Category Archives: neurotransmitter

On Dopamine and Ethics?

It has long been known that the hormone dopamine plays a key role in both addictive and learning behaviors. As with all neurotransmitters, “normal” functioning of these small molecules in the body depends on their receptors, of which there can be several types.

Two papers, published last year in the journal Science, both focus on particular dopamine receptor types, and behaviors associated with them. Work from Jeffrey Dalley’s lab1 uses PET to demonstrate that impulsive rats have significantly lower availability of two dopamine receptor types in a certain brain area, and further, that such impulsivity predicts how effective cocaine is in reinforcing behaviors thatt cause its administration. Meanwhile, experiments conducted by Tilmann Klein2 indicate that people with a certain allele of one type of dopamine receptor are significantly less able to learn a task via negative feedback.

The latter work, however, has implications beyond the nature of learning. This is the first example of a certain type of human behavioral trait about which one can make quantitative predictions through genomic analysis. This trait is one that has the potential to stratify people; categorize them before birth according their potential abilities.

This gene presents the possibility for exactly the sort of genetic profiling that many have predicted with dread. The former work mentioned above carries some of the same baggage, a PET scan based screening of individuals could potentially be done quite early, and it is not hard to imagine the development of genetic screens to forecast “dopamine receptor availability.” An interesting conclusion to the presentation of these ideas would be some exploration of the actual value of being able to learn from negative feedback. Are those less able to learn in this way simply more bold? Fearless? Risk-takers? More creative? Could there be some value in attempting to tailor one’s activities to one’s genetic disposition?

Just as the construction of the atomic bomb raised questions about the ethics of conducting certain types of research in the physical sciences, recent work such as that mentioned here does the same for neuroscience. As it gains widespread interest and makes increasingly strong predictions about human beings and their behaviors, the power of great neuroscientific research must be wielded as gingerly as any.

References:
1. Dalley, JW et al, (2007) Nucleus Accumbens D2/3 Receptors Predict Trait Impulsivity and Cocaine Reinforcement, Science, Vol. 315:1267-1270 | 
DOI: 10.1126/science.1137073

2. Klein, TA et al, (2007) Genetically Determined Differences in Learning from Errors, Science, Vol. 318:1642-1645 |
DOI: 10.1126/science.1145044

On Bodies and Brains

Being a dyed in the wool materialist, I believe that the cellular material hidden in our skulls creates our conscious experience. I was reminded this week, however, of just how irrevocably the body is involved in the generative process as well.

I read a piece of work which appears in the journal Cell, about a newly identified form of stimulated muscle contraction. Apparently, in the nematode Caenorhabditis Elegans, the muscles involved in expunging digested foosdtuffs from the body can be stimulated to do so directly by the intestinal tract1. Asim A. Beg et al, working in the lab of Erik M. Jorgensen, demonstrates that these muscles can be signaled that it’s time to go to work by a high proton concentration, i.e. an acid. Thus, as the gut works, and the space between the intestine and the muscles becomes acidified, the muscles contract.

Normally, muscles are only commanded to produce a force by the release of neurotransmitter from neurons that specifically innervate these tissues. In other words, the nervous system must tell muscles when it’s time to act. Of course the heart represents a notable counter-example, but there one finds specialized muscle cells that endogenously signal the heart to beat at regular intervals; native activity, not native stimulus response. The worm-gut case is unique because it is an example of the body bypassing the need for neural intervention.

I was further disabused of my cephalocentric ideology by listening to an old episode of RadioLab titled “Where am I?” That program contained several magnificent examples of the theme I’m expounding on, and one in particular that caught my attention.

The hosts of this not to be ignored radiological phenomenon had as a guest the science writer and scientist Robert M. Sapolsky (amongst others). He commented on a theory concerning brain-body interactions first attributed to William James. It goes like this: not only do the bodily physiological manifestations of emotional states precede conscious awareness of the source-stimulus, but those responses can themselves cause emotional experiences. I found this fascinating, and being a student of the brain, I wanted a little bit more information than the show had to offer, so I got in touch with Professor Sapolsky (at Stanford), and he gave me the following distillation (of the first part):

“The basic story is that sensory information (with the exception of olfaction) gets to the amygdala by way of the usual projections to the cortex, where there is classical sensory cortical processing, and with information eventually passed on to the amygdala. But there is an alternative pathway going straight from the thalamus to the amgdala, bypassing Cortex, so that information gets there sooner. So there’s the potential for amygdaloid activation in response to stimuli before there is conscious (i.e., cortical) awareness of the stimuli. So very fast, but because the cortex really does do all the important transformations of sensory information, this fast short-cut can be quite inaccurate.”

This explains how — by activating the amygdala — emotions and concomitant corporal responses can occur before conscious awareness of their origins, but the bit about the body informing the emotional state goes further. The idea there is that even if you’ve decided on some rational level that there’s nothing to be upset about, the body’s state can convince the brain that there is.

I am not sure of the mechanism that the brain employs to read off the emotional state of the body, but this interplay between mental and body implies a couple of things. First, our body has the ability to inform our brains how we’re feeling, which is especially remarkable in light of the example of the body working independently of the brain, and second that how we’re feeling comes before what we think.

I suppose my dream of some day existing as a brain floating in a tank of reddish liquid is going to turn out far duller than I had imagined.

References

1. Beg AA, Ernstrom GG, Nix P, Davis MW, Jorgensen EM. (2008) Protons Act as a Transmitter for Muscle Contraction in C. elegans. Cell, 11;132(1):149-60.