Synaptic plasticity: on the trail of the retrograde messenger

Arachidonic acid inhibits capacitative Ca2+ entry and activates non-capacitative Ca2+ entry in cultured astrocytes

Arachidonic acid (AA) plays important physiological or pathophysiological roles. Here, we show in cultured rat astrocytes that: (i) endothelin-1 or thapsigargin (Tg) induces store-depleted activated Ca(2+) entry (CCE), which is inhibited by 2-aminoethoxydiphenyl borane (2-APB) or La(3+); (ii) AA (10 microM) and other unsaturated fatty acids (8,11,14-eicosatrienoic acid and gamma-linoleic acid) have an initial inhibitory effect on the CCE, due to AA- or fatty acid-induced internal acid load; (iii) after full activation of CCE, AA induces a further Ca(2+) influx, which is not inhibited by 2-APB or La(3+), indicating that AA activates a second Ca(2+) entry pathway, which coexists with CCE; and (iv) Tg or AA activates two independent and co-existing non-selective cation channels and the Tg-induced currents are initially inhibited by addition of AA or weak acids. A possible pathophysiological effect of the AA-induced [Ca](i) overload is to cause delayed cell death in astrocytes.

In vivo31P NMR spectroscopy shows an increase in glycerophosphorylcholine concentration without alterations in mitochondrial function in the prefrontal cortex of medicated schizophrenic patients at rest

The (31)P NMR localised method was used to study the metabolism of phospholipid and high energy phosphate in the prefrontal cortex. The spectra were taken from patients with schizophrenia (11 males) receiving neuroleptic medication, and were compared to normal controls (15 males). Their spectral intensities were analysed using a non-linear least-squares method with a prior knowledge of the fixed chemical shifts and linewidths, leading to further resolution into resonances of glycerophosphorylethanolamine (GPE), glycerophosphorylcholine (GPC), phosphorylethanolamine (PE) and phosphorylcholine (PC). The metabolite concentrations were calculated referring to the spectral intensities of phosphate phantoms with known concentrations. T1 values of phantom and cerebrum were estimated from a series of localised inversion recovery spectra to correct for the signal saturation effects. The schizophrenic patients showed an increased concentration of GPC but not GPE, PE or PC. Furthermore, no difference was observed regarding the concentration of high-energy phosphates such as phosphocreatine, inorganic phosphate and ATP. The patients did not show any differences in mitochondrial function such as phosphorylation potential and the ratio of the rate of ATP synthesis. Thus, an increase in GPC concentration in the prefrontal cortex could be characteristic of the pathophysiology of schizophrenia with mild negative symptoms.

Similarity in perception: a window to brain organization

This paper presents a neural model of similarity perception in identification tasks. It is based on self-organizing maps and population coding and is examined through five different identification experiments. Simulating an identification task, the neural model generates a confusion matrix that can be compared directly with that of human subjects. The model achieves a fairly accurate match with the pertaining experimental data both during training and thereafter. To achieve this fit, we find that the entire activity in the network should decline while learning the identification task, and that the population encoding of the specific stimuli should become sparse as the network organizes. Our results, thus, suggest that a self-organizing neural model employing population coding can account for identification processing while suggesting computational constraints on the underlying cortical networks.

NMDA glutamate receptor role in the development of context-dependent and independent sensitization of the induction of stereotypy by amphetamine or apomorphine

We have been studying sensitization of psychostimulant-induced stereotyped behavior in mice using both a context-dependent and a context-independent paradigm. In the present study, we tested whether N-methyl-D-aspartate (NMDA) receptor antagonists prevent development of sensitization in either of these models. Male CF-1 mice were pretreated with 20 mg/kg (+)3-(2-carboxypiperazine-4yl)-propyl-1-phosphonic acid (CPP), 0.1 mg/kg (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohopten-5, 10-imine maleate (MK-801, dizocilpine maleate), or 25 mg/kg 7-nitroindazole 30 min before a single dose (context-dependent paradigm) or each of three daily doses (context-independent paradigm) of 14 mg/kg amphetamine or 40 mg/kg apomorphine. Two days following this pretreatment, mice were injected with 7 mg/kg amphetamine or 3 mg/kg apomorphine. The stereotyped behavioral response was enhanced in mice pretreated with amphetamine or apomorphine alone, indicating that sensitization had developed. Both CPP and MK-801 prevented the development of sensitization in the context-dependent model but not in the context-independent paradigm. 7-Nitroindazole did not attenuate development of sensitization in either model. The results suggest that activation of glutamatergic receptors is important in some sensitization paradigms but not others, indicating that glutamate can be important but is not always required for the development of sensitization.

