Endogenous noradrenaline and dopamine in nerve terminals of the hippocampus: differences in levels and release kinetics

Time course of the physiological stress response to an acute stressor and its associations with the primacy and recency effect of the serial position curve

Whether stress affects memory depends on which stress pathway becomes activated and which specific memory system is involved. The activation of the sympathetic nervous system (SNS), leads to a release of catecholamines. The activation of the hypothalamic-pituitary-adrenal (HPA) axis, leads to a release of glucocorticoids. In thus study, it was investigated whether SNS and/or HPA axis activation are associated with long-term memory (LTM) and/or working memory (WM) performance in humans. Thirty-three participants underwent the socially evaluated cold-pressor test. Salivary alpha-amylase (sAA) was used as a marker for the activation of the SNS and cortisol as marker for HPA axis activation. Memory was assessed by means of word lists with 15 words each. The primacy effect (i.e., the correctly recalled words from the beginning of the lists) of the serial position curve was considered as indicator for LTM. The recency effect (i.e., the correctly recalled words from the end of the lists) were used as estimator for WM performance. In sAA responders, the recency effect and, therefore, WM performance increased immediately after the stressor. This was not found in sAA non-responders. In cortisol responders, the primacy effect and, thus, LTM performance decreased 20 minutes after the stressor. No change in LTM performance was found in cortisol non-responders. Our study supports the assumptions that 1) SNS activation is associated with WM processes via stimulation of the prefrontal cortex, and 2) HPA axis activation is associated with LTM processes through interactions with the hippocampus.

Modulating the map: dopaminergic tuning of hippocampal spatial coding and interactions

Salient events activate the midbrain dopaminergic system and have important impacts on various aspects of mnemonic function, including the stability of hippocampus-dependent memories. Dopamine is also central to modulation of neocortical memory processing, particularly during prefrontal cortex-dependent working memory. Here, we review the current state of the circuitry and physiology underlying dopamine's actions, suggesting that--alongside local effects within hippocampus and prefrontal cortex--dopamine released from the midbrain ventral tegmental area is well positioned to dynamically tune interactions between limbic-cortical circuits through modulation of rhythmic network activity.

Modulation of muscimol state-dependent memory by α2-adrenoceptors of the dorsal hippocampal area

In the present study, the effects of bilateral intra-dorsal hippocampal (intra-CA1) injections of α2-adrenoceptor agonist and antagonist, on muscimol state-dependent memory were examined in mice. A single-trial step-down passive avoidance task was used for the assessment of memory retention in adult male NMRI mice. Administration of muscimol (0.1 μg/mouse, intra-CA1) 15 min before training or testing induced impairment of memory retention. Injection of the same dose of the drug 15 min before testing restored memory retention impaired under pre-training muscimol influence. Pre-test intra-CA1 administration of the α2-adrenoceptor agonist clonidine (0.5 and 1 μg/mouse) impaired memory retention, although the low dose of the drug (0.25 μg/mouse) did not affect memory retention. Pre-test intra-CA1 administration of the α2-adrenoceptor antagonist yohimbine (1 and 2 μg/mouse) improved memory retention, although the low dose of the drug (0.5 μg/mouse) did not affect memory retention. In other series of experiments, pre-test co-administration of certain doses of clonidine (0.125 and 0.25 μg/mouse, intra-CA1), doses which were ineffective when given alone, and muscimol (0.1 μg/mouse, intra-CA1) significantly inhibited muscimol state-dependent memory. Pre-test intra-CA1 administration of certain doses of yohimbine (0.25 and 0.5 μg/mouse), doses which were ineffective when given alone, improved pre-training muscimol (0.1 μg/mouse)-induced retrieval impairment. Moreover, pre-test co-administration of yohimbine (0.25 and 0.5 μg/mouse, intra-CA1) and muscimol (0.025 μg/mouse, intra-CA1), an ineffective dose, significantly restored the retrieval and induced muscimol state-dependent memory. It may be concluded that the α2-adrenoceptors of the dorsal hippocampal area play an important role in muscimol state-dependent memory.

