Responses of rat subicular neurons to convergent stimulation of lateral entorhinal cortex and CA1 in vivo

There has been Little electrophysiological examination of the afferent proj ection from lateral entorhinal cortex to dorsal subiculum. Here we provide evidence that synaptic inputs from lateral entorhinal cortex and CAI conver ge onto single dorsal subicular neurons in vivo. Subicular responses to CA1 stimulation consisted of excitation and/or long-duration inhibition. Neuro ns excited by CA1 activation usually showed inhibition to entorhinal stimul ation. The latter inhibition was usually of short duration, however, long d uration inhibition was seen in a significant proportion of responses. Entor hinal stimulation produced excitatory responses in four bursting cells and ii was these cells that also tended to show the longest inhibition. Only bu rsting cells could be driven antidromically by entorhinal stimulation. Bioc ytin-filled multipolar and pyramidal cells displayed excitation-inhibition sequences to CA1 and inhibition to entorhinal stimulation. These data stron gly suggest that subicular inhibitory neurons receive excitatory input from CA1 and display mutual inhibition. The source of entorhinal-evoked inhibit ion is less clear. The relative sparseness of observed entorhinal-evoked re sponses suggests that the input to dorsal subiculum from any one part of la teral entorhinal cortex is spatially restricted. These data show that excit ation-inhibition sequences can be seen in subicular pyramidal and multipola r cells and that single subicular neurons receive convergent inputs from CA 1 and entorhinal cortex. We show for the first time that bursting cells can be driven both orthodromically and antidromically by direct entorhinal sti mulation. These data support the existence of a reciprocal excitatory conne ction between lateral entorhinal cortex and dorsal subiculum and suggest fu rther that this connection may involve only bursting subicular neurons. (C) 2000 Elsevier Science B.V. All rights reserved.