Morphological and functional characterization of layer 5 neurons in rat medial prefrontal cortex, their synaptic microcircuitry and serotonin modulation

Rama, Ramya; Feldmeyer, Dirk (Thesis advisor); Spehr, Marc (Thesis advisor)

Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2023)
Book, Dissertation / PhD Thesis

In: Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien = Key technologies 267
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2022

Abstract

Medial prefrontal cortex (Cortex praefrontalis medialis, mPFC) is a part of the association cortex that serves to process and integrate synaptic inputs from sensory and motor cortical areas. The mPFC plays important roles in numerous cognitive processes, such as emotional processing, executive function, decision-making, spatial orientation, long-term memory and impulse control. Compared to other cortical areas, the mPFC has a particularly high density of serotonergic axons which originate from the dorsal and medial raphe nuclei; they play a crucial role in regulating the development and function of the neocortex. For this reason, a basic knowledge of the role of serotonin (5-HT) and its receptors (5-HTRs) function in the mPFC in modulating cortical activity is crucial.In rodents, approximately 60% of pyramidal neurons (PNs) possess 5-HT1ARs or 5-HT1BRs, particularly those in layer 5 (L5) of mPFC, ~80% of which express both receptor subtypes. In previous studies, L5 PNs were divided into two groups; the first group includes PNs that innervate brain areas within the telencephalon such as the cerebral cortex, the limbic system and the basal ganglia. In contrast, PNs of the second group project to brain areas outside the telencephalon such as the cerebellum, the brain stem and the spinal cord. Besides the PNs, neuronal activity of the neocortex is also under the control of the different types of inhibitory interneurons (INs); most of these IN also express 5-HTRs. PNs and INs in L5 form complex neural networks in the mPFC. In the present work, two different types of PNs and three types of INs in L5 of mPFC were identified through electrophysiological recordings with the patch-clamp technique in whole-cell mode and morphological reconstructions. PNs have been divided into adaptive-spiking (AS) and regular-spiking (RS) types, corresponding to intra- and extratelencephalic PNs. Based on the peak action potential frequency, L5 INs were classified into non-fast-spiking (nFS), regular-fast-spiking (rFS) and stuttering/intermittent burst-like fast-spiking (bFS) INs.In this work, 5-HT showed a cell type-specific effect on the electrophysiological properties of L5 PNs and INs. In AS PNs with high input resistance (Rin), 5-HT induced sustained depolarization through 5-HT2ARs activation and a sustained hyperpolarization in AS PNs with a low input resistance (Rin) that was mediated by 5-HT1ARs or 5-HT1BRs. RS PNs showed a biphasic 5-HT response with an initial transient hyperpolarization followed by a persistent depolarization. The initial transient hyperpolarization was caused by a Ca2+-dependent activation of potassium channels with low single-channel conductivity (so-called ‘small-conductance’ K+ channels; SK channels).5-HT had no effect on nFS INs, but caused a clear depolarization in rFS INs; in bFS INs, 5-HT induced either depolarization or hyperpolarization depending on the bFS IN subtypes. In INs a 5-HT induced depolarization was mediated either by 5-HT2ARs or 5-HT3ARs; a 5-HT evoked hyperpolarization was mediated by either 5-HT1ARs or 5-HT1BRs.To investigate the effects of 5-HT on synaptic transmission between the two cell types, pair recordings of synaptically coupled pairs of neurons in L5 of mPFC were performed. By activating 5-HT1BRs, 5-HT caused a decrease in the EPSP amplitude at all synaptic connections with presynaptic L5 PNs independent of the specific type. Concomitantly, an increase in the ‘paired-pulse ratio’ (PPR) was observed suggesting that 5-HT reduces the presynaptic release of the neurotransmitter glutamate by activating the 5-HT1BRs on the presynaptic terminals. In inhibitory synaptic connections, the activation of presynaptically localized 5-HT3ARs caused a potentiation of the IPSP amplitude together with a decrease in PPR suggesting a 5-HT-dependent increase in GABA release. A decrease in the IPSP amplitude observed in a different set of inhibitory connections could be attributed to presynaptic 5-HT1ARs or 5-HT1BRs.In summary, the 5-HT induced modulation of L5 PNs and INs in the PFC investigated here causes a cell type-specific change in excitability that decisively influences the function of the neuronal network of the mPFC. The results of this work can form the basis for further investigation of 5-HT mediated neuromodulation and the underlying 5-HTRs in the various layers of the mPFC and other cortical areas.

Institutions

• Chemosensation Laboratory [163310]
• JARA - BRAIN [080010]
• Department of Biology [160000]
• [535500-2]