Tachykinins amplify the action of capsaicin on central histaminergic neurons

Substance P (SP), a product of the tachykinin 1 (Tac1) gene, is expressed in many hypothalamic neurons. Its wake-promoting potential could be mediated through histaminergic (HA) neurons of the tuberomamillary nucleus (TMN), where functional expression of neurokinin receptors (NKRs) waits to be characterized. As in the process of nociception in the peripheral nervous system (PNS) capsaicin-receptor (transient potential vanilloid 1: TRPV1) signalling is amplified by local release of histamine and SP, we tested the involvement of tachykinins in the capsaicin-induced long-lasting enhancement (LLEcaps) of HA neurons firing by investigating selective neurokinin receptor ligands in the hypothalamic mouse brain slice preparation using patch-clamp recordings in cell-attached mode combined with single-cell RT-PCR. We report that the majority of HA neurons respond to SP (EC50 3 nM), express the SP precursor tachykinin 1 (Tac1) gene and at least one of the neurokinin receptors. Responses to selective agonists of three known neurokinin receptors were sensitive to corresponding antagonists. LLEcaps was significantly impaired by the neurokinin receptor antagonists, indicating that in hypothalamus, as in the PNS, release of tachykinins downstream to TRPV1 activation is able to boost the release of histamine. The excitatory action of SP on histaminergic neurons adds another pathway to the noradrenergic and orexinergic ones to synergistically enhance cortical arousal. We show NK1R to play a prominent role on HA neurons and thus the control of wakefulness.

Benzodiazepine-site pharmacology on GABAA receptors in histaminergic neurons

BACKGROUND AND PURPOSE

The histaminergic tuberomamillary nucleus (TMN) of the posterior hypothalamus controls the cognitive aspects of vigilance which is reduced by common sedatives and anxiolytics. The receptors targeted by these drugs in histaminergic neurons are unknown. TMN neurons express nine different subunits of the GABAA receptor (GABAA R) with three α- (α1, α2 and α5) and two γ- (γ1, γ 2) subunits, which confer different pharmacologies of the benzodiazepine-binding site.

EXPERIMENTAL APPROACH

We investigated the actions of zolpidem, midazolam, diazepam, chlordiazepoxide, flumazenil (Ro15-1788) and methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) in TMN neurons using mouse genetics, electrophysiological and molecular biological methods.

KEY RESULTS

We find the sensitivity of GABAA R to zolpidem, midazolam and DMCM significantly reduced in TMN neurons from γ2F77I mice, but modulatory activities of diazepam, chlordiazepoxide and flumazenil not affected. Potencies and efficacies of these compounds are in line with the dominance of α2- and α1-subunit containing receptors associated with γ2- or γ1-subunits. Functional expression of the γ1-subunit is supported by siRNA-based knock-down experiments in γ2F77I mice.

CONCLUSIONS AND IMPLICATIONS

GABAA R of TMN neurons respond to a variety of common sedatives with a high affinity binding site (γ2F77I) involved. The γ1-subunit likely contributes to the action of common sedatives in TMN neurons. This study is relevant for understanding the role of neuronal histamine and benzodiazepines in disorders of sleep and metabolism.

The bile steroid chenodeoxycholate is a potent antagonist at NMDA and GABA(A) receptors

The bile steroids (BS) cholic acid and chenodeoxycholic acid are produced in hepatocytes and in the brain. Nothing is known about neuronal actions of BS. Deficiency in a 27-hydroxylase enzyme coincides with reduced production of chenodeoxycholic acid (CDCA) and a relative increase in cholic acid in an inherited lipid storage disease, cerebrotendinous xanthomatosis, characterized by neurological dysfunctions, which can be treated by dietary CDCA. We have examined the modulation of hypothalamic network activity by nine common BS. Cholate and CDCA significantly reduced the firing of hypothalamic neurons and synchronized network activity with CDCA being nearly 10 times more potent. The synthetic BS dehydrocholate synchronized the activity without affecting the firing rate. Gabazine, a GABA(A) receptor antagonist, occluded synchronization by BS. Whole-cell patch clamp recordings revealed a block of NMDA- and GABA(A)-receptors by BS. Potencies of nine common BS differed between NMDA and GABA(A) receptors, however in both cases they correlated with BS affinities for albumin but not with their lipophilicity, supporting a direct action at ligand gated ion channels. GABAergic synaptic currents displayed a faster decay under BS. Our data provide new insight into extrahepatic functions of BS revealing their neuroactive potential.

Orexins/hypocretins and aminergic systems

Orexin/hypocretin neurones in the posterior hypothalamus are mutually connected with noradrenergic, serotonergic, dopaminergic, histaminergic, and cholinergic neurone systems. They activate these targets by direct post-synaptic and indirect pre-synaptic mechanisms and in turn receive inhibitory feedback and excitatory feed forward control. With respect to behavioural state control, orexin/hypocretin neurones are conducting the orchestra of biogenic amines. This review highlights the role of these players in the control of energy administration, sleep-wake architecture, cortical activation, plasticity, and memory functions in health and disease.

Orexins/hypocretins control bistability of hippocampal long-term synaptic plasticity through co-activation of multiple kinases

AIM

Orexins/hypocretins (OX/Hcrt) are hypothalamic neuropeptides linking sleep-wakefulness, appetite and neuroendocrine control. Their role and mechanisms of action on higher brain functions, such as learning and memory, are not clear.

METHODS

We used field recordings of excitatory post-synaptic potentials (fEPSP) in acute mouse brain slice preparations to study the effects of orexins and pharmacological inhibitors of multiple kinases on long-term synaptic plasticity in the hippocampus.

RESULTS

Orexin-A (OX-A) but not orexin-B (OX-B) induces a state-dependent long-term potentiation of synaptic transmission (LTP(OX)) at Schaffer collateral-CA1 synapses in hippocampal slices from adult (8- to 12-week-old) mice. In contrast, OX-A applied to slices from juvenile (3- to 4-week-old) animals causes a long-term depression (LTD(OX)) in the same pathway. LTP(OX) is blocked by pharmacological inhibition of orexin receptor-1 (OX1R) and plasticity-related kinases, including serine/threonine- (CaMKII, PKC, PKA, MAPK), lipid- (PI3K), and receptor tyrosine kinases (Trk). Inhibition of OX1R, CaMKII, PKC, PKA and Trk unmasks LTD(OX) in adult animals.

CONCLUSION

Orexins control not only the bistability of arousal states and threshold for appetitive behaviours but, in an age- and kinase-dependent manner, also bidirectional long-term synaptic plasticity in the hippocampus, providing a possible link between behavioural state and memory functions.

