PAIRED-PULSE FACILITATION OF IPSCs IN SLICES OF IMMATURE AND MATURE MOUSE
SOMATOSENSORY NEOCORTEX.
Fleidervish, I. A., and M. J. Gutnick.
Department of Physiology and Brain Research Center, Faculty of Health
Sciences, Ben-Gurion University of the Negev, Beersheva 84105, Israel.
APStracts 2:0084N, 1995.
SUMMARY AND CONCLUSIONS
1. Whole-cell recordings from layer V neurons of mouse somatosensory cortex
were made using a "blind" patch clamp technique. In slices from
immature (P6-P11) and juvenile (P18-P21) animals, inhibitory postsynaptic
currents (IPSCs) were evoked in all cells by extracellular stimulation at the
layer V-VI border. Monosynaptic IPSCs, with latency <2 ms, were isolated
pharmacologically by blockade of ionotropic glutamatergic transmission. IPSCs
were blocked by bicuculline methiodide and reversed near the predicted
equilibrium potential for Cl-. 2. IPSC characteristics were not different for
the two age groups. At 1.5-2 X threshold intensity (0.2 Hz), they fluctuated
in amplitude with occasional failures. At -70 or -80 mV, mean amplitudes were
-202 +/- 20 pA (mean +/- SEM) and -207 +/- 32 pA for immature (39 cells) and
juvenile (13 cells) cortex, respectively. Half rise times were 0.74 ? +/- ?
0.03 ms (n ? = ? 7 cells) in neonates and 0.67 ? +/- ? 0.04 ms (n ? = ? 7
cells) in juveniles. Decays were biexponential with ? ? 1 = ? 14.8 ? +/- 1.3
ms and t 2 = 59.0 +/- 7.4 ms (n = 7 cells) in neonates, and t 1 ? = 11.9 ? +/-
? 1.1 ms and t 2 ? = ? 55.5 +/- 4.2 ms (n =7 cells) in juveniles. 3. Pairs of
stimuli elicited paired-pulse facilitation (PPF) when delivered at brief
inter-stimulus intervals (ISI), and paired-pulse depression (PPD) at long ISI.
PPF, which was evident in 64% of immature cells and 38% of juvenile cells, was maximal (38 +/- 4% greater than the conditioning response) at 20-40 ms. PPD,
which was evident in 82% of immature cells and 87% of juvenile cells, was
maximal (29 +/- 2% smaller than the conditioning response) by 300 ms. In each
age group, some animals showed PPF without PPD. 4. In 36% of immature cells
and 62% of juvenile cells, the paired-pulse response was characterized by an
initial, deep depression followed by rapid return towards the control level by
20-40 ms, and subsequent depression with a slower time course typical of PPD.
In 3 of these neurons, the effect of lowering release probability on the early
depression was examined by lowering Ca 2+ to 0.5 mM and raising Mg 2+ to 3.5
mM; in each case, the early depression was replaced by PPF. In a different 3
neurons, 2 pairs of stimuli were delivered such that the second pair fell
during the peak of the late PPD induced by the first pair; in each case, PPF
of the second pair was clearly revealed in this way, suggesting a presynaptic
mechanism for late PPD. 5. This sequence of early PPF and late PPD may impose
a time window for facilitation of IPSCs in neocortex, such that an inhibitory
interneurone will be maximally effective when it fires brief (<300
msec) bursts of action potentials at intraburst frequencies greater than 25-50
Hz.
Received 9 February 1994; accepted in final form 16 March 1995.
APS Manuscript Number J87-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 19 April 1995.