Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC304
Selective decline in slowly adapting type I mechanoreceptors during development in rat sinus hair follicles in vitro
P. M. Cahusac1, S. Senok2
1. Psychology, University of Stirling, Stirling, United Kingdom. 2. College of Medicine, Alfaisal University, Riyadh, Saudi Arabia.
Figure 1. A plot of mean number of St I and St II units plotted according to the age of rat. ANOVA comparing the incidence of the two types of units from 112 experiments (112 different animals) over the last few years showed there was a statistically significant interaction between type of unit and time F(8,103) = 2.8, p = .008. The Figure shows that St I units in 6 week old animals were approximately 2.5 times as common compared with animals older than 14 weeks. Pairwise t test comparisons showed that St I units were significantly more common than St II units at the youngest mean age of 6 weeks (t(10) = 3.99, p = .003), while St II units were more common at mean age of 22 weeks (t(25) = 2.51, p = .02) - see asterisks on Figure, and St II units tended to be more common from that age up to the maximum age studied of 50 weeks.
Studies in primates, including humans, indicate that slowly adapting type I (SA I) mechanoreceptors are responsible for high resolution pattern discrimination by the fingertips (Johnson, 2001). Peripheral terminations of SA I afferents arborize in the epidermis giving rise to disc-like endings apposed to Merkel cells. In rodents, the highest concentration of Merkel nerve endings is found in sinus hairs (Halata et al., 2003). Prominent sinus hairs, present in many animals, are essential tactile organs used to sense the immediate environment: for hunting, food foraging and for spatial navigation. Behavioural studies have shown that the rat’s vibrissa system has a discriminative ability comparable to primate fingertips (Carvell & Simons, 1990). Like the role of SA I mechanoreceptors in primate fingertips, St I mechanoreceptors in the sinus hair follicle complex are likely to play a major role in the highly developed discriminative abilities of rodents and other whisker-bearing animals. It is therefore of considerable interest that of all the sensory nerve endings studied, Merkel endings at the level of the ring sinus (supplied by the deep vibrissal nerve) show the greatest decline and signs of degeneration from 6 weeks to aged (120 week old) rats (Fundin et al., 1997). During our experiments, carried out over the last few years using 6 - 50 week old rats kept in conventional cages, we found a dramatic drop in the probability of recording from St I units from the deep vibrissal nerve, see Figure 1). A precipitous decline occurred between 6 - 14 weeks, stabilizing thereafter (although further losses might be expected after 50 weeks during senescence). In contrast, the prevalence of St II units remained fairly constant over the age range. The decline in prevalence of St I units that we see from 6 - 14 weeks corresponds to the reduction in Merkel nerve endings seen in other studies from 6 - 120 weeks (Fundin et al., 1997). We hypothesise that there may be two components: an early decline from young adulthood due to a pruning of underused St I mechanoreceptors, and a much later decline only observed after 1 - 2.3 years associated with senescence. The decline in Merkel nerve endings may be linked with a reduction in neurotrophin 3 and trkC (reviewed by (Ulfhake et al., 2002)), and a loss of Merkel cells rather than the myelinated nerves that supply them, and as such it may be possible to preserve or restore peripheral cells (Merkel cells) as targets for therapy.
Where applicable, experiments conform with Society ethical requirements