Lithium treatment blocks long-term synaptic depression in the striatum

Case Studies in Neuroscience: Lack of inhibitory synaptic plasticity in the substantia nigra pars reticulata of a patient with lithium-induced tremor

Tremor is a well-known side effect from many psychiatric medications, including lithium and dopamine antagonists. In patients whose psychiatric symptoms are stabilized and only respond to certain medications, deep brain stimulation may offer relief of the consequent motor complications. We report the case of an elderly male with disabling tremor related to lithium therapy for bipolar affective disorder, who was subsequently treated with deep brain stimulation. In this patient, we obtained recordings from the substantia nigra pars reticulata and performed a high-frequency stimulation protocol that robustly elicits long-term potentiation (LTP)-like changes in patients with Parkinson's disease. We hypothesized that in this patient, who did not have Parkinson's disease, the levels of inhibitory plasticity would be much greater. However, we found an unanticipated lack of plasticity in the patient with lithium-induced tremor, compared with two de novo control patients with Parkinson's disease. This patient was successfully treated with deep brain stimulation in the vicinity of the ventral oral posterior nucleus, an area of the thalamus that receives inputs from the basal ganglia. We postulate that the lithium-induced blockade of LTP may bring about motor complications such as tremor while simultaneously contributing to the therapeutic mechanism for treating the symptoms of psychiatric disorders such as bipolar affective disorder.NEW & NOTEWORTHY Use of a dual-microelectrode technique enabled us to compare long-term potentiation (LTP)-like changes in a patient with lithium-induced tremor to that of patients with Parkinson's disease. This study corroborated the findings in rodent brain slices that chronic lithium treatment may block LTP. Whereas a deficit in LTP may underlie the therapeutic mechanism for treating psychiatric disorders such as bipolar affective disorder, it may simultaneously contribute to consequent appearance of tremor.

Dietary Zinc Supplementation Prevents Autism Related Behaviors and Striatal Synaptic Dysfunction in Shank3 Exon 13-16 Mutant Mice

The SHANK family of synaptic proteins (SHANK1–3) are master regulators of the organizational structure of excitatory synapses in the brain. Mutations in SHANK1–3 are prevalent in patients with autism spectrum disorders (ASD), and loss of one copy of SHANK3 causes Phelan-McDermid Syndrome, a syndrome in which Autism occurs in >80% of cases. The synaptic stability of SHANK3 is highly regulated by zinc, driving the formation of postsynaptic protein complexes and increases in excitatory synaptic strength. As ASD-associated SHANK3 mutations retain responsiveness to zinc, here we investigated how increasing levels of dietary zinc could alter behavioral and synaptic deficits that occur with ASD. We performed behavioral testing together with cortico-striatal slice electrophysiology on a Shank3^−/− mouse model of ASD (Shank3^{ex13–1616−/−}), which displays ASD-related behaviors and structural and functional deficits at striatal synapses. We observed that 6 weeks of dietary zinc supplementation in Shank3^{ex13–16−/−} mice prevented ASD-related repetitive and anxiety behaviors and deficits in social novelty recognition. Dietary zinc supplementation also increased the recruitment of zinc sensitive SHANK2 to synapses, reduced synaptic transmission specifically through N-methyl-D-aspartate (NMDA)-type glutamate receptors, reversed the slowed decay tau of NMDA receptor (NMDAR)-mediated currents and occluded long term potentiation (LTP) at cortico-striatal synapses. These data suggest that alterations in NMDAR function underlie the lack of NMDAR-dependent cortico-striatal LTP and contribute to the reversal of ASD-related behaviors such as compulsive grooming. Our data reveal that dietary zinc alters neurological function from synapses to behavior, and identifies dietary zinc as a potential therapeutic agent in ASD.

A network pharmacology-based strategy deciphers the underlying molecular mechanisms of Qixuehe Capsule in the treatment of menstrual disorders

BACKGROUND

QiXueHe Capsule (QXHC) is a Chinese patent drug that is extensively used for the treatment of menstrual disorders. However, its underlying pharmacological mechanisms have not been fully elucidated.

