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Sensitization to , effects of NMDA blockades Martin J. Acerbo*, Jennifer M. Lee, Juan D. Delius Allgemeine P.I'ychoiogie, Un.iversiti:it KOl'lsrall Z, 78457 KOllstal'lZ, Germany

Abstract

The dopamine apomorphine (apo) eli cits bouts of stereotyped pecking in pigeons, a response which increases with sllccessive apo injections. This sensitization is strongly context-specific and has been suggested to arise through a Pavlovian conditioning to both extern al and internal cues. We hypothetized that thi s involves dopamino-glutamatergic interactions and investigated the issue by inducing NMDA blockades with the antagonist dizocilpine (diz). A first experiment examined the effects that four different doses (ranging between 0.05 and 0.1 2 mg/kg) of di z co-administered with a standard dose of 0.5 mg/kg of apo had on the development of the incremented response and on the later expression of the conditioned pecking response. Both responses were impaired by doses of around 0.10 mg/kg diz. A second experiment assessed whether either a di z treatment or a diz plus apo co-treatment affected the development of a subsequent sensitization to apo. The first treatment had no effect on the latter sensiti zation. A part sensitization that arose with the second treatment did not transfer to the final sensitization. The last experiment examined whether the administration of diz had an immediate effect on the incremented responding to apo and on the conditioned response shown by already sensitized pigeons. No effect was apparent with the first treatment, but there was a marked response inhibition with the second treatment. It is concluded that NMDA glutamate receptors play an important role in apo-induced sensitization in pigeons which is compatible with the Pavlovian conditioning account of sensitization.

Keywords: Dizocilpine; MK-80 I; Apomorphine; Sensitization; Conditioning; Glutamate; Dopamine; Pigeons; Pecking

1. Introduction context-specific. It seems possible that this finding is due to a conditioning to the environmental cues that happen to A behavioral response increment that occurs in rodents be contingent with the drug experience [4,8,11,16,38]. upon repeated administ.ration of psychostimulant drugs We have proposed that the behavioral sensitization which such as and cocaine is widely suspected to pigeons show in response to repeated administration of apo be connected with their addictive potency in humans [47]. is largely, if not exclusively, due to a Pavlovian conditioning The sensitization phenomenon appears to be linked with the process. When a dose of about 0.5 mg/kg apo is adminis­ activation of dopaminoreceptive mechanisms which these tered for the first time to pigeons it occasions a bout of sev­ drugs are known to trigger [13,2],31,53,54]. Non-addictive, eral hundred pecks that lasts somewhat less than 1 h, an un­ more specific, and more direct dopamine than the conditioned response (UR). When the same dose is repeat­ two previously mentioned drugs, such as apomorphine (apo) edly injected, an incremented response (IR) develops and the and quinpirole are, however, also capable of producing a pecking in the experimental cage increases to an asymptotic sensitization effect of comparable nature [6,42,44,51,57]. level of several thousand pecks prodi.lced within approxi­ This may be due to the mes-telencephalic reward-signalling mately 1.5 h [17,60]. If later challenged with saline (sal) in system of vertebrates which is well known to largely rely the same cage, the pigeons exhibit a conditioned pecking re­ on transmission [14,40,49]. This fact may sponse (CR) of about a hundred pecks within about 30 min, a also be responsible for the finding that the sensitization response that gradually extinguishes with sal-treatment rep­ to dopamine agonists in rodents is often at least partly etitions [12,28,39]. Both the IR and the CR are markedly context-specific in the sense that they are only evident when the pigeons are exposed to the same experimental cage in • Corresponding author. Present. address: Department of Psychology, Biopsychology Program, University of Michiga n, Ann Arbor, USA. which they were sensitized. If the pigeons are placed in a E-mail address: [email protected] (M.J. Acerbo). sufficiently different cage so as to avoid a simple stimulus 202

