[Throwback Thursdays] The rat that became addicted to shocking its brain

All addictive substances exert their effects by harnessing the powerful reward system that is normally used to guide an animal’s behavior. One simplistic way to build a reward system is to have neurons that carry positive and negative values. These reward centers would then, and have been shown to, be the primary target of the addictive substances. To find these reward centers, researchers must devise a way to stimulate a brain region and ask the animal how it likes it. How can one do this?

 

3 rat self stimulating

Figure 1. A rat presses the lever to stimulate its own brain. Adapted from Olds 19581.

Olds and Milner did it, in 19542. They made a pivotal assumption that if stimulating a brain region is rewarding, the animal would like to do more of it. In this way, they can just hand over the switch to stimulate an animal’s brain region to the animal itself and see how much it presses the switch. At first, an animal may accidentally deliver the stimulation here and there, but if the stimulation is rewarding, the animal will soon to learn to press the switch more and more, forgetting to eat or sleep. Using this ingenious method, Olds and Milner found that stimulating a region in the middle of the brain called septum (which is now known to be important for stress and mood regulation) is highly addictive.  Over the course of 12 hours of experiments, the rats stimulated their septa incessantly at a rate of 6-30 times a minute. These self-stimulations stopped when the researchers set the voltage of the stimulation to 0, suggesting that this unnatural obsession was nevertheless goal-directed.

3 1000 presses

Figure 2. A rat pressed the bar more than 1000 times in an hour. Adapted from Olds and Milner2. The stimulation voltage is 0 in shaded regions. This rat peaked at 1920 presses/hour, about once every 2 seconds.

The legacy of self-stimulation did not stop here. By surveying the brain regions more systematically and changing the stimulation method from electrical to chemical (e.g. cocaine), generations of researchers have generated a comprehensive map of addiction in the brain3.

 

References:

  1. Olds, J. Self-Stimulation of the Brain. Science. 127, 315–324 (1958).
  2. Olds, J. & Milner, P. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J. Comp. Physiol. Psychol. 47, 419–27 (1954).
  3. Lüscher, C. & Malenka, R. C. Drug-evoked synaptic plasticity in addiction: from molecular changes to circuit remodeling. Neuron 69, 650–63 (2011).
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1 Comment

  1. What does cocaine do in the brain? | Harvard Neuroscience Blog

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