METHODS

A list of QXHC putative targets were predicted using MetaDrug. An interaction network using links between QXHC putative targets and the known therapeutic targets of menstrual disorders was constructed. QXHC candidate targets were also identified via calculating the topological feature values of nodes in the network. Additionally, molecular docking simulation was performed to determine the binding efficiency of QXHC compound-putative target pairs.

RESULTS

A total of 1022 putative targets were predicted for 311 chemical components containing in QXHC. Following the calculation of topological features of QXHC putative target-known therapeutic target of menstrual disorder network, 66 QXHC candidate targets for the treatment of menstrual disorders were identified. Functionally, QXHC candidate targets were significantly associated with several biological pathways, such as VEGF and Chemokine signaling pathways, Alanine/aspartate/glutamate metabolism, Long-term depression and T/B cell receptor signaling pathway. Moreover, molecular docking simulation demonstrated that there were 20 pairs of QXHC chemical component-candidate target had the strong binding free energy.

CONCLUSIONS

This novel and scientific network pharmacology-based study holistically deciphers that the pharmacological mechanisms of QXHC in the treatment of menstrual disorders may be associated with its involvement into hemopoiesis, analgesia, nutrients absorption and metabolism, mood regulation, as well as immune modulation.

Modifications of excitatory and inhibitory transmission in rat hippocampal pyramidal neurons by acute lithium treatment

The acute effects of high-dose Li(+) treatment on glutamatergic and GABAergic transmissions were studied in the "synaptic bouton" preparation of isolated rat hippocampal pyramidal neurons by using focal electrical stimulation. Both action potential-dependent glutamatergic excitatory and GABAergic inhibitory postsynaptic currents (eEPSC and eIPSC, respectively) were dose-dependently inhibited in the external media containing 30-150 mM Li(+), but the sensitivity for Li(+) was greater tendency for eEPSCs than for eIPSCs. When the effects of Li(+) on glutamate or GABAA receptor-mediated whole-cell responses (IGlu and IGABA) elicited by an exogenous application of glutamate or GABA were examined in the postsynaptic soma membrane of CA3 neurons, Li(+) slightly inhibited both IGlu and IGABA at the 150 mM Li(+) concentration. Present results suggest that acute treatment with high concentrations of Li(+) acts preferentially on presynaptic terminals, and that the Li(+)-induced inhibition may be greater for excitatory than for inhibitory transmission.

Enhancing effects of chronic lithium treatment on detour learning in chicks

Lithium is the first line of therapeutic drugs used to treat both mania and depression in bipolar disorder.Although a body of research suggests that lithium acts as a cognitive enhancer, other animal studies suggest that lithium induces cognitive deficits. Comparatively, the effects of lithium on cognitive behaviour in these studies are inconsistent and contradictory. Further investigations in different species of animals and behavioural tasks are important to evaluate the possibility that lithium may act as a cognitive enhancer. In the present study, the chicks were treated intraperitoneally with lithium chloride (120 mg/kg), and the effects of chronic lithium treatment on chick cognitive behaviour were examined using a detour learning task.Additionally, the effects of chronic lithium treatment on BDNF messenger RNA (mRNA) expression were measured in RTPCR. We found that chronic lithium treatment(120 mg/kg) had no effect on spontaneous motor activity or weight gain of the chicks and that the chicks had a general healthy appearance, while chronic lithium treatment significantly promoted the response latency of detour learning and BDNF mRNA expression. These results suggest that chronic lithium treatment may improve cognitive function.

