Aging and the hypothalamo-pituitary-testicular axis in the rat

Low-Dose Augmentation With Buprenorphine for Treatment-Resistant Depression: A Multisite Randomized Controlled Trial With Multimodal Assessment of Target Engagement

Background

The experimental therapeutics approach that combines a placebo-controlled clinical trial with translational neuroscience methods can provide a better understanding of both the clinical and physiological effects of pharmacotherapy. We aimed to test the efficacy and tolerability of low-dose augmentation with buprenorphine (BPN) for treatment-resistant depression, combined with multimodal assessment of target engagement.

Methods

In this multisite randomized clinical trial, 85 participants ≥50 years of age with a major depressive episode that had not responded to venlafaxine extended release were randomized to augmentation with BPN or placebo for 8 weeks. The primary outcome measure was the Montgomery-Åsberg Depression Rating Scale. In addition, three linked experiments were conducted to test target engagement: 1) functional magnetic resonance imaging using the monetary incentive delay task, 2) brain positron emission tomography of healthy participants using a novel kappa opioid receptor antagonist tracer [¹¹C]LY2795050, and 3) transcranial magnetic stimulation measure of cortical transmission after daily BPN administration.

Results

The mean ± SD dosage of BPN was 0.59 ± 0.33 mg/day. There were no significant differences between the BPN and placebo groups in Montgomery-Åsberg Depression Rating Scale changes over time or adverse effects. BPN administration had minimal effects on functional magnetic resonance imaging blood oxygen level-dependent responses in regions involved in reward anticipation and response, no significant displacement of kappa opioid receptor radioligand in positron emission tomography imaging, and no significant changes in transcranial magnetic stimulation measures of inhibitory and excitatory cortical transmission.

Conclusions

Our findings suggest a lack of clinical effect of low-dose BPN augmentation and lack of target engagement with this dosage and physiological probes.

Effects of hypothalamic peptides on the aging brain

The hypothalamic peptide hormones, TRH, LHRH (GnRH), CRH, GHRH, and GHIRH (somatostatin), influence the release of the anterior pituitary hormones, which in turn promote the release of target endocrine gland hormones and other metabolites. These latter compounds feed back to the brain to help control the secretion of the hypothalamic hormones. This is a dynamic interaction that is influenced by the aging process: Most of these hormones systems become less responsive with advancing age, due to decreased function of peptide-containing secretory neurons, a loss of hormone receptor sensitivity, and/or a reduction in the output of the target endocrine glands. That the hypothalamic peptides themselves can influence brain function is supported by the fact that most are found in areas of the brain other than the hypothalamus and that receptors for them exist in these other areas. For example, CRH is contained in a number of central neural systems that can influence behavior, including limbic areas, the hypothalamus, locus coeruleus, median raphé nuclei, and cortical interneurons. CRH has been shown to be anxiogenic in animal models, and its effect can be blocked by CRH receptor antagonists. CRH content in the locus coeruleus is particularly increased by stress and may influence norepinephrine neurotransmitter function in this structure. In aging there is a gradual reduction of the sensitivity of the brain to the negative feedback of corticosteroids, such that CRH secretion becomes somewhat increased under basal conditions. The behavioral effects of this change are unclear, however, as is the influence of stress-related activation of CRH, ACTH, and glucocorticoid secretion on behavior in the elderly. Other hypothalamic peptides have different patterns of change with aging, and some are markedly altered in pathological conditions; for example, in Alzheimer's disease the content of CRH and somatostatin in certain brain areas is decreased. However, whether the changes in hypothalamic peptides precede or follow the pathological behavioral changes, and how they participate in the changes, is still unclear.

Hormones and sexual behavior in relationship to aging in male rats

To determine if the age-related decline in male sex behavior is correlated with hormonal factors, a longitudinal study was conducted. Sexually experienced males were given mating tests every 2 months from 7 through 27 months of age. To study possible relationships between changes in behavior and alterations in hormone levels, blood samples were taken before and after these bimonthly tests. At 23 months, cross-sectional studies were also conducted comparing results to those obtained in 5-month-old males. Significant changes in mating behavior first appeared at 11 months; mount latency, intromission latency, ejaculation latency, postejaculatory interval, and intercopulatory interval were increased. Similarly, detectable decreases in testosterone (T) also occurred at this age. A significant decline in luteinizing hormone (LH) was not seen until 19 months. Correlational analyses revealed small (r less than or equal to -0.29) but significant negative correlations between T and parameters of mating behavior with age. When each age was examined separately, no significant correlations appeared. Plasma T was not predictive of behavioral performance. At 23 months, cross-sectional studies revealed deficits in mounting and penile reflex behavior but ejaculatory reflex capacity was unimpaired. At 28 months, males were decapitated. Only T levels showed a significant effect of age; estradiol, prolactin, and LH were unaffected when compared to 5-month-old males. The data suggest that although there are small and significant negative correlations between circulating testosterone and parameters of mating behavior with advancing age, it is unlikely that the observed decline in testosterone is the primary cause of the age-induced behavioral deficits. It is likely that the major causal factor(s) involves non-hormone-dependent changes within the CNS.