generalization, the two responses are virtuall y absent. This abl e NMDA receptors probably becomes exhausted when is even so when the pigeons were pre-familiarized with the both glutamate and di z are present for some length of time. different cage while being sal-treated [3 ,34]. Furthermore, As di z vanishes, the pool is repl eni shed by a s low unblocking when thus-treated pi geons are later given the choice, their of receptors and by a slow synthesis of new NMDA recep­ IR and CR pecking is nearly exclusively directed at visual tors, a process that may take some 18 h (G. Westbrook, per­ features characteristic of the cage which they experienced sonal communication). Apo, incidentall y, is a potent, com­ whi le apo injected rather than at visual features typical of the petitive agonist at D 1- and D2-type dopamine r ~ce ptor s, act­ cage which they experienced while sal was injected [33]. All ing within a few minutes after having been inj ected intra­ this suggests that the cage in which the pigeons repeatedly muscul arly. It has a half-li fe of about 20 min in mammalian experi enced the effects of apo comes to act as a CScagexapo neural tissue [41,50]. to which a pecking CR develops. The fact that this CR seen The results of three experiments are reported. The first in the relevant cage upon a sal chall enge is of a markedly experiment dealt with the effects that four different doses of lesser magnitude than the IR obtained when the pigeons are diz co-administered with the standard dose of apo had on challenged with apo in that same cage, is explained by the the development of the IR and the subsequent expression circumstance that the drug, besides acting as a US eliciting of the CR. The second experiment assessed whether either a UR, also induces an internal state that has demonstrable a diz alone or a diz plus apo pretreatment affected the de­ CSapo properties [19,20,30]. The large IR that arises during velopment of a subsequent sensitization IR to apo. The las t the sensitization to apo is apparently driven by an approx­ experiment examined whether the administration of di z had imately multiplicative stimulus compound CScagexapo. The an immediate effect on the expression of the IR and the CR CSapo component is naturally absent when a sal challenge in in previously apo-sensitized pigeons. the CScage yields the weak pecking CR. A similarly weak, partial CR due to the CSapo component alone can sometimes be obtained when the pigeons are apo-chall enged in a cage 2. Methods different from that in which they were sensitized [33]. How­ ever, this response is often obscured by the strong, though 2.1. Subjects and procedures somewhat variable, pecking UR directly elicited by the drug [3 ,9,18]. Drug naive adult pi geons (CoLumba Livia), bred from lo­ learning is commonly ascribed to cal homing'Yiock and weighing between 450 and 550 g were a neurophysiogical long-term potentiation (LTP) processes used. A week before the experiments began they were moved known to occur in many glutamatergic in vel'te­ from an outside aviary to individual 40 cm x 40 cm x 45 cm brates. The apo-induced conditioning that we hypothesize is stainless steel grid cages. These home cages were located in likely to involve a dopamine-enhanced LTP variant [7 ,3 5]. a well-ventilated and brightly lit (12 h on and 12 h off) room. The changes in synaptic underlying this variant are Animal maintenance and treatments compli ed with the stan­ nevertheless attributed to a lasting increase in glutamatergic dards and rules specified by the German animal welfare law. rather than dopaminergic synaptic transmission. The longer It should be mentioned that at one point we di scovered that term modifications associated with LTP are thought to be we had accidentally used some pi geons that descended from principall y due to an increase in AMPA receptor density inadvertently inbred pairs. These pairs had produced a hi gh in the post-synaptic membrane, an increase that can be fa­ proportion of abnormall y apo unresponsive offspring which cil itated by the co-activation of dopamine receptors [15]. were most probably homozygous for a recessive insensitiv­ In the present study we examine whether NMDA receptors ity mutant. The relevant birds (n = 5) were excl uded and actuall y play a role in th e sensitization to apo shown by replaced by an equal number of identically treated, normally pigeons. If so, that would constitute further evidence that outbred pigeons. the sensitization is based on a conditioning process. The The experimental cages used were modified standard matter was examined by administering the NMDA recep­ cages. Their inner back- and side-wall surfaces were lined tor blocker dizocilpine (diz), also known as MK-801 , dur­ with white panels speckled with dark green dots (0.8 mm in ing successive phases of the sensitization process. Diz binds diameter, about 10/100 cm2). These di stinctive cages were in a non-competitive manner to the ionotropic NMDA re­ located in a separate, brightly lit room equipped with a ceptors and near-permanently blocks their normal response video-camera and -recorder. Drug treatments were adminis­ to glutamate. This