Noradrenaline Release In Hippocampus Research Articles

Functional and binding properties of sigma receptors in rat cerebellum.

Autoradiographic studies have shown that sigma receptors are enriched in the locus coeruleus, the origin of noradrenergic projections to the cerebellum, as well as in the Purkinje, molecular, and granular layers and the interpositus cerebellar nucleus of the cerebellum itself. In contrast, the cerebellum is relatively poor in phencyclidine (PCP) binding sites, which have been historically confused with sigma sites. The high ratio of sigma to PCP receptors in cerebellum is advantageous for discriminating sigma-mediated physiological effects. sigma agonists and antagonists have been shown to regulate N-methyl-D-aspartate (NMDA)-stimulated norepinephrine release in hippocampus, which is innervated by locus coeruleus projections. We now report that sigma drugs also regulate norepinephrine release from cerebellum. In contrast to findings in the hippocampus, where regulation is via sigma 1 and sigma 2 receptors, sigma-mediated regulation in cerebellum seems to be primarily via sigma 1 receptors. In radioligand binding studies, we find that sigma receptors primarily of the sigma 1 type are present in the cerebellum. We further report that binding to sigma receptors in cerebellum is not affected by the addition of NMDA or glycine or by the presence of NMDA antagonists, suggesting that sigma receptors are not located within the NMDA-operated cation channel in this brain region.

Subtype-specificity of the presynaptic α 2-adrenoceptors modulating hippocampal norepinephrine release in rat

In vivo brain microdialysis and high-performance liquid chromatography with electrochemical detection were used to study the effect of different selective α 2-antagonists on hippocampal norepinephrine (NE) release in freely moving awake rat. Systemic administration (0.5 mg/kg i.p.) of either the α 2AD-antagonist BRL 44408 or the α- 2BC-antagonist ARC 239 did not significantly change the basal release of NE. At a higher dose (5 mg/kg i.p.) ARC 239 was still ineffective, whereas BRL 44408 caused a significant increase of the extracellular level of NE. Similar results were obtained from in vitro perfusion experiments. Rat hippocampal slices were loaded with [ 3H]NE and the electrical stimulation-evoked release of [ 3H]NE was determined. The α 2-antagonists were applied in a concentration range of 10 −8 to 10 −6 M. ARC 239 was ineffective, whereas BRL 44408 significantly increased the electrically induced release of [ 3H]NE. In agreement with the data of microdialysis and perfusion experiments. BRL 44408 displaced [ 3H]yohimbine from hippocampal and cortical membranes of rat brain with high affinity whereas ARC 239 was less effective. The p K i values of eight different α 2-adrenergic compounds showed a very good correlation ( r = 0.98, slope = 1.11 P < 0.0001) in hippocampus and frontal cortex where the α 2-adrenoceptors have been characterized as α 2D-subtype. Our data indicate that hippocampal NE release in rat is regulated by α 2D-adrenoceptors, a species variation of the human α 2A-subtype.

Amphetamine enhances memory retention and facilitates norepinephrine release from the hippocampus in rats

The present study investigated the effects of intra-hippocampal amphetamine on memory retention and the role of hippocampal norepinephrine (NE) in memory consolidation in rats. One-way inhibitory avoidance learning paradigm was adopted. Animals were trained to avoid the foot shock. The latency to step into the shock compartment was recorded as the retention measure. The ceiling score (full retention) was 600 s. Results indicated that intra-hippocampal injections of amphetamine produced a dose-dependent enhancement of memory retention with doses at 0.6 μg and 1.6 μg reaching a significant effect. The β-adrenergic blocker propranolol, at a dose which did not affect retention alone (80 ng), antagonized the memory-enhancing effect of amphetamine. Along with this memory-enhancing effect, amphetamine also elevated the level of NE release, and this effect was significant in animals not showing a full retention score (nonresponders) than in animals showing a full retention score (responders), as assayed by in vivo microdialysis. Within the control group, the responders also had a higher level of NE than the nonresponders. All these results are probably due to the fact that responders have a higher level of NE release than nonresponders. The effect of amphetamine on NE release is, therefore, not as obvious in responders. These results together support our hypothesis that NE plays a facilitatory role in the memory process and amphetamine enhances retention performance, at least in part, through facilitation of hippocampal NE release.

