Drug-induced brain monoamine depletion and its behavioral correlates in Caiman sclerops

Comp, Biochem. Phystol.. 1977, l~bL 57C. pp, 153 to 157. Pergamon Press. Printed in Great Britain

DRUG-INDUCED BRAIN MONOAMINE DEPLETION A N D ITS BEHAVIORAL CORRELATES IN C A I M A N S C L E R O P S E. DOSHI AND S. E. HUC,GINS Department of Biology, University of Houston, Houston, TX 77004, U.S.A. (Received 28 October 1976)

Abstract--1. Monoamine-depleting drugs were administered to the crocodilian Caiman sclerops, and their effect on brain concentrations of 5-hydroxytryptamine (5-HT), epinephrine (E), norepinephrinc (NE) and dopamine (DA) was observed. Physical activity was also quantitated following injection of the drugs. 2. Treatment with reserpine caused progressive depletion of all the monoamines but did not significantly affect activity levels. 3. Parachlorophenylalanine (PCPA) caused progressive depletion of 5-HT and DA. Physical activity was significantly increased on the third day following PCPA but was significantly reduced by the seventh day. 4. In animals pretreated with desmethylimipramine, 6-hydroxydopamine caused significant depletion of DA without any significant alteration of other monoamine concentrations. Activity was significantly reduced.

INTRODUCTION

duced by lesioning of the raphe nuclei. Experiments dealing with PCPA apparently have been limited to mammalian species. No reports have been found in the literature in which PCPA was administered to reptiles. In 1968 Tranzer & Thoenen reported that 6-hydroxydopamine (6-OHDA) destroyed terminal endings of peripheral sympathetic neurons. When injected into the brain both NE and DA neurons are destroyed (Ungerstedt, 1968). In order to produce a more selective depletion and/or destruction of only one division of central catecholamine neuronal systems, various techniques have been used. Administration of the tricyclic antidepressant desipramine before 6OHDA injection brings about a ,selective depletion of DA (Kostrzewa & Jacobowitz.. 1974). Most studies using 6-OHDA have also been performed on mammalian species, the only non-mammalian species referred to in the literature being a frog (Hopkins, 1971) in which 6-OHDA was injected into the dorsal lymph sac, resulting in some depletion of peripheral E and NE. Since so few reports exist in which monoaminedepleting drugs have been administered to non-mammalian species, the present study was undertaken to determine the effects of these drugs on monoamine levels and distribution in the brain of a reptile. General activity levels were also monitored following drug administration because drug-induced changes in monoamine levels in the mammalian brain can usually be correlated with changes in locomotor actw~ty.

Drugs which alter brain monoamine concentrations have been studied extensively and from many different aspects. These studies, however, have dealt almost exclusively with the effects of the drugs within the mammalian system. Reserpine has been shown to lower brain concentrations of both catecholamines and 5-hydroxytryptamine (5-HT), and this depletion is usually accompanied by sedation. In mammals, its action on brain catecholamines is very similar to its action on brain 5-HT both with regard to dose-response curves and to the time course of depletion (Shore, 1962). Very few reports were found in which reserpine was administered to non-mammalian species. Brodie & Bogdanski (1964) found the effect of reserpine in pigeons to bc similar to that in mzunrnzds, producing sedation and extensive depletion of both catecholamines and 5-HT. In frogs, however, reserpine failed to produce sedation and it appeared to act selectively to deplete brain 5-HT. Juorio (1969) reported that administration of reserpine to tortoises decreased brain dopamine concentration, but failed to produce sedation• Juorio (1973) also found that reserpine brought about depletion of E, NE, and DA from both the hypothalamus and cerebral hemispheres of tortoises. He suggested that the storage mechanisms for catecholamines in reptiles are similar to those existing in birds and mammals. The 5-HT depleting properties of parachlorophenylalanine (PCPA) were first described by Koe & Weissman (1966), who also found that it acted through blockade of the enzyme tryptophan hydroxylase to MATERIALS AND METHODS bring about a reduction in 5-HT synthesis. Delorme The species chosen for use in this study was the crocodiet al. (1966) found that in cats and rats PCPA led lian Caiman sclerops, the South American caiman. Animals to an almost total suppression of sleep, the effects were obtained from commercial sources and a colony was becoming evident about 24 hr after injection. The in- maintained in a large aquarium where they were fed regusomnia thus produced was very similar to that pro- larly with chicken cut into small pieces. All were young 153