GABAA-receptor modification in taurine transporter knockout mice causes striatal disinhibition

The Striatum is involved in the regulation of movements and motor skills. We have shown previously, that the osmolyte and neuromodulator taurine plays a role in striatal plasticity. We demonstrate now that hereditary taurine deficiency in taurine-transporter knock-out (TAUT KO) mice results in disinhibition of striatal network activity, which can be corrected by taurine supplementation. Modification of GABAA but not glycine receptors (taurine is a ligand for both receptor types) underlies this disinhibition. Whole-cell recordings from acutely isolated as well as cultured striatal neurons revealed a decreased agonist sensitivity of the GABAA receptor in TAUT KO neurons in the absence of changes in the maximal GABA-evoked current amplitude. The striatal GABA level in TAUT KO mice was unchanged. The amplitude enhancement of spontaneous IPSCs by zolpidem was stronger in TAUT KO than in wild-type (WT) animals. Tonic inhibition was absent in striatal neurons under control conditions but was detected after incubation with the GABA-transaminase inhibitor vigabatrin: bicuculline induced a larger shift of baseline current in WT as compared to TAUT KO neurons. Lack of taurine leads to reduced sensitivity of synaptic and extrasynaptic GABAA receptors and consequently to disinhibition. These findings help in understanding neuropathologies accompanied by the loss of endogenous taurine, for instance in hepatic encephalopathy.

Long-term depression of cortico-striatal synaptic transmission by DHPG depends on endocannabinoid release and nitric oxide synthesis

In models of early stage Parkinson's disease (PD), motor deficits are accompanied by excessive activation of striatal glutamate receptors. Metabotropic glutamate group I receptors (mGluR I) play an important but not well-understood role in PD progression. In mouse brain slices, bath application of the mGluR I agonist (RS)-DHPG (3,5-dihydroxyphenylglycine, 100 microm for 20 min) caused a long-term depression of corticostriatal transmission (LTD(DHPG)), which was reversed by three mGluR I antagonists: LY 367385, CPCCOEt and MPEP. LTD(DHPG) required nitric oxide (NO) synthesis as it was blocked by the broad-spectrum NO synthase (NOS) inhibitor Nomega-nitro-l-arginine (NL-Arg) and impaired under blockade of neuronal NOS and in endothelial NOS-deficient mice. Release of endocannabinoids (eCB) was critically involved in this form of striatal plasticity givem that the CB1 receptor antagonist AM251 prevented LTD(DHPG), while the CB1 agonist ACEA elicited LTD. The NO synthesis necessary for LTD(DHPG) induction occurred downstream of CB1 activation as ACEA-evoked LTD was also abolished by NL-Arg. These findings are relevant for the pathophysiology of PD, as they link the overactivation of group I mGluRs and striatal NO production.

Modafinil inhibits rat midbrain dopaminergic neurons through D2-like receptors

Modafinil is a well-tolerated medication for excessive sleepiness, attention-deficit disorder, cocaine dependence and as an adjunct to antidepressants with low propensity for abuse. We investigated the modafinil action on identified dopaminergic and GABAergic neurons in the ventral tegmental area (VTA) and substantia nigra (SN) of rat brain slices. Modafinil (20 microM) inhibited the firing of dopaminergic, but not GABAergic neurons. This inhibition was maintained in the presence of tetrodotoxin and was accompanied by hyperpolarization. Sulpiride (10 microM), a D2-receptor antagonist, but not prazosine (20 microM, an alpha1-adrenoreceptor blocker) abolished the modafinil action. Inhibition of dopamine reuptake with a low dose of nomifensine (1 microM) reduced the firing of DA neurons in a sulpiride-dependent manner and blunted the effect of modafinil. On acutely isolated neurons, modafinil evoked D2-receptor-mediated outward currents in tyrosine-hydroxylase positive cells, identified by single-cell RT-PCR, which reversed polarity near the K(+) equilibrium potential and were unchanged in the presence of nomifensine. Thus modafinil directly inhibits DA neurons through D2 receptors.

Effects of arousal- and feeding-related neuropeptides on dopaminergic and GABAergic neurons in the ventral tegmental area of the rat

Many neuropeptides regulate feeding and arousal; the ventral tegmental area (VTA) is likely to be one site where they act. We used whole-cell patch-clamp and single-unit extracellular recordings to examine the effects of such neuropeptides on the activity of VTA neurons. Substance P (SP; 300 nM) increased the firing rate of the majority of VTA dopaminergic and gamma-aminobutyric acid (GABA)ergic neurons, and induced oscillations in two dopaminergic cells. Corticotropin-releasing factor (CRF; 200 nM) excited the majority of VTA cells directly, whereas neuropeptide Y (NPY; 300 nM) directly inhibited a subset of dopaminergic and GABAergic cells. Consecutive application of several neuropeptides revealed that all the neurons were excited by at least one of the excitatory neuropeptides SP, CRF or/and orexins. Alpha-melanocyte-stimulating hormone had no effect on dopaminergic cells (at concentrations of 500 nM and 1 microM) and affected only a small proportion of GABAergic neurons. Ghrelin (500 nM), agouti-related peptide (1 microM); cocaine and amphetamine-related transcript (500 nM) and leptin (500 nM and 1 microM) did not modulate the firing rate and membrane potential of VTA neurons. Single-cell reverse transcription polymerase chain reaction analysis showed that all NPY receptors were present in VTA neurons, and all but one cell expressed NPY and/or at least one NPY receptor. CRF was expressed in 70% of dopaminergic VTA cells; the expression of CRF receptor 2 was more abundant than that of receptor 1. These findings suggest a link between the ability of neuropeptides to promote arousal and their action on VTA neurons.

Long-Lasting Enhancement of Corticostriatal Transmission by Taurine: Role of Dopamine and Acetylcholine

1. Taurine applied to mouse brain slices evokes a long-lasting enhancement (LLE) of corticostriatal synaptic transmission, LLE(TAU). 2. The occurrence of LLE(TAU) was significantly decreased in the presence of the specific antagonists at either D1 (SCH23390) or D2 (raclopride) dopamine (DA) receptors. 3. LLE(TAU) was prevented by scopolamine, a muscarinic antagonist, and significantly suppressed by the nicotinic antagonist mecamylamine. 4. Thus, dopaminergic and cholinergic mechanisms, in concert with the taurine transporter and glycine receptors, contribute critically to the induction of corticostriatal LLE(TAU).

Deficits in cortico-striatal synaptic plasticity and behavioral habituation in rats with portacaval anastomosis

Hepatic encephalopathy is characterized by disturbances of motor and cognitive functions involving the basal ganglia. So far no standards for assessment of neuropsychiatric abnormalities (disorders of sleep, mood, anxiety and personality) in subclinical hepatic encephalopathy have been defined. Using an animal model of mild (subclinical) hepatic encephalopathy we investigated now striatum-related behaviors and cortico-striatal synaptic plasticity in rats 2 months after introduction of a portacaval shunt and sham-operated matched controls. In a novel open field portacaval shunt rats displayed less locomotor activity; unlike controls they also showed no habituation to the field and no recall of the field environment after 24 h, indicative of cognitive deficit. The elevated-plus maze test indicated no differences in fear/anxiety in the portacaval shunt animals. Tetanic stimulation of cortical afferents in magnesium-free solution evoked an N-methyl-D-aspartate-dependent long-term potentiation in sham-operated animals. In portacaval shunt animals long-term potentiation was significantly impaired. Histamine, a potent modulator of cortico-striatal transmission, induced a larger long-term depression of field potentials in control compared with portacaval shunt rats. In conclusion, a combination of electrophysiological and behavioral approaches has revealed functional changes in cortico-striatal transmission. These data are relevant for understanding the mechanisms of motor and cognitive dysfunctions in hepatic encephalopathy patients and for the development of precise psychometric tests, evaluating cognitive deficits in subclinical hepatic encephalopathy.