Biochemical evidence for the regulation of central noradrenergic activity by 5-HT 1A and 5-HT 2 receptors: Microdialysis studies in the awake and anaesthetized rat

Here we have studied the effect of various 5-HT 1A and 5-HT 2 receptor-selective drugs on noradrenaline release in the hippocampus of anaesthetized and awake rats using microdialysis. In the anaesthetized rat, administration of the 5-HT 1A agonists buspirone, gepirone and ipsapirone increased noradrenaline levels in the microdialysates. However, the common metabolite of these compounds, 1-PP (an α-2 adrenoceptor antagonist with low affinity for 5-HT 1A receptors), also increased noradrenaline efflux whilst the 5-HT 1A receptor agonist 8-OH-DPAT and MDL 73005EF, which are not metabolized to 1-PP, did not. In the awake rat, buspirone but also 8-OH-DPAT increased noradrenaline efflux. A similar effect was observed in response to MDL 73005EF and the 5-HT 1A ligand NAN-190. Since the latter two drugs have weak intrinsic activity at the post-versus presynaptic 5-HT 1A receptor, a presynaptic mechanism (inhibition of 5-HT release) was implicated. The 5-HT 2 receptor may be important to this mechanism as noradrenaline increased following administration of the 5-HT 2 receptor antagonists, ritanserin and ICI 170,809. In conclusion, our data indicate that there are clear differences in the effects of 5-HT 1A and 5-HT 2 receptor-selective drugs on noradrenaline efflux in hippocampus of the anaesthetized versus awake rat. Our findings are reconcilable with the hypothesis that in the awake (but not anaesthetized) rat, release of noradrenaline in hippocampus is influenced by an inhibitory tone mediated via 5-HT 2 receptors. If this inhibitory tone is removed, either by decreasing 5-HT release through activation 5-HT 1A autoreceptors or by blocking postsynaptic 5-HT 2 receptors, noradrenaline release increases.

Hippocampal noradrenaline release is modulated by γ- and δ-hexachlorocyclohexane isomers: which mechanisms are involved?

The differential effects of γ- and δ-hexachlorocyclohexane isomers on 25 mM K +-evoked release of [ 3H]noradrenaline were studied in hippocampal slices treated with selected agents to activate or block L- and N-type Ca 2+ and Na + voltage-sensitive ion channels, Cl − transport and Ca 2+-dependent protein activity. At maximally effective concentrations, the L- and N-type Ca 2+ channel blockers nifedipine and ω-conotoxin, respectively, and the Na + channel antagonist tetrodotoxin did not modify the enhancement of K +-evoked [ 3H]noradrenaline release induced by γ-hexachlorocyclohexane. Likewise, under activation of protein kinase C by phorbol 12,13-dibutyrate (PDB) or inhibition of calmodulin by N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), the stimulatory effect of γ-hexachlorocyclohexane remained almost unchanged. The Cl − transport blocker 4,4-diisothiocyanato-stilbene-2,2′-disulfonic acid (DIDS) significantly reduced the effect of γ-hexachlorocyclohexane on [ 3H]noradrenaline release. The enhanced release in the presence of Bay K 8644, the L-type Ca 2+ channel activator, was significantly inhibited by nifedipine but not by δ-hexachlorocyclohexane. The combination of ω-conotoxin and tetrodotoxin with δ-hexachlorocyclohexane did not alter the [ 3H]noradrenaline release effects of each agent alone. Activation of protein kinase C in the presence of δ-hexachlorocyclohexane resulted in a reduction of the δ isomer effect and in a potentiation of the PDB effect. W-7 did not further facilitate the inhibition induced by δ-hexachlorocyclohexane alone. These data suggest that hexachlorocyclohexane isomers may modify K +-evoked [ 3H]noradrenaline release by interacting with presynaptic molecular processes involving changes in Cl − membrane permeability and intracellular Ca 2+ homeostasis.

Learned helplessness and in vivo hippocampal norepinephrine release

Hippocampal norepinephrine release was measured using in vivo microdialysis in rats before and after exposure to inescapable tail shock stress and after testing for learned helplessness. Rats that did not develop learned helplessness after stress had higher basal norepinephrine release after stress than rats developing learned helplessness or than control rats. After the shuttlebox test for learned helplessness, K +-stimulated norepinephrine release was lower in learned helpless than in nonhelpless or control rats. These results confirm an important role for the hippocampal noradrenergic system in differential behavioral responses to stress.

In vivo monoamine release during naloxone-precipitated morphine withdrawal.

There is much evidence from animal work suggesting that the release of noradrenaline (NA) in the brain increases during naloxone-precipitated morphine withdrawal, but the evidence in favour of changes in release of 5-hydroxytryptamine (5-HT) and dopamine (DA) is contradictory. Here we demonstrate, using in vivo microdialysis, that whilst there is a considerable increase (300%) in release of NA in hippocampus precipitated by naloxone in morphine-dependent rats, there is no change in the release of either 5-HT (in hippocampus) or DA (in nucleus accumbens). These results are consistent with suggestions that the symptoms of morphine withdrawal in rats are due primarily to an increase in central NA release.

