High-frequency stimulation of the medial prefrontal cortex decreases cellular firing in the dorsal raphe

Abstract

High-frequency deep brain stimulation (HFS-DBS) of the subcallosal cingulate (SCC) region has been investigated as a treatment for refractory forms of depression with a ~50% remission rate in open label studies. However, the therapeutic mechanisms of DBS are still largely unknown. Using anaesthetized Sprague Dawley rats, we recorded neuronal spiking activity in 102 neurons of the dorsal raphe (DR) before, during and after the induction of a 5-min HFS train in the infralimbic region (IL) of the medial prefrontal cortex (mPFC), the rodent homologue of the human SCC. The majority of DR cells (82%) significantly decreased firing rate during HFS (P < 0.01, 55.7 ± 4.5% of baseline, 35 rats). To assess whether mPFC-HFS mediates inhibition of DR cellular firing by stimulating local GABAergic interneurons, the GABAA antagonist bicuculline (Bic, 100 μm) was injected directly into the DR during HFS. Neurons inhibited by HFS recovered their firing rate during Bic+HFS (P < 0.01, n = 15, seven rats) to levels not different from baseline. Cells that were not affected by HFS did not change firing rate during Bic+HFS (P = 0.968, n = 7, three rats). These results indicate that blocking GABAA reverses HFS-mediated inhibition of DR neurons. As the cells that were not inhibited by HFS were also unaffected by HFS+Bic, they are probably not innervated by local GABA. Taken together, our results suggest that mPFC-HFS may exert a preferential effect on DR neurons with GABAA receptors.

Keywords: cellular firing; depression; dorsal raphe; high-frequency stimulation; medial prefrontal cortex; rat.

Fig. 1

(A) Diagram illustrating the location of recording electrodes in the dorsal raphe (DR) (ML 0 mm, AP +12 mm, and DV +7–8 mm relative to bregma in the 32° plane). The section of the brain on the left (40 μm) has been stained with cresyl violet and illustrates the recording electrode lesion (arrow). (B) Illustration (right) of the stimulating electrode location in the infralimbic region (IL) of the medial prefrontal cortex (mPFC) along with cresyl violet histological section (left) of the same region with electrode lesion (arrow).

Fig. 2

All recorded neurons in the dorsal raphe plotted based on spike duration (d in waveform inset, x-axis, in ms) and baseline firing rate (y-axis, Hz). The two insets are examples of a slow-firing (right) and fast-firing neuron (left). The distribution of frequencies revealed a significant correlation between cellular firing rate and spike width, where slower firing cells tended to have broader spikes, while fast spiking neurons were found to have shorter spike width (R = 0.330, P < 0.001, n = 102, 35 rats).

Fig. 3

Histograms showing significant decrease in firing rate of cells in the DR during 5 min of mPFC-HFS (P < 0.01, n = 102, 35 rats). Following HFS, the firing rate recovered to levels not significantly different from baseline (P = 0.389), but significantly different from HFS (P < 0.05). *P < 0.05, ** P < 0.01.

Fig. 4

Examples of slow-firing (A) and fast-firing (B) cells before, during and after HFS. Insets are 30-s segments of raw waveforms illustrating the change in spiking frequency. Note a significant decrease in firing rate during HFS and a recovery to baseline levels in both slow and fast neurons. Bottom graphs are vertical plots of all slow (C) and fast (D) neurons before, during and after HFS. Black lines in the middle of each group indicate mean firing rate, while grey lines are error bars (SEM). Note a significant decrease in firing rate during HFS in both slow (P < 0.05, n = 29) and fast neurons (P < 0.001, n = 73). *P < 0.05, ***P < 0.001.

Fig. 5

(A) Illustration of the effect of bicuculline (Bic, 100 μM direct microinjection) on a DR neuron that was inhibited by mPFC-HFS. Insets are 30-s segments of raw waveforms illustrating the change in spiking frequency. Note the recovery of the firing rate to near-baseline levels upon Bic administration. (B) Illustration of the lack of any effect of Bic on a DR neuron that was not affected by mPFC-HFS. (C) Histogram showing that neurons inhibited by HFS significantly increased their firing rate in response to Bic+HFS (P < 0.01, n = 15, seven rats) to levels not different from baseline (P = 0.783). (D) The neurons that were not inhibited by HFS did not change firing rate in response to Bic+HFS (P = 0.968, n = 7, three rats). **P < 0.01.

Fig. 6

Illustration of the effects of bicuculline (Bic) on neurons in the dorsal raphe (DR) that were inhibited by high-frequency stimulation (HFS) of the medial prefrontal cortex (mPFC). HFS activates the excitatory glutamatergic mPFC projections that preferentially innervate GABA neurons in the DR, thus increasing inhibitory GABAergic tone and decreasing cellular firing of recorded neurons (middle illustration). Local administration of Bic blocks GABA_A receptors on the recorded cells, disinhibiting the firing rate of the recorded cell, and thereby cancelling the inhibitory effect of HFS.