Lithium: a switch from LTD- to LTP-like plasticity in human cortex

Lithium, a simple cation, is the mainstay treatment of bipolar disorder. Deficient synaptic plasticity is considered one important mechanism of this disease. Lithium inhibits glycogen synthase kinase-3beta (GSK-3β), which is involved in the regulation of synaptic plasticity. In animal preparations, inhibition of GSK-3β by lithium up-regulated long-term potentiation (LTP) of excitatory synapses but down-regulated long-term depression (LTD). The effects of lithium on plasticity in the human brain are unexplored. We tested the effects of a single oral dose of 900 mg of lithium on LTP-/LTD-like plasticity in human motor cortex induced by established paired associative transcranial magnetic stimulation (PAS(LTP), PAS(LTD)) protocols. We studied 10 healthy adults in a placebo-controlled double-blind randomized crossover design. PAS-induced plasticity was indexed by change in motor evoked potential amplitude recorded in a hand muscle. In the placebo session, subjects were stratified, according to the known variability of the PAS(LTP) response, into PAS(LTP) 'LTP responders' and PAS(LTP) 'LTD responders' (n = 5 each). Lithium did not affect the PAS(LTP)-induced LTP-like plasticity in the 'LTP responders', but switched the PAS(LTP)-induced LTD-like plasticity in the 'LTD responders' to LTP-like plasticity. In contrast, lithium had no effect on the PAS(LTD)-induced LTD-like plasticity in the 'LTD responders'. We provide first-time evidence that lithium significantly modulates brain stimulation induced plasticity in human cortex. The switch from LTD- to LTP-like plasticity is best explained by the inhibitory action of lithium on GSK-3β. This conclusion is necessarily circumstantial because GSK-3β activity was not directly measured. We discuss that other important plasticity-related modes actions of lithium cannot explain our findings.

Neuroprotective effects of lithium treatment following hypoxic-ischemic brain injury in neonatal rats

PURPOSE

Increasing evidence indicates that lithium is a neuroprotective agent against transient focal and global ischemic injury in the adult animal. In the developing brain, lithium has shown protective effects against neuroapoptosis induced by drugs. This study was designed to investigate the neuroprotective effects of lithium on hypoxic-ischemic brain injury in the neonatal rat.

METHODS

Seven-day-old Sprague-Dawley rats underwent hypoxic-ischemic injury (HII) induced by ligation of the common carotid artery followed by exposure to ~2.5 h of hypoxia (~7% oxygen). After HII, rat pups were randomly assigned into two groups: a control group (n = 21), which received a daily subcutaneous injection of 0.9% normal saline for 14 days following HII; and a lithium group (n = 32), treated with daily injection of lithium chloride. N-acetylaspartate/creatinine, choline/creatinine, lipid/creatinine ratios at 1.3 ppm (Lip(1.3)/Cr) and 0.9 ppm (Lip(0.9)/Cr) lipid peaks were evaluated by proton magnetic resonance spectroscopy on the day of HII and on days 7 and 14 after HII. Infarct ratios based on magnetic resonance images were also determined at the same time points.

RESULTS

Seven days after HII, the Lip(1.3)/Cr and Lip(0.9)/Cr ratios as well as the infarct ratio were significantly lower in the lithium group than in the control group. The Lip(1.3)/Cr and Lip(0.9)/Cr ratios were significantly correlated with infarct ratio.

CONCLUSION

This study showed that post-HII treatment with lithium may have a neuroprotective effect in the immature brain. Further studies are needed to elucidate the mechanism of neuroprotective properties of lithium against HII-induced neonatal brain damage.

Synaptic plasticity in the basal ganglia

Activity-dependent synaptic plasticity occurs in several parts of the basal ganglia. Increasing evidence supports the hypothesis that activity-dependent plasticity underlies the acquisition, maintenance, and extinction of certain types of learning in the basal ganglia. This review focuses on synaptic plasticity in the corticostriatal pathway. As in other systems, both long-term potentiation and long-term depression have been described, and intracellular calcium signalling plays an important role in the induction of plasticity. However, intracellular calcium levels do not appear to be the dominating control factor. Dopamine, via intracellular signalling cascades, also plays a crucial role in determining the magnitude and direction of plasticity, and in modulating the requirements for induction. Endocannabinoids also play an important role in mediating presynaptic expression of synaptic depression. Recent studies have highlighted spike-timing dependent plasticity phenomena, which also involve dopamine and endocannabinoid signalling. Despite significant progress in recent years, many important questions remain unanswered, especially in relation to long-term potentiation. Of particular interest is the question of how to link the molecular and cellular mechanisms of synaptic plasticity to learning operations at the systems level, which are expressed behaviourally as reinforcement-related learning.