Effect of K+ channel blockers on the ?2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus

The question whether presynaptic alpha 2-adrenoceptors regulating noradrenaline release in hippocampus directly couple to tetraethylammonium chloride (TEA) or alpha-dendrotoxin (alpha-DTX)-sensitive K+ channels was investigated. Hippocampal slices, prelabelled with [3H] noradrenaline, were superfused in the presence of (+)-oxaprotiline and electrically stimulated with 4 pulses delivered at 100 Hz, in order to avoid autoinhibition due to released noradrenaline. TEA enhanced the evoked [3H]noradrenaline release in rabbit hippocampus in a concentration-dependent manner, yielding an approximately 4-fold increase at 30 mmol/l, whereas the spontaneous outflow of tritium was only slightly affected at this concentration. The alpha 2-adrenoceptor agonist clonidine, at 10-100 nmol/l inhibited the evoked [3H]noradrenaline release between 77% and 96%. The inhibitory effect of the alpha 2-agonist was distinctly diminished in the presence of 30 mmol/l TEA but was restored in low Ca2+/high Mg2+ buffer. Therefore, the diminution of the alpha 2-agonist effect by TEA observed in experiments with normal Ca2+ can be explained by an increase of the Ca2+ availability for the release process due to the prolongation of action potentials. In rabbit hippocampus alpha-DTX (10-200 nmol/l) did neither affect the evoked release of [3H]noradrenaline nor its alpha 2-agonist-induced modulation. However, in rat hippocampus alpha-DTX significantly increased the evoked transmitter release and diminished the effect of clonidine. Taken together, the present data for the rabbit hippocampus exclude the possibility that activation of presynaptic alpha 2-adrenoceptors inhibits depolarization-evoked [3H]noradrenaline release by inducing an outward K+ current through TEA- or alpha-DTX-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that 5-HT2 receptor activation decreases noradrenaline release in rat hippocampus in vivo.

1. Recent electrophysiological studies have shown that 5-HT2/5-HT1C receptor agonists inhibit the electrical activity of noradrenergic neurones in the rat locus coeruleus. Here we examine the effect of various agonists and antagonists of 5-HT2/5-HT1C receptors on noradrenaline release in hippocampus of anaesthetized rats using microdialysis. 2. Subcutaneous administration of the 5-HT2/5-HT1C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI: 0.2 and 0.5 mg kg-1), caused a marked decrease (50% of pre-drug levels 60 min after injection) of noradrenaline in hippocampal dialysates which was long-lasting (greater than 120 min). Noradrenaline output also decreased in response to administration of the structural analogue of DOI, 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB: 1 mg kg-1, s.c.). 3. The effect of DOI on noradrenaline output was prevented by pretreatment with the 5-HT2/5-HT1C receptor antagonist, ritanserin (0.4 mg kg-1, s.c.). Spiperone (0.2 and 1 mg kg-1, s.c.), a 5-HT2/dopamine D2 receptor antagonist which has low affinity for 5-HT1C receptors, also antagonized the effect of DOI (0.5 mg kg-1, s.c.). Sulpiride (50 mg kg-1, s.c.), a dopamine D2 receptor antagonist did not alter the response to DOI (0.5 mg kg-1, s.c.). 4. Both the non-selective 5-HT receptor agonist, quipazine (1 mg kg-1, s.c.), and the 5-HT-releasing agent, p-chloroamphetamine (2 mg kg-1, s.c.), decreased noradrenaline release in hippocampus and these effects were antagonized by pretreatment with ritanserin (0.4 mg kg-1, s.c.).5. Our data suggest that in vivo, noradrenaline release in hippocampus is inhibited by 5-HT2 receptor activation. This effect is probably associated with a decrease in noradrenergic neuronal activity.

Effect of naloxone-precipitated morphine withdrawal on noradrenaline release in rat hippocampus in vivo

Here we investigated the effect of naloxone-precipitated morphine withdrawal on the release of noradrenaline in hippocampus of the anaesthetised rat. Naloxone (1 mg/kg i.p.) injected 3 and 24 h, but not 3 weeks, after eight daily injections of morphine, induced an immediate increase in hippocampal noradrenaline release. This striking effect of naloxone was markedly attenuated by pretreatment with clonidine (0.1 mg/kg s.c.). These findings provide direct evidence for a marked release of noradrenaline in hippocampus during opiate withdrawal in vivo.

