The highest dose of the D4 antagonist L-745,870 (2000 μg/kg) moderately, but statistically significantly, increased position bias in the present spontaneous alternation paradigm compared to the lowest dose (20 μg/kg) (Fig. ). Rats injected with 2000 μg/kg L-745,870 also had a higher spatial bias than the saline group and the 200 μg/kg group, but these differences did not reach statistical significance. Thus, the hypothesis that L-745,870 would increase position bias was confirmed by the comparison of high and low L-745,870 doses, but not by the comparison of L-745,870 and saline. It was also the hypothesis, that the D4 antagonist would decrease the spatial alternation rate. There were, however, no significant differences in the alternation rate between the dosage groups for any parts of the session. Thus, the latter hypothesis was not confirmed.
This combination of results can only have been achieved because the effect of the drug doses tested on position bias was moderate. Had the increasing effect on position bias been more substantial, the alternation rate would have been decreased as a logical consequence. Control rats alternated at a relatively high rate (78%) during the first six trials of the experiment, and still over chance level (65%) when all 20 trials were considered. Thus, there should be no hindrance of showing a possible decrease in alternation rate due to floor effect. Alternation rates of 60–70% are usual in spontaneous alternation paradigms, depending on the behavioural strategy used by the rats [15
]. However, many versions of spontaneous alternation test regimes – regarding apparatus and procedure – have been described in the literature, some with alternation rates as high as 93% [e.g. [16
]]. Therefore it cannot be excluded that it may be possible to demonstrate a decreasing effect of the D4
antagonist – in the doses employed in the present study – on alternation rate, using another spontaneous alternation paradigm. However, the behavioural strategy used by rats in some spontaneous alternation paradigms with very high alternation rates probably differs from the "standard" spontaneous alternation strategy. For instance, Pych et al. reported that rats exhibiting 93% alternation in a Y-maze paradigm used a persistent turning strategy [16
]. Furthermore, the moderate alternation rate obtained with the present testing paradigm leaves room for increasing as well as decreasing effects of drugs on the alternation rate. An increasing effect of L-745,870 on alternation rate was a possibility in the present experiment, as the drug seemed to improve the behavioural shifting ability of rats in an earlier study [8
We cannot exclude that the highest drug dose (2 mg/kg L-745,870) could have mildly impaired locomotor activity, and that this motor impairment could have affected behaviour in the spontaneous alternation test. We did not measure the effect of L-745,870 on locomotor activity in the present study. However, visual observation suggested no obvious locomotor suppressing effects of the drug in any of the doses used. Patel et al. reported that 1 mg/kg orally dosed L-745,870 had no effect on locomotor activity in squirrel monkeys, while 10 mg/kg greatly reduced locomotor activity, induced sedation and mild parkinsonism [13
]. The motor impairing effects of L-745,870 are most likely due to the antagonist binding to D2
receptors, when the drug level in the brain is high [13
]. At the dose range used in the present study, L-745,870 has negligible action at other than D4
], and significant locomotor suppressing effects were therefore not expected in the experiment. Ideally, the highest dose of drugs used in behavioural pharmacology studies should have a noticeable effect on generalised behaviour. However, the L-745,870 doses of the present study were chosen, because the D4
antagonist is highly selective in these doses, and significant effects on performance of a learned alternation task have been found previously using comparable doses of L-745,870 (Zhang et al., 2004).
McFarland (1989) reported an increasing effect on position bias in a spontaneous alternation paradigm after administration of other drugs acting on the dopaminergic system, i.e. the dopamine releaser d
-amphetamine and the non-specific dopamine receptor agonist apomorphine [10
]. However, in these studies the increasing effect on position bias was followed by a decrease in alternation rate. It has been suggested, that the effects of d-
amphetamine and dopaminergic agonists on spontaneous alternation may be due to drug-induced behavioural perseveration [10
]. Since behavioural perseveration is one of the clearest manifestations of behavioural inflexibility that can be seen in behavioural experiments, we also analysed for repetitive position responding in the present study. There was, however, no increase in perseverative behaviour after administration of L-745,870 in the present study. Repetitive position responding for 5–10 trials in a row occurred in all groups – also the saline group – in a minority of the rats.
The effect of the D4
receptor antagonist on spontaneous alternation could also have been tested using a paradigm in which the attenuation by the D4
antagonist of behavioural disruptions induced by a non-specific indirect agonist such as d-
amphetamine was investigated. Such a paradigm could be applied to – and could prove to be valuable for – future studies of the effect of selective dopamine antagonists on spontaneous alternation, since d-
amphetamine has been demonstrated to disrupt normal behaviour in spontaneous alternation tests [12
A broad spectrum of doses of L-745,870 comparable to the doses used in the present study has earlier been shown to have either positive or negative effect on delayed alternation performance, depending on delay lengths as well as the baseline performance of the rats [14
]. The effects were interpreted as effects on working memory. Even though working memory is not the main topic of the present study, it is noteworthy that L-745,870 did not seem to interfere with working memory in the present study since the drug did not change the alternation rate. Working memory is involved in the rats' recollection of their latest response in spontaneous alternation. The inter-trial interval, however, was of shorter duration in the present study (15 s) than in the study of Zhang and colleagues [14
The dopaminergic drugs that have earlier been found to affect spontaneous alternation [17
] were not receptor-selective, and not much can therefore be concluded with certainty about the type of dopamine receptors involved. D1
receptor knockout mice have been tested in spontaneous alternation and did not differ from controls [19
], but functional compensation processes during development probably play a large role in knockout animals. It seems that the administration of dopaminergic agonists e.g. quinpirole [18
] and antagonists e.g. pimozide [17
] as well as selective depletions of dopamine can result in decreased spontaneous alternation rate [11
]. These results indicate that alternation behaviour is dependent upon an optimal balance in a distributed dopaminergic network.
In conclusion it has been demonstrated for the first time that the D4 receptor antagonist L-745,870 had an effect on position bias in a spontaneous alternation paradigm. It is too early to say if the effect on position bias found in the present study after administration of a relatively high dose of L-745,870 should be interpreted as a mild decrease in behavioural flexibility, related to the effects seen after prefrontal lesions or the administration of d-amphetamine. In future studies addressing this issue, it would be relevant to test higher doses of L-745,870 than 2 mg/kg in the present spontaneous alternation paradigm, and to test the effect of D4 antagonists on other spontaneous alternation paradigms, e.g. non-reinforced or discrete-trial paradigms. Paradigms using attenuation by D4 antagonists of dopamine agonist-induced disruption of behaviour should also be performed.