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Deficits in inhibitory avoidance after neurotoxic lesions of the ventral striatum are neurochemically and behaviorally selective
Beha~ioural Brain Research, 18 (1985) 279-283 Elsevier
279
BBR 00510
Short Communications Deficits in inhibitory avoidance after neurotoxic lesions of the ventral striatum are neurochemically and behaviorally selective R. S C H W A R T I N G ~ and R.J. CAREY 2
Jlnstitute o/ Psychology II1. University of Diisseldor£ Diisseldorf (F.R.G.) and z Veterans Administration Medical Center and State University of New York. Upstate Medical Center, Syracuse, NY 13210 (U.S.A.) (Received March 19th, 1985) Revised version received September 26th, 1985) (Accepted October 4th, 1985) Key words: inhibitory avoidance - locomotion - biogenic amines - ventral striatum
Inhibitory avoidance (step-in type) was investigated in rats subjected to neurochemical lesions of the ventral striatum. The neurotoxins 6-hydroxydopamine or 5,7-dihydroxytryptamine were used to produce selective depletions of either dopamine, norepinephrine or serotonin. Only lesions which decreased the dopamine content of the ventral striatum impaired post-shock step-in behavior. Measurement of footshock reactivity by the Flinch-Jump technique indicated that only serotonin depletion altered reactivity to footshocks. Assessment of open-field locomotor behavior showed that the dopamine-denervated rats were hypoactive (fewer rearings) compared to controls, whereas serotonin-depleted rats were hyperactive. It is concluded that the deficit in inhibitory avoidance behavior following ventral striatal dopamine loss was dissociated from its effect on locomotor activity.
One of the main target areas of dopaminergic neurons in the brain is the striatal complex, which, according to Heimer et al. 1°, is composed of the dorsal and ventral striatum. The dorsal part (caudate-putamen) receives its main dopaminergic input from the substantia nigra j3 and has a well-known and important role in the mediation of voluntary movement2. The ventral striatum (nucleus accumbens and olfactory tubercle) receives dopaminergic innervation from the ventral tegmental area ~3 and has received considerable attention in neuropharmacological theories of abnormal behavior such as the dopamine hypothesis of schizophrenia ~7. The prominence of the ventral striatum in these theoretical formulations has generated a number of experimental studies involving manipulations of ventral striatal dopamine in animals. One line
of empirical investigation has shown that manipulation of dopamine in the ventral striatum has an important modulatory influence on global motoric processes such as locomotion. Thus, in rats an increase in ventral striatal dopaminergic activity produces an increase in locomotion, whereas a dopaminergic deficiency in the ventral striatum diminishes both spontaneous and pharmacologically induced locomotion. A similar relationship has been suggested for the involvement of ventral striatal dopamine in reward behavior 14"1s. This possible involvement of ventral striatal dopamine in reward behavior suggests the possibility of an influence on fundamental adaptive behavioral processes such as learning and memory. Surprisingly, the influences of ventral striatal dopamine on learning and memory have received little experimental attention. Using an active
Correspondence: R. Schwarting, Institute of Psychology III, University of Dtisseldorf, Universit/itsstrasse 1~ D 4000 D/isseldorf, F.R.G. 0166-4328/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
280 avoidance learning situation Koob et al. 12 have shown that selective denervation of dopamine in the ventral striatum has a small but significant effect on acquisition of active avoidance. Another observation suggesting that ventral striatal dopamine might play a role in learning and memory comes from the report of Bracs et al.3, that pretrial, but not pre-retest, injection of dopamine in the ventral striatum interferes with learning in an inhibitory avoidance situation. Since an excess of ventral striatal dopamine affects acquisition of an inhibitory avoidance response, we chose to further study the influence of ventral striatal dopamine in learning and memory by examining the effect of a dopamine deficiency in this area on inhibitory avoidance learning. As the ventral striatum is also rich in the other biogenic amines, norepinephrine 1~ and serotonin 16, we decided to also investigate depletions of these amines. We used the intracerebral application of the neurotoxin 6-hydroxydopamine (6OHDA), together with the norepinephrine reuptake-inhibitor desmethylimipramine (DMI) or the dopamine reuptake-inhibitor benztropine, to selectively destroy dopaminergic