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WHY ARE THERE PARALLEL HIPPOCAMPAL - DIENCEPHALIC PATHWAYS FOR EVENT MEMORY?. Wellcome Trust Project JOHN AGGLETON SHANE O’MARA JONATHAN ERICHSEN SERALYNNE VANN. Presentation format. Introduction to synaptic plasticity and memory Hippocampal-diencephalic system (HDS) - PowerPoint PPT Presentation
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WHY ARE THERE PARALLEL HIPPOCAMPAL - DIENCEPHALIC
PATHWAYS FOR EVENT MEMORY?
Wellcome Trust Project
JOHN AGGLETON SHANE O’MARA
JONATHAN ERICHSEN SERALYNNE VANN
Presentation format
• Introduction to synaptic plasticity and memory• Hippocampal-diencephalic system (HDS)• Mammillary bodies (MB)• Anterior thalamic nuclei (ATN)• Lesion studies• Subiculum• Overview
Synaptic plasticity >> Memory• Different types of:
memory
memory circuits
plasticity involved cells
synaptic plasticity
plasticity induction
plasticity cellular targets
intracellular mechanisms
Plasticity
Sjöström and Nelson, Curr Op Neurobiol, 2002
Sjöström and Nelson, Curr Op Neurobiol 2002
Introduction
Hippocampal - diencephalic system
Lesion studies : the hippocampal -
diencephalic system is required for the encoding of episodic information.
Still the functional significance of this system for the memory formation is under construction.
HDS
Significance of the parallel thalamic projections (direct and via MB)
Hippocampal –diencephalic circuit (Papez’s circuit):
hippocampal formation → MB → anterior thalamus → cingulate cortex → parahippocampal gyrus → hippocampal formation
MB are part of the extended hippocampal - diencephalic system
HDS
Methods and approaches• Retrograde markers tracing• Lesion application + Behavioural
examination (learning tasks)• IEG activity immunohistochemistry• Field EPSP recording under
anaesthesia• Field EPSP + Lesion application
under anaesthesia• EEG under anaesthesia and in freely
moving rats• Unit recording under anaesthesia and
in freely moving rats• Others (fMRI, microdialysis)
• PPF• LTP• LTD• STDP • Theta
Methodology
Significance of the parallel thalamic projections (direct and via MB)
MMB involvement in spatial working memory tasks:
- lesion techniques - c-fos protein
expression - cytochrome oxidase
(CO) aczivity (Conejo et al., 2004).
MB
MB lesions – the mildest outcome Spatial deficits after MB damage are not as severe
as those found after hippocampectomy and are typically less severe than those associated with ATN damage.
Anatomical or functional bypassing possibility?
MB
MB function: relayers of hippocampal theta rhythm - to the ATN and beyond
• theta-related cells in the MB seem to be driven by descending projections from the hippocampus
• and are especially correlated with the CA1 theta generators.
MB
MB function: relayers of hippocampal theta rhythm - to the ATN and beyond
• Propolsals about the significance of this relay:
• 1. relaying of theta by the mammillary bodies might reduce interference by helping to separate encoding and retrieval (Hasselmo et al., 2002)
• 2. theta activity im parallel with head direction processing facilitates the transmission and plasticity of LMN – AD thalamus - retrosplenial information (Vertes et al., 2004).
• 3. theta oscillations of MMB facilitate the information current in Papez’s circuit
MB
ATN convergence has a gating effect on theta and its potential to act upon the retrosplenial
cortices and back upon the hippocampus.
Synaptic plasticity and hippocampo-diencephalic system
MB
Anterior thalamus and theta• Approximately 75% of ventral
ANT cells fire synchrony with hippocampal theta rhythm.
• Active locomotion increases peak firing rates of anterodorsal thalamic head direction cells.
The level of locomotor activity provides a statedependent modulation of the response magnitude of AD HD cells (Zugaro et al., 2001).
ATN
Mammillothalamic tract lesions
Vann and Aggleton, 2003; Vann et al., 2003: mammillothalamic tract lesions (MTTx) coordinates of the lesion relative to ear-bar zero were AP +4.2 and L +0.9, and the depth from top of cortex was +6.9 mm.
Lesion studies
Subicular coordinates
Transverse sections perpendicular to the long axis of the hippocampal formation(Ishizuka 2001).
Subiculum
PLASTICITY IN THE PROJECTION FROM THE ATN TO THE ANTERIOR CINGULATE CORTEX (Gemmell and O‘Mara, 2002)
Hippocampal - diencephalic plasticity, system approach
Overview
Regional specific long-term synaptic plasticity
Physiological substrate of hippocampal diencephalic plasticity – possible key to the encoding of temporal sequences (episodes).
Overview