The Relationship Between Plasma Oxytocin and Executive Functioning in Huntington’s Disease: A Pilot Study

INTRODUCTION

Huntington’s disease (HD) is an inherited neurodegenerative condition defined by progressively worsening cognitive and motor function, and diverse psychiatric symptoms [1, 2]. Patients with premanifest HD may display marked cognitive, psychiatric, and social difficulties prior to the onset of motor symptoms [3–5]. Cognitive decline in premanifest HD includes deficits in attention, visuospatial processing, social cognition, memory, executive functioning, and the timing and processing speed of these actions [6–9]. Verified measures of cognition specific to HD have been developed (The Unified Huntington’s Disease Rating Scale (UHDRS), HD-CAB), and in particular the Symbol Digit Modalities Test (SDMT) has been highlighted for its utility in disease progression [3, 10].

Oxytocin (OT) is a neuropeptide hormone produced in the hypothalamus and released by the posterior pituitary gland with identified roles in human behavior, including stress-response regulation and prosocial interactions [11]. Hypothalamic changes in patients with HD have been noted in neuroimaging and neuropathological studies [12, 13]. OT has become a popular yet highly debated research topic for its wide-ranging potential impact on behavior in various diseases, measured both in the cerebrospinal fluid and in plasma as a rough indicator of the brain OT system [14, 15].

Although significant differences in plasma OT levels between HD gene carriers and healthy controls have not been reported [16], OT appears to influence social cognition in patients with HD, particularly in the context of recognition of negative emotions [17, 18]. Recent studies have also suggested OT may modulate other cognitive processes such as executive functioning and components of learning and memory [19–21]. However, there is limited research among HD patients regarding the role of OT in cognitive domains apart from social cognition.

The aim of our study was to further investigate the relationship between OT plasma levels and cognition, specifically executive functioning, as measured by a subset of tests from the UHDRS [3].

METHODS

Participants with HD were recruited from the Huntington’s Disease Society of America (HDSA) Center of Excellence at UTHealth, and healthy controls were recruited from the local community from August 2017 to December 2018 by trained professionals. The genetic diagnosis of HD was confirmed by ≥36 CAG repeats. A movement disorder specialist evaluated all patients, and the clinical diagnosis of HD was based on the motor signs certainty, i.e., a Diagnostic Confidence Level (DCL) set to 4 in the UHDRS [22]. HD participants were not eligible for inclusion based on the following criteria: severe cognitive impairment as measured by a score of < 18 on the Mini-Mental State Examination (MMSE) to assess the ability to consent for research, concurrent major illnesses such as cancer, the use of anti-inflammatory medications, or antibiotic use in the previous four weeks. Healthy controls were recruited based on comparable age and sex to HD participants, and were not eligible if there was a personal history of any neurologic disease or severe psychiatric disorder. This study included 55 participants: 16 healthy controls, 18 premanifest HD, and 21 manifest HD. Institutional review board approval was received from the Research Ethics Committee of UTHealth, and written informed consent was obtained for all participants.

The clinical battery included motor, cognitive and behavioral assessments included in the UHDRS. The SDMT was performed to evaluate visuospatial attention, processing speed, and working memory, and the Verbal Fluency Test (VFT), to evaluate lexical retrieval efficiency [3]. These are widely-accepted measures for evaluating individuals with HD, particularly in the pre-manifest and early manifest stages [9]. Behavioral symptoms were assessed with the short version of the Problem Behaviors Assessment –short version (PBA-s) [23].

Participants were subjected to a blood draw on the same day of the clinical assessment. Ten milliliters of blood were drawn by venipuncture in vacuum tubes containing heparin. Whole blood samples were used for plasma obtainment within 2 h of the blood draw. These samples were centrifuged at 3,000 g for 10 min at 4°C, twice. Plasma was collected and stored at –80°C until assayed.

Plasma samples were analyzed by an investigator blinded to clinical information. OT levels in samples were examined utilizing a highly specific enzyme-linked immune absorbent (ELISA) assay kit (ADI-900-153A, Enzo Life Sciences, Farmingdale, NY) as per manufacturer’s kit manual instructions. Measurements were performed on serum samples (diluted 1 to 10), and the concentrations of samples were calculated based on the standard curve. Absorbance was read at 405 nm using the EnVision multimode plate reader (PerkinElmer, Waltham, MA).

