The research of the laboratory is focused on steroid hormone interactions with the central nervous system.   Throughout life, sex steroid hormones profoundly influence the structure and function of specific circuits that regulate reproduction and reproductive behaviors.  Previous work had focused on the regulation of neuropeptide and transmitter expression. However, relatively little is known about the mechanisms by which steroids affect postsynaptic activation and signal transduction.  The sex steroid hormone estrogen is also implicated in modulation of pain processing and neuroprotection.  Again, the mechanisms through which estrogen acts remain to be elucidated.

RESEARCH INTERESTS

STEROID MODULATION OF mu-OPIOID RECEPTOR (MOR) ACTIVATION
Estrogen treatment of ovariectomized rats initially has an inhibitory action on circuits mediating sexual receptivity (lordosis), but eventually induces sexual receptivity.  We have determined that an important component of this inhibition is due to the activation of MOR circuits in the medial preoptic area.  Taking advantage of G protein-coupled receptor (GPCR) internalization following activation of the receptor by an endogenous ligand, we determined that activation of MOR  is correlated with an inhibition of lordosis.  Behaviorally, progesterone augments estrogen action.  We have determined that progesterone relieves MOR-mediated inhibition, through the termination of opioid release in the medial preoptic area.

Is the MOR-inhibition dependent on estrogen receptors?  Working with Dr. Emilie Rissman, (University of Virginia), we have determined that estrogen activation of MOR circuits is dependent on the expression of the estrogen receptor-a (ERa).  Although MOR and opioid expression appears nominal in ERa knockout (ERaKO) mice, and MOR-selective opioids internalize MOR, estrogen does not induce internalization.  These results and the rapid time course of internalization suggest that the ERa is acting through a nongenomic mechanism.
 

REGULATION OF NEUROSTEROID BIOSYNTHESIS
Although it is well known that the brain can synthesize neurosteroids, it has been difficult to determine the function of these steroids in the regulation of reproduction.  We have recently determined that peripheral estrogen stimulates the synthesis of progesterone in the hypothalamus.  This increased in progesterone is restricted to the hypothalamus and is necessary for the initiation of the LH surge.  Examination of cells in vitro suggest that astrocytes are responsible for the estrogen-induced progesterone synthesis.  This response to estrogen may be an important component of estrogen positive feedback regulates the LH surge.  Males and aging females that do not exhibit positive feedback, that are lacking the ability to increase progesterone synthesis in the hypothalamus.
 

SIGNATURE OF STEROID ACTIVATION OF REPRODUCTIVE CIRCUITS
Although the common perception is that CNS circuits that regulate reproduction are highly redundant, data from several decades of research implies just the opposite. These circuits are easily disrupted.  A view more congruent with the data is that steroids activate (or inhibit) a particular sequence of neurochemical events that activate a set of circuits leading to the display of receptive behavior.  Peptidergic and especially opioid, CCK and NPY, circuits are central to this is activation.  These neuropeptides act on G protein-coupled receptors (GPCR).   Internalization of GPCR follows the activation of these receptors and thus can be used as an indication of receptor activation.  We are using receptor antibodies to track the internalization of specific receptors to determine the temporal and spatial signature of steroid activation of circuits involved in the regulation of sexual receptivity.  Our present data indicate that following estrogen stimulation NPY-Y1 circuits in the arcuate are activated.  These receptors are present on beta-endorphinergic neurons that project to the medial preoptic area where they release opioids to activate MOR which in turn modulate the release of CCK - a neuropeptide which stimulates lordosis in the medial preoptic area.
 
 

NONGENOMIC ACTIONS OF ESTROGEN MODULATION
To begin examining the nongenomic actions of estrogen on regulation of postsynaptic mechanisms, we have studied the response of Ca2+ in dorsal root ganglion (DRG) cells.  DRG cells provide an accessible and practical solution to quantitatively study the chemosensitive properties of estrogen-sensitive neurons. These cells express ERa and a number of other well characterized Ca2+ channels.  We have been studying the effects of estrogen on modulation of P2X receptors (ATP receptors) and activity of voltage gated Ca2+ channels (VSCC) using digital videomicroscopy for [Ca2+]i changes. Recent results indicate that 17-beta estradiol inhibited ATP-mediated [Ca2+]i responses and attenuated Ca2+ rise by acting on L-type VSCC in both male and female DRG neurons.