DIFFERENCES IN THE REFRACTORY PROPERTIES OF 2 DISTINCT INHIBITORY CIRCUITRIES IN-FIELD CA1 OF THE HIPPOCAMPUS

Extracellular reflections of IPSPs were examined in two distinct circu itries in field CA1 of the hippocampus. Stimulation in the stratum rad iatum in the presence of AMPA receptor antagonists elicited positive p otentials in the same stratum that were eliminated by picrotoxin, a bl ocker of GABA(A) receptors. Laminar profile analysis revealed that the response was maximal in the stratum radiatum at a point well distal t o the pyramidal cell body layer and had a negative reflection in the s tratum oriens. These field IPSPs presumably mediate the feedforward in hibition normally activated by the Schaffer-commissural projections to field CA1. Stimulation of the alveus produced an antidromic response followed by a much slower positive potential in recordings collected i n the pyramidal cell layer. The latter response was suppressed by AMPA receptor antagonists or picrotoxin, as expected for disynaptic, recur rent (feedback) inhibition. The laminar profile for the feedback field IPSPs had its maximum near the pyramidal cell layer and its negative dipole in the stratum radiatum. Feedforward IPSPs were inhibited by ab out 50% if they were preceded within 200 ms by a priming pulse while f eedback IPSPs were reduced by less than 20% under comparable condition s. The refractory effect was minimally dependent on stimulation intens ity but was strongly affected by an antagonist of GABA(B) receptors. A ttempts to modify IPSPs in the s. radiatum with long trains of low fre quency stimulation or with theta-burst stimulation were not successful , suggesting that GABAergic synapses do not have the plasticities foun d in their glutamatergic counterparts. These results indicate that int erneurons contacted by the extrinsic afferents of hippocampus form GAB Aergic synapses that differ in terms of spatial location and functiona l properties from the synapses generated by interneurons innervated by the recurrent collaterals of the pyramidal cells. The findings also s uggest that repetitive afferent activity, while reducing the influence of dendritic IPSPs on excitatory input, will leave feedback suppressi on of cell spiking largely intact.