HMBA Consensus Basal Ganglia Atlas: CN LGE GABA

Dorsomedial shell

Striatal projection neurons (SPNs) of the indirect pathway also known as: iSPNs, D2-SPNs Earlier name was “medium spiny neuron (MSN)”, from an era in which the distinctions between striatal projection neurons and interneurons were less apparent and connectivity was far less characterized than today.  Recent literature increasingly uses “Striatal projection neurons (SPN)” to distinguish these projection neurons from local striatal interneurons and to clarify their role as output elements in basal ganglia circuits. We also know today that very many kinds of neurons are “spiny”. “Spiny projection neurons (SPNs)” is also sometimes used, though again, many kinds of projection neurons have spines. Synonyms: D2-SPNs, indirect pathway SPNs, iSPNs Direct pathway SPNs (dSPNs) predominantly express dopamine D1 receptors and project directly to the main output nuclei of the basal ganglia, such as the substantia nigra pars reticulata (SNr) and internal globus pallidus (GPi). Indirect pathway SPNs (iSPNs) primarily express dopamine D2 receptors and project to the external segment of the globus pallidus (GPe), forming an indirect route to the output nuclei by relaying through the subthalamic nucleus (STN). I prefer “iSPNs” (emphasizing connectivity) over “D2-SPNs” (emphasizing a single marker) or “D2-MSNs”.

Striatal projection neurons (SPNs) of the direct pathway also known as: dSPNs, D1-SPNs Earlier name was “medium spiny neuron (MSN)”, from an era in which the distinctions between striatal projection neurons and interneurons were less apparent and connectivity was far less characterized than today.  Recent literature increasingly uses “Striatal projection neurons (SPN)” to distinguish these projection neurons from local striatal interneurons and to clarify their role as output elements in basal ganglia circuits. We also know today that very many kinds of neurons are “spiny”. “Spiny projection neurons (SPNs)” is also sometimes used, though again, many kinds of projection neurons have spines. Synonyms: D1-SPNs, indirect pathway SPNs, dSPNs Direct pathway SPNs (dSPNs) predominantly express dopamine D1 receptors and project directly to the main output nuclei of the basal ganglia, such as the substantia nigra pars reticulata (SNr) and internal globus pallidus (GPi). Indirect pathway SPNs (iSPNs) primarily express dopamine D2 receptors and project to the external segment of the globus pallidus (GPe), forming an indirect route to the output nuclei by relaying through the subthalamic nucleus (STN). I prefer “dSPNs” (emphasizing connectivity) over “D1-SPNs” (emphasizing a single marker) or “D1-MSNs”.

OK I disagree with the use of the word hybrid for 2 reasons. 1. They're not all DRD1/DRD2 positive 2. Hybrid implies that they're an offspring or fusion of two things, however I think this is incorrect. In my grad school work, I identified these cells as originating in the dLGE (like the MEIS2 cells which migrate into the OB, white matter, etc), but postnatally adopt a more MSN-like fate. I, and the original "eccentric" spiny neuron paper identify these cells as distinct from early development by being TSHZ1/FOXP2/OPRM1/CASZ1 high and FOXP1 low/negative (OTOF isn't a good marker for them), and it appears that that in adult primate this identity throughline is pretty clear. https://doi.org/10.1038/s41586-022-04510-w https://doi.org/10.1016/j.cell.2018.07.028 I think calling them eccentric is maybe not ideal, but would pay homage to the original discovery and what they've been known as. You could choose a different name, but I think "hybrid" as a descriptor for the subclass is specifically misleading due to its incompleteness and connotations in the evolutionary literature and so should be changed.