Vulnerability to stress and in vivo hippocampal norepinephrine release

Vulnerability to stress and in vivo hippocampal norepinephrine release

Adinazolam affects biogenic amine release in hippocampal CA 1 neuronal circuitry

The new triazolobenzodiazepine, adinazolam, which has dual anxiolytic and antidepressant activities, was studied for its effects on hippocampal CA 1 norepinephrine and serotonin release in chloral hydrate-anesthetized rats, with in vivo voltammetry. Norepinephrine signals were further characterized in vivo by the detection of a significantly increased norepinephrine signal (mean = 25.8%) ( p < 0.003) after intraperitoneal administration of the α 2 adrenoreceptor antagonist, yohimbine, and by the detection of a significantly decreased norepinephrine signal (mean = 20.1%) ( p < 0.037) after intraperitoneal administration of the α 2 adrenoreceptor agonist, clonidine. Time course studies showed that the anxiolytic-antidepressant drug adinazolam (10 mg/kg IP) significantly decreased hippocampal norepinephrine release (mean = 26.2%) ( p < 0.007). The norepinephrine signal was further significantly decreased by adinazolam (mean= 16.4%) ( p < 0.009) after an additional 2 mg/kg IP injection. Serotonin release, which was detected with norepinephrine in sequence, was also significantly decreased by adinazolam (10 mg/kg IP) (mean = 22.4%) ( p < 0.002). The supplemental dose of adinazolam (2 mg/kg IP), however, did not significantly alter serotonin release any further ( p < 0.307). The findings show that the mechanism of action of adinazolam occurs simultaneously on presynaptic release mechanisms for norepinephrine and for serotonin in CA 1 region of hippocampus. These findings implicate that noradrenergic and serotonergic release mechanisms may be responsible in part for the dual anxiolytic-antidepressant efficacy of adinazolam.

Electrical stimulation of the lateral habenula increases hippocampal noradrenaline release as monitored by in vivo microdialysis.

Hippocampal extracellular levels of noradrenaline (NA) were monitored with the microdialysis technique during electrical stimulation of the lateral habenula (LHb) in halothane anaesthetized rats. The steady state NA level was 20.8 +/- 4.6 fmole/15 min of perfusion (mean +/- SEM). Electrical stimulation of the LHb for 15 min (15 Hz, 0.5 mA) induced an immediate 228 +/- 48% increase in hippocampal NA release, compared to the pre-stimulation baseline (p less than 0.05). A second stimulation 90 min later induced a similar increase. The effect of LHb stimulation was completely abolished by a knife cut transecting the dorsal NA bundle either immediately rostral to the locus coeruleus or at the level of the parafascicular nucleus. This suggests that the effect was dependent on nerve impulses flow in the coeruleo-hippocampal NA neurons, and was not mediated, e.g., by a local spread of electricity into the hippocampus. Since the LHb has previously been shown to be a powerful activator of the mesencephalic raphe nuclei we tested whether the effect was mediated via the serotonergic system. However, the effect of LHb stimulation on hippocampal NA release persisted after 5,7-dihydroxytryptamine treatment and after complete radiofrequency lesions of the dorsal and central superior raphe nuclei. The present data suggest that electrical stimulation of the LHb can increase hippocampal NA release through an activation of the locus coeruleus, and that this effect is not dependent on the mesencephalic raphe nuclei. The results support the role of the LHb as a link for limbic and striatal forebrain activation of brain stem monoaminergic systems.

Characterization of hippocampal norepinephrine release as measured by microdialysis perfusion: Pharmacological and behavioral studies

The release of endogenous norepinephrine in hippocampus was studied in freely moving rats with microdialysis perfusion. Using a loop-style dialysis probe, the basal amount of norepinephrine collected in 15-min fractions averaged 12pg/25/gml. Correcting for recovery (21%), the concentration of norepinephrine in the extracellular fluid of hippocampus under resting conditions was estimated to be approximately 14nM. The alpha 2 adrenoceptor antagonist yohimbine (5.0mg/kg, i.p.) increased norepinephrine efflux to 230% of basal levels. Clonidine (0.3 mg/kg, i.p.), an alpha 2 adrenoceptor agonist, decreased norepinephrine efflux to 56% of baseline. Addition of the reuptake blocker desipramine (1.0μM) to the perfusate had no significant effect on norepinephrine efflux. However, increasing the K + concentration of the perfusate to 30 mM increased norepinephrine efflux to 196% of baseline, and this effect was increased nearly two-fold by the addition of desipramine to the perfusate (364% of baseline). Restraint stress and intermittent tailshock increased norepinephrine efflux to 213% and 234% of baseline, respectively. The results suggest that microdialysis is a useful way to study norepinephrine release in hippocampus and they permit several conclusions to be drawn. First, the data obtained with systemic administration of alpha 2 adrenoceptor drugs emphasize the fact that a variety of regulatory mechanisms exist that may affect transmitter levels in the extracellular fluid. Second, the ratio of extracellular to intracellular norepinephrine in hippocampal tissue is considerably higher than that reported for dopamine in striatum. Coupled with the small effect of norepinephrine uptake blockade, this suggests that nerve terminal density is an important factor in determining the concentration of catecholamines in the extracellular fluid. Finally, the observed stress-induced increase in norepinephrine efflux supports the hypothesis that the locus coeruleus noradrenergic system exerts an important central influence during stressful conditions.