blockage, however, only affects already tered on a once-a-day schedule. After having received these glutamate-activated, open channel receptors. In mammalian treatments (see the following for details) the pigeons were brain ti ssue diz reaches its maximum concentration some immediately and individually placed into the distinctive 30 min after it has been intramuscul arly administered and cages and videotaped for 20 min before they were returned persists there with a half-life of about 2 h [55]. The recep­ to their home cages. The videotapes were later reviewed tors blocked by diz are displaced to the periphery of the and the numbers of pecks per session were counted with a post-synaptic membrane and replaced within about 10 min tally-counter. The pecks of pi geons involve a quite di stinct, by viable NMDA receptors [48,52]. The overall pool of vi - easy to recognize motion pattern [29] and permit quite 203 accurate counts th at yield inter-observer agreement coeffi­ later a ll of th ese groups, whi ch will sometimes be referred to cients of rs > 0.85. Nibbling and preening responses th at as DizX + Apo/Sal groups, were additionall y injected with sometimes occurred in li eu of peckin g when th e reaction apo and videotaped. During th e second phase all the pi geons to apo was partly suppressed by co-administered diz were of these groups were inj ected with sal and videotaped fo r 3 disregarded. The experimental schedules to which the vari ­ days except for th e Apo-Di z lO/Sal pigeons which were so ous groups of pi geons were subjected invari abl y consisted treated fo r 6 days. Pigeons (n = 10) belonging to a Sal/Sal of two successive phases, mostl y involving two different control group were inj ected with sal and videotaped for 9 treatments. The abbrevi ated Xxx/Yyy-type names given to days (n = 5) or 10 days (n = 5). th e groups allude to these sequential treatments. Daily mean scores and standard errors were calcul ated for each of the 2.4. Pretreatment effects groups (see the foll owing). Because the data were often not normall y distributed, all statisti cal analyses were carried It seemed possible that th e second phase resul ts obtained out with non-parametri c Mann- Whitney U (between-group in the preceding experiment (see Section 3.2) might have compari sons) and Wilcoxon T (within-grou p compari sons) been due to an after-effect of th e di z treatments during the tests. fi rst phase. To check thi s, pigeons of a Diz/Apo group (n = 8) received 0. 10 mg/kg di z injections and were videotaped 2.2. Drug treatments 30 min later fo r 6 days. P igeons of a Sal/Apo control group (n = 6) were pre-treated with sal injecti ons in stead of diz Apo obtained from Teclapharm as a ready clinical so­ inj ecti ons. An additional Diz + Apo/Apo group (n = 6) of lu tion (10 mg/ml) was di lu ted with saline to a 1 mg/ml pigeons was fi rst administered 0.12 mg/kg diz and 20 mi n af­ solution prior to inj ection. Throughout thi s study we used terwards administered apo and then videotaped. The shorter a standard 0.5 mg/kg dose of apo. In earli er studi es it had th an usual delay and the compensatin g hi gher dose used been establi shed that doses between 0.2 and 2 mg/kg apo with thi s latter group was occasioned by a logistic constrain t. yield an orderly increasing set of dose-dependent sensiti za­ During the second phase all th e pi geons received apo onl y tion curves [5,27]. Dizocilpine(+)-maleate (MK-801 ), that injecti ons for a further 6 days, again being videotaped. is (5 R,1O S)-( + )-5-methyl- I 0, 11 -dihydro-5H-debenzo[a,d] cycl ohepten-5, I O-imine, was obtained from Tocris Cookson 2.5. Response retrieval and di ssolved in sal to yi eld a 0.2 mg/ml solution. Prelimi­ nary tri als showed that doses of more than 0.20 mg/kg had a In thi s experiment we examined whether di z would have strong sedating effect, often accompani ed by some postural an immedi ate effect on th e expression the IR and the CR in to ttering, whereas doses of 0.12 mg/kg or less yielded at pigeons that had previously been sensitized to apo. Pigeons most a sli ght sedation. Furthermore, in a pil ot ex periment belonging to an ApolDiz+Apo group (n = 8), an Apo/Diz+ doses up to 0. 12 mg/kg had no inhibitory effect whatsoever Sal group (n. = 7), and an Apo/Apo control group (n. = on the normal forage pecking of hungry pi geons. Based 6) were all injected with apo and videotaped for 6 days. on these fi ndings we began by employing doses ra ngin g During the second phase the Apo/Diz + Apo group received between 0.05 and 0.1 2 mg/kg di z. After diz inj ections the O. I 0 mg/kg diz and 30 min later the stand ard dose of apo pi geons spent 30 min (20 min in one instance: see the fo l­ for 4 days. T he Apo/Diz + Sal group received 0.1 0 mg/kg lowing) in their home cages to all ow fo r the drug-taking di z before receiving sal for 3 days. T he Apo/ Apo group effect before the next treatment step. Equivolume doses of continued to receive apo as before for a further 4 days. After sal were administered for control purposes. All inj ecti ons the sal ol' apo inj ecti ons the pigeons were videotaped. were given in tramuscul arl y (pectoral muscles).

