Striatal CREB, plasticity & alcohol-seeking behaviour by Dr Andrew Lawrence

Cyclic-AMP Response Element Binding protein (CREB) is thought to be necessary for the long-term consolidation of synaptic changes following experimental or drug-induced plasticity. We have developed a strategy to selectively delete the CREB gene in the striatal cells that receive dopaminergic input from the ventral tegmental area. Furthermore, this conditional deletion is achieved after weaning, thereby preventing any developmental compensation to confound data interpretation. This is predicted to alter induction of synaptic plasticity on striatal medium spiny neurons. If the current hypothesis of addiction is correct then this deletion of CREB from striatal medium spiny neurons will alter the reinforcing effects of drugs of addiction. We will test this hypothesis in our mice using alcohol as our reinforcer and comparing with saccharin as a “natural” reinforcer (the choice of saccharin over sucrose is because saccharin is devoid of calorific value, but equally palatable).

Confirmation of Genotype
We have bed mice and confirmed that in double transgenic mice, a critical exon of the CREB gene has been deleted. Furthermore, we have subsequently determined that this genetic manipulation results in a lack of CREB protein in target brain regions (basal ganglia) while expression of the CREB protein is normal in the rest of the brain.

Neurochemical Impact of CREB deletion
Subsequently, we examined the expression of a number of genes located on medium spiny neurons that are pertinent to natural reward, drug-seeking and/or synaptic plasticity. For example, we quantified the expression of dopamine D1 and D2 receptors, the opioid peptides enkephalin and dynorphin and signalling molecules such as DARPP32. In every case, there was no effect on expression of any of these genes as a consequence of the conditional CREB deletion. This gave us the confidence that our transgenic model was not complicated by altered expression of these critical genes. However, we also tested mutant mice for the expression of other CREB family transcription factors, and found that even following a post-natal deletion of CREB, we observed substantial up-regulation of the related molecule CREM. This compensatory response could clearly dampen any potential phenotype.

Two-Bottle Free Choice Drinking
We have examined mutant and control mice for their voluntary consumption of saccharin, water and alcohol. There is no difference in preference or consumption of the highly palatable saccharin solution between genotypes. For alcohol consumption, there is no difference between genotypes at a range of ethanol concentrations. These data suggest simplistically that taste perception is intact and the consummatory drive to self-administer either a natural or alcohol reward is essentially normal.

Conclusion: These data suggest that while we successfully deleted CREB from medium spiny neurons in the mature mouse brain, there was still rapid compensation in the form of CREM up-regulation that may take over roles previously driven by CREB. At present we are engaged is studies designed to overcome this compensation so that the true role of this transcription factor can be ascertained.