Dendritic GABA release depresses excitatory transmission between layer 2/3 pyramidal and bitufted neurons in rat neocortex

GABAergic, somatostatin-containing bitufted interneurons in layer 2/3 of rat neocortex are excited via glutamatergic excitatory postsynaptic potentials (EPSPs) by pyramidal neurons located in the same cortical layer. Pair recordings showed that short bursts of backpropagating dendritic action potentials (APs) reduced the amplitude of unitary EPSPs. EPSP depression was dependent on a rise in dendritic [Ca2+]. The effect was blocked by the GABA(B) receptor (GABA(B)-R) antagonist CGP55845A and was mimicked by the GABA(B)-R agonist baclofen. As presynaptic GABA(B)-Rs were activated neither by somatostatin nor by GABA released from axon collaterals of the bitufted cell, we conclude that GABA(B)-Rs were activated by a retrograde messenger, most likely GABA, released from the dendrite. Because synaptic depression was prevented by loading bitufted neurons with GDP-beta-S, it is likely to be caused by exocytotic GABA release from dendrites.

Effects of nitric oxide donors on basal and K+-evoked release of [3H]noradrenaline from rat cerebral cortex synaptosomes

We investigated the effects of nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside on basal and K+-evoked release of [3H]noradrenaline from superfused synaptosomes from the rat cerebral cortex. Both substances produced concentration-dependent increases in the release of the labeled transmitter under basal and depolarized conditions. The effects of the donors on basal release were Ca2+-independent but were not inhibited by the carrier-uptake blocker, desipramine; the effects were abolished by hemoglobin (an NO scavenger). Thirty-five minutes after stimulation with sodium nitroprusside, the synaptosomes were still responsive to KCl stimulation, indicating that the donor's effects were not caused by damage to the synaptosome membrane. The cGMP analogue, 8-bromo-cGMP, had no effect on basal release, and the enhanced release produced by sodium nitroprusside was not inhibited by the specific inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, indicating that NO's effects on basal release of the neurotransmitter are guanylate cyclase-independent. Both of the NO donors had more marked effects on release of [3H]noradrenaline during K+-stimulated depolarization. The NO-mediated increase in this case was partially antagonized by 10 microM LH-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, and 8-Br-cGMP was also capable of producing concentration-dependent increases in the K+-stimulated release of the transmitter. These findings indicate that the effects of the NO donors on [3H]noradrenaline release during depolarization are partially mediated by the activation of guanylate cyclase.

Memory, sleep and the evolution of mechanisms of synaptic efficacy maintenance

The origin of both sleep and memory appears to be closely associated with the evolution of mechanisms of enhancement and maintenance of synaptic efficacy. The development of activity-dependent synaptic plasticity apparently was the first evolutionary adaptation of nervous systems beyond a capacity to respond to environmental stimuli by mere reflexive actions. After the origin of activity-dependent synaptic plasticity, whereby single activations of synapses led to short-term efficacy enhancement, lengthy maintenance of enhancements probably was achieved by repetitive activations ("dynamic stabilization"). One source of selective pressure for the evolutionary origin of neurons and neural circuits with oscillatory firing capacities may have been a need for repetitive spontaneous activations to maintain synaptic efficacy in circuits that were in infrequent use. This process is referred to as "non-utilitarian" dynamic stabilization. Dynamic stabilization of synapses in "simple" invertebrates occurs primarily through frequent use. In complex, locomoting forms, it probably occurs through both frequent use and non-utilitarian activations during restful waking. With the evolution of increasing repertories and complexities of behavioral and sensory capabilities--with vision usually being the vastly pre-eminent sense brain complexity increased markedly. Accompanying the greater complexity, needs for storage and maintenance of hereditary and experiential information (memories) increased greatly. It is suggested that these increases led to conflicts between sensory input processing during restful waking and concomitant non-utilitarian dynamic stabilization of infrequently used memory circuits. The selective pressure for the origin of primitive sleep may have been a resulting need to achieve greater depression of central processing of sensory inputs largely complex visual information than occurs during restful waking. The electrical activities of the brain during sleep (aside from those that subserve autonomic activities) may function largely to maintain sleep and to dynamically stabilize infrequently used circuitry encoding memories. Sleep may not have been the only evolutionary adaptation to conflicts between dynamic stabilization and sensory input processing. In some ectothermic vertebrates, sleep may have been postponed or rendered unnecessary by a more readily effected means of resolution of the conflicts, namely, extensive retinal processing of visual information during restful waking. By this means, processing of visual information in central regions of the brain may have been maintained at a sufficiently low level to allow adequate concomitant dynamic stabilization. As endothermy evolved, the skeletal muscle hypotonia of primitive sleep may have become insufficient to prevent sleep-disrupting skeletal muscle contractions during non-utilitarian dynamic stabilization of motor circuitry at the accompanying higher body temperatures and metabolic rates. Selection against such disruption during dynamic stabilization of motor circuitry may have led to the inhibition of skeletal muscle tone during a portion of primitive sleep, the portion designated as rapid-eye-movement sleep. Many marine mammals that are active almost continuously engage only in unihemispheric non-rapid-eye-movement sleep. They apparently do not require rapid-eye-movement sleep and accompanying non-utilitarian dynamic stabilization of motor circuitry, because this circuitry is in virtually continuous use. Studies of hibernation by arctic ground squirrels suggest that each hour of sleep may stabilize brain synapses for as long as 4 h. Phasic irregularities in heart and respiratory rates during rapid-eye-movement sleep may be a consequence of superposition of dynamic stabilization of motor circuitry on the rhythmic autonomic control mechanisms. Some information encoded in circuitry being dynamically stabilized during sleep achieves unconscious awareness in authentic and var

