Influence of prolonged life span on receptor 'memory' in a unicellular organism, Tetrahymena.

The first interaction of the Tetrahymena with a hormone-like molecule-diiodo-tyrosine (T2) in the present study - gives rise to hormonal imprinting, the effect of which is being preserved by receptor 'memory' over as many as 500 generations. The first interaction with the hormone modifies the growth potential of the cell also without reexposure. In the present study, we followed up the fate of receptor 'memory' in Tetrahymena cells maintained without exchange of medium, and thereby inhibited in division, over the period of a year. The cells continued to display increased responsiveness - receptor 'memory' - over the first 3 months which normally cover the change of 500 generations, and to a lesser degree over the next 3 months (after 6 months) as well, but they showed loss of 'memory' after 9 and 12 months, as judged from the similarity of their growth response to the control. At 9 months, the cells were able to recover their original growth potential within 4 days of return to normal medium, but the 'memory' of the behavioral (growth response) modification induced by T2 returned, in a not fully convincing form, only after 48 days. Receptor 'memory' returned in 21 days. It follows that receptor 'memory' can equally persist by cell-cell transmission or within the same nondividing cell of extremely prolonged life span. Evidence has also been presented that loss of receptor 'memory' with ageing is only virtual, because it can be reactivated if the aged cells are 'rejuvenated' under appropriate conditions.