Differential cholinergic modulation of synaptic encoding and gain control mechanisms in rat hippocampus

The Effects of Light and the Circadian System on Rhythmic Brain Function

Life on earth has evolved under the influence of regularly recurring changes in the environment, such as the 24 h light/dark cycle. Consequently, organisms have developed endogenous clocks, generating 24 h (circadian) rhythms that serve to anticipate these rhythmic changes. In addition to these circadian rhythms, which persist in constant conditions and can be entrained to environmental rhythms, light drives rhythmic behavior and brain function, especially in nocturnal laboratory rodents. In recent decades, research has made great advances in the elucidation of the molecular circadian clockwork and circadian light perception. This review summarizes the role of light and the circadian clock in rhythmic brain function, with a focus on the complex interaction between the different components of the mammalian circadian system. Furthermore, chronodisruption as a consequence of light at night, genetic manipulation, and neurodegenerative diseases is briefly discussed.

Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

The disproportionate evolutionary expansion of the human cerebral cortex with reinforcement of cholinergic innervations warranted a major rise in the functional and metabolic load of the conserved basal forebrain (BF) cholinergic system. Given that acetylcholine (ACh) regulates properties of the microtubule-associated protein (MAP) tau and promotes non-amyloidogenic processing of amyloid precursor protein (APP), growing neocortex predicts higher demands for ACh, while the emerging role of BF cholinergic projections in Aβ clearance infers greater exposure of source neurons and their innervation fields to amyloid pathology. The higher exposure of evolutionary most recent cortical areas to the amyloid pathology of Alzheimer's disease (AD) with synaptic impairments and atrophy, therefore, might involve attenuated homeostatic effects of BF cholinergic projections, in addition to fall-outs of inherent processes of expanding association areas. This unifying model, thus, views amyloid pathology and loss of cholinergic cells as a quid pro quo of the allometric evolution of the human brain, which in combination with increase in life expectancy overwhelm the fine homeostatic balance and trigger the disease process.

In Vivo Plasticity at Hippocampal Schaffer Collateral-CA1 Synapses: Replicability of the LTP Response and Pharmacology in the Long-Evans Rat

Broad issues associated with non-replicability have been described in experimental pharmacological and behavioral cognitive studies. Efforts to prevent biases that contribute to non-replicable scientific protocols and to improve experimental rigor for reproducibility are increasingly seen as a basic requirement for the integrity of scientific research. Synaptic plasticity, encompassing long-term potentiation (LTP), is believed to underlie mechanisms of learning and memory. The present study was undertaken in Long-Evans (LE) rats, a strain of rat commonly used in cognitive behavioral tests, to (1) compare three LTP tetanisation protocols, namely, the high-frequency stimulation (HFS), the theta-burst stimulation (TBS), and the paired-pulse facilitation (PPF) at the Schaffer collateral-CA1 stratum radiatum synapse and to (2) assess sensitivity to acute pharmacology. Results: (1) When compared to Sprague-Dawley (SD) rats, the HFS using a stimulus intensity of 50% of the maximum slope obtained from input/output (I/O) curves elicited lower and higher thresholds of synaptic plasticity responses in SD and LE rats, respectively. The 2-train TBS protocol significantly enhanced the LTP response in LE rats over the 5- and 10-train TBS protocols in both strains, and the 5 × TBS protocol inducing a subthreshold LTP response was used in subsequent pharmacological studies in LE rats. The PPF protocol, investigating the locus of the LTP response, showed no difference for the selected interstimulus intervals. (2) Scopolamine, a nonspecific muscarinic antagonist, had a subtle effect, whereas donepezil, an acetylcholinesterase inhibitor, significantly enhanced the LTP response, demonstrating the sensitivity of the TBS protocol to enhanced cholinergic tone. MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist, significantly reduced LTP response, while memantine, another NMDA antagonist, had no effect on LTP response, likely associated with a weaker TBS protocol. PQ10, a phosphodiesterase-10 inhibitor, significantly enhanced the TBS-induced LTP response, but did not change the PPF response. Overall, the results confirm the strain-dependent differences in the form of synaptic plasticity, replicate earlier plasticity results, and report effective protocols that generate a relatively subthreshold margin of LTP induction and maintenance, which are suitable for pharmacology in the LE rat strain mainly used in cognitive studies.

Cholinergic Mechanisms in the Cerebral Cortex: Beyond Synaptic Transmission

Functional overviews of cholinergic mechanisms in the cerebral cortex have traditionally focused on the release of acetylcholine with modulator and transmitter effects. Recently, however, data have emerged that extend the role of acetylcholine and cholinergic innervations to a range of housekeeping and metabolic functions. These include regulation of amyloid precursor protein (APP) processing with production of amyloid β (Aβ) and other APP fragments and control of the phosphorylation of microtubule-associated protein (MAP) tau. Evidence has been also presented for receptor-ligand like interactions of cholinergic receptors with soluble Aβ peptide and MAP tau, with modulator and signaling effects. Moreover, high-affinity binding of Aβ to the neurotrophin receptor p75 (p75NTR) enriched in basalo-cortical cholinergic projections has been implicated in clearance of Aβ and nucleation of amyloid plaques. Here, we critically evaluate these unorthodox cholinergic mechanisms and discuss their role in neuronal physiology and the biology of Alzheimer's disease.