Inhibition of nitric oxide facilitates LH release from rat pituitaries

We examined the effects of nitric oxide (NO) modulators on rat pituitary LH content in vivo and studied their response to LHRH-stimulated LH secretion in vitro in ovariectomized adult female Sprague Dawley rats. Alzet mini pumps (flow rate 10 microl/h) delivering either normal saline (Group I, 1.2 mg nitroglycerin, a donor of NO (Group II) or 50 mg of nitro-L-Arginine methyl ester, a NO synthase (NOS) inhibitor (Group III), were subcutaneously implanted into experimental animals. Following 36 h infusion, pituitaries were removed and either frozen for LH quantitation, or fragmented and challenged in the superfusion system with 10 min pulses of LHRH (1 ng/ml) at 90 min intervals for 10 hours. LH was assayed by radio-immunoassay (RIA) in the homogenates of pituitaries and in aliquots of the superfusate collected every 10 mins. Significantly lower pituitary LH levels were noted in Group III (150.3 +/- 18.6 ng) in comparison to Groups I (215.6 +/- 5.5 ng; p<0.04) or II (221.2 +/- 14.9 ng; p<0.01), suggesting that low levels of NO stimulate LH secretion in vivo. The pituitary LH contents were not significantly different in Groups I and II. In vitro studies reveal that exogenous LHRH stimulated response, measured as average pulse response (90 minute period after LHRH), and total LH released during the 10 hour perfusion, was 290 +/- 23.6 ng and 1646.7 +/- 270.8 ng, respectively, in Group III; 57.9 +/- 3.1, and 344.7 +/- 24.3 ng in Group I, and 105.3 +/- 6.3, and 633.7 +/- 77.1 mg in Group II. Thus, our in vitro studies demonstrate significantly enhanced (p<0.05) LHRH- stimulated LH secretion in Group III in comparison to Groups I and II, while Group II shows higher responsiveness than Group I (p<0.05). The results of the current studies provide evidence that NOS inhibition facilitates pituitary LH secretion. The differential responses to LHRH-stimulated LH secretion in vitro in the 3 groups suggest a possible role of NO in modulating pituitary LHRH receptor concentrations. However, this will have to be tested by further studies.

Prenatal development of nicotinamide adenine dinucleotide phosphate-diaphorase activity in the human hippocampal formation

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry was used to study the development of the neurons metabolizing nitric oxide in the prenatal human hippocampal formation. Strongly reactive non-pyramidal neurons appeared in small numbers in the subplate at 15 weeks, and rapidly increased in this layer, as well as the cortical plate-derived layers between 17 and 24 weeks. The marginal zone also had a few NADPH-d cells at 15 weeks. The pattern of these darkly reactive cells stabilized by 28 weeks, with the somata distributed mostly at the border of the cortex and white matter in the entorhinal cortex and subiculum, or the alveus in Ammon's horn. Moderately stained non-pyramidal neurons appeared in the dentate gyrus by 17 weeks, and increased in this region and Ammon's horn up to 28 weeks. Small, lightly reactive non-pyramidal neurons were first seen by 32 weeks and increased in number by term. They were mainly distributed in layers II/III of the entorhinal cortex and stratum pyramidale of the subiculum and Ammon's horn. NADPH-d positive fibers in the marginal zone were mostly thin and developed between 20 and 28 weeks. In other cortical layers, thick processes from the darkly stained NADPH-d neurons appeared first, then fine fibers appeared more numerous, especially after 28 weeks. NADPH-d processes that arose from non-pyramidal cells were frequently apposed to blood vessels, including those in the hippocampal fissure. In addition, NADPH-d reactivity was also present in pyramidal and granule cells, but this staining was most pronounced between 15 and 24 weeks. The results show three types of distinctly stained NADPH-d interneurons in the fetal human hippocampal formation with different developmental courses and morphology. Also, hippocampal principal neurons transiently express NADPH-d at early fetal ages. Our data correlated with other findings suggest that nitric oxide may play a role in neuronal development in the hippocampal formation by modulating neuronal differentiation and maturation, and regulating blood supply.

