- Email: [email protected]
Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat
Neuroscience Letters 168 (1994) 111 114
ELSEVIER
NEURDSCIENCE LETTERS
Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat R Vezina**, D. Herv6, J. Glowinski, J.P. Tassin* (Tzaire dc Neuropharmacoh*gie.
I N S E R M U 114, ('oll@e ~h" I'}ance, I1, place Marcclin B(,rlhclot, ~5231 l>ari~ ('edcv 05. k)'anc'c
Received 7 October 1993: Revised version received 20 December 1993: Accepted 2(I December 1993
Abstract The locomotor response to nicotine was assessed four weeks following destruction of mesolimbic dopalnine t DA ~ neurons in rats by infusion of 6-hydroxydopamine into the ventral tegmental area. Resulting depletions of nucleus accumbens {N.Acc.) DA of up to 100% of control concentrations did not block the acute locomotor response to nicotine 10.4 mg/kg, base, s.c.). Such depletions also did not prevent the progressive enhancement of nicotine's locomotor effects when injections were repeated daily for nine days. These results suggest that mesolimbic D A is not necessary for the elicitation of locomotor activation by nicotine.
Key words. Sensitization: Nicotine; Psychomotor stimulant; 6-Hydroxydopamine: Dopamine: Ventral tegmental area: Locomotion
Acute systemic injections of nicotine produce increased locomotion in rats and repeating these injections leads to a more pronounced locomotor effect [4,17]. Different lines of evidence suggest that nicotine produces locomotion at least in part by activating mesolimbic dopamine (DA) neurons. These neurons express nicotinic acetylcholine receptors both on their perikarya and terminals [5,26] and several studies have shown that nicotine increases the rate of firing and the amount of burst firing in these cells as well as the DA transmission in the nucleus accumbens (N.Acc.) and anteromedial striatum. major terminal fields of AI0 DA neurons [for a review, see 11]. Importantly, acute injections of nicotine or of the nicotine agonist cytisine into the cell body or terminal regions of mesolimbic DA neurons (ventral tegmental area, VTA, and N.Acc., respectively), but not into other DA terminal fields, produce increased locomotion [21,22,24]. * Corresponding author. **Current address: Neurosciences, Loeb Medical Research Institute. Ottawa Civic Hospital. 1053 Carling Avenue, Ottawa, Ont. K1Y 4E9 Canada. 0304-3940/94/$7.00 ,c'. 1994 Elsevier Science Ireland Ltd. All rights reserved SSDI 0304-3940(93)E0884-X
Attempts to block nicotine-induced locomotion with DA receptor antagonists, oll the other hand, have yielded inconsistent results. DA receptor antagonists have been shown to block intra-VTA cytisine induced locomotion [21]. However, their effectiveness in blocking the locomotion induced by systemically injected nicotine has been equivocal [7,8,231, suggesting that nicotine might also produce some of its locomotor effects via non-DA-dependent mechanisms at other sites. This possibility is also supported by the lack of consistent evidence indicating that the enhancement of nicotine's locomotor effects following repeated injection is accompanied by a similar enhancement in mesolimbic DA neuron reactivity to the drug [2,12,19,24,27], as has been found to occur with psychomotor stimulant and opiate drugs [14]. The present experiment investigated the possibility that nicotine's acute locomotor effects and the progressive enhancement of these effects that occurs with repeated injection might be produced by this drug's actions on non-DA mechanisms. This was done by assessing the locomotor response to nicotine following destruction of the mesolimbic DA system in rats with bilateral in-
112
P
,,00 ~ 1000 o xz "~
I-Z
~
LESION
~
CONTROL
1 [ 'z []~(1 { '[
(t]l
•
! I
//.~
900
800
/ / /I
700
' I~ {'t~t'~ )'~[(It ( '~ { [
I -
-4
i
0
600
///i
{")
500
~//'~
0
400
-~ ///I ///t
T
//.4 ///I
0 L3 0
300
~~I ~
0
200
///1 //A ///1
SALINE
NICOTINE
///I //.4 //.4 // / / /1/ ~ / / A ///q
NICOTINE
9
Fig. l. L o c o m o t o r counts ( g r o u p mean 4- S.E.M.) obtained in l h after
animals were injected with saline and after the first and ninth injection of nicotine (0.4 mg/kg, base, s.c.). The counts shown %r saline represent the means of counts obtained after the two saline days. n = 8/group. • P < 0.025, nicotine 1 vs saline. **P < 0.0001, nicotine 9 vs saline. tP < 0.001, nicotine 9 vs nicotine 1.
