Abstract
Increased inflammation in major depressive disorder (MDD) has been associated with low functional connectivity (FC) in corticostriatal reward circuits and symptoms of anhedonia, relationships which may involve the impact of inflammation on synthesis and release of dopamine. To test this hypothesis while establishing a platform to examine target engagement of potential therapies in patients with increased inflammation, medically stable unmedicated adult MDD outpatients enrolled to have a range of inflammation (as indexed by plasma C-reactive protein [CRP] levels) were studied at two visits involving acute challenge with the dopamine precursor levodopa (L-DOPA; 250 mg) and placebo (double-blind, randomized order ~1-week apart). The primary outcome of resting-state (rs)FC in a classic ventral striatum to ventromedial prefrontal cortex reward circuit was calculated using a targeted, a priori approach. Data available both pre- and post-challenge (n = 31/40) established stability of rsFC across visits and determined CRP > 2 mg/L as a cut-point for patients exhibiting positive FC responses (post minus pre) to L-DOPA versus placebo (p < 0.01). Higher post-L-DOPA FC in patients with CRP > 2 mg/L was confirmed in all patients (n = 40) where rsFC data were available post-challenge (B = 0.15, p = 0.006), and in those with task-based (tb)FC during reward anticipation (B = 0.15, p = 0.013). While effort-based motivation outside the scanner positively correlated with rsFC independent of treatment or CRP, change in anhedonia scores negatively correlated with rsFC after L-DOPA only in patients with CRP > 2 mg/L (r = -0.56, p = 0.012). FC in reward circuitry should be further validated in larger samples as a biomarker of target engagement for potential treatments including dopaminergic agents in MDD patients with increased inflammation.
Introduction
A significant portion of patients with major depressive disorder (MDD) reliably exhibit evidence of increased inflammation, which has received considerable attention as one pathophysiologic pathway contributing to symptoms of depression and particularly anhedonia [1,2,3,4,5]. Neuroimaging work has revealed that exogenous administration of inflammatory cytokines or cytokine inducers (e.g., endotoxin, vaccination) in a laboratory environment alters activation of and functional connectivity (FC) between reward-related brain regions relevant to reduced motivation and anhedonia [6,7,8,9,10,11]. Preclinical and clinical evidence suggests that these functional MRI (fMRI) findings may be due to the impact of inflammation on the availability and release of dopamine, in part through limiting its synthetic precursors [12,13,14]. For example, patients receiving inflammatory cytokine therapy for hepatitis C virus exhibited a pattern of striatal fluorodopa (18F) uptake consistent with reduced dopamine availability and release that correlated with symptoms of depression including reduced motivation [8]. Our work in non-human primates also demonstrated that peripheral inflammatory cytokine-induced decreases in striatal dopamine release, which occur in association with reduced effort-based sucrose consumption, were reversed by administration of the dopamine precursor levodopa (L-DOPA) [15, 16]. As a growing body of literature also demonstrates relationships between increased endogenous inflammation, alterations in activity of and FC within reward circuits, and anhedonia [17,18,19,20,21], understanding potential dopaminergic mechanisms and therapeutic implications is important considering substantial evidence of treatment resistance in MDD with increased inflammation [22,23,24,25].
We previously reported that increased plasma C-reactive protein (CRP) concentrations, as well as inflammatory cytokines and their soluble receptors, were associated with low resting-state (rs)FC between the left ventral striatum (VS) and ventromedial prefrontal cortex (vmPFC), key structures of classic reward circuitry that receive mesolimbic and mesocortical dopamine input [26], that correlated with anhedonia in medically stable and unmedicated MDD patients [19]. Relationships between inflammation and low VS-vmPFC rsFC in association with symptoms of anhedonia were observed using both seed-to-voxel-wise and targeted seed-to-ROI approaches [19], corroborated by network analyses [27], and replicated in a sample of highly-traumatized women [28]. Relationships between endogenous inflammation and functional changes in reward circuits were further supported by reduced striatal activation during reward anticipation in depressed patients with higher CRP and other inflammatory markers [20, 21]. Despite evidence of low dopamine function in MDD [29,30,31] and preferential response to dopaminergic antidepressants in MDD patients with higher levels of CRP [22], whether alterations in reward circuits in MDD patients with higher inflammation and symptoms of anhedonia involve dopaminergic mechanisms and may respond to relevant therapies is currently unknown.