Social behaviour in sheep relates to behaviour and neurotransmitter responses to nociceptive stimuli

Sheep in the field display differences in social behaviour. These differences allow a division into three social groups with distinct behavioural occurrences and frequencies. The behavioural and neurotransmission responses of each of these groups to aversive stimuli were compared. Behavioural responses were seen to both forelimb electric shocks and thermal heating of the nose in all groups. These responses changed with stimulus repetition in a group-dependent manner. Microdialysis probe studies of neurotransmitter release in the somatosensory cortex indicated neurotransmitter responses to stimuli in all animals that varied with both animal group and stimulus repetition. Group 1 animals, aggressive and socially active, showed increases in gamma amino-4-butyric acid (GABA) with initial stimulus presentation; this increased with stimulus repetition. Behavioural responses to the stimuli decreased with repetition and nonstimulus-related behaviours, during the course of the experiment, increased. Both of these appeared dependent upon GABA. Group 2 animals, moderately aggressive and socially active, released opioid-like peptides (OLP) upon initial exposure to stimuli but, with repetition, switched to using GABA. Group 3 animals, nonaggressive and socially inactive, released OLP with initial and repeat stimuli. In groups 2 and 3, both GABA and OLP appear to reduce stimulus-related behaviour, but OLP appeared to also reduce nonstimulus-related behaviour and GABA increased these. Changes were independent of animal liveweight. Glutamate was released in response to stimuli in all 3 groups and, with repetition, fell in groups 1 and 2 but increased in group 3. An animal's social behaviour and status may predict its response to a stimulus.

Memory, sleep, and dynamic stabilization of neural circuitry: evolutionary perspectives

Some aspects of the evolution of mechanisms for enhancement and maintenance of synaptic efficacy are treated. After the origin of use-dependent synaptic plasticity, frequent synaptic activation (dynamic stabilization, DS) probably prolonged transient efficacy enhancements induced by single activations. In many "primitive" invertebrates inhabiting essentially unvarying aqueous environments, DS of synapses occurs primarily in the course of frequent functional use. In advanced locomoting ectotherms encountering highly varied environments, DS is thought to occur both through frequent functional use and by spontaneous "non-utilitarian" activations that occur primarily during rest. Non-utilitarian activations are induced by endogenous oscillatory neuronal activity, the need for which might have been one of the sources of selective pressure for the evolution of neurons with oscillatory firing capacities. As non-sleeping animals evolved increasingly complex brains, ever greater amounts of circuitry encoding inherited and experiential information (memories) required maintenance. The selective pressure for the evolution of sleep may have been the need to depress perception and processing of sensory inputs to minimize interference with DS of this circuitry. As the higher body temperatures and metabolic rates of endothermy evolved, mere skeletal muscle hypotonia evidently did not suffice to prevent sleep-disrupting skeletal muscle contractions during DS of motor circuitry. Selection against sleep disruption may have led to the evolution of further decreases in muscle tone, paralleling the increase in metabolic rate, and culminating in the postural atonia of REM (rapid eye movement) sleep. Phasic variations in heart and respiratory rates during REM sleep may result from superposition of activations accomplishing non-utilitarian DS of redundant and modulatory motor circuitry on the rhythmic autonomic control mechanisms. Accompanying non-utilitarian DS of circuitry during sleep, authentic and variously modified information encoded in the circuitry achieves the level of unconscious awareness as dreams and other sleep mentation.

Nicotine enhancement of fast excitatory synaptic transmission in CNS by presynaptic receptors

The behavioral and cognitive effects of nicotine suggest that nicotinic acetylcholine receptors (nAChRs) participate in central nervous system (CNS) function. Although nAChR subunit messenger RNA (mRNA) and nicotine binding sites are common in the brain, there is little evidence for synapses mediated by nAChRs in the CNS. To test whether, CNS nAChRs might modify rather than mediate transmission, the regulation of excitatory synaptic transmission by these receptors was examined. Nanomolar concentrations of nicotine enhanced both glutamatergic and cholinergic synaptic transmission by activation of presynaptic nAChRs that increased presynaptic [Ca2]i. Pharmacological and subunit deletion experiments reveal that these presynaptic nAChRs include the alpha 7 subunit. These findings reveal that CNS nAChRs enhance fast excitatory transmission, providing a likely mechanism for the complex behavioral effects of nicotine.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampal slices: facilitation by endogenous or exogenous nitric oxide

The involvement of nitric oxide (NO) in the evoked release of noradrenaline (NA) was studied in rat hippocampal slices preincubated with [3H]NA and stimulated with 3,4-diaminopyridine (3,4-DAP; 200 microM) for 2 min. The 3,4-DAP-evoked [3H]overflow was enhanced by the NO synthase substrate L-arginine, but not by D-arginine; it was reduced by the NO synthase inhibitor NG-nitro-L-arginine, which also antagonized the effects of L-arginine. The corresponding nitro derivative of D-arginine was inactive and unable to block the effects of L-arginine. Also drugs known to produce NO in-vitro, like sodium nitroprusside (SNP), 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP) enhanced the 3,4-DAP-evoked NA release. The NO scavenger hemoglobin showed no significant effects when given alone, but reduced or abolished, respectively, the facilitatory effects of SNP, or SNAP and L-arginine. The cyclic GMP derivatives 8-Br-cGMP and Sp-8-p-chlorophenylthioguanosine-3',5'-cyclic monophosphorothioate (Sp-8-pCPT-cGMPS) also acted facilitatory, whereas the corresponding Rp-enantiomer of the latter compound was inactive, but antagonized the effect of Sp-8-pCPT-cGMPS. NA release evoked by 3,4-DAP (10 microM) from rat hippocampus synaptosomes was not affected by L-arginine or NG-nitro-L-arginine but slightly increased by SNAP and Sp-8-pCPT-cGMPS. Antagonists at NMDA, non-NMDA and metabotropic glutamate receptors neither affected the 3,4-DAP-evoked NA release nor the facilitatory effect of L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-existence and interaction between facilitatory and inhibitory metabotropic glutamate receptors and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals

