Coincident glutamatergic and cholinergic inputs transiently depress glutamate release at rat schaffer collateral synapses

Association Between the Cholinesterase Inhibitor Donepezil and the Cholinergic Precursor Choline Alphoscerate in the Treatment of Depression in Patients with Alzheimer’s Disease

Background:

Depressive symptoms are common in Alzheimer’s disease (AD) patients and are associated with an increased functional decline. Selective serotonin reuptake inhibitor antidepressants showed a limited efficacy.

Objective:

The purpose of this work was to evaluate if a higher brain cholinergic stimulation induced by the association between the acetylcholinesterase inhibitor donepezil and the cholinergic precursor choline alphoscerate has any effect on depression in AD patients.

Methods:

Patients were selected among those recruited in the ASCOMALVA (association between the cholinesterase inhibitor donepezil and the cholinergic precursor choline alphoscerate in AD) trial. Depressive symptoms were investigated in 90 AD patients through the neuropsychiatric inventory at baseline and after 3, 6, 9, 12, 18, and 24 months of treatment. Patients were randomized in a group association therapy (45 subjects) receiving donepezil 10 mg plus choline alphoscerate 1,200 mg/day, and a group monotherapy (45 subjects) receiving donepezil 10 mg/day plus placebo. Based on the results of the MMSE at the recruitment patients were divided into 3 groups: severely impaired (score < 15); moderately impaired (score 19-16); mild-moderately impaired (score 24-20).

Results:

Depression symptoms were significantly lower (p < 0.05) in patients treated with donepezil plus choline alphoscerate compared to patients treated with donepezil alone. Subjects of the group having mild to moderate cognitive impairment were those more sensitive to the association treatment.

Conclusion:

Depression symptoms of AD patients in the mild to moderate stage probably could to benefit of a stronger cholinergic stimulation induced by associating donepezil with the cholinergic precursor choline alphoscerate.

NMDA receptor-dependent afterdepolarizations are curtailed by carbonic anhydrase 14: regulation of a short-term postsynaptic potentiation

In the hippocampus, extracellular carbonic anhydrase (Car) speeds the buffering of an activity-generated rise in extracellular pH that impacts H(+)-sensitive NMDA receptors (NMDARs). We studied the role of Car14 in this brain structure, in which it is expressed solely on neurons. Current-clamp responses were recorded from CA1 pyramidal neurons in wild-type (WT) versus Car14 knock-out (KO) mice 2 s before (control) and after (test) a 10 pulse, 100 Hz afferent train. In both WT and KO, the half-width (HW) of the test response, and its number of spikes, were augmented relative to the control. An increase in presynaptic release was not involved, because AMPAR-mediated EPSCs were depressed after a train. The increases in HW and spike number were both greater in the Car14 KO. In 0 Mg(2+) saline with picrotoxin (using a 20 Hz train), the HW measures were still greater in the KO. The Car inhibitor benzolamide (BZ) enhanced the test response HW in the WT but had no effect on the already-prolonged HW in the KO. With intracellular MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]-cyclohepten-5,10-imine maleate], the curtailed WT and KO responses were indistinguishable, and BZ caused no change. In contrast, the extracellular alkaline changes evoked by the train were not different between WT and KO, and BZ amplified these alkalinizations similarly. These data suggest that Car14 regulates pH transients in the perisynaptic microenvironment and govern their impact on NMDARs but plays little role in buffering pH shifts in the broader, macroscopic, extracellular space.

Metabotropic glutamate receptors regulate hippocampal CA1 pyramidal neuron excitability via Ca²⁺ wave-dependent activation of SK and TRPC channels