154

E. DOSHI

AND S. E. HU(i(;INS

males, one to three months of age, weighing 26.~65.7 g, and with length varying from 19.7 to 27.4 cm. All animals used in experimental procedures were given injections of drugs at about 12 noon. Reserpine was diluted

in sterile steroid suspending vehicle (SSV) (Koella et al., 19681; the dose injected was 5 mg/kg body weight given intraperitoneally. Control animals received an equivalent volume of SSV. Animals receiving reserpine were killed

Table 1. Effect of drugs on activity levels in Caiman Minimal activity

Drugs Control 1361 Reserpine (14l P C P A - 3 days (17) P C P A - 7 days 121J 6 - O H D A - 8 days (31)

14.97 929 23.55 6.48 10.55

+ 444. 4.

Low level activtty

204 1.53* 2.72* 1.52** 1,38 °*

19.13 I1 71 4065 590 529

sclerops

Medmm level activity

+ 380 4- 2 2 3 _+ 6.76 *° 4. 2 . 4 8 * ' " 4. 0.87"**

591 457 2024 I 86 1.13

High level activity

x 112 : ()85 ~ 3.73"** _, (181*** _+ 0.28"*"

3.64 2.71 1582 057 077

= 050 _- 0.52 = 382"" 7 : 0 - I 5 °°" 4. 0.34 °°°

Activity measured as average number of activity bursts/'hr + S.E.M. Number in parcnthescs indicates number of one-hour periods recorded. High level activity: Pen excursions in excess of 2cm. Medium level activity: Pen excursions between 1 and 2cm. Low level activity: Pen excursions greater than 1 mm and less than 1 cm and continuing for several pen movements. Minimal activity: Multiple excursions less than 1 mm or a single excursion less than 1 cm. *P < 0.05, **P < 0.01, ***P < 0.005. Table 2. Effect of reserpine on brain monoamine concentrations in Caimwl sclerops EPINEPItR1Nt! N u m b e r of days post-rejection

Brain region Control (N = 51 O l f a e t o D bulbs and smlks Cerebral hemispheres

Optic tcctum D]enc'ephalon M~dbram tegmentum Cerebellum Pons

Medulla W h o l e bram

019 0.07 0.10 028 0.21 0. I3 0.18 0.22 0.15

_+ 0.05 4-_ 0 0 1 4' 002 4' 0 0 3 4' 0 3 3 4- 0.04 + 0.03 _+ 0.03 _+ 0.02

I (N = 21

0.03 0.03 008 0.07 0.01 007 0.I0 0.05

<0.01 4- 0.01* _+ 0.01" 4' 0 . 0 1 ' ' ' + 0,01 °* 4- 0,01"* + 0.01** _+. 0.01* 4-0.01 *°°

2 IN = 21

3 [N - 2)

7 (N -: 2)

14 (N - I)

<0.01 <0.01 <()All 0.04 4" 0 0 1 " * " 0.06 ± 0,02 *° 0.01 4. 0 0 1 * " 0 0 4 4. 0 . 0 2 ' ' * 0.01 0 0 2 4. 0,003 *°°

<001 <0.01 <0.01 0.02 _* 0.01 * ' ° 0.06 _- 0t)2 °° <001 <00l <001 <001

<001 <001
0,06 0.02" 003 006* (1(15 005 006 0.05 ° 004 °

7 (N - 2)

14 (N = 1)

I)OPAMINE N u m b e r of days post-re.ice(ion

Brain region

Control (N = 5) Olfactor .', bulbs and stalks Cerebral hemLspheres O p t i c tectum I)lencephalon M l d b r a m legmen(urn ('crcbellum Pons Medulla W h o l e Brain

0.56 0.98 0.77 3,16 0.41 0.45 150 0.11 0.96

_~ 0.16 - 0.06 _- 0 0 5 = 023 Z 007 + 0.07 +_ 0 2 2 4' 0.03 4. 006

I (N = l) 0.01 058 0.35 035 0.30 0.36 0.33 047 050

± 0.01"* _+ 0 17* _+ 0 . 0 5 " " +_ 0.22 *°" _+ 0 30 4. 0,36 4- 0 . 0 5 " ' ' +_ 0.12* 4' 0.01***