We have investigated the interaction between facilitatory and inhibitory metabotropic glutamate receptors (mGluRs) and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals from young (3 weeks postnatal) rats. The adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) (1 microM) and the mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) (100 microM) inhibited Ca(2+)-dependent release of glutamate evoked by depolarization of synaptosomes with 30 mM KCl to 33 +/- 6 and 30 +/- 4% of control values, respectively. The CHA and L-AP4 inhibition of release was consistent with the reduction of a component of Ca2+ entry in nerve terminals which was also sensitive to omega-Aga-IVA. When the inhibitory agonists were co-applied at optimal concentrations, no additivity of the inhibitory effects on either glutamate release or [Ca2+]c was observed. The nerve terminals from young rats also exhibit the facilitatory pathway for glutamate release that is observed during 4-aminopyridine-evoked depolarization after stimulation of mGluRs with the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) in the presence of arachidonic acid (AA). The addition of ACPD or AA alone did not alter the ability of CHA and L-AP4 to reduce the release, however the co-application of AA and ACPD abolished the inhibitory effect induced by CHA and L-AP4 whether alone or in combination. These results indicate the co-existence of the three modulatory pathways of glutamate release and the dominant role of the ACPD/AA activated facilitatory pathway in its interaction with the inhibitory pathways activated by L-AP4 and CHA.

Nitric oxide synthase inhibitors block behavioral sensitization to methamphetamine in mice

Repeated administration of methamphetamine (1.0 mg/kg) once daily for 7 consecutive days resulted in an augmentation of the locomotor-activating effect of methamphetamine (0.5 mg/kg) challenged 72 h after the last injection. Administration of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (10 and 30 mg/kg), before daily methamphetamine injections dose dependently prevented the development of behavioral sensitization to subsequent methamphetamine challenge. The mice given another NO synthase inhibitor, NG-nitro-L-arginine methyl ester (100 mg/kg), before daily methamphetamine injections showed significantly less locomotor activity in response to 0.5 mg/kg methamphetamine challenge than the mice given daily methamphetamine alone. Such effects were not observed when the inactive isomer, NG-nitro-D-arginine methyl ester (100 mg/kg), was administered daily prior to methamphetamine. Both NO synthase inhibitors exerted the acute effect to reduce spontaneous and methamphetamine-stimulated locomotor activity, while neither spontaneous locomotion nor hyperlocomotion in response to 1.0 mg/kg methamphetamine was altered 72 h after repeated administration of NG-nitro-L-arginine (30 mg/kg) or NG-nitro-L-arginine methyl ester (100 mg/kg) alone once daily for 7 days. On the other hand, pretreatment with the NMDA receptor antagonist, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate), at 0.2 mg/kg also suppressed the development of sensitization to the locomotor-activating effect of methamphetamine. These findings suggest that NO formation possibly mediated by NMDA receptors is involved in mechanisms underlying the development of behavioral sensitization to methamphetamine.

Aldehyde fixation differentially affects distribution of diaphorase activity but not of nitric oxide synthase immunoreactivity in rat brain

The effect of aldehyde fixation on NADPH- and NADH-dependent diaphorase (d) histochemistry and nitric oxide synthase (NOS) immunocytochemistry in the brain was investigated by comparing the distribution of these enzymes in in situ nitrocellulose blots of unfixed brain sections with that in aldehyde-fixed brain sections. Substitution of NADPH by NADH yielded no gross differences in cellular distribution in the native blot, whereas in fixed sections NADH produced nonspecific staining of the entire section. In the in situ blot NADPHd histochemistry therefore visualized general nitroblue tetrazolium reductase (NBTr) activity, which was particularly strong in hippocampal pyramidal neurons and cerebellar Purkinje cells. Aldehyde fixation abolished the anatomical pattern of general NBTr activity and changed the histochemical distribution in that of the NADPHd activity associated with the distribution of NOS-I immunoreactivity (ir). Fixation intensified NADPHd histochem- ical staining in specific neurons, resulting in outstanding, Golgi-like staining of these neurons in several brain regions, whereas the general NBTr activity in pyramidal and Purkinje cells disappeared. In contrast to the histochemical diaphorase distribution, the distribution of NOS-I ir on blots and in aldehyde-fixed brain sections was similar. No NOS was observed in hippocampal pyramidal and cerebellar Purkinje neurons. In regions like cerebral and cerebellar cortex and striatum the applied anti NOS-I serum had a higher affinity for the native protein. It is concluded that aldehydes, rather than to progressively suppress NOS-unrelated enzymes, differentially elicit NADPHd activity in some groups of neurons while leaving NOS-ir unaffected.

