Research Interests


Laboratory of Reproductive Biology and Behavioral Neuroendocrinology

My research program investigates the inter-relationship of neural, endocrine and physiological mechanisms by which reproductive behavior and mating are regulated. During my research career this line of investigation has allowed me to work with several wide ranging vertebrate models including reptiles (snakes), mammals (ferret and the big brown bat) and birds (Japanese quail).

My early research focused on physiological and endocrine variations that occur during the reproductive cycles of several species of snakes. Snakes have been shown to be an excellent model system for this line of investigation. Specifically, snakes exhibit a wide array of reproductive strategies that have allowed them to invade environments from the tropics to near the Arctic Circle (live birth vs. egg layer, continuous vs. seasonal breeding). In addition, snakes in these various habitats utilize a number of distinct environmental cues to regulate courtship behavior and mating.

My initial research endeavor described the reproductive biology of a geographically isolated population of lined snakes (Tropidoclonium lineatum) in eastern Missouri. This investigation found the lined snake to be a rather unique species. I discovered that the male lined snake initiates spermatogenesis at 6-8 weeks of age and mate for the first time when they are just one year old. In addition, although the male lined snake in this population retains sperm during winter dormancy, no mating occurs in the spring. This mating pattern appears to be unique to my population as we have now demonstrated that lined snake populations located in Kansas, Oklahoma and Texas mate in the spring. The difference in mating season appears to be directly related to environmental temperature.

During my Ph.D. program, I investigated the influence of hypothalamic and gonadal hormones on the development and maintenance of secondary sexual characteristics in several species of water snakes. I found that spermatogenesis and the renal sexual segment (RSS) of male water snakes was directly related to the release of follicle stimulating hormone and lutenizing hormone and the subsequent release of testosterone. The RSS is an hypertrophied area of the kidney of squamate reptiles (lizards and snakes) that is directly related to mating. In addition, I conducted an electron microscopic examination of the seasonal changes of the sexual granule within the epithelia of the RSS. My research shows that the granules develop and are utilized within the cells of the epithelia and do not rupture from the apex of the cells as previous reported.

The focus of my current research program specifically investigates the neuroendocrine aspects of reproductive physiology and behavior in snakes. In this line of investigation, I have been able to delineate the neural pathways critical for the control of seasonal courtship behavior in the male red-sided garter snake, a species exhibiting a dissociated reproductive pattern, mating at a time when the gonads are quiescent. Within these pathways are sex steroid concentrating neurons, collections of neurons that respond to altering levels of circulating sex steroids. The presence of these sex-steroid concentrating neurons in a male red-sided garter snake is of considerable interest since circulating testosterone levels do not initiate courtship behavior and mating. To date, the only known stimulus initiating courtship and mating in the male red-sided garter snake is a period of low temperature dormancy followed by exposure to warm temperatures. Initiation of courtship behavior in a species exhibiting this dissociated reproductive pattern has been described as being independent of circulating hormone levels.

Investigations in my lab are based on the hypothesis that circulating levels of testosterone do actually play a role in the initiation of courtship behavior and mating, although indirectly through the aromatization of testosterone to estrogen. We have been investigating the role of aromatase activity within the pathways controlling courtship and mating, specifically the sex-steroid concentrating neurons. We have compared the volume of brain nuclei at various times during the active season and during dormancy. We have also found that by inhibiting aromatase activity during low temperature dormancy we could inhibit courtship behavior while the addition of estrogen during this time period resulted in the display of normal courtship behavior. Recently, using immunocytochemical techniques, we were able to label, for the first time in a reptile, aromatase immunoreactive cells (ARO-ir) (cells containing the aromatase enzyme) in the brain of the red-sided garter snake. In addition, we are currently investigating the presence and seasonal variation of ARO-ir cells in the spinal cord of the male red-sided garter snake.

Other studies being conducted in my lab are looking at the presence and seasonal distribution of nitric oxide synthase (NOS), estrogen receptors and dopamine receptors in the male red-sided garter snake brain. As a change of pace, we are examining the distribution of NADPH-diaphorase in the brain of the little brown bat (a histochemical procedure to examine the presence of NOS) New studies being initiated include an investigation of C-fos production in the nervous system of the female red-sided garter snakes following mating and the seasonal distribution of arginine vasotocin (AVT) in the brain and spinal cord of both male and female red-sided garter snakes. Finally, I have initiated a study that investigates the role of sex steroids, directly or indirectly, on the development of neural pathways critical for the control of reproductive behavior and mating during the perinatal period encompassing both fetal and neonatal life.

 

Krohmer's Home Page

Science Department

SXU Home Page