Life Sciences Yol . 5, pp . 11-16, 1966 . Printed in Great Britain.
HYPOTHALAMIC SELF-STIMULATION :
Pergamon Press Ltd .
ITS SUPPRESSION BY BLOCKADE OF NOREPINEPHRINE
BIOSYNTHESIS P.ND REINSTATEMENT BY METHAMPHETAMINC B . P . H . Poschel and F . W. Ninteman Parke,
Davis b Company
Research Division, Ann Arbor,
Michigan
(Received 3 September 1965 ; in final form 5 Iiovember 1965) PREVIOUS experiments in
this
available a sustained high sites .
series
(1,2,3) employed various drugs to make
level of free norepinephrine at central reactive
The resulting effects on self-stimulation behavior supported the
tentative conclusion that norepinephrine serves as an excitatory neurohormone for reward neurons of the lateral posterior hypothalamus .
It was
suggested that norepinephrine may act as a transmitter at the synapses concerned . or perhaps as a modulator of the actual
transmitter .
Another approach to testing the above notion drug DL-al ha-methyl-tyrosine sine hydroxylase (4), dopa,
(:~-MT) .
the enzyme
the rate-limiting step
is now made possible by the
This drug is a potent
involved
in
inhibitor of tyro-
the hydroxylation of tyrosine to
in the biosynthesis of catecholamines
(5,6) .
Thus . appropriate doses of .~-MT block the biosynthesis of cate:cholamines and consequently greatly lower the brain concentrations of norepinephrine and dopamine
in guinea pigs
affected .
Therefor :,
(6) and rats
(7) . while leaving serotonin levels un-
if norepinephrine
is
involved
in the functioning of the
reward system of the posterior lateral hypothalam~~s, appropriate doses of ~7-MT should suppretis the powerful region .
reward effect normally obtained from this brain
Furthermore . after this suppression has developed,
reinstate the reward effect at central
if
~eactive site, ($) .
it serves
amphetamine should
to mimic the action of norepinephrine
In the present paper we report experiments
which confirmed both of these expectations . 1?
12~
HYPO ~sT~~~TC SELF-STIMULATION
Yol . 5, No . 1
Method Five male albino rats of the Holtzman strain, weighing about 285 9m, were
implanted with permanent electrodes aimed at the medial
of the posterior lateral hypothalamus . fled histologically .)
electrode placements were veri
The electrodes were monopolar, were made of 30-gauge
platinum wire, and were completely the tip .
(AJ1
forebrain bundle
insulated except for a 0 .5 mm segment at
Approximately four weeks after surgery,
the subjects were trained to
press a lever to stimulate their brains electrically .
The lever was housed
in
a box with an open top and with wire-mesh flooring . The box provided an experimental space 8 in . wide, 11
in . long, and 18
in . tall .
Each depression of
lever delivered 60-cycle sine wave sti.mulatlon, 0 .4 sec . s.timulus .was generated by an high
resistance .
jects,
The
isolation transformer in series with a relatively
The intensity of stimulation was set at 50 NA for four sub-
and at 40 NA for the other animal .
employed
i n duration .
the
These high stimulus
in order to elicit a near-maximal
In the critical
levels were
reward effect from the hypothalamus .
tests which followed, seven days
control test and a drug test on each subject .
intervened between a
The control test was run prior
to the drug test in three subjects ; for the other subjects this order was reversed .
Each test ran at least S hrs .
(racemate) was administered the session . omitted,
However,
In the drug tests, 50 mg/kg of a-MT
immediately after the lst,
3rd,
and 5th hrs . of
for two of the subjects the third dose of the drug was
since self-stimulation had already been attenuated markedly by the
first two doses . suspended in 2 .5
All
injections were administered
cc of methocel
solution .
above procedure with pure methocel
I .P .,
50 mg of drug being
The control tests duplicated
the
solution serving as the placebo .
The action of methamphetamine against the effect induced by a-MT was tested
in two of the above subjects .