Dopamine transporter blockade increases LTP in the CA1 region of the rat hippocampus via activation of the D3 dopamine receptor

Dopamine has been demonstrated to be involved in the modulation of long-term potentiation (LTP) in the CA1 region of the hippocampus. As monoamine transporter blockade will increase the actions of endogenous monoamine neurotransmitters, the effect of a dopamine transporter (DAT) antagonist on LTP was assessed using field excitatory postsynaptic potentials recorded in the CA1 region of the rat hippocampal slice preparation. Application of the DAT-specific blocker GBR 12,935 produced a significant enhancement in LTP of Schaffer collateral synapses in the CA1 at concentrations as low as 100 nM. A selective D1/D5 dopamine receptor antagonist (SCH 23,390, 1 microM) did not affect the ability of GBR 12,935 to enhance LTP, whereas application of the D3 dopamine receptor antagonist U 99,194 (1 microM) blocked the GBR 12,935-induced enhancement in LTP. In addition, a D3 dopamine receptor agonist (7-OH-DPAT, 1 microM) caused a significant increase in LTP, an effect that was also blocked by U 99,194 (3 microM). These results suggest that either endogenously released dopamine (facilitated by DAT blockade) or exogenously applied dopamine agonist can act to increase LTP in the CA1 of the hippocampus via activation of the D3 subtype of dopamine receptor.

Modulation of calcium-evoked [3H]noradrenaline release from permeabilized cerebrocortical synaptosomes by the MARCKS protein, calmodulin and the actin cytoskeleton

In order to examine intracellular modulation of CNS catecholamine release, cerebrocortical synaptosomes were prelabeled with [3H]noradrenaline and permeabilized with streptolysin-O in the absence or presence of Ca(2+). Plasma membrane permeabilization allowed efflux of cytosol and left a compartmentalized pool of [3H]noradrenaline intact, approximately 10% of which was released by addition of 10(-5) M Ca(2+). Addition of activators or inhibitors of protein kinase C, as well as inhibitors of Ca(2+)-calmodulin kinase II or calcineurin, failed to change Ca(2+)-induced noradrenaline release. Evoked release from permeabilized synaptosomes deficient in the vesicle-associated phosphoprotein synapsin I was also unchanged. In contrast, addition of a synthetic 'active domain' peptide from the myristoylated, alanine-rich C-kinase substrate (MARCKS) protein increased, while addition of calmodulin decreased Ca(2+)-induced release from the permeabilized synaptosomes, the latter effect being reversed by a peptide inhibitor of calcineurin. Moreover, addition of the actin-destabilizing agent DNase I, as well as antibodies to MARCKS, appeared to increase spontaneous, Ca(2+)-independent release from noradrenergic vesicles. These results indicate that the MARCKS protein may modulate release from permeabilized noradrenergic synaptosomes, possibly by modulating calmodulin levels and/or the actin cytoskeleton.

A late phase of exocytosis from synaptosomes induced by elevated [Ca2+]i is not blocked by Clostridial neurotoxins

Treatment of rat cerebrocortical synaptosomes with botulinum toxin types E and C1 or tetanus toxin removed the majority of intact SNAP-25, syntaxin 1A/1B, and synaptobrevin and diminished Ca(2+)-dependent K+ depolarization-induced noradrenaline secretion. With botulinum toxin type E, <10% of intact SNAP-25 remained and K(+)-evoked release of glutamate and GABA was inhibited. The large component of noradrenaline release evoked within 120 s by inclusion of the Ca2+ ionophore A23187 with the K+ stimulus was also attenuated by these toxins; additionally, botulinium neurotoxin type E blocked the first 60 s of ionophore-induced GABA and glutamate exocytosis. However, exposure to A23187 for longer periods induced a phase of secretion nonsusceptible to any of these toxins (>120 s for noradrenaline; >60 s for glutamate or GABA). Most of this late phase of release represented exocytosis because of its Ca2+ dependency, ATP requirement, and sensitivity to a phosphatidylinositol 4-kinase inhibitor. Based on these collective findings, we suggest that the ionophore-induced elevation of [Ca2+]i culminates in the disassembly of complexes containing nonproteolyzed SNAP receptors protected from the toxins that can then contribute to neuroexocytosis.