Association between dichotomous variables was assessed with the Pearson Chi-Square test. All variables were tested for Gaussian distribution by the Shapiro-Wilk normality test. Comparisons between three groups were made by Kruskal-Wallis or ANOVA tests, according to the non-Gaussian or Gaussian distribution of the variables. Post-hoc analyses were used to determine significant differences between pairs of groups (ANOVA was followed by Tukey’s Multiple Comparison and Kruskal-Wallis test was followed by the Test Dunn’s Multiple Comparison Test). Mann–Whitney tests were used for two-group comparisons (premanifest HD vs. manifest HD) as data were determined not to follow a normal distribution. Spearman’s correlations were performed to examine the association between plasma levels of OT and the scores obtained in the clinical scales. All statistical tests were two-tailed and were performed using a significance level of α= 0.05.

RESULTS

Demographic and clinical data of study participants are shown in Table 1. There were no significant group differences regarding age, sex distribution, and educational level. As expected, manifest HD participants had worse scores in the motor and cognitive scales in comparison with premanifest HD and control groups. Premanifest HD participants performed worse than controls in the VFT, but not in the SDMT. Regarding the PBA-s, HD participants (both premanifest and manifest) had more severe scores for depression and executive function subscales.

The three groups presented similar plasma levels of OT (p = 0.772) (Fig. 1A). No significant correlation was observed for OT and UHDRS-Total Motor Score in any group. In the premanifest HD group, higher levels of OT were associated with higher scores in the SDMT (rho = 0.713, p = 0.001), and VFT (rho = 0.478, p = 0.045) (Fig. 1B, C); however, these correlations were not significant for healthy controls or the manifest HD group. Higher levels of OT were also associated with lower scores in the PBA-s depression subscale for both the premanifest (rho = –0.486, p = 0.041) and manifest (rho = –0.436, p = 0.048) HD groups (Fig. 1D, E); there was no significant association in healthy controls.

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Fig. 1

Plasma OT Associations. There were no significant differences in plasma OT levels among healthy controls, premanifest, and manifest HD participants (A). In premanifest HD, higher OT levels were associated with better performance on the Symbol Digit Modalities Test (B) and the Verbal Fluency Test (C). Higher OT levels were also associated with lower reported depressive symptoms in both premanifest (D) and manifest (E) HD participants. Three-group differences were assessed by the Kruskal-Wallis test. The Spearman correlation test was used to examine associations between plasma levels of OT and the scores obtained in the clinical scales. The horizontal bars in (A) show the mean and the standard error of the mean. Male participants are presented in red, and female participants are listed in black on the figure. HD, Huntington’s disease; OT, oxytocin; PBA, Problem Behaviors Assessment; SDMT, Symbol Digit Modalities Test; VFT, Verbal Fluency Test.

DISCUSSION

To our knowledge, this pilot study is the first to examine plasma OT levels and measures of executive functioning in HD patients. There was no significant difference in OT levels among healthy controls, premanifest HD, and manifest HD, corroborating a previous study [16]. In our sample, higher OT levels correlated with better performance on the SDMT and VFT. Of note, the SDMT has been highlighted as perhaps the best measure to track disease progression and proximity to the presentation of motor symptoms among pre-manifest HD subjects [10, 24]. Similar correlations were not observed in the manifest HD group. This is perhaps due to the hypothalamic neurodegenerative processes and related changes in OT dynamics in HD, or simply due to our limited sample size in this pilot project [25].

Additionally, among all HD gene carriers, those with higher OT levels reported less depressive symptoms. Depressive symptoms in HD are common, with studies estimating the prevalence of depression in HD patients to be between 40–50%[26, 27]. However, in our sample there was an observed trend of lower self-reported depression symptoms on average in the manifest HD group versus premanifest. It is possible that this reflects the phenomenon of anosognosia, or lack of self-awareness, of depressive symptoms in later stages of HD; this symptom has recently been highlighted as a barrier to adequately capturing psychiatric symptoms in HD patients [28]. Further research is needed to clarify the potential influence of OT in depressive symptoms in HD, recognizing that self-reported symptoms in HD patients are extremely valuable but require thoughtful operationalization.

This pilot study explored the role of OT in executive functioning in HD patients. The limitations of our study include the relatively small sample size, which did not allow for control of confounding factors (e.g., sex), its cross-sectional design preventing causality assumptions, and a limited cognitive battery. In addition, significant debate remains about the utility of plasma OT levels as a biomarker, including concerns related to disparities between OT plasma and cerebrospinal fluid levels, the short half-life of the hormone in serum, the complexities of the OT hormonal pathway, estrogen modulation of the OT pathways leading to sex differences in clinical assays, and potential modulation of OT due to HD treatment regimens [18, 29–31]. Future research on the role of OT in additional cognitive domains is necessary, with the hopes of improved understanding of OT in HD will enhance clinical knowledge and inform therapeutic developments.