Christiane Mourre

Sulfonylurea binding sites associated with ATP-regulated K+ channels in the central nervous system: autoradiographic analysis of their distribution and ontogenesis, and of their localization in mutant mice cerebellum.

The localization of a putative ATP-regulated K+ channel in normal rat and neurological mutant mice was studied by light microscopic quantitative autoradiography using a tritiated glibenclamide, an antidiabetic sulfonylurea. Glibenclamide binding sites presented a heterogeneous distribution in the rat central nervous system. Their density was particularly important in substantia nigra reticulata, septohippocampal nucleus, globus pallidus, neocortex, molecular layer of cerebellum, CA3 field and dentate gyrus of hippocampus. Conversely hypothalamic areas, medulla oblongata and spinal cord contained only low amounts of glibenclamide receptors. The ontogenesis of sulfonylurea binding sites was a postnatal phenomenon and seemed to correlate with the maturation of neuronal connectivity. In the cerebellum of neurological mutant mice, the autoradiographic patterns were different to that of wild-type cerebellum. In particular, in the molecular layer of weaver cerebellum, a decrease of 82% of binding site density suggested a presynaptic position of glibenclamide receptors in parallel fibers.

Axonal transport of the voltage-dependent Na+ channel protein identified by its tetrodotoxin binding site in rat sciatic nerves.

Na+ channels levels were measured in different segments of rat vagus and sciatic nerves by in vitro binding using a tritiated ethylene-diamine tetrodotoxin derivative ([3H]en-TTX). Binding sites were found to accumulate on both sides of a ligature tied on the sciatic nerve indicating an anterograde and retrograde axoplasmic transport of Na+ channels. Accumulation of Na+ channels at the ligature was time-dependent and appeared to occur through fast axoplasmic transport mechanisms. This accumulation on both sides of a ligature was also visualized by autoradiographic studies in longitudinal sections of sciatic nerves using [3H]en-TTX.

Apamin improves reference memory but not procedural memory in rats by blocking small conductance Ca2+-activated K+ channels in an olfactory discrimination task

Apamin blocks SK channels responsible for long-lasting hyperpolarization following the action potential. Using an olfactory associative task, the effect of an intracerebroventricular 0.3 ng apamin injection was tested on learning and memory. Apamin did not modify the learning of the procedure side of the task or the learning of the odor-reward association. To test reference memory specifically, the rats were trained on a new odor-association problem using the same procedure (acquisition session), and they were tested for retention 24 h later. Apamin injected before or after the acquisition session improved retention of the valence of a new odor pair. Apamin injected before the retention session did not affect the retrieval of the new valence. Thus, the results indicate that the blockage of apamin-sensitive SK channels facilitate consolidation on new-odor-reward association.

Specific hippocampal lesions indicate the presence of sulfonylurea binding sites associated to ATP-sensitive K+ channels both post-synaptically and on mossy fibers

The density of glibenclamide (GB) binding sites decreases considerably in the dentate gyrus (greater than 80%), the CA1 field (approximately 75%) and the stratum lucidum of CA3 field (approximately 70%) after intradentate colchicine injections in rat hippocampus. The density of GB receptors is unchanged in kainic acid damaged hippocampus. These data show that GB binding sites associated to adenosine triphosphate-sensitive K+ channels are mainly located in granular cells in both pre- and post-synaptic positions. They are present in mossy fibers.

Betahistine dihydrochloride interaction with the histaminergic system in the cat: neurochemical and molecular mechanisms

Drugs interfering with the histaminergic system facilitate behavioral recovery after vestibular lesion, likely by increasing histamine turnover and release. The effects of betahistine (structural analogue of histamine) on the histaminergic system were tested by quantifying messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) by in situ hybridization and binding to histamine H(3) receptors (mediating, namely, histamine autoinhibition) using a histamine H(3) receptor agonist ([(3)H]N-alpha-methylhistamine) and radioautography methods. Experiments were done in brain sections of control cats (N=6) and cats treated with betahistine for 1 (N=6) or 3 (N=6) weeks. Betahistine treatment induced symmetrical changes with up-regulation of histidine decarboxylase mRNA in the tuberomammillary nucleus and reduction of [(3)H]N-alpha-methylhistamine labeling in both the tuberomammillary nucleus, the vestibular nuclei complex and nuclei of the inferior olive. These findings suggest that betahistine upregulates histamine turnover and release, very likely by blocking presynaptic histamine H(3) receptors, and induces histamine H(3) receptor downregulation. This action on the histaminergic system could explain the effectiveness of betahistine in the treatment of vertigo and vestibular disease.

A new class of scorpion toxin binding sites related to an A-type K+ channel: pharmacological characterization and localization in rat brain.

