Title

Backpropagating Action Potentials in the Dendritic Tree of the Rat PFC Layer 5 Pyramidal Neurons: Dynamics, Dichotomy and Dopaminergic Modulation

Date of Completion

January 2012

Keywords

Biology, Neurobiology

Degree

Ph.D.

Abstract

Action potential (AP) backpropagation into the dendritic tree represents an important retrograde signal broadcasting the outcome of synaptic integration in the axon to synapses in the dendrites. Its relevance has been established to several physiological processes. Although well studied in apical dendrites, little is known how APs invade basal dendrites of the prefrontal cortical (PFC) pyramidal neurons. Using improved voltage-sensitive dye imaging technique we studied AP backpropagation in the basal dendrites of layer 5 pyramidal neurons of rat PFC. We found that in short- and medium-range of basal dendrites APs propagated actively with only modest changes in AP half-width or AP rise-time. Together with the lack of substantial amplitude boosting of the third AP in a high-frequency burst, our findings are inconsistent with the previous AP-failure model, but suggest that in short and medium-sized basal dendrites bAPs were not severely attenuated. Our results show that the AP failure concept does not apply to all basal dendrites of the rat PFC. The majority of synaptic contacts in the basilar dendritic tree receive significant AP-associated electrical and calcium transients. ^ Next, we asked whether and how the propagation of AP is favored in different dendrites of the same class, in different neurons of the same subtype and in sister branches of the same pyramidal neurons of the PFC. Using whole-field illumination combined with intracellular application of voltage- or calcium-sensitive dyes, we found that AP propagation in two sister branches (branching off the same parent dendrite) may differ drastically in efficacy and high-frequency filtering properties. The heterogeneity of AP backpropagation was not only detected among sister branches belonging to the same neuron, but among the neighboring layer 5 pyramidal neurons. In two nearby pyramidal cells the calcium electrogenesis in the dendritic tuft exhibited drastically different sensitivity to the AP firing. The efficacy of AP propagation is influenced by the branching pattern (topology), thickness (diameter) of individual dendritic branches and the distribution of dendritic A-type conductance. By applying adequate synaptic inputs into the dendritic tuft in the form of cholinergic stimulation and/ or LTP induction, we were able to effectively change the efficacy of AP propagation in the target dendritic branches, suggesting activity-dependent compartmentalized alteration might be an important drive for the heterogeneity of branch-specific excitability in the dendritic tree. Such findings might provide a new perspective for understanding the subcellular basis of intrinsic excitability and the role of local regulation of dendritic excitability in synaptic integration and memory. ^ Lastly, we investigated the modulation of AP backpropagation by synaptic-like dopaminergic stimulation. Using focal application of exogenous dopamine (DA) onto one dendritic branch while monitoring APs from all dendrites in the visual field using calcium- and/ or voltage-sensitive dye imaging, we found that DA suppressed AP-evoked calcium transients while it had little effect on voltage transients. Dopaminergic suppression of dendritic calcium transients was rapid (<500 ms) and restricted to the dopamine input site, and were fully mimicked by agonists, partially obscured by D1 antagonist. In comparison, we also studied GABA effect on AP-associated voltage and calcium transients. Contrary to DA, GABA prohibited AP propagation distally from the input site, and therefore excluded AP-associated calcium transient. We hypothesize that the mammalian neocortex employs two distinct strategies (two spatio-temporal patterns) for rapid (< 0.5 sec) modulation of dendritic calcium influx (DA and GABA). The DA-mediated spatio-temporal pattern of dendritic calcium suppression is expected to occur during phasic dopaminergic signaling, when midbrain dopaminergic neurons generate a transient (0.5 s) burst of action potentials in response to a salient event in the animal's environment. ^