Effect of chronic lithium treatment on the turnover of alpha2-adrenoceptors after chemical inactivation in rats

One of the most effective psychotherapeutic agents in the treatment of bipolar disease is lithium. Chronic lithium treatment affects some signal transduction mechanisms such as cAMP, cGMP, inositol 1,4,5 P(3), Gi protein, protein kinase C and can also modify gene expression in rat brain. In a previous study, we observed a greater inhibitory effect of lithium on cAMP production after blockade of alpha(2)-adrenoceptors in rat cerebral cortex. Here we examine the influence of chronic lithium treatment on turnover of alpha(2)-adrenoceptors after their inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in rat cerebral cortex. After treatment with lithium for 10 days (120 mg/kg/day, i.p.), there was a significant increase in the appearance and disappearance rate constants of these adrenoceptors and a significant reduction of their half-life. These results suggest that chronic lithium administration alters the alpha(2)-adrenoceptor turnover in rat brain.

Repeated electroconvulsive stimulation impairs long-term depression in the neostriatum

BACKGROUND

Increasing and consistent findings of structural and functional abnormalities in patients with mood disorders demonstrate a clear involvement of the neostriatum. Therefore, the beneficial effect of electroconvulsive stimulation (ECS) treatment of acute state of mood disorder may relate to changes in striatal synaptic plasticity.

METHODS

We studied the effect of ECS treatment on the reported long-term depression (LTD) of synaptic excitatory afferents to striatal medium spiny neurons. Stimulation of the white matter between the cortex and the striatum elicited excitatory postsynaptic potentials (EPSPs) in medium spiny neurons in rat corticostriatal slices while recording using the whole-cell patch-clamp technique.

RESULTS

The EPSPs evoked in striatal neurons undergo LTD with repeated stimulation trains, and LTD of this pathway is impaired in rats after ECS treatment for 1 week, similar to what is reported in chronic lithium treatment. Electroconvulsive stimulation did not affect intrinsic membrane properties or the occurrence of spontaneous EPSCs. Dose-dependent inhibition of the EPSPs by a nonselective agonist of metabotropic glutamate receptor did not change in rats after ECS treatment.

CONCLUSIONS

Our data suggest that the effectiveness of electroconvulsive therapy in mood disorders may be a consequence of LTD impairment of excitatory cortical afferents to striatal projecting neurons.

Inhibition of mitochondrial complex II induces a long-term potentiation of NMDA-mediated synaptic excitation in the striatum requiring endogenous dopamine

Abnormal involuntary movements and cognitive impairment represent the classical clinical symptoms of Huntington's disease (HD). This genetic disorder involves degeneration of striatal spiny neurons, but not striatal large cholinergic interneurons, and corresponds to a marked decrease in the activity of mitochondrial complex II [succinate dehydrogenase (SD)] in the brains of HD patients. Here we have examined the possibility that SD inhibitors exert their toxic action by increasing glutamatergic transmission. We report that SD inhibitors such as 3-nitroproprionic acid (3-NP), but not an inhibitor of mitochondrial complex I, produce a long-term potentiation of the NMDA-mediated synaptic excitation (3-NP-LTP) in striatal spiny neurons. In contrast, these inhibitors had no effect on excitatory synaptic transmission in striatal cholinergic interneurons and pyramidal cortical neurons. 3-NP-LTP involves increased intracellular calcium and activation of the mitogen-activated protein kinase extracellular signal-regulated kinase and is critically dependent on endogenous dopamine acting via D2 receptors, whereas it is negatively regulated by D1 receptors. Thus 3-NP-LTP might play a key role in the regional and cell type-specific neuronal death observed in HD.

