Photomicrograph
of serotonin containing axons (green) in close proximity to
hypoglossal nerve cells (red).
My
research involves how aging effects the neural control of breathing.
Very little is known about the effects of age on regions of the
brain and spinal cord that are involved in the control of respiration.
We believe that aging results in structural and functional changes
in these respiratory control areas that may predispose people to
breathing disorders. Moreover, we hypothesize that age-associated
changes in respiratory areas of the brain and spinal cord are quite
different in men and women.
I am
particularly interested in the role of the neurotransmitter serotonin
in the control of breathing. We have found that in male rats with
increasing age, serotonin levels decrease in areas of the brain
that help to keep the upper airway functioning normally. In contrast,
these changes are not seen in aging female rats. In other studies
on rats, we have recorded an age-associated decline in the normal
response to episodes of hypoxia in male rats. Once again, similar
changes were not seen in female rats. These data led us to conclude
that aging has a gender-specific effect on how serotonin can modulate
the respiratory control system. For
over a century it has been known that fluctuations in female hormone
levels associated with the estrus cycle and with pregnancy have
profound effects on breathing, and recent evidence supports a role
for the female gonadal hormones, estrogen and progesterone in the
neural control of breathing. The male gonadal hormone testosterone
may also be involved in the control of breathing, as it can be
converted to estrogen in the brain. Gonadal hormone levels decline
with age. We believe that there is a link between increasing age,
levels of gonadal hormones, serotonin, and the neural control of
breathing. Recent results from our laboratory show that levels
of serotonin in brain regions involved in the control of breathing
fluctuate during the rat estrus cycle, and decrease following gonadectomy.
The responses of the respiratory system to episodes of hypoxia
also change with hormone levels in both male and female rats. Taken
together, our data support the hypothesis that gonadal hormones
play a critical role in the maintenance of respiratory function
with increasing age through their action on the serotonergic system
in the brain. Furthermore, preliminary studies suggest that manipulations
of gonadal hormone levels can reverse or delay the age-associated
changes in the neural control of breathing. These ongoing studies
will provide a better understanding of the age and gender-related
contributions of the serotonergic system to sleep-associated breathing
disorders such as obstructive sleep apnea (OSA). OSA is an age-associated
disease that is far more prevalent in men than women, and also
more prevalent in women after menopause who are not taking hormone
replacement therapy.
Publications
Related to This Topic:
Behan,
M. and M. S. Brownfield. 1999 Age-related changes in serotonin
in the hypoglossal nucleus of rat: Implications for sleep-disordered
breathing. Neurosci. Lett. 267:133-136. [Abstract]
Fuller,
D.D., Baker, T.L., Behan, M. and G.S. Mitchell, 2001.
Expression of hypoglossal long term facilitation differs between sub-strains
of Sprague-Dawley rat. Physiol. Genomics 4;175-181 [Full
text]
Zabka,
A.G., Behan, M. and G.S. Mitchell, 2001.
Long term facilitation (LTF) of phrenic and hypoglossal motor output decreases
with age in male rats. J. Physiol. 531:509-514 [Full
text ]
Zabka,
A.G., Behan, M. and G.S. Mitchell, 2001. Time dependent hypoxic respiratory
responses in female rats are influenced by age and by the estrus cycle. J.
Appl. Physiol. 91:2831-2838 [Full
text]
Behan,
M., Zabka, A.G., and G.S. Mitchell. 2002. Age and gender effects on serotonin-dependent
plasticity in respiratory motor control. Respir. Physiol. and Neurobio. 131:65-77. [Abstract]
Behan,
M., Zabka, A.G., Thomas, C.F., and G.S. Mitchell, 2003. Sex steroid hormones
and the neural control of breathing. Respir. Physiology. and Neurobio. 136:249-263. [Abstract]
Behan,
M., and C.F. Thomas. 2005. Sex hormone receptors are expressed in identified
respiratory motoneurons in male and female rats. Neuroscience 130:725-734. [Abstract]
Zabka,
A.G., Mitchell, G.S. and M. Behan. 2005. Aging and gonadectomy
have similar effects on hypoglossal long-term facilitation
in male Fisher rats. J. Physiol. Mar 1;563(Pt
2):557-68. [Abstract]
Zabka,
A.G., Mitchell, G.S., and M. Behan. 2006. Conversion from testosterone
to estradiol is required to modulates respiratory long-term facilitation
in male rats. J. Physiol. 576: 903-912. [Abstract]
Seebart
B.R., Stoffel, R. T. and M. Behan. 2007. Age-related Changes in
the Serotonin 2A Receptor in the Hypoglossal Nucleus of Male and
Female Rats. Resp. Phys. Neurobiol. 158: 14-21. [Abstract]
Schwarz,
E.C., Thompson, J. M., Connor, N.P. and M. Behan. 2008. The effects
of aging on hypoglossal motoneurons in rats. Dysphagia (In Press)
Behan,
M., and J. Wenninger. 2008. Sex hormones and modulation of respiratory
motoneurons. Resp. Phys. Neurobiol. (In Press)