2.3. Sensitization acquisition 3. Results

We fi rst examined whether th e co-administrati on of di z 3. 1. Sensitization acquisition during th e sensitization acquisition phase would in terfere with the development of a pecking IR to apo and subse­ During the fi rst phase of the experiment the co-administra­ quentl y res ul t in a diminished pecking CR in response to sal. ti on of all four doses of di z reduced th e pecking induced by Pigeons (n = 10) constitutin g an Apo/Sal baseline group 0.5 mg/kg apo in an approx im ately dose-dependent manner. were inj ected with apo and videotaped for 6 days and th en The lesser pecking of the Diz5 + Apo/Sal group compared inj ected with sal for another 3 days (n = 6) or 6 days with the Apo/Sal group, however, di d not reach signi fi­ (n = 4) and videotaped in the dis ti nct cages. The pigeons cance. All the other Diz + Apo/Sal groups exhi bited sig­ of groups D iz5 + Apo/Sal (n = 5), Diz8 + Apo/Sal (n. = 5), ni fic antly less pecking than th e Apo/Sal group throughout Di z lO +Apo/Sal (n = 6), and D iz12+Apo/Sal (n = 5) were days 1- 6 (Ps < 0.05). However, the pecking score differ­ first inj ected with 0.05, 0.08, 0.10, and 0. 12 mg/kg diz, re­ ences between the Diz8 + Apo/Sal and Diz lO + Apo/Sal specti vely, and returned to their home cages. Thirty minutes groups were not significant. Fig. I accordingly presents 204

4000 100 , ,/ •, --.I 3000 75 I/ ~ I c I 'C apo • CD 'E • 0 0 I I ~ N ~I VI 2000 I I --- 50 VI • I / 1 I I\.l .-: Q 0 Q) /'I/ j dl,1O+·00 3 Co ...... S' r 1\11 'I k' 1000 I . diz12+apo I j. I ~/ I 25 T I ' I...... -j I!:?" II ' I /,1 --A-A ~I_. all~al ~ ... J.,...... -:t . ' .1 , o I 1-,"--' 0 . , =" .1-'-.-+-sa, + I I 6 7 10 days

Fi g. I. Sensitiza tion acqu isiti on, mean pec king scores ± S. E. Left: days 1- 6 of Apo/Sal Ce), Diz lO + Apo/Sa l C- ) and D iz l 2 + Apo/Sal C"') groups while injected with 0, 0. 10, or 0.1 2 Illg/kg di z, res pectively, and with apo. The Sal/Sal group C+ ) was inj ected throughout with sa l. Right: days 7- 10 whi le all groups were injected with sal. The Diz 12 + Apo/Sa l group was not tested on session 10. Note that two differi ng respon se sca les are valid for the ri ght and left secti ons of the graph .