Presynaptic alpha2-adrenoceptors control excitatory, but not inhibitory, transmission at rat hippocampal synapses

1. The effects of noradrenaline on neurotransmission at rat hippocampal synapses were investigated by recording autaptic currents in single neurons isolated on glial microislands. Noradrenaline reduced excitatory, but not inhibitory, autaptic currents in a pertussis toxin-sensitive manner, but the amine did not affect glutamate-evoked currents. 2. The inhibition of excitatory autaptic currents by noradrenaline was half-maximal at 0. 11 +/- 0.06 microM. The alpha2-adrenoceptor agonists UK 14 304 and clonidine were equipotent to noradrenaline in reducing these currents, whereas the alpha1-adrenoceptor agonist methoxamine and the beta-adrenoceptor agonist isoprenaline (isoproterenol) were ineffective. The reduction of excitatory autaptic currents by noradrenaline was not altered by the alpha1-adrenergic antagonist urapidil or the beta-antagonist propranolol, but reduced by the alpha2-antagonist yohimbine. The subtype-preferring antagonists rauwolscine and phentolamine (both at 0.3 microM) caused 9-fold and 36-fold rightward shifts in the concentration-response curve for the noradrenaline-dependent reduction of excitatory autaptic currents, respectively. Prazosine (1 microM) did not affect this concentration-response curve. 3. Noradrenaline reduced voltage-activated Ca2+ currents in excitatory, but not in inhibitory, microisland neurons. For comparison, the GABAB agonist baclofen reduced both excitatory and inhibitory autaptic currents and diminished voltage-activated Ca2+ currents in both types of neurons. The inhibition of Ca2+ currents by noradrenaline was half-maximal at 0.17 +/- 0.05 microM, and UK 14 304 and clonidine were equipotent to noradrenaline in reducing these currents. The noradrenaline-induced reduction of Ca2+ currents was antagonized by yohimbine, but not by urapidil or propranolol; the subtype-preferring alpha2-adrenergic antagonists displayed the following rank order of activity: phentolamine > rauwolscine > prazosine. 4. Noradrenaline did not affect K+ currents and failed to alter the frequency of miniature excitatory postsynaptic currents measured in mass cultures of hippocampal neurons. 5. These results show that noradrenaline regulates transmission at glutamatergic, but not at GABAergic, hippocampal synapses via presynaptic alpha2-adrenoceptors of the alpha2A/D subtype. This inhibitory action involves an inhibition of voltage-activated Ca2+ currents, but no modulation of spontaneous vesicle exocytosis or of voltage-activated K+ currents.

The regulation of forebrain dopamine transmission: relevance to the pathophysiology and psychopathology of schizophrenia

Since the discovery that the therapeutic efficacy of antipsychotic drugs was significantly correlated to their ability to block dopamine D2 receptors, abnormal dopamine transmission in the forebrain has been postulated to underlie psychosis in schizophrenia. In the past 15 years, an impressive amount of clinical and basic research aimed at the study of schizophrenia has indicated that prefrontal and temporal cortical abnormalities may be more important in the etiology of many of the symptoms of schizophrenia, including psychosis. However, the cortical systems that appear to have structural and/or metabolic abnormalities in schizophrenia patients potently regulate forebrain dopamine transmission through a number of mechanisms. In turn, dopamine modulates excitatory transmission mediated by frontal and temporal cortical projections to the basal ganglia and other regions. The present review summarizes the multiple interactions between forebrain DA systems and frontal and temporal corticostriatal transmission. It then examines the role of these interactions in normal behaviors and the psychopathology of schizophrenia.

Regulation of calcium-dependent [3H]noradrenaline release from rat cerebrocortical synaptosomes by protein kinase C and modulation of the actin cytoskeleton

