Impact of Cage Size and Enrichment (Tube and Shelf) on Heart Rate Variability in Rats

Abstract

Rats respond physiologically and behaviorally to environmental stressors. As cage conditions can be a  stressor, it is important that experimental results acquired from caged rats are not confounded by these  responses. This study determined the effects of cage size and cage enrichment (tube and shelf) on heart  rate variability (HRV) in rats as a measure of stress. Electrocardiogram data were collected from 5 male  Sprague-Dawley rats, each implanted with a radio-telemetric transducer to assess the ratio of the low to high  frequency components of the HRV power spectrum (LF/HF). This ratio reflects the degree of sympathetic  versus parasympathetic nervous activity and increases with decreasing HRV. Rats were housed for 3 weeks  in each of the following cage conditions: small un-enriched, small enriched, large un-enriched and large enriched.  Cage enrichment and/or larger cages did not significantly alter LF/HF values compared to the small,  un-enriched cage condition, when considered independent of the sleep/wake cycle. However, when results  were pooled for all cage conditions, LF/HF significantly increased during the wake cycle compared to the  sleep cycle. Further analysis showed that this difference was only statistically significant for the un-enriched  cage condition. Thus the presence of a tube and a shelf in a rodent cage can alter the diurnal rhythm of HRV  in rats and this should be taken into account when designing experiments in which HRV is an outcome.