Effect of growth hormone on memory in mice

In order to determine the effects of growth hormone (GH) on long term memory, two experiments were carried out, all involving a common paradigm. Mice were given massed training on a discrimination task and were injected with GH or control substances before and/or after training: retention was measured 4 weeks later. In Experiment 1, different groups of mice were injected with either GH or saline at five different time points relative to original learning (OL): 90, 30, and 5 min before, and 5 and 30 min after. The results showed a significant quadratic trend, in the form of a U-shaped curve for the GH-injected groups at retention. Maximal retention was observed at the 90 min pre-OL and the 30 min post-OL injection groups. Poorest retention was shown by the 5 min pre-OL injection group. In Experiment 2, GH, saline, and thyroid-stimulating hormone (TSH) were administered at 90 and 5 min pre-OL. In addition, a "state dependency" hypothesis was tested using additional groups of mice. The results again showed a severe drop in retention when GH was administered 5 min before OL; differences between the two pre-OL GH injection groups were less marked than in Experiment 1; TSH did not have the same action as GH; and finally, a state dependency interpretation was ruled out. Together the two experiments present results that indicate a selectively GH-induced suppression of memory when GH is administered in close proximity to OL. As the time lapse between the injection of drug and the learning task increases, the inhibitory effect on memory is reversed.