Estrogen

Estrogen, the “female sex steroid”, plays an important role in female reproduction in lots of different species. Before reproduction even happens, estrogen production ramps up in females to prepare them for breeding, and it helps maintain pregnancy in some mammals (something exploited by The Pill, by artificially maintaining high levels of estrogen and progesterone, it “tricks” the body into thinking it is pregnant). In the coolest of vertebrates, the birds, it even helps control egg size. Given all of the research that has gone into studying estrogen in female reproduction, you might think that this hormone is JUST restricted to its role in reproduction – but you would be dead wrong. Estrogen, in fact, isn’t just for making babies.

The main way that hormones influence the body is through receptors. Receptors are proteins that dangle off of the membranes of cell surfaces, and when hormones pair up with them stuff happens in the body. The receptor-estrogen duo’s main job is deciding what proteins get made, and these different proteins have different effects on the body.

Estrogen binds to receptors to initiate cell changes,
including protein production (photo:Medicineworld.org)

You can get a good idea of where in the body estrogen has an effect by finding places in the body that have estrogen specific receptors. That’s right, these receptors aren’t just anywhere. There are (not surprisingly) receptors in the gonads and there are receptors in different places in the brain. In some of these regions estrogen does influence reproduction (estrogen levels in the pituitary gland tell the body if it’s prego or not), but estrogen also affects memory and cognitive abilities, with receptors occurring in the hippocampus and in the striatum, two memory-related brain regions. Memory formation could (and really should) be a whole new blog post. However, I will say that a lot of what goes into making memories involves synaptic plasticity, where the connections between neurons are strengthened and weakened as memories are formed and (le sigh) lost. Guess what favorite hormone helps remodel synapses? That’s right, estrogen.

Synaptic plasticity: strengthening of connections between cells

So it seems that estrogen plays a part in how the brain processes the outside world, but is that the whole story? Of course not, it turns out there are also estrogen receptors in our sensory cells. Receptors are found in the eyes, nose and ears of species from humans to fish, suggesting that estrogen is affecting how the body receives signals from the outside world. In songbirds, this might explain why females with higher estrogen levels are choosier towards male songs. Estrogen changes the parts of the brain that are responsible for song reception.

 How’s that for a ubiquitous hormone. This “female sex steroid” is so darn omnipotent it makes me want to give a high five to girl power. Reproduction, sexual differentiation, cognition, memory and sensory reception, are there any pots it doesn’t have its thumbs in? Only time (and a lot more research) will tell.  

Sensors

Testing, testing, 1,2,3…  First blog post – I’d like to take the time to rehash some super cool stuff I learned at the 12^th Annual UNC Neuroscience Symposium. 

When I think of sensory receptors, I think of eyes, nose, ears, tongues – the obvious ones. However, I was recently exposed to the myriad of different receptors that organisms use, and I was introduced to the term “umwelt”. Umwelt refers to an organism’s perceptual world, and the umwelt of different species can vary quite a bit. While electric charges in general aren’t detectable by terrestrial organisms (i.e. electric currents can’t easily travel through air), lots of aquatic animals can sense small changes in electricity in the environment and can get information out of those changes.  These animals use electroreceptors on the skin that send signals to the brain when they sense electricity. Additionally, lots of long distance (and not so long distance) travelers – birds, sea turtles, bacteria - can get information from the earths magnetic field.

Oh Cool! a magnetotactic bacterium. That chain in the middle is magnetite (photo: nature.com)

The receptors that are used to sense magnetic fields are still a bit of a mystery, but one possible mechanism is through the use of magnetite, an iron-based molecule (the same one that is found in your compass needle). Magnetite is essentially a little magnet inside an animal’s body. It is thought to work by attaching to other sensory cells and pulling or pushing on them as it moves with the earth’s magnetic field.

An animal’s umwelt typically reflects aspects of its environment. Fish that live in caves have super poor vision but great touch receptors, while fish that in shallow streams have great vision. This general finding, that evolution of animal’s sensory abilities reflects its physical and social environment, helps explain why a word like umwelt is needed. Sensory perceptions are variable across different species.

Blind cave fish (notice the loss of pigment as well) (photo: nature manchester)

Well, for the most part they are. There are some types of sensory receptors that are common across lots of organisms. These kinds of receptors reflect the similar evolutionary history and similar environmental conditions from invertebrates to vertebrates. One such receptor was discussed by Cori Bargmann from Rockefeller University, in her talk on nematode social behavior. Sometimes nematodes clump together, this typically happens when they are in poor environments that are low in oxygen.  Bargmann's lab has found the nematode oxygen receptor cells. In most nematodes, they rapidly detect changes in oxygen levels. However, there are some types of nematodes that don’t clump together in low oxygen environments. Why are these nematodes different from the others? These nematodes have been living in laboratories for the past 50 years. In lab conditions, oxygen levels are about 10X higher than in the wild. These nematodes have lost their sensitivity to oxygen levels as they have evolved in this super cushy oxygen environment.

What do humans have in common with nematodes? Well, one thing is that we are also sensitive to oxygen levels. Another thing is that we can also adapt to different levels of oxygen. Andean and Tibetan human populations have evolved in lower oxygen levels, and are more efficient at utilizing oxygen compared to most human populations. So, while our umwelt may differ dramatically from other species, evolutionary processes continue to shape them, just like the nematode.