microinfusions of the cholinergic agonist carbachol was also tested. Carbachol produced hyperactivity which was attenuated by the coinfusion of isoproterenol. These results are consistent with the hypothesis that increases in diversive exploration and stimulus sampling induced by intrahippocampal norepinephrine are due to the activation of beta adrenergic receptors within the dentate gyrus.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. I. Receptors mediating the effect of microiontophoretically applied norepinephrine

The rat hippocampus receives a dense noradrenergic innervation originating exclusively from the locus coeruleus. The present electrophysiological study was undertaken to characterize the adrenoceptor mediating the suppressant effect of microiontophoretically applied norepinephrine (NE) on CA1 and CA3 dorsal hippocampus pyramidal neurons of the rat. The rank order of potency of microiontophoretically applied agonists, in suppressing the firing rate of hippocampus pyramidal neurons was: oxymetazoline greater than NE greater than phenylephrine greater than isoproterenol greater than clonidine. In the hippocampus, oxymetazoline was more potent than NE, whereas it was ineffective in the lateral geniculate nucleus where the effect of NE is mediated by an alpha 1-adrenoceptor. Low currents of clonidine antagonized the effect of NE suggesting that clonidine may exert a partial agonistic effect. The rank order of potency of i.v. administered adrenergic antagonists in blocking the suppressant effect of microiontophoretically applied NE was: idazoxan much greater than prazosin much greater than propranolol. Idazoxan also blocked the effect of oxymetazoline, phenylephrine, and isoproterenol but did not modify the effect of microiontophoretically applied gamma-aminobutyric acid (GABA). In addition, idazoxan, applied by microiontophoresis, readily blocked the suppressant effect of NE without affecting that of GABA. These results suggest that the suppressant effect of microiontophoretically applied NE on rat dorsal hippocampus pyramidal neurons is primarily mediated by alpha 2-adrenoceptors.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. II. Receptors mediating the effect of synaptically released norepinephrine

The present studies were undertaken to determine the nature of the receptors mediating the effects of endogenous norepinephrine (NE) released by stimulation of the locus coeruleus (LC) on the firing activity of dorsal hippocampus pyramidal neurons in the rat. Unitary activity of CA3 pyramidal neurons was recorded extracellularly. In most neurons, the LC stimulation produced a period of suppression, followed by a period of activation. The suppression was selectively blocked by prazosin, an alpha 1-adrenoceptor antagonist, whereas the activation was selectively blocked by propranolol, a beta-adrenoceptor antagonist. Idazoxan, an alpha 2-adrenoceptor antagonist, increased the period of suppression without affecting the period of activation. The effectiveness of microiontophoretic applications of NE on the same neurons was reduced by idazoxan, but was modified neither by propranolol nor prazosin. Lesion of the central noradrenergic system by intracerebroventricular 6-hydroxydopamine markedly decreased the NE content in the hippocampus in all rats but the effectiveness of the LC stimulation was reduced only in rats with a depletion greater than 90%. These results demonstrate that the suppressant effect of endogenous NE released by LC stimulation on hippocampus pyramidal neurons is mediated by an alpha 1-adrenoceptor and suggest that its late excitatory effect might involve beta-adrenoceptors. Since the effect of microiontophoretically applied NE on the same neurons is mediated by alpha 2-adrenoceptors, these data provide evidence that, in the rat hippocampus, postsynaptic alpha 1-adrenoceptors are intrasynaptic, whereas postsynaptic alpha 2-adrenoceptors are extrasynaptic.

Evidence for functional contact between cografted locus coeruleus and spinal cord in oculo: electrophysiological studies

The functional consequences of the locus coeruleus innervation of the spinal cord are not yet clearly understood. In a recent histological study it was shown that intraocular spinal cord grafts will become innervated by tyrosine hydroxylase-positive nerve fibers from a cografted locus coeruleus. In the present study we use this intraocular model of the descending coeruleo-spinal pathway to investigate functional contact between locus coeruleus and the spinal cord. We have pharmacologically characterized the receptor mediation of norepinephrine-induced, as well as locus coeruleus-mediated depressions of spinal cord neurons grafted in oculo. We found that electrical stimulation of the locus coeruleus part of the double grafts predominantly caused an inhibition of cografted spinal cord neurons. Norepinephrine-induced inhibition of the firing rate of single grafted spinal cord neurons was antagonized by phentolamine, an alpha-adrenergic antagonist, but was unaffected by timolol, a beta-adrenergic antagonist. Similarly, inhibition of the firing rate of grafted spinal cord neurons by stimulation of cografted locus coeruleus was antagonized by phentolamine but not by timolol. Interestingly, single spinal cord grafts were more sensitive to the depressant effects of perfused norepinephrine than was the spinal cord cografted with locus coeruleus. We conclude that spinal cord grafts can be functionally innervated by cografted locus coeruleus and that the noradrenergic inputs to spinal cord from cografted locus coeruleus are alpha-adrenergically mediated. Furthermore, the postsynaptic receptors in single spinal cord grafts appear to be supersensitive to norepinephrine application.