2 iN = 21

042 031 0.38 0.44 007 0.57 (I.29 0.35

<0.01 _'2 (} O5 " ° " 4. 0.O6"** ± 0,27 "°* _+ 0 04 4- 0 0 7 * * 4. 0,05"* 4- 0.19 4' 0 , 0 4 " ' *

3 (N - 21 001 4. 0.01 * °
t).12 OA7 (1"/3 034 061 030

<0.0l _+ 0.O6"** z 0.03*** _~ 0 0 5 " * " + 0.I0 <001 7:_ 0 2 8 " <001 ± 004***

<0.01 0.27*** 0 35* 0 28"** 0 45 0,47 037" 0 39 ° 031

5-HT N u m b e r of days post-inJection

Bra.m regmn

Control (N = 5) Olfactor) bulbs and stalks Cerebral hemispheres O p t i c rectum Dlcnccphalon

Midbrain legmen(urn Cerebellum Pon~ Medulla Whole Brain

(1.28 1.03 173 1.40 260 070 231 304 1.59

4' 0.09 _+ 0.10 _+ 0.32 4. 0.14 4' 0.24 4' 0.06 4" 0.22 4' 0 1 9 4. 0 1 0

I (N = 2) 0.13 0 58 I06 1.37 1.46 1.27 1.34 2.311 1.14

~ 0.09 4' 0.05"** 4. 0 3 8 4' 0 4 0 + 0.14"* + 0.65 4" 0.32"" 4' 1.37 4' 0.09**

2 (N = 2) 0,36 0.99 1.19 1.45 127 0.57 1.82 1.18 I 12

~_ 0.16 -'- O 19 4. 0.11 4' 0 5 1 4' 0.06"** 4' 0 3 5 4' 0 4 1 4' 0.06*** + O.(M***

3 (N :.. 31 009 0.53 088 11.41 (I.62 0.76 150 1.71 0.78

+ 0,05 4' 1.134 .+_ 0 0 9 ° .4_ 0.22"* : 0 10"** _- 0.04 7_ 0 2 9 " _+ 0.73 + 0.30 °

7 IN = 2) 0.58 1.53 2.11 1.49 I 41 I 83 224 21b I 63

l 025 4. O.06"** 4- I166 _4- 11.05 .= 0.24** = O.57 4- ().04* _4 11.71 + 0.15

14 (N - 11 0 14 0.97 1 47 148 1.15 ° 1.22 1.57 2.78 112

NOREPINEPHRINF N u m b e r of days post-injectmn

Flram region

Control (N = 5)

I (N = 2)

2 (N - 2)

3 (N = 2)

7 (N = 21

14 (N = I)

<0.01 ().()4 ~- 0.02 *)" 0.06 ± 002''' 011 4. O.Ol*'" 11,07 4. 0.003**" 0.04 + 0 0 3 " * * 0 1 3 + 0,03"* 0 0 5 4' 0 0 0 3 * ' * 0 0 6 4' 0 0 1 " * *

<0.01 0 ()I + 0.01"** 0.02 + 0.02"*" 0,05 4- 0 0 3 * * " O.(M + OIM*** <01)I 0 0 5 "_ 0 . 0 1 ' ' " 0.01 --_ 0 0 1 " * " 0.02 _- 0 0 1 " * *


<0.OI (),(M'* 005"** 001"** <0.01 <0.01 ()(he')" 0,03"** 003

Olfactory bulbs

and stalks Cerebral bern±spheres O p n c lectum Dlencephalon M l d b r a m tegmentum Cerebellum Pons Medulla Whole brain

0.12 0 29 0.20 042 0 30 0,34 0.32 0.25 0.29

4. 0.02 4:. 0.02 _+ 0.01 _+ 0 0 2 + 003 - 003 4" 0.04 4' 0 0 1 4' 0 0 2

0.04 0. II 0 14 0.12 0.13 0.09 0, II 014 011

X 0.03 ___ 0.03"** 4- 003 + 0,01**" 4. 0.03" 4. 0.01 "*° 4' O 0 1 * ' * 4' 0.02"*" 4' 0.02"**

N = Number of animals used. Concentrations are in ,ug/g wet weight of tissue _+ S.E.M. * P < 0.05. • * P < 0.01, *** P < 0.005.

