Localization and quantitation of dynorphin B in the rat hippocampus

Influence of intrastriatal infusion of dynorphin fragments on overflow of acetylcholine and dopamine in the rat brain

Dynorphin (DYN) fragments are the members of the endogenous opioid system and postulated ligands for the opioid receptors. Infusion of DYN(1-17) fragment into the rat dorsal striatum caused a significant increase in acetylcholine and decrease in dopamine overflow. Contrary to this, infusions of DYN(2-17) fragment into the rat dorsal striatum caused a significant increase in dopamine and decrease in acetylcholine overflow. Intrastriatal infusions of different doses of the acetylcholinesterase blocker, neostigmine, augmented acetylcholine and inhibited dopamine overflow in a dose-dependent manner. The opposing responses of the DYN fragments suggest that the N-terminal residue plays a key role in presynaptic neuromodulation.

The regulation of hippocampal dynorphin by neural/neuroendocrine pathways: models for effects of aging on an opioid peptide system

Previous research has demonstrated increased messenger RNA expression and peptide content in an opioid system localized to hippocampal dentate granule cells in aged rats. This altered regulation of dynorphin was correlated with the emergence of an age-related impairment in spatial learning. Considerable evidence exists for additional effects of aging on systems that provide input to the dynorphin-containing dentate granule cells. Such changes have been well documented for loss of perforant path innervation from entorhinal cortex, deterioration in septohippocampal cholinergic neurons, and high amounts of glucocorticoids that have, among their targets, receptors located in the dentate gyrus. Similar to the effects of aging on hippocampal dynorphin, age-related changes in each of these systems correlate with the severity of spatial learning impairment in aged rats. This raises the possibility that dysregulation of dynorphin in the aged brain is a reactive response to antecedant change(s) in this circuitry, a hypothesis that was examined by separately manipulating in young rats the three neural/neuroendocrine systems identified above. Of the three models examined only removal of the perforant path reproduced the effect of aging on dynorphin in the hippocampal formation. An immunotoxin was used in Experiment 1 to selectively remove septo-hippocampal cholinergic neurons in young rats. No alteration in hippocampal opioid peptides was produced by this treatment. Experiment 2 examined effects of exposure to excess corticosterone. Adrenalectomized rats exhibited a significant decrease in hippocampal dynorphin-A (1-8) content, which was reversed by corticosterone replacement at a concentration approximating normal basal levels. Dynorphin-A (1-8) content, however, was not reliably increased by exposure to excess corticosterone. In contrast, perforant path removal was found to reproduce the effect of aging on dynorphin content; either aspiration of the entorhinal cortex or knife-cut transections of the perforant path reliably increased hippocampal dynorphin content. These results support the conclusion that age-related deterioration in the septohippocampal cholinergic system and evaluated exposure to corticosterone are not sufficient to induce an elevation in hippocampal dynorphin content. Only removal of the perforant path innervation was found to reproduce the elevation in hippocampal dynorphin content observed in aged rats with hippocampal-dependent learning impairment.

Subcellular fractionation on Percoll gradient of mossy fiber synaptosomes: morphological and biochemical characterization in control and degranulated rat hippocampus

A method for preparation of hippocampal mossy fiber synaptosomes directly from the postnuclear pellet is presented. This method represents an adaptation of that previously described for the isolation of synaptosomes by centrifugation through Percoll gradients directly from the supernatant fraction. We have characterized by electron microscopy two fractions, PII and PIII, enriched in mossy fiber synaptosomes; fraction PIII had 75% mossy fiber synaptosomes with well-preserved morphology (large size 3 microns, complex morphology, high synaptic vesicle density, multisynapses), whereas fraction PII contained 12%. These fractions were enriched in lactate dehydrogenase activity indicating that the integrity of synaptosomes was preserved. Compared with the other synaptosomal fractions, these fractions showed greater levels of dynorphin A (1-8) immunoreactivity and endogenous zinc, which are particularly concentrated in hippocampal mossy fiber terminals. Furthermore, we prepared synaptosomes from adult hippocampus after neonatal irradiation, which destroys the majority of granule cells and associated mossy fibers. The levels of dynorphin and zinc decreased by 88 and 70% in fraction PII and by 95 and 90%, respectively, in PIII. These results suggest that the rapid Percoll procedure is convenient for the purification of mossy fiber synaptosomes.

