Metabotropic action of postsynaptic kainate receptors triggers hippocampal long-term potentiation

Long-term potentiation (LTP) in the rat hippocampus is the most extensively studied cellular model for learning and memory. Induction of classical LTP involves an NMDA-receptor- and calcium-dependent increase in functional synaptic AMPA receptors, mediated by enhanced recycling of internalized AMPA receptors back to the postsynaptic membrane. Here we report a physiologically relevant NMDA-receptor-independent mechanism that drives increased AMPA receptor recycling and LTP. This pathway requires the metabotropic action of kainate receptors and activation of G protein, protein kinase C and phospholipase C. Like classical LTP, kainate-receptor-dependent LTP recruits recycling endosomes to spines, enhances synaptic recycling of AMPA receptors to increase their surface expression and elicits structural changes in spines, including increased growth and maturation. These data reveal a new and, to our knowledge, previously unsuspected role for postsynaptic kainate receptors in the induction of functional and structural plasticity in the hippocampus.

Homeostatic synaptic scaling is regulated by protein SUMOylation

Homeostatic scaling allows neurons to alter synaptic transmission to compensate for changes in network activity. Here, we show that suppression of network activity with tetrodotoxin, which increases surface expression of AMPA receptors (AMPARs), dramatically reduces levels of the deSUMOylating (where SUMO is small ubiquitin-like modifier) enzyme SENP1, leading to a consequent increase in protein SUMOylation. Overexpression of the catalytic domain of SENP1 prevents this scaling effect, and we identify Arc as a SUMO substrate involved in the tetrodotoxin-induced increase in AMPAR surface expression. Thus, protein SUMOylation plays an important and previously unsuspected role in synaptic trafficking of AMPARs that underlies homeostatic scaling.

Inhibition of post-synaptic Kv7/KCNQ/M channels facilitates long-term potentiation in the hippocampus

Activation of muscarinic acetylcholine receptors (mAChR) facilitates the induction of synaptic plasticity and enhances cognitive function. In the hippocampus, M₁ mAChR on CA1 pyramidal cells inhibit both small conductance Ca²⁺-activated KCa2 potassium channels and voltage-activated Kv7 potassium channels. Inhibition of KCa2 channels facilitates long-term potentiation (LTP) by enhancing Ca²⁺calcium influx through postsynaptic NMDA receptors (NMDAR). Inhibition of Kv7 channels is also reported to facilitate LTP but the mechanism of action is unclear. Here, we show that inhibition of Kv7 channels with XE-991 facilitated LTP induced by theta burst pairing at Schaffer collateral commissural synapses in rat hippocampal slices. Similarly, negating Kv7 channel conductance using dynamic clamp methodologies also facilitated LTP. Negation of Kv7 channels by XE-991 or dynamic clamp did not enhance synaptic NMDAR activation in response to theta burst synaptic stimulation. Instead, Kv7 channel inhibition increased the amplitude and duration of the after-depolarisation following a burst of action potentials. Furthermore, the effects of XE-991 were reversed by re-introducing a Kv7-like conductance with dynamic clamp. These data reveal that Kv7 channel inhibition promotes NMDAR opening during LTP induction by enhancing depolarisation during and after bursts of postsynaptic action potentials. Thus, during the induction of LTP M₁ mAChRs enhance NMDAR opening by two distinct mechanisms namely inhibition of KCa2 and Kv7 channels.

Facilitation of long-term potentiation by muscarinic M(1) receptors is mediated by inhibition of SK channels

Muscarinic receptor activation facilitates the induction of synaptic plasticity and enhances cognitive function. However, the specific muscarinic receptor subtype involved and the critical intracellular signaling pathways engaged have remained controversial. Here, we show that the recently discovered highly selective allosteric M₁ receptor agonist 77-LH-28-1 facilitates long-term potentiation (LTP) induced by theta burst stimulation at Schaffer collateral synapses in the hippocampus. Similarly, release of acetylcholine by stimulation of cholinergic fibers facilitates LTP via activation of M₁ receptors. N-methyl-D-aspartate receptor (NMDAR) opening during theta burst stimulation was enhanced by M₁ receptor activation, indicating this is the mechanism for LTP facilitation. M₁ receptors were found to enhance NMDAR activation by inhibiting SK channels that otherwise act to hyperpolarize postsynaptic spines and inhibit NMDAR opening. Thus, we describe a mechanism where M₁ receptor activation inhibits SK channels, allowing enhanced NMDAR activity and leading to a facilitation of LTP induction in the hippocampus.