or noradrenergic neurons, respectively4. We used 5,7-dihydroxytryptamine (5,7-DHT) to deplete serotonin 9. Four groups of male Sprague-Dawley rats were used. Two groups were administered bilateral injections of 6-OHDA (6 #g/2#l, free base) into the ventral striatum. In one group (DA, n = 8), 6-OHDA was combined with a systemic injection of DMI (25 mg/kg), and in the other group (NE, n = 11) 6-OHDA was combined with an injection of benztropine (10 mg/kg). The third group (5-HT, n = 7) was administered bilateral injections of 5,7-DHT (8/~g/2/~1) together with systemic DMI and benztropine. Stereotaxic coordinates were 2.5 mm anterior to bregma, 1.5 mm lateral, and 7.0 mm below dura, with the incisor bar 1.0 mm above the interaural line. The unoperated control group (Co, n = 7) received the anesthetic (0.3 ml/kg equithesin) plus systemic DMI and benztropine. On days 9,10 and 11 postlesion all animals had 10 minutes of free exploration in an observation box which was an open 60 × 60 cm wooden en-
closure with a wire mesh floor subdivided into four 30 x 30cm quadrants. Two behavioral measures were recorded during the 10-min period on day 11 : (1) the number of quadrants of the test box entered (crossings) and (2)the number of times the rat reared up on its hind legs (rearings). Sixteen days after surgery all rats were tested for inhibitory avoidance using a conventional step-in apparatus (as described in ref. 6). Latencies to enter the dark compartment of this apparatus were measured before and 24 h after the presentation of a 0.5 mA footshock (5 s duration) in the dark compartment. Reactivity to electric footshock was measured in an Evans Flinch-Jump Box one week "after inhibitory avoidance testing: Flinch and jump thresholds to footshocks of increasing and decreasing intensities were observed (steps of 0.1 mA; for details see ref. 6). After completion of behavioral testing all rats were killed and brain samples of the ventral (nucleus accumbens and olfactory tubercle) and dorsal striatum (caudate-putamen) were assayed for dopamine, norepinephrine and serotonin using high-performance liquid chromatography with electrochemical detection (for details see ref. 5). Statistical analysis was performed for between group comparisons (lesion vs control) with the t-test (two-tailed for locomotor activity and flinch-jump thresholds, and one-tailed for biochemical assays) and the Wilcoxon Mann-Whitney U-test (two-tailed for inhibitory avoidance). Fig. 1 illustrates the results of locomotor activity testing (10-min observation period, postlesion day 11). Compared to controls, significant differences in locomotor activity were measured in the dopamine- and serotonin-depleted animals, but not in the norepinephrine-depleted group (crossings t = 1.43, d f = 16, n.s.; rearings t = 2.06, d f = 16, n.s.). Whereas the serotonin-lesioned animals showed significantly more rearings ( t = 2 . 5 7 , d f = 12, P < 0 . 0 5 ) and crossings (t = 2.34, df = 12, P < 0.05) than controls, dopamine-lesioned rats showed fewer rearings (t = 2.86, df = 13, P < 0.05) and no difference in crossings (t = 0.83, df = 13, n.s.). Before footshock all groups showed the same
281 03
~"
711 ii':!~!ii
30
Flinch
021 01
0 o
10
~{
Jump
0.6-
E 05
DA
NE
5-HT
Co
Fig. 1. Locomotor activity during a 10-min observation
period 11 days after lesions of the ventral striatum. Stippled bars: number of rearings; open bars: number of crossings (means, S.E.M.). Bottom line indicates groups: DA, 6-OHDA plus systemic DMI (n = 8); NE, 6-OHDA plus systemic benztropinc (n = 11); 5-HT, 5,7-DHT plus systemic DMI and benztropine (n = 7). Asterisks indicate statistical difference (t-test, P < 0.05) compared to controls (Co).
short latencies to step in. Twenty-four hours after the 0.5-mA footshock, only the dopamine-lesioned group showed an avoidance deficit (Wilcoxon/Mann-Whitney U-test, z = 2.20, P < 0.05) compared to controls; that is, the latencies from this group were virtually unchanged from preshock latencies whereas all other groups showed a modest but reliable increase in latencies
Pre Shock
3
G 2Ahrs after 05mA Shock
2 ,A-
DA
NE
5-HT
Co
Groups
Fig. 2. Median latencies to enter the dark compartment of the step-in apparatus before and 24 h after receiving a 0.5 mA footshock in that compartment. Groups as in Fig. 1. Asterisks indicate statistical differences (Wilcoxon/MannWhitney U-test, P < 0.05) compared to controls.
o
J~ 04
II
030.201DA
NE 5-HT Groups
Co
Fig. 3. Footshock reactivity as observed in the Evans Flinch-Jump Box. Mean thresholds (mA, S.E.MDto emit a flinch or jump response in ten series of increasing and decreasingfootshock intensities (0.1 mA steps). Groups as in Fig. 1. Asterisks indicate statistical differences (t-test, P < 0.05) compared to controls.
