A STUDY OF THE BARIUM-SENSITIVE AND BARIUM-INSENSITIVE COMPONENTS OF THE ACTION OF THYROTROPIN-RELEASING-HORMONE ON LUMBAR MOTONEURONS OF THE RAT ISOLATED SPINAL-CORD
Nd. Fisher et A. Nistri, A STUDY OF THE BARIUM-SENSITIVE AND BARIUM-INSENSITIVE COMPONENTS OF THE ACTION OF THYROTROPIN-RELEASING-HORMONE ON LUMBAR MOTONEURONS OF THE RAT ISOLATED SPINAL-CORD, European journal of neuroscience, 5(10), 1993, pp. 1360-1369
The electrophysiological action of thyrotropin-releasing hormone (TRH)
on rat spinal motoneurons was studied in vitro using single-electrode
voltage- and current-clamp techniques. In current-clamp conditions TR
H elicited a slowly developing depolarization, associated with a large
input resistance increase and sustained neuronal firing; the primary
metabolites of TRH were ineffective. Under voltage-clamp conditions in
the presence of tetrodotoxin, TRH evoked a large inward current (I(TR
H); peaking at approximately -40 mV) associated with a large input con
ductance fall. Only 44% of cells displayed I(TRH) reversal; when the c
hord conductance values of these cells were plotted against membrane p
otential, a bell-shaped relation occurred, indicating voltage-dependen
t block by TRH of a persistent conductance active over a wide range of
membrane potentials. I(TRH) reversal values were shifted to more posi
tive levels in high K+ solution in Nernstian fashion; hence a large pr
oportion of the TRH response is suggested to be mediated by the block
of a K+ conductance (I(K(T))). I(K(T)) (and its voltage-dependent bloc
k by TRH) was resistant to certain K+ channel antagonists (tetraethyla
mmonium, Cs+, 4-aminopyridine or apamin), but was depressed by Ba2+. T
he Ba2+-resistant fraction of I(TRH) was attenuated by Cd2+, Mn2+ or C
o2+, indicating that it probably involved a Ca2+-sensitive inward curr
ent. Concomitant application of Ba2+ and Cd2+ induced a near-total blo
ck of the response to TRH. It is suggested that suppression of I(K(T))
, associated with the onset of a Ca2+-sensitive current, can explain t
he excitatory effect of TRH on rat spinal motoneurons.