Our functional macrocircuit mapping demonstrates that a circuit exists for communication from frontal cortices to V1/V2 and PtA through claustrum. Although claustrum projection neurons are glutamatergic (Brand
1981; Braak and Braak
1982; Hur and Zaborszky
2005; Watakabe et al.
2014; Kim et al.
2016), the downstream impact of claustrum activity on cortical layers in rodents is unclear. As such, we injected AAV-ChR2 into claustrum (Fig.
3a) and performed whole-cell recordings across the layers of PtA while stimulating claustrum afferents. Our recordings were restricted to PtA because of the denser number of claustrum cells projecting to PtA relative to V1/V2 (see Fig.
1). Neurons were filled with neurobiotin to allow for post hoc discrimination of pyramidal neurons from interneurons, and slices were stained post hoc for ChR2 (Fig.
3b, c). To identify the cortical layers from which neurons were recorded, we used a 4′,6-Diamidino-2-phenylindole (DAPI) stain. Representative responses from each cortical layer are shown in Fig.
3d and summary data are given in the last four rows of Table
1. Notably, we detected AP generation in approximately half of all neurons recorded, excluding those in layer V, which we identified to be pyramidal in all cases (
n = 10 of 10). All of the neurons recorded from layers II/III were also all pyramidal (
n = 11 of 11). Neuron sub-types recorded from layers IV and VI were heterogeneous: pyramidal neurons (layer IV:
n = 1 of 10; layer VI:
n = 1 of 14), spiny neurons without any detectable apical dendrite (layer IV:
n = 5 of 10; layer VI:
n = 4 of 14), and aspiny neurons (layer IV:
n = 4 of 10; layer VI:
n = 6 of 14). In layer VI, 3 of 14 neurons recorded were unable to be classified.