A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensi venom, sequenced and chemically synthesized (sBmTX3). The A‐type current of striatum neurons in culture completely disappeared when 1 μM sBmTX3 was applied (K d=54 nM), whereas the sustained K+ current was unaffected. 125I‐sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol mg−1 of protein, K d=0.21 nM). A panel of toxins yet described as specific ligands for K+ channels were unable to compete with 125I‐sBmTX3. A high density of 125I‐sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.

Apamin, a blocker of the calcium-activated potassium channel, induces neurodegeneration of Purkinje cells exclusively

Following acute intracerebroventricular injections of 1 ng of apamin and chronic apamin infusion (0.4 ng/microl, 0.5 microl/h, 14 days), the rat brains exhibited bilateral damage only in the cerebellum. The argyrophilic cells were Purkinje cells in copula pyramis, flocculus, paraflocculus, and paramedian lobules. These data demonstrate that the inactivation of small conductance Ca2+-activated K+ channels by apamin induces a non-limbic neurodegeneration.

Distribution of voltage-dependent Na ÷ channels identified by high-affinity receptors for tetrodotoxin and saxitoxin in rat and human brains: quantitative autoradiographic analysis

The localization of a putative voltage-dependent Na+ channel in adult rat and human brain was studied by light microscopic quantitative autoradiography using a tritiated derivative of tetrodotoxin ([3H]enTTX) and tritiated saxitoxin [( 3H]STX). Equilibrium binding experiments in the whole rat brain gave dissociation constants of 7.0 nM ([3H]enTTX) and 5.0 nM ([3H]STX). The dissociation constant for the binding of [3H]STX in the different human brain regions was near 1.5 nM. Autoradiograms demonstrated a heterogeneous distribution of toxin binding sites in the brain with a very good correlation of the mapping of tetrodotoxin and saxitoxin receptors. With the exception of a few regions, the same type of cartography was observed for human and rat brain structures. If toxin receptors were present in all brain regions, their density was particularly important in cerebral cortex, hippocampus, lateral septum and molecular layer of cerebellar cortex. Conversely, the medulla oblongata contained only low amounts of binding sites.

Kv4 channels sensitive to BmTX3 in rat nervous system: autoradiographic analysis of their distribution during brain ontogenesis.

The binding site distribution of sBmTX3, a chemically synthesized toxin originally purified from the venom of the scorpion Buthus martensi, was investigated in adult and developing rat brain, using patch‐clamp experiments and quantitative autoradiography. The molecular basis of these sBmTX3 sites was analysed by electrophysiology on transient Kv currents recorded in mammalian transfected cells. The rapidly activating and inactivating Kv4.1 current was inhibited by sBmTX3 (IC50, 105 nm). The inhibition was less effective on Kv4.2 and Kv4.3 channels and the toxin did not affect other transient currents such as Kv1.4 and Kv3.4. The distribution of the 125I‐sBmTX3 binding sites was heterogeneous, with a 113‐fold difference between the highest and the lowest densities in adult rat brain. The site density was particularly important in the caudate–putamen and accumbens nucleus, thalamus, hippocampal formation and cerebellum. The affinity of sBmTX3 remained constant during brain ontogenesis. The level of sBmTX3 binding sites was very low in prenatal and postnatal stages to postnatal day (P)12 but drastically increased from P15 in the major part of the studied structures except in the CA3 hippocampal field where the density was very high from P6. Thus, the distribution of sBmTX3 binding sites in rat brain and its electrophysiological characteristics suggest that sBmTX3 specifically binds to the Kv4 subfamily of K channels.

Differential effects of two blockers of small conductance Ca2+-activated K+ channels, apamin and lei-Dab7, on learning and memory in rats.

SK channels are responsible for long-lasting hyperpolarization following action potential and contribute to the neuronal integration signal. This study evaluates the involvement of SK channels on learning and memory in rats, by comparing the effects of two SK channel blockers, i.e., apamin which recognizes SK2 and SK3 channels, and lei-Dab7 which binds SK2 channels only. lei-Dab7 totally competes and contests apamin binding on whole brain sections (IC(50): 11.4 nM). Using an olfactory associative task, intracerebroventricular blocker injections were tested on reference memory. Once the task was mastered with one odor pair, it was then tested with a new odor pair. Apamin (0.3 ng), injected before or after the acquisition session, improved new odor pair learning in a retention session 24 hours later, whereas lei-Dab7 (3 ng) did not significantly affect the mnesic processes. These results indicated that the blockage of SK channels by apamin facilitates consolidation on new odor associations; lei-Dab7, containing only SK2 subunits, remains without effect suggesting an involvement of SK3 channels in the modulation of the mnesic processes.