Chapter - I

Introduction

It is an established fact that one's life

is influenced greatly by p^st experiences. Generally

the term memory is used to refer to the consequences

of past experiences. Foi . any learning to result in

a changed behaviour the learning stimuli will have

to be stored somewhere in the brain. The act of demons-

trating memory is very of.ten a retrieval of stored

information. However/ the nature of the triggers for

memory retrieval from its stored state into performance

are not very well understood. One type of trigger

appears to be an equivalency or at least a similarity

in the biochemical state oif the brain to the state

in which the original learning occured.

Learning that is,i acquired under a specific

condition and later retrie^ved only when the same con-

dition is reestablished is called state dependent

learning (SDL). When SDL occurs/ a behavioural response

learned while the animal is drugged will thereafter

be performed well whenever the animal is again drugged,

but will be performed p cEtorly or not at all during

tests without drug*. Conversely/ a response learned

while undrugged will be adequately performed without

drug and will be poorly performed when the drug is

present. Engrams acquired in one drug state are said

to be dissociated and hence irretrievable in anpther

drug state.

The most convincing demonstration' of state

dependent performance has been provided by the obser-

vations that an organism behaves adaptively is a speci-

fied situation under both normal and abnormal conditions

but the type of behaviour displayed at a moment is

determined by a particular state imposed at that stage.

The first example of state dependent learning was

provided by Lashley (1917) who trained rats in a maze ro- vcv *

after administration of strychlne or caffeine and

found that strych'ine enhanced the rate of acquisition I of the maze habit/ while caffeine resulted in slower

acquisition. In comparision to the control group both

the drug groups took longer to relearn the maze when

tested subsequently without the drug. Girdan and Culler

(1937) and Girden (1942a, b, c; 1947) conditioned

a number of autonomic and muscular responses in dogS/

cats and monkeys injected with crude curare or dihydro-

^ -erythroidine HBR. The conditioned response thus

established could not be subsequently demonstrated

in the undrugged state, but reappeared after the admi-

nistration of the drug^ Case and Funderbank(1947)

reported similar dissociation with physostigmine.

These findings suggest that severe response

decrement can be produced by a difference in the state

of the central nervous system at the time of training

and testing. Some investigators have proposed that

this decrement could be due to changes in the stimulus

as perceived by the subject. Carson (1957) demonstrated

that the rats could acquire a conditioned avoidance

response (CAR) quite rapidly in the period following

ECS but demonstrated no retention when tested several

days later. As distortion in perception of the stimuli

could occur in the period following ECS, this effect

can only be attributed to the difference in the state

of the CNS due to the gradual return of the perturbed

nervous system to a more normal state.

The most convincing evidence against the

stimulus change explanation for dissociation of learning

has been provided in a series of experiments by Overton

(1964). He used the saving method to evaluate the

degree of state dependency and argueid that if learning

in the drugged and non-drugged state is completely

dissociated, it shou-ld be possible to develop different

response tendencies in the two states concurrently

by alternate training trials under the two conditions.

It was found that rats in the experimental group learned

to turn toward one goal box when drugged and opposite

goal box when not drugged. Since the difference in

the amount of training trials was not significants

it is apparent that no transfer of training occured

between the two experimental conditions.

In another investigation by the same investi-

gator/ it was found that not all dissociation phenomena

are the result of some single process such as changes

in arousal level. A set of depressant drugs including

pentobarbital/ phenobarbital, alcohol, urethane and

meprobamate produced a state in which learning was

partly dissociated from learning in the non-drug state.

These substances were approximately equivalent in

their actions and their actions were interchangeable.

Anticholinergic drugs including atropine and scopolamine

were found to produce a state in which learning was

partially dissociated both from the non-drug and de-

pressant drug state. The effects of atropine which

produced this state neither mimicked nor antagonized

the dissociative effects of the depressants. This

study also showed that the amount dissociation of

learning caused by a particular substance may vary

sharply from task to task. Since there is no reason

to believe that the action of a given drug on the

state of various brain regions will vary with the

task, this finding illustrates that involvement of

brain regions in the mediation of learned responses

varies from task to task.