Comparative effects of intrahippocampal injection of dynorphin A(1-8), dynorphin A(1-13), dynorphin A(1-17), U-50,488H, and dynorphin B on blood pressure and heart rate in spontaneously hypertensive and normotensive Wistar-Kyoto rats

We previously demonstrated centrally mediated hypotensive and bradycardic effects of dynorphin A(1-8) (DA1-8) on microinjection into various areas of the hippocampal formation (HF) of both anesthetized and conscious male normotensive and spontaneously hypertensive rats (SHR). The purpose of the present study was to determine whether other dynorphin fragments also had this activity. We microinjected DA1-8, dynorphin A(1-13), dynorphin A(1-17), dynorphin B (DB), and the nonpeptide kappa-opioid agonist U-50,488H into HF areas previously found to react to DA1-8, at doses ranging from 0.05 to 50 nmol. The subjects were male SHR and normotensive Wistar-Kyoto (WKY) rats in which arterial pressure and heart rate were monitored. Dose-related centrally mediated hypotension and bradycardia were found in both strains with all agents used, except for DB, which had no effects. Similarly injected drug vehicle was also without effect. In general, the responses were greater in SHR than in WKY rats. Preinjection of the active HF areas with 2 nmol of nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, which itself had no blood pressure or heart rate effects, abolished both the decrease in blood pressure and heart rate of all dynorphins and U-50,488H. The results demonstrated the equivalent abilities of all the dynorphin fragments studied, except DB, to cause HF-mediated hypotension and bradycardia. The results with U-50,488H and nor-BNI strongly implicate kappa-opiate receptor activation of the HF in these effects.

Evidence for the corelease of dynorphin and glutamate from rat hippocampal mossy fiber terminals

Hippocampal mossy fiber (MF) nerve endings may be isolated in a subcellular fraction (P3) that releases both prodynorphin-derived peptides and glutamate (Glu) in a calcium-dependent manner when depolarized. However, this isolation procedure does not yield a pure preparation of MF synaptosomes. The present study evaluates the proportion of dynorphin (Dyn) and Glu that is released from synaptosomes in the P3 fraction that are of MF origin. We have addressed this issue by determining the degree to which a selective lesion of the dentate granule cell/MF system in vivo concomitantly reduces the exocytosis of Dyn and Glu from the P3 subcellular fraction. Unilateral injections of colchicine into the dentate gyrus resulted in a substantial and selective degeneration of the granule cell/MF pathway in the rat hippocampal formation. The overall integrated density of the Timm-stained band, which corresponds to the position of the MF terminal field, was estimated to be reduced by 75%. After this extensive loss of MF boutons, the K(+)-evoked release of Dyn and Glu from the P3 fraction was reduced by 95 and 51%, respectively. The loss of Timm staining and evoked Dyn release indicate that colchicine effectively eliminated MF synaptosomes from the P3 fraction. Those subcellular entities that were not destroyed by colchicine comprised approximately 50% of the protein and evoked Glu release measured by using the P3 fraction. In addition, the present results demonstrate that the inhibitory potency of the kappa opioid agonist U-50,488H was not altered by the elimination of MF boutons from this synaptosomal preparation. This finding indicates that U-50, 488H is capable of suppressing Glu exocytosis from both MF and non-MF synaptosomes. These results are consistent with the hypothesis that Dyn peptides and Glu are co-released from hippocampal MF terminals.

Three-dimensional analysis of the structure and composition of CA3 branched dendritic spines and their synaptic relationships with mossy fiber boutons in the rat hippocampus

This paper is the third in a series to quantify differences in the composition of subcellular organelles and three-dimensional structure of dendritic spines that could contribute to their specific biological properties. Proximal apical dendritic spines of the CA3 pyramidal cells receiving synaptic input from mossy fiber (MF) boutons in the adult rat hippocampus were evaluated in three sets of serial electron micrographs. These CA3 spines are unusual in that they have from 1 to 16 branches emerging from a single dendritic origin. The branched spines usually contain subcellular organelles that are rarely found in adult spines of other brain regions including ribosomes, multivesicular bodies (MVB), mitochondria, and microtubules. MVBs occur most often in the spine heads that also contain smooth endoplasmic reticulum, and ribosomes occur most often in spines that have spinules, which are small nonsynaptic protuberances emerging from the spine head. Most of the branched spines are surrounded by a single MF bouton, which establishes synapses with multiple spine heads. The postsynaptic densities (PSDs) occupy about 10-15% of the spine head membrane, a value that is consistent with spines from other brain regions, with spines of different geometries, and with immature spines. Individual MF boutons usually synapse with several different branched spines, all of which originate from the same parent dendrite. Larger branched spines and MF boutons are more likely to synapse with multiple MF boutons and spines, respectively, than smaller spines and boutons. Complete three-dimensional reconstructions of representative spines with 1, 6, or 12 heads were measured to obtain the volumes, total surface areas, and PSD surface areas. Overall, these dimensions were larger for the complete branched spines than for unbranched or branched spines in other brain regions. However, individual branches were of comparable size to the large mushroom spines in hippocampal area CA1 and in the visual cortex, though the CA3 branches were more irregular in shape. The diameters of each spine branch were measured along the cytoplasmic path from the PSD to the origin with the dendrite, and the lengths of branch segments over which the diameters remained approximately uniform were computed for subsequent use in biophysical models. No constrictions in the segments of the branched spines were thin enough to reduce charge transfer along their lengths.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of dynorphin-containing neurons on dissociated dentate gyrus cell cultures

