Neuromodulation:
acetylcholine and
memory consolidation
Michael E. Hasselmo
Trends in Cognitive Sciences – Vol. 3, No. 9, September 1999
Why „two-stage model“ of long-term memory storage?
- traditional concept of „long-term“ memory can be divided into
intermediate – term episodic memory  sensitive to lesions of hippocampus and
long – term episodic and semantic memory  not sensitive to hipp. lesions1,2
 suggestion, that these properties of memory might result from a twostage process for long-term memory formation4-7
Two-stage model of long-term memory formation (Hasselmo).
{
information from environment
rapid encoding
active waking
EEG: theta
(excitatory connections between CA3 pyramids are
strengthened that are activated by the event4,7-9)
{
quiet waking,
SWS
EEG: SPWs
intermediate-term episodic representation
re-activation of the stored representations including
spread of activity across the strengthened excitatory
connections6,10
further strengthening of CA3-CA3 and CA3-CA1
connections4,9
activation of neurons representing the event in EC5
and association neocortex11
slower strengthening of the excitatory
associative synaptic connections between
neurons representing these features in
neocortex
 Formation of links between these neurons that
could be described as semantic or long-term episodic
memories in neocortex4,7,8
Neuropsychological evidence for the „two-stage model“(1)
- Ribot‘s law: the most recently encoded episodic memories are the most sensitive
to hippocampal damage (process: „temporally graded amnesia“3)
evidence:
 in subjects with damage to hippocampal subregions the temporal extent of
retrograde amnesia depends on number of regions involved2
 subjects having been treated with electroconvulsive therapy1
 monkeys trained on object recognition prior to a hippocampal lesion, impairment was
- greater, when lesion followed training within 0 – 25 days
- minor in monkeys with lesion 75 days after training12
 in rats having been trained to associate a particular testing location (=context) with a
tone-shock pairing, the ability to associate was
- greater if hippocampal lesions are performed 0 – 2 days after tone-shock pairing
- reduced if lesions were applied 5 – 10 days later13
(location [=context] learning depends on hippocampus; tone-shock pairing depends on amygdala)
 neuropsychological data supporting the „two-stage model“
Neuropsychological evidence for the „two-stage model“(2)
criticism on concept of hippocampal activity to guide the formation of
neocortical associations14,15:
 animal studies: extensive lesions: flat gradient of retrograde amnesia
partial lesions: temporally graded retrograde amnesia14
 human data: hippocampal damage caused a very long-term retrograde amnesia15
(but: these lesions included subiculum and EC whereas
very short-tem retrograde amnesia occurs when damage is restricted to
specific hippocampal subfields2)
 thus, consolidation does not neccessarily result in formation of links in association
neocortex, but could instead result in strengthening of representations within
hippocampus itself or within the EC
 „multiple-trace“ hypothesis15 possible also to be valid!
(= reactivation of memories and formation of additional representations within the
hippocampal formation; in this scenario memories are reactivated in CA3 and guide
strengthening of associations in CA1 or the EC4)
 but: also the process of forming multiple traces requires two stages with different
dynamics: first for encoding of new sensory information
second for reactivation of an old memory to form additional traces
 whole process takes place within the hippocampus
Computational evidence for the „two-stage model
- diverse computational models describing the interaction between hippocampus and
neocortex when reactivating representations for consolidation7,8,16-18
- key feature of these models:
 initial storage of an association in an „associative matrix“ (= component of hippocampus)
which requires capacity for rapid synaptic modification in the hippocampus
(LTP as a physiological correlate?9)
 then, stored representations from this matrix are repetitively retrieved to activate
units in association neocortex
(this repetitive reactivation results in gradual strengthening of the connections between
neurons in the associative neocortex)
= slow process compared to synaptic modification within the hippocampus
(experimentally supported by observation that LTP in rat association neocortex is
enhanced when stimulation is spaced and repeated over a series of days20)
Computational evidence for the „two-stage model (2)
- why two stages?