Impaired insulin signaling and mechanisms of memory loss

Insulin is secreted from the β-cells of the pancreas and helps maintain glucose homeostasis. Although secreted peripherally, insulin also plays a profound role in cognitive function. Increasing evidence suggests that insulin signaling in the brain is necessary to maintain health of neuronal cells, promote learning and memory, decrease oxidative stress, and ultimately increase neuronal survival. This chapter summarizes the different facets of insulin signaling necessary for learning and memory and additionally explores the association between cognitive impairment and central insulin resistance. The role of impaired insulin signaling in the advancement of cognitive dysfunction is relevant to the current debate of whether the shared pathophysiological mechanisms between diabetes and cognitive impairment implicate a direct relationship. Here, we summarize a vast amount of literature that suggests a strong association between impaired brain insulin signaling and cognitive impairment.

Neurotrophin receptor p75 mediates the uptake of the amyloid beta (Aβ) peptide, guiding it to lysosomes for degradation in basal forebrain cholinergic neurons

A fascinating yet perhaps overlooked trait of the p75 neurotrophin receptor (p75(NTR)) is its ability to bind ligands with no obvious neurotrophic function. Using cultured basal forebrain (BF) neurons, this study demonstrates selective internalization of amyloid β (Aβ) 1-42 in conjunction with p75(NTR) (labelled with IgG192-Cy3) by cholinergic cells. Active under resting conditions, this process was enhanced by high K(+) stimulation and was insensitive to inhibitors of regulated synaptic activity-tetrodotoxin or botulinum neurotoxins (BoNT type/A and/B). Blockade of sarco-endoplasmic reticulum (SERCA) Ca(2+) ATPase with thapsigargin and CPA or chelation of Ca(2+) with EGTA-AM strongly suppressed the endocytosis of p75(NTR), implicating the role of ER released Ca(2+). The uptake of IgG192-Cy3 was also reduced by T-type Ca(2+) channel blocker mibefradil but not Cd(2+), an indiscriminate blocker of high voltage-activated Ca(2+) currents. A strong co-localization of IgG192-Cy3 with late endosome (Rab7) or lysosome (Lamp1) qualifier proteins suggest these compartments as the primary destination for internalized IgG192 and Aβ. Selective uptake and labeling of BF cholinergic cells with IgG192-Cy3 injected into the prefrontal cortex was verified also in vivo. The significance of these findings in relation to Aβ clearance in the cerebral cortex and pathophysiology of Alzheimer's disease is discussed.

Intrinsic voltage dynamics govern the diversity of spontaneous firing profiles in basal forebrain noncholinergic neurons

Spontaneous firing and behavior-related changes in discharge profiles of basal forebrain (BF) neurons are well documented, albeit the mechanisms underlying the variety of activity modes and intermodal transitions remain elusive. With the use of cell-attached recordings, this study identifies a range of spiking patterns in diagonal band Broca (DBB) noncholinergic cells of rats and tentatively categorizes them into low-rate random, tonic, and cluster firing activities. It demonstrates further that the multiplicity of discharge profiles is sustained intrinsically and persists after blockade of glutamate-, glycine/GABA-, and cholinergic synaptic inputs. Stimulation of muscarinic receptors, blockade of voltage-gated Ca(2+)-, and small conductance (SK) Ca(2+)-activated K(+) currents as well as chelating of intracellular Ca(2+) concentration accelerate low-rate random and tonic firing and favor transition of neurons into cluster firing mode. A similar trend towards higher discharge rates with switch of neurons into cluster firing has been revealed by activation of neuropeptide Y (NPY) receptors with the NPY or NPY(1) receptor agonist [Leu(31),Pro(34)]-NPY. Whole cell current-clamp analysis demonstrates that the variety of spiking modes and intermodal transitions could be induced within the same neuronal population by injection of bias depolarizing or hyperpolarizing currents. Taken together, these data demonstrate the intrinsic and highly variable character of regenerative firing in BF noncholinergic cells, subject to powerful modulation by classical neurotransmitters, NPY, and small membrane currents.

Cholinergic control of visual categorization in macaques

Acetylcholine (ACh) is a neurotransmitter acting via muscarinic and nicotinic receptors that is implicated in several cognitive functions and impairments, such as Alzheimer’s disease. It is believed to especially affect the acquisition of new information, which is particularly important when behavior needs to be adapted to new situations and to novel sensory events. Categorization, the process of assigning stimuli to a category, is a cognitive function that also involves information acquisition. The role of ACh on categorization has not been previously studied. We have examined the effects of scopolamine, an antagonist of muscarinic ACh receptors, on visual categorization in macaque monkeys using familiar and novel stimuli. When the peripheral effects of scopolamine on the parasympathetic nervous system were controlled for, categorization performance was disrupted following systemic injections of scopolamine. This impairment was observed only when the stimuli that needed to be categorized had not been seen before. In other words, the monkeys were not impaired by the central action of scopolamine in categorizing a set of familiar stimuli (stimuli which they had categorized successfully in previous sessions). Categorization performance also deteriorated as the stimulus became less salient by an increase in the level of visual noise. However, scopolamine did not cause additional performance disruptions for difficult categorization judgments at lower coherence levels. Scopolamine, therefore, specifically affects the assignment of new exemplars to established cognitive categories, presumably by impairing the processing of novel information. Since we did not find an effect of scopolamine in the categorization of familiar stimuli, scopolamine had no significant central action on other cognitive functions such as perception, attention, memory, or executive control within the context of our categorization task.