Orexins/hypocretins cause sharp wave- and theta-related synaptic plasticity in the hippocampus via glutamatergic, gabaergic, noradrenergic, and cholinergic signaling

Orexins (OX), also called hypocretins, are bioactive peptides secreted from glucose-sensitive neurons in the lateral hypothalamus linking appetite, arousal and neuroendocrine-autonomic control. Here, OX-A was found to cause a slow-onset long-term potentiation of synaptic transmission (LTPOX) in the hippocampus of young adult mice. LTPOX was induced at Schaffer collateral-CA1 but not mossy fiber-CA3 synapses, and required transient sharp wave-concurrent population field-burst activity generated by the autoassociative CA3 network. Exogenous long theta-frequency stimulation of Schaffer collateral axons erased LTPOX in intact hippocampal slices but not mini slices devoid of the CA3 region. Pharmacological analysis revealed that LTPOX requires co-activation of ionotropic and metabotropic glutamatergic, GABAergic, as well as noradrenergic and cholinergic receptors. Together these data indicate that OX-A induces a state-dependent metaplasticity in the CA1 region associated with sharp-wave and theta rhythm activity as well as glutamatergic, GABAergic, aminergic, and cholinergic transmission. Thus, orexins not only regulate arousal threshold and body weight but also threshold and weight of synaptic connectivity, providing a molecular prerequisite for homeostatic and behavioral state-dependent control of neuronal plasticity and presumably memory functions.

Multiple GABAA receptor subtypes regulate hippocampal ripple oscillations

High-frequency oscillations (140-200 Hz) were recorded in behaving rats from the CA1 area of the hippocampus. As generation of these synchronous patterns is assumed to depend on coordinated interneuronal inhibition, we studied the interference of benzodiazepines with the fine structure and occurrence of ripple oscillations. The nonselective GABAA receptor alpha-subunit agonist, diazepam, lowered the frequency of ripple oscillations and reduced their occurrence, amplitude and duration. Zolpidem, an alpha1-subunit selective benzodiazepine elevated ripple duration but acted similar to diazepam in other respects. The nonselective alpha-subunit benzodiazepine antagonist, flumazenil, reduced ripple numbers, amplitude and duration. Wavelet based analysis of the dynamics of intraripple frequency revealed a dramatic decay within a ripple. Only diazepam (1 mg/kg) accelerated this intraripple frequency accommodation. The effects were not due to increased behavioural activity and alertness as evident from vigilance state control. The results suggest a differential role of GABAA receptor subtype specific inhibitory mechanisms in the mediation and fine-tuning of the network synchronization during approximately 200 Hz hippocampal oscillations.

Changes in motoric, exploratory and emotional behaviours and neuronal acetylcholine content and 5-HT turnover in histidine decarboxylase-KO mice

Histamine has been implicated, inter alia, in mechanisms underlying arousal, exploratory behaviour and emotionality. Here, we investigated behavioural and neurochemical parameters related to these concepts, including open-field activity, rotarod performance and anxiety, as well as brain acetylcholine and 5-HT concentrations of mice deficient for the histidine decarboxylase (HDC) gene. These mice are unable to synthesize histamine from its precursor histidine. The HDC-knockout mice showed reduced exploratory activity in an open-field, but normal habituation to a novel environment. They behaved more anxious than the controls, as assessed by the height-fear task and the graded anxiety test, a modified elevated plus-maze. Furthermore, motor coordination on the rotarod was superior to controls. Biochemical assessments revealed that the HDC-knockout mice had higher acetylcholine concentrations and a significantly higher 5-HT turnover in the frontal cortex, but reduced acetylcholine levels in the neostriatum. These results are suggestive of important interactions between neuronal histamine and these site-specific neurotransmitters, which may be related to the behavioural changes found in the HDC-deficient animals.

AMPA receptor properties and coexpression with sodium-calcium exchangers in rat hypothalamic neurons

The histaminergic tuberomamillary (TM) nucleus, a center for the regulation of wakefulness, is excited by glutamatergic, aminergic and peptidergic inputs. AMPA receptor properties in relation to their expression were investigated in acutely isolated TM neurons with the help of whole-cell patch-clamp recordings combined with single-cell RT-PCR. The mRNAs encoding for the AMPA receptor GluR2 (100% of the neurons) and GluR1 (75%) were the most frequently detected, followed by the mRNA for GluR4 (56%), whereas GluR3 cDNA amplification did not yield a PCR product in any neuron. Flip splice variants prevailed over flop, in keeping with a strong glutamate-response potentiation by cyclothiazide. The expression pattern of AMPA subunits in their two splice variants was correlated with the different subtypes of Na+/Ca2+ (NCX) and Na+/Ca2+/K+ (NCKX) exchangers: glutamate receptor subunits GluR1-4 displayed no coordinated pattern with NCX. However, NCKX2 mRNA occurred only in TM cells with a fast desensitizing glutamate response, where it was coexpressed with the GluR4 subunit in the flop splice variant. NCKX3 mRNA was detected in neurons with fast or slow desensitization of glutamate responses. AMPA receptors in TM neurons were Ca2+-impermeable. As reverse Na+/Ca2+ exchange contributes to the immediate rise in intracellular calcium resulting from glutamate receptor activation, we suggest that the coordinated expression of NCKX2 with the fast desensitizing AMPA receptor-type reflects either a receptor-exchanger coupling or separate mechanisms for maintaining calcium homeostasis in neurons with fast or slow glutamate responses.

Behavioural parameters in aged rats are related to LTP and gene expression of ChAT and NMDA-NR2 subunits in the striatum

Striatal parameters were assessed for their relevance to age-related behavioural decline. Forty aged rats (28-30 months) were tested in the water maze and open field. Of these, seven superior and seven inferior learners were compared with each other in terms of levels of in vitro short- and long-term potentiation (STP and LTP), and gene expression of choline acetyltransferase (ChAT) as well as of the NMDA-NR2A-C subunits assessed by quantitative RT-PCR. Results revealed that the superior as compared with the inferior learners had higher levels of ChAT mRNA in the striatum. For the superior group, ChAT mRNA was correlated with escape on to the cued platform in the water maze, whereas level of LTP was predictive of place learning in the water maze and rearing activity in the open field. For the inferior group, expression of NR2A and NR2B was positively correlated with place learning and probe trial performance in the water maze. The results show that individual differences in various behaviours of aged rats were accounted for by variability in striatal parameters, i.e. LTP, ChAT and NMDA-NR2 subunit mRNA. Notably, the correlations found were heterogeneous amid the groups, e.g. variability in place learning was explained by variability in levels of LTP in the superior learners, but in levels of NR2A-B mRNA in the inferior group.

