Introduction
In part I of this series we discussed the history and uses of the
nootropic agent piracetam. Piracetam significantly improves memory in
animal models and humans when memory is compromised and moderately
improves memory in healthy humans and animals. Despite decades of
research, the mechanism of action of piracetam is still largely
elusive. Part II will discuss the effects piracetam has on the brain
and their proposed mechanisms.
Global effects
The effects of piracetam are centrally mediated. This is supported by
the fact that it reverses the action of central, but not peripheral
anticholinergics, among other things [1]. Piracetam also improves both
cerebral blood flow and glucose utilization and alters EEG activity
[1-2]. This last effect has been researched in some detail.
Spontaneous EEG readings indicate that piracetam causes a decrease in
delta, theta, and beta activity and an increase in alpha activity [3].
Alpha wave activity is usually associated with a state of being awake
and alert, yet relaxed, and is commonly elevated in those practicing
activities such as yoga and meditation [4-5]. Piracetam also changes
the late components of event-related potentials (ERPs), which are
responsible for information processing [3].
One way in which multichannel EEG recordings are classified is by the
measure of global dimensional complexity. A newer and related scale is
global omega-complexity. These scales are correlated with mental
activity, vigilance levels, and drug effects, and are said to measure
"the degree of cooperation of the processes generating the electric
field of the brain" [3]. A decrease in complexity usually represents an
increase in the brain's functional state, and single doses of piracetam
decrease both measures with a U-shaped dose response curve and the
largest decrease seen at 2.4 g [3]. Although this information is useful
for establishing dose-response curves, the usefulness of correlating
changes in global complexity with behavior and cognition is debatable.
Piracetam also facilitates the interhemispheric transfer of
information, as evidenced by early studies in both humans and animals
[6-7]. The importance of this to the activity of piracetam, and whether
this is a specific effect or a result of piracetam's effect at large,
is unknown.
Because of the wide variety of effects, it has been argued that the
mechanism of action of piracetam is nonspecific. This is supported by
the fact that a single common mechanism of the racetams has yet to be
established. However, the fact that the dose-response curve for
piracetam is bell-shaped would indicate a specific mode of action [1].
We will now discuss the action of piracetam at the molecular level, to
see if it sheds any light on this debate.
GABA
Since piracetam is a cyclic derivative of GABA, and the GABAergic
system plays a role in learning and memory, it seems to be the most
likely system to be effected by piracetam. However, piracetam shows no
GABA receptor binding until extremely high concentrations are reached,
and likewise does not effect uptake of GABA or levels of GABA in the
brain or plasma [1, 6]. Piracetam does significantly decrease the
I-wave peak size, an effect seen with GABAergic drugs such as
benzodiazepines, but this could be due to alterations in
neurotransmitters other than GABA which suppress generation of I-waves
[8].
Although GABAergic mechanisms do not appear to play a significant role
in piracetam's activity, there is some evidence of an effect relating
to GABA-B receptors. In one study, piracetam had a similar
pharmacological profile to the GABA-B antagonist CGP 35348, as it
reduced analgesia caused by both direct and indirect GABA-B receptor
activation and amnesia from direct GABA-B activation [6]. It was
hypothesized that piracetam had an effect on G-proteins or
G-protein-mediated effects which caused it to have action similar to
GABA-B antagonists. However, piracetam reduces analgesia caused by
non-GABA related drugs as well, so the evidence for a GABAergic
mechanism is still weak.
Monoamines
Piracetam can alter monoamine levels and turnover in many ways. In
general, piracetam most reliably leads to an increase in dopamine
levels in animals. One study using 20 and 100 mg/kg found no
significant effect on dopamine levels [12], but other studies with
100-600 mg/kg find increased levels and turnover of dopamine in various
areas of the brain [1, 13-14]. This means that any effect piracetam has
on dopamine levels in normal doses is likely to be insignificant.
