A Purkinje cell Timing Mechanism. On the Physical Basis of a Temporal Duration Memory

Abstract: The standard view of neural signaling is that a neuron can influence its target cell by exciting or inhibiting it. Learning is thought to involve strengthening or weakening synaptic connections. For most behaviors, the brain must learn to produce precisely timed activity patterns. Learned response timing is indispensable for a wide range of tasks and requires learning of interstimulus intervals (ISIs). The learning mechanism thought to accomplish this combines time-varying patterns of activity in the pre-synaptic neural network with changes in synaptic strength between the pre-synaptic neurons active at the end of the ISI and the post-synaptic neuron. Timing-dependent learning can be studied in eyeblink conditioning. If a neutral conditional stimulus is paired with an unconditional blink-eliciting stimulus, at an ISI of fixed duration, it acquires the ability to elicit a blink that peaks near the end of the ISI. Cerebellar Purkinje cells that control the blink acquire adaptively timed pauses in spontaneous firing, conditioned Purkinje cell responses, that interrupt their tonic inhibition of cerebellar nuclear cells and cause excitatory output that generates the overt blink. Most models assume the generation of a time code instantiated in varying patterns of activity in the presynaptic granule cells that represent the passage of time. However, we show here (paper I) that a cerebellar Purkinje cell can learn to respond to a specific input with adaptively timed pauses without such a temporally patterned input. Training Purkinje cells with direct stimulation of their presynaptic fibers, and pharmacological blocking of interneurons shows that the timing mechanism is intrinsic to the cell itself and not an emergent property of the network. That an individual neuron can learn temporal relationships suggests the existence of intracellular temporal duration memory. We demonstrate that this Purkinje cell memory is triggered by the metabotropic glutamate receptor 7 (paper II) and that the timed voltage response in large part is produced by the G-protein activated K+ channel family Kir3/GIRK (paper III). The implication is that a learned and adjustable timing of a metabotropic signaling cascade constitutes a physical memory of temporal duration. A theoretical model (paper IV) describes how this could be accomplished by a learning mechanism that selects among a finite number of regulatory proteins, those which bestow the intracellular signaling cascade with latencies to activation and deactivation that matches the ISI. The results presented in this thesis show that the traditional view of learning as a change in synaptic strength is insufficient. Finally, because Purkinje cells directly control the conditioned eyeblink we believe that, to our knowledge, this is the first time that a causal link can be shown between a learned and timing-dependent behavior and not only a single neuron’s memory, but also the specific activating receptor of said memory and the specific ion channel that puts it into effect.