Temporal pattern of hippocampal high-frequency oscillations during sleep after stimulant-evoked waking

Hippocampal ripple oscillations (140-200 Hz) are believed to be critically involved in the consolidation of memory traces during slow-wave sleep (SWS). We investigated the temporal pattern of ripple occurrence in relation to sleep phases following different types of waking. Amphetamine, the atypical wakening drug modafinil or non-pharmacological sleep deprivation lead to an increased ripple occurrence ("rebound") during the subsequent SWS episode. Waking of the same duration evoked by amphetamine or sleep deprivation led to a ripple rebound of similar extent (approximately 200%). The mean intraripple frequency was also elevated by up to 20 Hz during SWS following all treatments. Ripple amplitude was significantly increased only in experiments with amphetamine. Ripple occurrence but not intraripple frequency clearly correlated with the antecedent waking duration independent of treatment. Recovery of ripple occurrence and frequency to the pretreatment level during SWS depended on SWS duration. At the end of the recovery period paradoxical sleep (PS) acted like waking, elevating ripple occurrence during subsequent SWS episodes. On the other hand, PS decreased ripple occurrence if recovery from the rebound was not yet complete. Thus occurrence and structure of ripple oscillations are regulated by the timing and duration of previous SWS, PS and waking episodes.

Co-ordinated expression of 5-HT2C receptors with the NCX1 Na+/Ca2+ exchanger in histaminergic neurones

The different roles of Na+/Ca2+ (NCX) exchangers and Na+/Ca2+/K+ (NCKX) exchangers in regulation of the ionic homeostasis in neurones are poorly understood. We have previously shown that serotonin excites histaminergic tuberomamillary (TM) neurones by activation of 5-HT2C-receptors and Na+/Ca2+ exchange. With the help of single-cell RT-PCR (sc-RT-PCR) we have now determined the coexpression pattern of different subtypes of NCX and NCKX with serotonin receptors. The majority of TM neurones express NCX1, NCX2 and NCKX3. Serotonin 2C receptor-mRNA was detected in 70% while 5-HT2A mRNA was found in only 10% of TM neurones. In all neurones expressing the 5-HT2C receptor NCX1-mRNA was present. Double immunostaining revealed the presence of the NCX1 protein in histidine decarboxylase-positive neurones. In the majority of TM neurones one or two out of five isoforms, NCX1.4, NCX1.5, NCX1.7, NCX1.14, NCX1.15, were detected by cDNA sequencing and/or by restriction analysis. The alternative splicing region is important for the Ca2+ sensitivity and presumably for the modulation of NCX1 function by second messengers. We conclude that several exchanger-subtypes can be coexpressed in single neurones and that TM cells are heterogeneous with respect to their calcium homeostasis regulation.

Taurine-induced long-lasting enhancement of synaptic transmission in mice: role of transporters

Taurine, a major osmolyte in the brain evokes a long-lasting enhancement (LLETAU) of synaptic transmission in hippocampal and cortico-striatal slices. Hippocampal LLETAU was abolished by the GABA uptake blocker nipecotic acid (NPA) but not by the taurine-uptake inhibitor guanidinoethyl sulphonate (GES). Striatal LLETAU was sensitive to GES but not to NPA. Semiquantitative PCR analysis and immunohistochemistry revealed that taurine transporter expression is significantly higher in the striatum than in the hippocampus. Taurine transporter-deficient mice displayed very low taurine levels in both structures and a low ability to develop LLETAU in the striatum, but not in the hippocampus. The different mechanisms of taurine-induced synaptic plasticity may reflect the different vulnerabilities of these brain regions under pathological conditions that are accompanied by osmotic changes such as hepatic encephalopathy.

High-frequency oscillation in the hippocampus of the behaving rat and its modulation by the histaminergic system

The histaminergic neurons located in the posterior hypothalamus modulate whole brain activity in a manner dependent on behavioral state. We have investigated their influence on high-frequency oscillation (200-Hz ripples) in the hippocampal CA1 region of freely moving rats. The occurrence of these ripples, assumed to be involved in memory trace formation, was markedly enhanced after injection of the H1-antagonists pyrilamine and ketotifen in a lateral ventricle, indicating a tonic activity of the histaminergic system. The H2- and H3-antagonists cimetidine and thioperamide were ineffective. We suggest a mediation of these effects through blocking the known histaminergic excitation of septal neurons. Histamine administered by the intracerebroventricular route had an inhibitory action on ripples. H1-receptor activation, which has been shown to inhibit learning and memory, thus shifts hippocampal activity away from high-frequency oscillation toward theta activity.

Water maze performance, exploratory activity, inhibitory avoidance and hippocampal plasticity in aged superior and inferior learners

In 28- to 30-month-old rats, in vitro short-term and long-term potentiation (STP and LTP) were measured in area CA1 of the hippocampus in seven superior and seven inferior learners, that were selected from a pool of 40 rats based on water maze escape performance over a period of 9 days. The aim was to examine whether levels of STP and LTP could account for group differences in learning of water maze escape, spatial preference and wall (thigmotaxis)-avoidance and in short-term retention of an inhibitory avoidance task. There was no significant group difference in open-field exploration, i.e. the number of rearings. In contrast to expectation, the superior and inferior learners did not differ significantly from each other in levels of STP and LTP. However, variability in escape and spatial learning, but not thigmotaxis-avoidance learning, was significantly predicted by variability in STP and LTP in the superior group. Also, open-field exploratory rearings were significantly correlated with STP and LTP as well as with maze escape learning in the superior group. The results show that, in the aged superior group, levels of CA1 STP and LTP coincided with residual water maze escape and spatial preference learning as well as open-field exploration, i.e. behavioural expressions known to be related to hippocampal functioning, but not with learning to avoid thigmotaxis in the maze. The lack of such correlations in the inferior group may be due to the severe impairment in escape and spatial preference learning and/or the influence of yet unknown third variables on these relationships.

[Hippocampal ripple oscillations (200 Hz) in mechanisms of memory consolidation]

A current status of knowledge about high-frequency (140-200 Hz) ripple oscillations in the CA1 hippocampal subfield is summarized and considered in the context of two-stage model of the hippocampal memory processing. A large body of evidence suggests highly-selective recruitment of pyramidal cells and interneurons in the generation of the oscillatory pattern after co-operative sharp-wave-related discharge of CA3 pyramidal neurons. We also discuss a role of transmission via gap junctions in the mechanisms of ripple oscillations as well as their adaptive aminergic (histaminergic) modulation. Patterns of neuronal firing in the hippocampus observed during ripple oscillations reproduce space-dependant neuronal activity from the previous waking period. Together with a data about efficacy of high-frequency stimulation for induction of synaptic modification it points out a role for ripples in the formation of long-term memory. Focal ultra fast ripples (up to 500 Hz) have been shown to participate in the development of temporal lobe epilepsy.