Drug-induced brain monoamine depletion at 24hr, 48 hr, 3, 7 and 14 days post-injection. Activity was recorded on the third day following injection. PCPA was also diluted in SSV and the dosage given was 300 mg/kg body weight injected intraperitoneally, with controls receiving an equivalent volume of SSV. Animals receiving PCPA were killed at 3, 7 and 14 days. Activity was recorded on the third and seventh days following injection. The caimans used in the study with 6-OHDA were pretreated with desmethylimipramine (20 mg/kg dissolved in 0.9% saline, injected intraperitoneally) followed 1 hr later by intracisternal administration of 25/~1 of a solution containing 100/zg of 6-OHDA per 251d of vehicle (0.9% saline + 0.4 mg ascorbic acid per ml]. Controls received an equivalent volume of saline intraperitoneally followed one hour later by intracisternal administration of 25/al of vehicle solution. Activity was measured on the seventh day and animals were killed on the eighth day following injection. Activity was measured using a box in which a platform was mounted on polyurethane foam blocks so that movement of the animal would cause the platform to move. A geophone, of the type commonly used in geophysical exploration, was anchored to the platform and connected through a high-gain amplifier to a physiograph. Activity was quantitated by measuring both amplitude of movement and time spent in movement. Table 1 lists the criteria used in determining the various levels of activity. The procedures used for division of the brain and assay for 5-HT and catecholamines were identical to those described by Doshi & Huggins (1977). Statistical comparisons between experimental and control values were performed using Student's t test.

RESt,:I,TS

When reserpine was administered, both catecholamines and 5-HT were progressively depleted (Table 2). The pattern of depletion was much the same for the three catecholamines, concentrations being about half control levels on the first day after injection of the drug, roughly one-fourth control levels on the second day, and around one-tenth of control levels

155

on the third day. On the seventh day, E (Table 2a) and N E (Table 2b) had further decreased to less than 1% of control levels, but DA (Table 2c) had increased somewhat, reaching about one-third of the control level. By the fourteenth day, the concentrations of all the catecholamines had increased over the values found on the third and seventh days, being at least 20--30% of control levels. Reserpine-induced depletion of 5-HT (Table 2d) was not nearly as complete as was depletion of catecholamines; the greatest depletion appeared to occur on the third day, the concentration present at this time being about half the control level. Administration of PCPA (Table 3) brought about significant and progressive depletion of 5-HT over a fourteen day period, although concentrations never dropped much below 50~o of control values (Table 3al. PCPA did not significantly alter either E or NE concentrations in any region of the brain, and the values obtained were very similar to controls. On the other hand, DA concentrations decreased significantly following administration of PCPA, whole brain DA concentration falling to roughly 20% of control level on the seventh day (Table 3b). When 6 - O H D A was administered to caimans pretreated with desmethylimipramine, there was no significant alteration of levels of E, NE or 5-HT, but whole brain DA concentration decreased significantly to about 37% of control value (Table 4). Table 1 summarizes the results of the activity studies on caimans which had received the various drugs. The effect of reserpine on activity levels was not statistically significant, but there appeared to be a slight tendency toward reduced activity. When animals were given PCPA, the number of activity bursts at all levels doubled or tripled on the third day, but dropped to less than half control values by the seventh day. In animals given 6 - O H D A activity dropped to less than half that of control animals.

Table 3. Effect of PCPA on brain monoamine concentrations in Cai,nan sclerops 5-Hr N u m b e r of days post-injection

Brain region

Olfactory bulbs and stalks Cerebral hemispheres O p t i c lecture I)iencephalon M i d b r a i n legmen(urn Cerebellum Pons Medulla Whole brain

Control (N = 5) 0.28 +_ 0 0 9 103 .+_ 0 10 173 4- 0 3 2 1.40 4- 0 14 2.60 4- 0.24 0.70 4- 0 0 6 2.71 4- 0.22 3.0A 4- 0 19 1.59 4- 0 IO

(N = 4) 4- 0.07 o*° _+ 0.0~ ° ± 0.16" _+ 0.17"*" _+ 0.16"** = 024 = 027" : O31" : 0.12**

7 (N = 4) 0.24 ___ ().IN* 082 + 005 0 3 8 4- 0.24* 0,92 4- 0.15" 1.12 + 024*** 0.76 + 0.13 1.45 + 0.52" 1.40 4- 0.49* 0.85 + 0.04"**

14 (N = 4) 0.25 4- 0.004 0.71 4- 0 0 6 ° 0.48 4- 0.09"* 0.70 i 0.21* 0.87 4- 0 2 5 °°* 0.51 4- 0.24 I 18 4- 0.27"** I 08 4- 0 3 2 **° 0.75 + 0.09 *°"