jections of 6-hydroxydopamine (6-OHDA) into the VTA. Male Sprague-Dawley rats (250-300 g, Charles River, France) were anaesthetized with ketamine (150 mg/kg; Imalgene, Iffa-M6rieux, France), mounted on a stereotaxic apparatus and given bilateral injections into the VTA (A/P -4.3 m m from bregma, L + 0.5 m m from the midline and - 8 . 7 m m from skull). H a l f the animals (lesion, n = 8) received 6 - O H D A - H C I (4 #g/1 yl/side: RBI) and half (control, n = 8) received the 6 - O H D A vehicle. The 6 - O H D A was diluted in an isotonic solvent containing NaCI (9 mg/ml) and ascorbic acid (0.2 mg/ml) and adjusted to a p H of 4.5. Remaining 6 - O H D A lesion procedures were as described previously [13]. Behavioral testing began 4 weeks later. L o c o m o t o r activity was measured in circular corridors and was estimated daily by recording photocell beam interruptions during a 60 min period. On the first two days, rats were injected with saline (1 ml/kg, i.p.}. On each of the following nine days, rats were injected with (-)-nicotine bitartrate (0.4 mg/kg, base, s.c.). L o c o m o t o r counts obtained after saline were averaged over the two days for each animal. At the conclusion of the experiment, amine levels (DA and noradrenaline, NA) were estimated by H P L C - E D in the N.Acc. (DA) and medial prefrontal cortex (mPFC; DA and NA) as described previously [27]. Fig. 1 shows the locomotor counts obtained after animals were injected with saline and after the first and ninth injection of nicotine. It can be seen that, in comparison to saline, nicotine produced an acute increase in locomotion after the first injection and that this effect was significantly greater after the ninth injection. More importantly, these effects were seen in both control and
l.,':tcr~
16,~; : 19{J4 'I ] [ ]
114
6-OHDA-lesioned animals. A between-within analysis oF variance conducted on these data showed a significant effect of injection (F2,2s = 27.29, P < 0.0001 }. Post-hoe Scheff6 comparisons [16] revealed that locomotor counts after the ninth injection of nicotine were significantl} higher than alter the first (P < 0.001) and that both were significantly higher than counts obtained alter saline (P < 0.0001 and 0.025, respectively). The lesions produced by the intra-VTA injections of 6 - O H D A depleted N.Acc. DA from 68 to 100% of control values. Three animals showed depletions of N.Acc. DA ranging from 68 to 86% (68, 83 and 86%) while the remaining animals showed greater than 95% depletions. Two animals showed a greater than 99% depletion of DA in the N.Acc. These lesions affected m P F C levels of DA and NA to a lesser but more varied extent tTable 1). The locomotor response to nicotine of these subgroups as well as that of the control group expressed as percent change from their locomotor response to saline is shown in Fig. 2. It can be seen that, like control animals, all the lesioned animals showed an acute increase in locomotion following the first nicotine injection and that, in all cases, this effect became greater with repeated injections of the drug. Increases in N.Acc. DA have been tied to both the acute and sensitized locomotor effects of opiate and psychomotor stimulant drugs [t4]. And while nicotine has been shown to activate VTA-N.Acc. DA neurons and increase N.Acc. D A release acutely, these increases do not appear to parallel the enhancement of nicotine's locomotor effects brought on by repeated injection. The present findings indicate that VTA-N.Acc. DA neurons and N.Acc. D A are, in fact, not necessary to produce either the acute or the sensitized locomotor effects of nicotine, indeed, destruction of these neurons by intraVTA 6 - O H D A and the subsequent depletion of N.Acc. DA up to 100% of control concentrations did not prevent nicotine from producing either of these effects. Such 6 - O H D A induced depletions of N.Acc. DA, on the other Table l Amine depletions produced by injection of 6-hydroxydopamineinto the ventral tegmental area N. Acc. DA
mPFC DA
mPFC NA
Control (n = 8)
126 _+8
0.84 -+0.03
1.57 _+0.05
Lesion
68 86% > 95% > 99%
19~0% 55-84% 80-84%
54-92% 65 93% 70-92%
(n = 3) (n = 5) (n = 21
Control group amine concentrations (mean _+S.E.MJ in the nucleus accumbens (dopamine)and the medial prefrontal cortex {dopamineand noradrenaline) are expressed as ng/mg protein. The lesion subgroup concentrations are expressed as % depletions from these values. Punches were taken bilaterally and amine concentrations calculated as the mean of both sides. Differencesbetween sides never exceeded 25% of control values.
