GABAergic inhibitory interneurons (INs) account for approximately 10-20% of neurons in the rodent cortex. Of these, about 20-30% express a peptide called somatostatin (SOM). Most study into the function of SOM INs has focused on one class in particular called the Martinotti cells. Martinotti cells have axons that ascend to the most superficial layer of the cortex, contacting distal sites on the dendrites of excitatory pyramidal cells. Recent papers, like this one from Massimo Scanziani’s group, highlight the role of layer 2/3 SOM INs in mediating surround suppression of layer 2/3 pyramidal neurons. Feedback inhibition from SOM INs is recruited via excitatory input coming from the horizontal axons of pyramidal neurons. In this way, inhibition from SOM INs scales with the spread of excitation in layer 2/3.
Given what was known about SOM INs, researchers in Bernardo Rudy’s group at NYU were surprised to find that when they optogenetically silenced the activity of SOM INs in brain slices (using conditionally-expressed halorhodpsin in the SOM-Cre mouse line), layer 4 pyramidal neurons fired less. Meanwhile, layer 2/3 pyramidal neurons fired more, as one would expect upon removing a source of inhibition.
The key to this result lies in the distinct pattern of connections that layer 4 SOM INs exhibit compared to their Martinotti cousins.
Analysis of layer 4 SOM INs revealed that their axonal arbors are restricted to layer 4, the layer of the cortex that receives thalamic input. In addition, layer 4 SOM INs exhibit electrophysiological properties that are distinct from Martinotti cells: notably, they have a more hyperpolarized resting potential and can fire at much higher rates. What about their functional connectivity?
Dual recordings between SOM and either pyramidal neurons or fast-spiking (FS) GABAergic INs revealed that SOM INs were much more likely to be connected to FS INs than pyramidal neurons. The connection probability from SOM INs to FS INs in layer 4 was surprisingly high: 62%. Furthermore, measures of unitary IPSCs and IPSPs in response to SOM IN spiking revealed that response amplitude was consistently higher in FS INs compared to pyramidal neurons. This finding was also (rather heroically) confirmed by exciting a SOM IN and simultaneously recording responses in a FS IN and pyramidal neuron. Keep in mind that only SOM INs were fluorescently-tagged, so that FS INs had to be hunted for the old-fashioned way – by taking a guess, patching a selected neuron, and testing its spike properties.
Importantly, short term dynamics were similar across the two types of synapses. Unitary IPSCs evoked by SOM INs were moderately depressing in both FS INs and pyramidal neuron synapses confirming that FS INs really do “see” more inhibition than pyramidal neurons, whether SOM INs fire sparsely or repetitively. When the same experiments were performed in layer 2/3 the results were flipped: SOM INs were more likely to contact pyramidal neurons, and they inhibited them more strongly than FS INs. This set of experiments was repeated using a sexier approach (*cough* optogenetics) that produced the same results. Experimenters conditionally expressed ChR2 in SOM INs by injecting a virus encoding floxed ChR2 into the SOM Cre mouse line. They then recorded the light-evoked IPSCs in FS INs and pyramidal neurons.
Given that the major role of layer 4 SOM INs, as defined by their functional connectivity, is to inhibit FS INs, it makes sense that silencing SOM INs reduces pyramidal neuron firing in layer 4. Based on connectivity, one would predict that silencing layer 4 SOM INs would remove inhibition placed on FS INs. Once disinhibited, FS INs would fire more and inhibit pyramidal neurons to a greater extent.
Indeed, researchers showed that optogenetically silencing SOM INs increased the number of spikes that layer 4 FS IN s fired during UP-states. Finally, to demonstrate that FS INs are the potent inhibitors of layer 4 pyramidal neurons scientists silenced FS INs using halorhodopsin expressed conditionally in the PV-Cre mouse line. Inhibiting PV INs significantly increased pyramidal neuron firing, supporting the notion that layer 4 SOM INs could indirectly excite pyramidal neurons by inhibiting PV fast-spiking interneurons.
Most studies on inhibitory neuronal circuits have focused on the control that GABAergic interneurons can exert over excitatory neuronal activity, but we are beginning to uncover the interactions among GABAergic interneurons. These interactions may potentially form disinhibitory circuits that could serve to gate the transmission of information to excitatory pyramidal neurons. Andres Luthi’s lab reported the existence of a disinhibitory microcircuit that facilitates learning of an associative fear memory. Foot shock excites layer 1 INs in auditory cortex that in turn inhibit FS INs in layer 2/3. This results in disinhibition of pyramidal neurons that was necessary for acquisition of the auditory fear memory.
Because all of the experiments regarding layer 4 SOM INs were performed in brain slices, we don’t know when this disinhibitory circuit could be activated in the intact brain. Previous studies suggest that layer 4 SOM INs only receive weak thalamocortical input, so it is unlikely that this disinhibitory circuit is recruited directly by thalamic activity. However, layer 4 SOM INs are potently activated by cholinergic inputs. An appealing possibility is that during heightened states of arousal cholinergic inputs excite layer 4 SOM INs. In turn, layer 4 SOM INs inhibit FS INs, thereby facilitating transmission of sensory information from the thalamus to the cortex. This study raises interesting questions to be addressed in vivo.
*EDIT* thanks to Nick Olivas who reminded me of this 2012 paper studying the function of SOM INs in vivo during active whisking. It’s interesting to note that researchers found that layer 2/3 SOM INs become hyperpolarized during active whisking, increasing the excitability of pyramidal neuron dendrites. If at the same time layer 4 SOM INs are excited, layer 2/3 pyramidal firing could be driven further by increasing the excitatory relay from layer 4 pyramidal neurons onto readily-excitable layer 2/3 neurons.