Modulation of serotonin dynamics in the dorsal raphe nucleus via high frequency medial prefrontal cortex stimulation.

The subcallosal cingulate (SCC) region, or its rodent homologue the medial prefrontal cortex (mPFC), and midbrain dorsal raphe (DR) are crucial nodes of the widespread network implicated in emotional regulation. Stimulation of the SCC is being explored as a potential treatment for depression. Because modulation of the 5-HT system is the most common pharmacological means of treating depression, we sought to establish 5-HT's role in the mPFC-DR projection. Using anaesthetized mice, we recorded neuronal activity in 49 neurons of the DR before, during, and after high frequency stimulation (HFS) of the mPFC. The majority of DR cells (74%) significantly decreased firing rate during HFS (p<0.001, 65.7±9.4% of baseline, 14 mice). To see the effect of mPFC-HFS on 5-HT neurons, we used transgenic mice with expression of the channelrhodopsin fusion protein directed to the 5-HT neuronal population. Neurons were categorized as 5-HT based on their excitatory response to blue light stimulation (p<0.05, n=11). Our main finding was that identified 5-HT neurons in the DR were clearly inhibited by HFS, albeit non-selectively. Lastly, we used fast scan cyclic voltammetry (FSCV) to investigate the effects of mPFC-HFS on the release and reuptake of electrically stimulated 5-HT in the DR of C57BL/6J mice. Serotonin clearance was significantly faster following 5min HFS (2.3±1.0s, n=5, p<0.05) when compared to control levels (3.7±1.0s, n=5), indicating less release or more efficient 5-HT reuptake. Taken together, these findings imply that mPFC stimulation alters 5-HT activity dynamics in the DR. Such altered 5-HT dynamics may modulate the potential therapeutic mechanisms of SCC/mPFC stimulation.

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

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.

Uncoupling PSD-95 interactions leads to rapid recovery of cortical function after focal stroke.

Since the most significant ischemic sequelae occur within hours of stroke, it is necessary to understand how neuronal function changes during this time. While histologic and behavioral models show the extent of stroke-related damage, only in vivo recordings can illustrate changes in brain activity during stroke and validate effectiveness of neuroprotective compounds. Spontaneous and evoked field potentials (fEPs) were recorded in the deep layers of the cortex with a linear microelectrode array for 3 hours after focal stroke in anesthetized rats. Tat-NR2B9c peptide, which confers neuroprotection by uncoupling the PSD-95 protein from N-methyl-D-aspartate receptor (NMDAR), was administered 5 minutes before ischemia. Evoked field potentials were completely suppressed within 3 minutes of infarct in all ischemic groups. Evoked field potential recovery after stroke in rats treated with Tat-NR2B9c (83% of baseline) was greater compared with stroke-only (61% of baseline) or control peptide (Tat-NR2B-AA; 67% of baseline) groups (P<0.001). Electroencephalography (EEG) power was higher in Tat-NR2B9c-treated animals at both 20 minutes and 1 hour (50% and 73% of baseline, respectively) compared with stroke-only and Tat-NR2B-AA-treated rats (P<0.05). Tat-NR2B9c significantly reduces stroke-related cortical dysfunction as evidenced by greater recovery of fEPs and EEG power; illustrating the immediate effects of the compound on poststroke brain function.

High-frequency cortical activity associated with postischemic epileptiform discharges in an in vivo rat focal stroke model.

OBJECT: The postischemic brain has greater susceptibility to epileptogenic activity than physiologically healthy tissue. Epileptiform discharges are thought to exacerbate postischemic brain function. The aim of this study was to develop an in vivo focal stroke model in rats to characterize epileptiform activity. METHODS: The authors developed a parasagittal 8-channel intracortical microelectrode array to obtain recordings of cortical oscillations of local field potentials following partial middle and anterior cerebral artery occlusion. All experiments were done in urethane-anesthetized Sprague-Dawley rats. RESULTS: Theta runs (TRs), ranging in duration from 5 seconds to 5 minutes, were observed in 62% of animals within 1 hour of occlusion. High-frequency oscillations (HFOs) in the high gamma range (80-120 Hz) were observed 5-15 seconds before each TR and terminated at the onset of the discharge. Periodic epileptiform discharges (PEDs) were detected in 54% of rats following ischemia. The PEDs consisted of an early negative slow wave, a high-amplitude positive spike, and a short negative slow wave. Transient HFOs in the low gamma range (30-70 Hz) occurred during the first negative wave and the rising phase of the positive spike of the PED. CONCLUSIONS: These recordings provide the first intracortical evidence of a high-frequency component that could be an important element for diagnosis and intervention in postischemic epileptogenic activity. The early onset also suggests that HFOs could serve as a reliable method of detecting small epileptiform events and could be used as a consideration in deciding whether antiepileptic medications are appropriate as part of a patient's poststroke care.