Elucidation of the Central Serotonin Metabolism Pathway in Rhesus Macaques (Macaca mulatta) with Self-injurious Behavior

Macaques with self-injurious behavior (SIB) have been used as a model of human SIB and have previously been shown to respond to treatments targeting enhancement of central serotonin signaling, whether by supplementation with tryptophan, or by inhibiting synaptic reuptake. Decreased serotonin signaling in the brain has also been implicated in many human psychopathologies including major depression disorder. A disturbance in tryptophan metabolism that moves away from the production of serotonin and toward the production of kynurenine has been proposed as a major etiological factor of depression. We hypothesized that in macaques with SIB, central tryptophan metabolism would be shifted toward kynurenine production, leading to lower central serotonin (5-hydroxytryptamine). We analyzed tryptophan metabolites in the cerebral spinal fluid (CSF) of macaques with and without SIB to determine whether and where tryptophan metabolism is altered in affected animals as compared with behaviorally normal controls. We found that macaques with SIB had lower CSF concentrations of serotonin than did behaviorally normal macaques, and that these deficits were inversely correlated with the severity of abnormal behavior. However, our results suggest that this decrease is not due to shifting of the tryptophan metabolic pathway toward kynurenine, as concentrations of kynurenine were also low. Concentrations of IL6 were elevated, suggesting central inflammation. Determining the mechanism by which serotonin function is altered in self-injurious macaques could shed light on novel therapies for SIB and other disorders of serotonin signaling.

Severity and Distribution of Wounds in Rhesus Macaques (Macaca mulatta) Correlate with Observed Self-Injurious Behavior

Self-injurious behavior (SIB) occurs within laboratory-housed NHP at low frequency but can have a devastating effect on animal research and wellbeing. One barrier to the study and clinical management of these cases is the cost of equipment and personnel time to quantify the behavior according to the current standard of observation and to score remotely obtained video recordings. In studies of human SIB, in which direct observation is difficult or prohibited, researchers have demonstrated that quantifying the tissue damage resulting from SIB can be a useful proxy to represent the underlying behavior. We hypothesized that the nature of wounds resulting from SIB in NHP could be used in a similar manner to measure the abnormal behavior. Using a cohort of rhesus macaques with high-incidence SIB, we examined severity, distribution, and number of wounds and compared them with observed incidences of SIB during a 12-wk experiment. We found that the number, severity, and distribution of physical wounds were associated with the incidences of biting behavior observed during the 2 wk prior to measurement. We also found that an increased number of wounds was associated with increased severity. Animals with wounds of moderate severity were more likely to also have severe wounds than were macaques with wounds that were lower than moderate in severity. This work is the first representative study in NHP to find that behavioral SIB correlates with physical wounding and that increases in the frequency and number of the body regions affected correlates with the severity of wounding.

Self-Injurious Behavior Secondary to Cytomegalovirus-Induced Neuropathy in an SIV-Infected Rhesus Macaque (Macaca mulatta)

A 3.5-y-old, female rhesus macaque (Macaca mulatta) inoculated with SIVmac239 presented 8 mo later for inappetence and facial bruising. Physical examination revealed a superficial skin abrasion below the left eye, bruising below the left brow, and epistaxis of the left nostril. There were no significant findings on CBC, serum chemistry, urinalysis, or radiographs. Differential diagnoses included infectious etiologies, self-injurious behavior, immune-mediated dermatitis, and neoplasia. Lack of response to antibiotic and analgesic therapy and observations of the macaque made it apparent that the skin lesions were self-inflicted. The excoriations rapidly progressed to extend over the nose, and the left palpebrae became edematous. Euthanasia was elected because the macaque appeared to be experiencing continued discomfort despite analgesic therapy. Histopathologic examination revealed systemic cytomegalovirus (CMV) infection involving the facial nerves, periocular nerves, meninges, and perimesenteric lymph nodes. CMV is a common infection in macaques, with adult seroprevalence close to 100% in most colonies. Infection in immunocompetent animals is usually asymptomatic but can cause significant clinical disease in immunodeficient hosts. CMV is associated with a painful peripheral neuropathy in human AIDS patients, and analgesic treatment is often unsatisfactory. Peripheral neuropathy secondary to CMV should be considered as an underlying cause of self-injurious behavior in SIV-infected macaques. Macaques affected by other diseases and disorders may also be at risk for development of painful peripheral neuropathies.