The effects that active phorbol esters, staurosporine, and changes in actin dynamics, might have on Ca2+ -dependent exocytosis of [3H]-labelled noradrenaline, induced by either membrane-depolarizing agents or a Ca2+ ionophore, have been examined in isolated nerve terminals in vitro. Depolarization-induced openings of voltage-dependent Ca2+ channels with 30 mM KCl or 1 mM 4-aminopyridine induced limited exocytosis of [3H]noradrenaline, presumably from a readily releasable vesicle pool. Application of the Ca2+ ionophore calcimycin (10 microM) induced more extensive [3H]noradrenaline release, presumably from intracellular reserve vesicles. Stimulation of protein kinase C with phorbol 12-myristate,13-acetate increased release evoked by all secretagogues. Staurosporine (1 microM) had no effect on depolarization-induced release, but decreased ionophore-induced release and reversed all effects of the phorbol ester. When release was induced by depolarization, internalization of the actin-destabilizing agent DNAase I into the synaptosomes gave a slight increase in [3H]NA release and strongly increased the potentiating effect of the phorbol ester. In contrast, when release was induced by the Ca2+ ionophore, DNAase I had no effect, either in the absence or presence of phorbol ester. The results indicate that depolarization of noradrenergic rat synaptosomes induces Ca2+ -dependent release from a releasable pool of staurosporine-insensitive vesicles. Activation of protein kinase C increases this release by staurosporine-sensitive mechanisms, and destabilization of the actin cytoskeleton further increases this effect of protein kinase C. In contrast, ionophore-induced noradrenaline release originates from a pool of staurosporine-sensitive vesicles, and although activation of protein kinase C increases release from this pool, DNAase I has no effect and also does not change the effect of protein kinase C. The results support the existence of two functionally distinct pools of secretory vesicles in noradrenergic CNS nerve terminals, which are regulated in distinct ways by protein kinase C and the actin cytoskeleton.

A presynaptic N-methyl-D-aspartate autoreceptor in rat hippocampus modulating amino acid release from a cytoplasmic pool

A possible role of the N-methyl-D-aspartate receptor (NMDA-R) as a presynaptic autoreceptor was investigated using Percoll-purified hippocampus nerve terminals (synaptosomes). This preparation contained only a neglectable amount of postsynaptic structures. Two main effects of NMDA were observed. First, NMDA dose-dependently (10-100 microM) and in the absence of Mg2+, stimulated basal release of aspartate and glutamate, but not of GABA. MK801 (10 microM), an open NMDA-R-channel blocker, reduced this effect even below control levels, indicating endogenous NMDA-R activation. By superfusing synaptosomes, which prevents a tonic receptor occupation, also basal GABA release was stimulated by NMDA. The NMDA-induced potentiation of amino acid superfusate levels was blocked both by MK801 and Mg2+ (1 mM), was slow in onset and returned to baseline after NMDA-removal. The NMDA-effect was also found in the absence of extracellular Ca2+, suggesting that amino acids were released from a non-vesicular (cytoplasmic) pool. Secondly, in KCl-depolarized synaptosomes exposed to 1 mM Mg2+, NMDA did not affect the release of the amino acids. MK801, however, reduced the KCl-evoked Ca2+-independent release of aspartate and glutamate, but not of GABA. L-trans-PDC, the selective inhibitor of the glutamate/aspartate transporter, prevented this MK801-effect, suggesting a coupling between NMDA-Rs and these transporters. These data provide evidence for a presynaptic NMDA autoreceptor in rat hippocampus. We speculate on the role of this NMDA-R to depolarize the presynaptic membrane by Na+-entry, which may induce reversal of amino acid transporters and thereby releasing amino acids from a cytoplasmic pool.

Antipsychotic regulation of hippocampal dopamine receptor messenger RNA expression

Medial temporal lobe structures including the hippocampus and entorhinal cortex have been implicated in the pathophysiology of schizophrenia. Markers of dopaminergic neurotransmission indicate that these regions receive dopaminergic innervation. Accordingly, dysfunction of dopaminergic neurotransmission within the hippocampus and associated cortical areas may be associated with schizophrenia. Little is known, however, about the expression and regulation of dopamine receptors in these regions. We determined the effects of 14 days of clozapine or haloperidol treatment on dopamine receptor messenger RNA (mRNA) expression in medial temporal regions of the rat brain by in situ hybridization. These two drugs had different effects in the hippocampus and entorhinal cortex, particularly a dissociation of their effects on D2 and D3 receptor mRNA expression. There was a parallel down-regulation of D4 mRNA by both drugs. D1 and D5 transcripts were not regulated by either treatment. These results suggest a differential pattern of regulation of D2-like receptor expression by clozapine and haloperidol in some medial temporal lobe structures. These drugs also appear to cause changes in the expression of these transcripts that differ from what has been reported in the striatum, adding to a growing literature suggesting that hippocampal and striatal dopamine receptors are differentially regulated.