Activation of mitogen-activated protein kinase and arachidonate release via two G protein-coupled receptors expressed in the rat hippocampus

Platelet-activating factor and somatostatin receptors, two G protein-coupled receptors expressed in the rat hippocampus, were analyzed for the downstream signaling pathways in Chinese hamster ovary cells stably expressing each receptor. Ligand stimulation to each CHO cell line induced (1) inhibition of forskolin-induced accumulation of cAMP, (2) arachidonate release, and (3) activation of mitogen-activated protein kinase and MAP kinase kinase. In contrast, inositol phosphate breakdown was seen only in the PAF-stimulated CHO cells. The induction of these signals accompanied no detectable Ras activation. Suppression of the signals by pertussis toxin was almost complete for the somatostatin receptor but partial for the PAF receptor, suggesting that the somatostatin receptor couples only with PTX-sensitive G protein, while the PAF receptor couples with both PTX-sensitive and -insensitive G proteins. A model of G protein-mediated signaling pathways was proposed in which the signals from Gi and those from Gq converge at MAP kinase kinase and lead to arachidonate release. The present system using CHO cells is useful for analyzing signaling pathways from G proteins to MAP kinase kinase and will thereby provide clues for understanding the mechanisms underlying the physiological and pathological events mediated by PAF, somatostatin, and other G protein-coupled receptors in the central nervous system and other tissues.

Sleep and dynamic stabilization of neural circuitry: a review and synthesis

A common mechanism is advanced for the lengthy stabilization of neural circuitry encoding information of both hereditary and experimental origin. Stabilization is proposed to occur through the following means and interrelationships. Synaptic function is intrinsically plastic because of greatly restricted entry of essential, relatively short-lived molecules into synaptic terminals. Alterations that accompany synaptic transmission transiently facilitate this entry ("facilitated entry"). Synaptic efficacy is enhanced as the concentration of these molecules increases following a transmission event but subsequently declines if depletion of the molecules occurs without commensurate replacement. Accordingly, if lengthy persistence of information encoded by enhancements of synaptic efficacy is to be achieved, the enhancements must be reinforced repeatedly by synaptic transmission ("dynamic stabilization"). Synapses of circuits not in frequent functional use are thought to be dynamically stabilized by spontaneous, internally generated, "non-utilitarian" excitations occurring primarily during rest or sleep. In species with complex, highly developed brains, requirements for dynamic stabilization of infrequently used circuits apparently cannot be met during rest, a restriction that may underlie the origin of sleep. Dynamic stabilization of infrequently used motor circuits of endotherms appears to occur predominantly during REM sleep.

NADPH diaphorase (nitric oxide synthase) in a part of the chick brain involved in imprinting

The intermediate and medial part of the hyperstriatum ventrale (IMHV) is a memory system in the chick forebrain in which certain learning-related changes occur after imprinting. We have enquired whether NADPH-diaphorase, a presumed marker for nitric oxide synthase, is present in sections through the IMHV of chicks, either trained with an imprinting stimulus or dark-reared. Very few NADPH-d-positive cells were found in the IMHV (0.37 +/- 0.07 S.E.M. cells per 0.3 mm x 0.9 mm sampling frame), in contrast to the palaeostriatum augmentatum (PA) (19.47 +/- 0.77). Some stained cells in the PA were closely associated with blood vessels. The results do not support the hypothesis that learning-related changes in the IMHV depend on nitric oxide acting as a retrograde neuronal messenger.

No enhancement by nitric oxide of glutamate release from P2 and P3 synaptosomes of rat hippocampus

Effects of nitric oxide on glutamate (Glu) release in long-term potentiation (LTP) were investigated by superfusion of conventional (P2) and large (P3) synaptosomes prepared from the rat hippocampus. Basal releasing rates of endogenous Glu from P2 and P3 fractions were 103.6 and 85.2 pmol/min/mg protein, respectively. Exposure to a depolarizing concentration of KCl (30 mM) evoked 3.58- and 4.52-fold increases in releasing rates of Glu from P2 and P3 fractions, respectively. Although the perfusion with sodium nitroprusside (NP, 10(-3) M), a nitric oxide-releasing agent, failed to augment the K(+)-evoked releases of Glu from P2 and P3 synaptosomes, NP enhanced that from slices of the hippocampus by 39% without changing basal release. Similarly, 8-bromoguanosine 3':5'-cyclic monophosphate (10(-4) M) increased the K(+)-evoked release of Glu from slices by 30%, but not from either synaptosomes. When synaptosomes were prepared from the hippocampus which was pretreated with two trains of electrical field stimulation (100 Hz, 0.1 ms, for 2 s), K(+)-evoked releases of Glu from P2 and P3 synaptosomes were increased by 15% and 23%, respectively. Although nitric oxide is postulated to function as a retrograde messenger to maintain LTP, present results suggest that nitric oxide may not directly act upon nerve terminals to enhance glutamate release, but that interventions of glias and short neurons may be involved in the presynaptic mechanism of LTP.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of arachidonic acid on [3H]D-aspartate outflow in the rat hippocampus

