V. Tonkovic-capin et al., Differential modulation of respiratory neuronal discharge patterns by GABA(A) receptor and apamin-sensitive K+ channel antagonism, J NEUROPHYS, 86(5), 2001, pp. 2363-2373
The discharge patterns of respiratory neurons of the caudal ventral respira
tory group (cVRG) appear to be subject to potent GABAergic gain modulation.
Local application of the GABA(A) receptor antagonist bicuculline methochlo
ride amplifies the underlying discharge frequency (F-n) patterns mediated b
y endogenous excitatory and inhibitory synaptic inputs. Gain modulation can
also be produced by alterations in the amplitude of spike afterhyperpolari
zations (AHPs) mediated by apamin-sensitive small-conductance Ca2+-activate
d K+ (SK) channels. Since methyl derivatives of bicuculline (BICm) also hav
e been shown to reduce the amplitude of AHPs, in vitro, it is possible that
the BICm-induced gain modulation is due to a block of SK channels. The pur
pose of these studies was to determine the mechanisms by which BICm produce
s gain modulation and to characterize the influence of SK channels in the c
ontrol of respiratory neuron discharge. Six protocols were used in this in
vivo study of cVRG inspiratory (I) and expiratory (E) neurons in decerebrat
e, paralyzed, ventilated dogs. The protocols included characterizations of
the neuronal responses to 1) BICm and apamin on the same neuron, 2) BICm du
ring maximum apamin-induced block of AHPs, 3) apamin during maximum BICm-in
duced gain modulatory responses, 4) the specific GABA(A) receptor antagonis
t, (1) beta -hydrastine, 5) the specific GABA(A) receptor agonist, muscimol
, and 6) the GABA uptake inhibitor, nipecotic acid. For protocols 3, 5, and
6, only E neurons were studied. Four-barrel micropipettes were used for ex
tracellular single neuron recording and pressure ejection of drugs. Cycle-t
riggered histograms were used to quantify the F-n patterns and to determine
the drug-induced changes in the gain (slope) and offset of the F-n pattern
s. Compared to apamin at maximum effective dose rates, BICm produced a 2.1-
fold greater increase in peak F-n and a 3.1-fold greater increase in averag
e F-n. BICm and apamin produced similar increases in gain, but the offsets
due to apamin were more negative. The responses to hydrastine were similar
to BICm. During maximum apamin block, BICm produced an additional 112 +/- 2
2% increase in peak F-n. Conversely, apamin produced an additional 176 +/-
74% increase in peak F-n during the maximum BICm-induced response. Muscimol
and nipecotic acid both decreased the gain and offset of the discharge pat
terns. Taken together, these results suggest that the gain modulatory effec
t of BICm is due to a reduction of GABA(A)-ergic shunting inhibition rather
than a reduction in AHPs by block of SK channels in canine cVRG neurons.