An isocratic assay for norepinephrine, dopamine, and 5-hydroxytryptamine using their native fluorescence by high-performance liquid chromatography with fluorescence detection in discrete brain areas of rat

A rapid and simple isocratic chromatographic procedure for simultaneous determination of norepinephrine (NE), dopamine (DA), and 5-hydroxytryptamine (5-HT) using their native fluorescence by high-performance liquid chromatography coupled to fluorescence detection (HPLC-FD) is described. Since the present procedure does not involve sample prepurification, the recovery of monoamines was more than 97% (n = 12) and within a given run, coefficient of variation was less than 3.1% (n = 12). Accordingly, use of an internal standard is not mandatory. In a single chromatographic run, levels of NE, DA, and 5-HT can be determined in less than 30 min. The minimum concentration of monoamines which could be detected by this method was found to be 250 pg for NE and DA and 100 pg for 5-HT. The validity of the method was confirmed by the estimation of levels of monoamines in the hypothalamus and striatum of rat brain following treatment with clorgyline, a monoamine oxidase inhibitor.

Distribution of dopamine immunoreactivity in the rat, cat and monkey spinal cord

In the present study, the distribution of dopamine (DA) was identified light microscopically in all segments of the rat, cat, and monkey spinal cord by using immunocytochemistry with antibodies directed against dopamine. Only fibers and (presumed) terminals were found to be immunoreactive for DA. Strongest DA labeling was present in the sympathetic intermediolateral cell column (IML). Strong DA labeling, consisting of many varicose fibers, was found in all laminae of the dorsal horn, including the central canal area (region X), but with the exception of the substantia gelatinosa, which was only sparsely labeled, especially in rat and monkey. In the motoneuronal cell groups DA labeling was also strong and showed a fine granular appearance. The sexually dimorphic cremaster nucleus and Onuf's nucleus (or its homologue) showed a much stronger labeling than the surrounding somatic motoneurons. In the parasympathetic area at sacral levels, labeling was moderate. The remaining areas, like the intermediate zone (laminae VI-VIII), were only sparsely innervated. The dorsal nucleus (column of Clarke) showed the fewest DA fibers, as did the central cervical nucleus, suggesting that cerebellar projecting cells were avoided by the DA projection. In all species, the descending fibers were located mostly in the dorsolateral funiculus, but laminae I and III also contained many rostrocaudally oriented fibers. It is concluded that DA is widely distributed within the spinal cord, with few differences between species, emphasizing that DA plays an important role as one of the monoamines that influences sensory input as well as autonomic and motor output at the spinal level.

Effects of immobilization stress on hippocampal monoamine release: modification by mivazerol, a new alpha 2-adrenoceptor agonist

Mivazerol is a new and selective alpha 2-adrenoceptor agonist which has demonstrated anti-ischemic effects, both in animals and in patients with myocardial ischemia. In the present study, mivazerol was evaluated for its ability to inhibit the release of catecholamines and serotonin (5-HT) in the hippocampus of freely moving rats, and also was compared to clonidine. In vivo microdialysis in combination with high-performance liquid chromatography (HPLC) was employed. Intravenous administration of mivazerol (8.0 micrograms/kg) had no effect on basal outflow of norepinephrine (NE), dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). In contrast, clonidine (8.5 micrograms/kg, i.v.) attenuated the basal release of DOPAC, which has been proposed to reflect NE biosynthesis, suggesting that clonidine has an inhibitory effect on NE synthesis. In addition, both mivazerol and clonidine decreased the spontaneous release of 5-HT, which provided further evidence that alpha 2-adrenoceptors in the hippocampus modulate 5-HT. Sixty-min immobilization stress significantly increased the release of NE (177 +/- 28%), DA (209 +/- 46%) and DOPAC (337 +/- 72%). Mivazerol (2.5, 8.0 and 25 micrograms/kg, i.v.) completely prevented the immobilization stress-induced enhancement of NE, DA and DOPAC, which was equi-effective to clonidine at a dose of 8.5 micrograms/kg, i.v. These findings demonstrate that mivazerol has a profound modulatory effect on stress-induced neurotransmitter release in the hippocampus, at dose levels reported to protect against myocardial ischemia.

