Cholinergic effects on spatial exploration and its memory

Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity

The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases.

The frequency of spontaneous seizures in rats correlates with alterations in sensorimotor gating, spatial working memory, and parvalbumin expression throughout limbic regions

Cognitive deficits and psychotic symptoms are highly prevalent in patients with temporal lobe epilepsy (TLE). Imaging studies in humans have suggested that these comorbidities are associated with atrophy in temporal lobe structures and other limbic regions. It remains to be clarified whether TLE comorbidities are due to the frequency of spontaneous seizures or to limbic structural damage per se. Here, we used the pilocarpine model of chronic spontaneous seizures to evaluate the possible association of seizure frequency with sensorimotor gating, spatial working memory, and neuropathology throughout limbic regions. For TLE modeling, we induced a 2-h status epilepticus by the systemic administration of lithium-pilocarpine. Once spontaneous seizures were established, we tested the locomotor activity (open field), spatial working memory (eight-arm radial maze), and sensorimotor gating (prepulse inhibition of acoustic startle). After behavioral testing, the brains were sectioned for hematoxylin-eosin staining (cell density) and parvalbumin immunohistochemistry (GABAergic neuropil) in the prefrontal cortex, nucleus accumbens, thalamus, amygdala, hippocampus, and entorhinal cortex. The animal groups analyzed included chronic epileptic rats, their controls, and rats that received lithium-pilocarpine but eventually failed to express status epilepticus or spontaneous seizures. Epileptic rats showed deficits in sensorimotor gating that negatively correlated with the radial maze performance, and impairments in both behavioral tests correlated with seizure frequency. In addition to neuronal loss at several sites, we found increased parvalbumin immunostaining in the prefrontal cortex (infralimbic area), thalamus (midline and reticular nuclei), amygdala, Ammon's horn, dentate gyrus, and entorhinal cortex. These tissue changes correlated with seizure frequency and impairments in sensorimotor gating. Our work indicates that chronic seizures might impact the inhibitory-excitatory balance in the temporal lobe and its interconnected limbic regions, which could increase the likelihood of cognitive deficits and interictal psychiatric disorders.

A Role in Learning for SRF: Deletion in the Adult Forebrain Disrupts LTD and the Formation of an Immediate Memory of a Novel Context

Whereas significant insight exists as to how LTP-related changes can contribute to the formation of long-term memory, little is known about the role of hippocampal LTD-like changes in learning and memory storage. We describe a mouse lacking the transcription factor SRF in the adult forebrain. This mouse could not acquire a hippocampus-based immediate memory for a novel context even across a few minute timespan, which led to a profound but selective deficit in explicit spatial memory. These animals were also impaired in the induction of LTD, including LTD triggered by a cholinergic agonist. Moreover, genes regulating two processes essential for LTD-calcium release from intracellular stores and phosphatase activation-were abnormally expressed in knockouts. These findings suggest that for the hippocampus to form associative spatial memories through LTP-like processes, it must first undergo learning of the context per se through exploration and the learning of familiarity, which requires LTD-like processes.

Progressive age-related impairment of cognitive behavior in APP23 transgenic mice

Transgenic APP23 mice expressing human APP(751) with the K670N/M671L mutation, were compared at ages 3, 18 or 25 months to non-transgenic littermates in passive avoidance and in a small and large Morris maze. The task in the smaller pool habituated their flight response to the platform. Impairments in passive avoidance and small pool performance in APP23 mice were clearly age-related. In the larger Morris maze APP23 mice at all ages were impaired in latency and distance swum before finding the platform. Identical performance of 18-month APP23 and controls in a visible platform condition indicates that the Morris maze performance deficit was not due to sensory, motor or motivational alterations. At age 3 months both groups initially unexpectedly avoided the visible platform, suggesting that in young mice neophobia may contribute significantly to performance in cognitive tests. In conclusion, APP23 mice exhibit both early behavioral impairment in the large Morris maze as well as impairments in passive avoidance and small pool performance that are marked only in old age.

Cholinergic modulation of inhibitory avoidance impairment induced by paradoxical sleep deprivation

1. Male Wistar rats were submitted to paradoxical sleep deprivation for 96 hr by a modified multiple platform technique. 2. Training of step-through inhibitory avoidance was performed immediately after the last day of paradoxical sleep deprivation. Twenty-four hr after training the animals were submitted to the retention test. 3. In Experiment 1, pilocarpine (4 mg/kg, i.p.) or atropine (4 mg/kg, i.p.) were administered daily during the paradoxical sleep deprivation period. Pilocarpine, but not atropine, reversed the impairment induced by PS deprivation. 4. In Experiment 2, pilocarpine (4, 8 and 12 mg/kg, i.p.) was injected 1 hr before training in order to verify if the reversal of memory impairment was an effect secondary to residual enhanced blood levels of pilocarpine during training. Acute treatment with pilocarpine, in any dose, did not reverse the impairment produced by paradoxical sleep deprivation 5. Activation of the cholinergic system during the period of deprivation is able to prevent memory deficits induced by paradoxical sleep deprivation.

Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.