The effects of LSD-25 on the amplitudes of evoked potentials in the hippocampus of the cat

Life Sciences Yol . 5, pp . 259-266, 1966 . Printed is Great Britain.

Pergamoa Press Ltd .

TAE SFPBCTS OF L3D-25 OM THB AI~LITIIDSB OF EVOI~D POTgITIALB Ill THE HIPPOCAMPU3 OF THF CAT Alvin M. Revsin end Alvin Armatroaq Pharmacology-Biochemistry Laboratory Civil Aeromedical Institute Federal Aviatiaa Agcy Oklahoma City, Oklahoma (8eceived 6 December 1965) Admiaietrntion of reserpine to cats causes aeiaure activity is hippocsapua (1,2) sad a facilitntian of the atq~gdelo-hippocampel evoked potential (3) . These actiaaa of reserpine as the hippocampea seas related to the depletion of brain aerotoain (5HT) induced by the drug (4,5) .

5HT end tryptaeiiae are

inhibitory to hippocaupal aeurogee (6,T), end dapletica of sa inhibitory "neurohorsooe" would be expected to increase the excitability of the hippocesipua .

Zf 5HT is an inhibitory "neurohorsnne", than l~aergic acid diethyla-

mide (LSD), which entsgasisea the stoma of 5BT at moat peripheral synapses (8), should have effects m the hippocatpua Which are siailar or identical to thota of reserpiae.

Iadeed, leisure activity has bees found in cat hippocnpus

following admlniatrstim of small doses of LSD (1,2),

fhe folloviag expari-

meats wre perforsed to deteraine whether the effects of LSD m evoked potentials in the hippocs~pua wre similar to throe of reserpine. Methods Twelve adult cats wighing 2 .5 - 3.5 kg wre'used .

The aaisala wre

anesthetised with ether end placed in a atereotazic iastnmaat .

The meseaceph-

elic reticular foraatim at A 1 .5 was then destroyed by a series of electrolytic fulruratims, end the ether was diacmtinued.

The resulting ~memscioua

modified cerveau isole' cat could be kept in good condition for about a week, with rectal temperatures maintained at 35 °C - 36 °C, sad routiae antibiotic and fluid therapy.

Thus, when desirable, the effects of repeated drug admin259

26p

ISD-25 ON EVOSED POTENTIAL

Yol . 5, No . 3

istrationa could he compared in one animal, free from possible anesthetic interference .

Stimulating and recording electrode pairs were made from

insulated stainless steel wire .

The tip dimensions were 0,25

the tip to tip distance we~ about 1 .Omm .

x 0.50mm;

The electrode pairs were placed

bilaterally in ventral hippocampue and in the right lateral amygdalar nucleus end the right septal area .

Electrode positioning was guided by stereotaxic

coordinates (9) and by observation of evoked potential characteristics during placement.

After final positioning the electrodes were fixed to the skull with

dental acrylic .

All electrode positions were confirmed histologically after

the experiment . Spontaneous electrical activity end evoked responses were routinely recorded from right ventral hipnocampus .

Stimulation of ar~vgdale, septum and

cantrelateral hippocnmpus produced evoked potentials in the hippocampus, which were derived from three independent pathways and which terminated in characteristic intro-hippocampal synaptic fields (10) .

By analyzing the drug effects in

these 3 pathways the existence of selective drug actions could be determined . Previous experience with this preparation has shown that hippocampal excitability varies markedly end unpredictably for some time after electrode implantation .

Most of the fluctuation in excitability occurs within 8-10

hours of elect;ode positioning, although full stabilization of excitability is not seen until 5-7 days after implantation .

For this reason recordiaPe were

delayefl for at leant 18 hours after electrode positioning and were seldom taken for more thaw 8 hours continuously .

In general, control records were

taken for 2 to 3 hours, the drug was injected, and the recordings continued from

4

to

6

hours thereafter .

Stimulus intensities used ranged from threshold

to intensities which evoked responses of 1/2 maximal amplitude .

Stronger

stimuli were avoided due to the risk of triggering~hippocampal seizures . An average response computer (Nuclear Data ND-800) was used in order to simplify data analysis .

125 evoked responses were averaged Por each data

Qol . 5, No . 3 point .

261

LBD-25 OA EYOSED POTF~TIAI~

In control recordings taken over as

from the mean potential amplitude xere =5x.

8

hour period the sa:isma de~iatioas

Therefore, in the axparisants !a

which drugs were Riven, changes in mean evoked response amplitude of less then llOx of the mean control amplitudes rare not caaeiderad significant.

Changes

of 10-20Z wen considered "possibly significeat'~ and changes of sore than 20K were considered "significant" (p<0 .01) . The amplifying, stimulating, photographic, end ink recording systsss were canventiaanl .

ISD end pentobarbital were obtained cosssreial~r .

The drugs

were dissolved in 0 .9x 1faC1 and injected intraveaouslyr, Results Characteristics of the Ebokad Potentials .