Play caging benefits the behavior of singly housed laboratory rhesus macaques (Macaca mulatta)

This study addresses a recommendation in The Guide for the Care and Use of Laboratory Animals to provide singly housed nonhuman primates with intermittent access to large, enriched (play) caging. Research on the potential benefits of this type of caging is limited. The present study examines the effects of play caging on behavior, activity, and enrichment use. Singly housed, adult male, rhesus macaques (n = 10) underwent a baseline phase in their home cages, a 2-wk treatment phase with housing in play cages, and a posttreatment phase after returning to their home cages. Each subject underwent focal behavioral observations (n = 10; duration 30 min each) during each study phase, for a total of 150 h of data collection. Results showed increases in locomotion and enrichment use and a trend toward decreased abnormal behavior while subjects were in the play cage, with the durations of these behaviors returning to baseline levels after treatment. Anxiety-related behaviors decreased between the treatment and posttreatment phases but not between baseline and treatment, suggesting that outside factors may have influenced the decline. During the treatment phase, subjects spent more time in the upper quadrants of the play caging and preferred a mirror and forage boards as forms of enrichment. The greatest behavioral improvement occurred during the first week in the play cage. This study provides evidence to support the benefits of play caging for singly housed rhesus macaques.

Effects of extended-release injectable naltrexone on self-injurious behavior in rhesus macaques (Macaca mulatta)

Self-injurious behavior (SIB) is a spontaneous behavior that threatens the health and wellbeing of multiple species. In humans, the opioid antagonist naltrexone hydrochloride has been used successfully to modulate the endogenous opioid system and reduce the occurrence of SIB. This study is the first to assess the efficacy of extended-release naltrexone in the pharmacologic treatment of SIB in rhesus macaques (Macaca mulatta). In an acute pharmacokinetic study of 4 macaques, we determined the mean naltrexone plasma concentration was maintained above the therapeutic level (2 ng/mL) after administration of a single dose (20 mg/kg) of 28-d extended-release naltrexone throughout the release period. For a subsequent treatment study, we selected 8 singly housed macaques known to engage in SIB. The study comprised a 4-wk baseline phase; an 8-wk treatment phase, during which each macaque received 2 doses of extended-release naltrexone 28 d apart; and a 4-wk posttreatment phase. Plasma samples were collected and analyzed weekly for naltrexone concentrations throughout the treatment and posttreatment phases. In addition, total of 6 h of video was analyzed per animal per phase of the study. Compared with baseline phases, both the frequency and the percentage of time spent displaying SIB decreased during the treatment phase, and the percentage of time remained decreased during the posttreatment phase. In contrast, extended-release naltrexone did not alter the expression of other abnormal, anxiety-related, or agonistic behaviors nor were levels of inactivity affected. The present study supports the use of naltrexone in the treatment of SIB in rhesus macaques.

Dose-finding study of fluoxetine and venlafaxine for the treatment of self-injurious and stereotypic behavior in rhesus macaques (Macaca mulatta)

The short-term effects on rates and durations of self-injurious behavior and self-directed stereotypies associated with various doses of fluoxetine (FLX) and venlafaxine (VEN) were examined in rhesus macaques. Adult male macaques (Macaca mulatta; n = 17; age, 7 to 15 y) with at least 1 episode of severe SIB within the past 5 y were randomized to treatment with FLX (n = 6), VEN (n = 6), or placebo (PLC, n = 5), administered by voluntary consumption of medication provided in fruit-flavored tablets. After 4-wk baseline and 4-wk placebo lead-in phases, doses were increased monthly for 4 mo (FLX: 0.5, 2.0, 4.0, and 8.0 mg/kg; VEN: 2.0, 4.0, 8.0, and 16.0 mg/kg). Animals in the PLC condition received similar nonmedicated fruit-flavored tablets. Focal behavioral observations, plasma drug levels, and neurochemical data were obtained. Results indicated that rates and percentage time spent self-biting declined at all doses of FLX, with the greatest effect seen at 2.0 mg/kg. For VEN, percentage time spent self-biting was significantly lower only at the 4.0 mg/kg dose. Treatment-induced reductions in platelet serotonin and cerebrospinal fluid 5-hydroxyindoleacetic acid (CSF 5HIAA) concentrations were substantially greater in the FLX-treated condition than in the VEN-treated condition. Plasma FLX and norfluoxetine levels increased with FLX dose; plasma levels of VEN were low and not dose-related. Fluoxetine at a dose of 2.0 mg/kg daily was most efficacious in reducing SIB, and the observed reductions in platelet serotonin and CSF 5HIAA levels indicated substantial bioeffect at this dose. Treatment with VEN was marked by noncompliance, low bioeffect, and low efficacy.

L-tryptophan and correlates of self-injurious behavior in small-eared bushbabies (Otolemur garnettii)

Self-injurious behavior (SIB) among captive primates is a recurring problem for those who manage such facilities. Its prevalence highlights the need for research evaluating the effectiveness of potential treatment approaches. In the present study, 4 wk of dietary supplementation with L-tryptophan (100 mg daily) was evaluated for the treatment of self-inflicted wounds in 22 small-eared bushbabies, a prosimian primate, with a history of SIB. The treatment significantly reduced stereotypy and was associated with a reduction in wound area and severity. In terms of physiologic measures, preexisting high levels of cortisol were reduced in bushbabies with SIB, whereas serotonin concentrations were increased after 4 wk of treatment. Results indicate that L-tryptophan as a dietary supplement may be a viable adjunct to standard husbandry procedures for animals exhibiting maladaptive behaviors such as stereotypy and SIB.