In the dentate gyrus, the synthesis of the opioid peptide, dynorphin, is modulated by a variety of stimuli. In order to elucidate the cellular and molecular mechanisms regulating the synthesis of dynorphin in the hippocampus, we have established a routine primary cell culture of dentate granule neurons and identified granule-like neurons by a characteristic marker, dynorphin, in these cultures. Cultures were prepared from 7-day-old rat pups and maintained in medium with 2% fetal bovine serum. These cultures contained approximately 20% neurons and survived for over 4 weeks. After 2 weeks in culture, neurons expressing dynorphin-A and its messenger RNA were detected using immunocytochemistry and in situ hybridization, respectively. In dentate cultures, enkephalin-, cholecystokinin-, neuropeptide Y- and substance P-positive cells were observed in addition to dynorphin-positive cells with immunocytochemistry. The results suggest that dentate gyrus cell cultures provide a valid in vitro model for studying molecular mechanisms regulating prodynorphin gene expression.

Modulation of Glutamate Release From Hippocampal Mossy Fiber Nerve Endings By Arachidonic Acid And Eicosanoids

Arachidonic acid has been implicated in normal synaptic transmission processes, including those related to the development of hippocampal long-term synaptic potentiation. Hippocampal mossy fiber (MF) synaptosomes were used to investigate the role of arachidonate in the evoked accumulation of presynaptic Ca2+ and the release of endogenous glutamate, since these nerve terminals express long-term potentiation and selectively release glutamate as the excitatory transmitter. It was demonstrated that membrane depolarization evoked the accumulation of Ca2+, the release of glutamate, and the production of unesterified arachidonic acid. These events may be functionally related, since exogenous arachidonate and phospholipase A2 activation mimicked the effects of depolarization on Ca2+ availability and glutamate release, while secretion processes were attenuated in the presence of phospholipase A2 inhibitors. In addition, pretreatment of the nerve terminals with arachidonate or melittin allowed for the facilitated release of glutamate in response to a subsequent depolarizing stimulus. Inhibition of cyclooxygenase or lipoxygenase activities also potentiated presynaptic responses to membrane depolarization. In contrast, 12-lipoxygenase products attenuated the depolarization-evoked accumulation of intraterminal free Ca2+ and glutamate release. It is suggested that arachidonic acid acts as a positive modulator of mossy fiber secretion processes, including those involved in the increased glutamate release required for the induction of long-term potentiation, while 12-lipoxygenase metabolites provide negative feedback signals designed to limit neurotransmitter secretion.

A presynaptic role for protein kinase C in hippocampal mossy fiber synaptic transmission

It has been suggested that the maintenance of long-term potentiation (LTP) in the hippocampal mossy fiber (MF) synapse involves a presynaptic mechanism that does not require the activation of protein kinase C (PKC), since this enzyme appears to be absent in the MF presynaptic terminals. In the present study the authors evaluated this proposal by directly comparing the metabolic properties of hippocampal MF synaptosomes and a conventional P2B synaptosomal preparation prepared from the same hippocampal tissue. Protein kinase C-dependent histone phosphotranferase activity was found to be comparable in MF and P2B synaptosomes. Western blot analysis was performed using antisera prepared against four of the PKC isoforms, and the results demonstrate that the alpha, beta, and gamma PKC isoforms are present in relatively equivalent amounts in these two subcellular fractions. However, the cytosolic fraction derived from the hippocampal MF synaptosomes appeared to contain a greater amount of the PKC-epsilon isoform when compared to the P2B synaptosomal preparation. Four distinct endogenous substrates present in the MF synaptosomes are shown to be phosphorylated in response to PKC activation. A functional role for PKC in the hippocampal MF nerve endings seems to be indicated by the finding that 4 beta-phorbol 12,13-dibutyrate (PDBu) and 4 beta-phorbol 12,13-diacetate produce a dose-dependent potentiation of the K(+)-evoked release of endogenous glutamate and dynorphin B, while the inactive 4-alpha-phorbol was without effect. The PDBu-induced enhancement of transmitter release was blocked by the PKC inhibitor, staurosporine. In addition, PDBu significantly facilitated the rise in cytosolic free calcium that immediately followed depolarization of the MF synaptosomal membrane. It is concluded that hippocampal MF presynaptic terminals possess a variety of PKC isoforms and that their activation may have an important facilitory influence on MF synaptic transmission and plasticity.