The aim of this study was to investigate the effect of arachidonic acid on [3H]d-aspartate outflow in rat hippocampus synaptosomes and slices. Arachidonic acid 1) increased basal outflow of [3H]d-aspartate in both synaptosomes and slices, and 2) increased K(+)-evoked overflow in slices but not in synaptosomes. The latter effect was dependent (at least in part) on arachidonic acid metabolism, most likely mediated by lipo-oxygenase metabolites and free radical production. It was prevented by nordihydroguairetic acid but not by indomethacin, and was significantly reduced by free radical scavengers (superoxide-dismutase and catalase). This effect was dependent upon stimulation since it could not be observed after a continuous perfusion of arachidonic acid in the absence of stimulation. Furthermore, it was long-lasting since a 30 min perfusion of arachidonic acid was sufficient to exert a significant effect on a stimulation following termination of the application.

Deficits in working memory following inhibition of hippocampal nitric oxide synthesis in the rat

In order to elucidate the roles of hippocampal nitric oxide (NO) synthesis in working and reference memory performance of rats, the effects of intrahippocampal injections of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on this behavior were examined with a three-panel runway task. In the working memory task, L-NAME, injected bilaterally at 10 and 32 micrograms/side into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). This effect of intrahippocampal L-NAME (32 micrograms/side) on working memory was attenuated by concurrent injection of 100 micrograms/side L-arginine, the precursor of NO. Intrahippocampal injection of the inactive isomer D-NAME at doses up to 32 micrograms/side had no effect on the number of working memory errors. In the reference memory task, neither L-NAME nor D-NAME affected the number of errors when injected into the hippocampus at doses up to 32 micrograms/side. These results suggest that processes mediated by NO synthesis in the hippocampus are involved in working memory, but not in reference memory.

Ionotropic and metabotropic components of electrophysiological response of cerebellar Purkinje neurons to excitatory amino acids

Cerebellar Purkinje neurons possess AMPA ((RS)-alpha-amino-3-hydroxyl-5- methyl-4-isoxazolepropionic acid)-sensitive ionotropic glutamate receptors (AMPA GluRs) and ACPD ((1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid)-sensitive metabotropic glutamate receptors (mGluRs). The contributions of these receptors to responses elicited by dual receptor activation in cultured cerebellar Purkinje neurons were delineated by quantitative analysis of agonist-induced single unit activity. Responses to co-activation using Quis or AMPA + ACPD were biphasic, consisting of a dramatic increase in firing rate (excitatory phase) followed by a temporary decrease (inhibitory phase). In half of the cells tested bursting was induced during both the excitatory and inhibitory phases and the duration of each phase was prolonged relative to responses observed in non-bursting cells. Quantitative comparisons of these responses and responses produced by selective activation of AMPA GluRs and mGluRs revealed that: (a) AMPA GluRs mediated the dramatic changes in firing rate, (b) mGluRs mediated the dramatic increases in bursting and the extended duration of each phase and (c) these AMPA GluR and mGluR mediated effects were largely additive when simultaneously activated. Nevertheless, interactions did occur with repeated co-activation of AMPA GluRs and mGluRs, as indicated by selective changes in the mGluR-mediated bursting component of the response. Such modulation may contribute to synaptic regulation of Purkinje neuron excitability, for example, that associated with long term depression.

Mechanisms involved in the neuroprotective activity of a nitric oxide synthase inhibitor during focal cerebral ischemia

We have reported previously that posttreatment with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the nitric oxide synthase, reduced the volume of cortical and striatal infarct induced by middle cerebral artery occlusion in rats. In the present study, we investigated the mechanisms by which L-NAME (3 mg/kg i.p.) is neuroprotective in this model of cerebral ischemia. First, we have shown the reversal of the neuroprotective effect of L-NAME by a coinjection of L-arginine. Second, in order to determine by which mechanism nitric oxide exacerbates neuronal damage produced by focal cerebral ischemia, we studied the effect of the inhibition of nitric oxide synthase by L-NAME on the histological consequences of a focal injection of N-methyl-D-aspartate (NMDA) in the striatum, and on the striatal overflow of glutamate and aspartate induced either by K+ depolarization or by focal cerebral ischemia. We have found that L-NAME treatment reduced the excitotoxic damage produced by NMDA injection. By using microdialysis, we have shown that the K(+)- and the ischemia-induced glutamate efflux was reduced by 52 and 30%, respectively, after the L-NAME treatment. These results indicate that nitric oxide synthesis induced by the NMDA receptor overstimulation is one of the major events leading to neuronal damage. One possible mechanism by which nitric oxide may contribute to the excitotoxic process is by facilitating the ischemia-induced glutamate overflow.

