Proteomics and behavior
Sabine Spijker, Vrije Universiteit
crose. To study the long-term effects of ly regulated for such a long time afWhen studying the behavioral and sucrose self-administration, the invester the training period ended. She was neuronal effects on rats of drugs of tigators sacrificed the rats 21 days after also surprised that many neurotransabuse, researchers often use sucrose the training ended and extracted the mission proteins were identified, beadministration as a control. But suproteins from the rats’ mPFCs. cause the researchers did not specificrose itself is a reward that is very reWhen mPFC proteins were separated cally enrich their samples for synaptic inforcing for rats, says Sabine Spijker on 2DE gels, Spijker and co-workers obcompartments. Some of the regulatat Vrije Universiteit (The Netherlands). served that 66 protein spots differed in ed proteins, such as tubulin and actin Sucrose administration may not be abundance between the samples from isoforms and some of the growth-rethe best control because it elicits some the sucrose rats and the control rats. To lated molecules, could be involved in of the same learning behaviors as the identify these proteins, the researchchanging the morphology of neurons, drugs of abuse. “We thought it would be ers excised the spots, digested the proshe says. good to look first at the sucrose effect teins with trypsin, and analyzed the The researchers also examined itself . . . because if you use it as a conpeptides by MALDI MS/MS. Of the promRNA levels to see whether gene extrol, what does it do?” says Spijker. teins that were differentially regulated, pression was affected. ComplementaPrevious studies by ry DNA (cDNA) was other groups on susynthesized from total crose self-adminismRNA from mPFCs. tration described a Quantitative real-time few molecular effects, PCR was performed on such as changes in dothe cDNA with primpamine receptor levers specific for genes els, opioid peptide that encode the regulated proteins. On expression, and dopaCue light mine receptor bindly five of the genes ing, but Spijker says were differentially exthat no one had inpressed; this may be vestigated the molecdue to differences in ular response to suthe lifetimes of some crose in detail until of the mRNAs and the now. As described in proteins, says Spijker. the previous issue of Another possibility is Active nose poke hole JPR (2006, 5, 147–154), that, in some instancSpijker and co-workes, regulation occurs ers at Vrije Universi at the level of transteit and Vrije Univerlation of mRNA into protein or at the levsiteit Medical Center used proteomics and el of posttranslational modifications. genomics methods to Sweet reward. Rat in an operant conditioning apparatus. Rats are trained to poke investigate the longThe long-term at the active nose poke hole to receive sucrose pellets when a light cue is given. A term effects of sucrose changes in gene exrat will still poke the hole in response to the light cue weeks after the training has self-administration pression and protein ended. on the rat medial preabundance in rats that frontal cortex (mPFC), which is known to play a role in reward processing. The researchers found that several proteins were differentially regulated in the brains of rats that had been trained to self-administer sucrose, even 21 days after the final training session. One group of rats was trained over a 16-day period to poke at a hole in response to a light to receive sucrose pellets. A control group received the same cue and was placed in the same type of apparatus but received no sucrose. At the end of the training period, all of the rats were returned to normal cages and no longer received cues or su-
28 were confidently identified. The levels of 11 proteins were lower in sucrose self-administering rats than in the control rats, and the levels of 17 proteins were higher. Proteins involved in cytoskeletal organization, energy metabolism, oxidative stress, neurotransmission, and neuronal outgrowth and differentiation were among those identified. “We didn’t expect so many [changes],” says Spijker. “The extent of the changes was really quite tremendous.” For example, she explains that metabolism proteins were not expected to be differential-
were trained to selfadminister sucrose made Spijker and colleagues wonder about the implications for the use of sucrose as a control in addiction studies. “One conclusion you can draw is that you should be careful . . . that you take the right control,” she says. She suggests that researchers include additional controls to account for the changes occurring in rats rewarded with sucrose. Most importantly, she adds, sucrose has substantial effects on protein levels in the brain in the long term, but further studies are necessary to find out what these changes really mean. —Katie Cottingham
Journal of Proteome Research • Vol. 5, No. 2, 2006 229