1~ {'ezina et aL / Neuroseience Letter,s 16,~¢ r 1994 J 11 1-114
o'1 z D
ca')
~/_i-
2 O0
o~°' :~",g'l"-/i/"/°o -"-1~0100 0
'J
_1 ~'~
~, 50
""~1/",, /,,'
© • • •
~ //' /
tJ
CONTROL ( n = 8 ) >99% DEPLETED ( n = 2 ) >95% DEPLETED ( n = 5 ) 6 8 - 8 6 % DEPLETED ( n = 3 )
I
I
I
t
I
1
3
5
7
9
NICOTINE INJECTIONS (days) Fig. 2. Locomotor response to nicotine as a function of dopamine depletion in the nucleus accumbens. Animals with 6-hydroxydopamine lesions were subdivided according to the extent of dopamine depletion exhibited in the nucleus accumbens. Locomotor counts were obtained in 1 h and are shown as mean % change from counts obtained after saline by the different groups. These were: control, 277; > 99% depleted, 158: >95% depleted, 376:68 86% depleted, 331.
hand, have been shown to completely block the locomotor response to amphetamine up to 33 days post-lesion [9]. Interestingly, the present results differ from those of an earlier report indicating that N.Acc. 6-OHDA lesions blocked nicotine induced locomotion two weeks following surgery [3]. It is difficult to reconcile these results with those reported here. The intra-N.Acc, infusions of 6-OHDA may have produced nonspecific lesions that interfered with actions of nicotine in this site. However. these lesions no longer blocked but rather enhanced nicotine's locomotor effects 4 weeks following surgery. It is conceivable that nicotine's acute effects on VTAN.Acc. DA neurons have been responsible for some aspects of drug-induced locomotion or other behavioral effects not detected in the present paradigm. Alternatively, nicotine may produce locomotion by activating VTA-N.Acc. DA neurons [21] as well as by acting on non-DA systems. Interestingly, morphine also elicits locomotion via mesolimbic DA dependent and non-DA dependent mechanisms [15] but only the locomotion produced by the former shows sensitization with repeated injection [28]. Nicotine's actions on mesolimbic DA neurons do not appear to contribute to the induction of sensitization since both the acute and sensitized locomotion produced by this drug can occur in the absence of DA. Furthermore, prior exposure to nicotine in nonlesioned animals does not produce cross-sensitization to either morphine [27], amphetamine [2] or cocaine [25]. Cross-sensitization has been well established to occur between these latter drugs and to reflect underlying changes in the reactivity of mesolimbic DA neurons [141.
113
Nicotinic acetylcholine receptors are expressed in several brain sites and only a fraction of these are linked to DA neurons [5,6]. Beside those investigating the involvement of DA neurons, few studies have explored the interactions of nicotine with the other neurotransmitter systems [1] and their role in the production of locomotor effects. Cytisine was reported to produce locomotion after injection into the N.Acc. and it is not known whether this effect is DA-dependent [22]. The hippocampus and cortex are also possible sites of production of locomotion effects since acetylcholine release in both areas was shown to be positively correlated with locomotor activity [10]. Repeated injections of nicotine have been reported to increase, in a way that parallels this drug's production of locomotion, the number of nicotine receptor binding sites in several brain regions including the frontal and parietal cortex and hippocampus [17,18]. However, this increased binding has been linked to a desensitization of nicotinic receptors to agonists [18]. On the other hand, repeating nicotine injections increases DA utilization (DOPAC/DA) in mPFC [27] and it is possible that transmission of other systems is affected in this region as well. In conclusion, while it is clear that nicotine exerts important effects on mesolimbic DA neurons and that these are linked to some of this drug's behavioral effects, the present results indicate that nicotine also produces potent locomotor effects by acting on non-DA systems. These remain to be elucidated. This work was supported by grants from INSERM, Philip Morris Europe and MRC of Canada. [1] Balfour, D.J.K.. The effects of nicotine on brain neurotransmitter systems, Pharmacol. Ther.. 16 (19g2) 269 292. [2] Bcnwell, M.E. and Balfour, D.J.K., The effects of acute and repeated nicotine treatment on nuclcu~ accumbens dopamine and locomotor actwit_v, Br. J. Pharmacol., 1(15 (1992) 849 S56. [3] Clarke, P.B.S., Fu, D.S., Jakubo~ic, A. and Fibiger, tl.C., Evidence that mesolimbic dopaminergic activation underlies the locomotor stimulant action of nicotine in ruts, J. Pharmacol. Exp. Then, 246(1988) 701 70S. [4] Clarke, P.B.S. and Kumar. R., The effects of mcotme on locomotor activity in nontolerant