In the mammalian cortex, it is generally assumed that the output information of neurons is encoded in the number and the timing of action potentials. Here, we show, by using direct patchclamp recordings from presynaptic hippocampal mossy fiber boutons, that axons transmit analog signals in addition to action potentials. Excitatory presynaptic potentials result from subthreshold dendritic synaptic inputs, which propagate several hundreds of micrometers along the axon and modulate action potential-evoked transmitter release at the mossy fiber-CA3 synapse. This combined analog and action potential coding represents an additional mechanism for information transmission in a major hippocampal pathway.

Action potentials in nonmyelinated axons are considered to contribute substantially to activity-dependent brain metabolism. Here we show that fast Na+ current decay and delayed K+ current onset during action potentials in nonmyelinated mossy fibers of the rat hippocampus minimize the overlap of their respective ion fluxes. This results in total Na+ influx and associated energy demand per action potential of only 1.3 times the theoretical minimum, in contrast to the factor of 4 used in previous energy budget calculations for neural activity. Analysis of ionic conductance parameters revealed that the properties of Na+ and K+ channels are matched to make axonal action potentials energy-efficient, minimizing their contribution to activity-dependent metabolism.

The mossy fiber-CA3 pyramidal neuron synapse is a main component of the hippocampal trisynaptic circuitry. Recent studies, however, suggested that inhibitory interneurons are the major targets of the mossy fiber system. To study the regulation of mossy fiber-interneuron excitation, we examined unitary and compound excitatory postsynaptic currents in dentate gyrus basket cells, evoked by paired recording between granule and basket cells or extracellular stimulation of mossy fiber collaterals. The application of an associative high-frequency stimulation paradigm induced posttetanic potentiation (PTP) followed by homosynaptic longterm potentiation (LTP). Analysis of numbers of failures, coefficient of variation, and paired-pulse modulation indicated that both PTP and LTP were expressed presynaptically. The Ca 2؉ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N ,N -tetraacetic acid (BAPTA) did not affect PTP or LTP at a concentration of 10 mM but attenuated LTP at a concentration of 30 mM. Both forskolin, an adenylyl cyclase activator, and phorbolester diacetate, a protein kinase C stimulator, lead to a long-lasting increase in excitatory postsynaptic current amplitude. H-89, a protein kinase A inhibitor, and bisindolylmaleimide, a protein kinase C antagonist, reduced PTP, whereas only bisindolylmaleimide reduced LTP. These results may suggest a differential contribution of protein kinase A and C pathways to mossy fiber-interneuron plasticity. Interneuron PTP and LTP may provide mechanisms to maintain the balance between synaptic excitation of interneurons and that of principal neurons in the dentate gyrus-CA3 network.T he ␥-aminobutyric acid-ergic inhibitory interneurons control the activity of cortical neuronal networks by feedback and feed-forward inhibition (1), synchronize the collective activity of principal neuron ensembles (2), and regulate plasticity at glutamatergic synapses between principal neurons (3). Inhibition mediated by a single interneuron can be very powerful because of the efficacy of interneuron output synapses (4) and the large number of postsynaptic target cells innervated (1).The strength of interneuron excitation at principal neuroninterneuron synapses is a key factor for maintaining the balance between excitation and inhibition in cortical neuronal networks (5-9). However, the mechanisms that regulate the efficacy of these synapses are not understood. Although recent studies indicated that principal neuron-interneuron synapses are capable of plastic changes in synaptic strength, the direction of plasticity and the possible dependence on interneuron subtype, temperature, brain region, and stimulation paradigm remains highly controversial (10-21).In the hippocampal mossy fiber system, synapses on interneurons are much more abundant than those on principal cells (22), which further emphasizes the importance of synaptic interneuron excitation and plasticity. At mossy fiber synapses on CA3 pyramidal neurons, a high-frequency stimulation paradigm (HFS) induces various forms of enhancement of synapti...

