Relation of exploratory behavior of rats in elevated plus-maze to brain receptor binding properties and serum growth hormone levels.

Forty-five male Wistar rats were selected according to their behavior in the elevated plus-maze. They were separated as follows: animals with low exploratory activity ('anxious'), an 'intermediate' group and animals having high exploratory activity ('non-anxious'). Various receptor binding studies and hormonal assays were also performed in these selected rats. The affinity of 5-hydroxytryptamine 5-HT2A receptors in the frontal cortex was lower in the 'anxious' rats compared to home-cage controls and 'non-anxious' animals. Moreover, the number of cholecystokinin (CCK) receptors in the hippocampus was significantly elevated in the 'anxious' group compared to home-cage control animals. The blood levels of growth hormone (GH) were significantly lower in the 'non-anxious' rats compared to 'anxious' counterparts. In conclusion, it seems likely that the decreased exploratory activity of rats is related to the increased 5-hydroxytryptamine (5-HT) and CCK mediated neurotransmission in the brain. The different serum levels of GH in the selected rats probably reflect alterations in the activity of 5-HT and CCK.