Fen. Lebeau et al., CONTRIBUTION OF GABA-MEDIATED AND GLYCINE-MEDIATED INHIBITION TO THE MONAURAL TEMPORAL RESPONSE PROPERTIES OF NEURONS IN THE INFERIOR COLLICULUS, Journal of neurophysiology, 75(2), 1996, pp. 902-919
1. To determine the contribution of inhibition to the generation of th
e temporal response patterns of neurons in the inferior colliculus (IC
), the effects of iontophoretically applied gamma-aminobutyric acid (G
ABA), glycine, and the GABA(A) and glycine receptor antagonists, bicuc
ulline and strychnine were studied on 121 neurons in the IC of urethan
-anesthetised guinea pig. 2. The neurons temporal discharge patterns w
ere classified into six categories on the basis of their peristimulus
time histograms (PSTHs). 1) Onset units fired at the stimulus onset an
d could be divided into two subtypes: narrow (1-2 spikes only) or broa
d (response lasting up to similar to 30 ms). 2) Pauser units had a pre
cisely timed onset peak separated from a lower level of sustained acti
vity by either a marked reduction or complete cessation of firing. 3)
Chopper units had three or more clearly defined peaks near stimulus on
set or evidence of regularly spaced peaks over the duration of the sti
mulus. 4) Onset-chopper units had three clearly defined peaks at onset
but no sustained firing. 5) On-sustained units had a clearly defined
single onset peak followed by a lower level of sustained activity. 6)
Sustained units fired throughout the stimulus, but lacked an onset pea
k. 3. Iontophoretic application of GABA and glycine produced a dose-de
pendent reduction in firing rate in 76% (42/55) and 79% (11/14) of uni
ts, respectively. Application of bicuculline or strychnine increased t
he discharge rate in 91% (64/70) and 94% (16/17) of neurons, respectiv
ely. 4. The effects of bicuculline and strychnine on PSTH class were s
tudied in detail on 70 neurons. Changes in discharge rate were accompa
nied by changes in PSTH in 49% (34/70) of neurons tested with bicucull
ine and 41% (7/17) tested with strychnine. Pauser units were the most
affected with 69% changing their PSTH class, but some units in all PST
H classes, except the chopper group, exhibited changes in PSTH pattern
after application of bicuculline. The majority of units (similar to 5
0%) that changed PSTH pattern in the presence of bicuculline became ch
opper units. Units of all PSTH classes could become choppers, but the
proportion of units showing this change was dependent on the unit's co
ntrol response pattern. All seven units that changed PSTH class with s
trychnine also became choppers. Changes in PSTH, including the appeara
nce of a chopper pattern, did not depend on either a unit's control di
scharge rate or the magnitude of the change in discharge rate induced
by the antagonists. 5. Bicuculline and strychnine had no significant e
ffect on latency for units in the chopper, onset-chopper, onset, pause
r, and on-sustained groups. A few sustained and unclassified units tha
t had long predrug latencies did show marked reductions in latency whe
n tested with bicuculline. 6. The majority of units did not fire spont
aneously, and neither bicuculline or strychnine produced a significant
increase in spontaneous rate. 7. In many units, the changes in firing
rate did not occur equally over the duration of the response. Firing
rates at the onset and in the last quarter of the sustained response w
ere compared. Three effects of bicuculline and strychnine were observe
d. For 80% of units the largest change in firing rate occurred in the
sustained response, while in 14% of units the change was greatest at o
nset. The change in onset and sustained firing rate for the remaining
6% of units was equal. The different effects of the antagonists on the
onset and sustained components of the response varied with the unit's
control PSTH type. This suggests that there are differences in the ti
ming and strength of the inhibitory inputs impinging on different neur
ons. 8. Histological verification of recording sites was possible for
59/70 units of which 68% were located in the central nucleus of the IC
(CNIC). All the reported effects occurred in the CNIC, but similar ch
anges also were seen in the dorsal cortex and external cortex of the I
C. 9. We conclude that GABAergic and glycinergic inhibition exerts a s
trong influence on the temporal firing pattern of collicular units. Af
ter the application of either bicuculline or strychnine, the majority
of units become choppers, showing that many IC neurons have the capaci
ty to fire with a regular discharge pattern. These inhibitory effects
may be mediated both by temporal variations in the interaction of exci
tatory and inhibitory inputs, or by the influence of these inputs on s
pecific membrane conductances.