th e resul ts of onl y the ApolSal, Di z I 0 + ApolSal, and (Ps < 0.05) than the ApolSal group of the prev ious experi ­ Diz 12 + ApolSal group s. Of the latter two groups, the ment whi ch is inc luded in Fig. 2 for comparison. However, pecking scores pertaining to sessions 2- 6 and 4- 6, re­ the present Diz+ApolApo group, although similarl y treated specti vely, were nevertheless significantly hi gher (Ps < with 0.12mg/kg diz, pecked signifi cantly more during days 0.05) than those of the Sal/Sal group, also show n in 3- 6 ( Ps < 0.05), than the Diz 12 + ApolSal group of Fig. I. Fig. I . The peck.ing of the sa id Di z + Apol Apo group was in fact During the second phase of the ex periment when all not signifi cantly different fro m that of the DizlO + ApolSal groups were treated with sa l the ApolSal group yielded group shown in Fig. 1. This might have been because, as a sizeabl e pecking response during day 7 co mpared with explained in Section 2.4, the interval between the diz and the Sal/Sal group (P < 0.01), a response which, however, apo injections employed with the present Diz + ApolApo declin ed over the next two days . This decrease (extinc­ gro up was shorter than that normall y used. During the sec­ tion) from days 7 to 9 was sign ifi cant ( P < 0.01). All ond phase the D izl Apo, Diz + Apol Apo, and the Sal/ Apo four DizX + ApolSal groups yielded lesser peck.i ng scores groups all exhibited statistica ll y indistinguishab le sensiti ­ than the ApolSal group on day 7. This was significant in tizati.on courses. They were sli ghtly but not sign ifica ntl y the case of the Diz8 + ApolSal, DizlO + ApolSal, and lower than the sensitization courses ev inced by the ApolSal Diz 12 + ApolSal groups CPs < 0.05). The responding of group during the first phase (compare also with Fig. 3, left). all the DizX + ApolSal groups tended to increase slightly This establishes that the diz treatments experienced by the after day 7 but not signifi cantl y so. In any case, the dif­ Diz/Apo and Diz + ApolApo groups during the first phase ference between their scores and those of the ApolSal did not affect the IR development during a subseq uent sen­ group were no longer significant on day 8 and all sub­ siti zation to apo phase. Co ncern ing the Diz/Apo group, the sequen t days. However, from day 8 onwards the pecking outcome agrees with the circumstance th at the NMDA recep­ scores of the DizX + ApolSal groups were all signifi ca ntl y tor blocking effect of diz pers ists for only 18 h (see Section hi gher than those of the Sal/S al group (Ps < 0.05). The I ). It is worth noting, however, that the partial IR that the co-administration of diz during the sensitization phase thus Diz + ApolApo group had de,veloped during the first phase to a development of a dimin ished IR and later also to did not transfer to the second phase (day 6 to day 7 drop, the expression of a reduced CR. P > 0.0l). A tentative explanation of thi s finding is offered later in the Section 4. 3.2. Pretreatment effects 3.3. Response expression Fig. 2 depicts the resul ts of thi s experiment. As was ex­ pected both the Dizl Apo and the Sa il Apo pigeons pecked The res ults of thi s experiment are shown in Fig. 3. ve ry little during the first phase. From day 3 onwards the During the first phase all the groups responded in a ve ry Di z+ApolApo group pecked signi Rca ntly more (Ps < 0.01) similar manner. In fact, the sensitization courses of groups than th e formerly menti oned grQ ups but significantly less Apo/Diz+Apo and Apo/Diz+Sal were nearl y identical and 205

4000 .1 , 3000 ./1--. E: I~I ' E apo T o N 2000 diz+apo I I/) ~/ ~ .!I<: a.~ ' ,/i-- ' 1000 I/ -,---I .I /' diz ~ , sal /1- 1., .·_ .. iAo_- .•- J ,~ •• ,,_ ._. o ~.' - •., ~'"" - ....'. =-.·'-. sal'".. T ,-+-+-+-+-. I 6 7 12 days

Fig. 2. Pretreatment effects, mean pecking scores ± S.E, Left: days 1-6 of groups Diz/Apo (+), Sal/Apo (A), Diz + Apo/Apo (_), and Apo/Sal (e) while they were inj ected with diz, sa l, cli z and apo, or apo, respectively. Ri ght: days 7- 12, scores while the three first groups were inj ected with apo and the last one with sa l. Note that here the same j'esponse scale is valid for both the right and the left secti ons of the graph.

4000

3000 75 &:: 'E o N 2000 50 'i ~ ~w a. I::l 25 3 1000 :r

o o

~--r---~--~---r---'I I 6 7 10 days

Fig. 3, Response expression. Mean pecking scores ± S.E. Right: days 1- 6 of groups Apo/Apo (_), ApolDiz + Apo (A,A), Apo/Diz + Sal (A,..-), and Apo/Sal Ce) while all groups were treated with apo, The responses of groups Apo/Diz + Apo and Apo/Diz + Sal are shown pooled (see text). Right: days 7- 10 of above groups while'they were treated with apo, diz plus sal, diz plus apo, and sal, respectively. The Apo/Diz + Sal group was not tested on day 10. Note that an expanded scale is valid for the responses of the Apo/Sal and Apo/Diz + Sal groups during days 7-10, are shown pooled for simplicity's sake. None of the sensiti­ 4. Discussion zation courses differed significantly from that of the Apo/Sal group shown for comparison, The co-administration of diz Several of the results reported above show that a first ad­ which the Apo/Diz + Apo experienced during the second ministration of a medium dose of apo in pigeons elicits a phase (days 7-10) did not affect the apo-sensitized peck­ sizeable bout of stereotyped pecking (UR) and that a re­ ing (compare with the Apo/Apo group), This indicates that peated treatment with the same dose of apo treatment yields once the IR is established, it can no longer be appreciably a progressively increasing pecking response (IR), Fmther­ influenced by an NMDA receptor blockade. In contrast with mote, the Apo/Sal group (Fig, 1) demonstrates that, when that finding, the Apo/Diz + Sal group pecked very little challenged with sal, pigeons previously sensitized with apo during the second phase, significantly less in fact than the exhibit a pecking CR. This confirms findings of several ear­ Apo/Sal group did during the same phase (Ps < 0.05) . A lier studies in which it was additionally shown that the ex­ comparable diz treatment could thus suppress the expres­ pression of both the IR and CR is strongly dependent on sion of the weak CR but could not appreciably inhibit the the pigeons being tested in the same distinctive cage in much stronger IR. which they experienced the original sensitization-treatment 206