(Fig. 2). Fig. 3 shows that the neurotoxic lesions did not alter footshock reactivity, except for the 5-HT group where the threshold for the flinch response was significantly higher than for control animals (t-test; t = 2.30, df = 12, P < 0.05). The results of the biochemical analysis demonstrated that each lesion selectively depleted one biogenic amine in the ventral striatum (Table I). The injection of 6 #g of 6-OHDA, together with systemic D M I , depleted dopamine to 52% of control levels (t-test; t = 6.71, d f = 13, P < 0.001) without affecting norepinephrine or serotonin, whereas 6 #g of 6-OHDA, together with systemic benztropine, depleted norepinephrine (62% of control levels; t = 6.09, df = 16, P < 0.001) and not dopamine or serotonin. FinallY, the injection of 8 ~g of 5,7-DHT with systemic D M I and benztropine resulted in a small but significant depletion (t = 2.62, df = 12, P < 0.05) of serotonin. Neither lesion produced any statistically significant changes in biogenic amine levels in the dorsal striatum.
282 TABLE I
Content of biogenic amines (#gig tissue, means S.E.M.) measured by high-performance liquid chromatography with electrochemical detection; ventral striatum includes nucleus accumbens and ofactor.v tubercle, dorsal striatum : caudate-putamen: Groups
Ventral striatum DA NE
5 -H T
Dorsal striatum DA NE
5 -H T
DA
6 - O H D A plus systemic D M I (n = 8)
1.64 + 0.29**
1.40 + 0.05
1.92 + 0.25
12.26 + 1.60
0.64 + 0.08
1.34-+ (!.36
NE
6 - O H D A plus systemic benztropine (n = 11)
2.87 + 0.18
0.90 4:_ 0.05**
2.01 + 0.37
13.39 _+ 1.80
0.54 + 0.03
1.35-+ 0.38
5-HT
5,7-DHT plus systemic D M I and benztropine (n = 7)
3.68 +_ 0.41
1.58 _+ 0.06
1.63 + 0.21'
14.10 +_ 1.30
0.53 7~ 0.05
157 -~ 0.41
Co
controls (n = 7)
3.18 + 0.34
1.46 + 11.04
2.07 _+ 0.20
14.20 _+ 1.70
0.62 ~: 0.08
1.49 4:0.33
* P < 0.05 c o m p a r e d to controls (t-test). ** P < 0.001 c o m p a r e d to controls (t-test).
The present study provides some further evidence that a disturbance in dopaminergic neurotransmission in the ventral striatum may influence learning and memory processes. One important feature of the present findings is that the effect was selective for dopamine, as depletions ofnorepinephrine or serotonin did not affect inhibitory avoidance performance. This result appears to parallel observations regarding the dorsal striaturn where norepinephrine and serotonin have relatively minor effects compared with dopamine ~. In evaluating the results of the present experiment, note must be taken that the amine depletions, while selective and substantial were only partial, particularly for serotonin. Nevertheless these depletions resulted in locomotor changes which are in accord with earlier studies 7"11. Secondly, it seems important to observe that although inhibitory avoidance behavior was present in the control, norepinephrine and serotonin groups, the avoidance latencies were quite short. This suggests that the testing situation was near threshold for observing a significant hthibitory avoidance effect. Certainly more systematic manipulations of the testing situation are needed to adequately assess the magnitude of the influence of dopamine denervation on inhibitory avoidance. It could be argued that the dopamine-depleted
rats in this study, while hypoactive at the time of activity testing (2 weeks postlesion), were hyperactive at the time of avoidance testing (3 weeks postlesion), thus resulting in impaired avoidance. This argument cannot be ruled out by our experimental data. However, it is not supported by our general behavioral observations at the time of avoidance testing, as no indication of hyperactivity was seen in dopamine depleted animals. Despite these limitations, the present study is important because it indicates that the effect of a ventral striatal dopamine lesion on locomotor activity is dissociated from an effect on a simple learning task. That is, reduced activity levels as an explanation for the deficits observed can be excluded. Firstly, the preshock latencies to step-in were the same in dopamine-lesioned and unlesioned animals, and secondly, reduced activity should, if at all, result in better performance in the inhibitory avoidance task, since appropriate performance requires that the rat does not locomote (does not step in). In fact, the finding that dopamine stimulation of the ventral striatum (nucleus accumbens) does not impair inhibitory avoidance performance 3 is an analogous dissociation. Because this treatment typically produces hyperactivity 8, an impairment in inhibitory avoidance should have been observed if the influence of ventral striatal do-