1. discussed in terms of modification of existing semantic representations in the neocortex3
example: network established that codes a range of items of semantic knowledge
here: „birds can fly“ („robins are birds that fly… sparrows are birds that fly…“)
if such a network were sequentially trained with a new item inconsistent with previous input:
„penguins are birds; penguins swim but do not fly“
 this new information could interfere with the previous learned knowledge and
could lead to the belief that all birds swim
but:
if the new information is stored as an episodic memory in hippocampal formation and later interleaved with other examples during training of neocortical representations during consolidation
„robins are birds that fly… sparrows are birds that fly… penguins are birds that
swim… eagles are birds that fly“
 new information can be incorporated as an exception in semantic networks
 interleaved learning of multiple different examples is necessary for the formation of
efficient representation in a number of different models (e.g. 21,22)
2. in models focussing on formation of representations in multi-layer hierachical networks23,24 alternate
phases of dominant feedforward versus dominant feedback connection have been used
 examples of computational models of memory consolidation favouring two-stage architecture
Electrophysiological evidence for the „two-stage model“
 hippocampal networks appear to reactivate external stimuli they have responded to
during active waking also during quiet waking and slow-wave sleep
(experimental evidence:
- place cells that fired during waking have an enhanced probability to fire again in a
subsequent sleep episode25
- pairs of place cells that code adjacent positions during active waking show more
correlated firing during subsequent SWS6,10,11)
 experiments demonstrating direction of flow of activity in the hippocampal formation:
- during active waking (theta present): extensive neuronal activity in layer II of the
EC (= layer providing input to the hipocampus)5
- during quiet waking and slow-wave sleep, SPWs selectively activate hippocampal
target structures in the EC („deep“ layers V and VI) but not superficial layers5
- also demonstration of correlation between sharp wave-associating ripples an sleep
spindles of prefrontal cortex26
 suggests that hippocampus could be inducing co-activation of
neurons in neocortical regions, which could form new crossmodal
associations27
Neuromodulation during sleep-wake cycle
 the two stage model requires different dynamic states during each stage
 during active waking: predominant influence of EC on CA3
Feedback connections arising from CA3 are functional but do not dominate over
the feedforward connection
during quiet waking and slow-wave sleep: predominant influence
of CA3 on CA1 and EC
 how are these different dynamic states modulated?
Hypothesis: modulation mediated by acetylcholine,
which shows parallel fluctuations during different stages
of waking and sleep
Changes in cortical neuromodulation
during waking, slow-wave sleep and REM sleep
hippocampus
locus coeruleus
frontal cortex
Fig. 3. Histogram bars represent levels of neuromodulators (shown at left) in cortical structures
relative to levels measured during waking (here normalized to be the same for all
neuromodulators). Acetylcholine (Ach) levels are based on microdialysis measurements in
hippocampus[32 and 33]. Levels of acetylcholine do not increase as much in the cortex during
REM sleep [33]. For norepinephrine (NE), level of unit activity in the locus coeruleus is shown
[75 and 85]. Serotonin (5HT) levels are based on microdialysis in the frontal cortex [77].
Neuromodulation during sleep-wake cycle
- active waking (AW)
- defined by behaviour (exploration, sniffing, extensive rearing-up) and
EEG (large-amplitude theta31 in hippoc. and EC)
- microdialysis measurements: high levels of ACh release32,33
- influence of hippoc. on EC is weak during AW
(as shown by - low spiking activity in deep layers of EC5
- small amplitude of EC field potentials arising
from hippoc. connections36,37)
- effects of ACh could provide a specific mechanism for weakening the
hippocampal feedback during AW
(experimental evidence from brain slice or intact animal:
suppressive effect of ACh at excit. glut. synapt. transmission at CA3  EC
feedback connections24,38-45
but weaker effects on feedforward connections to hippocampus42-44,46,47  Fig. 4)
- specific effects of ACh (brain slices):
- suppresses transmission at excitatory recurrent collaterals in CA344,48
- suppresses transmission at Schaffer collaterals38,39,43,49,50
- suppresses transmission at CA1  Subiculum connection45
Neuromodulation during sleep-wake cycle
- active waking (AW) 2
- effects of ACh (in vivo data)
- CA3  CA1 connection is weakened (reduced EPSPs) at
- local ACh-infusion40-42
- stimulation of cholinergic innervation from the medial septum54
- sensory stimulation46 (effect blocked by atropine42)
 Taken together, ACh-mediated suppression of excitatory feedback would act
to reduce the influence of hippocampus on EC and other cortical areas
during active waking
But: no total suppression of feedback from hippocampus
 Sufficient feedback for retrieval of relevant stored information
ACh does not suppress most of feedforward connections to hippocampus
- PP input to CA1 much weaker suppressed than CA3CA1 connection
within the same slice43
- direct ECCA3 input also less suppressed48
Neuromodulation during sleep-wake cycle
- active waking (AW) 3
- specific cholinergic effects on dentate gyrus:
- effects of ACh on mossy fiber  CA3 synapse
- cholinergic suppression also present 57, but weaker than associational inputs
to CA3 radiatum at comparable doses43
- no cholinergic suppression in the outer molecular layer of dentate gyrus47,58,59,
(PP input from lateral EC)
- cholinergic suppression does appear
- in the middle molecular layer47(input from medial EC)
- in the inner molecular layer59 (commissural input from contralatral dentate gyrus)
- cholinergic influence on PP input to dentate gyrus is weak (experimentally supported
by fact that angular bundle-evoked EPSPs in the dentate gyrus are larger during
high ACh-levels of active waking than during low levels of SWS)36,60
- medial septum-stimulation leads to increases in population spike activity in the dentate gyrus
while having no systematic effect on EPSPs61,62
(possible explanation for this enhanced spiking: GABAergic input from MS;
absence of an effect on EPSPs suggests that feedforward input is not suppressed)
- postsynaptic effects of ACh (which would enhance the response of neurons
to feedforward input64):
- depolarization of pyramidal cells65,66
- suppression of spike frequency adaptation67,68
Neuromodulation during sleep-wake cycle
- active waking (AW) 4
functional purpose of suppressing the feedback to EC during active waking?