Electrophysiological effects of norepinephrine on Purkinje neurons in intraocular cerebellar grafts: alpha- vs beta-specificity

The present study investigates the receptor specificity of the electrophysiological effects of norepinephrine (NE) on cerebellar Purkinje neurons. Intraocular cerebellar grafts were utilized to allow both superfusion and local administration of selective adrenergic agonists and antagonists. Fetal cerebellar anlagen (E13-15) were homologously transplanted to the anterior chamber of the eye of adult recipient rats and allowed to mature in the eye for at least 5 weeks. Spontaneous activity of Purkinje neurons was recorded extracellularly in the intraocular grafts. Superfusion of 5 microM NE caused elevations of the spontaneous firing rate. Superfusion of 30 microM NE caused depressions, which were occasionally preceded by an excitation. Iontophoretic application of NE to grafted Purkinje neurons primarily caused depressions of the spontaneous discharge rate. Thus, the NE-induced excitations previously reported from in vitro slices are not anomalies of the in vitro slice preparation, but can be observed with superfusion of NE in our in vivo preparation as well. In general, the excitations caused by low doses of superfused NE were blocked by timolol, a specific beta-adrenergic antagonist, while the depressions caused by 30 microM superfused NE or iontophoretically applied NE were blocked by the specific alpha-adrenergic antagonist phentolamine. Large doses of sotalol were found to block both excitatory and depressant responses while lower doses only antagonized the NE-induced excitations. Taken together, these results suggest that the inhibitory effects of NE on PUrkinje neuron firing rate in intraocular cerebellar grafts in vivo are mediated via an alpha-adrenergic receptor mechanism, while the excitations caused by NE may be beta-mediated.

Systemic phenoxybenzamine but not beta-adrenergic antagonists block noradrenergic inhibition of cerebellar Purkinje and hippocampal pyramidal neurons

Previous pharmacological characterization of central noradrenergic receptors has been interpreted as favoring beta-type receptors on cerebellar Purkinje neurons and hippocampal pyramidal neurons. However, the recent development of additional noradrenergic antagonists suitable for single neuron analysis, prompted an initial re-evaluation of alpha and beta adrenergic receptors in these two cell populations. In contrast with earlier data based on local antagonism of iontophoretric or synaptically released norepinephrine (NE), we now find that systemic phenoxybenzamine, the alpha antagonist, was effective in blocking responses to NE in cerebellum and hippocampus, whereas systemic beta antagonists metoprolol, ICI 118.551 [correction of ICI 181.551], or proprandol did not interfere with local NE responses at systemic doses that altered spontaneous discharge rates. These preliminary data suggest that a more complete re-evaluation of the nature of central noradrenergic response mechanisms may be warranted.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. II. An in vitro study of the effects of iontophoretically applied norepinephrine on LH neuronal responses to gamma-aminobutyric acid (GABA)

The preceding studies demonstrated that norepinephrine (NE) can consistently augment synaptically mediated (70%) and gamma-aminobutyric acid (GABA)-induced (69%) inhibitory responses of lateral hypothalamic (LH) neurons in vivo. The present experiments further characterized the interactions of NE with LH neuronal responses to GABA in terms of alpha- and beta-receptor mechanisms and demonstrated the utility of the in vitro LH tissue slice preparation as a model for future extra- and intracellular studies of NE modulatory phenomena. Extracellular activity of LH cells was recorded from diencephalic slices (450 microns) incubated in artificial cerebrospinal fluid at 33 degrees C. Interactions between iontophoretically applied NE, isoproterenol (ISO) or phenylephrine (PE) and responses of LH neurons (n = 64) to GABA microiontophoresis were quantitated and characterized using computer-generated ratemeter and histogram records. This analysis revealed two distinct actions of NE on GABA-induced responses of LH neurons. In 8 of 32 cells tested (25%), locally applied NE markedly enhanced inhibitory responses to GABA iontophoresis in a manner identical to that observed in vivo. However, in 20 cells (62.5%), iontophoretic application of NE produced a clear antagonism of GABA responses. NE also exerted dual effects on the background firing rate of LH neurons, causing both inhibition and excitation. Overall, in those cells where NE administration increased spontaneous discharge, it either antagonized or had no effect on GABA-mediated inhibition. In contrast, spontaneous firing rate was never elevated above control levels in those cases where NE potentiated GABA responses. Additional experiments demonstrated that the GABA potentiating actions of the benzodiazepine, flurazepam, were preserved in LH tissue slice preparations. In addition, iontophoretic application of the beta-agonist, ISO, routinely suppressed the spontaneous activity of LH neurons and mimicked the facilitating action of NE on GABA. Likewise, microiontophoretic application of 8-bromo cyclic adenosine monophosphate (AMP) enhanced GABA-induced inhibition of LH firing rate in each of 11 cells tested. On the other hand, local administration of the alpha agonist, PE, routinely produced NE-like antagonism of GABA inhibition along with increases in spontaneous firing rate. Taken together these findings indicate that the commonly observed in vivo phenomena of NE augmentation of GABA and suppression of LH neuron spontaneous firing can be demonstrated in vitro, and most likely result from activation of beta adrenoceptors and subsequent elevation of cyclic AMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Norepinephrine decreases synaptic inhibition in the rat hippocampus

The effects of norepinephrine (NE) on inhibitory synaptic potentials were studied on CA1 pyramidal neurons in the hippocampal slice in vitro. Norepinephrine caused the appearance of multiple population spikes in the CA1 region of the hippocampal slice, reminiscent of the actions of gamma-aminobutyric acid (GABA) antagonists. Intracellular recording revealed that NE causes a marked and reversible reduction in inhibitory postsynaptic potentials (IPSPs) recorded in CA1 pyramidal cells. This reduced IPSP results in a larger intracellular excitatory postsynaptic potential (EPSP), which can cause the cell to fire more than one action potential. This disinhibitory effect of NE appears to be mediated by an alpha-receptor, and occurs at a site presynaptic to the pyramidal cell, since NE does not change the reversal potential of the IPSP nor does it affect the amplitude or the reversal potential of iontophoretic GABA responses. In addition to reducing evoked IPSPs, NE causes an increase in the frequency of spontaneous IPSPs, suggesting that inhibition of interneuronal firing may not account for this disinhibitory action of NE.