(At the time we decided to run this
test, most of the subjects had already been in Fig .
l .
subjects .)
run in the experiment surtmarized
Exhaustion of our supply of a-MT prevented testing additional The animals received methamphetamine hydrochloride (0 .5 mg/kg,
Yol .
5,
No .
1
IiYPCTHAI~ALQC SELF-STIIIIILA,TICN
I .P .) immediately after the 8th hr . of the a-MT session . receiving methamphetamine,
the animals were
13
Directly after
returned tq the test chamber and
were run overnight . The tests were programmed autanatically by relay circuitry, and leverpresses were recorded continuously by a cumulative recorder . Results To assess the effect produced by ~-MT, the number of self-stimulations made by the animals during each hour of the test was tabulated . the mean scores
from this tabulation of the data
A graph of
is presented in Fig .
1 .
The control scores show good consistency, although a gradual slight decrease in response rate occurred over the 8 hr . test, probably reflecting an accumu-
FIG .
I
Mean number of self-stimulations made each hr . during the tests . Alpha -MT (54 mg/kg) or placebo was administered (I .P .) immediately after the lst, 3rd, and 5th hrs . However, the final (5th hr .) infection was omitted for two of the five subjects . Of primary significance, Fig . 1 shows that under the influence of AMT the rate of self-stimulation gradually declined and then virtually stopped . Analysis of variance indicated that the two curves different from parallel
( .001
in Fig . 1 are significantly
level of confidence) .
The effect methamphetamine had against the suppression induced by ~-MT is shown in Fig . 2 .
Reproduced are segments of the self-stimulation records
14
HYPOT$ALAMIC SELF-STIMULATION
YT 11.
ILl11 - Ii fFFEIf II111N11f1
IIIH 11 IUJI -/f
'
I
Yol .
5,
No .
~ill/Ilfil111f
//f,
rJ'
'
r F-HI
I~~ Î ~~~1~~~
_-
1 i I __ i ;~ ;rr
'r
1r~
1
-_
._
FIG . 2 Cunulatlve records of self-stimulation from two subJects . Records read from left to right . Each reset of recorder pen equals 55o self-stimulations . First record shown for both subJects is 1 hr . long ; second record, 3 hrs . long . The arrow Indicates the point where methamphetamine (0 .5 mg/kg) was given I .P . In the case of rat E-133, the methamphetamine was given 3 hr . after the final dose of a-MT . This interval was 5 hrs . for rat E-149, since this~sub)ect received only the first two doses of cx-MT . of the two animals employed in this phase of the study .
In these records,
each reset of the recorder pen represents 550 self-stimulations . segment of behavior shown for each animal
The first
is 1 hr . long ; it shows the rate
of self-stimulation rtlalntained during the lst hr . of the test, prior to any drug .
The second segment is 3 hrs . long .
It first shows the final
(8th)
hour of self-stimulation under cx-MT, when the behavior had virtually stopped . Then, at the point indicated by the arrow, 0 .5 mg/kg of methamphetamine hydrochloride was administered . ~ansued in the next 2 hrs .
The remainder of the record shows the effect that It is apparent that the drug, even at the small
dose employed, counteracted the suppressive affect of ~-MT . Discussion For the following reasons, a-MT appears to suppress self-stimulation because it lowers the level of brain cetecholamines .
First, other experiments
show that the dosing regimen used in the present study should reduce brain catecholamlnes substantially, but not affect serotonin levels at all
(6,7) .
Second, the rate at which the behavioral effect developed in the present study (Fig . 1) is consistent with the rate at which brain catecholamines fell
Yol . 5, No . 1
HYPOTHALAIQC SELF-~TIYULATION
in the other experiments (6,7) . that catecholamines play an self-stimulation .