Effect of prenatal malnutrition on release of monoamines from hippocampal slices

The effect of prenatal protein malnutrition on release of monoamine neurotransmitters, their precursors and metabolites, from hippocampal slices was investigated in 15, 30, 90 and 220 days old male rats. The release of dopamine and its metabolites, tryptophan, and 5-hydroxyindoleacetic acid from hippocampal slices of malnourished rats was greater than release from control slices at all ages studied. Malnutrition also significantly increased the release of normetanephrine but only in the 220 day age group. Potassium-induced depolarization increased release of tyrosine, normetanephrine and 5-hydroxyindoleacetic acid less from slices of malnourished than from control rats. The release of norepinephrine, normetanephrine, serotonin and 5-hydroxyindoleacetic acid increased significantly with age while the release of tyrosine, 3,4-dihydroxyphenylacetic acid and homovanillic acid decreased significantly with age. Age was also significantly associated with the effectiveness of potassium-induced depolarization in increasing release of tyrosine, norepinephrine, normetanephrine, tryptophan, serotonin and 5-hydroxyindoleacetic acid.

Dopamine D2-like receptors labeled by [3H]YM-09151-2 in the rat hippocampus: characterization and autoradiographic distribution

Dopamine D2-like receptor labeled by [3H]YM-09151-2 in the rat hippocampus proper was examined by in vitro receptor autoradiography. In the dorsal hippocampus, [3H]YM-09151-2 bindings were high in the whole layers of CA1, the stratum pyramidale of CA4 and the stratum molecular of gyrus dentatus, moderate in the stratum oriens of CA3 and hilus of the gyrus dentatus, and low in remaining CA3 and the subiculum. In the ventral hippocampus, the binding densities were high in the stratum oriens and the stratum radiatum of CA1, the stratum pyramidale of CA4, and the stratum moleculare of gyrus dentatus, moderate in the stratum lacnosum moleculare of CA1 and the hilus of the gyrus dentatus. Saturation analysis using hippocampal sections demonstrated that the Kd value was about five times higher than that using striatal sections. The rank order potency of competition on [3H]YM-09151-2 binding by dopaminergic ligands in the hippocampus was YM-09151-2 > (+)-butaclamol > dopamine > sulpiride > SCH-23390; which shows the appropriate dopamine D2-like receptor profile. The hippocampal [3H]YM-09151-2 binding did not represent serotonergic receptors (5-HT1A and 5-HT2) and sigma receptor, since Ki values of ketanserine, serotonin, 8-OH-DPAT and DTG were much lower than D2-like receptor antagonists. These findings suggest tha [3H]YM-09151-2 binds hippocampal D2-like receptor site with different association kinetics of striatal D2-like receptor site, and demonstrates widespread distribution of D2-like receptor in the hippocampus with distinct region-specific profile.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.

Haloperidol reduces K(+)-evoked Ca(2+)-dependent D-[3H]aspartate release from rat hippocampal slices

Rat hippocampal slices preloaded with D-[3H]aspartate, a non metabolizable analogue of L-glutamate, were superfused with artificial CSF. Depolarization was induced by 53.5 mM K+, in the presence of Ca2+ (1.3 mM) or Mg2+ (5 mM) to determine the Ca2+ dependent release. Haloperidol added in the superfusion medium at 100 microM reduced by about 60% the Ca2+ dependent release of D-[3H]aspartate. This drug at 20 microM or 100 microM inhibited the non-activated glutamate dehydrogenase (GDH) but had no effect on GDH activated by ADP (2 mM) or leucine (5 mM). In addition no effect was observed on phosphate activated glutaminase (PAG) in the presence either of 20 mM or 5 mM phosphate. These results indicate that the effect of haloperidol is exerted on presynaptic mechanisms regulating neurotransmitter release.

Modulation of dopamine and noradrenaline release and of intracellular Ca2+ concentration by presynaptic glutamate receptors in hippocampus

