Monday question: Do neuromodulators have a unifying role?

We would never say that glutamate or GABA, the “basic” neurotransmitters, have a particular function. So why do attempt to give modulators like dopamine or oxytocin a defined role? Dopamine, for instance, is not only spread across the brain, but is also in the retina!

Or take oxytocin, “the love molecule”. It is not only involved in social behaviors but also cross-modal plasticity and modulating hippocampal fast spiking interneurons.

Do either dopamine or oxytocin – or any other neuromodulator for that matter – have a unifying function? Or did the brain evolve multiple independent uses for the modulators?

The basic unit of human relationship is the pair

Posting has been light (ie, nonexistent) because I’ve been preparing for/been at a conference.  While I was gone, I read the book Escape From Camp 14, about someone who not only was born and raised in a North Korean political prison, but also managed to escape from it.  Not only was the story interesting, but there was a lot of good stuff in it relating to how the human brain interacts with the environment.  Take this:

“It was in the pairs that the prisoners kept alive the semblance of humanity,” concluded Elmer Luchterhand, a sociologist at Yale who interviewed fifty-two concentration camp survivors shortly after liberation.

Pairs stole food and clothing for each other, exchanged small gifts, and planned for the future.  If one member of a pair fainted from hunger in front of an SS officer, the other would prop him up.

“Survival … could only be a social achievement, not an individual accident,” wrote Eugene Weinstock, a Belgian resistance fighter and Hungarian-born Jew who was sent to Buchenwald in 1943.

The death of one pair often doomed the other.  Women who knew Anne Frank in the Bergen-Belsen camp said that neither hunger nor typhus killed the young girl who would become the most famous diarist of the Nazi era.  Rather, they said, she lost the will to live after the death of her sister, Margot.

There was a bit more to the quote, but the book has already been returned to the library and I only have Google Books to quote from.  The point here is that the pair-bond seems to be the basic unit of human relationship.  This shouldn’t be too surprising; humans are generally monogamous on the order of a few years at a time.  But this pair-bonding isn’t solely romantic, but also extends to friendships.  What we know about pair-bonding comes primarily from work on prairie voles who are a uniquely monogamous species of vole.  This monogamy (or should I say, “monogamy”) connect to the neurohormone oxytocin.  Oxytocin seems to stimulate pair-bonding and social recognition.  It unfortunately gets a lot of press as the ‘love hormone’, even though it can have some darker effects.

Escapees from North Korea also seem to share certain personality traits that make it hard for them to prosper as refugees: they have a hard time holding down a job, they refuse to take personal responsibility for their actions, they are exceedingly suspicious of others, etc.  Not too surprising, obviously.  But taken together this illustrates certain facts about how the brain interacts with the environment to create personality: some things are hardwired in pretty solidly, like pair-bonding.  Others are plastic and interact with the environment, albeit in stereotyped ways.  In order to fully understand the brain, we will have to understand how interactions with the environment create neural mechanisms – the neuroscience of ecology.

Why you’ll become an alcoholic unless you get more sex

One very social behavior involves a man and a woman who love each other very much (hint: I’m talking about sex).  Flies who love each other very much obviously also mate, although you may not know that they undergo a courtship ritual first – not just any ol’ fly is getting to home plate.  That’s a behavior I’ll talk more about in a future post.  What I want to talk about here is instead what happens to that unlucky guy who, know matter how hard he tries, isn’t getting any.

A recent paper looked at this very question by taking a bunch of flies, and either having one group that either had a lot of sex or were rejected.  And can I say how awesome this sounds?  Listen to the protocol: one group of male flies experienced 1-hour sessions of sexual rejection three times a day for four days.  Another group experienced six-hour sessions of mating with multiple receptive virgin females for four days.  Let’s just say that you probably couldn’t do this kind of science in people.

The flies were then given the choice between food with alcohol and food without alcohol.  When the flies were sexually satisfied, they went without the alcohol; the flies were rejected needed that extra beer.  It turns out even virgin flies choose the alcohol – though they like it less than the rejected flies – which means that it is the lack of sex that mainly influences how much they need to drink.  If these same flies are allowed to mate?  Then they don’t need the alcohol anymore!

Desire, motivation and addiction in the brain are normally associated with the neural chemical dopamine.  But in this paper they looked at a neural peptide instead.  In humans, the neural peptide Y regulates alcohol consumption, as does all kinds of stress like PTSD and early maternal separation.  The equivalent peptide in fly is neural peptide F (NPF).  When they measured the amount of NPF in these flies, they found that it matched the desire for alcohol: the sexually rejected males had the lowest amount of NPF, the virgins had a little more, and the mated males had the most.  By decreasing the amount of NPF with siRNA or artificially activating it, they were able to control how much the flies wanted the alcohol.

So what is happening in the brain in response to sex?  Sex releases this neuropeptide – NPF in flies, NPY in humans – and the peptide is rewarding!  You love it (no surprise there)!  The peptide probably sets in motion changes in the larger reward system, modifying dopamine transmission over the course of many days.  This reveals the importance of investigating how we interact with our environment and fellow creatures in order to understand how our brain really works.

Reference

Shohat-Ophir, Kaun, Azanchi, Huberlein.  Sexual deprivation increases ethanol intake in Drosophila.  Science (335) 1351-1355.  DOI: 10.1126/science.1215932

See also the perspective.