DOPAMINE N u m b e r of days post-injection

Brain region

Olfactory bulbs a n d stalks Cerebral hemispheres O p t i c tectum Diencephalon Midbram tegmentum Cerebellum Pons Medulla W h o l e brain

3 0.47 0.72 078 084 I 24 0.65 1.79 1.85 0.98

Control IN - 5) 0.56 _+ 0.16 0 9 8 4- 0 0 6 0.77 _T 0.1)5 3.16 -.- 0.23 0.41 = 0.07 0.45 z 0.07 1.50 _- 0 2 2 0.11 _- 0 0 3 0 9 6 a__ 0 0 6

3(N = 2) <0.01 0 5 7 ~_ 005*** 0.74 _+ 0.06 0.90 _.- 0.17"*" 0.75 _.- O19 0.38 4- 0 3 7 2.12 _* 0.55 0.14 _~ 0 0 2 1).68 + 0 0 3 * *

7 0.13 018 0.99 0.15 0.68 0.10 0.03 I).48 024

(N = 2) _+ 0 . 1 2 ' 4- 0.17"** ~- 0.02 z 0.15"** 4- 0.17 4- 0,09* _+ 0.03*** 4- O I 8 _+ 0.05***

14 ([06 0.24 0,36 0.54 088 005 0.64 I).68 042

(N - 2) _+ 0.06 ° _+ 0.06 °*° + 0.(k5 *°* 4- 0.39"** _'2 0.21" 4- 0.04 °°* Z 0.60 4- 0.19 4- 0.18"

N = Number of animals used. Concentrations are in .ug/g wet weight of tissue _+ S.E.M. * P < 0.05, • * P < 0.01, *** P < 0.(X)5.

156

E . D O S H I AND S. E . H U G G I N S

Table 4. Effect of 6-OHDA on brain dopamine concentrations in Caiman sclerops Brain region Olfaclor) bulbs and stalks Cerebral hemispheres Opttc tectum l)iencephalon Midbrain tegmentum Cerebellum Pons Medulla Whole brain

Control (N = 5) 0.56 (},98 1}77 3 16 041 0.45 150 0.11 0.96

± 0,16 _+ 0,06 _+ (}.05 _+ 023 4- 007 _+ 0.07 4- 0.22 +_ 0.03 4- 0.06

6-Hydroxydopamine (N = 2) <001 0.36 + 0.09**" 0.19 + 0.18" 0.27 + 0.17"** 0.66 - 0.23 0.39 : 0.37 0 78 : 0.4~ 0,06 : 0.05 0.35 .7- 0.12"**

N = Number of animals. Concentrations are in /,(g/g wet weight of tissue + S.EM. * P < 0.05, ** P < 0.01. • ** P < 0.005. DISCUSSION