Tissue plasminogen activator controls multiple forms of synaptic plasticity and memory

Induction of long-term depression (LTD) in rat striatal slices revealed that this form of synaptic plasticity is coupled to an increased expression of tissue-plasminogen activator (t-PA) mRNA, as detected by the mRNA differential display technique. To further investigate the involvement of this gene in synaptic remodelling following striatal LTD, we recorded electrical activity from mice lacking the gene encoding t-PA (t-PA-KO) and from wild-type (WT) mice. Tetanic stimulation induced LTD in the large majority of striatal neurons recorded from WT mice. Conversely, LTD was absent in a significant proportion of striatal neurons obtained from mice lacking t-PA. Electrophysiological recordings obtained from hippocampal slices in the CA1 area showed that mainly the late phase of long-term potentiation (LTP) was reduced in t-PA-KO mice. Learning and memory-related behavioural abnormalities were also found in these transgenic mice. Disruption of the t-PA gene, in fact, altered both the context conditioning test, a hippocampus-related behavioural task, and the two-way active avoidance, a striatum-dependent task. In an open field object exploration task, t-PA-KO mice expressed deficits in habituation and reactivity to spatial change that are consistent with an altered hippocampal function. Nevertheless, decreased rearing and poor initial object exploration were also observed, further suggesting an altered striatal function. These data indicate that t-PA plays a critical role in the formation of various forms of synaptic plasticity and memory.

Glutamate-triggered events inducing corticostriatal long-term depression

Repetitive activation of corticostriatal fibers produces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of synaptic plasticity might be considered the possible neural basis of some forms of motor learning and memory. In the present study, intracellular recordings were performed from rat corticostriatal slice preparations to study the role of glutamate and other critical factors underlying striatal LTD. In current-clamp, but not in voltage-clamp experiments, brief focal applications of glutamate, as well as high-frequency stimulation (HFS) of corticostriatal fibers, induced LTD. This pharmacological LTD and the HFS-induced LTD were mutually occlusive, suggesting that both forms of synaptic plasticity share common induction mechanisms. Isolated activation of either non-NMDA-ionotropic glutamate receptors (iGluRs) or metabotropic glutamate receptors (mGluRs), respectively by AMPA and t-ACPD failed to produce significant long-term changes of corticostriatal synaptic transmission. Conversely, LTD was obtained after the simultaneous application of AMPA plus t-ACPD. Moreover, also quisqualate, a compound that activates both iGluRs and group I mGluRs, was able to induce this form of pharmacological LTD. Electrical depolarization of the recorded neurons either alone or in the presence of t-ACPD and dopamine (DA) failed to mimic the effects of the activation of glutamate receptors in inducing LTD. However, electrical depolarization was able to induce LTD when preceded by coadministration of t-ACPD, DA, and a low dose of hydroxylamine, a compound generating nitric oxide (NO) in the tissue. None of these compounds alone produced LTD. Glutamate-induced LTD, as well as the HFS-induced LTD, was blocked by L-sulpiride, a D2 DA receptor antagonist, and by 7-nitroindazole monosodium salt, a NO synthase inhibitor. The present study indicates that four main factors are required to induce corticostriatal LTD: (1) membrane depolarization of the postsynaptic neuron; (2) activation of mGluRs; (3) activation of DA receptors; and (4) release of NO from striatal interneurons.