and toleram rat~, gr. J. Pharmacol., 7g (1983) 329 337. [5] Clarke. P.B.S. and Pert, A., Autoradiographic evidence for nicotine receptors on nigrostnatal dopaminergic neurons, Brain Res., 348(1985) 355 358. [6] Clarke, P.B.S., Pert, C.B. and Pert, A., Autoradiographic distribution of nicotine receptors m rat brain, Brain Res.. 323 (1984) 390 395. [7] Cline, E.J. and Ksir. C.. Nicotine effect on locomotor activity is not reduced by dopamine receptor blockade. Soc. Neurosci. Abstr., 14 (1988) 1137. [8] Corrigall, W.A. and Coen, K.M., Selective dopamine antagonists reduce nicotine self-administration, PsycflopharmacoIogy, 104 (1991) 171 176. [9] Creese, 1. and Iversen, S.D., The pharmacologmal and anatomical substrates of the amphetamine response in the rat, Brain Rcs., 83 (1975) 419 436,
114
P li'zhm er a/ /,Veurosclemc L,,llers 168 : / 9 9 4 , I l l 114
[10] Day, J., Damsma, G. and Fibiger, H.C., Cholinergic activity in the rat hippocampus, cortex and striatum correlates with locomotor activity: an in vivo microdialysis study, Pharmacol. Biochem Behav., 38 11991) 723~729. [11] Grenhoff, J. and Svensson, T.H., Pharmacology of nicotine, Br. J. Addict., 84 (1989) 477~92. [12] Harsing, L.G., Jr., Sershen, H. and Lajtha, A., Dopamme efllux from striatum after chronic nicotine: evidence for autoreceptor desensitization, J. Neurochem., 59 (1992) 48-54. [13] Herv& D.. Studler, J.-M., Blanc, G., Glowinski, J. and Tassin, J.-P., Partial protection by desmethylimipramine of the mesocortical dopamine neurones from the neurotoxic effect of 6-hydroxydopamine injected in ventral mesencephalic tegmentum. The role of noradrenergic innervation, Brain Res., 383 11986) 47 53, [14] Kalivas, RW. and Stewart, J., Dopaminc transmission in drug-and stress-induced sensitization of motor activity. Brain Res, Rev., 16 (1991) 223- 244. [15] Kalivas, RW., Widerlov, E., Stanley, D., Breese, G. and Prange, A.J., Enkephalin action on the mesolimbic system: a dopaminedependent and a dopamine-independent increase in locomotor activity, J. Pharmacol. Exp. Ther., 227 (1983) 229 ~237. [16] Kirk, R.E., Experimental Design: Procedures for the Behavioral Sciences, Brooks/Cole, Belmont, 1968. [17] Ksir. C., Hakan, R.L., Hall, D.R and Kellar, K.J., Exposure to nicotine enhances the behavioral stimulant effect of nicotine and increases binding of [3H]acetylcholine to nicotine receptors. Neuropharmacology, 24 11985) 527 531. [18] Lapchak, RA., Araujo, D.M., Quirion, R. and Collier, B., Effect of chronic nicotine treatment on nicotinic autoreceptor function and N-[3H]Methylcarbamylchotine binding sites in rat brain, J. Neurochem., 52 (1989) 483 491. [19] Lapin, E.R, Maker, H.S., Sershen, H. and Lajtha, A., Action of nicotine on accumbens dopamine and attenuation with repeated administration, Eur. J. Pharmacol., 160 11989) 53 50,
[20] Mitchell, S.N., Brazell, M.R. Joseph, M.H.. Alavijeh, M.S. and Gray, J.A., Regionally specific effects of acute and chronic nicotine on rates of catecholamine and 5-hydroxytryptamine synthesis m rat brain, Eur. J. Pharmacol., 167 (1989) ~;11 322. [21] Museo, E. and Wise, R.A., Locomotion induced by ventral tegmental microinjection of a nicotine agonist. Pharmacol. Biochem. Beha~., 35 (1990) 735-737. [22] Museo, E. and Wise, R.A., Microinjection of a nicotine agonist into dopamine terminal fields: effects on locomotion. Pharmaco{. Biochem. Behav., 37 (1990) 113 116. [23] Reavilk C. and Stolerman, I.R, Interaction of nicotine w~th dopaminergic mechanisms assessed through drug discrimination and rotational behaviour in rats. J. Psychopharmacok, 1 fl9871 264-273. [24] Reavill, C. and Stolerman, I.R, Locomotor activity in rats aRer administering nicotinic agonists intracerebrally, Br. J. Pharmacol.. 99 (1990) 273 278. [25] Schenk. S., Snow, S. and Horger, B.A., Pre-exposure to amphetamine but not to nicotine sensitizes rats to the motor activating effect of cocaine, Psychopharmacology, 103 I 1991) 62 -66. [261 Schwartz, R.D., Lehmann, J. and Kellar, K.J., Presynaptic nicotine cholinergic receptors labeled by [3H]acetylcholine on catecholamine and serotonin axons in brain, J. Neurochem,, 42 11984) 1495 1498. [27] Vezina, P., Blanc, G., Glowinski, J. and Tassin. J.-R, Nicotine and morphine differentially activate brain dopamine in prefrontocortical and subcortical terminal fields: effects of acute and repeated injections, J. Pharmacol. Exp. Ther., 261 (1992t 484,490. [28] Vezina, P., Kalivas, RW. and Stewart, J,, Sensitization occurs to the locomotor effects of morphine and the specific opioid receptor agonist, DAGO, administered repeatedly to the ventral tegmental area but not to the nucleus accumbens, Brain Res., 417 {1987) 51 58.