In the caiman, the effect of reserpine was a progressive depletion of all monoamines. The lowest levels of 5-HT and DA were reached on the third day following administration while E and NE were lowest on the seventh day. On the fourteenth day, concentrations of all monoamines had risen somewhat, possibly indicating that recovery had begun. Depletion of E and NE was virtually complete, being more than 99% on the seventh day. Approximately 85°,', of DA was depleted on the third day, indicating that either DA is not as completely depleted as the other catecholamines or that the time of maximum depletion occurred between the third and seventh days, at a time when no measurement was made. Depletion of 5-HT did not appear as complete or as systematic as that of the catecholamines, since concentrations never fell much below 50% of control level. This would seem to indicate that reserpine may be at least partially selective for depletion of catecholamines in the caiman brain. This pattern is exactly the opposite of that observed by Brodie et al. (1964) in the frog, where reserpine appeared to deplete 5-HT stores selectively. In general, the brain regions which were depleted most thoroughly and systematically were those areas which contained the highest levels of each amine initially. Overall physical activity levels were not significantly affected by reserpine, although a general trend toward a reduction in activity could be noted. This finding is in agreement with that of Juorio (1973) who observed no gross changes in the behavior of tortoises given reserpine, and of Brodie & Bogdanski (1964) who observed no sedation in reserpinized frogs. When PCPA was administered to the caimans the 5-HT depletion it produced was somewhat more consistent and of slightly greater magnitude than the depletion produced by reserpine. Whole brain 5-HT progressi~clx dccrca.~cd during the fourteen days following injection of the drug, depletion being greatest on the fourteenth day and nearly 60~o complete. No significant changes occurred in E and NE levels during this period; however, DA was significantly depleted, this process being more than 80% complete on the seventh day, at which time 5-HT depletion was not yet 50"
in the areas which in controls contain high levels of the amine. The mechanism of action of PCPA has been shown to be through inhibition of the enzyme tryptophan hydroxylase (Koe & Weissman, 1966), leading to selective 5-HT depletion in the mammal. The initial step in catecholamine synthesis also involves hydroxylation of an amino acid, tyrosine, to form DOPA. In the mammalian system, amino acid decarboxylase acts upon 5-hydroxytryptophan to form 5-HT as well as upon D O P A to form DA. It is conceivable that in the reptile there is a single enzyme which operates in a similar fashion to hydroxylate both tryptophan and tyrosine, and which is inhibited by PCPA. Activity levels in caimans which had received PCPA were found to be extremely high, with the animals in a constant state of arousal, on the third day following injection. This is the same type of behavior observed in mammals which have received PCPA and in which catecholamine levels remain within the normal range. At this time DA levels were still about three-fourths of normal while 5-HT depletion was somewhat more complete in the caimans. On the seventh day, the activity level of caimans receiving PCPA was reduced significantly below the control value. At this time, DA levels were reduced to less than 20'~, of control while 5-HT depletion was less than 50°<,i complete. It seems, then, that there may exist some connection between selective DA depletion and reduced motor activity in caimans. When caimans were pretreated with desmethylimipramine and subsequently were injected with 6-OHDA, there resulted a depletion of DA in all regions of the brain; whole brain DA level had dropped by more than 60.°/,i at the end of eight days. Levels of E. NE and 5-HT were not significantly affected. Activity in 6-OHDA-treated caimans was significantly lower than in controls, the levels being similar to those observed in PCPA-treated animals in which DA was depleted. This further suggests a possible role for DA in motor functioning and/or arousal pathways in the caiman.

REFERENCES

BRODIE B. B. & BOODANSKID. F. (1964) Biogenic amines and drug action in the nervous system of various vertebrate classes. Pro9. Brain Res. 8, 234-242. D E L O R M E F., F R O M E N T J. L. & J O U V E T M . ( 1 9 6 6 ) S u p p r e s s i o n du sommcil par la p-chloromethamphetamine ct la

p-chlorophenylalanine. C. r. S~;anc. Soc. Biol. 160, 2347. DOSHI E. & HtJ(~lys S. E. (1977) Monoamine distribution in the brain of Caiman sclerops. Comp. Biochem. Physiol. (in press.) HOPKINS C. R. (1971) Localization of adrenergic fibers in the amphibian pars intermedia by electron microscope autoradiography and their selective removal by 6-hydroxydopamine. Gen. comp. Endocrinol. 16, 112-120. JUOR10 A. V. 11969) The distribution of dopamine in the brain of a tortoise, Geochelone chilensi.~ (Gray). J. Physiol., Lond. 204, 503- 509. JUOR10 A. V. (1973) The distribution of catecholamines in the hypothalamus and other brain areas of some lower vertebrates. J. Neurochem. 20, 641-645.

Drug-induced brain monoamine depletion KoE B. K. & WEtSSMA~ A. (1966) p-chlorophenylalanine, a specific depletor of brain serotonin. J. Pharmacol. exp. Ther. 154, 499. KOELLA W. P., FELDKrEIN A. & CZICMAN J. S. (1968) The effect of p-chlorophenylalanine on the sleep of cats. Electroenceph. Clin. Neurophysiol. 25, 481-490. KOSTRZEWA R. M. & JACOBOW]'I~ZD. M. (1974) Pharmacological actions of 6-hydroxydopaminc. Pharmac. Ret:. 290). 200-275.

157

SltoRt P. A. (1962) Release of scrotonin and catccholamines by drugs. Plmrmac. Ret,. 14, 531 550. TRANZER J. P. & TtIOENEN H. (1968) An electron microscopic study of selective acute degeneration of sympathetic nerve terminals after administration of 6-hydroxydopamine. Experentia 24, 155--156. UrqGERSTH)T U. (19681 6-hydroxydopamine-induced degeneration of central monoamine neurons. Eur. J. Pharmacol. 5, 107-110.