- suppression is not total, as recently stored memories from hippocampus are still
accessible for retrieval
- but: hazard that the strength of connections neccessary for the strong transmission of
stored memories back to CA1 and EC would dominate over afferent input
 this could distort the initial perception of sensory information, causing
interference during learning in temporal structures
and – if retrieval activity is sufficiently strong – would cause hallucinations
(as mediated by the influence of cholinergic antagonists at high doses)
- thus: partial cholinergic suppression of excitatory feedback allows cued retrieval
without hallucinatory retrieval
Behavioural effects of cholinergic antagonists
- during active waking, high levels of ACh could result in a predominant feedforward
flow of information, allowing normal interaction with the environment
- what happens, when – during waking – reduction of the cholinergic effects is induced
by administration of a muscarinic receptor antagonist, e.g. scopolamine?
- prediction 1: enhancement of feedback effects in the cortex and this will interfere
with the feedforward sensory input to hippocampus
 low doses of scop. impair encoding of new information in human subjects
but do not effect (even slightly enhance!) retrievala,b
 higher doses would cause stronger feedback effects, which might be
the reason for the described hallucinogenic effects of muscarinic antagonistsc-e
- prediction 2: when cholinergic modulation is blocked after learning of paired associates,
this should interfere with learning of subsequent overlapping paired associates
 preliminary supporting data (then still unpublished):
- learning of related paired associates (A-B, lotion-bottle)
- then: injection of scopolamine
- new paired associates, either/or not overlapping (A-C, lotion-oil vs. D-E, kitchen-spoon)
- then test: greater impairment after learning of overlapping paired associates
Neuromodulation during sleep-wake cycle
- quiet waking
- defined by behaviour (grooming, eating5,30,33) and EEG (Sharp waves in hippocampus5,37)
- microdialysis measurements: decrease of levels of ACh
(~60% of those during active waking33)
- this decrease would release glutamatergic synapses from cholinergic suppression
(experimental evidence:
evoked synaptic potentials are much larger in CA1 or in EC
during quiet waking than during active waking60 or
when a rat is not being presented with sensory stimulation46)
- possibly drop of cholinergic inhibition also explains generation of SPWs and
- stronger feedback from CA3 to EC would then allow SPWs to spread
from CA3  CA1  EC4,5,37
consistent: finding that „retrohippocampal“ structures (deep layers of EC) show much
stronger spiking activity during SPWs of quiet waking
than during theta activity of active waking5
Neuromodulation during sleep-wake cycle
- slow-wave sleep (1)
- defined by EEG pattern (delta waves in neocortex)
- microdialysis measurements: decrease of levels of ACh
(~33% of those during active waking32,33)
- this drop in ACh levels would further release glutamatergic synapses from
cholinergic suppression  even stronger excitatory feedback than during quiet waking
(= very large increase in the effect of excitatory recurrent connections within
CA3 and excitatory feedback connections from CA3 to CA1 and EC)
- this drop could therefore underly the increase of SPW activity during SWS4,37
- the relese of suppression of excitatory transmission could contribute to the greater
tendency of cells to fire together during SWS if they fired in the previous waking period6,10,11
- loss of cholinergic suppression should also enhance the spread of activity in response
to stimulation –
shown for CA1 and EC
( increased magnitude of evoked synaptic potentials during SWS36,30)
Neuromodulation during sleep-wake cycle
- slow-wave sleep (2)
- what functional role could this enhancement of excitatory feedback have?
 provide the appropriate dynamics for the formation of additional traces within CA3 and
CA1, and could allow the hippocampus to „train“ the EC or association neocortex
on the basis of previously encoded associations
 not all information for episodic memories is stored in the hippocampus but CA3 region
provides a mechanism for linking together disparate information
from multiple regions of association neocortex;
 during waking, these links can be used for cued retrieval of recently stored information
by neocortex, but they should not be able to strongly drive neocortical activity
to the level of distorting sensory inputs (hazard of interference leading to hallucinations!)