ELSEVIER
NEURDSCIENCE LETTERS
Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat R Vezina**, D. Herv6, J. Glowinski, J.P. Tassin* (Tzaire dc Neuropharmacoh*gie.
I N S E R M U 114, ('oll@e ~h" I'}ance, I1, place Marcclin B(,rlhclot, ~5231 l>ari~ ('edcv 05. k)'anc'c
Received 7 October 1993: Revised version received 20 December 1993: Accepted 2(I December 1993
Abstract The locomotor response to nicotine was assessed four weeks following destruction of mesolimbic dopalnine t DA ~ neurons in rats by infusion of 6-hydroxydopamine into the ventral tegmental area. Resulting depletions of nucleus accumbens {N.Acc.) DA of up to 100% of control concentrations did not block the acute locomotor response to nicotine 10.4 mg/kg, base, s.c.). Such depletions also did not prevent the progressive enhancement of nicotine's locomotor effects when injections were repeated daily for nine days. These results suggest that mesolimbic D A is not necessary for the elicitation of locomotor activation by nicotine.
Key words. Sensitization: Nicotine; Psychomotor stimulant; 6-Hydroxydopamine: Dopamine: Ventral tegmental area: Locomotion
Acute systemic injections of nicotine produce increased locomotion in rats and repeating these injections leads to a more pronounced locomotor effect [4,17]. Different lines of evidence suggest that nicotine produces locomotion at least in part by activating mesolimbic dopamine (DA) neurons. These neurons express nicotinic acetylcholine receptors both on their perikarya and terminals [5,26] and several studies have shown that nicotine increases the rate of firing and the amount of burst firing in these cells as well as the DA transmission in the nucleus accumbens (N.Acc.) and anteromedial striatum. major terminal fields of AI0 DA neurons [for a review, see 11]. Importantly, acute injections of nicotine or of the nicotine agonist cytisine into the cell body or terminal regions of mesolimbic DA neurons (ventral tegmental area, VTA, and N.Acc., respectively), but not into other DA terminal fields, produce increased locomotion [21,22,24]. * Corresponding author. **Current address: Neurosciences, Loeb Medical Research Institute. Ottawa Civic Hospital. 1053 Carling Avenue, Ottawa, Ont. K1Y 4E9 Canada. 0304-3940/94/$7.00 ,c'. 1994 Elsevier Science Ireland Ltd. All rights reserved SSDI 0304-3940(93)E0884-X
Attempts to block nicotine-induced locomotion with DA receptor antagonists, oll the other hand, have yielded inconsistent results. DA receptor antagonists have been shown to block intra-VTA cytisine induced locomotion [21]. However, their effectiveness in blocking the locomotion induced by systemically injected nicotine has been equivocal [7,8,231, suggesting that nicotine might also produce some of its locomotor effects via non-DA-dependent mechanisms at other sites. This possibility is also supported by the lack of consistent evidence indicating that the enhancement of nicotine's locomotor effects following repeated injection is accompanied by a similar enhancement in mesolimbic DA neuron reactivity to the drug [2,12,19,24,27], as has been found to occur with psychomotor stimulant and opiate drugs [14]. The present experiment investigated the possibility that nicotine's acute locomotor effects and the progressive enhancement of these effects that occurs with repeated injection might be produced by this drug's actions on non-DA mechanisms. This was done by assessing the locomotor response to nicotine following destruction of the mesolimbic DA system in rats with bilateral in-
112
P
,,00 ~ 1000 o xz "~
I-Z
~
LESION
~
CONTROL
1 [ 'z []~(1 { '[
(t]l
•
! I
//.~
900
800
/ / /I
700
' I~ {'t~t'~ )'~[(It ( '~ { [
I -
-4
i
0
600
///i
{")
500
~//'~
0
400
-~ ///I ///t
T
//.4 ///I
0 L3 0
300
~~I ~
0
200
///1 //A ///1
SALINE
NICOTINE
///I //.4 //.4 // / / /1/ ~ / / A ///q
NICOTINE
9
Fig. l. L o c o m o t o r counts ( g r o u p mean 4- S.E.M.) obtained in l h after
animals were injected with saline and after the first and ninth injection of nicotine (0.4 mg/kg, base, s.c.). The counts shown %r saline represent the means of counts obtained after the two saline days. n = 8/group. • P < 0.025, nicotine 1 vs saline. **P < 0.0001, nicotine 9 vs saline. tP < 0.001, nicotine 9 vs nicotine 1.
