The Integration of Top-down and Bottom-up Inputs to the Striatal Cholinergic Interneurons

Abstract

Background: Cholinergic interneurons (ChIs) are important for learning and memory. They exhibit a multiphasic excitation-pause-rebound response to reward or sensory cues indicating a reward, believed to gate dopamine-dependent learning. Although ChIs receive extensive top-down inputs from the cortex and bottom-up inputs from the thalamus and midbrain, it is unclear which inputs are involved in the development of ChI multiphasic activity.

Methods: We used a single-unit recording of putative ChIs (pChIs) in response to cortical and visual stimulation to investigate how top-down and bottom-up inputs regulate the firing pattern of ChIs.

Results: We demonstrated that cortical stimulation strongly regulates pChIs, with the maximum firing rate occurring at the peak of the inverted local field potential (iLFP), reflecting maximum cortical stimulation. Pauses in pChIs occurred during the descending phase of iLFP, indicating withdrawal of excitatory cortical input. Visual stimulation induced long pauses in pChIs, but it is unlikely that bottom- up inputs alone induce pauses in behaving animals. Also, the firing pattern of ChIs triggered by visual stimulation did not correlate with the iLFP as it did after cortical stimulation. Top-down and bottom-up inputs independently regulate the firing pattern of ChIs with similar efficacy but notably produce a well-defined pause in ChI firing.

Conclusion: This study provides in vivo evidence that the multiphasic ChI response may require both top-down and bottom-up inputs. The findings suggest that the firing pattern of ChIs correlated to the iLFP might be a useful tool for estimating the degree of contribution of top-down and bottom-up inputs in regulating the firing activity of ChIs.

Keywords: Cholinergic interneurons; bottom-up input; firing pattern; integration.; pauses; top-down input.

Fig. (1)

Top-down and bottom-up pathways to the striatum. Electrical stimulation of the cortex can activate the glutamatergic top-down input to the striatum. The injection of BIC, which disinhibits the SC, opens subcortical bottom-up pathways to the striatum. These pathways involve the glutamatergic pathway through the thalamus and dopaminergic inputs from the SNc. The red arrows represent glutamatergic pathways, and the green arrow represents dopaminergic pathways.

Fig. (2)

Visual inputs induce varying pauses in pChIs during the effect of BIC-induced dis-inhibition of the SC. (A) Contralateral visual stimulation was applied with BIC injected into the SC to regulate pChIs firing pattern. (B) Visual stimulation was applied at time 0 s and repeated every 5 s. The average firing pattern (dark blue line) of six pChIs in the first five minutes recording showed an initial excitation around 0.5 s after visual stimulation and a pause between 1s and 2 s following visual stimulation. pChIs then showed a rebound following the “pause.” The Orange dashed line is the average firing rate of the pChI during baseline. Light blue lines are (Mean ± SD). (C) A representative pChI recorded during BIC injection and the light flash protocol. Each sweep represents the spikes of the cell over a period of 5 seconds. The BIC ejection and light flash were commenced at the same time (the 50^th sweep) (horizontal line). The left panel shows the VEP in the deep layers of the SC during the recording. Each dot in the left panel represents the average of 5 consecutive VEPs. The middle panel is a z-scored raster figure. Warmer colours indirectly represent a higher instantaneous firing rate, and colder colours represent lower instantaneous firing rates. The vertical dashed line indicates the time of the light flash during each sweep. Each dot in the right panel shows the average firing rate of the cell during that sweep. (D) Upper Raster data shows that visual stimulation alone can induce an excitation-pause-rebound firing pattern of a pChI. The same visual stimulation induces a VEP in the SC (Middle) but a very weak fluctuation of the iLFP in the striatum; there is no clear correlation between the iLFP and the firing pattern of the pChI (lower).

Fig. (3)

The firing patterns of pChIs were locked to the iLFP regardless of local disinhibition of the SC with BIC. (A) Contralateral cortical stimulation was applied with BIC injected into the SC to regulate the pChIs firing pattern. (B) Local injection of BIC into the SC desynchronized the striatal LFP caused by urethane anaesthesia. (C) Raster data shows that cortical stimulation alone can trigger multiphasic responses in a representative pChI (from n = 13 pChIs) during the BIC effect (upper). The iLFP recorded in the striatum and the SC showed a similar pattern due to cortical stimulation (lower). (D) Left: A representative pChI was recorded after BIC injection with the cortical stimulation protocol. Each sweep represents the spikes of the cell over a period of 5 seconds. Warmer colors indirectly represent a higher instantaneous firing rate, and colder colors represent lower instantaneous firing rates in this z-scored raster figure. The vertical dashed line indicates the time of the cortical stimulation during each sweep. Right: the averaged iLFPs; each iLFP is an average of 60 sweeps during the ChI recordings shown in the left panel. (E) The firing pattern of 60 sweeps from the representative pChI is shown (upper) with the associated iLFPs (lower) stretched to align the same phases of the iLFP (red arrows in D left panel). Despite the recordings being selected right after BIC injection (blue) and when the BIC effect was weaker (orange), the firing pattern of the ChI locked to the same phases of the iLFP and was highly correlated (Pearson correlation, P = 5.128*10^-10).

Fig. (4)

Top-down and bottom-up inputs may contribute equally to the firing pattern of pChIs. (A) Paired contralateral cortical and visual stimulation was applied with BIC injected into the SC to regulate pChIs firing pattern. (B) The firing pattern of a representative pChI is influenced by both cortical stimulation and visual stimulation under the effect of BIC. Each sweep represents the spikes of the cell over a period of 5 seconds. Warmer colors indirectly represent a higher instantaneous firing rate, and colder colors represent lower instantaneous firing rates in this z-scored raster figure. In the first 100 sweeps, visual stimulation (yellow dashed line) and cortical stimulation (red dashed line) were paired at a 100 ms interval. Visual stimulation was ceased after the 100^th sweep (orange line). The firing pattern of the representative ChI changed immediately after the visual stimulation was ceased. (C) Raster data shows that paired cortical and visual stimulation alone can trigger multiphasic responses in a representative pChI (from n = 3 pChIs) during the BIC effect (upper). The iLFP recorded in the striatum and the SC showed distinct patterns due to cortical stimulation (lower). (D) Under the effect of BIC, the firing patterns of the same ChI that received visual stimulation (yellow) alone and cortical stimulation (blue) alone were aligned at the same timing used in the pairing protocol. i.e., cortical stimulation is at time 0, and visual stimulation is at time -0.1 s. (E) The average of the two firing patterns induced by cortical and visual stimulation alone (green) was similar to the real firing pattern obtained during pairing protocol (orange), especially the timing and amplitude of the excitation (Pearson correlation, P = 1.363 * 10^-6). However, there is a clearer and more sharply defined pause in firing induced by combined stimulation.