Hormonal signaling systems and their role in sex drive

Two University of Edinburgh professors from the Centre for Integrative Physiology describe how "our understanding of how the brain processes information is undergoing a radical shift as we begin to recognize the implications of hormonal signaling systems within the brain itself," explained Gareth Leng.

Mike Ludwig added that research in many centers has made it "apparent that hypothalamic neurons employ a vast diversity of chemical signals to communicate information, including more than 60 peptides. Many of these peptides, when released within the brain, trigger coherent, specific, complex behaviors, including sexual and social behaviors. Increasingly we are recognizing that peptide signals play a key role in information processing that is quite unlike the role played by conventional neurotransmitters."

Ludwig and Leng are scheduled to speak at the American Physiological Society's 2005 Conference, "Neurohypophyseal Hormones: From Genomics and Physiology to Disease," plus the latest developments toward clinical applications in Steamboat Springs, Colorado.

Leng and Ludwig also are scheduled to participate July 19 in the symposium, "Central release and actions of NH hormones," chaired by Quentin Pittman, University of Calgary, and Larry Young, Emory University School of Medicine.

"Melanocortin and oxytocin in facilitated sexual responses." Gareth Leng, Celine Caquineau, Nancy Sabatier, Alison Douglas, University of Edinburgh.

Leng described the now-accepted notion that "release of oxytocin and vasopressin in the brain occurs from dendrites, which are conventionally thought to be structures that receive information rather than transmit information."

For instance, Leng described how alpha-MSH, another peptide neurohormone, can stimulate oxytocin (OT) release from dendrites while blocking "normal" OT release at the axonal release sites. "The important new experiment here," he said, "was to find a link between endogenous alpha-MSH - that is, alpha-MSH produced within the brain - sexual behavior and oxytocin." Pairing sexually experienced male rats with sexually receptive females, the researchers injected a certain receptor antagonist (MC4) and then recorded their behavior.

All the control (non-treated) rats mated very rapidly. Of the rats injected with the receptor antagonist, about one-third showed no interest in mating. The others mated, but later when compared with the controls. Laboratory tests showed that the control rats had a lot of specific protein (Fos) in magnocellular oxytocin cells, less but some Fos in the antagonist-treated rats that mated late, but none at all in the antagonist-treated rats that didn't mate.

"These experiments provide pretty strong evidence that the effects of alpha-MSH on male sexual behavior are the result of OT release from the dendrites of the magnocellular neurons," Leng stated. "More importantly to me at least, they show that inside the brain there is a system of alpha-MSH-producing neurons that plays a key role in regulating sex drive," Leng concluded. In a broader sense, he added, "it also shows that alpha-MSH can act as a kind of 'switch' to change the site of oxytocin's release, either to the brain, or to the blood where it circulates to the body's periphery."