283
pamine manipulation on inhibitory avoidance were simply a manipulation of hyper- or hypoactivity. This dissociation between the behavioral effects ofdopamine denervation of the ventral striaturn on activity level and on the inhibitory avoidance task is of particular importance because other behavioral effects of dopaminergic denervation of the ventral striatum such as in reward behavior have produced directed changes in performance ~4 which are isomorphic with the directed changes in locomotion. Thus, it is difficult to rule out the possibility that the effects are only secondary to the locomotor effects. In summary, the present study suggests that the ventral striatum may influence behavior in ways not directly linked to motoric processes. Additional studies are needed to confirm and extend the present findings. Questions that need to be addressed include whether the observed avoidance deficit is dependent on time after lesion, whether this deficit occurs when other avoidance paradigms are used, and whether total depletions of ventral striatal dopamine also result in deficits in inhibitory avoidance behavior. This work was supported by Grant Hu 306/4-1 from the Deutsche Forschungsgemeinschaft and V.A. Medical Research and N.I.M.H. Grant M.H. 33959. I Bcrnheimcr. H., Birkmayer, W., Hornykiewicz, O., Jcllinger, K. and Seitelberger, F., Brain dopamine and the syndromes of Parkinson and Huntington, J. Neurol. Sci., 20 (1973) 415-455. 2 Bernheimer, H. and Hornykiewicz, O., Herabgesetzte Konzentration der Homovanillins~.ure im Gehirn von parkinsonkranken Menschen als Ausdruck der St6rung des zentralen Dopaminstoffwechsels, Klin. Wochenschr., 43 (1965) 711-715. 3 Bracs, P.U., Gregory, P. and Jackson, D.M., Passive avoidance in rats: disruption by dopamine applied to the nucleus accumbens, Pa:vchopharmacology, 83 (1984) 70-75. 4 Breese, G.R. and Taylor, T.D., Depletion of brain noradrcnaline and dopamine by 6-hydroxydopamine, Br. J. Pharmacol.. 42 (1971) 88-99.
5 Carey, R.J., Differential effects of limbic versus striatal dopamine loss on motoric function, Behav. Brain Res., 7 (1983) 283-296. 6 Carey, R.J. and Miller, M., Absence of learning and memory deficits in the vasopressin-deficient rat (Brattleborn strain), Behav. Brain Res., 6 (1982) 1-13. 7 Carter, C.J. and Pycock, CJ., The effects of 5,7-dihydroxytryptamine lesions of extrapyramidal and mesolimbic sites on spontaneous motor behaviour, and amphetamine-induced stereotypy, Naunyn- Schmiedeberg ~"Arch. Pharmacol., 308 (1979) 51-54. 8 Costall, B. and Naylor, R.J., The behavioural effects of dopamine applied intracerebrally to areas of the mesolimbic system, Eur. J. Pharmacol., 32 (1975) 87-92. 9 Gersson, S. and Baldessarini, R.J., Selective destruction of serotonin terminals in rat forebrain by high doses of 5,7-dihydroxytryptamine, Brain Re.v.. 85 (1975) 140-145. 10 tlcilncr, L, Switzcr, R.D. and van t tocscn, G.W., Ventral striatum and ventral pallidum. Components of the motor system? TINS, 3 (1982) 83-87. 11 Knob, G.F., Riley, S.J., Smith, S.C. and Robbins, T.W., Effects of 6-hydroxydopamine lesions of the nucleus accumbens septi and olfactory tubercle on feeding, locomotor activity, and amphetamine anorexia in the rat, J. Comp. Physiol. Psychol., 92 (1978) 917-927. 12 Knob, G.F., Simon, H., Herman, J.P. and LeMoal, M., Neuroleptic-like disruption of the conditioned avoidance response requires destruction of both the mesolimbic and nigrostriatal dopamine systems, Brain Res., 303 (1984) 319-329. 13 Lindvai1, O. and BjOrklund, A., Anatomy of the dopaminergic neuron systems in the rat brain. In P.J. Roberts, G.N. Woodruff and L.L. Iversen (Eds.), Advances in Biochemical Psvchopharmacology, Vol. 19, Raven Press New York, 1978, pp. 1-23. 14 Lyness, W.H., Friedle, N.M. and Moore, K.E., Destruction of dopaminergic nerve terminals in nucleus accumbens: cffect on D-amphetamine self-administration, Phar#nacol. Biochem. Behav.. 11 11979) 553-556. 15 Roberts, D.C.S., Knob, G.F., Klonoff, P. and Fibiger, H.C., Extinction and recovery of cocaine self-administration following 6-hydroxydopamine lesions of the nucleus accumbens, Pharmacol. Biochem. Behav., 12 (1980) 781-787. 16 Saavedra, J.M., Brownstein, M. and Palkovits, M., Serotonin distribution in the limbic system of the rat, Brain Res., 79 (1974) 437-441. 17 Snyder, S.H., Banerjee, S.P., Yamamura, H.J. and Greenberg, D., Drugs, neurotransmitters, and schizophrenia, Science, 184 (1974) 1243-1253. 18 Versteeg, D.H.G., van der Gugten, J,, de Jong, W. and Palkovits, M., Regional concentrations of noradrenaline and dopamine in rat brain, Bra#7 Res., 113 (1976) 563-574.
