Persistent Activation of cAMP-Dependent Protein Kinase and the Induction of Long-Term Memory

The last decade has witnessed a remarkable growth in our understanding of the molecular basis of memory formation. Among the many important advances that have contributed to this understanding was a pair of landmark papers in 1995 demonstrating that functional enhancement of cAMP-dependent transcription via cAMP response element binding protein (CREB) enhanced both the induction of long-term synaptic facilitation (LTF) in Aplysia (Bartsch et al., 1995) and the formation of long-term memory (LTM) for olfactory conditioning in Drosophila (Yin et al. 1995xYin, J.C.P., Del Vecchio, M., Zhou, H., and Tully, T. Cell. 1995; 81: 107–115Abstract | Full Text PDF | PubMed | Scopus (471)See all ReferencesYin et al. 1995; reviewed by Carew 1996xCarew, T.J. Neuron. 1996; 16: 5–8Abstract | Full Text | Full Text PDF | PubMed | Scopus (141)See all ReferencesCarew 1996). Now a new paper has come on the scene that directly implicates persistent activation of cAMP-dependent protein kinase (PKA) in the induction of long-term memory for olfactory learning in the honeybee (Muller, 2000 [this issue of Neuron]). This paper provides the first direct link between a specific pattern of training required for the induction of LTM, the temporal dynamics of PKA activation induced by that training pattern, and the induction of LTM in intact animals.To appreciate the significance of the paper by Muller, a bit of background would be helpful. It is now well established that increases in cAMP and the activation of PKA play a critical role in the induction of long-term synaptic, cellular, and behavioral changes implicated in LTM in Aplysia (Bacskai et al. 1993xBacskai, B.J., Hochner, B., Mahaut-Smith, M., Adams, S.R., Kaang, B.-K., Kandel, E.R., and Tsien, R.Y. Science. 1993; 260: 222–226CrossRef | PubMedSee all ReferencesBacskai et al. 1993), Drosophila (Davis et al. 1995xDavis, R.L., Cherry, J., Dauwalder, B., Han, P.L., and Skoulakis, E. Mol. Cell. Biochem. 1995; 149: 271–278CrossRef | PubMed | Scopus (104)See all ReferencesDavis et al. 1995), honeybee (Fiala et al. 1999xFiala, A., Muller, U., and Menzel, R. J. Neurosci. 1999; 19: 10125–10134PubMedSee all ReferencesFiala et al. 1999), and mice (Abel et al. 1997xAbel, T., Nguyen, P.V., Barad, M., Deuel, T.A.S., and Kandel, E.R. Cell. 1997; 88: 615–626Abstract | Full Text | Full Text PDF | PubMed | Scopus (808)See all ReferencesAbel et al. 1997). One of the key consequences of PKA activation is the activation of CREB, which has been implicated in LTM formation in Aplysia (Bartsch et al., 1995, Bartsch et al. 1998xBartsch, D., Casadio, A., Karl, K.A., Serodio, P., and Kandel, E.R. Cell. 1998; 95: 211–223Abstract | Full Text | Full Text PDF | PubMed | Scopus (239)See all ReferencesBartsch et al. 1998), in Drosophila (12xYin, J.C.P., Wallach, J.S., Del Vecchio, M., Wilder, E.L., Zhou, H., Quinn, W.G., and Tully, T. Cell. 1994; 79: 49–58Abstract | Full Text PDF | PubMed | Scopus (665)See all References, 11xYin, J.C.P., Del Vecchio, M., Zhou, H., and Tully, T. Cell. 1995; 81: 107–115Abstract | Full Text PDF | PubMed | Scopus (471)See all References), and in rodents (Silva et al. 1998xSilva, A.J., Kogan, J.H., Frankland, P.W., and Kida, S. Annu. Rev. Neurosci. 1998; 21: 127–148CrossRef | PubMed | Scopus (857)See all ReferencesSilva et al. 1998). A final bit of background relevant to the Muller paper concerns the temporal dynamics of PKA activation. In Aplysia sensory neurons, repeated pulses of serotonin give rise to mechanistically distinct phases of persistent activation of PKA (Muller and Carew 1998xMuller, U. and Carew, T.J. Neuron. 1998; 21: 1423–1434Abstract | Full Text | Full Text PDF | PubMed | Scopus (93)See all ReferencesMuller and Carew 1998) and prolonged phosphorlyation of CREB (Bartsch et al. 1998xBartsch, D., Casadio, A., Karl, K.A., Serodio, P., and Kandel, E.R. Cell. 1998; 95: 211–223Abstract | Full Text | Full Text PDF | PubMed | Scopus (239)See all ReferencesBartsch et al. 1998). Thus, there is considerable evidence linking cAMP and CREB to LTF and LTM formation in several systems. Moreover, different patterns of training that produce distinct phases of memory also produce specific temporal patterns of PKA activation. Now, we turn to the honeybee.The temporal dynamics and associative nature of olfactory learning in the proboscis extension reflex (PER) of the honeybee has been extremely well characterized on behavioral, anatomical, and cellular levels (7xHammer, M. and Menzel, R. J. Neurosci. 