Investigation Of The Effects Of Diazepam And Amphetamine On Rat Locomotor Activity In The Home Cage And Open Field Using An Automated Approach

Dara Bree1,2, Cliona O’Mahony1,2, Bradley Morris1,2, Daniel Broom1,4, John Kelly1,2, Michelle Roche3,2, David Finn1,2. 1Pharmacology and Therapeutics, School of Medicine, National University of Ireland, Galway, Galway, Ireland, 2Neuroscience Cluster and Centre for Pain Research, NCBES, National University of Ireland, Galway, Galway, Ireland, 3Physiology, School of Medicine, National University of Ireland, Galway, Galway, Ireland, 4Research and Development, Covidien, North Haven, CT, USA.

Locomotor activity is routinely monitored in behavioural pharmacology studies, either as a primary endpoint or as a potential confound. Diazepam and amphetamine are psychoactive drugs which produce locomotor suppressant and stimulant effects, respectively (Dunne et al., 2007). The aim of the present study was to further characterise the effects and timecourse of these two drugs on rat locomotor activity in both the home cage and open field environments. Twenty-four male Lister-Hooded rats (225-250g, Charles River UK) were housed individually on a reverse 12:12h light:dark cycle (lights on at 20.00h) at 22 ± 1 °C. Twenty days later, rats received a 5 min exposure to the open field (43 x 43 x 43 cm, 70-90 lux) between 09.00h and 17.00h to assess baseline activity. Horizontal and vertical locomotor activity in the open field were monitored by the Opto-M3 Triple Axis System (Columbus Instruments, USA) which employs infra-red beam break technology. Automated tracking of distance moved (horizontal activity) and manual rating of rearing (vertical activity) were also performed using Ethovision XT7 (Noldus, The Netherlands). At 23h post-baseline, separate groups of rats (n = 8) received a single subcutaneous injection of either saline vehicle, diazepam (5mg kg-1) or amphetamine (4mg kg-1). Rats were then re-exposed to the open field for 5 min at 1, 2, 4 and 24h post-injection. When not in active testing, home cage activity was monitored continuously during the experiment using the Opto-M3 Dual Axis System. Data are shown as mean ± SEM and were analysed by repeated measured ANOVA and Student-Newman-Keuls post-hoc test (P<0.05 significant). There were no significant differences between the 3 experimental groups in baseline horizontal (H) or vertical (V) activity (Opto-M3 beam counts) in the home cage (H/V: Saline 954±149/285±56, Diazepam 944±118/317±60, Amphetamine 1242±204/ 310±57) or open field (H/V: Saline 1031±50/259±13, Diazepam 863±48/217±10, Amphetamine 966±47/224±16). Compared with saline controls, diazepam significantly reduced activity in the open field at 1h (H: 952±74 vs 203±45, P<0.001; V: 227±16 vs 11±4, P<0.001), 2h (H: 954±70 vs 196±56 P<0.001; V: 213±27 vs 24±10, P<0.001) and 4h (H: 859±65 vs 367±52, P<0.001; V: 164±23 vs 44±6, P<0.001) post-injection. Diazepam also significantly reduced home cage activity assessed by Opto-M3 over the 4 hour post-injection period, compared with saline-treated controls (H: 4875±472 vs 1636±117, P<0.05; V: 1840±229 vs 564±267, P<0.05). Amphetamine had no significant effect on open field activity but significantly increased home cage activity over the 4 hour post-injection period (H: 4875±472 vs 10023±835, P<0.05; V: 1840±229 vs 4076±679, P<0.05), as assessed by Opto-M3. At 24h post-injection, home cage and open field activity in all groups had returned to baseline levels. Opto-M3 outputs were corroborated by data generated using Ethovision XT7 and there was a strong positive correlation between the two methods of analysis (H: r2 = 0.74; V: r2 = 0.80). In conclusion, both diazepam and amphetamine profoundly affect horizontal and vertical locomotor activity over a period of at least 4h. The results also suggest that 5 min open field exposure is insufficient to detect the stimulant effects of amphetamine and that continuous home cage monitoring may be more useful for detecting locomotor stimulant properties of drugs.

Dunne, F. et al. (2007) Prog Neuropsychopharmacol Biol Psychiatry 31(7), 1456-1463.