279
BBR 00510
Short Communications Deficits in inhibitory avoidance after neurotoxic lesions of the ventral striatum are neurochemically and behaviorally selective R. S C H W A R T I N G ~ and R.J. CAREY 2
Jlnstitute o/ Psychology II1. University of Diisseldor£ Diisseldorf (F.R.G.) and z Veterans Administration Medical Center and State University of New York. Upstate Medical Center, Syracuse, NY 13210 (U.S.A.) (Received March 19th, 1985) Revised version received September 26th, 1985) (Accepted October 4th, 1985) Key words: inhibitory avoidance - locomotion - biogenic amines - ventral striatum
Inhibitory avoidance (step-in type) was investigated in rats subjected to neurochemical lesions of the ventral striatum. The neurotoxins 6-hydroxydopamine or 5,7-dihydroxytryptamine were used to produce selective depletions of either dopamine, norepinephrine or serotonin. Only lesions which decreased the dopamine content of the ventral striatum impaired post-shock step-in behavior. Measurement of footshock reactivity by the Flinch-Jump technique indicated that only serotonin depletion altered reactivity to footshocks. Assessment of open-field locomotor behavior showed that the dopamine-denervated rats were hypoactive (fewer rearings) compared to controls, whereas serotonin-depleted rats were hyperactive. It is concluded that the deficit in inhibitory avoidance behavior following ventral striatal dopamine loss was dissociated from its effect on locomotor activity.
One of the main target areas of dopaminergic neurons in the brain is the striatal complex, which, according to Heimer et al. 1°, is composed of the dorsal and ventral striatum. The dorsal part (caudate-putamen) receives its main dopaminergic input from the substantia nigra j3 and has a well-known and important role in the mediation of voluntary movement2. The ventral striatum (nucleus accumbens and olfactory tubercle) receives dopaminergic innervation from the ventral tegmental area ~3 and has received considerable attention in neuropharmacological theories of abnormal behavior such as the dopamine hypothesis of schizophrenia ~7. The prominence of the ventral striatum in these theoretical formulations has generated a number of experimental studies involving manipulations of ventral striatal dopamine in animals. One line
of empirical investigation has shown that manipulation of dopamine in the ventral striatum has an important modulatory influence on global motoric processes such as locomotion. Thus, in rats an increase in ventral striatal dopaminergic activity produces an increase in locomotion, whereas a dopaminergic deficiency in the ventral striatum diminishes both spontaneous and pharmacologically induced locomotion. A similar relationship has been suggested for the involvement of ventral striatal dopamine in reward behavior 14"1s. This possible involvement of ventral striatal dopamine in reward behavior suggests the possibility of an influence on fundamental adaptive behavioral processes such as learning and memory. Surprisingly, the influences of ventral striatal dopamine on learning and memory have received little experimental attention. Using an active
Correspondence: R. Schwarting, Institute of Psychology III, University of Dtisseldorf, Universit/itsstrasse 1~ D 4000 D/isseldorf, F.R.G. 0166-4328/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
280 avoidance learning situation Koob et al. 12 have shown that selective denervation of dopamine in the ventral striatum has a small but significant effect on acquisition of active avoidance. Another observation suggesting that ventral striatal dopamine might play a role in learning and memory comes from the report of Bracs et al.3, that pretrial, but not pre-retest, injection of dopamine in the ventral striatum interferes with learning in an inhibitory avoidance situation. Since an excess of ventral striatal dopamine affects acquisition of an inhibitory avoidance response, we chose to further study the influence of ventral striatal dopamine in learning and memory by examining the effect of a dopamine deficiency in this area on inhibitory avoidance learning. As the ventral striatum is also rich in the other biogenic amines, norepinephrine 1~ and serotonin 16, we decided to also investigate depletions of these amines. We used the intracerebral application of the neurotoxin 6-hydroxydopamine (6OHDA), together with the norepinephrine reuptake-inhibitor desmethylimipramine (DMI) or the dopamine reuptake-inhibitor benztropine, to selectively destroy dopaminergic or noradrenergic neurons, respectively4. We used 5,7-dihydroxytryptamine (5,7-DHT) to deplete serotonin 9. Four groups