Synaptic restoration by cAMP/PKA drives activity-dependent neuroprotection to motoneurons in ALS

Excessive excitation is hypothesized to cause motoneuron (MN) degeneration in amyotrophic lateral sclerosis (ALS), but actual proof of hyperexcitation in vivo is missing, and trials based on this concept have failed. We demonstrate, by in vivo single-MN electrophysiology, that, contrary to expectations, excitatory responses evoked by sensory and brainstem inputs are reduced in MNs of presymptomatic mutSOD1 mice. This impairment correlates with disrupted postsynaptic clustering of Homer1b, Shank, and AMPAR subunits. Synaptic restoration can be achieved by activation of the cAMP/PKA pathway, by either intracellular injection of cAMP or DREADD-Gs stimulation. Furthermore, we reveal, through independent control of signaling and excitability allowed by multiplexed DREADD/PSAM chemogenetics, that PKA-induced restoration of synapses triggers an excitation-dependent decrease in misfolded SOD1 burden and autophagy overload. In turn, increased MN excitability contributes to restoring synaptic structures. Thus, the decrease of excitation to MN is an early but reversible event in ALS. Failure of the postsynaptic site, rather than hyperexcitation, drives disease pathobiochemistry.

Presynaptic serotonin 2A receptors modulate thalamocortical plasticity and associative learning

Higher-level cognitive processes strongly depend on a complex interplay between mediodorsal thalamus nuclei and the prefrontal cortex (PFC). Alteration of thalamofrontal connectivity has been involved in cognitive deficits of schizophrenia. Prefrontal serotonin (5-HT)2A receptors play an essential role in cortical network activity, but the mechanism underlying their modulation of glutamatergic transmission and plasticity at thalamocortical synapses remains largely unexplored. Here, we show that 5-HT2A receptor activation enhances NMDA transmission and gates the induction of temporal-dependent plasticity mediated by NMDA receptors at thalamocortical synapses in acute PFC slices. Expressing 5-HT2A receptors in the mediodorsal thalamus (presynaptic site) of 5-HT2A receptor-deficient mice, but not in the PFC (postsynaptic site), using a viral gene-delivery approach, rescued the otherwise absent potentiation of NMDA transmission, induction of temporal plasticity, and deficit in associative memory. These results provide, to our knowledge, the first physiological evidence of a role of presynaptic 5-HT2A receptors located at thalamocortical synapses in the control of thalamofrontal connectivity and the associated cognitive functions.

Restoring synaptic vesicles during compensatory endocytosis

In the CNS (central nervous system), nerve cells communicate by transmitting signals from one to the next across chemical synapses. Electrical signals trigger controlled secretion of neurotransmitter by exocytosis of SV (synaptic vesicles) at the presynaptic site. Neurotransmitters diffuse across the synaptic cleft, activate receptor channels in the receiving neuron at the postsynaptic site, and thereby elicit a new electrical signal. Repetitive stimulation should result in fast depletion of fusion-competent SVs, given their limited number in the presynaptic bouton. Therefore, to support repeated rounds of release, a fast trafficking cycle is required that couples exocytosis and compensatory endocytosis. During this exo-endocytic cycle, a defined stoichiometry of SV proteins has to be preserved, that is, membrane proteins have to be sorted precisely. However, how this sorting is accomplished on a molecular level is poorly understood. In the present chapter we review recent findings regarding the molecular composition of SVs and the mechanisms that sort SV proteins during compensatory endocytosis. We identify self-assembly of SV components and individual cargo recognition by sorting adaptors as major mechanisms for maintenance of the SV protein complement.

Three Types of Striational Antibodies in Myasthenia Gravis

Myasthenia gravis (MG) is caused by antibodies that react mainly with the acetylcholine receptor on the postsynaptic site of the neuromuscular junction. A wide range of clinical presentations and associated features allow MG to be classified into subtypes based onautoantibodystatus. Striational antibodies, which react with epitopes on the muscle proteins titin, ryanodine receptor (RyR), and Kv1.4, are frequently found in MG patients with late-onset and thymoma. Antititin and anti-RyR antibodies are determined by enzyme-linked immunosorbent assay or immunoblot. More recently, a method for the detection of anti-Kv1.4 autoantibodies has become available, involving 12–15% of all MG patients. The presence of striational antibodies is associated with more severe disease in all MG subgroups. Anti-Kv1.4 antibody is a useful marker for the potential development of lethal autoimmune myocarditis and response to calcineurin inhibitors. Detection of striational antibodies provides more specific and useful clinical information in MG patients.

The effect of glucagon on vasoconstriction and vascular escape from nerve- and norepinephrine-induced constriction of the hepatic artery of the cat

Glucagon, in the anesthetized cat, was capable of dilating the hepatic artery to the same extent and in a dose-dependent manner when administered directly into the hepatic artery or into the portal vein. Portal venous infusions of glucagon did not inhibit nerve- or norepinephrine-induced vasoconstriction of the hepatic artery in contrast to previous reports in the dog. Rather, at certain doses, glucagon mildly potentiated the vasoconstriction induced by both constrictor stimuli. Vascular escape from nerve- and norepinephrine-induced constrictor responses was found to be inhibited by glucagon in a dose-dependent manner. Glucagon infusion is the first intervention reported to modulate vascular escape in the hepatic artery. Owing to its similar effects on nerve- and exogenous norepinephrine-induced responses, glucagon appears to be acting at a postsynaptic site. Therefore, we suggest that in the cat, glucagon is not an inhibitory modulator of nerve- and norepinephrine-induced vasoconstriction, but rather may potentiate the constrictor response in a postsynaptic manner.Key words: glucagon, hepatic artery, modulation, vascular escape, vasoconstriction.

Sympathetic Vasoconstriction as a Mechanism of Action of Ouabain in Forearm Arterioles of Hypertensive Patients

1. The interaction of ouabain, a Na+/K+ adenosine 5′-triphosphatase inhibitor, with sympathetic mechanisms of vasoconstriction, as well as its possible site(s) of action, were investigated in forearm arterioles of patients with uncomplicated hypertension. 2. Intra-arterial infusion of ouabain per se decreased forearm blood flow without changes in systemic arterial pressure or contralateral flow. However, the vasoconstrictor effect of the glycoside was abolished after local pre-treatment with either phentolamine, a competitive α-adrenoceptor antagonist, or bretylium tosylate, a neurotransmitter blocker. 3. To exclude a non-specific effect due to the vasodilatation, a similar protocol was performed using histamine, which acts independently of sympathetic mechanisms. The vascular effect of ouabain was maintained in spite of histamine-induced increases in forearm blood flow even greater than those obtained from either blocker. 4. To discriminate between pre- and post-synaptic site(s) of action of ouabain, exogenous noradrenaline was infused intra-arterially after inactivation of local neurotransmitter release by bretylium, thus causing direct postsynaptic vascular α-adrenoceptor stimulation. Under these conditions, noradrenaline decreased forearm blood flow irrespective of the presence or absence of ouabain. 5. Thus, local sympatholysis by drugs acting on different levels of the sympathetic neuroeffector junction abolished the effect of ouabain, whereas histamine did not influence it. The data provide positive evidence for an effect of ouabain on sympathetically mediated vasoconstriction. This action is apparently not exerted at a postsynaptic site but possibly by enhancing neurotransmitter release. 6. If a circulating endogenous ouabain-like Na+/K+ adenosine 5′-triphosphatase inhibitor is relevant to the development of hypertension in man, it might act through a similar mechanism.