MK-0677 and exogenous GH raise serum IGF-1 but leave CSF IGF-1 and GH unchanged in dogs
Background
Growth hormone (GH) and insulin-like growth factor-1 (IGF-1) are key anabolic hormones with well-established peripheral signaling roles, yet their presence and regulation within the central nervous system (CNS) remain poorly understood. Circulating GH drives dose-related increases in serum IGF-1 in dogs, but whether these systemic elevations translate to changes in cerebrospinal fluid (CSF) concentrations was entirely unknown prior to this work. Understanding this compartmental relationship matters because CNS-derived GH and IGF-1 may serve distinct neurotrophic and neuroendocrine roles independent of peripheral signaling. The blood-brain barrier (BBB) is known to restrict passage of many large molecules, but formal demonstration of BBB exclusion for GH and IGF-1 in a living animal model had not been established. GHSR agonism via MK-0677 — a potent oral GH secretagogue — offered a pharmacological tool to push systemic GH and IGF-1 to supraphysiological levels and probe whether the CNS compartment responds.
Study Design
12 normal young adult male dogs were randomized by body weight into 3 groups (n = 4 each). Group 1 received vehicle control. Group 2 received MK-0677 (5 mg/kg/day) orally in deionized water. Group 3 received porcine GH (pGH) at 0.1 IU/kg/day via subcutaneous injection. Blood and CSF samples were collected at baseline and on days 7 and 15 of treatment. All samples were analyzed using a validated radioimmunoassay (RIA) for both IGF-1 and GH. Primary endpoints were serum and CSF concentrations of IGF-1 and GH, plus body weight gain across the treatment period.
Results
Administration of both MK-0677 and pGH produced statistically significant (P ≤ 0.05) increases in body weight gain and in serum IGF-1 and GH levels relative to controls. Despite these robust peripheral elevations, neither treatment produced a significant (P > 0.05) increase in CSF IGF-1 or GH on either day 7 or day 15. CSF IGF-1 values ranged from 1.2 to 2.0 ng/ml across all dogs with minimal variation between the three serial samples per dog, irrespective of treatment group. CSF GH levels were very low across all groups, ranging from < 0.98 ng/ml to 2.4 ng/ml, and again showed no treatment-related changes.
Serum IGF-1 in vehicle control dogs was approximately 100-fold higher than CSF IGF-1 levels, directly demonstrating a blood-brain barrier for circulating IGF-1 in normal dogs.
The stability of CSF IGF-1 and GH across vehicle controls throughout the study period further confirms that the CNS compartment maintains tightly regulated, autonomous hormone concentrations independent of peripheral GH/IGF-1 axis status.
Key Findings
- MK-0677 (5 mg/kg/day oral) and pGH (0.1 IU/kg/day SC) both significantly raised serum IGF-1 and GH (P ≤ 0.05)
- CSF IGF-1 remained stable at 1.2–2.0 ng/ml across all groups and time points regardless of treatment
- CSF GH levels stayed very low (< 0.98 to 2.4 ng/ml) in all dogs with no treatment effect (P > 0.05)
- Serum IGF-1 was approximately 100-fold higher than CSF IGF-1 in vehicle control dogs
- Both MK-0677 and pGH administration produced significant body weight gain (P ≤ 0.05)
Why It Matters
Systemic GH secretagogue use — including MK-0677 — does not elevate CNS IGF-1 or GH levels, which has direct implications for researchers and clinicians who hypothesize that peripheral GH/IGF-1 elevation might produce CNS-mediated benefits such as neuroprotection or cognitive enhancement. Any such CNS effects would need to arise through indirect mechanisms rather than direct hormonal transport across the blood-brain barrier. Protocols stacking MK-0677 with GH for purported central nervous system effects should not assume CSF hormone changes as the mechanism. The finding that CSF GH and IGF-1 are likely of CNS origin suggests the GH/IGF-1 axis in the brain is functionally decoupled from the periphery, which has broader implications for interpreting neurological studies of these hormones. Clinical translation of GH secretagogues for CNS indications will require entirely different mechanistic rationales and CNS-penetrant delivery strategies.
TitrateLab articles covering MK-677
Written by TitrateLab, separately from the study summarized above.