Group I metabotropic glutamate receptors (mGluRs) play an essential role in cognitive function. Their activation results in a wide array of cellular and molecular responses that are mediated by multiple signalling cascades. In this study, we focused on Group I mGluR activation of IP3R-mediated intracellular Ca2+ waves and their role in activating Ca2+-dependent ion channels in CA1 pyramidal neurons. Using whole-cell patch-clamp recordings and high-speed Ca2+ fluorescence imaging in acute hippocampal brain slices, we show that synaptic and pharmacological stimulation of mGluRs triggers intracellular Ca2+ waves and a biphasic electrical response composed of a transient Ca2+-dependent SK channel-mediated hyperpolarization and a TRPC-mediated sustained depolarization. The generation and magnitude of the SK channel-mediated hyperpolarization depended solely on the rise in intracellular Ca2+ concentration ([Ca2+]i), whereas the TRPC channel-mediated depolarization required both a small rise in [Ca2+]i and mGluR activation. Furthermore, the TRPC-mediated current was suppressed by forskolin-induced rises in cAMP. We also show that SK- and TRPC-mediated currents robustly modulate pyramidal neuron excitability by decreasing and increasing their firing frequency, respectively. These findings provide additional evidence that mGluR-mediated synaptic transmission makes an important contribution to regulating the output of hippocampal neurons through intracellular Ca2+ wave activation of SK and TRPC channels. cAMP provides an additional level of regulation by modulating TRPC-mediated sustained depolarization that we propose to be important for stabilizing periods of sustained firing.

Synapse- and subtype-specific modulation of synaptic transmission by nicotinic acetylcholine receptors in the ventrobasal thalamus

The rodent thalamic ventrobasal complex (VB) which is a subdivision of somatosensory thalamus receives two excitatory inputs through the medial lemniscal synapse, which is a sensory afferent synapse, and the corticothalamic synapse from layer VI of the somatosensory cortex. In addition, the VB also receives cholinergic inputs from the brain stem, and nicotinic acetylcholine receptors (nAChRs) are highly expressed in the VB. Little is known, however, how acetylcholine (ACh) modulates synaptic transmission at the medial lemniscal and corticothalamic synapses in the VB. Furthermore, it remains unclear which subtype of nAChRs contributes to VB synaptic transmission. We report here that the activation of nAChRs presynaptically depressed corticothalamic synaptic transmission, whereas it did not affect medial lemniscal synaptic transmission in juvenile mice. This presynaptic modulation was mediated by the activation of nAChRs that contained α4 and β2 subunit, but not by α7 nAChRs. Moreover, galanthamine, an allosteric modulator of α4β2α5 nAChR, enhanced the ACh-induced depression of corticothalamic excitatory postsynaptic currents (EPSCs), indicating that α4β2α5 nAChRs at corticothalamic axon terminals specifically contribute to the depression of corticothalamic synaptic transmission.

Inositol-1,4,5-trisphosphate receptor-mediated Ca2+ waves in pyramidal neuron dendrites propagate through hot spots and cold spots

We studied inositol-1,4,5-trisphosphate (IP(3)) receptor-dependent intracellular Ca(2+) waves in CA1 hippocampal and layer V medial prefrontal cortical pyramidal neurons using whole-cell patch-clamp recordings and Ca(2+) fluorescence imaging. We observed that Ca(2+) waves propagate in a saltatory manner through dendritic regions where increases in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) were large and fast ('hot spots') separated by regions where increases in [Ca(2+)](i) were comparatively small and slow ('cold spots'). We also observed that Ca(2+) waves typically initiate in hot spots and terminate in cold spots, and that most hot spots, but few cold spots, are located at dendritic branch points. Using immunohistochemistry, we found that IP(3) receptors (IP(3)Rs) are distributed in clusters along pyramidal neuron dendrites and that the distribution of inter-cluster distances is nearly identical to the distribution of inter-hot spot distances. These findings support the hypothesis that the dendritic locations of Ca(2+) wave hot spots in general, and branch points in particular, are specially equipped for regenerative IP(3)R-dependent internal Ca(2+) release. Functionally, the observation that IP(3)R-dependent [Ca(2+)](i) rises are greater at branch points raises the possibility that this novel Ca(2+) signal may be important for the regulation of Ca(2+)-dependent processes in these locations. Futhermore, the observation that Ca(2+) waves tend to fail between hot spots raises the possibility that influences on Ca(2+) wave propagation may determine the degree of functional association between distinct Ca(2+)-sensitive dendritic domains.

Endocannabinoids modulate encoding of sequential memory in the rat hippocampus

RATIONALE

This report investigated the role of endocannabinoids in the encoding of task-relevant information by ensembles of hippocampal neurons under conditions in which the CB1 receptor antagonist, rimonabant, was administered during performance of a short-term memory delayed non-match to sample (DNMS) task in rats.

