CrimePsych

This article was written with the help of Macquarie University and relevant staff who assisted in providing the apparatus, methods etc. for the data collection.

Abstract
In an attempt to examine the effects of cocaine on dopamine and horizontal locomotor activity, an experiment was conducted on Acheta domestica (crickets). In a double-blind design 121 crickets were randomly assigned to groups before baseline data collection commence. After which, they were then injected with either cocaine or saline only (vehicle). In both the pre and post-treatment timeframes horizontal locomotor activity was measured for twenty minutes. Statistically significant results were found to exist between the saline and cocaine groups, with the cocaine group displaying a much higher amount of horizontal locomotor activity than their counterparts. These results suggest, as is supported by other evidence, that cocaine blocks the Dopamine Transporter.

Dopamine, in the human brain, has multiple roles that it is involved in, ranging from locomotor activity to mood, attention and learning, as well as our internal motivation and reward system to name but a few. Although not as prevalent in other species as it is in humans, dopamine still plays a role in other vertebrates and invertebrates. When considering locomotor activity, dopamine helps the signals reach their desired destination by acting as a neurotransmitter, a lack of dopamine biosynthesis is associated with diseases such as Parkinson’s Disease in humans where a person loses the ability to make smooth and controlled movements.

Similarly, in crickets (acheta domestica), dopamine appears to not only play a role in locomotor activity but also in aggression (Stevenson, Hofmann, Schoch & Schildberger) Cocaine is a world renowned drug of abuse, and as a result, thousands of research papers have been written on its effects, some using rats or mice (Ritz, Lamb, Goldberg & Kuhar; Tilley et. al; Carroll, Anderson & Morgan; Catlow & Kirstein) others focusing their attention on various types of insect (Donly & Caveney; Dimitrijevic, Dzitoyeva & Manev; Bainton et. al) with some even interested in caterpillars that feed on cocaine rich coca-plants (Chen, Wu, Wei, Han & Gu).

Cocaine effects the brain by interfering with our pleasure and reward system; when we experience a pleasurable event, a large amount of dopamine is released from neurons into the synapse of a cell. Normally, dopamine is then transported back to the pre-synaptic side of the synapse and either broken down or reconstituted for later use by the Dopamine Transporter (DAT). However cocaine blocks the Dopamine Transporter from operating, subsequently increasing the amount of dopamine in the synapse, creating a wide range of behaviours.

In addition to these general studies of cocaine, a number of researchers have been interested in the effect of cocaine on locomotor activity. For example, Chang et. al. found that in fruit-fly (Drosophila melanogaster) they could induce stereotypic grooming and locomotor activity by overexpressing certain elements within the dopamine system, results that have been backed up by previous research on Drosophila (McClung & Hirsh) as well as on mice (Itzhak & Martin).

In this particular study we will be looking at the effect of cocaine on crickets (acheta domestica) horizontal locomotor activity. It is theorised that those crickets injected with the cocaine solution will produce significantly more horizontal locomotor activity than those in the control group, who will be administered the vehicle only (saline).

Method
Subjects
For this experiment, adult male crickets (acheta domestica) were assigned to one of two groups; cocaine (n=61) or saline (vehicle, n=60). The procedure was conducted as a double-blind experiment where only the researcher who was injecting the crickets knew what each cricket was receiving.

Apparatus
Crickets were housed 25 per plastic tub, which contained a damp sponge and water soaked carrots for sustenance. All crickets had never been used in any sort of drug treatment in the past and were obtained directly from Biosupplies, NSW.

Procedure
In this experiment we were examining any increase to a cricket’s horizontal locomotor activity, which for the purpose of this experiment was any forward or backwards movement on the flat surface of a 2 litre round glass container. Any grooming, or vertical movement (attempting to climb the sides) was not counted towards the cricket’s total scores.

Firstly, crickets were placed in their own glass containers and then given 1 minute to settle before the baseline data collection began. Each cricket was observed for 20 minutes, timed with digital stopwatches, with one researcher timing the horizontal locomotor activity and one researcher indicating when each minute had concluded, and recording the results.

After 20 minutes each cricket was injected through their abdomen using a 29 Ga hypodermic needle and 1ml syringe with either 20 µl of saline or cocaine (100 µg/ml, dissolved in saline), which was obtained from Clifford Hallum Pharmaceuticals, NSW. After injection the crickets were placed back in their original glass container and again given 1 minute to settle before experimental data collection began.

In both the baseline and post-treatment data collection the same researchers worked together, as did they have the same task for both procedures; one timing horizontal locomotor activity, the other indicating when each minute had concluded and recording the results.

Results
The hypothesis that those crickets injected with the cocaine would exhibit a larger amount of horizontal locomotor activity than those injected with saline was supported by our results. An unpaired t-test was conducted on the total data following treatment which resulted in a statistically significant result; t = 3.757, p < 0.005. Further support can be seen by looking at the means of the post-treatment data with those crickets treated with cocaine (mean = 110.72, SD = 77.69) quite obviously exhibiting more horizontal locomotor behaviour than those injected with saline alone (mean = 58.20, SD = 76.05).

As can be witnessed in the two graphs above, Figure 1 contains all our pre-treatment data, in which there was no significant difference between the two groups while in Figure 2, a significant difference can be seen between the cocaine and saline (control) groups. As time passes, the differences begin to get smaller, but by the end of the 20 minutes the group treated with cocaine are still higher than those treated with saline only.

Again a significant difference can be observed between the cocaine and saline only groups. Figure 3 shows our baseline (pre-treatment) results with no significant difference witnessed between the two groups, however upon introduction of the treatments a marked difference is easily seen between the two groups; Figure 4 shows that the group administered the cocaine had significantly higher horizontal locomotor activity than the saline only group.

Discussion
As indicated by the results obtained, the hypothesised relationship between cocaine and horizontal locomotor activity is supported, that is, when cocaine is introduced into a crickets nervous system it will affect the dopamine system resulting in increased locomotor activity.

The results from our research fall in line with those already stated (Chang et. al; McClung & Hirsh; Itzhak & Martin) but in addition to these other support can also be found. For example, in their review paper Rothenfluh and Heberlein found various studies that have been conducted that support the idea that cocaine increases locomotor activity in various animals, as well as increases or decreases in other types of behaviour. Other research has confirmed what we observed in our study, with additional gender differences also being noted in rats that are intravenously administered cocaine a large number of times (Harrod, Booze, Welch, Browning & Mactutus).

In the past, most research into cocaine and dopamine has been conducted on mice or rats, with a more recent trend towards Drosophila, with crickets not having been used in this type of experiment before. In fact, it could be foreseen that crickets, with their simple anatomical structure yet still possessing many of the neurotransmitters we as humans have, begin to be used more frequently for cocaine related research. It is likely, after all, that Acheta domestica are more easily managed than Drosophila!

That being said, one can envision a limitation of using crickets for this type of research, especially when an attempt to generalise to humans is the ultimate outcome; dopamine is only found in very small amounts in crickets as they primarily use octopamine. This could likely impact the generalisable nature of such studies, or even bring about criticism of results that are generalised to humans.

In closing, our results emphasise the effect of cocaine on dopamine, such that even a creature with significantly smaller amount of it in their nervous system can still experience a large response when it is administered. In time, it is inevitable that further information regarding dopamine and drug abuse will be uncovered and that more research results will be generalised to humans. Further research may endeavour to examine whether octopamine plays any role at all in the horizontal locomotor activity in an attempt to establish if cocaine only affects the Dopamine Transporter in crickets.