Thus it appears that learning acquired under the influence of a particular drug is generally best

retrieved when tested under the influence of same

drug.. This effect is attributed to the stimulus proper-

ties of the drugs and may occur independent of their

influences on acquisition or memory storage. In recent

years/ it has been proposed that learning might depend

on states induced by endogenous substances. State

dependent influence of endogenous substances have

been observed by a number of investigators (Gray,

1975; Spear/ 1978; Zornetzer, 1978; Izquierdo, 1980;

Riccio and Concannon, 1981; Izquierdo et al. / 1982;

Izquierdo and Dias, 1983). During and after various

forms of training there is a release of central and

peripheral cate'^holamines (Gold and Delanoy, 1981)

brain -endorphin (Izquierdo et al., 1982)/ and possi-

bly pituitary ACTH (Gold and Delanoy/ 1981; Riccio

and Concannon/ 1981). Neuroharaoral and hormonal changes

during test sessions have been much less investigated,

but there are reasons to think that they may be smaller

than those that occur during training. The central

p-endorphine release certainly is smaller during test

sessions (Iz^ierdo et al w 1982) and possibly the

hypersecretion of peripheral catecholamines and ACTH

is also less pronounced during testing^ particularly

if this does not involve the use of footshocks (Gold

and Delanoy/ 1981). Also, the administration of small

doses of epinephrine (E)^ /2-endorphin/ or ACTH prior

to testing counteract the amnesia caused by the same

substances when given after training (izquierdo and

Dias, 1983).

It is, therefore, possible that since the

treatments which affect memory are physiological

stressors/ they have their effect due to an excessive

neuroendocrine response. However, it was general

through that these treatments affect learning and

memory through direct influences on brain processes

underlying the storage or consolidation of recently

acquired information (McGaugh,1966; and HerZ/ 1972),

One of the most intriguing feature of these

treatments is that their impairing and enhancing effects

are t ime dependent. The magni tude of these effects

on memory decreases with a subsequent increase in

the time between training and treatment. However,

this decrement does not necessarily reflect the time

required for memory formation (Gold and McGaugh, 1975).

This view is supported by a number of investigators

who have reported that the temporal gradient of sus-

ceptibility to modification varies with the severity

of a particular treatment (Alpern and McGaugh^ 1968;

Gherkin, 1969; Gold, Macri and McGaugh, 1983; Ma'fi

and Albeftrt/ 1983). The interval after training during

which a treatment can alter retention performance

seems to be an indicant of the treatment's effective-

ness rather than that of a time required for memory

formation.

On the basis of these investigations the

hypothesis of memory modulation (Gold and McGaugh,

1975)/ which focuses attention on conditions under

which memory can be altered/ was formulated. It seems

that some biologically adaptive responses to training

such as alterations in arousal level, autonomic function

or neuro-endocrine activity might be correlated with

memory storage. From this perspective it appears that

memory/ in untreated animals/ might be modulated by

certain endogeneous responses to training (GoId/van

Buskirk and Haycock/ 1977).

The discovery of hormones dates back, to 400B.<^.

STRESSOR

d

ASCENDING RETICULAR ACTIVATING SYSTEM

CORTICAL ASSOCIATION AREAS VIA SENSES

JTOMATIC NERVOUS SYSTEM

ADRENAL MEDULLA

ANTERIOR PITUITARY POSTERIOR PITUITARY

ACTH \f^SOPRESSlN

EPINEPHRINE AND

NOREPINEPHRINE

ADRENAL CORTEX

OTHER GLANDS

JL KIDNEY

^CORTICOIDS

GENERAL -resisTKNCE

OTHER HORMONE AND ELECTROLYTE

CHANGES

CRF = CORTICOTROPIN RELEASING HORMONE ACTH = ADRENOCORTICOTROPIC HORMONE

FIG I A GENERAL OUTLINE OF THE'^M^AJOR STRUCTURES AND THEIR SECRETIONS DURING THE STRE^SS REACTION.

when Hippocampus postulated that health is dependent

on the proper balance .of humors in the body. However/

the historyfof endocrinology, actually begins with

Berthold (1849) who studied the function and mode

of action of the testers in cockerels. He reported

a humoral effect indepen<3ent of any specific neural

control. Since then a -large number of hormones have

been . identified, Hoirmones have extremely diverse

but specific functions which may be limited to a parti-

cular or extended to more/ organs. Their secretion

is triggered off by certain stimuli which tend to

displace the internal, state of homeostasis. The source

ot the stressor may be external or Internal, Generally

the thyroid/ parathyroid; pancreas and posterior

pituitary glands main^aij:t the balance of body. However/

if displacement is more intense/ then a more dramatic

endocrine response-th^. stress reaction - occurs. This

reaction is specific/ dominated by an increase in

specific hormonal secretions of two endocrine glands-

the pituitory and the .^adrenals. These stress related

•r>euro-endocrine responjses are depicted in Figure 1.