jections of 6-hydroxydopamine (6-OHDA) into the VTA. Male Sprague-Dawley rats (250-300 g, Charles River, France) were anaesthetized with ketamine (150 mg/kg; Imalgene, Iffa-M6rieux, France), mounted on a stereotaxic apparatus and given bilateral injections into the VTA (A/P -4.3 m m from bregma, L + 0.5 m m from the midline and - 8 . 7 m m from skull). H a l f the animals (lesion, n = 8) received 6 - O H D A - H C I (4 #g/1 yl/side: RBI) and half (control, n = 8) received the 6 - O H D A vehicle. The 6 - O H D A was diluted in an isotonic solvent containing NaCI (9 mg/ml) and ascorbic acid (0.2 mg/ml) and adjusted to a p H of 4.5. Remaining 6 - O H D A lesion procedures were as described previously [13]. Behavioral testing began 4 weeks later. L o c o m o t o r activity was measured in circular corridors and was estimated daily by recording photocell beam interruptions during a 60 min period. On the first two days, rats were injected with saline (1 ml/kg, i.p.}. On each of the following nine days, rats were injected with (-)-nicotine bitartrate (0.4 mg/kg, base, s.c.). L o c o m o t o r counts obtained after saline were averaged over the two days for each animal. At the conclusion of the experiment, amine levels (DA and noradrenaline, NA) were estimated by H P L C - E D in the N.Acc. (DA) and medial prefrontal cortex (mPFC; DA and NA) as described previously [27]. Fig. 1 shows the locomotor counts obtained after animals were injected with saline and after the first and ninth injection of nicotine. It can be seen that, in comparison to saline, nicotine produced an acute increase in locomotion after the first injection and that this effect was significantly greater after the ninth injection. More importantly, these effects were seen in both control and
l.,':tcr~
16,~; : 19{J4 'I ] [ ]
114
6-OHDA-lesioned animals. A between-within analysis oF variance conducted on these data showed a significant effect of injection (F2,2s = 27.29, P < 0.0001 }. Post-hoe Scheff6 comparisons [16] revealed that locomotor counts after the ninth injection of nicotine were significantl} higher than alter the first (P < 0.001) and that both were significantly higher than counts obtained alter saline (P < 0.0001 and 0.025, respectively). The lesions produced by the intra-VTA injections of 6 - O H D A depleted N.Acc. DA from 68 to 100% of control values. Three animals showed depletions of N.Acc. DA ranging from 68 to 86% (68, 83 and 86%) while the remaining animals showed greater than 95% depletions. Two animals showed a greater than 99% depletion of DA in the N.Acc. These lesions affected m P F C levels of DA and NA to a lesser but more varied extent tTable 1). The locomotor response to nicotine of these subgroups as well as that of the control group expressed as percent change from their locomotor response to saline is shown in Fig. 2. It can be seen that, like control animals, all the lesioned animals showed an acute increase in locomotion following the first nicotine injection and that, in all cases, this effect became greater with repeated injections of the drug. Increases in N.Acc. DA have been tied to both the acute and sensitized locomotor effects of opiate and psychomotor stimulant drugs [t4]. And while nicotine has been shown to activate VTA-N.Acc. DA neurons and increase N.Acc. D A release acutely, these increases do not appear to parallel the enhancement of nicotine's locomotor effects brought on by repeated injection. The present findings indicate that VTA-N.Acc. DA neurons and N.Acc. D A are, in fact, not necessary to produce either the acute or the sensitized locomotor effects of nicotine, indeed, destruction of these neurons by intraVTA 6 - O H D A and the subsequent depletion of N.Acc. DA up to 100% of control concentrations did not prevent nicotine from producing either of these effects. Such 6 - O H D A induced depletions of N.Acc. DA, on the other Table l Amine depletions produced by injection of 6-hydroxydopamineinto the ventral tegmental area N. Acc. DA
mPFC DA
mPFC NA
Control (n = 8)
126 _+8
0.84 -+0.03
1.57 _+0.05
Lesion
68 86% > 95% > 99%
19~0% 55-84% 80-84%
54-92% 65 93% 70-92%
(n = 3) (n = 5) (n = 21
Control group amine concentrations (mean _+S.E.MJ in the nucleus accumbens (dopamine)and the medial prefrontal cortex {dopamineand noradrenaline) are expressed as ng/mg protein. The lesion subgroup concentrations are expressed as % depletions from these values. Punches were taken bilaterally and amine concentrations calculated as the mean of both sides. Differencesbetween sides never exceeded 25% of control values.