of male Sprague-Dawley rats were used. Two groups were administered bilateral injections of 6-OHDA (6 #g/2#l, free base) into the ventral striatum. In one group (DA, n = 8), 6-OHDA was combined with a systemic injection of DMI (25 mg/kg), and in the other group (NE, n = 11) 6-OHDA was combined with an injection of benztropine (10 mg/kg). The third group (5-HT, n = 7) was administered bilateral injections of 5,7-DHT (8/~g/2/~1) together with systemic DMI and benztropine. Stereotaxic coordinates were 2.5 mm anterior to bregma, 1.5 mm lateral, and 7.0 mm below dura, with the incisor bar 1.0 mm above the interaural line. The unoperated control group (Co, n = 7) received the anesthetic (0.3 ml/kg equithesin) plus systemic DMI and benztropine. On days 9,10 and 11 postlesion all animals had 10 minutes of free exploration in an observation box which was an open 60 × 60 cm wooden en-
closure with a wire mesh floor subdivided into four 30 x 30cm quadrants. Two behavioral measures were recorded during the 10-min period on day 11 : (1) the number of quadrants of the test box entered (crossings) and (2)the number of times the rat reared up on its hind legs (rearings). Sixteen days after surgery all rats were tested for inhibitory avoidance using a conventional step-in apparatus (as described in ref. 6). Latencies to enter the dark compartment of this apparatus were measured before and 24 h after the presentation of a 0.5 mA footshock (5 s duration) in the dark compartment. Reactivity to electric footshock was measured in an Evans Flinch-Jump Box one week "after inhibitory avoidance testing: Flinch and jump thresholds to footshocks of increasing and decreasing intensities were observed (steps of 0.1 mA; for details see ref. 6). After completion of behavioral testing all rats were killed and brain samples of the ventral (nucleus accumbens and olfactory tubercle) and dorsal striatum (caudate-putamen) were assayed for dopamine, norepinephrine and serotonin using high-performance liquid chromatography with electrochemical detection (for details see ref. 5). Statistical analysis was performed for between group comparisons (lesion vs control) with the t-test (two-tailed for locomotor activity and flinch-jump thresholds, and one-tailed for biochemical assays) and the Wilcoxon Mann-Whitney U-test (two-tailed for inhibitory avoidance). Fig. 1 illustrates the results of locomotor activity testing (10-min observation period, postlesion day 11). Compared to controls, significant differences in locomotor activity were measured in the dopamine- and serotonin-depleted animals, but not in the norepinephrine-depleted group (crossings t = 1.43, d f = 16, n.s.; rearings t = 2.06, d f = 16, n.s.). Whereas the serotonin-lesioned animals showed significantly more rearings ( t = 2 . 5 7 , d f = 12, P < 0 . 0 5 ) and crossings (t = 2.34, df = 12, P < 0.05) than controls, dopamine-lesioned rats showed fewer rearings (t = 2.86, df = 13, P < 0.05) and no difference in crossings (t = 0.83, df = 13, n.s.). Before footshock all groups showed the same
281 03
~"
711 ii':!~!ii
30
Flinch
021 01
0 o
10
~{
Jump
0.6-
E 05
DA
NE
5-HT
Co
Fig. 1. Locomotor activity during a 10-min observation
period 11 days after lesions of the ventral striatum. Stippled bars: number of rearings; open bars: number of crossings (means, S.E.M.). Bottom line indicates groups: DA, 6-OHDA plus systemic DMI (n = 8); NE, 6-OHDA plus systemic benztropinc (n = 11); 5-HT, 5,7-DHT plus systemic DMI and benztropine (n = 7). Asterisks indicate statistical difference (t-test, P < 0.05) compared to controls (Co).
short latencies to step in. Twenty-four hours after the 0.5-mA footshock, only the dopamine-lesioned group showed an avoidance deficit (Wilcoxon/Mann-Whitney U-test, z = 2.20, P < 0.05) compared to controls; that is, the latencies from this group were virtually unchanged from preshock latencies whereas all other groups showed a modest but reliable increase in latencies
Pre Shock
3
G 2Ahrs after 05mA Shock
2 ,A-
DA
NE
5-HT
Co
Groups
Fig. 2. Median latencies to enter the dark compartment of the step-in apparatus before and 24 h after receiving a 0.5 mA footshock in that compartment. Groups as in Fig. 1. Asterisks indicate statistical differences (Wilcoxon/MannWhitney U-test, P < 0.05) compared to controls.
o
J~ 04
II
030.201DA
NE 5-HT Groups
Co
Fig. 3. Footshock reactivity as observed in the Evans Flinch-Jump Box. Mean thresholds (mA, S.E.MDto emit a flinch or jump response in ten series of increasing and decreasingfootshock intensities (0.1 mA steps). Groups as in Fig. 1. Asterisks indicate statistical differences (t-test, P < 0.05) compared to controls.
