Exp Brain Res (1987) 67:615-622 Ex mental Bra nResearch Springer-Verlag 1987 Synaptic organization of olfactory inputs and local circuits in the entorhinal cortex: a current source density analysis in the cat Th. Van Groen, F.H. Lopes da Silva, and W.J. Wadman Department of Zoology, University of Amsterdam, Biological Centrum Gebouw II, Kruislaan 320, NL-1098 SM Amsterdam, The Netherlands Summary. The distribution of the olfactory afferents within the ventrolateral part of the entorhinal cortex (EC) was studied by means of field potentials evoked by stimulation of the olfactory bulb (OB) and the olfactory cortex (PPC). Depth profiles of the field potentials evoked by OB or PPC stimulation were studied using current source density analysis. After OB or PPC stimulation an early superficial sink-deep source configuration was found, which some time later reversed into a superficial source-deep sink. Both OB and PPC activated mainly the superficial dendrites of the cells of layers II and III. In layers II and III evidence for strong recurrent inhibition was found, using double pulse stimulation. The results indicate that there exists a common basic design of the synaptic organization of the olfactory areas of the base of the brain extending to the EC. Key words: Entorhinal area - Current source density - Olfactory afferents - Cat Introduction The entorhinal cortex (EC) is known to receive different sensory inputs, especially an olfactory path- way arising from the olfactory bulb (OB) and pre- piriform cortex (PPC) (Room et al. 1984; Boeijinga and Van Groen 1984). The depth profiles of the field potentials evoked in the entorhinal cortex by stimula- tion of the OB and the anterior part of the PPC (PPCa) show a characteristic pattern. The PPCa profile is characterized by an initial small positivity followed by a large negative wave at the surface of the EC which changes polarity in deeper layers. The OB profile resembles that of the PPCa, but with a smaller amplitude and a larger onset-latency (Boei- Offprint requests to: F.H. Lopes da Silva (address see above) jinga and Van Groen 1984). In order to investigate the functional organization of the physiological ele- ments that generate these field potentials, we calcu- lated the current-source density (CSD) as a function of depth in the EC. Taking into account the restric- tions imposed by anatomical and histological know- ledge, it is possible to interpret the CSD profiles in terms of functional synaptic processes. We report here the results of such a CSD analysis and discuss them in relation to structural data of the entorhinal cortex and related cortical areas (Krettek and Price 1977 and Room et al. 1984). Methods Eight adult cats were used. These received an initial intramuscular injection of Ketaset (ketamine 20 mg/kg), were intubated and placed in a stereotaxic frame. The animals were artificially ventilated with a mixture of filtered air and halothane (about 1.5%), the ECG and expired COz concentration (Capnograph: E. Jaeger) were measured continuously. In this way rather constant anesthesia was maintained. This is important since we could only obtain depth profiles by sequential recording from successive locations (see below). Stimulation electrode bundles (4 stainless steel electrodes, d = 0.1 ram, cut sharp with an intertip distance of 0.5 ram) were placed in the olfactory bulb, a vue, and in the prepiriform cortex under stereotaxic guidance (coordinates derived from the atlas of Snider and Niemer 1965). Stimulation bundles were placed in the anterior prepiriform cortex (PPCa). The depth of the PPC electrodes was established electrophysiolo- gically by way of olfactory bulb stimulation (Boeijinga and Van Groen 1984). The stimulation sites lay at opposite sides of layer II and did not reach the lateral olfactory tract (see Fig. IB or Boeijinga and Van Groen 1984). Unit activity recorded in the EC to PPC stimulation showed only unit responses with rather variable latencies and no indication of antidromic activation (Boeijinga and Van Groen 1984). To place recording electrodes in the EC an array of cannulas (2 x 4) was lowered under stereotaxic guidance until the tips of the cannulas were 10 mm above the EC surface. The cannulas were used to guide the recording electrodes (bundles of 2 stainless steel electrodes, d = 0.1 mm, cut sharp with an intertip distance of 0.5 ram). Each electrode was lowered until it reached the base of the brain, and then it was fixed to the cannulas. Hereafter the cannulas