1~ {'ezina et aL / Neuroseience Letter,s 16,~¢ r 1994 J 11 1-114
o'1 z D
ca')
~/_i-
2 O0
o~°' :~",g'l"-/i/"/°o -"-1~0100 0
'J
_1 ~'~
~, 50
""~1/",, /,,'
© • • •
~ //' /
tJ
CONTROL ( n = 8 ) >99% DEPLETED ( n = 2 ) >95% DEPLETED ( n = 5 ) 6 8 - 8 6 % DEPLETED ( n = 3 )
I
I
I
t
I
1
3
5
7
9
NICOTINE INJECTIONS (days) Fig. 2. Locomotor response to nicotine as a function of dopamine depletion in the nucleus accumbens. Animals with 6-hydroxydopamine lesions were subdivided according to the extent of dopamine depletion exhibited in the nucleus accumbens. Locomotor counts were obtained in 1 h and are shown as mean % change from counts obtained after saline by the different groups. These were: control, 277; > 99% depleted, 158: >95% depleted, 376:68 86% depleted, 331.
hand, have been shown to completely block the locomotor response to amphetamine up to 33 days post-lesion [9]. Interestingly, the present results differ from those of an earlier report indicating that N.Acc. 6-OHDA lesions blocked nicotine induced locomotion two weeks following surgery [3]. It is difficult to reconcile these results with those reported here. The intra-N.Acc, infusions of 6-OHDA may have produced nonspecific lesions that interfered with actions of nicotine in this site. However. these lesions no longer blocked but rather enhanced nicotine's locomotor effects 4 weeks following surgery. It is conceivable that nicotine's acute effects on VTAN.Acc. DA neurons have been responsible for some aspects of drug-induced locomotion or other behavioral effects not detected in the present paradigm. Alternatively, nicotine may produce locomotion by activating VTA-N.Acc. DA neurons [21] as well as by acting on non-DA systems. Interestingly, morphine also elicits locomotion via mesolimbic DA dependent and non-DA dependent mechanisms [15] but only the locomotion produced by the former shows sensitization with repeated injection [28]. Nicotine's actions on mesolimbic DA neurons do not appear to contribute to the induction of sensitization since both the acute and sensitized locomotion produced by this drug can occur in the absence of DA. Furthermore, prior exposure to nicotine in nonlesioned animals does not produce cross-sensitization to either morphine [27], amphetamine [2] or cocaine [25]. Cross-sensitization has been well established to occur between these latter drugs and to reflect underlying changes in the reactivity of mesolimbic DA neurons [141.
113
Nicotinic acetylcholine receptors are expressed in several brain sites and only a fraction of these are linked to DA neurons [5,6]. Beside those investigating the involvement of DA neurons, few studies have explored the interactions of nicotine with the other neurotransmitter systems [1] and their role in the production of locomotor effects. Cytisine was reported to produce locomotion after injection into the N.Acc. and it is not known whether this effect is DA-dependent [22]. The hippocampus and cortex are also possible sites of production of locomotion effects since acetylcholine release in both areas was shown to be positively correlated with locomotor activity [10]. Repeated injections of nicotine have been reported to increase, in a way that parallels this drug's production of locomotion, the number of nicotine receptor binding sites in several brain regions including the frontal and parietal cortex and hippocampus [17,18]. However, this increased binding has been linked to a desensitization of nicotinic receptors to agonists [18]. On the other hand, repeating nicotine injections increases DA utilization (DOPAC/DA) in mPFC [27] and it is possible that transmission of other systems is affected in this region as well. In conclusion, while it is clear that nicotine exerts important effects on mesolimbic DA neurons and that these are linked to some of this drug's behavioral effects, the present results indicate that nicotine also produces potent locomotor effects by acting on non-DA systems. These remain to be elucidated. This work was supported by grants from INSERM, Philip Morris Europe and MRC of Canada. [1] Balfour, D.J.K.. The effects of nicotine on brain neurotransmitter systems, Pharmacol. Ther.. 16 (19g2) 269 292. [2] Bcnwell, M.E. and Balfour, D.J.K., The effects of acute and repeated nicotine treatment on nuclcu~ accumbens dopamine and locomotor actwit_v, Br. J. Pharmacol., 1(15 (1992) 849 S56. [3] Clarke, P.B.S., Fu, D.S., Jakubo~ic, A. and Fibiger, tl.C., Evidence that mesolimbic dopaminergic activation underlies the locomotor stimulant action of nicotine in ruts, J. Pharmacol. Exp. Then, 246(1988) 701 70S. [4] Clarke, P.B.S. and Kumar. R., The effects of mcotme on locomotor activity in nontolerant