Scent of a [sexually receptive partner]

Usually, the mainstream media jumps all over scientific reports relating to gender issues. LGBT science is hot stuff. I might have missed it, but a recent report in Nature (Vol 448, August 30, 2007, p. 1009) should have made a splash, but didn’t — perhaps because the findings can’t be boiled down into a simple sound byte, or the information is a little too technical, or folks are too quick to disregard any relationship between mice and humans.

Here’s the question, and it is, arguably, the central question of gender: what controls our preference for a male or female mate? In the August 30 Nature, Kimchi et al. (no, I’m not joking about the name) report that pheromone sensing controls both partner preference and mating behavior. First, some definitions:

In vertebrates, pheromones are recognized by neurons located in two sensory tissues in the nasal cavity, the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) . . . . Previous work had shown that deletion of the gene encoding TRPC2, a cation channel expressed only in VNO neurons, profoundly diminishes pheromone-evoked activity in these neurons.

Here’s the deal: since the 80s, scientists have had the ability to create “knockout mice,” mice lacking function in one particular gene. (The inventors of the technique recently won the Nobel Prize for it.) In the old days, if you wanted to investigate the function of the VNO, you’d have to surgically ablate it. But that would open up a raft of confounding variables — perhaps the behavioral changes were due to some other effect of surgery, not to destruction of the VNO. But with genetic techniques, you have a truly fine scalpel to dissect structure and function. Trpc2 knockout mice allow us to look at the behavior of mice which have not had surgery, still have VNO neurons, but lack VNO neurons’ responsiveness to pheromones. They are (nearly) ‘pheromone-blind’ mice.

Are you still with me? Great. Because now we get to the good stuff: the behavior of Trpc2 knockout mice.

Male mice lacking the Trpc2 gene do not distinguish between males and females, mating with animals of either sex. Moreover, in contrast to normal males, these mutant mice do not fight with intruder males.

. . . .

Now, Kimchi et al. find that Trpc2-deficient females also fail to distinguish between males and females among their conspecifics [members of the same species] in terms of mating preference. Unexpectedly, however, they found that mutant females behave like Trpc2-deficient males, sniffing, pursuing, and mounting mice of either sex . . . . These findings suggest that the VNO detects pheromones that normally prevent female mice from displaying male-typical sexual behavior.

Females can also be from Mars, Nirao M. Shah and S. Marc Breedlove, Nature News & Views, 30 August, pages 999-1000

How much of this, if any of it, can be generalized to humans? If you search for articles on “olfaction” (the sense of smell) and “libido,” or “anosmia” (loss of the sense of smell) and “libido,” you’ll find a raft of testimonial-quality evidence, but there’s precious little in peer-reviewed journals. A recent review (abstract) looked primarily at evidence from animal studies. However, Swedish scientists have found, using positron emission tomography (a scan which highlights metabolic activity in the brain), that lesbians respond to the putative pheromones AND and EST the way heterosexual men do. Similarly, homosexual men respond to AND the way heterosexual women do.

I don’t think a coherent picture has yet emerged explaining all the intricacies of olfaction and its effects on human sexual preferences. Fascinating topics like this make me wish I were back in the biomedical research biz, though. It’s even relevant to my turf — ear, NOSE, and throat.

Enjoy your Sunday.

D.