The hippocampus is a brain structure that is involved in memory formation, where inputs (contextual information) are processed here and converted to outputs (firing of pyramidal cells). The firing of pyramidal cells are regulated by more than 20 types of inhibitory interneurons, and we still don't know exactly how this works.
I am studying one type of interneuron, called the neurogliaform cells (NGFCs), to see how they function within the hippocampal circuit. NGFCs form synapses onto the distal tufts of pyramidal cell apical dendrites alongside excitatory inputs from the entorhinal cortex. NGFCs also express neuronal nitric oxide synthase (nNOS), are often synaptically coupled, and fire during theta oscillations in vivo.
Show figure of hippocampal interneuron subtypes
Peter Somogyi and Thomas Klausberger, The Journal of Physiology, 2005
What I have found, a summary
In this project, I describe a novel feature of synaptic communication between these interneuron types, hereafter referred to as the firing induced suppression of inhibition (FSI). Specifically, I found that when a theta-associated activity patterns were evoked in NGFCs from rodent hippocampal slices, the cells showed a transient reduction in unitary IPSP amplitude. My data suggest that FSI requires the backpropagation of action potentials, calcium influx through L-type calcium channels, nNOS activity within the dendrites of interneurons, and activation of NO-sensitive guanylyl cyclase (NOsGC) receptors, which are present on presynaptic terminals. My results also demonstrate the physiological impact of this phenomenon by showing that when FSI takes place the strength of incoming excitatory postsynaptic potentials onto NGFC is transiently sharpened. Specifically, FSI indirectly increased the amplitude of EPSPs. Thus FSI may enhance spatial and temporal summation of excitatory inputs to NGFCs, thus regulating their inhibition of pyramidal cells.
Here are the details
1. In vivo firing induces suppresion of inhibition (FSI)
I have recorded the unitary inhibitory postsynaptic potentials (uIPSPs) or currents (uIPSCs) from synaptically-coupled putative NGFCs or other interneurons with the soma in the SLM of the rat hippocampus in vitro. Neurons were recorded with a high chloride internal solution were used to increased the signal of these events and as a result these events were depolarising potentials or inward currents. A firing sequence (60 s duration, average frequency and CV values: 8.3 Hz, 0.75; stimulation protocol) was recorded in a rat NGFC in vivo during theta oscillations (Fuentealba et al., 2010) and which was injected in current clamp configuration in a postsynaptic interneuron of the SLM recorded in vitro. I have found that shortly after (125 or 250 ms) the firing of the postsynaptic cell from the in vivo sequence, there was a transient decrease in the amplitude of the uIPSPs recorded in current clamp or voltage clamp configuration. The kinetics of baseline and decreased uIPSCs evoked before and immediately after the stimulation protocol was similar, suggesting that presynaptic mechanisms were more likely than postsynaptic mechanisms to be responsible for the decrease of the synaptic responses.
In summary, I have found that in vivo NGFC firing during hippocampal theta rhythm induces a DSI-like event (FSI) recorded from pairs of interneurons with the soma in the SLM. The following experiments have been designed to study the cell types and mechanisms involved in this phenomenon.
2. What happens during in vivo firing?
FSI is temporally-correlated with the postsynaptic firing, but is postsynaptic cell firing required for FSI? I have studied this question by using a subthreshold protocol and found that a subthreshold protocol was unable to induce FSI in all cell pairs tested. This result suggests that FSI was initiated by the postsynaptic neuron firing, indicating that active processes were likely to be involved. According to this result, Action potential backpropagation (bAP) could be one of the active processes that occur, namely bAP travels along the postsynaptic dendrites in order to trigger FSI. To test this hypothesis, I have visualised bAPs in interneurons of the SLM by using single cell voltage imaging and they show robust action potential backpropagation
A possible function for bAP is to activate downstream enzymes that are responsible for the synthesis of retrograde signalling molecules. According to this idea, I have tested the hypothesis that the postsynaptic bAPs may elicit an increase in the dendritic calcium concentration. Using calcium imaging I have found that cell pairs showing FSI have higher dendritic calcium level.
Using pharmcology I have found this calcium influx is indeed through calcium channels (L-type) and not from stores.
3. Which messenger(s) is responsible for FSI?
Using pharmacology I have found that nitric oxide is responsible for FSI. FSI is blocked by the application of the nNOs inhibitor L-NAME, NOs-GC antogonist ODQ, and potentiated by the nitric oxide precursor L-arginine.
4. What are the physiological implications of FSI?
To test the physiological role of FSI on single cell integration, I have injected a train of EPSPs as synaptic conductance by using dynamic clamp (dEPSPs) in postsynaptic NGFCs. Since NGFCs have short dendrites and are biophysically compact, the somatic injection of unitary dEPSPs should represent a realistic representation of EPSPs evoked by stimulation of one or a few fibres present in the SLM, such as the perforant path from the entorhinal cortex. I observed that when FSI occurred, the amplitude of the dEPSPs was significantly increased compared to dEPSPs elicited in the presence of control synaptic inhibition before or after FSI (note, normal chloride internal solution was used). Thus, FSI transiently increased the size of the dEPSPs thereby promoting the temporal/spatial integration of the uEPSPs elicited by stimulation of perforant path/other excitatory fibres present in the SLM.