Effects of adrenocortical steroids on long-term potentiation in the limbic system: basic mechanisms and behavioral consequences

Hippocampal structures are a major target for adrenal steroid hormones, and hence these neural regions are some of the most likely mediators of the effects of adrenocortical steroids on behavior. Memory disturbance, in particular biasing toward negative contents, are part of the symptomatology presented by depressive patients. In turn, a sizeable subset of depression also presents with hypercortisolemia. Adrenocortical hormones are also known to affect memory processes. Hippocampal formation is essential for declarative memory. We thought it appropriate then to study the effects of adrenal steroids on long-term potentiation, a putative memory mechanism in the hippocampus. Two clearly distinguished components of the evoked response to perforant path stimulation can be studied in the hippocampus: the excitatory postsynaptic potential (EPSP) which denotes the graded depolarization of the somatodendritic region of the neuron and the population spike (PS), a manifestation of the all-or-none-discharge of the cell action potential. Corticosterone had a significant depressant effect on the EPSP component of the evoked response immediately and 15 min after injection. Thereafter EPSP amplitudes were within normal values. Corticosterone significantly decreased the PS immediately after the train, the component remaining low 30 min after the train. 5 alpha-Dihydrocorticosterone (a ring A-reduced metabolite of corticosterone) significantly reduced the PS component of the response at all times after injection. 18-Hydroxydeoxycorticosterone and deoxycorticosterone significantly decreased both EPSP and PS components of the evoked response from the time of infusion. Contrary to expectation, tetrahydrodeoxycorticosterone was ineffective in decreasing and if anything, enhanced the development of long-term potentiation. 18-Hydroxydeoxycorticosterone 21-acetate behaved like vehicle, except for the first 30 min after injection when the EPSP was decreased. Allotetrahydroprogesterone decreased all EPSP's values and had no effect in the PS development in comparison with vehicle. The suggestion is made that the study of steroidal effects on hippocampal LTP can serve as a preclinical model of some aspects of depression in a specific subset of the disease.

Involvement of nitric oxide synthase in 3,4-diaminopyridine-evoked noradrenaline release in rat hippocampus

The release of tritiated noradrenaline, evoked by stimulation of rat hippocampus slices with 3,4-diaminopyridine (200 microM, 2 min), was enhanced by the nitric oxide (NO) synthase substrate, L-arginine (but not by D-arginine), by NO donors (sodium nitroprusside, 3-morpholino-sydnonimine), and by 8-Br-cGMP. The effect of L-arginine was stereospecifically antagonized by NG-nitro-L-arginine, which given alone was inhibitory. From these findings we conclude that endogenously formed NO facilitates 3,4-diaminopyridine-evoked noradrenaline release in rat hippocampus.

Arachidonic acid stimulates glucose uptake in cerebral cortical astrocytes

Arachidonic acid (AA) has recently been shown to influence various cellular functions in the central nervous system. Here we report that AA increases, in a time- and concentration-dependent manner, 2-deoxy-D-[1-3H]glucose ([3H]2DG) uptake in primary cultures of astrocytes prepared from the cerebral cortex of neonatal mice. This effect is mimicked by an unsaturated fatty acid such as linolenic acid, while palmitic and arachidic acids, two saturated fatty acids, are inactive. Pharmacological agents that increase the endogenous levels of AA by stimulating AA release (melittin) or by inhibiting its reacylation (thimerosal) also promote [3H]2DG uptake by astrocytes. We also report that norepinephrine (NE) stimulates the release of [3H]AA from membrane phospholipids, with an EC50 of 3 microM; this effect is accompanied, with a temporal delay of approximately 4 min, by the stimulation of [3H]2DG uptake, for which the EC50 of NE is 1 microM. Since the cerebral cortex, the brain region from which astrocytes used in this study were prepared, receives a massive noradrenergic innervation, originating from the locus coeruleus, the effects of NE reported here further stress the notion that certain neurotransmitters may play a role in the regulation of energy metabolism in the cerebral cortex and point at astrocytes as the likely targets of such metabolic effects.

Arachidonic acid in cell signaling

Important advances have recently been made in our understanding of the arachidonic acid cascade. The molecular characterization of different forms of phospholipase A2 indicates that multiple pathways are involved in the release of arachidonic acid evoked by physiological or pathological stimuli. Moreover, studies on the expression of enzymes that metabolize arachidonic acid reveal the potential participation of the eicosanoids in central aspects of cell regulation, such as control of mitogenesis. Finally, cloning of the first eicosanoid receptors is a major step towards elucidating the diverse cellular functions exerted by these bioactive lipids.