hippocampal physiology Flashcards
what kind of memory is the hippocampus necessary for/what kind of memories cannot be formed without a functional hippocampus
hippocampus necessary for explicit LTM formation; cannot form new explicit (episodic or semantic) memories if it isn’t functional
what kind of memory is unaffected with hippocampal damage (4)
- STM
- WM
- any semantic information that was well-learned before
- implicit memory
types of explicit memories (2) and explain
- episodic memories: past personal experiences (who, what, where, when, why)
- semantic memories: common knowledge, facts, ideas and concepts (names, colors, sounds of letters, capitals, any other basic facts)
fate of semantic and episodic memories over time
semantic: become hippocampal independent over time; seem to be permanently stored in the cortex
episodic: remain dependent of the hippocampus
what suggested that neurons in the hippocampus encode episodic memories
electrical stimulation of hippocampus evokes specific episodic memories
how are long term explicit memories formed
- memory encoding: cortical sensory systems are activated during experience -> hippocampus takes a snapshot of the activity
- memory retrieval event: hippocampus reactivates same cortical pattern of sensory activity as when the memory was encoded
pattern completion/memory recall mechanism of CA3 neurons (4)
- firing of CA3 neurons initially only driven by external inputs
- all CA3 neurons are interconnected, but activity of few cells doesn’t propagate to entire population because initially all CA3->CA3 synapses are too weak to trigger APs
- when few CA3 neurons activated at same time (partial cues of memory) = fire together wire together event -> connections grow stronger & synaptic connections can now drive APs
- now, when even only 1 CA3 neurons fires, entire ensemble will fire (all other neurons involved in memory will be activated)
what does the fact that explicit memories become independent from the hippocampus mean
the memory will still be there if the hippocampus is damaged because it is now stored in the cortex
what happens during the years when memories are dependent on the hippocampus
hippocampal activity (during recall events and sleep) gradually reorganize synaptic weights in cortex so that semantic info (and very old episodes) can eventually be retrieved without hippocampus)
how does memory retrieval driven by hippocampus affect connections
by replaying events, connections within the cortex become stronger and more stable so that eventually, the hippocampus isn’t need anymore
explain the ‘T maze/probe’ test and what do results suggest
- mouse learns it needs to turn right (towards window) to find food
- maze is flipped -> with minimal training = turns towards window; with overtraining = turns right (not where the food would be, automatically turns right)
* suggests that explicit spatial memory is initially learned (turns toward the window is maze flipped); over time, stimulus-response memory dominates
in the ‘t maze’ test, what do (a) BG lesions disrupt; (b) hippocampal lesions disrupt
(a) disrupts implicit stimulus-response learning (automatically turning right)
(b) disrupts explicit spatial learning (turning towards window)
place cells
neurons that only fire when mouse is in a particular position
activity of place cells (a) across several days (b) in different rooms
(a) position encoded by activity of place cell is consistent over several days
(b) position encoded by activity of place cell in room 1 doesn’t give info about position it would encode in room 2
what does the existence of place cells suggest
that animals understand the geometry of space -> if go south, east, north, know that to get back to same position, needs to go west
what feature do place cells develop when animals explore narrow passageways
become direction-specific -> going in different direction seems like a different location for animal?
when rats were trained to turn left and then right on alternating trials to receive food, what was observed in the hippocampal formation and what does it imply
when animal was reaching decision point, different hippocampal cells were active depending on if animal had last turned right or left -> some place cells encode position, some place cells encode more abstract things (contextual info/state of the animal)
other than spatial position, what is place cell activity influenced by (4)
- location and identity of nearby objects
- time of day
- valence of position in room (reward/punishment)
- location where animal is planning to go/where it came from
TET-OFF cFOS-ChR2 system (5)
- BAC transgene = cFOS gene promoter drives expression of bacterial transcription factor tTA
- tTA protein activity inhibited by doxycycline (mixed in animal’s food)
- when dox removed (1 day), tTA protein binds TRE promoter to drive ChR2 expression
- any cells that read cFOS gene promoter when dox absent will express ChR2
- ChR2 protein lasts few weeks
how used optogenetics to stimulate specific hippocampal cells (limit to neuron encoding one place) (3) and results of experiment (and is necessary and sufficient?)