The characteristics of the poten-

tials evoked in the hippocnmpua by stimulation of asprgdala, saptus, and coatralaternl hippocempus have bean described by msgy authors (10) . potentials obtained in the present study are illustrated is Mgure l.

Typical The

eepto-hippocempel evoked potential (SÜEP) and the traps-hippocempal evoked potential (THEP) are very similar in latency and wave-shape . hippocampal evoked potential (A}iEP) usual

The aeggdalo-

differs fry 3ftEP and THEP in

latency, duration, end vaveshape, although the potentials shorn in Mgure 1 differ leas markedly than is usual.

The pathway for the AüBP is poly-synaptic,

unlike the mono- or oliRO-synaptic SAEP and TAEP,

The intro-hippocampal

terminations producing AüEP are those of the pertorant pathway sad differ considerably from the eynsptic sites producing SHEP sad TftSP (10), Sakes Effects .

Administration of 1-2m1 of physiological saline to two cats

in this series did not change any of the evoked potential amplitudes . Pentobarbital Effects .

Pentobarbital, in doses of 20mR/kg,was injected into

3 cats on 5 separate deya .

This xas done so that the activity pattern, if any,

of L3D-25 could be compared to the patterns induced by n drug of ]morn pharmacological effectiveness . about 35x.

Pentobarbital reduced the amplitude of the Af1EP by

AmplituA.ea of the SHEP and the ^'HFP were also reduced, but only by

15f end lOx respectively .

^_'here was, however, considerable variability in the

262

L8D-25 qI EPOEED POTENTIAL

decree of rtsponse to pentobarbital .

Yol . 5, No . 3

Smaller doses of pentobarbital (lOmg/kg)

could produce a elisht facilitation (15x) of AHED, with minimal effects oa SHE cad THEP,

hacilitatiao of hippocaspal electrical activity at low doses of

pentobarbital end inhibitiaa st hisher doses vas also reported by Bradley and lfichoLca (11) . LSD 25 .

Doses of 50~r/k;ç of Lt3D-25 were without effect on the potentials

CONTROL

LSD

I

HR. AFTER

2 .5

HR .

AFTER

zoo rnw

AHEP

aIO. 1 ßippoce :ipal evoked potentials before and at two times after adeinietration of LSD . SHEP-eepto-hippoca :apal evoked potential ; ilQD~trena-hippoeaspal evoked potential ; A![EP~amyp~dalo-hippocampal evoked potential . The individual records above are tracinigs from the oa~puter write-o t .

vol . 5,

Fo . 3

studied.

LSD in doses of 100-250p/kg induced a marked facilitation (>40~) of

IAD-25 O~f EPUSED POTENTIAL

the AIDD' in 5 aninala.

263

These doses of LSD either had ao effect upon SREP and

TBaP responses or canoed s slight (10-15x) inhibition (lrig. 1) .

The AiIBP

facilitation vu aeon an~r following the first injection of LSD-25 .

llu~ther

injections of LßD-25 vere .vithout effect as the evoked potentials unless tonic drug levalr were reached.

Doses of LSD-25 larger than 250Y/kg caused a rapid

sad ocrplete inhibition of all hippocanpal electrical activity, spontaneous and evoked .

lllthwtgh cost aaieials appeared in good condition for at least two

hours alter injection of these large doses of LSD-25, the animals died three to seven hours after injection.

In two inatencer eases recovery of the SHEP sad

THL~P vas seen two to three hours after injection of b00Y/kg of LSD-25, althot~t the ewisals died 1-3 hours thereafter . The responses to LSD-25 were variable . of the AHSP after 100-250Y/k8 of LSD-25 .

Five animals shoved facilitation

Rwever, two cases of inhibition end

one of ao change vera observed after injection of 25~/k8 of LSD-25 .

In the

latter 3 CYM , however, relatively wall doses of LSD-25 (100-250Y/kg) givra the nert dq were sufficient to induce cosplete electrical silarce in hippocampes and, after $ hours, death .

One cat givm 100Y/]cg in divided doses,

2 hours apart, also vmt into electrical silence and died 3.5 hours after the second LSD-25 injection . Mscursion The presart results indicate that LSD her a dipharic action on the A}iEP . LSD is doses of 100-250Y/ig facilitated the At~P ; doses f250y/kg inhibited the Al~P .

Previous studies ar the effects of LSD upon apontaaeour electrical

activity is the hippouempus have shove a similar diphesic dose-response characteristic (1,2) .

Caasidernble variability in the response of the AHI:P Lo

LSD vas seen, but vu not unexpected .

The preparations certainly differed

considerab~ is their "amsitivity" tovarda LSD, for the lethal done of the drug ;caged from 100-800y/kg.

Given a dipharic done-response characteristic,

evm small variations in "sensitivity" cea cause marked variations is drug

264

LSD-25 OA EIIOSED FOT$ATIAL

Vol . 5, No . 3

effects is the region of transition from excitation to inhibition .

