Enhanced D 1 and D 2 Inhibitions Induced by Low-Frequency Trains of Conditioning Stimuli : Differential Effects on Hand T-Reflexes and Possible Mechanisms

Mechanically evoked reflexes have been postulated to be less sensitive to presynaptic inhibition (PSI) than the H-reflex. This has implications on investigations of spinal cord neurophysiology that are based on the T-reflex. Preceding studies have shown an enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1 Hz was applied to the nerve of the antagonist muscle. The main questions to be addressed in the present study are if indeed T-reflexes are less sensitive to PSI and whether (and to what 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 soleus muscle. 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 either electrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanical percussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioning electrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSI as compared to a single conditioning pulse, for both reflexes (T and H), regardless of C-T-intervals. Moreover, the conditioning train of pulses (with respect to a single conditioning pulse) was proportionally more effective for Treflexes as compared to H-reflexes (irrespective of the C-T interval), which might be associated with the differential contingent of Ia afferents activated by mechanical and electrical test stimuli. A conceivable explanation for the enhanced PSI effect in response to a train of stimuli is the occurrence of homosynaptic depression at synapses on inhibitory interneurons interposed within the PSI pathway. The present results add to the discussion of the sensitivity of the stretch reflex pathway to PSI and its functional role. PLOS ONE | DOI:10.1371/journal.pone. 0121496 March 25, 2015 1 / 16 OPEN ACCESS Citation: Mezzarane RA, Magalhães FH, Chaud VM, Elias LA, Kohn AF (2015) Enhanced D1 and D2 Inhibitions Induced by Low-Frequency Trains of Conditioning Stimuli: Differential Effects on Hand TReflexes and Possible Mechanisms. PLoS ONE 10 (3): e0121496. doi:10.1371/journal.pone. 0121496 Academic Editor: Shawn Hochman, Emory University, UNITED STATES Received: August 19, 2014 Accepted: February 1, 2015 Published: March 25, 2015 Copyright: © 2015 Mezzarane et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All data files are available from the figshare database: http://dx.doi. org/ 10.6084/m9.figshare. 1300992. Funding: RAM was supported by grants from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (476830/2013-3) (http://www. cnpq.br/), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (http://www. capes.gov.br/), and a fellowship from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (2010/15522-4) (http://www.fapesp.br/). FHM was a recipient of a fellowship from FAPESP