Glutamatergic Projection Neurons Shape Neural Sequence Generation in the Premotor Cortex of Zebra Finches

Abstract

Neural circuits in the brain maintain a delicate equilibrium between excitation and inhibition, yet ensuring the proper functioning of neural microcircuits and the pre- cise manifestation of this balance at the mesoscopic level of neuronal populations remains less well understood. Moreover, neural circuits often exhibit sequences of activity that relies on excitation and inhibition, but the contribution of local net- works to their generation remains unclear. This study investigates neural sequence generation within the HVC (High Vocal Center) of the adult zebra finch forebrain. This nucleus is an analogue of the mammalian premotor cortex and plays a crit- ical role in the execution of temporally precise courtship songs. The topology of the HVC network is heterogeneous and comprised of three neural populations that has distinct in vitro and in vivo electrophysiological responses: Glutamatergic basal ganglia – projecting (HV CX ) and forebrain - projecting (HV CRA) cortical neurons as well as gabaergic interneurons (HV CINT ). While the excitatory and inhibitory connections between these three classes are reported, it’s largely unknown how they collaborate to orchestrate ongoing song and generate one of the most temporally- precise neural sequence known in nature to date. In this project, we used elimination based search methods and machine learning algorithms to unravel the underlying cytoarchitecture of the HVC neural network. Our network searches are based on biologically realistic constraints reported in the literature that are pertinent to the pharmacological nature of the synaptic connec- tions among the three different classes of HVC neurons as well as other anatomical and intrinsic constraints, like their approximate numbers in the nucleus, the precise timing as well as number of bursts, the frequencies of spiking during song etc. Con- sistent with a seminal experimental work reported earlier which remained shrouded in mystery, our simulations confirmed that irrespective of HV CX neurons’ intrinsic or synaptic properties, glutamatergic HV CRA and gabaergic HV CINT neurons are sufficient to drive motor activity, encode for the temporal precision in the neural se- quence generated, and orchestrate the neurons’ firing patterns. These findings show that the HVC network’s connector hubs are preferentially excitatory and suggest a potential mesoscopic organization of the excitation-inhibition balance within the nucleus.