CONTROL DE ESPECIES EXÓTICAS INVASORAS

The primary aim of the experiment was to determine whether (i) Lister Hooded rats could be used in the place of Sprague Dawley rats in L-DOPA and dyskinesia experiments, and (ii) whether lesion rats that had received chronic L-DOPA treatment

83 could be tested in operant chambers following acute administration of a low dose of acute L-DOPA without the presence of disabling dyskinesias. To this end, lesion rats from the two strains were chronically treated with L-DOPA and compared in their AIMs and motor response to a low dose of acute L-DOPA. In addition, their performance on a simple discrimination and reversal learning task following acute administration of either L-DOPA or saline was compared. Based on the data

Figure 3.6. The mean number of sucrose pellets obtained by Lister Hooded (n=21) and Sprague Dawley (n=20) rats on a sucrose pellet consumption test allowing rats to panel press for new pellets with a 0 s (A) or 5 s (B) ITI. “On” and “off” refer to lesion rats administered 1 mg/kg acute L-DOPA (“on”) or 1 ml/kg saline (“off”) prior to testing. The error bars show the standard error of the mean. ITI=Intertrial interval.

presented in this section, it was concluded that (i) lesion Lister Hooded rats could be used in dyskinesia experiments, and (ii) that it was possible to administer a low L-DOPA dose to lesion rats that had previously received chronic L-DOPA treatment testing in operant chambers without inducing AIMs of such magnitude so as to interfere with operant performance.

Following chronic L-DOPA treatment, acute administration of 10 or 1 mg/kg L-DOPA induced AIMs in lesion rats of both strains. Interestingly, Lister Hooded rats exhibited more pronounced AIMs than Sprague Dawley rats when administered acute L-DOPA post chronic treatment. The onset of LID and expression of AIMs have both been linked to sensitisation and activation of D1 type receptors (Aubert et al, 2005; Mela et al, 2012). Previous studies have suggested that rat strains differ in regard to their dopamine receptor and dopamine transporter binding (Zamudio et al, 2005). While strain differences between Lister Hooded and Sprague Dawley rats‟ D1 receptors have not, to my knowledge, been specifically explored, Lister Hooded rats

84 show a greater locomotor response to amphetamine than Sprague Dawley rats, a phenomenon which has been speculated to be due to differences in the two strains‟ dopamine receptor sensitivity (McDermott & Kelly, 2008). Hence, whereas it was beyond the scope of the current experiment to determine the reasons for the strain difference in AIMs scores, it may be speculated that is was due to differences in Lister Hooded and Sprague Dawley rats‟ D1 receptor sensitivity.

A primary aim of the experiment was to determine whether Lister Hooded rats could be used in L-DOPA and dyskinesia experiments instead of Sprague Dawley rats, which are more common in such studies (e.g. Monville et al, 2005; Breger et al, 2013). Although the strains differed in the magnitude of their AIMs scores, LID was successfully induced in all lesion L-DOPA treated rats. Previous studies on rats and humans have shown between-species consistency in the mechanisms underlying LID (e.g. Andersson et al, 1999; Lindgren et al 2011; Verhagen et al, 1998; Breger et al, 2013). Thus, while there were strain differences in the magnitude of AIMs expression these are unlikely to reflect differences in the mechanisms underlying LID and AIMs in Lister Hooded versus Sprague Dawley rats. For this reason, and because Lister Hooded rats are preferred in operant studies where high visual acuity is necessary, it was considered valid to use Lister Hooded rats in lieu of Sprague Dawley rats in later experiments where LID onset and operant behaviour were explored simultaneously.

Because later experiments would administer acute L-DOPA to lesion rats who had developed LID prior to operant testing, the current experiment also tested whether a low dose of acute L-DOPA could improve motor function in lesion rats that had developed LID without simultaneously causing dyskinesias of such magnitude as to interfere with operant performance. In line with previous data (e.g. Lindgren et al, 2007) it was demonstrated that decreasing the acute L-DOPA dose from 10 mg/kg to 1 mg/kg reduced AIMs magnitude without completely abolishing the dyskinetic response to L-DOPA; suggesting a modest yet still measurable motor effect of the lower dose.

However, while it has previously been shown that acute L-DOPA doses higher than that used here improve 6-OHDA lesion rats‟ performance on the adjusting step test (8 mg/kg; Olsson et al, 1995) the current data did not show improved forelimb function following administration of 1 mg/kg L-DOPA. Neither did it show an effect of acute L-DOPA on spontaneous activity as measured by the number of beam breaks recorded in activity chambers. The latter could be caused by limitations to the

85 apparatus used as the activity cages used were designed for mice and not rats (width at bottom of cage=20 cm; length at bottom of cage=36 cm). There was, however, improvement on the vibrissae test which, together with the low but measurable dyskinetic response to 1 mg/kg L-DOPA, suggested that the dose was sufficient to induce a motor response with minimal observable AIMs. Based on the data, the lower dose of 1 mg/kg L-DOPA dose was subsequently used in the simple discrimination task. Whilst it was not possible to conduct dyskinesia scoring during operant testing, rats were closely monitored before and after the operant task and, when possible, on video screens whilst in the operant boxes. Such observations did not reveal any instances of AIMs preventing rats‟ from performing the task.

In line with the hypothesis and previous findings by e.g. Robbins and colleagues (1990), data from the simple discrimination task showed impaired performance in lesion relative to intact rats, albeit subtle. However, contrary to the hypothesis, the current data only showed a marginal effect of acute L-DOPA on operant performance. This may either suggest that the task was not sensitive to the psychopharmacological effects of the drug, or that the ability for all lesion rats in the current experiment to acquire the simple discrimination rule caused a ceiling effect which limited the possibility of observing a beneficial effect of L-DOPA. It is possible that more complex operant tasks are needed in order to observe a robust effect of lesion and acute L-DOPA in rats with unilateral 6-OHDA, MFB lesions.

Following operant testing, rats underwent two versions of a sucrose consumption test, using a 0 s and a 5 s ITI respectively. The 5 s ITI version was considered less sensitive to motor impairments, as the longer ITI enabled both intact and lesion rats to complete consumption of an acquired sucrose pellet before they were able to panel press for another pellet. Therefore, motor deficits were considered less likely to affect the total number of sucrose obtained by rats in the 5 s ITI than in the 0 s ITI version of the task. The data showed that neither the lesion nor acute L-DOPA affected the total number of sucrose pellets consumed by the rats. This was true for both the 0 s and 5 s ITI version of the task. Thus, there was no evidence for an effect of lesion, acute L-DOPA, or strain on lesion rats‟ willingness to consume sucrose pellets in the current experiment.

To conclude, the aim of the experiment was to determine whether Lister Hooded rats could be used in experiments combining the study of operant behaviour and dyskinesia, despite dyskinesia being more commonly studied in Sprague Dawley

86 rats (e.g. Monville et al, 2005; Breger et al, 2013). The experiment also aimed to determine whether a low dose of acute L-DOPA could produce a motor response in lesion rats that had been chronically treated with L-DOPA without simultaneously causing disabling AIMs. Based on the described data, it was decided to (i) use Lister Hooded rats in subsequent experiments incorporating both dyskinesia and operant testing (Chapters 7-8), and (ii) use the 1 mg/kg L-DOPA dose when acute L-DOPA was administered pre-operant testing to lesion rats that had previously received chronic L-DOPA treatment in Chapter 7.

3.3. Experiment 2: Using non-pharmacological tests to determine