A critical role of the nitric oxide/cGMP pathway in corticostriatal long-term depression

High-frequency stimulation (HFS) of corticostriatal glutamatergic fibers induces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of LTD can be mimicked by zaprinast, a selective inhibitor of cGMP phosphodiesterases (PDEs). Biochemical analysis shows that most of the striatal cGMP PDE activity is calmodulin-dependent and inhibited by zaprinast. The zaprinast-induced LTD occludes further depression by tetanic stimulation and vice versa. Both forms of synaptic plasticity are blocked by intracellular 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylyl cyclase, indicating that an increased cGMP production in the spiny neuron is a key step. Accordingly, intracellular cGMP, activating protein kinase G (PKG), also induces LTD. Nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and 7-nitroindazole monosodium salt (7-NINA) block LTD induced by either HFS or zaprinast, but not that induced by cGMP. LTD is also induced by the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine. SNAP-induced LTD occludes further depression by HFS or zaprinast, and it is blocked by intracellular ODQ but not by L-NAME. Intracellular application of PKG inhibitors blocks LTD induced by HFS, zaprinast, and SNAP. Electron microscopy immunocytochemistry shows the presence of NOS-positive terminals of striatal interneurons forming synaptic contacts with dendrites of spiny neurons. These findings represent the first demonstration that the NO/cGMP pathway exerts a feed-forward control on the corticostriatal synaptic plasticity.

Synaptic plasticity and physiological interactions between dopamine and glutamate in the striatum

Several electrophysiological studies have addressed the interaction between glutamate and dopamine within the striatum. Although the results obtained from these studies were often conflicting, more recently the characterization of new forms of synaptic plasticity in the basal ganglia provided a possible integrative explanation of the different electrophysiological data regarding the interaction between these transmitters. In this review we will try to summarize and discuss the available data concerning the possible impact of the functional role of D1 and D2 receptor activation on the modulation of the glutamatergic corticostriatal pathway. Moreover, we will also describe the function of the striatum in the integration of glutamatergic and dopaminergic inputs to produce long-term changes of synaptic efficacy (long-term depression, long-term potentiation). Finally, we will consider the implication of the interaction between dopamine and glutamate in the regulation of energetic metabolism whose failure is responsible for neuronal death.

Abnormal synaptic plasticity in the striatum of mice lacking dopamine D2 receptors

Dopamine D2 receptors (D2Rs) are of crucial importance in the striatal processing of motor information received from the cortex. Disruption of the D2R gene function in mice results in a severe locomotor impairment. This phenotype has analogies with Parkinson's disease symptoms. D2R-null mice were used to investigate the role of this receptor in the generation of striatal synaptic plasticity. Tetanic stimulation of corticostriatal fibers produced long-term depression (LTD) of EPSPs in slices from wild-type (WT) mice. Strikingly, recordings from D2R-null mice showed the converse: long-term potentiation (LTP). This LTP, unlike LTD, was blocked by an NMDA receptor antagonist. In magnesium-free medium, LTP was also revealed in WT mice and found to be enhanced by L-sulpiride, a D2R antagonist, whereas it was reversed into LTD by LY 17555, a D2R agonist. In D2R-null mice this modulation was lost. Thus, our study indicates that D2Rs play a key role in mechanisms underlying the direction of long-term changes in synaptic efficacy in the striatum. It also shows that an imbalance between D2R and NMDA receptor activity induces altered synaptic plasticity at corticostriatal synapses. This abnormal synaptic plasticity might cause the movement disorders observed in Parkinson's disease.

On the functions of lithium: the mood stabilizer

Lithium, despite its simple structure, has numerous biological effects. It also has a remarkable therapeutic effect in the prophylactic treatment of manic depression, and is finding a role in controlling aggressive and self-mutilating behavior. The special feature of lithium is that it only acts on overactive systems to bring them back to normal, without affecting the stable system. The mechanisms of action of this simple cation are still largely unknown although the inositol depletion theory is the most widely accepted model. A recent paper described a different molecular mechanism for its effect on development, which may also explain its action in manic depression.