[3 ,28,33,34]. As ex plained in the Section 1 these and other veal any suppressive diz after-effect. But the reader should studies [12,39,60] have led us to postu late that the IR is the keep in mind that thi s latter remark does not invalidate the complete learned response shown when the fu ll CScagexapo response strength and the diz state arguments forwarded in compound is present while the much weaker CR is only the following paragraphs. a part-response exhibited when the CScage component is As might be expected according to the standard LTP present. A complementary part-response is elicited by the model, the second phase co-administration of di z and apo CSapo component when th e pi geons are treated with apo in in the Apo/Diz + Apo group (Fig. 3) had no effect on a cage different from that in which they were previously the ex pression of the sensiti zation IR previously established sensiti zed. This small pecking response is difficult to ascer­ through an apo-alone treatment. But in contrast to the stan­ tain because it is often obscured by variations of the much dard model's predictions it was found with the Apo/Diz+Sal stronger UR directly elicited by apo through its unquestion­ group (Fig. 3) that the administration of the same dose of diz able US quality [19,33]. Regardl ess of this, the sensitization inhibited the expression of a pecking CR in response to a sa l to apo in pigeons is wel l accounted for by a Pavlovian con­ challenge. It is possible that the difference in strength be­ ditioning process. In as much as this kind of learning ap­ tween the IR (more than 2000 pecks/20 min) and the CR (l ess pears to be neurally based on modifications of glutamatergic than 100 pecks/20 min) is the source of the disparity. The synapses and the onset of these modifications, in turn, are di z administration, in agreement with the model-deviating dependent on the activation of synaptic NMDA receptors, findin gs mentioned earlier, might have impaired the retrieval the co-administration of di z (MK-80l), an NMDA receptor of the weak CR but not have affected the much stronger IR. blocker, should have inhibitory effects on the development The IR impairment observed in the Diz + Apo/Apo group and ex pression of pecking IRs and CRs. (Fig. 2) during the first phase agrees, as already noted, with The role usuall y ascribed to the ionotropic NMDA recep­ what was seen in the earlier discussed DizX + Apo/Sal tors in LTP conditioning is that their channel opens when groups (Fig. 1). However, the fact that at the beginning presyn aptically released glutamate binds to them whi le the of the second phase (day 7) the pecking response of the post-synaptic membrane is depolarized as a resu lt of si­ Diz + Apo/Apo group was not different from that of the pre­ multaneous activation of non-NMDA glutamate receptors. viously unsensitized Diz/Apo and Sal/Apo groups (Fig. 2) The inflow of that follows triggers a com­ is puzzling. This means that the part-sensitization that the plex biochemical cascade [23,32]. In dopamine-facilitated Diz + Apo/Apo group had shown at the end of the first LTP, another such biochemical cascade may be tri ggered phase did not transfer to the second phase. An explanation by dopamine D I receptor activation [35,46]. When the two may be that diz, besides partially blocking the sensitiza­ cascades converge they lead to the phosphorylation of a nu ­ tion during the first phase, was also acting as a component clear CREB protein [LO] . When so activated this protein CSdiz that made the part-sensitization to apo diz-state de­ probably promotes the -controlled synthesis of AMPA pendent (i.e. the controlling stimulus here was effectively glutamate receptors. These receptors are thought to play a a triple CScagexapo xdi z compound). When the CSdiz com­ critical role in the establishment of lon g-term [43]. ponent was left out during the second phase the previously In any case, viable NMDA receptors are assumed not to be acquired part-IR went missing and a new IR controlled by any longer involved when the synaptic enhancement phase the simpler CScagexapo began to build up. In rodents it has is over [23,32]). However, thi s latter view has been occa­ been demonstrated that in drug discrimination experiments sionally upset when NMDA receptor antagonists have been