Long-lasting enhancement of corticostriatal neurotransmission by taurine

Taurine occurs at high concentrations in the forebrain and its distribution varies with (patho)physiological conditions; however, its role in neural function is poorly understood. We have now characterized its effects on corticostriatal synaptic transmission. Bath application of taurine (10 mm) to slices obtained from mice and rats exerted a biphasic action on corticostriatal field potentials. The fast and reversible inhibition by taurine was accompanied by a depolarization and conductance increase in medium spiny neurons and was sensitive to gamma-aminobutyric acid (GABA)A and glycine receptor (GlyR) antagonists. A long-lasting enhancement (LLETAU) of field potentials was recorded after taurine withdrawal. The LLETAU was not prevented by N-methyl-d-aspartate (NMDA)- or by GABAA receptor-antagonists, but was sensitive to the GlyR-antagonist strychnine and blocked by the competitive taurine uptake inhibitor guanidinoethylsulphonate (GES, 1 mm). GES at 10 mm evoked an enhancement of field potentials similar to LLETAU. LLETAU depended on protein kinase C activation as it was blocked by chelerythrine, but was unaffected by trifluoperazine, and thus independent of calmodulin. LLETAU was significantly smaller in juvenile than in mature rodents. Activation of GlyRs and the specific taurine transporter by taurine evoke a long-lasting enhancement of corticostriatal transmission.

Orexin/hypocretin excites the histaminergic neurons of the tuberomammillary nucleus

The hypothalamic orexin (hypocretin) neuropeptides are associated with the regulation of sleep and feeding, and disturbances in orexinergic neurotransmission lead to a narcoleptic phenotype. Histamine has also been shown to play a role in the regulation of sleep and feeding. Therefore, we studied the relationship between the orexin and histamine systems of the CNS using electrophysiology, immunocytochemistry, and the reverse transcriptase (RT)-PCR method. Both orexin-A and orexin-B depolarized the histaminergic tuberomammillary neurons and increased their firing rate via an action on postsynaptic receptors. The depolarization was associated with a small decrease in input resistance and was likely caused by activation of both the electrogenic Na(+)/Ca(2+) exchanger and a Ca(2+) current. In a single-cell RT-PCR study using primers for the two orexin receptors, we found that most tuberomammillary neurons express both receptors and that the expression of the orexin-2 receptor is stronger than that of the orexin-1 receptor. Immunocytochemical studies show that the histamine and orexin neurons are often located very close to each other. The contacts between these two types of neurons seem to be reciprocal, because the orexin neurons are heavily innervated by histaminergic axons. These results suggest a functional connection between the two populations of hypothalamic neurons and that they may cooperate in the regulation of rapid-eye-movement sleep and feeding.

The mechanism of spontaneous firing in histamine neurons

Histaminergic neurons project to virtually the whole central nervous system and display regular firing related to behavioral state. Electrophysiological studies of histaminergic neurons show that these neurons fire in a beating pacemaker pattern, which is intrinsic to individual neurons. Onset of an action potential occurs as the result of a slow depolarizing potential, which consists of voltage dependent calcium current(s) and non-inactivating sodium current. The calcium component is a voltage-dependent current activated by the return to threshold following the afterhyperpolarization (AHP) while the sodium current appears to be persistent. The action potential is followed by an AHP, which limits firing rate. The AHP is due to two potassium currents, one voltage-, the other calcium-dependent; it determines the amount of voltage-dependent currents available for activation. We show original results indicating that calcium current can be activated during AHP-like ramps and that the amount of calcium current near threshold is strongly dependent on the membrane potential and on the size of the AHP. The amount of calcium entering during the action potential will determine the duration of the AHP and thus, the firing rate.

Expression and function of glycine receptors in striatal cholinergic interneurons from rat and mouse

Although glycine receptors are widely expressed in the forebrain their function is obscure. We studied their activation by two possible endogenous ligands, glycine and taurine, and demonstrate a different expression pattern of glycine receptors in neostriatal cholinergic interneurons from two rodent species. Single-cell-reverse transcription-polymerase chain reaction analysis of glycine receptor-subunit expression was combined with whole-cell recordings from acutely isolated cholinergic interneurons. All cells expressed the alpha2-glycine receptor subunit, the majority (72%) in mice but none in young and aged rats expressed the alpha3-subunit. The beta-subunit expression was associated with both a higher efficacy and a higher potency of the partial agonist taurine. Cells expressing the alpha3-subunit displayed a slower desensitization of taurine responses than of glycine responses, in contrast to cells expressing the alpha2-, beta-subunits where desensitization time constants were similar. Glycine responses were reduced by preapplication of taurine; this effect was more pronounced in cells lacking the alpha3-subunit. We demonstrate interspecies differences and heterogeneity in expression and function of glycine receptors within the same neuronal population in the neostriatum.

Defective hippocampal mossy fiber long-term potentiation in endothelial nitric oxide synthase knockout mice

Mossy fiber long-term potentiation (mfLTP) was compared in hippocampal slices prepared from wild-type mice and mice lacking functional endothelial nitric oxide synthase (eNOS(-/-) mice) using field potential recording. In the presence of D-2-amino-5-phosphonovaleric acid (AP5, 50 microM), the mfLTP induced by tetanic stimulation (100 Hz, 1 sec) was significantly reduced in knockouts (n = 8) in comparison with wild-type (n = 8). Similarly, potentiation induced by forskolin (30 microM) or 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP, 100 microM) was less pronounced in knockouts. However, in wild-types the mfLTP-induced in the presence of the nonselective pharmacological inhibitor of NOS (N-nitro-L-Arginine, 100 microM, n = 6) was not significantly different from control (n = 8). Thus, eNOS is not directly involved in mfLTP, but lack of eNOS during development leads to a deficit downstream of adenylyl cyclase.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Glycine receptor mediated responses in rat histaminergic neurons

Patch-clamp whole-cell recordings were made in the hypothalamic tuberomammillary (TM) nucleus from isolated histaminergic neurons, identified by their expression of histidine decarboxylase. We compared strychnine-sensitive glycine-mediated currents with maximal currents activated by gamma-Aminobutyric acid (GABA, 0.5 mM) which were blocked by gabazine. The maximal glycine response (1 mM) in histaminergic cells with larger somata (25 microm) was about half of the maximal GABA response whereas in the cells with a smaller soma size (19.5 microm) the glycine response was absent or very small. We conclude that histaminergic cells are heterogeneous with respect to their sensitivity to glycine and this correlates with their size.

Orexin A excites serotonergic neurons in the dorsal raphe nucleus of the rat

Orexin A (10-300 nM) strongly excited dorsal raphe serotonergic neurons maintained in vitro. The depolarization persisted in the presence of tetrodotoxin (TTX, 0.5 microM) and was associated with an increase in input resistance. These results have relevance in the context of food intake regulation and the disease, narcolepsy.

