Abstracts

RATIONALE: Cortical interneuronal activity plays important roles in epileptogenesis, and in generation of network oscillations that may underlie several brain functions. Reductions in GABAergic output from interneurons, resulting from increased inhibition or decreased excitation of these cells, might be an important factor in the development or spread of epileptogenic discharge in cortical networks. There is extensive morphological evidence that neocortical interneurons can synapse on themselves, however a functional role for such autaptic innervation has not been established, raising the possibility that these anatomically-defined autapses are nonfunctional. We therefore tested the hypothesis that autaptic contacts on interneurons are functional and that their activity can modulate interneuronal excitability.
METHODS: We obtained whole cell recordings from fast spiking (FS) and low threshold spike (LTS) layer V interneurons in sensorimotor cortical slices from P13-20 rats. Cells were identified as multipolar neurons lacking apical dendrites, and electrophysiologically characterized in current-clamp.
RESULTS: Brief command steps applied to voltage-clamped FS neurons elicited an escaped action current followed by an inward current that proved to be GABAergic, as it was completely and reversibly blocked by 10 [mu]M gabazine, a GABAA receptor blocker, and was enhanced by the GABAA receptor modulator clonazepam. Application of either extracellular Cd2+ or intracellular BAPTA, two manipulations known to prevent the release of neurotransmitter from presynaptic terminals, blocked these GABAergic currents, thus indicating their synaptic origin. Autaptic GABAergic responses were common in FS interneurons (48 of 57 cells; 84.1%), highly reliable (failure rate = 0.03[plusminus]0.001; n = 20) and large in size (peak current amplitude = -352.3[plusminus]70.9 pA; n = 20). In contrast, no action-current evoked, gabazine-sensitive response was detectable any of 25 recorded LTS interneurons, indicating that this interneuronal subtype has few, if any functional autaptic contacts or that they are present at remote locations. The autaptic activity in FS cells substantially changed action-potential waveform, particularly during spike after-hyperpolarization, and played a crucial role in repetitive firing.
CONCLUSIONS: These results indicate that autaptic interneuronal innervation is functional and is selectively present in FS interneurons where it modulates the pattern of spiking. The activation of an autaptic conductance during neuronal firing is of particular interest because it is a mechanism that would regulate interneuronal output, particularly during spike trains, such as those occurring during epilepsy. Modifications of autaptic synaptic efficacy might give rise to long-term alterations in interneuronal firing properties. Moreover, the amount of cortical self-innervation, either excitatory or inhibitory, may be enhanced in epileptic tissue, which has been shown to undergo intense axonal sprouting and de novo synaptogenesis, thus leading to a possible change in the amount of either excitatory and inhibitory autaptic connections.
[Supported by: NIH Grant NS 12151 from the NINDS and the Pimley Research and Training Funds.]