This study examined whether pharmacological challenge with L-DOPA could increase FC in VS-vmPFC reward circuitry in medically stable, unmedicated MDD patients with increased inflammation. To increase rigor and reproducibility, and consistent with our goal of not only understanding mechanisms but also establishing a platform for future studies, a targeted a priori method was used to calculate left VS-vmPFC FC (see above) [19, 28, 32]. To further expand translational potential of results, mean concentrations of plasma CRP (which have clinical relevance, test-retest and inter-lab reliability, and correspond to increased levels of other inflammatory markers in both blood and cerebrospinal fluid) [4] assessed during participant screening were used as the primary measure of inflammation. To confirm that patients with higher CRP had increased concentrations of other inflammatory markers, plasma inflammatory cytokines and their soluble receptors were also measured [4, 19, 28].
We hypothesized that patients with higher (but not lower) levels of CRP would respond with higher VS-vmPFC rsFC after L-DOPA with respect to placebo as the primary outcome, and that similar relationships would be observed for task-based (tb)FC during reward anticipation in the monetary incentive delay (MID) task. It was also hypothesized that rsFC after L-DOPA with respect to placebo would correlate with objective assessments of motivation (effort expenditure for rewards task [EEfRT]) and symptoms of anhedonia (Snaith-Hamilton Pleasure Scale [SHAPS]), and that these relationships would depend on levels of CRP.
Methods
Participants
Fifty-seven participants (18–65 years) in a current MD episode determined by Structured Clinical Interview for DSM-5 (SCID-5) [33] were enrolled, and 40 participants with available and analyzable fMRI scans after challenge with both L-DOPA and placebo were included herein (see Supplementary Fig. S1, Supplementary Table S1 and Supplement for data quality exclusions). All subjects were tested for drugs of abuse at screening and pre-scans. All patients were free of psychotropic medications or those affecting the immune system at the time of study entry (see Supplement for details). No subjects were removed from treatment for the purposes of this study, and all subjects were monitored for significant worsening and suicidal risk. Patients were medically stable per medical history, physical exam, and laboratory testing. High sensitivity (hs)CRP was assessed over 2–5 screening visits; values >10 mg/L were retested at 2-week intervals to ensure stability and rule out infection. To maximize a range of values for statistical analyses, patients were recruited to represent a range of inflammation levels from low to high as distributed across mean plasma CRP concentrations 0-1, >1-2, >2-3, and >3 mg/L (20–27.5%/group; see Supplement). The study was registered (Clinicaltrials.gov, NCT02513485) and shared (NIMH Data Archive, #2540). All procedures were approved a priori by the Emory University Institutional Review Board. All participants provided written informed consent.
Study design and participation
Using a design previously employed in healthy controls that underwent fMRI to assess the effects of L-DOPA on rsFC in corticostriatal circuits [34], patients underwent fMRI and behavioral assessments on separate visits involving acute challenge with L-DOPA (250 mg with 25–50 mg carbidopa; see Supplement for details) and identically encapsulated placebo administered on separate visits spaced ~1-week apart using a double-blind, randomized, cross-over design (Fig. 1). Resting-fMRI and self-reported anhedonia (SHAPS) were collected before and after acute L-DOPA or placebo challenge, and task-fMRI (MID) and objective motivation (EEfRT, outside of scanner) were collected post-L-DOPA or placebo. Although MID and EEfRT have test-retest reliability [35,36,37], conducting them only post-L-DOPA or placebo administration limited same-day carry-over or practice-effects, fatigue, and promoted task sensitivity (see Supplement and below). Additionally, MID and EEfRT were practiced prior to the first scan visit….