Presynaptic ionotropic GABA A receptors have been suggested to contribute to the regulation of cortical glutamatergic synaptic transmission. Here, we analyzed presynaptic GABA A receptor-mediated currents (34°C) recorded from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs from young and adult animals, GABA puff application activated currents that were blocked by GABA A receptor antagonists. The conductance density of 0.65 mS ⅐ cm 2 was comparable to that of other presynaptic terminals. The singlechannel conductance was 36 pS (symmetrical chloride), yielding an estimated GABA A receptor density of 20 -200 receptors per MFB. Presynaptic GABA A receptors likely contain ␣ 2 -subunits as indicated by their zolpidem sensitivity. In accordance with the low apparent GABA affinity (EC 50 ϭ 60 M) of the receptors and a tight control of ambient GABA concentration by GABA transporters, no tonic background activation of presynaptic GABA A receptors was observed. Instead, extracellular high-frequency stimulation led to transient presynaptic currents, which were blocked by GABA A receptor antagonists but were enhanced by block of GAT 1 (GABA transporter 1), indicating that these currents were generated by GABA spill-over and subsequent presynaptic GABA A receptor activation. Presynaptic spill-over currents were depressed by pharmacological cannabinoid 1 (CB1) receptor activation, suggesting that GABA was released predominantly by a CB1 receptor-expressing interneuron subpopulation. Because GABA A receptors in axons are considered to act depolarizing, high activity of CB1 receptor-expressing interneurons will exert substantial impact on presynaptic membrane potential, thus modulating action potential-evoked transmitter release at the mossy fiber-CA3 synapse.

Presynaptic elements of axons, in which action potentials (APs) cause release of neurotransmitter, are sites of high densities and complex interactions of proteins. We report that the presence of K v 3 channels in addition to K v 1 at glutamatergic mossy fiber boutons (MFBs) in rat hippocampal slices considerably limits the number of fast, voltage-activated potassium channels necessary to achieve basal presynaptic AP repolarization. The ϳ10-fold higher repolarization efficacy per K v 3 channel compared with presynaptic K v 1 results from a higher steady-state availability at rest, a better recruitment by the presynaptic AP as a result of faster activation kinetics, and a larger single-channel conductance. Large-conductance calcium-and voltage-activated potassium channels (BK Ca ) at MFBs give rise to a fast activating/fast inactivating and a slowly activating/sustained K ϩ current component during long depolarizations. However, BK Ca contribute to MFB-AP repolarization only after presynaptic K v 3 have been disabled. The calcium chelators EGTA and BAPTA are equally effective in preventing BK Ca activation, suggesting that BK Ca are not organized in nanodomain complexes with presynaptic voltage-gated calcium channels. Thus, the functional properties of K v 3 channels at MFBs are tuned to both promote brevity of presynaptic APs limiting glutamate release and at the same time keep surface protein density of potassium channels low. Presynaptic BK Ca channels are restricted to limit additional increases of the AP half-duration in case of K v 3 hypofunction, because rapid membrane repolarization by K v 3 combined with distant calcium sources prevent BK Ca activation during basal APs.

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Transcranial magnetic stimulation (TMS) allows the testing of various inhibitory processes in human motor cortex. Here we aimed at gaining more insight into the underlying physiology by studying the interactions between short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI). SICI and SAI were examined in a slightly contracting hand muscle of healthy subjects by measuring inhibition of a test motor-evoked potential conditioned by a sub-threshold motor cortical magnetic pulse (S1) or an electrical pulse (P) applied to the ulnar nerve at the wrist, respectively. SICI alone and SAI alone had similar magnitude when S1 intensity was set to 90% active motor threshold and P intensity to three times the perceptual sensory threshold. SICI was reduced or even disinhibited when P was co-applied, and SAI was reduced or disinhibited when S1 was co-applied. These interactions did not depend on the exact timing of arrival of P and S1 in motor cortex. A control experiment with a S1 intensity lowered to 70% active motor threshold excluded a contribution by short-interval intracortical facilitation. Finally, SICI with co-applied P correlated linearly with SICI alone with a slope of the regression line close to 1 whereas SAI did not correlate with SAI when S1 was co-applied with a slope of the regression line close to zero. Data indicate that S1 largely eliminates the effects of P when applied together, suggesting dominance of S1 over P. Findings strongly support the idea that SICI and SAI are mediated through two distinct and reciprocally connected subtypes of GABAergic inhibitory interneurons with convergent projections onto the corticospinal neurons. Furthermore, dominance of S1 over P is compatible with the notion that the SICI interneurons target the corticospinal neurons closer to their axon initial segment than the SAI interneurons. Abbreviations ADM, abductor digiti minimi; AMT, active motor threshold; M1, muscarinic type 1; MEP, motor-evoked potential; P, conditioning electrical stimulus applied to the ulnar nerve for testing short-latency afferent inhibition; S1, sub-threshold conditioning magnetic pulse for testing short-interval intracortical inhibition; S2, supra-threshold test pulse for eliciting a test motor-evoked potential; SAI, short-latency afferent inhibition; SICI, short-interval intracortical inhibition; TMS, transcranial magnetic stimulation.

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