Classical approaches versus modern molecular techniques to dissect neuronal heterogeneity in the dorsal horn: opposite ends of the same spectrum?

The work contained within this thesis addresses a critical question across the field of neuroscience – how to identify unique cell types that serve specific roles, and perform specific functions, in the central nervous system (CNS). While this is relevant in all disciplines of neuroscience, the model used in this work focuses on the dorsal horn (DH) of the spinal cord, an area that processes sensory information. Traditionally, DH neurons can be divided into two broad types, those that relay information from the DH to higher cortical regions and those that form modulating networks within the DH itself. The latter, referred to as interneurons, are often the focus of pain and sensory research but no definitive atlas or taxonomy for the region exists. In the past, DH neurons (including interneurons) have been subdivided based on morphological features, neurochemical profiles, or electrophysiological properties. This has produced an extensive and, in many cases, confounding literature on DH neuron heterogeneity. To date, the multiple methodologies have not arrived at a nomenclature consensus for DH interneuron subtypes. More recently, advances in molecular and genetic sequencing approaches have been implemented, using the transcriptional profile of individual neurons (or their nuclei) in conjunction with bioinformatic analysis, to distinguish different cell types. This has also produced multiple identification schemes for differentiating DH neuron types. The key objective of this thesis is to assess the relationship between classical (morphology,neurochemical, and electrophysiological) and transcriptional (molecular, genetic) VII classification systems of DH neurons. A previously identified DH population that expresses Calretinin (CR) was chosen as a test group, as there is an extensive literature on this population and past functional studies characterizing these cells suggested discrete CR subpopulations exist.