A general feature of memory formation is its progression through different phases. Although evidence from invertebrates and mammals shows that distinct molecular mechanisms contribute to the multiphasic process of memory formation, little is known about the biochemical pathways underlying the induction and maintenance of these different memory phases and the memory retrieval. Our aim is to understand how consolidation, maintenance and memory retrieval change the temporal dynamics of signalling cascades and how these cascades contribute to the distinct aspects of learning.

(1) Relation between stimulation parameters, induction of memory phases and the underyling signalling pathways

The activation of cAMP-dependent protein kinase (PKA) plays a key role in the induction of long-term neuronal and behavioral changes in Drosophila, Aplysia, mice and the honeybee. In all of these model systems long-lasting neuronal changes and long-term memory (LTM) are most reliably induced by repeated spaced training sessions or repeated spaced stimulations. Repeated training however, seems to be not absolutely necessary since in the pond snail Lymnea, a single associative learning trial is sufficient to induce an LTM. Together with Dr. G. Kemenes (University of Sussex, Brighton UK) we presently investigate whether the key function of the cAMP/PKA-pathway in LTM formation also applies for single-trial induced LTM formation.

(2) Dynamics of memory formation and the network of the underyling signalling cascades

In addition to the cAMP/PKA pathway, blocking of signalling cascades, like MAPK and CaMKII during training also impair memory formation. This indicates that a network of serial and parallel molecular events underlies the multiphasic process of memory formation. Using pharmacological tools we identify the time windows during consolidation that require the activity of distinct signalling pathways. This provides the basis for the identification of the serial and parallel  interaction between the distinct signalling pathways durcing memory consolidation (Anke Friedrich, Irina Plekhanova, Ulf Thomas).

(3) Signalling cascades‘ contribution to memory retrieval

Although retrieval is a critical phase in accessing behavioral changes during memory formation, little is known about the molecular processes induced by retrieval. Do they differ from processes induced during acquisition? Do they change during the progression from labile short-lived forms to long-lived stable forms of memory? By monitoring changes in the activity of signalling cascades induced by memory retrieval we are starting to investigate the possible contribution of several signalling cascades to retrieval. Moreover, we are screening for inhibitors of neurotransmission and signalling cascades that selectively interfere with retrieval.

(4) Temporal dynamics of second messenger-mediated processes in different neuronal circuits involved in olfactory learning and their interaction

In the honeybee, as in mammals, the neuronal changes associated with learning and memory formation occur in several brain structures. Both the antennal lobes and the mushroom bodies contribute to different features of olfactory learning. To analyze the contribution of each of these neuronal networks with respect to learning and memory formation we determine the temporal dynamics of signalling cascades in the antennal lobes and the mushroom bodies in parallel. By local and independent manipulation of these second messenger cascades in the antennal lobes or the mushroom bodies we aim to understand the temporal interaction of these neuronal networks (Irina Plekhanova, Ulf Thomas).

(5) Search for the signalling pathways mediating effects of circadian rhythm and satiation on learning

It is well known that learning and memory formation is influenced by intrinsic and extrinsic parameters, such as circadian rhythm and satiation. We started a thorough analysis of the effects of circadian rhythm and satiation on learning and memory formation and the underlying signal cascades. Based on the knowledge of the function of second messenger cascades in the induction and maintenance of memory, we specifically investigate the influence of circadian rhythm and satiation on the temporal dynamics of learning-induced changes of the second messenger cascades. We aim to identify pathways by which intrinsic and extrinsic parameters interfere with learning processes (A. Friedrich).

Techniques:

We apply a broad spectrum of techniques, covering behavioral analysis as well as sophisticated specialized biochemical techniques. The combination of a shock-freezing technique with a fast and specific in vitro phosphorylation plays a key role in investigating in vivo-induced changes in enyzme activities. In addition to stimulation of intact animals, we use primary cell cultures to study questions concerning transmitter-induced changes of second messenger cascades in single neurons.The production of monoclonal antibodies against purified proteins and short peptides provides us with a powerful tool for our investigations. Using immunological and ELISA techniques we study the localization and determine the amount of proteins.

Supported by the Deutsche Forschungsgemeinschaft, SFB 515 and GRK 120

Selected Publications:

Müller, U. (1996) Inhibition of nitric oxide synthase impairs a distinct form of long-term memory in the honeybee, Apis mellifera. Neuron 16, 541-549.

Menzel, R. and Müller, U. (1996) Learning and memory in honeybees: From behavior to neural substrates. Annu. Rev. Neurosci. 19, 379-404.

Müller, U. (1997a) Neuronal cAMP-dependent Protein Kinase Type II is Concentrated in Mushroom Bodies of Drosophila melanogaster and the Honeybee, Apis mellifera. J. Neurobiol. 33, 33-44.

Müller, U. (1997b) The nitric oxide system in insects. Prog. Neurobiol. 51, 363-381.

Müller, U. (1997c) Insect 86 kDa protein kinase C substrate is a filament interacting protein regulated by Ca2+/calmodulin and phosphorylation. Brain Res. 757, 24-30.

Menzel, R., Geiger, K., Joerges, J., Müller, U., and Chittka, L. (1998) Bees travel novel homeward routes by integrating separately acquired vector memories. Animal Behav. 55, 139-152.

Grünbaum, L. and Müller, U. (1998) Induction of a specific olfactory memory leads to a long-lasting activation of protein kinase C in the antennal lobe of the honeybee. J. Neurosci. 18, 4384-4392.

Müller, U, and Carew, T. J. (1998) Serotonin induces temporally and mechanistically distinct phases of persistant PKA activity in Aplysia sensory neurons. Neuron  21,1423-1434.

Müller, U. (1999) Second messenger pathways in the honeybee brain: Immunohistochemistry of protein kinase A and protein kinase C. Microsc.Res.Tech. 45, 165-173.

Fiala, A., Müller, U. and Menzel, R. (1999) Reversible downregulation of protein kinase A during olfactory learning using antisense technique impairs long-term memory formation in the honeybee, Apis mellifera. J. Neurosci. 19, 10125-10134.

Homberg, U. and Müller, U. (1999) Neuroactive substances in the antennal lobe. In: Insect Olfaction, ed. B.S. Hansson, Springer Verlag, Heidelberg, Germany.

Müller, U. (2000) Signal transduction pathways in well-defined models of learning and memory: Drosophila and honeybee. In Cerebral Signal Transduction: From First to Fourth Messengers (M.E.A. Reith, ed.) Humana Press, p. 73-105.

Müller, U. (2000) Prolonged activation of cAMP-dependent protein kinase during conditioning induces long-term memory in honeybees. Neuron 27, 159-168.

Malun, D., Plath, N., Giurfa, M., Moseleit, A.D., and Müller, U. (2002) Hydroxyurea-Induced Partial Mushroom Body Ablation in the Honeybee Apis mellifera: Volumetric Analysis and Quantitative Protein Determination. J.Neurobiol. 50, 31-44.

last updated March 18, 2002