 during SWS, the associative links formed in the hippocampus need to be reactivated
and need to influence other brain areas in a strong enough manner to drive the
slower modifications of neocortical synapses
 neurons coding an association in the hippocampus must be able to drive cells in EC
and neocortex without any assistance of sensory input – reduction of cholinergic
suppression might provide the opportunity for this strong feedback influence;
additionally: physiological activity during SWS has been proposed to be appropriate
for modification of synaptic components70
Neuromodulation during sleep-wake cycle
- slow-wave sleep (3)
- evidence for the importance of SWS for episodic memories:
 subjects are better at retrieval of word lists if they learn the list before falling asleep
and are tested on retrieval when awakened in the middle of the night, than
if they learn the list after some hours of sleep and are tested in the morning71
 this suggests greater importance of SWS for memory consolidation because
most SWS occurs in the early part of the night
 if semantic memory is constantly being reshaped during the consolidation process
in SWS, then sustained disruption of SWS should impair semantic memory
 certain types of epileptic syndromes display constant seizure activity during
SWS72,73 which can cause severe impairments of memory function
(also delusion, loss of language function)73
Neuromodulation during sleep-wake cycle
- REM sleep (1)
- defined by EEG pattern (similar to that observed during waking) and rapid eye movements, muscular
atonia, muscle twitches31, ponto-geniculo-occipital waves33,74 and theta oscillations
in the hippocampus
- microdialysis measurements: ACh levels in the hippocampus increase to levels above those seen
during active waking32,33  levels of ACh resemble those seen during waking
- contrasting: levels of other neuromodulators:
- noradrenergic neurons projecting to neocortex decrease in activity to low levels during SWS and
show no activity during REM sleep75,76
- serotonergic neurons projecting to neocortex also decrease activity during REM (recordings from
raphe nuclei75 and microdialysis of serotonin in frontal cortex77)
Function of this change in neuromodulatory state?
- norepinephrine suppresses feedback excitatory synaptic transmission in the piriform cortex,
with a much weaker effect on feedforward transmission78
- norepinephrine does not suppress feedback within hippocampal formation80
 Hypothesis:
- during waking: high levels of ACh and norepinephrine shut down recurrent connections in neocortex
- during REM sleep: high levels of ACh in hippocampus might suppress feedback there, the lower
levels of ACh and very low levels of norepinephrine in cortex might allow spread of activity in
neocortex without strong influence from hippocampus
Neuromodulation during sleep-wake cycle
- REM sleep (2)
Theoretical framework:
 waking involves initial encoding of episodic representations in the hippocampus
 SWS would allow transmission of episodic representations from hippocampus to EC and
association cortex
 REM would allow neocortical structures to undergo a re-analysis, in which this episodic
information would be re-interpreted in relation to previous semantic representations
 REM could represent the development of new feedforward representations for behavior
(prediction: REM would therefore also be important for procedural tasks
 experiment: evidence, that performance in procedural memory task seems to
depend on quantity of REM sleep81-84)
 process of altering neocortical representations during REM sleep could utilize the
information clamped on the neocortex by hippocampal feedback during SWS
 indeed: perceptual learning task performance correlated most strongly with
SWS during first quarter of the night and
REM sleep during the last quarter of the night83,84
Two-stage model of long-term memory formation
Fig. 1.. (A) During active waking, information coded by neocortical structures flows through the entorhinal cortex and
dentate gyrus (DG) into hippocampal region CA3 (connections less sensitive to modulation by ACh; thick arrows).
Here, synaptic modification forms an intermediate-term representation, binding together different elements of an
episodic memory. Connections suppressed by ACh modulation (thin arrows) to region CA1, entorhinal cortex and
association cortex are strong enough to mediate immediate retrieval, but do not overwhelm the feedforward
connectivity. (B) During quiet waking or slow-wave sleep, memories are reactivated in region CA3 during EEG
phenomena termed sharp waves. These waves of activity flow back through region CA1 to entorhinal cortex[5] and
neocortex [26]. This will enable the slow consolidation (formation of separate traces) of long-term episodic memory
in hippocampal region CA1, entorhinal cortex and association neocortex, and might underlie modification of
semantic memory within circuits of association neocortex.
Two-stage model of long-term memory formation
Fig. 1.. (A) During active waking, information coded by neocortical structures flows through the entorhinal cortex and
dentate gyrus (DG) into hippocampal region CA3 (connections less sensitive to modulation by ACh; thick arrows).
Here, synaptic modification forms an intermediate-term representation, binding together different elements of an
episodic memory. Connections suppressed by ACh modulation (thin arrows) to region CA1, entorhinal cortex and
association cortex are strong enough to mediate immediate retrieval, but do not overwhelm the feedforward
connectivity. (B) During quiet waking or slow-wave sleep, memories are reactivated in region CA3 during EEG
phenomena termed sharp waves. These waves of activity flow back through region CA1 to entorhinal cortex[5] and
neocortex [26]. This will enable the slow consolidation (formation of separate traces) of long-term episodic memory
in hippocampal region CA1, entorhinal cortex and association neocortex, and might underlie modification of
semantic memory within circuits of association neocortex.