- day that animal is off dox -> place A get shocked, place B normal -> active place cells get labeled with ChR2
- back on dox -> animal that was in place A is put in place B & animal that was in place B is put in place A
- shine light to activate ChR2 on place cells
* results = animal A (shocked) freezes when light is shined, even if in different place (not encoded by place cells) because place cells activated are for shock room (A); animal B doesn’t freeze when light is shined + shock because place cells activated are for room without shock (B) -> firing of hippocampal neurons active during cFC is necessary and sufficient to retrieve associated freezing behavior
what is the problem with the optogenetic stimulation of place neurons (TET-OFF system) (2) and what is solution to show that palce cell activity underlies memory-guided spatial navigation (stimulation and behavior)
- tons of neurons were photostimulated; unclear if any of them are actually place cells
- evidence suggests that cFOS expression in hippocampus mostly restricted to cells that encode general context (rather than specific spatial position within context)
* solution = stimulate just cells encode particular position within a context (position in rom, not whole room) and behavioral output needs to be specifically associated with that position (rather than larger context)
what is an ‘all-optical’ approach and what does it permit
simultaneous 2-photon calcium imaging and optogenetics in head-fixed mice as perform virtual reality spatial navigation task -> stimulation of specific hippocampal place cells to assess their causal contribution to behavior guided by spatial memories
aspects of confocal 1-photon microcopy (5)
- blue light shined in
- green light emitted
- light emitted is blurry, but 2nd lens focuses it so isn’t blurry to the eye
- light is shined in all layers
- all layers are photobleached after light shined
aspects of 2-photon microscopy (5)
- 2 beams of light at 1/2 wavelength of energy of blue light
- 2 beams focus on 1 spot so wavelength adds up and activates FP
- specific spot, not all layers
- no photobleaching because exposures the length of fs
- not blurry because focused on 1 spot
methodology of all-optical experiment (4)
- mice express both GCaMP6 and excitatory opsin C1V1 in CA1 neurons
- after virus injection, cortical tissue above hippocampus removed
- replace skull with glass over hippocampus
- mice trained to perform virtual reality spatial navigation task -> 2-photon imaging and optogenetic stimulation of CA1 pyramidal cells
explain behavioral task of virtual reality spatial navigation task (5)
- one area designated as reward area
- mice trained to wait 3 sec in area and lick min 3x to receive reward
- 10 sec white screen timeout penalty if lick > 10x outside of reward zone or if ran into back wall of track
- 5 sec dark screen timeout after successful trials
- after timeout, stop licking for 3 sec to start new trial
GCaMP recordings of CA1 cells during virtual reality spatial navigation task and central hypothesis derived
different place cells were active at different moments/places along track (start position, reward zone, etc.)
* hypothesis = stimulation of similarly tuned place cells will bias mouse behavior toward that which is normally exhibited in the location of those place cells’ fields -> would provide evidence for causal role for place cell activity in guiding spatial behavior and supporting spatial memory
place cell classification and activity in virtual reality spatial navigation task (2)
- start-zone place cells -> most active during (a) stable high running speed and (b) low lick rate
- reward-zone place cells -> most active during (a) decelerating running speed and (b) high lick rate
(a) responsiveness; (b) average response magnitude (c) specificity of 2-photon opto-stimulation of specific neurons
(a) all types of neurons had same rate of response
(b) all types of neurons had ~ increased fluorescence than normal
(c) neuron type stimulated was specific
behavioral effect (licking behavior) of targeted stimulation (mid-track) of (a) reward-PC before reward zone; (b) start-PC; (c) non-PC
(a) increase in # of error-trials -> mice stopped to lick when neurons were stimulated (before reward zone) because thought were in reward zone
(b) didn’t significantly alter licking behavior
(c) didn’t significantly alter licking behavior
magnitude of behavioral effect of photostimulation and # of neurons in virtual reality spatial navigation task
of reward-PC that responded to stimulation correlated with magnitude of increase in lick rate
behavioral effect (time spent) of targeted stimulation (mid-track) of (a) reward-PC; (b) start-PC; (c) non-PC
(a) mice stopped earlier than normal so spent less time in reward zone compared to unstimulated trials
(b) mice stopped after reward zone (ran into the wall) -> think they’re at the beginning so continue to run
(c) no change of running speed or time spent in reward zone
what does the fact that stimulation of start-PC evokes change in behavior after stimulation has ceased (run into wall) suggest
lasting impact on neural activity that is not fully reset by visual cues in environment
amount of licking in reward zone after (a) reward-PC stimulation; (b) start-PC stimulation mid-track
(a) decrease of licking in reward zone
(b) decrease of licking in reward zone because increase of running beyond reward zone
what can stimulation of a single place cell affect
activity of other place cells
how assess how stimulation affected activity of non-targeted place cells
- identified place cells that had largest different in activity bw stim and non-stim trials after mice passed through stim zone
- picked cells that fired more than normal after stim and cells that fired less than normal after stim
cell activity of (a) enhanced cells; (b) suppressed cells of non-targeted place cells after stimulation and what do results indicate
(a) activity of some slowly goes back to baseline -> expected; some spike after stim onset
(b) activity of some slowly goes back to baseline (but less activity than control); some spike after stim, but less than control
* indicates place cell stimulation alters larger place cell network in enduring manner -> place cells may preferentially interact with members of same map, communicating through local interneurons to guide network dynamics
why do the suppressed non-targeted place cells all represent cells that are normally active after the stim zone
cells that normally fire there are less sure (of where they are?) so they fire less
long term behavioral effects of stimulation of reward-PC
at the end of sessions, mice decelerated before the simulation point -> suggests that reward-PC stimulation triggers changes in network that cause animal to anticipate stimulation zone (s if it were reward zone)
long term network effects of stimulation (3)
- reward-PC switches place they represent -> now activated at stim point
- non-PC becomes PC when stim and a bit after stim
- start-PC switches place they represent only post-stim
pre-post stim correlation -> how much does PC activity change because of stimulation? (do cells spike at same place it used to)
- ‘no stim’ has good correlation -> spiking hasn’t changed pre-post stimulation
- when stim start-PC and rew-PC -> start-PC, rew-PC and non-PC decreased correlation with themselves -> they all don’t spike at place they used to be (because network got confused)
general conclusion
optogenetic stimulation of specific place cell populations is sufficient to bias behavior toward that of associated with the location of their place fields -> demonstrates causal role for place cell activity in guiding spatial navigation and supporting spatial memory
what indicates that representation of space in hippocampus is efficient and sparse
errors in spatial navigation were produced by stimulation of small number of place cells
stimulation-induced neural plasticity as a result of driving reward-PCs (2)
- spatial learning
- not limited to targeted neurons (connections, long-term)
what does hippocampal encoding of spatial position enable
formation of more detailed memories that allow navigation through both external world and within internal cognitive models