OBJECTIVE

The influence of endocannabinoids on the encoding of task relevant information was determined via examination of the firing patterns of ensembles of CA1/CA3 hippocampal neurons during individual trials while rats performed a DNMS task.

MATERIALS AND METHODS

Multivariate discriminant analysis of the firing patterns of ensembles of hippocampal neurons was used to extract trial-specific codes for task-relevant information under different types of trial sequences.

RESULTS

It was discovered that rimonabant blocked an inherent hippocampal memory encoding bias used by all animals. This bias was characterized as the preferential encoding of sample information on individual trials based on the similarity (i.e., same or different) and duration of the delay in the preceding trial.

CONCLUSIONS

The results indicate that endocannabinoids are a major influence on the strategic encoding biases of hippocampal ensembles and that pharmacological blockade of CB1 receptors facilitated performance by eliminating such influences.

Short-term memory is modulated by the spontaneous release of endocannabinoids: evidence from hippocampal population codes

Population codes derived from ensembles of hippocampal neurons were assessed to determine whether endocannabinoids were active when rats performed a delayed-nonmatch-to-sample (DNMS) short-term memory task. Multivariate discriminant analyses of the firing patterns of ensembles of CA1 and CA3 hippocampal neurons extracted representations of information encoded at the time of the sample response (SmR codes) during individual DNMS trials. The 'strength' or distinctiveness of trial-specific SmR codes in normal sessions was compared with sessions in which either rimonabant, the well-characterized cannabinoid CB1 receptor antagonist, or WIN 55212-2 (WIN-2), a cannabinoid CB1 receptor agonist, were administered. Results show that performance on trials with delay intervals longer than 10 s was facilitated by rimonabant (2.0 mg/kg) owing to a significantly increased frequency of trials with stronger SmR codes. In contrast, WIN-2 (0.35 mg/kg) suppressed the strength of SmR codes necessary to perform trials with delays greater than 10 s. The positive influence of rimonabant on performance indicated that the action of endocannabinoids was to reduce SmR code strength, resulting in trials that were at risk for errors if the delay exceeded 10 s. Thus endocannabinoids, like exogenously administered cannabinoids, reduced hippocampal encoding necessary to perform long-delay trials. The findings therefore indicate a direct relationship between the actions of endocannabinoids on hippocampal processes and the ability to encode information into short-term memory.

Increased thalamocortical synaptic response and decreased layer IV innervation in GAP-43 knockout mice

The growth-associated protein, GAP-43, is an axonally localized neuronal protein with high expression in the developing brain and in regenerating neurites. Mice that lack GAP-43 (GAP-43 -/-) fail to form a whisker-related barrel map. In this study, we use GAP-43 -/- mice to examine GAP-43 synaptic function in the context of thalamocortical synapse development and cortical barrel map formation. Examination of thalamocortical synaptic currents in an acute brain slice preparation and in autaptic thalamic neurons reveals that GAP-43 -/- synapses have larger alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor (AMPAR)-mediated currents than controls despite similar AMPAR function and normal probability of vesicular release. Interestingly, GAP-43 -/- synapses are less sensitive to blockade by a competitive glutamate receptor antagonist, suggesting higher levels of neurotransmitter in the cleft during synaptic transmission. Field excitatory postsynaptic potentials (EPSPs) from GAP-43 -/- thalamocortical synapses reveal a reduced fiber response, and anatomical analysis shows reduced thalamic innervation of barrel cortex in GAP-43 -/- mice. Despite this fact synaptic responses in the field EPSPs are similar in GAP-43 -/- mice and wild-type littermate controls, and the ratio of AMPAR-mediated to N-methyl-d-aspartate receptor (NMDAR)-mediated currents (AMPAR:NMDAR ratio) is larger than normal. This suggests that GAP-43 -/- mice form fewer thalamocortical synapses in layer IV because of decreased anatomical innervation of the cortex, but the remaining contacts are individually stronger possibly due to increased neurotransmitter concentration in the synaptic cleft. Together, these results indicate that in addition to its well known role in axonal pathfinding GAP-43 plays a functional role in regulating neurotransmitter release.