Since each learning situation is a stressful

situation in itself/ i•t^is possible that the peripheral

endocrine changes i.e. the release of pituitary and

adrenal hormones, resulting from the training procedure

10

used in various tasks/ modulate memory. Inhibitory

avoidance training is followed by release of adrenaline

from the adrenal medulla, noradrenaline from the sym-

pathetic nerve endings and ACTH from the pituitary

gland (Gold et al., 1981). Various forms of aversive

and nonaversive training are also accompanied by the

depletion of brain catecholamines (Gold and van Buskirk/

1978).

Naka.jima (1975) reported that the effect

of cycloheximide (CXM), a protein synthesis inhibitor^

on retention is due to its influence on certain hormones

(e.g. corticosteroids) and not due ' to inhibition of

protein synthesis inhibition. This fact is supported

by the results of their experiment in which it was

observed that subcutaneous injection of CXW shortly

before training in a passive avoidance task did not

result in amnesia in adrenalectomized rats. Thus it

appears that secretions of the adrenal glands mediate

the CXM produced amnesia. Flexner et al. (1972)

pointed out that vasopressin (VP) , a pituitary hormone^

is also capable of blocking the memory dusruptive

effects of certain amnestic agents. It antagonizes

the memory disruption produced by the protein synthesis

inhibitor, puromycin.

li

Further support for the role of endocrine

changes in memory modulation is provided by' a number

of investigations in which various hormones were found

to facilitate memory when administered in a less intense

dose while higher dose caused amnesia. In an investi-

gation by Gold and van Buskirk (1975)/ rats received

an injection of saline or one of the levels of E.

Retention performance was tested 24 hours later. Animals

that received injections of loWer doses of E had

retention latencies significantly higher than those

of the saline injected controls.' A higher E dose

appeared to produce amnesia in many animals. Similarly

Gold and van Buskirik (1976) found that lower doses of

4norepinephrine {NE) and ACTH enhanced retention after

high footshock while higher doses caused amnesia.

Low doses of VP enhanced retention of an inhibitory

avoidance response..

Thus a number of hormones have been reported

to modulate memory processes when administered peri-

pherally. Of these hormones, the most effective evidence

in this regard has been obtained in studies of the

effects of E on memory.

E is the major active principle of the adrenal :nedulla. Its jecretion level varies as a function

of the general level of stimulation of the nervous

system. Very little or none is secreted under basal

conditions (sleep)/ but even relatively mild activity

such as walking produces a marked secretery activity.

Strong stimuli/ particularly if prolonged/ painful

or emotional, flood the entire organism with E and

NE (Vane, VSolsten^holme and Connor 1960). There is

extensive evidence that peripheral E can' regulate

the mechanism responsible for the storage of new memo-

ries (Gold and Zornetzear, 1983; McGaugh/ 1983; McGaugh

and Gold^ 1986). This view is supported by the demon-

strations that post-training injections of E can enhance

memory for both avoidance (Gold and van Buskirk* 1975/

1976/ 1978a/ 1978b; Gold, Van Buskirk and Haycock/

1977; Izquierdo and DiaS/ 1983 a/b; Introini-Collision

and McGaugh ^ 1986) and appetitive tasks (Sternberg

et al./ 1985), enables classical conditioning to occur

under deep anesthesia (Weinberger, Gold^and Sternberg,

1984; Gold^ Weinberger, and Sternberg, 1985),

retards rapid forgetting seen in juvenile (Gold, Murphy

and Cooley, 1982) and aged (Sternberg, Martinez/ McGaugh

and Gold, 1985) rodents, and enhances the development

of long term potentiation (Gold/ Delanoy and Menin,

1984).

13

The effect of peripherally injected E has been found to be dose

dependent. Memory enhancement is seen at moderate doses and

amnesia observed at high doses (Gold and van Buskirk/ 1975/

1976; Gold, van Buskirk and Haycock, 1977).

Furthermore, it has been found that the effect of E is time dependent-a negative relationship exists between the effectiveness of a dose and increase in the training treatment interval.