Cocaine enhancement of long-term potentiation in the CA1 region of rat hippocampus: lamina-specific mechanisms of action

There is an expanding body of work characterizing dopaminergic modulation of synaptic plasticity in the hippocampus CA1 region, an area known to be involved in learning and memory. However, in vitro studies to date have focused almost exclusively on the proximal and distal apical dendritic layers (strata radiatum and lacunosum moleculare, respectively). In this report, we establish that dopaminergic activity can enhance long-term potentiation (LTP) in the basal dendritic layer (stratum oriens) of CA1 in the rat hippocampal slice preparation. Application of the D(1/5) agonist SKF38393 (20 microM) significantly increased the magnitude of basal LTP of the fEPSP response following high-frequency stimulation of the Schaffer collateral/commissural inputs in the stratum oriens layer. In addition, endogenous dopamine (DA) activity facilitated by the presence of cocaine (6 muM) was also capable of enhancing the magnitude of basal LTP. Prior application of the D(1/5) antagonist SKF83566 (2 muM) prevented this effect of cocaine, indicating that endogenously released dopamine was exerting its LTP-enhancing effect in stratum oriens via activation of D(1/5) receptors. This final result stands in contrast with the previously characterized effects of cocaine on apical LTP in the stratum radiatum, which instead have been shown to require D(3) receptor activation. These observations demonstrate that dopaminergic mechanisms resulting in the enhancement of hippocampal LTP are lamina specific at Schaffer collateral/commissural synapses in the CA1 region. Synapse 2010. (c) 2010 Wiley-Liss, Inc.

Selective potentiation of crossed vs. uncrossed inputs from lateral geniculate nucleus to visual cortex by the basal forebrain: potential facilitation of rodent binocularity

Cholinergic projections originating in the basal forebrain (BF) play important roles in the heterosynaptic facilitation of synaptic strength in various sensory cortices, including the primary visual cortex (V1). Here, using urethane-anesthetized rats, we find that pairing burst stimulation of the BF with single pulse stimulation of the lateral geniculate nucleus (LGN) does not consistently increase field postsynaptic potentials (fPSPs) in V1 elicited by ipsilateral LGN stimulation. However, longer latency fPSPs recorded in V1 in response to stimulation of the contralateral LGN, reflecting crossed, polysynaptic inputs, show significant potentiation when paired with preceding BF stimulation. This synaptic enhancement requires relatively short time intervals between paired BF burst and LGN pulse stimulation (40 ms) and is abolished by systemic or local V1 muscarinic receptor blockade (scopolamine), while systemic nicotinic receptor blockade (mecamylamine) is ineffective. Together, these data provide evidence for a differential capacity for cholinergic/muscarinic-dependent plasticity induction among different signals in V1, with inputs reaching V1 from the contralateral LGN exhibiting potentiation in the face of stable strength in ipsilateral LGN-V1 projections. This preferential readiness for potentiation in crossed fiber systems could serve to amplify binocular responses in V1 elicited by synchronized excitation of ipsi- and contralateral LGN neurons.

Differential neuropsychopharmacological influences of naturally occurring tropane alkaloids anisodamine versus scopolamine

Two naturally occurring tropane alkaloids, anisodamine and scopolamine, structurally dissimilar in one OH group, are well established as muscarinic acetylcholine receptor (mAChR) antagonists in clinic and basic research. However, experimental evidence for central effects of anisodamine is limited and conflicting compared with that of scopolamine. In the present study, Morris water maze test, long-term potentiation (LTP) recording and receptor radioligand binding assays were used to explore the disparity in neuropsychopharmacological influences of anisodamine versus scopolamine and possible mechanisms. Anisodamine, at 10-40-fold higher doses than those of scopolamine, did not produce any spatial cognitive deficits as scopolamine, but tended to improve cognition at the repeated high doses. LTP in vivo was then adopted to predict BBB permeability of the muscarinic antagonists following systemic drug administration. Contrary to scopolamine, anisodamine did not influence the formation of LTP in the CA(1) region of rat hippocampus at 40-fold higher dose than that of scopolamine. Additionally, receptor radioligand binding assays (RRLBA) revealed that the binding affinity of anisodamine to mice brain mAChR was much lower than that of scopolamine. The findings suggested that anisodamine did not impair cognition nor depress LTP primarily due to its poor BBB permeability. This work enlarged knowledge of structure-activity relationship among tropane alkaloids, meanwhile providing evidence for more reasonable drug prescription in clinic.