Serotonin excites tuberomammillary neurons by activation of Na(+)/Ca(2+)-exchange

We have studied the effects of serotonin on the histaminergic neurons in the hypothalamic tuberomammillary nucleus. Intracellular recordings of the membrane potential were made with sharp electrodes from superfused rat hypothalamic slices. We found that serotonin increased the firing rate of the neurons to 224% of the control rate and depolarized them dose-dependently. Insensitivity to tetrodotoxin indicated a postsynaptic effect, which was unrelated to any conductance change. The involved receptor appeared to be a 5-HT2C receptor. The depolarization was strongly dependent on temperature and replacement of extracellular Na(+) with Li(+) or with N-methyl-D-glucamine suppressed the depolarization. Pretreatment with Ni(2+), 2',4'-dichlorobenzamil or KB-R7943 strongly attenuated the effect. These features indicate that the depolarization is the result of activation of an electrogenic Na(+)/Ca(2+)-exchanger which leads to an net inward current. These results support the view that the Na(+)/Ca(2+)-exchanger can play a role in determining the excitability of neurons. The results also provide a functional connection between two transmitter systems, the histaminergic and serotonergic, which modulate many physiological functions in the brain.

Histamine H(3) receptors depress synaptic transmission in the corticostriatal pathway

The effect of histamine on the main input to the striatum - the corticostriatal pathway - was studied using electrophysiological techniques in brain slices from rats and mice. Field potentials (FPs) were recorded in the striatum following stimulation at the border of the striatum and the cortex. Bath application of histamine caused a pronounced and long-lasting depression of FPs in rat slices with an IC(50) of 1.6 microM and a maximal depression of around 40%. In mouse slices histamine also depressed FPs, but to a lesser extent and more transiently. Further experiments in rat slices showed that histamine H(3) receptors were responsible for this depression since the selective H(3) receptor agonist R-alpha-methylhistamine (1 microM) mimicked the action of histamine whilst the selective H(3) receptor antagonist, thioperamide (10 microM) blocked the depression caused by histamine application. The histaminergic depression was probably not mediated indirectly through interneurons since blockade of GABA(A), GABA(B), nicotinic and muscarinic receptors or nitric oxide synthase did not prevent the histamine effect. Intracellular recordings from medium spiny neurons in the striatum revealed that histamine did not affect postsynaptic membrane properties but increased paired-pulse facilitation of excitatory synaptic responses indicating a presynaptic locus of action.

Functions of neuronal adenosine receptors

Endogenous adenosine in nervous tissue, a central link between energy metabolism and neuronal activity, varies according to behavioral state and (patho)physiological conditions, it may be the major sleep propensity substance. The functional consequences of activation of the four known adenosine receptors, A1, A2A, A2B and A3, are considered here. The mechanisms and electrophysiological actions, mainly those of the A1-receptor, have been extensively studied using in vitro brain-slice preparations. A1-receptor activation inhibits many neurons postsynaptically by inducing or modulating ionic currents and presynaptically by reducing transmitter release. A1-receptors are almost ubiquitous in the brain and affect various K+ (Ileak, IAHP), mixed cationic (Ih), or Ca2+ currents, through activation of Gi/o-proteins (coupled to ion channels, adenylyl cyclase or phospholipases). A2A-receptors are much more localized, their functional role in the striatum is only just emerging. A2B- and A3-receptors may be affected in pathophysiological events, their function is not yet clear. The cAMP-PKA signal cascade plays a central role in the regulation of both neural activity and energy metabolism. Under conditions of increased demand and decreased availability of energy (such as hypoxia, hypoglycemia and/or excessive neuronal activity), adenosine provides a powerful protective feedback mechanism. Interaction with adenosine metabolism is a promising target for therapeutic intervention in neurological and psychiatric diseases such as epilepsy, sleep, movement (parkinsonism or Huntington's disease) or psychiatric disorders (Alzheimer's disease, depression, schizophrenia or addiction).

Opposite modulation of histaminergic neurons by nociceptin and morphine

We have studied the effects of nociceptin/orphanin FQ on the histaminergic neurons in the tuberomammillary (TM) nucleus and compared them with the actions of opioid agonists. Intracellular recordings of the membrane potential were made with sharp electrodes from superfused rat hypothalamic slices. Nociceptin strongly inhibited the firing of the TM neurons. In the concentration range 10-300 nM, nociceptin hyperpolarized the neurons in a dose-dependent and reversible manner. Insensitivity to tetrodotoxin indicated a postsynaptic effect which was associated with decreased input resistance. Voltage-current plots suggested the involvement of a potassium conductance which was highly sensitive to Ba(2+) and decreased by Cs(+), in keeping with the activation of an inwardly rectifying potassium channel. Morphine (20-100 microM) depolarized the TM neurons and increased their firing, and this effect was blocked by tetrodotoxin. Dynorphin A(1-13) at 100-300 nM did not affect the TM neurons. Nociceptin and morphine modulate the activity of the TM neurons, and most likely histamine release, in opposite ways. Histamine has an antinociceptive effect in the brain and may be involved in opioid-induced analgesia. Nociceptin might therefore influence pain transmission by inhibiting opioid-induced histamine release from the TM nucleus and also modulate other physiological mechanisms which have been ascribed to the histaminergic system.

Differential modulation of AMPA receptors by cyclothiazide in two types of striatal neurons

The modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazol-propionate (AMPA) receptor-mediated currents by cyclothiazide was investigated in acutely isolated cells from rat striatum with whole-cell patch-clamp recording. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) was used to identify medium spiny and giant aspiny neurons and to determine their AMPA receptor subunit composition mostly in separate experiments. After pretreatment with cyclothiazide, kainate-induced AMPA responses were more strongly potentiated in medium spiny than in giant aspiny neurons; cyclothiazide induced a ninefold leftward shift in the kainate concentration-response curve for medium spiny neurons (not giant aspiny neurons). The EC50s for the cyclothiazide potentiation did not differ substantially between medium spiny neurons and giant aspiny neurons. The recovery of kainate-activated currents from modulation by cyclothiazide was slower for medium spiny neurons than for giant aspiny neurons. Medium spiny neurons expressed GluR-A, GluR-B and GluR-C, but not GluR-D subunits in both flip and flop splice variants. All giant aspiny neurons expressed GluR-A and GluR-D, exclusively in the flop form, half of them also expressed GluR-B and GluR-C. This is in keeping with slow and fast desensitization kinetics in medium spiny neurons and giant aspiny neurons, respectively, and differences in cyclothiazide modulation. The rate of cyclothiazide dissociation from the AMPA receptor, activated by glutamate, was approximately 90 times slower in medium spiny neurons than in giant aspiny neurons. In giant aspiny neurons (not medium spiny neurons) this rate was strongly dependent on the presence of an agonist; 1 mM glutamate increased it 30-fold. Thus, two major cell groups in the striatum display distinct AMPA receptor compositions carrying specific properties of glutamate responses. Excitatory transmission will thus be differentially affected by cyclothiazide-type compounds.

Mice deficient in endothelial nitric oxide synthase exhibit a selective deficit in hippocampal long-term potentiation

Long-term potentiation, a persistent increase in synaptic efficacy, may require a retrograde signal originating in the postsynaptic cell that induces an increase in presynaptic neurotransmitter release. We have constructed a mouse homozygous for a targeted null mutation in the endothelial isoform of nitric oxide synthase and report that long-term potentiation in the CA1 region of these mice is entirely absent under weak stimulation conditions. Application of a membrane-permeant guanosine-3',5'-cyclic monophosphate analogue during tetanus fails to compensate for this deficit, suggesting that nitric oxide produced by endothelial nitric oxide synthase may affect long-term potentiation through a cascade that does not include guanylyl cyclase. We also report that strong tetanic stimulation can induce robust long-term potentiation in these mice which is not blocked by pharmacological inhibitors of nitric oxide synthase. Furthermore, mice lacking endothelial nitric oxide synthase show no shift in the frequency-response curve for the induction of long-term potentiation. Basal synaptic transmission, paired-pulse facilitation and the electrical properties of CA1 cells in these mice were similar to controls. These results support a selective role for endothelial nitric oxide synthase in long-term potentiation, but also demonstrate that nitric oxide synthase is not involved in this process under all conditions.

