Reduced Wheel Running Activity in β1/β2 Adrenergic Receptor Knockout Mice is not Caused by Anxiety

Abstract

Mice lacking β1 and β2 adrenergic receptors (β1/β2ADR ‐/‐) exhibit reduced locomotor activity as measured by telemetry transmitter. In the present experiments we have examined whether the reduced locomotor activity resulted of augmented anxiety levels. Wild type (WT) and β1/β2ADR ‐/‐ mice were housed in running wheel (RW) cages for a 2 week period at a 12h light:12h dark (LD) cycle followed by a 2 week period at 12h dark: 12h dark (DD) conditions. The 24 hour average of RW revolutions/min was significantly lower in β1/β2ADR‐/‐ mice (LD: 3 ¡± 1; DD: 4 ± 1; n=7) compared to WT mice (LD: 18 ± 3; DD: 17 ± 4; n=8; p < 0.05). The activity showed a circadian pattern in both genotypes, but the periodic strength was significantly greater in WT (LD: 1117 ± 152; DD: 842 ± 135; n=8) than in β1/β2ADR‐/‐ (LD: 561 ± 114; DD: 414 ± 105; n=7; p < 0.05). To study anxiety levels behavior was analyzed in an open field test by videotracking for 5 min. 30‐35 min after placement into the open field, total distance traveled was 1382 ± 283 cm in WT (n=4) and 823 ± 150 cm (n=4; p<0.05) consistent with reduced locomotor activity. However, β1/β2ADR‐/‐ mice were found to spend 73 ± 8 % of total time in the open inner field while WT mice only spent 25 ± 9 % in the open field preferring proximity of walls for 75% of the time (n=4; p<0.05). In conclusion, absence of β1/β2ADR causes reduced voluntary locomotor activity and strength of circadian rhythmicity despite reduced levels of anxiety.