Enhanced D1 and D2 inhibitions induced by low-frequency trains of conditioning stimuli: differential effects on H and T-reflexes and possible mechanisms

• Grant number: 15/05133-4
• Support Opportunities: Regular Research Grants - Publications - Scientific article
• Field of knowledge: Engineering - Biomedical Engineering - Bioengineering
• Principal Investigator: André Fábio Kohn
• Grantee: André Fábio Kohn
• Host Institution: Escola Politécnica (EP). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

Mechanically evoked reflexes have been postulated to be less sensitive topresynaptic inhibition (PSI) than the H-reflex. This has implications on investigations ofspinal cord neurophysiology that are based on the T-reflex. Preceding studies have shownan enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1Hzwas applied to the nerve of the antagonist muscle. The main questions to be addressed inthe present study are if indeed T-reflexes are less sensitive to PSI and whether (and towhat extent and by what possible mechanisms) the effect of low frequency conditioning,found previously for the H-reflex, can be reproduced on T-reflexes from the soleusmuscle. We explored two different conditioning-to-test (C-T) intervals: 15 and 100 ms(corresponding to D1 and D2 inhibitions, respectively). Test stimuli consisted of eitherelectrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanicalpercussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioningelectrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSIas compared to a single conditioning pulse, for both reflexes (T and H), regardless of CT-intervals. Moreover, the conditioning train of pulses (with respect to a singleconditioning pulse) was proportionally more effective for T-reflexes as compared to Hreflexes(irrespective of the C-T interval), which might be associated with the differentialcontingent of Ia afferents activated by mechanical and electrical test stimuli. Aconceivable explanation for the enhanced PSI effect in response to a train of stimuli is theoccurrence of homosynaptic depression in inhibitory interneurons interposed in the PSIpathway. The present results add to the discussion of the sensitivity of T-reflexes to PSIand its functional role. (AU)