Primary afferent terminal excitability in the normal and spastic mutant mouse spinal cord

A Munc18-1 mutant mimicking phosphorylation by Down Syndrome-related kinase Dyrk1a supports normal synaptic transmission and promotes recovery after intense activity

Phosphorylation of Munc18-1 (Stxbp1), a presynaptic organizer of synaptic vesicle fusion, is a powerful mechanism to regulate synaptic strength. Munc18-1 is a proposed substrate for the Down Syndrome-related kinase dual-specificity tyrosine phosphorylation-regulate kinase 1a (Dyrk1a) and mutations in both genes cause intellectual disability. However, the functional consequences of Dyrk1a-dependent phosphorylation of Munc18-1 for synapse function are unknown. Here, we show that the proposed Munc18-1 phosphorylation site, T479, is among the highly constrained phosphorylation sites in the coding regions of the gene and is also located within a larger constrained coding region. We confirm that Dyrk1a phosphorylates Munc18-1 at T479. Patch-clamp physiology in conditional null mutant hippocampal neurons expressing Cre and either wildtype, or mutants mimicking or preventing phosphorylation, revealed that synaptic transmission is similar among the three groups: frequency/amplitude of mEPSCs, evoked EPSCs, paired pulse plasticity, rundown kinetics upon intense activity and the readily releasable pool. However, synapses expressing the phosphomimic mutant responded to intense activity with more pronounced facilitation. These data indicate that Dyrk1a-dependent Munc18-1 phosphorylation has a minor impact on synaptic transmission, only after intense activity, and that the role of genetic variation in both genes in intellectual disability may be through different mechanisms.

Evidence for glutamate as a neuroglial transmitter within sensory ganglia

This study examines key elements of glutamatergic transmission within sensory ganglia of the rat. We show that the soma of primary sensory neurons release glutamate when depolarized. Using acute dissociated mixed neuronal/glia cultures of dorsal root ganglia (DRG) or trigeminal ganglia and a colorimetric assay, we show that when glutamate uptake by satellite glial cells (SGCs) is inhibited, KCl stimulation leads to simultaneous increase of glutamate in the culture medium. With calcium imaging we see that the soma of primary sensory neurons and SGCs respond to AMPA, NMDA, kainate and mGluR agonists, and selective antagonists block this response. Using whole cell patch-clamp technique, inward currents were recorded from small diameter (<30 µm) DRG neurons from intact DRGs (ex-vivo whole ganglion preparation) in response to local application of the above glutamate receptor agonists. Following a chronic constriction injury (CCI) of either the inferior orbital nerve or the sciatic nerve, glutamate expression increases in the trigeminal ganglia and DRG respectively. This increase occurs in neurons of all diameters and is present in the somata of neurons with injured axons as well as in somata of neighboring uninjured neurons. These data provides additional evidence that glutamate can be released within the sensory ganglion, and that the somata of primary sensory neurons as well as SGCs express functional glutamate receptors at their surface. These findings, together with our previous gene knockdown data, suggest that glutamatergic transmission within the ganglion could impact nociceptive threshold.

Spontaneous activity induced in isolated mouse spinal cord by high extracellular calcium and by low extracellular magnesium

Spontaneous discharges were observed in recordings from dorsal and ventral roots (DRs and VRs) of hemisected mouse spinal cords in vitro when bath [Ca2+] was raised from the control level of 1.2 to 1.8 or 2.4 mmol/l, and when bath [Mg2+] was lowered resembling drug-induced 'interictal' discharges described earlier. Maximum discharge frequency was reached at 2.4 or 3.6 mmol/l [Ca2+] while at higher concentrations mean frequency diminished but mean amplitude still increased somewhat. The frequency distribution of wave amplitudes suggests recruitment of neurons by groups or assemblies. The pacemaker is located in the dorsal spinal quadrant. The GABAA receptor antagonists picrotoxin and bicuculline in low concentration blocked the spontaneous activity, indicating an obligatory GABAergic link in the pacemaker circuit. The NMDA antagonists D.L-APV and D-APV in high concentration variably depressed but did not abolish spontaneous activity. The propensity of spinal neuron assemblies for self-paced discharge may contribute to the pathologic irritability of injured spinal cords.

Pathophysiology of the spinal cord studied in vitro

We describe the use of isolated hemisected mouse spinal cords for pathophysiological investigations and analyze the responses evoked and recorded with suction electrodes in spinal roots. Dorsal root (DR) recordings from preparations in control solution show a directly evoked fiber volley (FV); an early postsynaptic spike generated by neurons in spinal gray matter and picked up by volume conduction (DRR1); and a 'slow' dorsal root potential (DRP). The 'conventional' dorsal root reflex (here termed DRR2) was absent or very small in control medium but became very prominent in elevated bath [Ca2+]. DRP and DRR2 but not DRR1 are depressed by GABAA antagonists. Recordings from VR contain the electrotonically conducted VRepsp and superimposed monosynaptic reflex discharge (VRR1). Rarely in control medium but regularly in elevated bath [Ca2+] a GABA-dependent late reflex (VRR2) appears (see also Duchen, 1986). The effects of varying bath concentrations of K+, Ca2+ and Mg2+ on evoked responses are briefly summarized. Irregularly timed spontaneous discharges appear in DR and VR recordings when [Ca2+] is elevated above 1.8 or 2.4 mM, and when [Mg2+] is lowered to 0.4 mM. In hypoxic solution synaptically transmitted responses fail in 10 to 20 min, but persist longer when [Ca2+] is elevated. Unexpectedly, spreading depression (SD)-like responses were recorded in some preparations during hypoxia. Following hypoxia, after synaptically transmitted responses recovered, spontaneous activity developed in DR and VR recordings.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)