Whole cell patch clamp analysis of non-synaptic and synaptic signaling in rodent and human supragranular neocortex

Abstract

The neocortex is the evolutionarily youngest part of the cerebral cortex and at the same time considered to be one of the most complex biological structures. Besides its crucial role in sensory perception and motor execution, this organ makes humans capable of performing sophisticated cognitive tasks. These cognitive abilities make neuroscientists so eager --and able-- to decipher the workings of the human brain. Although the ultimate goal of neuroscience is to solve the human brain, most of the research is currently devoted to understanding the nervous system of simpler and more available model organisms, such as rodents', which still proves to be quite ambitious a task. The two most prominent cell types of the human brain are the neuronal and the glial cells. Glial cells are the major class of non-excitable cells in the brain. Among glia, astrocytes are the most extensively studied due to the fact that they comprise the most abundant and diverse glia class. Despite their diversity, their functions are shared: they are crucial for the extracellular ionic homeostasis, neurotransmitter uptake, synapse formation, regulation of blood–brain-barrier, and the development of the nervous system. The cortical neurons can be divided in two main groups: the excitatory projecting pyramidal cells and the inhibitory local interneurons. The local GABAergic interneurons constitute the minor fraction of neurons in the neocortex (\numrange{10}{20} \%) but are instrumental for normal brain function. Despite their small numbers, these interneurons display extremely diverse morphological, electrophysiological and molecular properties. Our research group focuses on the function, input and output of various interneuron types in the human and rodent neocortex. In my doctorate study, we examined two very distinct synaptic connections. First, the output of a unique inhibitory interneuron, the neurogliaform cell (NGFCs). These GABAergic neurons were suggested to specialize in acting on GABA receptors on compartments of the neuronal surface which do not receive synaptic junctions, through a unitary form of volume transmission. The GABA released via volume transmission can effectively reach the extrasynaptic GABAA and GABAB receptors on virtually all neuronal processes within the axonal cloud of the NGFC. In the same vein, we hypothesized that neurogliaform interneurons might act on non-neuronal elements of the surrounding cortical tissue without establishing synaptic contacts. In my doctorate study, we show that there is a cell type selective unitary transmission from NGFCs to astrocytes with an early, GABAA receptor and GABA transporter-mediated component and a late component that results from the activation of GABA transporters and neuronal GABAB receptors. We could not detect Ca2+ influx in astrocytes associated with unitary GABAergic responses. Our experiments identify a presynaptic cell-type-specific, GABA-mediated communication pathway from individual neurons to astrocytes, assigning a role for unitary volume transmission in the control of ionic and neurotransmitter homeostasis. In the second part of my work, we compared a ubiquitous excitatory synaptic connection between the human and the rat because classic theories link cognitive abilities to synaptic properties and human-specific biophysical features of synapses might contribute to the unparalleled performance of the human cerebral cortex. We performed paired recordings and multiple probability fluctuation analysis in pyramidal cell to basket cell synaptic connections in acute slice preparation from both species. These experiments revealed similar quantal sizes, but four times more functional release sites in human pyramidal cell to fast-spiking interneuron connections compared to rats. These connections were mediated on average by three synaptic contacts in both species. Each presynaptic active zone contains 6.2 release sites in human, but only 1.6 in rats. Electron microscopy and electron microscopic tomography showed that an active zone harbors 4 docked vesicles in human, but only a single one in rats. Our results reveal a robust difference in the biophysical properties of a well-defined synaptic connection of the cortical microcircuit of human and rodents. Our research had two main conclusions. First, we discovered of a novel interneuron-to-astrocyte signalling pathway, the function of which is probably to stabilize the widespread inhibitory action of neurogliaform interneurons through unitary GABAergic volume transmission. And second, we compared a ubiquitous synaptic connection in the human and rodent neocortex and found the biophysical differences in their workings that make human synapses more efficient.