diz has a CS-Iike effect [37]. Nevertheless, this complica­ found to also impair the retrieval of previously well-learned tion does not detract in the least from the replicated finding responses (e.g. [2,25,26]). The synaptic mechanisms respon­ that diz co-administration interferes with the development sible for these model-anomalous findings are not yet prop­ of a pecking IR in response to repeated apo doses. The ques­ erly understood. tion whether this effect came about through inhibition of a The results pertaining to the various DizX + Apo/Sal CSapo-IR, or of a CScage -IR linkage formation, or even both, groups (Fig. I) indicate that a co-administration of a suit­ can not be properly decided. By the way, we can also not able dose of diz inhibits the acquisition of the IR and cur­ totally discount the possibility that the reduced CRs shown tails the magnitude of the CR expressed afterwards. This by the DizX + Apo/Sal groups (Fig. 1) might be also partly accords with predictions that follow from the above LTP due to thi s hypothetical CSdiz dropout effect. model. However, the very low day 1 pecking shown by the DizlO + Apo/Sal and Dizl2 + Apo/Sal groups (see also the Diz + Apo/Apo group, Fig. 2) suggests that the pecking UR 5. Conclusion elicited by apo is already inhibited by diz co-administration. This is an effect not foreseen by the LTP model. It may We have shown that the development of the IR and the be that this dopaminergically triggered pecking response in­ later expression of a CR that are characteristic of the peck­ volves an intervening glutamatergic transmission link. The ing response sensitization arising with repeated apo admin­ CR impairment, in any case, is unlikely to be due to any istrations is impaired by NMDA receptor blockades brought lingering di z since the Diz/Apo group (Fig. 2) did not re- about by a co-administration of diz. These findings were ex- 207 pected on th e bas is th at the sensiti zati on to apo in pi geons Acknowledgements depends on a dopamine-induced, LTP-li ke, classical condi­ tioning process. But we also found th at th e retri eval of th e The research was supported by grants from the Deutsche CR eli cited by the environmental context was similarly in ­ Forschungsgemein schaft, Bonn. M.J.A. was the recipient of hibited by diz bl ockades. Thi s finding does not agree with the a Landesgraduierten Baden-WUrttemberg grant. We thank standard LTP model whi ch assumes that the producti on of Ines Kru g and Stephani e Iskra for technical assistance. previously conditi oned responses is independent of NMDA receptors. However, contrary to th e th eory, it is a frequent empirical finding that NMDA receptor antagoni sts can in­ References terfere with response retri eva l. Our results in any case agree with the hypoth esis that sensiti zation to apo in pi geons is [ I] Acerbo MJ, Delius JD. SC H-23390 blockade of behav ioura l sensi­ based on a process leading to an alteration of g lutamatergic tizati on due to apomorphine, submitted. transmi ssion mechani sms. The results of two further stud­ [2] Acerbo MJ , Gargiulo PA, Krug I, Delius JD. Behav ioural co nse­ ies involving the administrati on of dopamine D I and D2 re­ quences of dopaminergic stimulation and glu ta­ ceptor antagoni sts in pi geons being sensitized or in pi geons maLergic blocking in pigeons. Behav Brain Res 2002; 136: 17 1- 7. already sensitized to apo [1 ,3] agree with th e above conclu ­ [3] Acerbo MJ , Godoy A M , Delius JD. Haloperi dol bl ocks the acq ui ­ sion in as much as they indicated that the sensitization to sition, but not the retri eva l of a conditioned response induced by apomorphine. Behav Pharm aco l, in press. apo does not entail a major change in dopaminergic trans­ [4.1 Anagnostaras SG, Schallert T, Robinso n T E. Memory processes gov­ mi ssion. ernin g amphetamine- induced psychomotor sensitization. Neuropsy­ Although a large nUlnber of studies have investi gated chopharmacology 2002;26:703-15. whether the sensitizati on in rodents to amphetamine and co­ [51 Basten-KreFrt A. A pomorphin-induziertes Verhalten bei Tauben caine is affected by di z treatments, onl y a smaller number (Apomorphine- induced behav iour in pi geons). Diplomthes is. Bochum , Ruhr-Uni versity, Germany; 1977. has examined how the sensitization to apo is