1995; 15: 1617–1630PubMedSee all References, 8xMenzel, R. J. Comp. Physiol. A. 1999; 185: 323–340CrossRef | Scopus (315)See all References). Of critical importance for the paper under consideration is that a single conditioning trial (pairing an odor with a sucrose reward) gives rise to a memory that decays over days, while three pairing trials produce memory (LTM) that is much longer lasting—virtually permanent (for an excellent review, see Menzel 1999xMenzel, R. J. Comp. Physiol. A. 1999; 185: 323–340CrossRef | Scopus (315)See all ReferencesMenzel 1999). Finally, several anatomical regions of the bee brain have been implicated in the induction of LTM for the PER, including the antennal lobes (AL).Using the honeybee, Muller combined clever experimental techniques with the power of the PER preparation to provide a major advance in forging links between cellular mechanisms and behavioral LTM. First, by blocking cAMP in the brain at different times before or after training, he established that formation of LTM requires PKA activity during a narrow time window, during conditioning and not longer than 25 min afterward. Moreover, Muller showed that a single pairing trial (which does not produce LTM) leads to a transient activation of PKA in the ALs, whereas three pairing trials (which do produce LTM) lead to a persistent activation of PKA in the ALs. Most convincingly, by means of local photolysis release of caged cAMP in the ALs , Muller was able to artificially prolong the PKA activation induced by a single pairing trial and, in so doing, test the hypothesis that the persistence of PKA activation, specifically, was linked to the formation of LTM. He found that a single pairing trial coupled with the prolongation of local PKA activation gave rise to LTM. This powerful result reflects a “gain of function” (see Carew 1996xCarew, T.J. Neuron. 1996; 16: 5–8Abstract | Full Text | Full Text PDF | PubMed | Scopus (141)See all ReferencesCarew 1996) and provides compelling evidence that PKA not only plays a critical role in LTM formation but that a specific temporal pattern of PKA activation is essential. Finally, a caveat: the magnitude of LTM induced by local activation of PKA in the ALs does not reach the level of memory induced by multiple training trials, suggesting that other brain areas (e.g., the mushroom bodies) or other molecular cascades contribute to the overall expression of LTM in the intact animal.A final series of experiments shed light on the upstream events that lead to persistent PKA activation during conditioning trials. Previous work from Muller's laboratory had implicated the activity of nitric oxide synthase (NOS) in LTM formation. In the present study, Muller showed that the prolonged activation of PKA in the ALs induced by multiple training trials was blocked by inhibition of NOS. Additional biochemical experiments showed that NO activates PKA in the ALs via cGMP. Finally, and again most convincingly, Muller showed that local uncaging of cGMP in the ALs (by photolysis) could also combine with a single pairing trial in the induction of LTM. Taken together, these data support the hypothesis that the NO/cGMP system within the ALs contributes to the prolongation of PKA activation induced by multiple conditioning trials.Elucidating the cellular and molecular mechanisms underlying long-term memory is a daunting task. Part of the problem is that this endeavor ideally requires experiments that traverse several levels of analysis, from behavior, to systems of neurons, to synapses, to molecules. No preparation exists that has equal power at each of these levels. Thus, progress in this field requires combining the insights gained from several preparations and several levels of analysis. The paper by Muller provides important links between bona fide behavioral memory and the dynamics of a specific second messenger system in a restricted brain region. These insights certainly add to the growing body of evidence implicating identified molecular cascades, such as PKA-mediated CREB activation, in the induction of long-lasting memories.