(Fig. 2). Fig. 3 shows that the neurotoxic lesions did not alter footshock reactivity, except for the 5-HT group where the threshold for the flinch response was significantly higher than for control animals (t-test; t = 2.30, df = 12, P < 0.05). The results of the biochemical analysis demonstrated that each lesion selectively depleted one biogenic amine in the ventral striatum (Table I). The injection of 6 #g of 6-OHDA, together with systemic D M I , depleted dopamine to 52% of control levels (t-test; t = 6.71, d f = 13, P < 0.001) without affecting norepinephrine or serotonin, whereas 6 #g of 6-OHDA, together with systemic benztropine, depleted norepinephrine (62% of control levels; t = 6.09, df = 16, P < 0.001) and not dopamine or serotonin. FinallY, the injection of 8 ~g of 5,7-DHT with systemic D M I and benztropine resulted in a small but significant depletion (t = 2.62, df = 12, P < 0.05) of serotonin. Neither lesion produced any statistically significant changes in biogenic amine levels in the dorsal striatum.
282 TABLE I
Content of biogenic amines (#gig tissue, means S.E.M.) measured by high-performance liquid chromatography with electrochemical detection; ventral striatum includes nucleus accumbens and ofactor.v tubercle, dorsal striatum : caudate-putamen: Groups
Ventral striatum DA NE
5 -H T
Dorsal striatum DA NE
5 -H T
DA
6 - O H D A plus systemic D M I (n = 8)
1.64 + 0.29**
1.40 + 0.05
1.92 + 0.25
12.26 + 1.60
0.64 + 0.08
1.34-+ (!.36
NE
6 - O H D A plus systemic benztropine (n = 11)
2.87 + 0.18
0.90 4:_ 0.05**
2.01 + 0.37
13.39 _+ 1.80
0.54 + 0.03
1.35-+ 0.38
5-HT
5,7-DHT plus systemic D M I and benztropine (n = 7)
3.68 +_ 0.41
1.58 _+ 0.06
1.63 + 0.21'
14.10 +_ 1.30
0.53 7~ 0.05
157 -~ 0.41
Co
controls (n = 7)
3.18 + 0.34
1.46 + 11.04
2.07 _+ 0.20
14.20 _+ 1.70
0.62 ~: 0.08
1.49 4:0.33
* P < 0.05 c o m p a r e d to controls (t-test). ** P < 0.001 c o m p a r e d to controls (t-test).
The present study provides some further evidence that a disturbance in dopaminergic neurotransmission in the ventral striatum may influence learning and memory processes. One important feature of the present findings is that the effect was selective for dopamine, as depletions ofnorepinephrine or serotonin did not affect inhibitory avoidance performance. This result appears to parallel observations regarding the dorsal striaturn where norepinephrine and serotonin have relatively minor effects compared with dopamine ~. In evaluating the results of the present experiment, note must be taken that the amine depletions, while selective and substantial were only partial, particularly for serotonin. Nevertheless these depletions resulted in locomotor changes which are in accord with earlier studies 7"11. Secondly, it seems important to observe that although inhibitory avoidance behavior was present in the control, norepinephrine and serotonin groups, the avoidance latencies were quite short. This suggests that the testing situation was near threshold for observing a significant hthibitory avoidance effect. Certainly more systematic manipulations of the testing situation are needed to adequately assess the magnitude of the influence of dopamine denervation on inhibitory avoidance. It could be argued that the dopamine-depleted
rats in this study, while hypoactive at the time of activity testing (2 weeks postlesion), were hyperactive at the time of avoidance testing (3 weeks postlesion), thus resulting in impaired avoidance. This argument cannot be ruled out by our experimental data. However, it is not supported by our general behavioral observations at the time of avoidance testing, as no indication of hyperactivity was seen in dopamine depleted animals. Despite these limitations, the present study is important because it indicates that the effect of a ventral striatal dopamine lesion on locomotor activity is dissociated from an effect on a simple learning task. That is, reduced activity levels as an explanation for the deficits observed can be excluded. Firstly, the preshock latencies to step-in were the same in dopamine-lesioned and unlesioned animals, and secondly, reduced activity should, if at all, result in better performance in the inhibitory avoidance task, since appropriate performance requires that the rat does not locomote (does not step in). In fact, the finding that dopamine stimulation of the ventral striatum (nucleus accumbens) does not impair inhibitory avoidance performance 3 is an analogous dissociation. Because this treatment typically produces hyperactivity 8, an impairment in inhibitory avoidance should have been observed if the influence of ventral striatal do-
