Facilitatory effect of the intra-hippocampal pre-test administration of MT3 in the inhibitory avoidance task
Felipe DiehlL FURSTENAUDEOLIVEIRAGonzalo M. SánchezC CAMBOIMLucas de Oliveira AlvaresV LANZIOTTICarlos ČerveñanskýEdgar KornisiukD JERUSALINKYJorge Alberto Quillfeldt
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The physiological properties of hippocampal neurons are commonly investigated, especially because of the involvement of the hippocampus in learning and memory. Primary hippocampal cell culturing allows neuroscientists to examine the activity and properties of neurons at the individual cell and single synapse level. In this video, we will demonstrate how to isolate and grow primary hippocampal cells from newborn rats. The hippocampus may be isolated from each newborn animal in as short as 2 to 3 minutes, and the cultures can be maintained for up to two weeks. We will also briefly demonstrate how to use these hippocampal neurons for ratiometric calcium imaging. While this protocol describes the process for the hippocampus, with little to no modification, it can be applied to other regions of the brain.
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The hippocampal circuitry undergoes attentional modulation by the cholinergic medial septum. However, it is unclear how septal activation regulates the spatial properties of hippocampal neurons. We investigated here what is the functional effect of selective-cholinergic and non-selective septal stimulation on septo-hippocampal system. We show for the first time selective activation of cholinergic cells and their differential network effect in medial septum of freely-behaving transgenic rats. Our data show that depolarization of cholinergic septal neurons evokes frequency-dependent response from the non-cholinergic septal neurons and hippocampal interneurons. Our findings provide vital evidence that cholinergic effect on septo-hippocampal axis is behavior-dependent. During the active behavioral state the activation of septal cholinergic projections is insufficient to evoke significant change in the spiking of the hippocampal neurons. The efficiency of septo-hippocampal processing during active exploration relates to the firing patterns of the non-cholinergic theta-bursting cells. Non-selective septal theta-burst stimulation resets the spiking of hippocampal theta cells, increases theta synchronization, entrains the spiking of hippocampal place cells, and tunes the spatial properties in a timing-dependent manner. The spatial properties are augmented only when the stimulation is applied in the periphery of the place field or 400 – 650ms before the animals approached the center of the field. In summary, our data show that selective cholinergic activation triggers a robust network effect in the septo-hippocampal system during inactive behavioral state, whereas the non-cholinergic septal activation regulates hippocampal functional properties during explorative behavior. Together, our findings uncover fast septal modulation on hippocampal network and reveal how septal inputs up-regulate and down-regulate the encoding of spatial representation.
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The medial septum/diagonal band of Broca complex (MSDB) is a key structure that modulates hippocampal rhythmogenesis. Cholinergic neurons of the MSDB play a central role in generating and pacing theta-band oscillations in the hippocampal formation during exploration, novelty detection, and memory encoding. How precisely cholinergic neurons affect hippocampal network dynamics in vivo , however, has remained elusive. In this study, we show that stimulation of cholinergic MSDB neurons in urethane-anesthetized mice acts on hippocampal networks via two distinct pathways. A direct septo-hippocampal cholinergic projection causes increased firing of hippocampal inhibitory interneurons with concomitantly decreased firing of principal cells. In addition, cholinergic neurons recruit noncholinergic neurons within the MSDB. This indirect pathway is required for hippocampal theta synchronization. Activation of both pathways causes a reduction in pyramidal neuron firing and a more precise coupling to the theta oscillatory phase. These two anatomically and functionally distinct pathways are likely relevant for cholinergic control of encoding versus retrieval modes in the hippocampus.
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In this study, I investigate the role that hippocampal inhibitory cells (intemeurons) have on the synchronization of oscillations between the two hemispheres of the hippocampus. My study focuses in particular on the ripple oscillations, because this network activity is highly synchronous between left and right hippocampus. My hypothesis is that a subset of hippocampal intemeurons might establish axonal connections from the hippocampal area in which the somata reside towards the contralateral side, hence regulating inter-hippocampal ripple discharges. I address this hypothesis injecting in one side of the hippocampus substance P fragment, a peptide that increases the activity of subsets of inhibitory neurons in rat hippocampus, and the antimalarial Quinine whose roles as gap-junction blocker has been well established by numerous publications. Simultaneous recording from both hippocampi are thus compared to investigate whether ipsilateral injected drugs affect hippocampal ripple activity recorded contralaterally. I found that ripple oscillations are indeed affected by injection of the abovementioned drugs: Quinine increases length and decreases Inter Ripple Interval (I.R.I.) in both injected and contralateral hippocampus; on the other hand, SP decreases the average amplitude of the ripple episode, but increases the duration of the ripple event. Most importantly, many of the perturbations observed were preserved between injected and contralateral hippocampus. Since the drugs I employed affect mainly inhibitory neurons, I propose that long-range projecting inhibitory neurons located in the injected hippocampus are responsible for carrying the drugs' effects to the contralateral hippocampus. In conclusion, my results seem to indicate that long-range projecting intemeurons are involved in transmitting ripple synchronization information across the two hippocampi.
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Abstract Cholinergic denervation of the hippocampus by medial septal (MS) lesions results in the ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglia, into the hippocampus. To determine the effect of hippocampal sympathetic ingrowth (HSI) [ 3 H]‐QNB (L‐quinuclidinyl [benzilic‐4, 4(n)] benzilate) binding was assessed in the dorsal and ventral hippocampus four weeks after MS lesions. In dorsal hippocampus, HSI was found to signignificantly increase the number (B max ) of [ 3 H]‐QNB binding sites and to normalize the decrease in affinity found in animals with MS lesions plus ganglionectomy (i. e., no ingrowth). In ventral hippocampus, HSI was found to normalize the increased number of binding sites and decreased affinity found in animals with MS lesions without ingrowth. No effect on either K d or B max was found in animals that had undergone ganglionectomy with sham MS lesions. These results suggest that HSI can induce changes in hippocampal muscarinic cholinergic receptors. © 1994 Wiley‐Liss, Inc.
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Abstract Although the hippocampus is generally considered a cognitive center for spatial representation, learning and memory, increasing evidence supports its roles in regulation of locomotion. However, the neuronal mechanisms of hippocampal regulation of locomotion and exploratory behavior remain unclear. Here we found that the inhibitory hippocampo-septal projection bi-directionally controls locomotion speed of mice. Pharmacogenetic activation of these septum-projecting interneurons decreased locomotion and exploratory behavior. Similarly, activation of the hippocampus-originated inhibitory terminal in the medial septum reduced locomotion. On the other hand, inhibition of the hippocampus-originated inhibitory terminal increased locomotion. The locomotion-regulative roles were specific to the septal projecting interneurons as activation of hippocampal interneurons projecting to the retrosplenial cortex did not change animal locomotion. Therefore, this study reveals a specific long-range inhibitory output from the hippocampus in the regulation of animal locomotion.
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