Reviewer #3 (Public Review):
Summary:
Kokinovic et al. presents an interesting paper that addresses an important gap in knowledge about the differences in the development of direct and indirect pathway striatal neurons in the striosome and matrix compartments. The division of the striatum into 4 distinct populations, striosome-dSPNs, striosome-iSPNs, matrix-dSPNs, and matrix-iSPNs is important, but rarely done. This study records all four populations across early development and shows differences in action potential characteristics and intrinsic properties. They also suppress striosome activity during postnatal development and evaluate the characteristics of adult dopaminergic neurons in control and previously striosome-quieted conditions.
Strengths:
The striatal electrophysiology is beautifully and carefully done and shows important developmental differences between neural subtypes.
The idea to test the striatonigral connection is a good idea.
Weaknesses:
The authors didn't actually test the striatonigral connection. The experiments they do instead don't convincingly show that the striosomal or even striatal connection to the dopaminergic neurons is altered after postnatal striosome suppression.
Major concerns:
(1) mIPSCs are measured and are reduced after chemogenetic suppression of striosomal neurons during development. This is an interesting finding, but these mIPSCs could be coming from any inhibitory input onto the SNc neurons. It is unlikely that most of the mIPSCs are coming from the striosomal inputs. The GPe is much more likely to be the source of these mIPSCs than the striatum because the GPe inputs form synapses nearer the soma and have a higher probability of release (Evans et al., 2020). dSPNs inhibit GPe neurons through a non-canonical pathway (Cui et al., 2021; Spix et al., 2021) and striosomes also inhibit the SNr (McGregor et al., 2019). The striatum has the potential to disinhibit SNc neurons through both the SNr or the GPe (Evans, 2022), and modification of the striosome-SNr or striosome-GPe connections during development could be what is causing the mIPSC changes. To claim that the striosome-SNc connection is altered, a direct test of this connection is necessary.
(2) The dopaminergic neurons recorded seem to be randomly selected, but the striosomes do not inhibit all SNc dopamine neurons. They selectively inhibit the ventral tier SNc neurons (Evans et al., 2020). In the present manuscript, it is impossible to know which subpopulation of SNc neurons was recorded, so it is impossible to tell whether the dopaminergic neurons recorded are the ones expected to receive striosomal input.
(3) Very similarly, the striosomes selectively wrap around the "SNr dendrite" of SNc neurons that participate in striosome-dendron bouquets (Crittenden et al., 2016). However, not all SNc neurons have prominent SNr dendrites (Henny et al., 2012). In the morphological images of Supplemental Figure 3, it looks like the recorded cells sometimes have an SNr dendrite and sometimes don't (but it is hard to tell because the medial-lateral rostral-caudal axis is not labeled in the images). The presence or absence of the "SNr dendrite" is a strong determinant of whether an individual dopaminergic neuron receives striosomal inhibition or not (Evans et al., 2020). As above, not knowing whether the neurons recorded have SNr dendrites makes it impossible to know whether they should be receiving striosomal input at all.
(4) It's quite interesting that the dendron-bouquet structure is intact even after striosomal activity suppression, as cannabinoid receptor knockout greatly disrupts the structural integrity of bouquets (Crittenden et al., 2022). However, going along with point 3, the gephyrin puncta analysis only at the somas is very limiting. The striosome-SNc relevant puncta would be primarily on the SNr dendrite. Gephyrin density on the SNr dendrites or in bouquets would be much more informative than density on the soma.
(5) The authors claim that "CNO didn't affect the shape of the DA neuron dendritic tree", but more information about the morphological analysis should be added. It is not clear how the sholl analysis was conducted or whether a full 3D reconstruction was made. This claim seems to be based on only one dendritic measurement (sholl analysis), but many other dendritic or morphological features could be altered.
Crittenden, J.R., Tillberg, P.W., Riad, M.H., Shima, Y., Gerfen, C.R., Curry, J., Housman, D.E., Nelson, S.B., Boyden, E.S., & Graybiel, A.M. (2016) Striosome-dendron bouquets highlight a unique striatonigral circuit targeting dopamine-containing neurons. Proc. Natl. Acad. Sci. U.S.A., 113, 11318-11323.<br />
Crittenden, J.R., Yoshida, T., Venu, S., Mahar, A., & Graybiel, A.M. (2022) Cannabinoid Receptor 1 Is Required for Neurodevelopment of Striosome-Dendron Bouquets. eNeuro, 9, ENEURO.0318-21.2022.<br />
Cui, Q., Du, X., Chang, I.Y.M., Pamukcu, A., Lilascharoen, V., Berceau, B.L., García, D., Hong, D., Chon, U., Narayanan, A., Kim, Y., Lim, B.K., & Chan, C.S. (2021) Striatal Direct Pathway Targets Npas1+ Pallidal Neurons. J Neurosci, 41, 3966-3987.<br />
Evans, R.C. (2022) Dendritic involvement in inhibition and disinhibition of vulnerable dopaminergic neurons in healthy and pathological conditions. Neurobiol Dis, 172, 105815.<br />
Evans, R.C., Twedell, E.L., Zhu, M., Ascencio, J., Zhang, R., & Khaliq, Z.M. (2020) Functional Dissection of Basal Ganglia Inhibitory Inputs onto Substantia Nigra Dopaminergic Neurons. Cell Rep, 32, 108156.<br />
Henny, P., Brown, M.T.C., Northrop, A., Faunes, M., Ungless, M.A., Magill, P.J., & Bolam, J.P. (2012) Structural correlates of heterogeneous in vivo activity of midbrain dopaminergic neurons. Nat. Neurosci., 15, 613-619.<br />
McGregor, M.M., McKinsey, G.L., Girasole, A.E., Bair-Marshall, C.J., Rubenstein, J.L.R., & Nelson, A.B. (2019) Functionally Distinct Connectivity of Developmentally Targeted Striosome Neurons. Cell Rep, 29, 1419-1428.e5.<br />
Spix, T.A., Nanivadekar, S., Toong, N., Kaplow, I.M., Isett, B.R., Goksen, Y., Pfenning, A.R., & Gittis, A.H. (2021) Population-specific neuromodulation prolongs therapeutic benefits of deep brain stimulation. Science, 374, 201-206.