The information on serotonergic and adrenergic transmission is even
less conclusive. 20 mg/kg decreased brain 5-HT levels, while 100 mg/kg
has been reported to increase total brain 5-HT but decrease 5-HT in the
striatum [12-13]. 600 mg/kg increased serotonin levels and turnover in
the cortex, but decreased both in the striatum, brain stem and
hypothalamus [14]. Doses of 20 mg/kg and 600 mg/kg were reported to
increase norepinephrine levels, while 100 mg/kg caused a decrease [1,
12, 14].
This all goes to show that piracetam produces widely differential
effects on the levels and turnover of monoamines in the brain, which
are probably very condition-specific. Other studies indicate that
piracetam decreases serotonin, dopamine, and norepinephrine brain
concentrations in rats, or increases levels of all three [1, 9].
Piracetam yields differential effects on MAO as well, with studies
finding either net inhibition or net stimulation [1, 10]. All of this
taken together seems to indicate that any effect on monoamines probably
occurs far downstream of the original effect, and this would be in line
with the observation that no direct effect on dopaminergic,
serotonergic, or adrenergic transmission has been found [11].
Glutamate
The glutamatergic system is the only receptor system which piracetam
seems to have a direct effect on [1, 8, 15]. Piracetam binds to some
glutamate receptors, although this may not be at all relevant in normal
doses [1]. Despite this, one review contends that the glutamatergic
system may play a central role in piracetam's nootropic activity [1].
One of the main supporting pieces of evidence for this hypothesis is
that the memory-enhancing effects of piracetam can be easily blocked by
NMDA channel blockers. Both ketamine and MK-801 antagonize the
nootropic effect. In a Russian study, it was found that piracetam
potentiated the response to glutamate and aspartate through the glycine
site of the NMDA receptor, and this effect may be obscured by the
presence of glycine. This would indicate a direct effect at the glycine
site, and another 2-pyrrolidinone derivative, HA-966, exhibits
antagonistic properties at the glycine site [1]. Again, this effect has
not been demonstrated to be relevant in normal doses. Piracetam also
prevents the age-related decrease in NMDA receptor density [16], but
this could easily be due to nonspecific action.
Piracetam may also have important effects on AMPA receptors. As is the
case with NMDA receptors, it increases the density of AMPA receptor
sites [1, 16]. More importantly, piracetam increases the efficiency,
but not potency, of AMPA-induced calcium influx and antagonizes the
effect of the L-type calcium channel blocker nifedipine. This
represents a promising and largely unexplored mode of action, since
positive modulation of AMPA receptors is also an effect shared by other
cognition enhancers such as the ampakines and aniracetam.
Acetylcholine
The majority of the secondary literature reports that piracetam
operates primarily via a cholinergic mechanism, which is not
surprising, since the role of the central cholinergic system in
learning and memory is fairly well-established, while the role other
systems play is not as well-known [17]. There is evidence both for and
against the proposition that cholinergic transmission plays an
important role in the memory enhancing action of piracetam.
The most reliable effect piracetam has on the cholinergic system is a
restoration of muscarinic receptor density in aged rats and mice [1, 3,
15-16]. Studies on whether muscarinic receptor density is
increased by piracetam in normal animals are contradictory, with one
showing an effect and another showing none [1, 16]. However, as is the
case with NMDA and AMPA receptors, an increase in receptor density does
not imply a specific effect on that system, but could occur downstream
of a different effect such as altered membrane fluidity. Another piece
of evidence presented to support a cholinergic mechanism of piracetam
is the fact that piracetam reverses scopolamine-induced amnesia, and
scopolamine is an anticholinergic [1, 17-18]. Again, this does not
necessarily show a specific action on the cholinergic system,
especially since piracetam reverses the effects of many amnestic agents
which do not exert their effects via the cholinergic system.
Additionally, reversal of scopolamine-induced amnesia generally only
implies that a drug may be useful for treating dementia, and would not
explain piracetam's effects in healthy individuals [19].
