Another breakthrough towards the understanding of
Another breakthrough towards the understanding of the mechanisms responsible for stress-induced relapse to drug seeking behaviour is the evidence published by the group of Wise  in which they show that the activation of VTA CRF2 and not CRF1 receptors is responsible for stress-mediated relapse to cocaine seeking. This data has been surprising , ,  as previous studies have discarded the participation of CRF2 receptors in stress-induced relapse to drug seeking . The evidence to discard the role of CRF2 receptors was obtained by administering 1 and 10μg AS-30 intra cerebroventricularly, a CRF2 receptor selective peptide antagonist . Caution should be taken when interpreting these results as no positive control was used to prove that enough AS-30 was effectively reaching the VTA under the conditions used or that it selectively blocked CRF2 receptors in vivo. Recently, it was shown that it was necessary to inject at least 20μg of AS-30 into the lateral ventricle to observe the effect of AS-30 upon submissive defensive behaviour , supporting the possibility that the amount of AS-30 used by Lu et al.  was not sufficient. A similar approach to discard the participation of CRF2 receptors was undertaken in the work of Bruijnzeel et al. . In this case, astressin-B, another peptide antagonist of the CRF2 receptor was used by intra cerebroventricular administration without controlling for the effectiveness of this type of injection. It is well known that compounds injected into the lateral ventricle diffuse heterogeneously to different b catenin inhibitor regions and can diffuse unevenly to each hemisphere. In this regard, the reverse perfusion of a CRF2 receptor antagonist through a microdialysis probe directly installed in the VTA, as performed by Wang et al.  is a more reliable strategy. Hopefully, in the near future it will be possible to utilize non-peptide CRF2 receptor antagonists that cross the blood brain barrier and/or diffuse more readily through the ventricles to provide more definitive proof that CRF acts through CRF2 receptors in the VTA to transduce cocaine-dependence related phenomenon.
The second observation that has been used to argue against a role of VTA CRF2 receptors in stress-induced relapse to cocaine seeking it is the known distribution of CRF1 and CRF2 receptors. Specifically, no CRF2 receptor mRNA is detected in the VTA by in situ hybridization , . The only report that has shown expression of CRF2 receptors in the VTA used single cell RT-PCT to quantify CRF2 receptor mRNA in dopaminergic VTA neurons . However, the results of Wang et al.  can be nicely explained by the presence of CRF2 receptors in VTA glutamatergic nerve terminals originating in neurons outside the VTA. VTA glutamatergic afferents originate from several cortical and subcortical brain regions , some of which express CRF2 receptor mRNA . We have observed the presence of CRF2 receptors in VTA glutamatergic nerve terminals of subcortical origin  using immunofluorescence with antibody against CRF2 receptors in VTA synaptosomes devoid of postsynaptic elements .
CRF-dependent sensitization of VTA glutamate release after repeated administration of cocaine has also been documented in an electrophysiological study recording from glutamatergic synapses in VTA containing slices from mice repeatedly treated with cocaine . In this case, both pre and postsynaptic modifications were reported. However, the CRF receptor type involved in presynaptic modifications was not analyzed further. Interestingly, a non-contingent administration of cocaine was used suggesting that the CRF-mediated sensitization of VTA glutamate release reported in cocaine-experienced rats ,  is not related to the contingency of the auto-administration process, but to the repeated presence of cocaine itself.
Possible role of CRF-BP in CRF/CRF2 receptor actions in the VTA