The retention impairment effect of post train-

ing E might be a consequence of any of several influ-

ences-a^ an interference with storage mechanisms/b)

state dependency (Izuierdo and Dias/ 1983 a/bj^c)

release of brain p - endrophin (Carrasco et al / Izui-

erdo, 1982/1984) and the consequent establishment

of state-dependency on p -endorphin (Izquierdo and % Dias/ 1985; Izuierdo and NettO/ 1985a/ 1985b). A

Evidence against the first explanation is

available from studies (XzquierdO/ 1984; Introini-

Collison and McGaugh/ 1986) in which good retention

performance is seen after post training E/ if the

animals are also injected with E before the retention

test. Alternatives b and c are not mutually exclusive/

and there is evidence in favour of both. In favour

of alternative (b) i.e. State dependency on E, is

14

the finding that the impairment of retention caused

by post training E can be reversed by another injec-

tion of ACTH/ tyraminejor j3 -endorphin given shortly

before the retention test (izquierdo/ 1984); although

E is more effective in reversing E amnesia (Izquierdo

and Dias/ 1983b/ izquierdo/ 1984). in favour of alter-

native (c) -an effect mediated by ^ endorphin are

the findings that novel training experiences are acco-

mpanied by a release of brain yj-endorphin, (Izquiredo

et al./ 1984; Izquierdo^f^Netto, 1985 a) which sets

up a form of state dependency; retention test perfor-

mance can be enhanced by a post training injectiono^

opiate blockers/ including naloxone or naltrexone

(Messing et al., 1979; Izquierdo, 1979/ 1980/ 1984;

Gallagher, 198i; Izquierdo and Dias> 1985; Izquierdo

and Netto, 1985a, 1985b) or by pretest administration

of 0 -endorphin (Izquierdo, 1980, 1984;Izquierdo and

Dias, 1985; Izquierdo and McGaugh, 1985b; Izquierdo

and Netto, 1985b). These treatments would all be expe-

cted to attenuate the differences between the " -

endorphin state" after training and at the time of

retention testing. This state dependency interpretation

is based on evidence indicating that |3-endorphin is

released only after training. Training but not test

experiences, are followed by a decrease in brain

lo

/3-endorphin immunoreactivity/ which is not attri-

butable to inhibition of synthesis or increased intra-

granular degradation (Iz^uierdo et al.; 1984).

Thus the support for an important role of

peripheral E in modulating memory is considerable.

In addition to effects on memory/ the hormone also

has other central nervous actions/ such as influences

on electrographic arousal/ amygdala kindling/ cerebral

blood flow and central nonadrenergic activity. However/

the mechanisms by which the hormone might act on the

CNS/ to affect these functi6ns or to modulate memory

presents a problem because E does not cross the blood

brain barrier (BBB),

However/ E need not cause an alteration in

brain chemistry to produce behavioural effects. The

effects may be due to an influence on some peripheral

system.

Earlier it was suggested that the effect

of NE AND E might be due to the blockade of the adrenal

medullary secretions or the sympathtic response to

training (Gold/ McCarty and Sternberg/ 1981). However,

adrenalectomy does not cause amnesia in all cases

(Sternberg et al.; 1981). Nor does sympathetic blockade

with bretylium/ a peripheral sympathetic NE antagonist

lb

results in attenuation af this amnesia (Sternberg/

Gold and McGaugh, 1982).

There is extensive;evidence that hepatic glycogen

degradation is rapidly stimulated by a variety of horm-

ones which are released in response to different stimuli

(Goldfien, 1958; Exton and" Harper, 1975; Forsling et

al./ 1977). Stress states cause the release of a wide

range of hormones which exert catabolic effects on

the liver andSis found, tctv be one of them. It raises

blood sugar levels by a. direct action on the release

of glycogen from the liver. It stimulates hepatic glyco-

gen degradation at concentrations of M and greater

(Sherline et al./ 1972; Exton and Harper, 1975).

Recent evidence' suggests that hyperglycemia

subsequent to release or' injection of E contributes

to the hormone's actions on memory. E raises blood

sugar levels by a direct action on the release of glyc-

ogenfromtheliver.

The level of circulating glucose regulates the output of both insulin and glucagon. Low blood

sugar stimulates glucagon secretion while high blood

^ugar stimulates insulin "gecretion. However, there

is another mechanism regul-ating the output of is let's

1 7

Adrenaifne + + LIVER glycogen

Figure B Showing t.e «echan.s™ of resynthesising thelacti, acias to. iiver glycogen by E.

18

secretion, BS^-'l^ C-r*^ I*)

ZWs _ "T*..-J . _ ^These endocrine

t issues are richly innervated by the autonomic nerves

{Woods and Porte/ 1974/ Gerich et al., 1975) and respond

to sympathetic stimulation or the hormone E. E increases

the output of glucagon (o< - adrenergic activity) while

inhibiting insulin secretion ( adrenergic activity).

In this way, the important hyperglucemic action is favoured

in emergency situations. Under extreme stress, E not

only provides glucose to the circulation by glycogenolysis

but preferentially preserves it for utilization by the

brain since it simultaneously decreases insulin release.