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pamine manipulation on inhibitory avoidance were simply a manipulation of hyper- or hypoactivity. This dissociation between the behavioral effects ofdopamine denervation of the ventral striaturn on activity level and on the inhibitory avoidance task is of particular importance because other behavioral effects of dopaminergic denervation of the ventral striatum such as in reward behavior have produced directed changes in performance ~4 which are isomorphic with the directed changes in locomotion. Thus, it is difficult to rule out the possibility that the effects are only secondary to the locomotor effects. In summary, the present study suggests that the ventral striatum may influence behavior in ways not directly linked to motoric processes. Additional studies are needed to confirm and extend the present findings. Questions that need to be addressed include whether the observed avoidance deficit is dependent on time after lesion, whether this deficit occurs when other avoidance paradigms are used, and whether total depletions of ventral striatal dopamine also result in deficits in inhibitory avoidance behavior. This work was supported by Grant Hu 306/4-1 from the Deutsche Forschungsgemeinschaft and V.A. Medical Research and N.I.M.H. Grant M.H. 33959. I Bcrnheimcr. H., Birkmayer, W., Hornykiewicz, O., Jcllinger, K. and Seitelberger, F., Brain dopamine and the syndromes of Parkinson and Huntington, J. Neurol. Sci., 20 (1973) 415-455. 2 Bernheimer, H. and Hornykiewicz, O., Herabgesetzte Konzentration der Homovanillins~.ure im Gehirn von parkinsonkranken Menschen als Ausdruck der St6rung des zentralen Dopaminstoffwechsels, Klin. Wochenschr., 43 (1965) 711-715. 3 Bracs, P.U., Gregory, P. and Jackson, D.M., Passive avoidance in rats: disruption by dopamine applied to the nucleus accumbens, Pa:vchopharmacology, 83 (1984) 70-75. 4 Breese, G.R. and Taylor, T.D., Depletion of brain noradrcnaline and dopamine by 6-hydroxydopamine, Br. J. Pharmacol.. 42 (1971) 88-99.
5 Carey, R.J., Differential effects of limbic versus striatal dopamine loss on motoric function, Behav. Brain Res., 7 (1983) 283-296. 6 Carey, R.J. and Miller, M., Absence of learning and memory deficits in the vasopressin-deficient rat (Brattleborn strain), Behav. Brain Res., 6 (1982) 1-13. 7 Carter, C.J. and Pycock, CJ., The effects of 5,7-dihydroxytryptamine lesions of extrapyramidal and mesolimbic sites on spontaneous motor behaviour, and amphetamine-induced stereotypy, Naunyn- Schmiedeberg ~"Arch. Pharmacol., 308 (1979) 51-54. 8 Costall, B. and Naylor, R.J., The behavioural effects of dopamine applied intracerebrally to areas of the mesolimbic system, Eur. J. Pharmacol., 32 (1975) 87-92. 9 Gersson, S. and Baldessarini, R.J., Selective destruction of serotonin terminals in rat forebrain by high doses of 5,7-dihydroxytryptamine, Brain Re.v.. 85 (1975) 140-145. 10 tlcilncr, L, Switzcr, R.D. and van t tocscn, G.W., Ventral striatum and ventral pallidum. Components of the motor system? TINS, 3 (1982) 83-87. 11 Knob, G.F., Riley, S.J., Smith, S.C. and Robbins, T.W., Effects of 6-hydroxydopamine lesions of the nucleus accumbens septi and olfactory tubercle on feeding, locomotor activity, and amphetamine anorexia in the rat, J. Comp. Physiol. Psychol., 92 (1978) 917-927. 12 Knob, G.F., Simon, H., Herman, J.P. and LeMoal, M., Neuroleptic-like disruption of the conditioned avoidance response requires destruction of both the mesolimbic and nigrostriatal dopamine systems, Brain Res., 303 (1984) 319-329. 13 Lindvai1, O. and BjOrklund, A., Anatomy of the dopaminergic neuron systems in the rat brain. In P.J. Roberts, G.N. Woodruff and L.L. Iversen (Eds.), Advances in Biochemical Psvchopharmacology, Vol. 19, Raven Press New York, 1978, pp. 1-23. 14 Lyness, W.H., Friedle, N.M. and Moore, K.E., Destruction of dopaminergic nerve terminals in nucleus accumbens: cffect on D-amphetamine self-administration, Phar#nacol. Biochem. Behav.. 11 11979) 553-556. 15 Roberts, D.C.S., Knob, G.F., Klonoff, P. and Fibiger, H.C., Extinction and recovery of cocaine self-administration following 6-hydroxydopamine lesions of the nucleus accumbens, Pharmacol. Biochem. Behav., 12 (1980) 781-787. 16 Saavedra, J.M., Brownstein, M. and Palkovits, M., Serotonin distribution in the limbic system of the rat, Brain Res., 79 (1974) 437-441. 17 Snyder, S.H., Banerjee, S.P., Yamamura, H.J. and Greenberg, D., Drugs, neurotransmitters, and schizophrenia, Science, 184 (1974) 1243-1253. 18 Versteeg, D.H.G., van der Gugten, J,, de Jong, W. and Palkovits, M., Regional concentrations of noradrenaline and dopamine in rat brain, Bra#7 Res., 113 (1976) 563-574.