Evolution of time coding systems

The neurotrophin receptor p75 is a low-affinity receptor that binds neurotrophins. To investigate the role of p75 in the survival and function of central neurons, p75 null-mutant and wild type litter mate mice were tested on behavioral tasks. Null mutants showed significant performance deficits on water maze, inhibitory avoidance, motor activity, and habituation tasks that may be attributed to cognitive dysfunction or may represent a global sensorimotor impairment. The p75 null-mutant and wild type litter mate mice were assessed for central cholinergic deficit by using quantitative stereology to estimate the total neuronal number in basal forebrain and striatum and for subpopulations expressing the high-affinity tyrosine receptor kinase A (trkA) neurotrophin receptor and choline acetyltransferase (ChAT). In the adult brain, cholinergic neurons of the basal forebrain receive target-derived trophic support, whereas cholinergic striatal neurons do not. Adult p75 null-mutant mice had significant reduction of basal forebrain volume by 25% and had a corresponding significant loss of 37% of total basal forebrain neurons. The basal forebrain population of ChAT-positive neurons in p75-deficient mice declined significantly by 27%, whereas the trkA-positive population did not change significantly. There was no significant change in striatal volume or in striatal neuronal number either in total or by cholinergic subpopulation. These results demonstrate vulnerability to the lack of p75 in adult central neurons that are neurotrophin dependent. In addition, the loss of noncholinergic central neurons in mice lacking p75 suggests a role for p75 in cell survival by an as yet undetermined mechanism. Possible direct and indirect effects of p75 loss on neuronal survival are discussed.