To investigate the role of phosphatases in synaptic plasticity using genetic approaches, we generated transgenic mice that overexpress a truncated form of calcineurin under the control of the CaMKIIalpha promoter. Mice expressing this transgene show increased calcium-dependent phosphatase activity in the hippocampus. Physiological studies of these mice and parallel pharmacological experiments in wild-type mice reveal a novel, intermediate phase of LTP (I-LTP) in the CA1 region of the hippocampus. This intermediate phase differs from E-LTP by requiring multiple trains for induction and in being dependent on PKA. It differs from L-LTP in not requiring new protein synthesis. These data suggest that calcineurin acts as an inhibitory constraint on I-LTP that is relieved by PKA. This inhibitory constraint acts as a gate to regulate the synaptic induction of L-LTP.

To obtain rapidly inducible and reversible expression of transgenes in the forebrain of the mouse, we have combined the reverse tetracycline-controlled transactivator (rtTA) system with the CaMKIIalpha promoter. We show that doxycycline induces maximal gene expression in neurons of the forebrain within 6 days and that this expression can be reversed by removal of doxycycline. Using calcineurin as a test transgene, we show that doxycycline-induced expression impairs both an intermediate form of LTP (I-LTP) in the hippocampus and the storage of spatial memory. The reversibility of the rtTA system in turn allowed us to examine the effects of the transgene on memory retrieval after normal storage had occurred. This examination suggests that retrieval requires some of the same molecular components required for storage.

Increasing evidence shows that the rostral agranular insular cortex (RAIC) is important in the modulation of nociception in humans and rats and that dopamine and GABA appear to be key neurotransmitters in the function of this cortical region. Here we use immunocytochemistry and path tracing to examine the relationship between dopamine and GABA related elements in the RAIC of the rat. We found that the RAIC has a high density of dopamine fibers that arise principally from the ipsilateral ventral tegmental area/substantia nigra (VTA/SN) and from a different set of neurons than those that project to the medial prefrontal cortex. Within the RAIC, there are close appositions between dopamine fibers and GABAergic interneurons. One target of cortical GABA appears to be a dense band of GABAB receptor-bearing neurons located in lamina 5 of the RAIC. The GABAB receptor-bearing neurons project principally to the amygdala and nucleus accumbens with few or no projections to the medial prefrontal cortex, cingulate gyrus, the mediodorsal thalamic nucleus or contralateral RAIC. The current anatomical data, together with previous behavioral results, suggest that part of the dopaminergic modulation of the RAIC occurs through GABAergic interneurons. GABA is able to exert specific effects through its action on GABAB receptor-bearing projection neurons that target a few subcortical limbic structures. Through these connections, dopamine innervation of the RAIC is likely to affect the motivational and affective dimensions of pain.