1. We studied the release of [3H]-dopamine and [3H]-noradrenaline (NA) from hippocampal synaptosomes induced by glutamate receptors and the associated Ca2+ influx through Ca2+ channels. The release of tritiated neurotransmitters was studied by use of superfusion system and the intracellular free Ca2+ concentration ([Ca2+]i) was determined by a fluorimetric assay with Indo-1 as a probe for Ca2+. 2. Presynaptic glutamate receptor activation induced Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA from rat hippocampal synaptosomes. Thus, L-glutamate induced the release of both neurotransmitters in a dose-dependent manner (EC50 = 5.62 microM), and the effect of 100 microM L-glutamate was inhibited by 83.8% in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX), but was not affected by 1 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801). 3. Other glutamate receptor agonists also stimulated the Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA as follows: N-methyl-D-aspartate (NMDA), at 200 microM, released 3.65 +/- 0.23% of the total 3H catecholamines, and this effect was inhibited by 81.2% in the presence of 1 microM MK-801; quisqualate (50 microM), S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (100 microM) or kainate (100 microM) released 1.57 +/- 0.26%, 1.93 +/- 0.17% and 2.09 +/- 0.22%, of the total 3H catecholamines, respectively. 4. The ionotropic glutamate receptor agonist, AMPA, induced an increase in the [Ca2+]i which was inhibited by 58.6% in the presence of 10 microM CNQX. In contrast, the increase in [Ca2+]i due to stimulation by glutamate was not sensitive to CNQX or MK-801.5. Nitrendipine, at I JAM, did not inhibit the neurotransmitter release induced by AMPA, but, both 0.5 micro M -conotoxin GVIA (w-CgTx) and 100 nM w-Aga IVA reduced catecholamine release to 49.03 +/- 3.79% and 46.06 +/- 10.51% of the control, respectively. In the presence of both toxins the release was reduced to 12.58 +/- 4.64% of the control.6. The results indicate that activation of presynaptic glutamate receptors of the NMDA and non-NMDA type induces the release of [3H]-dopamine and [H]-NA from rat hippocampal synaptosomes and that the release induced by AMPA involves the activation of N- and P-type Ca2" channels which allow the influx of Ca2" that triggers the 3H catecholamines release.

Loss of dopamine D2 receptors varies along the rostrocaudal axis of the hippocampal complex in Alzheimer's disease

The anatomy of the hippocampus, including the organization of its intrinsic neural circuits and afferents, is organized along a rostrocaudal axis. Dopamine D2 receptors are expressed in specific regions of the hippocampal complex (hippocampal subfields, entorhinal cortex, perirhinal cortex) and show differential expression along this axis. The dentate gyrus and CA3/CA4 subfields show higher numbers of D2 receptors in the rostral than in the caudal levels. In contrast, the subiculum shows the reverse gradient. We report here that Alzheimer's disease (AD) is associated with reduced expression of the dopamine D2 receptor, but the effects differ with respect to the rostrocaudal axis and area within the hippocampal complex. The number of D2 receptors is significantly reduced in the molecular layer of the dentate gyrus, CA3 subfield, and subiculum. For the dentate gyrus and subiculum, there were greater losses at more rostral levels. The CA3/CA4 subfields showed the greatest losses caudally. The entorhinal cortex, which shows only modest expression of D2 receptors in controls, does not exhibit reduced numbers in AD. The external laminae of the rostral perirhinal cortex showed more significant losses than more caudally in this cortical field. The regions showing loss of D2 receptors do not typically contain neuritic plaques, neurofibrillary tangles, or significant neuron loss. Thus other mechanisms must account for the unique gradient of D2 receptor loss in the hippocampus. The regions of reduced expression of dopamine D2 receptors do correlate well with the terminal zone of the dentate association pathway, the afferents from the amygdala and perirhinal cortex, and the sources of those afferents within the amygdala and perirhinal cortex. The specific patterns of reduced D2 receptor expression in AD are likely to contribute significantly to the disrupted information flow into and out of the hippocampus and, thus, of functions subserved by this system.

Dopamine D2 receptor expression in hippocampus and parahippocampal cortex of rat, cat, and human in relation to tyrosine hydroxylase-immunoreactive fibers