Most researchers also report that piracetam increases brain
acetylcholine release in animals, particularly in the hippocampus,
while others (including some in which cognitive enhancement is
demonstrated) find no effect on acetylcholine [1, 3, 11, 16, 20].
Piracetam may also increase high-affinity choline uptake, although the
majority of the research has failed to confirm this [1]. Taken as a
whole, the data indicates that piracetam generally increases
cholinergic transmission, but this effect is not necessary for it to
improve memory. This is further evidenced by the fact that piracetam
has many qualities independent of drugs that improve memory primarily
through cholinergic mechanisms – it can increase memory consolidation
when administrated up to eight hours after a trial, as opposed to two,
and unlike some cholinomimetics, the action of piracetam is reported to
be steroid-dependent [1, 21].
Corticosteroids
Taken as a whole, it would seem that if there is a specific mechanism
of action for piracetam, it does not involve an action specific to any
neurotransmitter system. It could be that piracetam relies on a
multitude of simultaneous events to improve memory, but this does not
seem congruous with the wide variety of situations in which it proves
effective. So it follows that the effects may not be mediated by a
neurotransmitter system, but a different system in the brain.
A consistent observation in the literature has been that the cognitive
effects of piracetam are steroid-dependent [1, 21]. In rats, it was
reported that adrenalectomy, aminoglutethimide, and epoxymexrenone all
interfere with the nootropic effect of piracetam, even though they did
not decrease learning ability in and of themselves. When aldosterone or
corticosterone were administered to adrenalectomised rats, the
nootropic effect was restored. On the other side of the spectrum, high
doses of aldosterone or corticosterone abolished the memory improving
effects of piracetam.
The relationship between piracetam, corticosteroids, and memory was
further researched in a passive avoidance model in day-old chicks. In
this model, strong training results in avoidance for days or even
weeks, while weak training results in avoidance for only a few hours.
It was found that strong training, but not weak training, resulted in
an elevation of corticosteroid levels, as well as corresponding changes
in the synthesis of cell adhesion molecules and synaptic connectivity
in some forebrain regions. Corticosteroid receptor antagonists also
interfered with long-term retention with strong training, while
intracerebral corticosterone facilitated learning from weak training
[21]. This is in opposition to the earlier results which implied that
reduced corticosteroid levels did not significantly effect memory and
learning. This is also in line with earlier studies that indicated that
the stress response and corticosterone release are key factors in
long-term memory consolidation [21].
When piracetam was introduced into this model, some interesting effects
were seen. First, after weak training, piracetam caused a small
increase in plasma corticosterone. This may functionally indicate that
piracetam renders the physiological response to weak training closer to
that of strong training, in other words, facilitation of long-term
storage of what would otherwise be short-term memories. The second
finding was in agreement with earlier findings, and indicated that
corticosteroid antagonists (for both type I and type II receptors)
prevented piracetam from facilitating long-term storage.
These effects must be interpreted with caution. First of all, although
they show that the effects of piracetam are dependent on
corticosteroids, they also show that memory and learning in general is
dependent on corticosteroids. Also, elevated corticosteroid levels are
by no means desirable, and block the effects of many memory improving
drugs. A small elevation in corticosteroid levels is a necessary aspect
of long-term memory consolidation, but that does not mean that it is
the only aspect, or that the relationship is linear. Finally, there is
conflicting data – although this study found piracetam increased
corticosterone in response to a learning trial, an earlier study in
rats which did not involve a learning trial found that piracetam
decreased both basal and morphine-induced serum corticosterone levels
[22]. This could be due to difference in species or other experimental
conditions. These results go to show that the nootropic effect of
piracetam can be blocked by corticosteroid antagonists, but the
evidence for a specific mechanism of action of piracetam involving
corticosteroids is still weak – and again, nothing has been
demonstrated at the molecular level, so these effects could just be
further downstream.
