Antidromically-conditioned E-S potentiation is not blocked by GABA A inhibitors

Campbell, L. W., Jester, J. M., Sejnowski, T. J.
Society for Neuroscience Abstracts, 19:1328 (1993)

ABSTRACT

Long-term potentiation (LTP) of the population spike in excess of that predicted by increase in the slope of the field excitatory post-synaptic potential (EPSP) has been described as a component of theta-burst LTP (Exp Brain Res 79:633-641). An increase in the ratio of excitation to inhibition, a reduction of tonic inhibition, and a decrease in spike threshold are thought to contribute to this EPSP-to-spike (E-S) potentiation. Associative LTP using antidromic stimulation as the conditioning stimulus produces a novel kind of E-S potentiation: a stable increase in the population spike amplitude, but no change in the slope of the EPSP (Soc Neuro Abstr 17:533.15). Do changes in GABA mediated synaptic inhibition underlie the associative form of E-S potentiation as they are thought to do in theta-burst E-S potentiation?

The EPSP and population spike were recorded from the CA1 layer of rat hippocampal slices. The antidromic conditioning stimulus was 50 bursts of 5 pulses at 100 Hz with an interburst interval of 200 ms delivered to the alveus. The Schaffer-collateral pathway was stimulated once per antidromic burst. In some slices the GABAA blockers picrotoxin (10 M) and/or bicuculline (10 M) were added to the bath prior to testing. When paired together, the antidromic and orthodromic stimuli produced a potentiation of the population spike, even in the presence of the GABAA blockers (142.6% ń 13.8; mean ń s.e.m). In contrast, the E-S potentiation component of theta-burst LTP is blocked by GABAA inhibitors.

Intracellular recordings from the CA1 layer reveal an increase in excitability following associative E-S potentiation. Paired t-tests suggest that a depolarization of the resting membrane potential (RMP) accounts for the increased excitability (baseline -63.8 ń 0.8 mV, 15 min. post tetanus -61.2 mV ń 1.1; mean RMP ń s.e.m, P = 0.04, F1 = 9.08). Spike threshold, input resistance, and time constant of the membrane were not statistically affected by associative E-S potentiation.

ELECTRODE PLACEMENT

PARADIGM


LTP	Associative E-S potentation
Potentiation of population spike usually develops over a period of several minutes and in some cases up to 60 minutes	Potentiation of population spike is often immediate
Slope of EPSP is potentiated	Slope of EPSP is not potentiated

GABAa INHIBITORS

Population spike potentiation is similar in control slices and slices treated with the GABAA inhibitors picrotoxin and/or bicuculline
Slope of the EPSP is also similar in control and GABAA inhibited slices

GABAa statistics
	control mean ą s.e.m	n		GABAa inhibited mean ą s.e.m	n	P	T
Population spike peak % potentiation	136.5 ą 17.6	22		142.6 ą 13.8	19	.78	-.28
EPSP slope peak % potentiation	96.2 ą 2.2	22		99.8 ą 1.5	19	.17	-1.42

Example graph showing similar associative E-S potentiation in a control (left) and a bicuculline-treated (right) slice.

INTRACELLULAR EXCITABILITY

Intracellular input-output (I/O) curves for injected current were performed before, 5 minutes after, and 15 minutes after applying the paired stimulation that produces E-S potentiation.
Graphed above are Na spike counts, from one cell, calculated from I/O curves before (upper) and 15 minutes after (lower) paired tetanization
A two-tailed T-test was used to compare the total number of action potentials generated across the entire I/O curve at baseline versus the 5 and 15 minute I/O curves. One cell lacked a 15 minute I/O curve

	n	# AP's mean ą s.e.m	T	P
baseline	6	10.8 ą 2.4	-.75	.49
5 min		12.6 ą 2.9
baseline	5	10.8 ą 2.9	-3.4	0.3
15 min.		15.0 ą 3.9

RESTING MEMBRANE POTENTIAL

Resting membrane potential
	n	RMP (mV) mean ą s.e.m	T	P
control	6	-63.8 ą .7	-2.2	.08
5 min.		-62.2 ą .9
control	5	-63.8 ą .8	-3.0	.04
15 min.		-61.2 ą 1.1

Preliminary intracellular recordings show a depolarization of the resting membrane potential at 15 minutes. The depolarization is less pronounced at 5 than at 15 minutes post tetanus.

SPIKE THRESHOLD AND IR

Spike threshold and input resistance are not significantly affected by associative E-S potentiation. Spike threshold was calculated from I/O curves (shown below). Input resistance was measured before the tetanus and between 5 and 15 minutes after the tetanus

SUMMARY

The GABAA inhibitors picrotoxin and bicuculline do not block or attenuate associative E S potentiation.
Somal intracellular recordings detect an increase in Na spike generation during depolarizing constant current injections. The increase is statistically significant 15 minutes, but not 5 minutes, after the paired tetanus.
The resting membrane potential depolarizes following the paired tetanus. The depolarization is statistically significant 15 minutes, but not 5 minutes, after the tetanus.
Spike Threshold and input resistance are not affected by E-S potentiation.

CONCLUSIONS

Associative E-S potentiation is not blocked by GABAA inhibitors, unlike the E-S component of LTP
The excitability increase seen at the soma during associative E-S potentiation may be explained by a depolarization of the resting membrane potential, but probably not by changes in Na spike threshold or input resistance