A detailed study comparing the distribution of D2 receptors and tyrosine hydroxylase-immunoreactive fibers in the hippocampus and parahippocampal cortices of the rat, cat, and human was conducted. The distribution of [125I]epidepride binding to D2 receptors along the transverse and longitudinal axes of the hippocampus and parahippocampus differed among the species. In rat hippocampus, the number of sites was highest in septal portions of lacunosum-moleculare of CA1 and stratum moleculare of the subiculum. Virtually no binding to D2 receptors existed in the temporal hippocampus. For the cat hippocampus, the highest binding existed in the inner one-third of the molecular layer of the dentate gyrus (DG). There were also significant numbers of D2 receptors in strata radiatum and oriens of the CA subfields, with almost undetectable levels in lacunosum moleculare and subiculum. The number of sites was higher in the septal than temporal hippocampus. In the human hippocampus, highest binding was observed in the molecular layer of DG and the subiculum, with lower levels in strata oriens and lacunosum-moleculare of CA3, and very low binding in CA1. The histochemical demonstration of the pattern of mossy fibers revealed an organization complementary to that of D2 receptors in cat and human. In none of the species was there significant expression of D2 receptors in the entorhinal cortex, except in the caudal extreme of this region in the rat. In that region a trilaminar pattern was exhibited that continued into the perirhinal cortex. A trilaminar pattern of D2 receptor expression was observed in the perirhinal cortex of all species, with the highest values in the external and deep laminae and low expression in the middle laminae. The organization of dopamine fibers was assessed by comparing the distribution of tyrosine hydroxylase-positive and dopamine beta-hydroxylase-immunoreactive fibers in these same regions. It revealed consistent mismatches between the pattern of D2 receptor expression and dopaminergic innervation in all three species. The implications for this mismatch are discussed. It is hypothesized that the distribution of D2 receptors, and not of dopamine fibers, determines what neural systems dopamine influences in the hippocampal complex.

Haloperidol and loxapine but not clozapine increase synaptic responses in the hippocampus

Effects of two typical neuroleptics haloperidol and loxapine, and an atypical antipsychotic clozapine on excitatory synaptic transmission were examined in the CA1 area of rat hippocampal slices. Haloperidol (10 mumol) and loxapine (10 mumol) increased extracelllar evoked field potentials by an average of 57 and 125% of the control level respectively. Clozapine (50 mumol) induced a transient depression (21% of control) of the response followed by a small augmentation (10%). Examination of haloperidol actions using the whole cell clamp technique showed an increase in the amplitude of the excitatory postsynaptic current (e.p.s.c.) in response to the drug without any apparent changes in the resting membrane current. These data suggest that haloperidol and loxapine may act by enhancing excitatory synaptic transmission in some areas of the brain, and that novel antipsychotic clozapine differs in its mechanism of action.

Studies on the role of B-50 (GAP-43) in the mechanism of Ca(2+)-induced noradrenaline release: lack of involvement of protein kinase C after the Ca2+ trigger

The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca(2+)-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (> 60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10(-8) to 10(-5) M Ca2+. The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca(2+)-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca(2+)-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca(2+)-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10(-5) M), failed to inhibit Ca(2+)-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca(2+)-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca(2+)- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA.

Characterization of the release of Met-enkephalin from isolated nerve terminals: release kinetics and cation-dependence

The release of the neuropeptide Met-enkephalin (Met-ENK) from isolated nerve terminals (synaptosomes) of the rat forebrain was characterized with respect to the subcellular distribution, the release upon addition of various stimulatory agents, the release kinetics, the cation-dependence of release and the relationship between Met-ENK release and elevations of the intraterminal free Ca(2+)-concentration ([Ca]i). A highly specific radioimmunoassay was developed for determination of Met-ENK (H-Tyr-Gly-Gly-Phe-Met-OH). Truncated and elongated forms of Met-ENK, Leu-enkephalin, beta-endorphin and dynorphin displayed negligible cross-reactivity. Met-ENK-like immunoreactivity (Met-ENK-LI) is enriched in the purified synaptosomal fraction of rat forebrain homogenates and is released in a strictly Ca(2+)-dependent manner upon chemical depolarization or stimulation with the Ca2+ ionophore, ionomycin. A correlation exists between the release of Met-ENK-LI and the elevations of [Ca]i. Barium ions are able to replace Ca2+ in triggering Met-ENK-LI release. The release of Met-ENK-LI is initiated within 20 s after depolarization and is terminated after 3-5 min, although depolarization and [Ca]i elevation are maintained. At this time, > 90% of the initial Met-ENK-LI is still present inside the synaptosomes. Repolarization and renewed stimulation again evokes Ca(2+)-dependent release of this retained Met-ENK-LI. It is concluded that Met-ENK release from isolated nerve terminals is exocytotic, and that exocytosis is terminated by a regulatory mechanism in synaptosomes after 3-5 min of depolarization, a process which can be reversed by repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)