15
Third, previous studies (1,2,3) have indicated
important role in lateral posterior hypothalamic
Therefore,
cessation of self-stimulation after a-MT seems
to be associated with a deficit of brain catecholamines, under which even intense stimulation of the lateral posterior hypothalamus seems unable to yield a satisfactory amount of . reward . This effect
is not equivalent to general sedation, because the animals
in the present study did not appear particularly sedeted even when selfstimulation had virtually stopped .
Weissman and Kce (7) also failed
observe sedation of gross behavior
in rats after a-MT .
to
In fact, they employed
higher doses of a-MT than used here, and their drug was in the levo form--the active
isomer with respect to enzyme
We expected
inhibition .
that methamphetamine would reinstate self-stimulation after
its suppression by a-MT, because the amphetamines seem to mimic the effect of norepinephrine at central reactive sites (8) .
Our positSve finding,
conflicts with the report of Weissman and Kce (7) .
however,
Using doses of d-amphet-
amine up to 10 times greater than the dose of methamphetamine employed here, these investigators reported that amphetamine stimulation is blocked by ~-MT . The exact reason for the discrepancy is not clear . will
Probably an explanation
be found in the very different aspects of behavior tested . Alpha-MT affects norepinephrine and dopamine equally (6,7), so it
clear which amine is critical
for hypothalamic self-stimulation .
grounds,
however, a good case can be made for norepinephrine .
nephrine
is highly concentrated
the reward system
On other
Thus, norepi
the hypothalamus and other structures of
(9), while most of the brain's dopamine
the corpus striatum (10) . stimulation of
in
is not
Moreover,
is concentrated in
recent work has indicated that electrical
the lateral hypothalamus
(or the amygdala) which elicits excited
emotional behavior, also causes a depletion of brain norepinephrine while leaving dopamine levels unaffected
(11) .
Yhus, of the two amines, norepineph-
rine appears more closely associated with neural
systems which subserve
16
üYPOTHALAHIC SELF-STIlIiTLATION
motivational and emotional processes .
If this
is
Vol . 5, No . 1 true, norepinephrine should
also be more closely associated with reward phenomena . Summary Rats were trained to press a lever for highly rewarding electrical lation of the lateral posterior hypothalamus . with alpha -methyl-tyrosine, a potent
stimu-
The animals were then treated
inhibitor of tyrosine hydroxylase .
drug gradually suppressed self-stimulation behavior almost completely .
The This
affect appeared to be associated with a decrease in brain norepinephrine levels resulting fran the blockade of catecholamine biosynthesis .
The suppres-
sion of self-stimulation was antagonized by a small dose of methamphetamine . This may mean that methamphetamine can mimic, at
least in part, the central
action of catecholamines . Acknowledgment We wish to thank Merck b Co .,
Inc . for their generosity
in supplying
DL-al ha-methyl-tyrosine . References 1 . B . P . H . Posche l and F. W . Ninteman, Life Sciences 2,
782 (1963) .
2 . B . P . H . Posche l and F. W . Ninteman, Life Sciences
903 (1964) .
3 . B . P . H . Poschel, (1965) "
3,
F. W . Ninteman and S . C . Stanat, Life Sciences 4, 53
4 . T. Nagatsu, M . Levitt and S . Udenfriend, J . Blol . Chem . 239,
2910
(1964) .
5 . M . Lovitt, S . Spector, A . Sjoerdsma and 5 . Udenfriend, _J . Pharmacol . Exptl . Thera . 148, I (1965) . Exptl 6. S . Spector, A . Sjoerdsma and S . Udenfriend, _J . Pharmacol . . The rap . 147, 86 (1965) . 7. A . Weissman and B . K . Koe,
life Sciences 4,
1037
(1965) "
8 ~B . B . Brodie and P. A. Shore, Ann . N. Y . Acad . Sçi . 66, 631 9 . M . Vogt, J . Physlol . ~,
451
10 . A. Bertler and E . Rosengren, 11 . D . J . Rois and L .-M. Gunne,
(1954) . Experientia ~,
10 (1959) "
Science 14~, 450 ( .1965) .
(1957) "