Phorbol ester-mediated enhancement of hippocampal noradrenaline release: which ion channels are involved?

Enhancement of neurotransmitter release following phorbol ester-induced activation of protein kinase C (PKC) may be mediated by changes in ion conductance through the presynaptic membrane. This question was studied with rabbit hippocampal slices preincubated with [ 3H]noradrenaline ([ 3H]NA). NA release was evoked by pulses of either high K + or Ca 2+ (in the presence of high K +), or by electrical field stimulation. 4β-Phorbol 12,13-dibutyrate (PDB) increased and polymyxin B (PMB) reduced the K +-evoked NA release independent of the K + concentration used for depolarization. The effects of PDB and PMB were not reduced by tetrodotoxin. PDB still enhanced the NA release triggered by short Ca 2+ pulses in depolarized, axon terminal membranes (30 mM K + and no Ca 2+). The electrically evoked NA release was markedly enhanced by PDB even in the absence of Cl − in the medium or in the presence of the K + channel blockers, tetraethylammonium, 4-amino- and 3,4-diaminopyridine. The inhibitory effect of the Ca 2+ channel blocker, Cd 2+, remained almost unchanged in the presence of PDB. It is concluded that PKC activation facilitates NA release in the hippocampus but not via presynaptic changes in Na +, K + or Cl − currents. Whether phorbol ester mediates an increased intracellular Ca 2+ availability, or whethr a triggering ‘normal’ Ca 2+ influx simply initiates, and synergistically supports, the PKC-mediated reactions leading to enhanced exocytosis, cannot be decided from the results of the present experiments.

ω-Conotoxin GVIA and pharmacological modulation of hippocampal noradrenaline release

The tritium overflow evoked by electrical stimulation of rabbit hippocampal slices labeled with [ 3H]noradrenaline was inhibited by ω-conotoxin GVIA, a peptide modulator of the N-type voltage-sensitive calcium channel (N-VSCC). The magnitude of this inhibition was unchanged in the presence of substances which interact with N- and/or L-VSCCs (cadmium, neomycin, (−)- and (+)-202−791), α 2- adrenoceptors (idazoxan, UK-14304), protein kinase C (4β-phorbol-12,13-dibutyrate) or potassium channels (4-aminopyridine). This finding suggests that the attenuation of calcium-dependent neurotransmitter release by ω-conotoxin GVIA is relatively insensitive to alterations of such release effected by other substances.

Adenosine: An endogenous modulator of hippocampal noradrenaline release

In slices of hippocampus from the rabbit, preincubated with [ 3H]noradrenaline and then continuously superfused, the modulation of the release of noradrenaline by adenosine receptors was studied. Electrical field stimulation of the slices elicited a release of [ 3H]noradrenaline which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. From the order of potency: cyclohexyladenosine > (−)phenylisopropyladenosine [(−)PIA] > 5′- N-ethylcarboxamide-adenosine (NECA) > 2-chloro-adenosine > adenosine (+)phenylisopropyladenosine > ATP, the inhibitory adenosine receptor was classified as A 1- (R i-) receptor. The effect of the agonists was strongly reduced by adenosine receptor antagonists, the methylxanthines. A role for endogenous adenosine in the modulation of hippocampal noradrenaline release is supported by these findings: (1) that blockade of adenosine receptors by methylxanthines, especially by 8-phenyltheophylline, increased, whereas (2) inhibition of the uptake of adenosine decreased the evoked release of noradrenaline and (3) that deamination of endogenous extracellular adenosine by addition of adenosine deaminease to the medium enhanced the evoked transmitter release. Inhibitors of endogenous adenosine deaminase and 5'-nucleotidase were without effect. It is concluded that release of noradrenaline in the hippocampus is inhibited at the level of the noradrenergic neve terminals by endogenous adenosine via A 1 (or R i) receptors.