Intraseptal administration of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid induces immediate early gene expression in lateral septal neurons

Prior work has shown that activation of metabotropic glutamate receptors can induce burst firing and a form of NMDA receptor independent long term potentiation in lateral septal slice preparations. To study this phenomenon in vivo we used the expression of immediate early gene products as markers for increased neuronal activity following intraseptal injection of the metabotropic agonist 1S,3R-ACPD. Intraseptal injection of 1S,3R-ACPD induced the expression of Fos-like, Jun B-like and Krox24-like immunoreactivity in lateral septal neurons in a dose-dependent fashion. Immediate early gene product expression peaked at 4 to 6 h post-injection and then declined to baseline. Immediate early gene expression was diminished by co-injection of L-AP3 and was not elicited by intraseptal injection of L-AP4, cysteine sulfinic acid or DHPG. Immediate early gene expression was not diminished by chronic lithium treatment but was diminished by chronic treatment with the phospholipase A(2) inhibitor quinacrine. Co-injection of the phospholipase A(2) inhibitor NDGA partially suppressed the induction of immediate early gene expression. Metabotropic glutamate receptors regulate lateral septal neuron excitability in vivo and some of their effects may be mediated by activation of phospholipase A(2). Alternatively, arachidonic acid may play a permissive role in the effects of metabotropic glutamate receptors on lateral septal neurons.

The corticostriatal projection: from synaptic plasticity to dysfunctions of the basal ganglia

Corticostriatal transmission has an important function in the regulation of the neuronal activity of the basal ganglia. The firing activity of corticostriatal neurones excites striatal cells via the release of glutamate. Presynaptic receptors that are located on corticostriatal terminals and that regulate the release of glutamate in the striatum have been postulated for dopamine and glutamate. Activation of these receptors may exert a negative feed-back on the striatal release of glutamate. High-frequency activation of corticostriatal fibres causes either long-term depression or long-term potentiation of excitatory transmission depending on the subclass of glutamate receptor that is activated. These forms of synaptic plasticity could be involved in motor learning. Alterations in striatal synaptic plasticity might be implicated in Parkinson's disease and Huntington's disease.

Synaptic transmission and modulation in the neostriatum

The neostriatum is the entryway into the basal ganglia and is the site of many of the neurological defects involving basal ganglia function. Thus, it is important to understand the regulation of synaptic transmission at afferent synapses innervating the neostriatum. Cortical glutamatergic and nigral dopaminergic afferent input impinge on neurons in the neostriatum, providing the most significant afferent inputs to this structure. Our understanding of the mechanisms involved in transmission and modulation of transmission at these synapses has greatly increased. It is now apparent that the corticostriatal glutamatergic inputs produce rapid depolarization of striatal neurons via activation of ionotropic AMPA-type glutamate receptors. In addition, transmission is modulated by a number of presynaptic, G-protein-coupled receptors but, surprisingly, relatively little evidence of postsynaptic modulation has been observed. Corticostriatal synapses also express certain forms of plasticity, most notably short- and long- term synaptic depression (STI) and LTD, respectively). It appears that LTD may involve convergent actions of glutamate and dopamine. Striatal LTD may have important roles in information storage and motor set selection in the striatum. However, some aspects of synaptic transmission in the striatum remain unclear. In particular, the exact physiological roles of dopaminergic nigrostriatal input and the role of NMDA-type glutamate receptors are not well understood. In addition, intrastriatal synaptic connections have received relatively little attention as compared with extrinsic input to the neostriatum. Future studies will need to focus on elucidating these aspects of neostriatal function.