U-50,488H inhibits dynorphin and glutamate release from guinea pig hippocampal mossy fiber terminals

The selective kappa opioid agonist U-50,488H was tested for its ability to modulate the potassium-induced rise of cytosolic Ca2+ in, and transmitter release from, guinea pig hippocampal mossy fiber synaptosomes. U-50,488H dose dependently inhibited the potassium-induced rise in synaptosomal free Ca2+ levels. This inhibition was attenuated by the selective kappa opioid antagonist nor-binaltorphimine, but was insensitive to naloxone and the sigma opioid antagonist ICI 174,864. U-50,488H also dose dependently depressed the potassium-induced release of L-glutamate and dynorphin B-like immunoreactivity from mossy fiber synaptosomes in a nor-binaltorphimine-sensitive manner. This is the first report to confirm the presence of a presynaptic kappa opioid receptor in the hippocampal mossy fiber-CA3 synapse and the nature of its influence on neurotransmitter release. The present results may be used to suggest that endogenous dynorphin peptides interact with this kappa opioid receptor to autoregulate the excitatory mossy fiber synaptic input.

Relationship between regulation of morphine-induced EEG effects and changes in naloxone sensitivity

The present data indicate that pretreatment with i.c.v. injection of dynorphin, morphine and dynorphin/morphine resulted in quantitative and qualitative changes in EEG power spectra in rats given i.c.v. morphine 24 h later. Correlated changes in sensitivity to antagonism of these EEG effects by naloxone were also found. Rats were implanted with cortical EEG electrodes and i.c.v. and i.v. cannulas. I.c.v. injections of morphine (20 micrograms/rat) produced high-voltage, slow-wave EEG bursts (1-10 Hz) associated with behavioral stupor which lasted about 2 h. Injections of i.c.v. morphine in rats pretreated with i.c.v. dynorphin (20 micrograms/rat), morphine (20 micrograms/rat) or dynorphin/morphine 24 h earlier, produced quantitative increases in absolute EEG spectral power. Injections of i.c.v. morphine in rats pretreated with i.c.v. dynorphin/morphine 24 h earlier, also produced qualitatively different EEG power spectra with a predominant peak in the 4-6 Hz band, similar to the EEG power spectra seen after acute administration of kappa opioids. After 20 min of morphine-induced high voltage EEG bursts, i.v. naloxone was given in sequential doses (0.0025, 0.0125, 0.025, 0.050 mg/kg) every 3 min until the EEG bursts were suppressed for 20 min. Relatively low doses of naloxone suppressed morphine-induced EEG bursts in rats that received i.c.v. H2O/H2O pretreatment. Slightly higher, but significant, doses of naloxone suppressed morphine-induced EEG bursts in rats that received i.c.v. H2O/morphine or dynorphin/H2O pretreatment. Moreover, a 10-fold increase in naloxone dose was needed to suppress EEG bursts in rats that received dynorphin/morphine pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Microinjection of dynorphin into the hippocampus impairs spatial learning in rats

The effect of hippocampal dynorphin administration on learning and memory was examined in spatial and nonspatial tasks. Bilateral infusion of dynorphin A(1-8) (DYN; 10 or 20 micrograms in one microliters) into the dorsal hippocampus resulted in a dose-related impairment of spatial working memory in a radial maze win-stay task. Subsequent experiments found that acquisition of a reference memory task in the water maze was impaired by DYN injections (20 micrograms/microliters) in the dorsal hippocampus, but not in the ventral hippocampus, and that this impairment could be blocked by naloxone. In a nonspatial task, posttraining DYN injections in the dorsal hippocampus had no effect on retention of step-through passive avoidance. These results suggest that dynorphin specifically interferes with spatial learning and memory, and that this effect is mediated by opioid receptors in the dorsal hippocampus.