Properties of thiamin transport in isolated perfused hearts of chronically alcoholic guinea pigsThis article is one of a selection of papers published in the special issue Bridging the Gap: Where Progress in Cardiovascular and Neurophysiologic Research Meet

The aim of this study was to determine the mechanism of transport of 14C-thiamin in the hearts of healthy (nonalcoholic) and chronically alcoholic guinea pigs. We used the single-pass, paired-tracer dilution method on isolated and retrogradely perfused guinea pig hearts. The maximal cellular uptake (Umax) and total cellular uptake (Utot) of 14C-thiamin were determined under control conditions and under influence of possible modifiers. We tested how the presence of unlabeled thiamin, metabolic inhibitors, or absence of sodium ions influence the transport of 14C-thiamin. The results of our experiments show that the transport of 14C-thiamin is specific and energy-dependent and that its properties are significantly changed under the influence of chronic alcoholism. The latter effect occurs by increase in both Umax and Utot, as a manifestation of a compensatory mechanism in thiamin deficiency.

Ryanodine receptors, voltage-gated calcium channels and their relationship with protein kinase A in the myocardium

We present a review about the relationship between ryanodine receptors and voltage-gated calcium channels in myocardium, and also how both of them are related to protein kinase A. Ryanodine receptors, which have three subtypes (RyR1-3), are located on the membrane of sarcoplasmic reticulum. Different subtypes of voltage-gated calcium channels interact with ryanodine receptors in skeletal and cardiac muscle tissue. The mechanism of excitation-contraction coupling is therefore different in the skeletal and cardiac muscle. However, in both tissues ryanodine receptors and voltage-gated calcium channels seem to be physically connected. FK-506 binding proteins (FKBPs) are bound to ryanodine receptors, thus allowing their concerted activity, called coupled gating. The activity of both ryanodine receptors and voltage-gated calcium channels is positively regulated by protein kinase A. These effects are, therefore, components of the mechanism of sympathetic stimulation of myocytes. The specificity of this enzyme's targeting is achieved by using different A kinase adapting proteins. Different diseases are related to inborn or acquired changes in ryanodine receptor activity in cardiac myocytes. Mutations in the cardiac ryanodine receptor gene can cause catecholamine-provoked ventricular tachycardia. Changes in phosphorylation state of ryanodine receptors can provide a credible explanation for the development of heart failure. The restoration of their normal level of phosphorylation could explain the positive effect of beta-blockers in the treatment of this disease. In conclusion, molecular interactions of ryanodine receptors and voltage-gated calcium channels with PKA have a significant physiological role. However, their defects and alterations can result in serious disturbances.

Mechanism of 3-deazaguanine transport in the rat heart

The aim of this study was to determine the mechanism of transport of 3-deazaguanine in the rat heart. We used single-pass, paired-tracer dilution method on isolated and retrogradely perfused rat hearts. The maximal cellular uptake (Umax) and total cellular uptake (Utot) of 3-deazaguanine were determined under control conditions and under influence of possible modifiers. Both Umax and Utot were significantly reduced in the presence of unlabeled 3-deazaguanine (from 19.57 +/- 2.02% to 8.14 +/- 1.19% and from 16.49 +/- 3.65% to 4.70 +/- 1.96%, n=6, respectively). The presence of pyrimidine nucleoside thymidine caused the reduction of both Umax and Utot (from 20.03 +/- 3.76% to 13.58 +/- 3.16% and from 16.43 +/- 3.58% to 11.94 +/- 3.13%, n=6, respectively). Also, we tested the effect of the absence of sodium ions in perfusion solution (both Umax and Utot, significantly reduced from 17.95 +/- 2.73% to 16.67 +/- 2.16% and from 16.68 +/- 2.97% to 14.81 +/- 3.04%, n=6, respectively) and the effect of dinitrophenol (both Umax and Utot significantly reduced from 19.09 +/- 3.68% to 10.58 +/- 3.14% and from 16.86 +/- 3.84% to 7.10 +/- 3.11%, n=6, respectively). The results of self- and cross-inhibition studies show that the transport of 3-deazaguanine is saturable, energy- and sodium-dependent and that 3-deazaguanine uses endogenous transport systems for thymidine and adenosine for its own transport.