Like glucagon, E increases the amount of active

phosphorylase in liver cells. It also promotes release

of acetate from muscle and glyceral from fat and these

provide materials for glycogenesis in the liver. Figure

II shows how the muscle glycoge^^ is broken down anaerO-

bically, in vigorous exercise, to lactic acid which

diffM^ii^^ into the blood and is resynthesisedfeglycogen

in the liver; with the help of epinephrine.

Several reports indicate that peripheral post-

trial injections of glucose also enhance memory storage

in a dose and time- dependent manner (Gold^ 1986; Gold

Vogt and Hall, 1986; Hall and Gold, 1986). A3 with E,

the dose response curve for glucose effects on memory

is an inverted U-function in which intermediate doses

enhance memory storage and the effects are time dependent (Gold,1986)

r>

SiG\o/gio>Ji act-iov o-f E" is -jreiea&e oVcujlatiV\g ^glucos^evels / it is possible that glucose represents part of the physiology by which E acts on memory. Cons-

istent with this view, Hall and Gold (1986) found that

plasma glucose levels show a footshock intensity related

increase soon after inhibitory avoidance training.

Further^ memory enhancing doses of glucose and E result

in increases (about 30%) in plasma glucose levels which

are similar to each other and to the values seen after

a training footshock. Thus these findings indicate

that relatively small increases in plasma glucose levels^

well within normal physiological limits, are corre-

lated with later retention performance. The similarities

in the behavioural properties of glucose and E and

the close relationship between post training glucose

level after footshock/ glucose or E injections/ there-

fore, suggest that hyperglycemia may be an important

component of E's action which modulate memory storage.

Further support for this view comes from the

findings in which memory enhancement with glucose was

found to be unaffected by pretreatment/ with adrenergic

antagonists (Gold^ Vogt and Hall, 1986). These findings

suggest that glucose modulation of memory storage is

beyond a relevant/ perhaps peripheral/ adrenergic rece-

ptor. Also, White and Messier (1988) found that glucose

can retroactively and non-contingently improve retention

in demeduJlated animals and do not'support the hypothesis

that the memory-improving action of glucose is due

to its effect on the adrenal medulla. Rather the effect

of glucose might be due to its action on the CNS. Unlike

E/ glucose is readily transported into the central

nervous system (Oldendorf^ 1971; PardL^idge and Oldendorf ^ t-M Cs y (X^t

1975). It is possible then that glucose^ directly on

the CNS to enhance memory. Consistent with this view,

Lee, Graham and Gold (1988)found that intraventricular

glucose injections can enhance memory storage for inhib-

itory avoidance training. The dose response curve has

an inverted Q form, as it does for peripheral injec-

tions of .E, glucose and several other memory.enhancing

treatments. Further the effects of glucose on memory are

time dependent. The findings are consistent with the

possibility that E responses to training modulate memory

by increasing circulating glucose levels and that

the increase in circulating glucose subsequent to increase

in E has central actions that regulate memory storage.

Ventricular glucose levels increase in response to

increases in circulating glucose levels. Unless glucose

acts directly at circumventricular sites (Cooper,

Beaty, Oppenheimer, Goodner and Petersdorf^ 1968),

21

elevations in ventricular glucose that follow circu-

lating glucose levels are not likely to contribute

substantially to neuronal glucose availability. Such

increases would be accomplished by cerebral vascu-

larization, which has far greater surface area in

contact with the CNS through which transfer of glucose

to neurons can be accomplished (G jedde, "J t ^ sen Silver /

1980; Hochwald, Gandhi and Goldman, 1983; Hochwald,

McShee and Ferguson, 1985). On the other hand/ injec-

tion of glucose directly into the lateral ventricle

TOiay be sufficient to increase glucose availability

in widespread brain regions. An additional possibility

is that central injections of glucose engage peripheral

sympathetic activation/ resulting in increases in

circulating E or glucose levels that mediate the effects

on memory.

Thus the behavioural and pharmacological findings

together with assessments of post-training plasma

glucose level support the view that the effects of

E on memory are mediated by its action on glucose.

The optimal memory enhancing doses of E and glucose,

derived from full behavioural dose-response curves

which have an inverted U-form; both result in elevated

9 ^

but physiologically rea'sonable plasma glucose levels.

Amnestic doses of E andi^g-^cose elevate glucose levels

to a significantly greajter and apprently supraphysio-

logical extent than do facilitating doses. Thus memory im/^t , modulation by E^oe relat-ed to the liberation of hepatic

glucose.

With this background we may pass on to the next chapter in which the effect of E and glucose

on memory has been re viewed-

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