affected by di z [6] Batti st i JJ , Uretsky NJ, Wallace LJ . Importance of environmental treatments. These studies have fairly consistentl y shown th at contex t in the develop ment 0 1' am phetamine- or apomorphine- in duced th e expression of a CR after an apo-sensiti zati on is largely stereotyped be hav ior after single and multiple doses. Phannaco l prevented when di z is administered before the challenge with Biochem Behav 2000;66:67 1- 7. sal [6,22,24,56]. Opinions about whether di z has an analo­ [7] Bell K, Duffy P, Kalivas pw. Context-spec ific enhancement of glutamate transmi ss by cocaine. Neuropsychophannacology gous effect on the emergence of the IR in response to apo are 2000;23 :335-44. not so un animous. One study found that a pre-administration [8] Bonate PL, Swann A, Silverm an PB . Context-dependent cross­ of diz did not prevent the emergence of an apo-induced loco­ sensitiza tion between coca ine and amphetam ine. Life Sc i 1997 ;60: motory rotati on IR in unilaterall y substanti a ni gra lesioned PLJ - 7. rats [24]. Another study found that co-administrati on of di z [9] Bouton ME. Contex t, time, and memory retrieva l in the interfe rence paradigms of Pav lov ian learning. Psychol Bull 1993; I 14:80-99. immedi ately before apo did not impair and perh aps even en­ [1 0] Brami-Cherrie K, Valjent E, Ga rcia M , Pages C, Hipskind RA , hanced a locomotor activity and stereotyped behavior IR in a Caboche 1. Dopamine indu ces a PI3- kinase- in dependent acti va ti on context-dependent manner [56]. Nevertheless, several other of Akt in stri atal : a new route to cAMP response element­ studies report th at the administrati on of di z 30 min before bi nding protein phosphorylati on. J Neurosci 2002;22:89 1 1- 2 1. the injection of either apo or amphetamine prevented the de­ [ I II Browman KE, Bad ian i A, Rob inson TE. T he in fl uence of environ­ ment on the in ducti on of sensitizati on to the psychomotor activating velopment of a locomotor IR in rodents [6,22,24,36,45]. The effects 0 1' intravenous coca ine in rats is dose-dependent. Psychophar­ discrepancies could of course be due to the di ffering strain s, maco logy 1998; 137 :90- 8. procedures, and drugs th at were employed. A revi ew ad­ [1 2] Burg B, Hasse C, Lindenblalt U, Delius JD. Sensitiza ti on to and co n­ duces th at although a co-administrati on of diz often inhibits ditioning with apomorphine in pigeons. Pharmaco l Biochem Behav the sensiti zation to amphetamine and cocaine in rodents, it 1989;34:59- 64. [1 3"1 Cador M , Bjijou Y, Stinus L. Evidence of a co mplete in dependence can sometimes induce a sensiti zed locomotory response on of the neurobi olog ica l substra tes for the in ducti on ancl expression of its own whi ch th en adds to that produced by the psychostim­ behav ioral sensitiza ti on to amphetamine. Neurosc ience 1995;65:385- ul ants [58,59]. Nevertheless, the author's overall conclusion 95. is that NMDA receptors playa central role in the devel­ [ 14] Cador M , Tay lor JR, Robbin s TW. Potentiati on of the effects of opment of sensiti zati on to amphetamine and cocaine. Even rewa rd-related stimuli by dopaminerg ic-dependent mecha ni sms in though we have some qualms about equatin g too easil y the the nucleus accum bens. Psychopharmacology 199 1; 104:377-85. [ 15] Centonze 0 , Picconi B, Gubellini P, Bernardi G, Ca labres i P. sensitizati on of psychostimulant-induced locomotor acti vity Dopaminergic contro l of synaptic pl asticity in the dorsa l . in rodents with the sensitizati on of apo-induced pecking be­ Eur J Nelll'osc i 200 I ; 13: I 07 1- 7. hav ior in pi geons it would seem that with respect to the in­ [1 6] Crombag HS, Badiani A, M aren S, Robinson T E. The role of con­ vo lvement of NMDA receptors there are some in teresting textual versus drug-associated cues in promot ing the inducti on of parall els. The av ian apo-sensitization/conditi oning prepara­ psychomotor sensiti zat ion to intravenous ampheta min e. Behav Brain Res 2000; 11 6: 1-22. ti on may in any case be a useful model fo r the study of the [1 7] Delius JD. The peck ing of the pi geon: free for all. In : Lowe CF, dopaminergic-glutamatergic synaptic interacti ons th ought to Ri chelle M, Black man DE, Bradshaw, CM , ed itors. 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