and toleram rat~, gr. J. Pharmacol., 7g (1983) 329 337. [5] Clarke. P.B.S. and Pert, A., Autoradiographic evidence for nicotine receptors on nigrostnatal dopaminergic neurons, Brain Res., 348(1985) 355 358. [6] Clarke, P.B.S., Pert, C.B. and Pert, A., Autoradiographic distribution of nicotine receptors m rat brain, Brain Res.. 323 (1984) 390 395. [7] Cline, E.J. and Ksir. C.. Nicotine effect on locomotor activity is not reduced by dopamine receptor blockade. Soc. Neurosci. Abstr., 14 (1988) 1137. [8] Corrigall, W.A. and Coen, K.M., Selective dopamine antagonists reduce nicotine self-administration, PsycflopharmacoIogy, 104 (1991) 171 176. [9] Creese, 1. and Iversen, S.D., The pharmacologmal and anatomical substrates of the amphetamine response in the rat, Brain Rcs., 83 (1975) 419 436,
114
P li'zhm er a/ /,Veurosclemc L,,llers 168 : / 9 9 4 , I l l 114
[10] Day, J., Damsma, G. and Fibiger, H.C., Cholinergic activity in the rat hippocampus, cortex and striatum correlates with locomotor activity: an in vivo microdialysis study, Pharmacol. Biochem Behav., 38 11991) 723~729. [11] Grenhoff, J. and Svensson, T.H., Pharmacology of nicotine, Br. J. Addict., 84 (1989) 477~92. [12] Harsing, L.G., Jr., Sershen, H. and Lajtha, A., Dopamme efllux from striatum after chronic nicotine: evidence for autoreceptor desensitization, J. Neurochem., 59 (1992) 48-54. [13] Herv& D.. Studler, J.-M., Blanc, G., Glowinski, J. and Tassin, J.-P., Partial protection by desmethylimipramine of the mesocortical dopamine neurones from the neurotoxic effect of 6-hydroxydopamine injected in ventral mesencephalic tegmentum. The role of noradrenergic innervation, Brain Res., 383 11986) 47 53, [14] Kalivas, RW. and Stewart, J., Dopaminc transmission in drug-and stress-induced sensitization of motor activity. Brain Res, Rev., 16 (1991) 223- 244. [15] Kalivas, RW., Widerlov, E., Stanley, D., Breese, G. and Prange, A.J., Enkephalin action on the mesolimbic system: a dopaminedependent and a dopamine-independent increase in locomotor activity, J. Pharmacol. Exp. Ther., 227 (1983) 229 ~237. [16] Kirk, R.E., Experimental Design: Procedures for the Behavioral Sciences, Brooks/Cole, Belmont, 1968. [17] Ksir. C., Hakan, R.L., Hall, D.R and Kellar, K.J., Exposure to nicotine enhances the behavioral stimulant effect of nicotine and increases binding of [3H]acetylcholine to nicotine receptors. Neuropharmacology, 24 11985) 527 531. [18] Lapchak, RA., Araujo, D.M., Quirion, R. and Collier, B., Effect of chronic nicotine treatment on nicotinic autoreceptor function and N-[3H]Methylcarbamylchotine binding sites in rat brain, J. Neurochem., 52 (1989) 483 491. [19] Lapin, E.R, Maker, H.S., Sershen, H. and Lajtha, A., Action of nicotine on accumbens dopamine and attenuation with repeated administration, Eur. J. Pharmacol., 160 11989) 53 50,
[20] Mitchell, S.N., Brazell, M.R. Joseph, M.H.. Alavijeh, M.S. and Gray, J.A., Regionally specific effects of acute and chronic nicotine on rates of catecholamine and 5-hydroxytryptamine synthesis m rat brain, Eur. J. Pharmacol., 167 (1989) ~;11 322. [21] Museo, E. and Wise, R.A., Locomotion induced by ventral tegmental microinjection of a nicotine agonist. Pharmacol. Biochem. Beha~., 35 (1990) 735-737. [22] Museo, E. and Wise, R.A., Microinjection of a nicotine agonist into dopamine terminal fields: effects on locomotion. Pharmaco{. Biochem. Behav., 37 (1990) 113 116. [23] Reavilk C. and Stolerman, I.R, Interaction of nicotine w~th dopaminergic mechanisms assessed through drug discrimination and rotational behaviour in rats. J. Psychopharmacok, 1 fl9871 264-273. [24] Reavill, C. and Stolerman, I.R, Locomotor activity in rats aRer administering nicotinic agonists intracerebrally, Br. J. Pharmacol.. 99 (1990) 273 278. [25] Schenk. S., Snow, S. and Horger, B.A., Pre-exposure to amphetamine but not to nicotine sensitizes rats to the motor activating effect of cocaine, Psychopharmacology, 103 I 1991) 62 -66. [261 Schwartz, R.D., Lehmann, J. and Kellar, K.J., Presynaptic nicotine cholinergic receptors labeled by [3H]acetylcholine on catecholamine and serotonin axons in brain, J. Neurochem,, 42 11984) 1495 1498. [27] Vezina, P., Blanc, G., Glowinski, J. and Tassin. J.-R, Nicotine and morphine differentially activate brain dopamine in prefrontocortical and subcortical terminal fields: effects of acute and repeated injections, J. Pharmacol. Exp. Ther., 261 (1992t 484,490. [28] Vezina, P., Kalivas, RW. and Stewart, J,, Sensitization occurs to the locomotor effects of morphine and the specific opioid receptor agonist, DAGO, administered repeatedly to the ventral tegmental area but not to the nucleus accumbens, Brain Res., 417 {1987) 51 58.