The strength and specificity with which a neuron forms synapses is a fundamental question in evaluating its function. To address this for striatal medium spiny neurons (MSNs), Chuhma et al used an optogenetic approach reminiscent of the aforementioned Bardi et al study.
First, they bred a mouse that selectively expressed ChR2 in striatal MSNs. Then, they recorded (in brain slices) the responses of target neurons before and just after shining light. If the neuron’s response to the light had an amplitude exceeding the mean + 2 SD of its baseline, then that neuron was counted as a synaptic connection.
For example, of the 22 neurons the researchers tested in the pars reticulata of the substantia nigra, 21 (or 95%) had an amplitude exceeding the baseline-derived cutoff following light exposure. Here’s the overall summary of what they found:
As the authors note in the discussion, the magnitude and strength of MSN-MSN connections indicate that these cells likely play a large role in determining striatal output. Their selectivity for particular cell subtypes is also striking. It’d be interesting to see if this was altered at all in a model of Huntington’s disease, which heavily affects striatal MSN processing.
Chuhma N, et al. 2011 Functional Connectome of the Striatal Medium Spiny Neuron. J Neuro, doi: 10.1523/JNEUROSCI.3833-10.2011.