Furthermore,

previous studies 'of the effects of drugs m the AHED in chronic preparatione has shove that, althou~ drug affects art stable and reproducible in assy one aaiaal, theta is considerable variation in the drug affects among the eaimsla (3) .

There vas ao apparut reasaa for the.iatra~saimal variability, in this

stuff or the previous one (3), either in Leans of behavioural responses to the drug or is fetes of siethodological variations such as electrode location . The LSD-induced increeas in hippocampal electrical activity seen in previous studies (1,2) has beu attributed to s direct ezcitut effect of the drug ao hippocae~pa~,aeuranes .

However, it is also possible that the hippocsspal

excitation caused by LSD is due to inhibition of inhibitory pathways within or afferut to the hippocampus.

If ao, and noting the diphasic dose-response

characteristic of L9D, the following hypothesis can be fors~eds LSD is purely inhibitory to ell or most neurones in hippocampaa, but certain inhibitory calls or sidings are the most sensitive to the drug.

Therefore, nt low doses

of LSD spec excitatory phuomsna mqy be seen, but higher doses will be purely inhibitory .

There is eviduce for this hypothesis .

The effects of LSD on the THSP and the SHSP are purely inhibitory

(Fin,, 1) .

This confirms and extends previous work showing that LSD blocks septo-hinpocarpal Ireassdssion (12 ) .

Therefore, the drug can not have n direct, excita-

tory adios on hippocampal aeuraoes in general .

Also, the "inhibition of

inhibition", if present, cannot be a Reneral intro-hipnocnmpal ohenonierson, since, again, no excitation phase hoe been seen in SRfm or :^HF;P .

An r~entioncd

above, LSD in sufficient quantities, inhibited hinpocampal electrofenesis (l, 2, l2) .

The doses of LSD used in these exnerimenta (1,2,12) were not

reported to be toxic, so Lhe observed inhibition orobablY represents n nore :al pharmacological effect of LSD .

In the present exnerinents, L:9D inhibtte~l

hippocampal evoked potentials at doses >250Y/kw,

indeed, con~lete hi~nac!urnal

electrical silence could be produced by these lame doses .

!t~wPVer, these

voi .

5, No . 3

LsD-25 ON EVOKED POTENTIAL

265

doses were lethal, or nearly so, and the inhibition we found could be a toxic effect of the drug,

However, nlthouc~ the electrical silence developed within

15 minutes of drun, injection, the condition of the arsimal did not begin to deteriorate °or several hours . related .

Thus, the inhibition and toxicity may not be

Furthermore, experiments on the et`fects of amphetamine in reserpinized

cats have shown that complete hippocampal electrical silencs over a period of hours can be induced by drug doses which are neither lethal nor toxic (3) . Coa sequently, we feel that the inhibition of hippocampal electrical activity sees in the present study was a pharmacological action of LSD and not a toxic effect, There is also some indirect evidence for an inhibitory action of LSD in hippocampus,

The present study and previous work (11) have indicated that pentobar-

bit al, a general neuronal inhibitor, had a diphasic action in the hippocampua which resembles that of ISD, The data noted above supports the hypothesis that LSD is an inhibitory agent is the hippocampus .

The facilitation of evoked activity seen following

low doses of L^>D was seen only in AH~P,

Therefore, it is probable that the

structures most sensitive to LSD inhibition in the hinpocampus are inhibitory neurones in the nolvsvna?~tic aigvgdalo-hiDpocampal pathway, possibly in the entorhinal area (1), The original hypothesis which stimulated this work was that L .SD and reaernine should have similar actions on the hippocampus since both could be taken as functional antaoonista of SHT,

However, from the nrecedinR discus

sion, it :seems probable that L3D is an inhibitor of hipnocamnal neurones, as is 51îT (6,7),

Thus, LSD may be a 51iT mimic in this system, as it is at some

peripheral. synapses (8),

Therefore, the original hypothesis is untenable and

the present experiments do not help explain the mode of action or reaerpine on the hinpocamnus, '>ummnrv I : P in doses of 100-?50y/ka Pacilitgted the amygdnlo-üippocamnal evoked potential,

noses enunl to or larre~ ttinn ?~Cv/kg inhibitc~i the rr;ygdalo-hippc~-

Pol . 5, No . 3

LBD-25 ON E90gED POTENTIAL

266

orpal reaponaa, the traaa-hippocaeipal evoked response end the septo-hippocrpal word potentiel .

The data is interpreted to indicate that LSD is a

pura~r inhibitory coapound in the hippocamous. References

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l,

W, R, Achy, F, R . Bell, end A, J . Dennia,

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A, R, Jasper, and C. Ajmone-!darsan, A Stereotaxic Atlas of the Diencephalaa of the Cat . Yntianal Research Council of Canada, Ottawa (1954)

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J. D. Green, Physiol, Rev, 44, 561 (1964),

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