Have you ever wondered how animals look or hunt for food? The simple response is they just follow their instincts, but what are the mechanisms behind their body’s response to look for food, and keep looking until they are satisfied? What is behind the neurology of motivation?
Previous research has found that environmental gustatory cues would provide information about the type and quality of available food, but only internal nutrient levels would ultimately suppress the circuits to the neurology of motivation. An international team of researchers led by the scientists from the Technical University of Munich set up various experiments involving the common fruit fly to determine which sensory cues are prioritized to influence a mammalian animal’s behaviour and internal motivation.
Both hungry and fed flies were tethered to a spherical treadmill and exposed to an odour source as a repeated stimulus. The activity of the flies was recorded at various intervals such as pre- and post-stimulus and then categorized into levels of motivation.
The researchers found two sets of behaviours during odour stimulation. Upon exposure, the flies sped up to almost double their average speed and suppressed turning behaviour to head straighter. This suggested that they were tracking the odour stimulus and had increased motivation. When the stimulation was terminated, the flies significantly reduced their speed and almost stopped before regaining the average speed. They also increased their turning behaviour, which was attributed to them searching for the initial odour stimulus.
Over the span of 10 repeated trials, the researchers further identified an evolution in the flies’ behaviour. The acceleration in speed upon stimulus onset had gradually increased and turning behaviour was more efficiently suppressed. In the end, the data provided insight into the flies’ neurology of motivation as the behaviour to track food was significantly intensified over the course of the experiment.
In addition, the researchers conducted experiments to investigate any changes in the flies’ behaviour induced by their hunger state. Flies were not intrigued or motivated to track odours when they were fed. However, those who were starved for 24 and 48 hours almost doubled their speed as well as tracking time when exposed to the odour stimulus. This demonstrated that starvation time was correlated with both tracking speed and time. The longer the flies were starved, the longer and faster they pursued the food odour.
The results published in CellPress identified two neurotransmitters that were key to the neurology of motivation in mammals: norepinephrine and octopamine. By taking advantage of the smaller and simpler structure in a fly brain’s neural network, the researchers found it easier to sift and identify specific neurons and their function. The presence of octopamine led to flies immediately slowing down or even stopping their odour tracking, whereas the presence of dopamine significantly increased their tracking speed and consequently, motivation.
The conclusions drawn from the study identified the circuit significant to the neurology of motivation that influenced the flies’ behaviour based on a need- and state-dependent manner. As similar neurotransmitters and circuits exist in the human mammalian brain, the hope of the study was to further understand mechanisms and one day, understand and treat human conditions such as addiction.
Written by Stephanie Tsang
Sayin, Sercan, et al. “A Neural Circuit Arbitrates between Persistence and Withdrawal in Hungry Drosophila.” Neuron, vol. 104, no. 3, 27 Aug. 2019, doi:10.1016/j.neuron.2019.07.028.
Battenberg, Andreas. “The Secret of Motivation.” EurekAlert!, 26 Sept. 2019, www.eurekalert.org/pub_releases/2019-09/tuom-tso092619.php.
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