Ion channels
In 1993, Gouliaev et al. [1] thoroughly reviewed the available research
on piracetam's mode of action. For most systems of the brain, their
conclusions were similar to the ones here, although research from the
past ten years has also been integrated. They concluded that the many
different biological effects of piracetam (and other racetam compounds)
were more than likely secondary to a more specific primary effect, and
hypothesized that this effect was mediated by ion carriers or ion
channels. A specific effect at the ion channel level would be expected
to have an impact in every system of the brain, and in the case of
piracetam, this net effect would coincide with an increase in memory
and learning capability. There is also a large amount of experimental
evidence supporting this proposition, although most of it stems from
the assumption that the racetams share a similar mechanism of action.
The most promising target, especially given research in the last ten
years, seems to be in the regulation of pre- and postsynaptic calcium
levels by a change in calcium and/or potassium ion channel conductance.
Both calcium and potassium antagonists have shown nootropic activity,
and are respectively associated with a prevention of calcium ion entry
into the cell (which prevents the cell from overload) and an increase
of calcium influx into presynaptic nerve terminals. A study with
isolated snail neurons using concentrations of piracetam similar to
those that enhance cognition in vivo found that it suppressed both
high-threshold Ca2+ and K+ currents. Both of these effects
independently are associated with improved cognition, but when put
together, the end result would be either an increase or decrease in
intracellular calcium depending on cell type. [23]
Various other effects on ion channels have been noted with piracetam
and other racetam compounds [1, 23], but there are too many to cover
here, and the mechanism described above appears to be the most
significant. However, we once again encounter the same weakness in this
theory as we did with the others, because it would seem that if this
was the primary action of piracetam, then it would be a single action,
as opposed to many. Additionally, the evidence for a structure-activity
relationship [1] seems contrived. Despite this, this is one of the
better theories, as it would explain the majority of the properties of
piracetam, and our inability to pin the mechanism of action on a single
neurotransmitter system.
Membrane fluidity
Both in vitro in mouse, rat, and human tissue and in vivo in rodents,
piracetam increases brain membrane fluidity [1, 24]. Decreased membrane
fluidity has been implicated in a wide variety of neurological deficits
associated with aging, such as changes in signal transduction, enzyme
activities, receptor numbers, and receptor functions [24]. Piracetam
particularly increases membrane fluidity in rodents that have decreased
membrane fluidity to begin with, such as aged or scopolamine treated
rats, and in these cases it brings membrane fluidity to the level of
healthy controls [1, 24]. This increase occurs in most, but not all
areas of the brain [24]. Although the literature reports no increase in
membrane fluidity in normal animals, it is noteworthy that the data
always shows a nonsignificant trend toward an increase.
This increase in membrane fluidity is one of the best working theories
for piracetam's mechanism of action. For one, it would explain the lack
of toxicity, as it brings membrane fluidity to a certain, optimal
level, as opposed to increasing fluidity independent of other factors
[24]. This also explains why piracetam is most effective in situations
where brain function is normally impaired [25]. Secondly, given that
piracetam changes membrane fluidity at a very rapid rate in vitro, the
idea that this effect is downstream from a different effect such as
altered lipid peroxidation or cholesterol/phospholipid ratios has been
dismissed [24]. Next, a specific structure-activity relationship has
been theorized. There is evidence from multiple experiments indicating
that piracetam partitions into the phospholipid bilayer of brain
membranes and interacts with phosphate head groups, thus changing
membrane properties [3, 8, 11, 15, 24]. Piracetam also affects blood
cell membranes, which is presumably the cause for the positive
cardiovascular effects [11, 25-26]. Finally, this model effectively
explains most, if not all of the known effects of piracetam [16, 26].
Conclusion
Despite decades of research, there is still no commonly accepted
mechanism of action for piracetam. A number of theories have emerged
over the years, some stronger than others. The most likely explanation
is that piracetam increases membrane fluidity, especially in
compromised situations. Hopefully, further research will lead to more
conclusive answers.
For questions or comments regarding this article, email
dvdtlsn@bulknutrition.com.
Posted with permission from
Mind
and Muscle MagazineNo part of this article may be reproduced in any form without the permission of David Tolson or Mike McCandless. 