GABAB-receptor-mediated control of GABAergic inhibition in rat histaminergic neurons in vitro

The onset of slow wave sleep may require an inhibition of histaminergic neurons by GABAergic afferents from the ventrolateral preoptic area. We have utilized electrophysiological methods in an in vitro brain slice preparation to examine the role of GABAB receptor activation in GABAergic synaptic inhibition in histaminergic neurons of the tuberomammillary nucleus. Tetrodotoxin blocked evoked GABAergic IPSPs but not miniature IPSPs or IPSCs. Evoked IPSPs varied in amplitude and exhibited failures of transmission. Baclofen reduced the amplitude of evoked IPSPs in all experiments and often caused an increase in failures of transmission. Responses elicited by application of exogenous GABA were insensitive to baclofen treatment. The action of baclofen was blocked by CGP-35348 (100 microm), a GABAB receptor antagonist, which also enhanced the amplitude of evoked IPSPs. The frequency of spontaneous and miniature IPSPs and IPSCs was reduced by baclofen. However, the amplitude distribution of mIPSCs was not altered. We conclude that GABA release onto TM neurons is under presynaptic control via GABAB receptors. This presynaptic control of transmission to tuberomammillary neurons may reduce inhibition, increasing histamine release and enhancing wakefulness.

Long-term suppression of synaptic transmission by tetanization of a single pyramidal cell in the mouse hippocampus in vitro

1. The consequences of stimulating a single pyramidal cell in the CA1 area of the hippocampus for synaptic transmission in the stratum radiatum were investigated. 2. Tetanic activation of single pyramids caused by depolarizing current injection, but not an equal number of distributed action potentials, reduced excitatory transmission by 20 %, with a delayed onset, for more than 1 h. 3. EPSPs in the tetanized pyramidal cells were increased for equally long periods but this was not the cause of the field EPSP reduction. Spontaneous somatic IPSPs were not affected; evoked IPSPs were decreased in the tetanized cell. 4. Paired pulse facilitation of the field EPSPs was unchanged. 5. The field EPSP reduction was markedly diminished by a knife cut along the base of pyramidal cells in CA1. 6. The addition of antagonists of GABA, NMDA and metabotropic glutamate receptors blocked or diminished the field EPSP slope reduction evoked by intracellular stimulation. 7. Simultaneous recordings revealed long-lasting excitations of interneurons located in the outer oriens layer as a result of single pyramid tetanization. 8. Intense firing of small numbers of pyramidal cells can thus persistently inhibit mass transmission through the hippocampus. This effect involves activation of interneurons by glutamate receptors.

On the mechanism of histaminergic inhibition of glutamate release in the rat dentate gyrus

1. Histaminergic depression of excitatory synaptic transmission in the rat dentate gyrus was investigated using extracellular and whole-cell patch-clamp recording techniques in vitro. 2. Application of histamine (10 microM, 5 min) depressed synaptic transmission in the dentate gyrus for 1 h. This depression was blocked by the selective antagonist of histamine H3 receptors, thioperamide (10 microM). 3. The magnitude of the depression caused by histamine was inversely related to the extracellular Ca2+ concentration. Application of the N-type calcium channel blocker omega-conotoxin (0. 5 or 1 microM) or the P/Q-type calcium channel blocker omega-agatoxin (800 nM) did not prevent depression of synaptic transmission by histamine. 4. The potassium channel blocker 4-aminopyridine (4-AP, 100 microM) enhanced synaptic transmission and reduced the depressant effect of histamine (10 microM). 4-AP reduced the effect of histamine more in 2 mM extracellular calcium than in 4 mM extracellular calcium. 5. Histamine (10 microM) did not affect the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and had only a small effect on their frequency. 6. Histaminergic depression was not blocked by an inhibitor of serine/threonine protein kinases, H7 (100 microM), or by an inhibitor of tyrosine kinases, Lavendustin A (10 microM). 7. Application of adenosine (20 microM) or the adenosine A1 agonist N6-cyclopentyladenosine (CPA, 0.3 microM) completely occluded the effect of histamine (10 microM). 8. We conclude that histamine, acting on histamine H3 receptors, inhibits glutamate release by inhibiting presynaptic calcium entry, via a direct G-protein-mediated inhibition of multiple calcium channels. Histamine H3 receptors and adenosine A1 receptors act upon a common final effector to cause presynaptic inhibition.

Impairment of neocortical long-term potentiation in mice deficient of endothelial nitric oxide synthase

The role of the possible retrograde messenger nitric oxide (NO) in the induction of long-term potentiation (LTP) was studied in supragranular layers of somatosensory cortical slices obtained from adult mice. High-frequency stimulation produced a slowly rising, long-lasting (50 min) and significant (P < 0.001) increase in the extracellular synaptic response by 23%. The induction of LTP was independent from activation of N-methyl-D-aspartate (NMDA) receptors, but prevented by bath application of NG-nitro-L-arginine methyl ester (L-NAME), indicating that one or several of the different NO synthases (NOS) produced NO within the postsynaptic neuron. No LTP could be induced in knockout mice lacking the endothelial NOS (eNOS) isoform. These data suggest that eNOS is involved in an NMDA receptor-independent form of LTP in the rodent cerebral cortex.

Differential modulation of hippocampal signal transfer by tuberomammillary nucleus stimulation in freely moving rats dependent on behavioral state

Tuberomammillary histamine neurons (TM) in the posterior hypothalamus project to extensive parts of the brain, including the hippocampal formation. The purpose of the present experiments was to investigate whether activation of the TM modulates signal transfer from the perforant pathway (PP) or ventral hippocampal commissure (VHC) to the dentate gyrus (DG) in freely moving rats. Paired pulses of electrical stimulation were delivered to PP or VHC, and evoked field potentials (fEPSPs and pop spikes) were recorded in the DG. Before activating PP or VHC, the TM was triggered by electrical stimulation. Experimentation was performed during four behavioral conditions: exploration, grooming, awake immobility, and slow-wave sleep. Electrical activation of the TM was found to modify dentate fEPSPs evoked by PP or VHC stimulation without generating a field potential by itself. Train stimulation of the TM (100 Hz, 500 ms) preceding paired pulses on the hippocampus by 50 ms decreased dentate fEPSPs in dependence of the ongoing behavior and the pathway stimulated. During exploration but not consummatory behavior, the PP signal was reduced when preceded by TM stimulation; during consummatory behavior but not exploration, the VHC signal was reduced. In contrast to other hippocampal afferents which increase pop spikes but leave fEPSPs unchanged, TM stimulation decreased dentate fEPSPs without affecting pop-spike activity. Thus, the TM-histaminergic system seems to modulate signal processing in the dentate gyrus in a specific way, exerting an inhibitory action on the entorhinal input only during learning-related exploratory behavior.