Action of GP 47779, the active metabolite of oxcarbazepine, on the corticostriatal system. I. Modulation of corticostriatal synaptic transmission

Oxcarbazepine (OCBZ) is the keto-analogue of carbamazepine (CBZ). In humans, OCBZ is rapidly and almost completely metabolized to 10, 11-dihydro-10-hydroxy-CBZ (GP 47779), the main metabolite responsible for the drug's antiepileptic activity. The corticostriatal pathway is involved in the propagation of epileptic discharges. We characterized the electrophysiological effects of GP 47779 on striatal neurons by making intracellular recordings from corticostriatal slices. GP 47779 (3-100 microM) produced a dose-dependent inhibition of glutamatergic excitatory postsynaptic potentials (EPSPs). This effect was not coupled either with changes of the membrane potential of these cells or with alterations of their postsynaptic sensitivity to excitatory amino acids (EAA) suggesting a presynaptic site of action. GP 47779 reduced the current-evoked firing discharge only at concentrations > 100 microM. GP 47779 did not affect the presynaptic inhibitory action of adenosine, showing that presynaptic adenosine receptors were not implicated in the GP 47779-mediated reduction of corticostriatal EPSPs. Our data indicate that GP 47779 apparently acts directly on corticostriatal terminals to reduce the release of EAA, probably by inhibiting high-voltage-activated (HVA) calcium (Ca2+) currents (described in the accompanying article). The inhibitory action of GP 47779 on corticostriatal transmission may contribute to the antiepileptic effects of this drug.

Transmitter release associated with long-term synaptic depression in rat corticostriatal slices

Using a corticostriatal slice preparation, we have recently shown that tetanic stimulation of the corticostriatal pathway produces long-term depression (LTD) of striatal excitatory synaptic transmission. In the present study we have analysed the relationship between LTD and the striatal release of different endogenous transmitters. Samples of perfusate were collected via a small cannula placed just above the surface of the striatal slice close to the recording electrode, and were analysed by HPLC. The high-frequency stimulation (100 Hz, three trains, 3 s duration, 20 s interval) used to induce LTd caused a significant but transient increase in the release of both excitatory (aspartate and glutamate) and inhibitory (glycine and GABA) amino acid transmitters. Tetanic stimulation also produced a significant, but transient increase in the release of endogenous dopamine. We conclude that the tetanic stimulation of the corticostriatal pathway is able to induce a large but transient release of excitatory amino acids and of dopamine, whose participation in the induction of striatal LTD has been demonstrated previously. Moreover, the maintenance of this form of synaptic plasticity does not seem to require a sustained change in transmitter release.

Neurones in the medullary raphe nuclei attenuate the cardiovascular responses evoked from the dorsolateral periaqueductal grey matter

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsolateral part of the periaqueductal grey matter (PAG; 10 s trains of 1 ms pulses at 80 Hz, 40-80 microA) evoked a pressor response accompanied by tachycardia. Both components of the response were attenuated following microinjection of 200 nl 0.1 M D,L-homocysteic acid into the caudal pole of the nucleus raphe magnus (NRM; n = 12) and into the nucleus raphe obscurus (NRO; n = 22) to selectively activate neuronal perikarya. Microinjection of 200 nl 165 mM NaCl into the same region (n = 15) had no effect. The attenuation of the midbrain-evoked cardiovascular responses lasted for 10-20 min and was independent of changes in resting blood pressure and heart rate. The maximum reduction in the pressor component of the midbrain-evoked responses was similar following stimulation in NRM (-35.4%) and NRO (-36.7%). However, the reduction in the midbrain-evoked tachycardia was greater following stimulation in NRM (-62.8%) compared to NRO (-27.2%). These results indicate that neurones in NRM and NRO may be involved in modulating the level of excitability of the midbrain defence area in the PAG and/or its efferent pathway.

Synaptic plasticity: LTP and LTD

Long-term potentiation (LTP) is a synaptic enhancement that follows brief, high-frequency electrical stimulation in the hippocampus and neocortex. Recent evidence suggests that induction of LTP may require, in addition to postsynaptic Ca2+ entry, activation of metabotropic glutamate receptors and the generation of diffusible intercellular messengers. A new form of synaptic plasticity, homosynaptic long-term depression (LTD) has also recently been documented, which, like LTP, requires Ca2+ entry through the NMDA receptor. Current work suggests that this LTD is a reversal of LTP, and vice versa, and that the mechanisms of LTP and LTD may converge at the level of specific phosphoproteins.