Enhancement of fear extinction with deep brain stimulation: evidence for medial orbitofrontal involvement

Abstract

Deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VC/VS) reduces anxiety, fear, and compulsive symptoms in patients suffering from refractory obsessive-compulsive disorder. In a rodent model, DBS-like high-frequency stimulation of VS can either enhance or impair extinction of conditioned fear, depending on the location of electrodes within VS (dorsal vs ventral). As striatal DBS activates fibers descending from the cortex, we reasoned that the differing effects on extinction may reflect differences in cortical sources of fibers passing through dorsal-VS and ventral-VS. In agreement with prior anatomical studies, we found that infralimbic (IL) and anterior insular (AI) cortices project densely through ventral-VS, the site where DBS impaired extinction. Contrary to IL and AI, we found that medial orbitofrontal cortex (mOFC) projects densely through dorsal-VS, the site where DBS enhanced extinction. Furthermore, pharmacological inactivation of mOFC reduced conditioned fear and DBS of dorsal-VS-induced plasticity (pERK) in mOFC neurons. Our results support the idea that VS DBS modulates fear extinction by stimulating specific fibers descending from mOFC and prefrontal cortices.

Figure 1

Stereological analysis of cortical neurons projecting through dorsal– and ventral–ventral striatum (VS). (a) Left: deep brain stimulation (DBS) electrode placements in VS. Circle diameter indicates the estimated spread of current from electrode tip. DBS of dorsal–VS was applied dorsal to anterior commissure (orange circles) and DBS of ventral–VS was applied ventral to anterior commissure (green circles). Right: freezing plots for Sham (n=16), dorsal–VS DBS (n=6), and ventral–VS DBS (n=8) groups. Dorsal–VS DBS reduced freezing on day 2 and facilitated extinction recall on day 3. Ventral–VS DBS had the opposite effects. Data are shown in blocks of two trials as mean±SEM. *p<0.05. Adapted from Figure 1b and c of Rodriguez-Romaguera et al, 2012. (b) Left: representative micrographs showing the spread of wheat germ agglutinin (WGA) into dorsal–VS (top) or ventral–VS (bottom). Right: Injection sites and spread of all cases used in Figure 2c. (c) Illustration of the cortical maps generated using non-biased stereology techniques. Sample obtained from a rat with a WGA injection into ventral–VS.

Figure 2

Medial orbitofrontal cortex (mOFC) projects through dorsal–ventral striatum (VS), whereas infralimbic (IL) and anterior insular (AI) cortices project through ventral–VS. (a) Stereological maps of cortical neurons that project through dorsal–VS (orange dots) at various anterior–posterior (AP) levels from bregma. Top left inset: micrographs showing the spread of wheat germ agglutinin (WGA) into dorsal–VS that generated the stereological maps. The boundary of the cortical area counted is defined using orange lines. (b) Stereological maps of cortical neurons that project through ventral–VS (green dots) at various AP levels from bregma. Top left inset: Micrographs showing the spread of WGA into ventral–VS that generated the stereological maps. The boundary of the cortical area counted is defined using green lines. (c) Comparison of fraction of cells (% of total cells counted) retrogradely labeled by WGA infusions into either dorsal–VS or ventral–VS (n=4). Data presented as mean±SEM. *p<0.05, **p<0.01. AId, dorsal portion of anterior insular; AIv, ventral portion of anterior insular; Cg1, cingulate area 1; cVLO, caudal portion of ventrolateral orbitofrontal; MO, medial orbitofrontal; PL, prelimbic; rVLO, rostral portion of ventrolateral orbitofrontal.

Figure 3

Inhibition of medial OFC (mOFC) neurons reduces fear expression. (a) Left: representative micrograph showing the spread of muscimol (MUS) in mOFC. Right: placement of cannula tips within the mOFC. (b) Freezing plots for saline (SAL, n=8) and MUS (n=6) groups. MUS reduced freezing on day 2 and did not alter extinction retrieval on day 3. Data are shown in blocks of two trials as mean±SEM. *p<0.05.

Figure 4

Deep brain stimulation (DBS) of dorsal–ventral striatum (VS) increases pERK expression in medial OFC (mOFC) neurons. (a) DBS electrode placements in ventral striatum. Circle diameter indicates the estimated spread of current from electrode tip. DBS of dorsal–VS was applied dorsal to the anterior commissure (orange circles) and DBS of ventral–VS was applied ventral to the anterior commissure (green circles). (b) Representative micrograph showing pERK labeling in mOFC after DBS of dorsal-VS. (c) Comparison of pERK density (counts/0.1 mm²) after rats underwent 3 h of Sham DBS (n=9), dorsal–VS DBS (n=4) or ventral–VS DBS (n=4). Data presented as mean±SEM. **p<0.01.