ATP release, adenosine formation, and modulation of dynorphin and glutamic acid release by adenosine analogues in rat hippocampal mossy fiber synaptosomes

Using a hippocampal subcellular fraction enriched in mossy fiber synaptosomes, evidence was obtained indicating that adenosine derived from a presynaptic pool of ATP may modulate the release of prodynorphin-derived peptides. and glutamic acid from mossy fiber terminals. Synaptosomal ATP was released in a Ca2+-dependent manner by K+-induced depolarization. The rapid hydrolysis of extracellular [14C]ATP in the presence of intact mossy fiber synaptosomes resulted in the production of [14C]adenosine. Micromolar concentrations of a stable adenosine analogue, 2-chloroadenosine, inhibited the K+-stimulated release of both dynorphin B and dynorphin A(1-8). 2-Chloroadenosine failed to suppress the evoked release of glutamic acid, measured in these same superfusates, unless the mossy fiber synaptosomes were pretreated with D-aspartic acid to deplete the cytosolic, Ca2+-independent, pool of this acidic amino acid. In synaptosomes pretreated in this manner, release of the remaining Ca2+-dependent pool of glutamic acid was significantly inhibited by NiCl2, 2-chloroadenosine, 5'-N-ethylcarboxamidoadenosine, cyclohexyladenosine, and R(-)-N6(2-phenylisopropyl)adenosine, but not by ATP. 2-Chloroadenosine-induced inhibition was reversed when the external CaCl2 concentration was raised from 1.8 mM to 6 mM. 8-Phenyltheophylline, an adenosine receptor antagonist, effectively blocked the inhibitory effects of 2-chloroadenosine on mossy fiber synaptosomes and significantly enhanced the K+-evoked release of both glutamic acid and dynorphin A(1-8) when added alone to the superfusion medium. These results support the proposition that depolarized hippocampal mossy fiber synaptosomes release endogenous ATP and are capable of forming adenosine from extracellular ATP, and that endogenous adenosine may act at a presynaptic site to inhibit the further release of glutamic acid and the prodynorphin-derived peptides.

Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Neurobehavioral genetics endeavors to trace the pathways from genetic and environmental determinants to neuroanatomical and neurophysiological systems and, thence, to behavior. Exploiting genetic variation as a tool, the behavioral sequelae of manipulating these neuronal systems by drugs and antisera are analyzed. Apart from research in rats, this paper deals mainly with the genetically-influenced regulation in mice of exploratory behaviors that are adaptive in novel surroundings and are hippocampally-mediated. Special attention is paid to neuropeptidergic, GABAergic, and cholinergic synaptic functions in the mouse hippocampus. The behaviorally different inbred mouse strains C57BL/6 and DBA/2 show opposite reactions (reductions and increases, respectively, in exploration rates) to peripheral and intrahippocampal injections with agents that interfere with peptidergic, cholinergic, and GABAergic neurotransmission. These findings can be explained by an interdependent over-release of opioids, arrested GABA release, and excess acetylcholine in the hippocampal neuronal network of DBA/2 mice, as compared to C57BL/6 mice where these systems are functionally well balanced. Very similar results have been obtained with the lines SRH and SRL, derived from C57BL/6 and DBA/2, and genetically selected for rearing behavior. Most probably, the opioids act to disinhibit exploratory responses. An additional genetic approach is mentioned, in which four inbred mouse strains and one derived heterogeneous stock are used for estimating genetic correlations between structural properties of the hippocampal mossy fibers and levels of hippocampal dynorphin B, on the one hand, and frequencies of exploratory responses to environmental novelty, on the other.

A genotype-dependent hippocampal dynorphinergic mechanism controls mouse exploration

Following microinjections with two dilutions of anti-dynorphin B antiserum into the hippocampal CA3 region, adult male mice from the inbred strains DBA/2 and C57BL/6 were individually tested for various exploratory behaviors in a novel environment and compared to preimmune serum control animals. Treatment augmented vertically-oriented exploratory acts in strain DBA/2 and reduced the scores in strain C57BL/6 so that strain differences originally present between the controls were reversed or eliminated after antiserum. These opposite effects indicate that a hippocampal dynorphinergic mechanism is involved in the regulation of novelty-induced behavior in mice and that its modulatory function depends on the genotype. It is concluded that DBA/2 animals exposed to novelty, as compared to C57BL/6, are characterized by an over-release of hippocampal dynorphin B which is neutralized in part by small amounts of antibody.