Facilitation of learning after lesions of the tuberomammillary nucleus region in adult and aged rats

The tuberomammillary nucleus (TM) located in the posterior part of the hypothalamus is the main source of neuronal histamine in the central nervous system. Recent work from our laboratories has indicated an involvement of the TM region in neuronal plasticity and reinforcement processes. In the present study, we investigated the effects of TM lesions on the performance of adult and aged Wistar rats in a set of learning tasks, which differed in terms of complexity and reward contingencies (habituation learning, inhibitory avoidance, discrimination learning, Morris water maze). An improvement was found in every test applied, indicating that TM lesions seem to generally enhance learning and memory capacities independent of the special demands of a given task. Age-related learning deficits were strongly diminished. Immunohistochemistry revealed that the excitotoxic lesions used to destroy the TM region led to a marked decrease in the number of histamine-positive neurons in the vicinity of the injection site, indicating an involvement of the brain histaminergic system in the observed behavioral changes.

High-affinity copper block of GABA(A) receptor-mediated currents in acutely isolated cerebellar Purkinje cells of the rat

The actions of Cu2+ ions on GABAA receptor-mediated currents in acutely isolated Purkinje cells from rat cerebellum were studied using the whole-cell patch-clamp technique and a rapid perfusion system. Bath application of Cu2+ reduced currents induced by 2 microM gamma-aminobutyric acid (GABA) in a concentration-dependent manner with an IC50 of 35 nM. The Cu2+-induced block of GABA responses was not voltage-dependent. Increasing the GABA concentration (from 2 to 50 microM) decreased the blocking effect of Cu2+. Dose-response analysis for activation of GABAA receptors revealed a twofold decrease in apparent affinity for GABA in the presence of 0.1 microM Cu2+. Recovery from the block required several minutes after removal of Cu2+ from the medium. The block was removed by histidine, which preferentially forms complexes with Cu2+, or by other chelating substances. Application of 10 microM histidine immediately before application of 2 microM GABA completely relieved the block of GABA responses produced by 0.1 microM Cu2+. The effect of histidine was concentration-dependent with an EC50 of 0.75 microM. The results demonstrate that Cu2+ is a potent inhibitor of GABA-evoked responses in rat Purkinje cells. Copper may be an endogenous synaptic modulating factor. Cu2+ toxicity, notably in Wilson's disease, could result to some extent from chronic GABAA receptor blockade.

Suppression of long-term potentiation in hippocampal slices by copper

Cu(2+)-ions are known to interfere with gamma-aminobutyric acid (GABA)- and glutamate-operated ion channels from experiments with isolated neurons. Such actions are likely involved in the pathophysiology of Wilson's disease. We have now studied the effects of Cu2+ in the CA1 region of hippocampal slices. Field excitatory postsynaptic potential (EPSP) slopes in the CA1 region were unaffected by 1 microM Cu2+ but were depressed by 10 microM (to 85%) and 100 microM (to 50%). A paired-pulse test revealed no difference in facilitation in the presence or absence of Cu2+, indicating a postsynaptic action. A late component of intracellularly registered EPSPs in Mg(2+)-free solution was also reduced by Cu2+. The N-methyl-D-aspartate (NMDA) component of the field EPSP, isolated by adding CNQX and bicuculline in Mg(2+)-free solution, was reduced to 69% of control by 1 microM and to 50% of control by 10 microM Cu2+. Long-term potentiation, evoked by 3 x 50 pulses at 100 Hz, 20 s interval amounted to 132 +/- 11% 90 min after tetanization under control conditions but was absent in the presence of 1 microM Cu2+ in the bath. Thus low concentrations of copper can selectively reduce NMDA-mediated potentials and synaptic plasticity.

Histamine increases the bursting activity of pyramidal cells in the CA3 region of mouse hippocampus

An excitatory action of histamine was investigated by intracellular recording in the CA3 region of hippocampal slices. Bath application of histamine or impromidine, a H2 receptor agonist, had the following effects: (1) a depolarisation in 60% and no changes in membrane potential in 40% of the CA3 pyramids; (2) single cell firing and burst activity were evoked or more than doubled when spontaneously present; (3) the bursts were prolonged and often followed by afterdischarges instead of the normal afterhyperpolarisations (AHPs); (4) synaptic stimulation evoked large bursts instead of excitatory synaptic potentials (EPSPs) and primary burst responses became prolonged. CA3 bursts may play a decisive role in memory trace formation, their facilitation and potentiation is in keeping with a positive role of the histaminergic system in attention and learning.

Adenosine A1 receptor-mediated depression of corticostriatal and thalamostriatal glutamatergic synaptic potentials in vitro

Electrophysiological recordings in rat brain slices have been used to study the actions of adenosine on striatal neurons and striatal excitatory amino acid neurotransmission originating in the cortex or the thalamus. Adenosine had no effects on membrane properties of striatal neurons. Adenosine and the A1 agonist N6-Cyclopentyl adenosine reduced EPSPs of both cortical and thalamic origin by more than 50%. Depression of EPSPs was associated with an increase in paired-pulse facilitation, suggesting a presynaptic locus of action. EPSP depression was blocked by the A1 antagonist, 8-Cyclopentyl-1,3-dipropyl xanthine. The A2 agonist 5'-(N-cyclopropyl)-carboxamidoadenosine had no effect on excitatory amino acid neurotransmission. The A1 antagonist alone enhanced the synaptic component of the evoked field potential (23 +/- 12%). These results indicate that endogenous adenosine, acting via A1 receptors, limits striatal glutamatergic neurotransmission, serving a modulatory and neuroprotective role.

Histaminergic system in co-cultures of hippocampus and posterior hypothalamus: a morphological and electrophysiological study in the rat

Neurons of the tuberomammillary nucleus in the posterior hypothalamus diffusely project to most parts of the central nervous system, where their main transmitter, histamine, modulates the excitability of the target neurons. The development of a histaminergic hypothalamo-hippocampal pathway and its function were studied in organotypic co-cultures. Immunocytochemistry for histidine decarboxylase, the specific synthesizing enzyme, stained clusters of neurons in the hypothalamic tuberomammillary area. Immunolabelled varicose processes innervated the co-cultured hippocampus and established a few synaptic contacts on dendrites. Cultured tuberomammillary neurons displayed their typical membrane properties and were spontaneously active. In hippocampal pyramidal cells of the CA3 region the long-lasting afterhyperpolarization was reduced by histamine or impromidine and increased by the H2 antagonist cimetidine, but not by the H1 antagonist mepyramine. The membrane potential was depolarized in presence of an H2 agonist and hyperpolarized by an H2 antagonist. In single hippocampal cultures histamine antagonists did not affect afterhyperpolarization and membrane potential. Histaminergic neurons retain their main morphological and physiological characteristics in slice cultures and establish a functional connection with co-cultured target cells.