CHurrents in Spiny versus Aspiny Hilar Neurons

Charles T. Livsey and Stefano Vicini FIDIA-Georgetown Institute for the Neurosciences Georgetown University School of Medicine Washington, D.C. 20007 Summary In the hilar region of the rat hippocampus, large sponta- neous excitatory postsynaptic currents (sEPSCs) medi- ated by non-NMDA glutamate receptors are present in both excitatory spiny mossy cells and inhibitory aspiny hilar interneurons, making these neurons ideal candi- dates for a comparative study using the tight seal whole cell recording technique. Although sEPSCs have similar amplitude distributions, the rise and decay times are significantly slower in spiny versus aspiny neurons. Simi- lar kinetic differences are observed in synaptic currents evoked by mossy fiber stimulation. These results dem- onstrate a physiological difference between the excita- tory drive to excitatory and inhibitory neurons in the hilus that certainly contributes to differences in synaptic strength and that may be applicable to other brain re- gions. Furthermore, since the development or modifica- tion of individual spines or groups of spines may affect synaptic strength, these results may be pivotal in estab- lishing a role for spines in modulating synaptic activity. Introduction Neurons of the rat hippocampal hilus are strategically located between the dentate gyrus and region CA3, where they are thought to regulate hippocampal activ- ity (Scharfman et al., 1990; Scharfman, 1991) and to be involved in the pathogenesis of epilepsy (Scharfman and Schwartzkroin, 1990; Sloviter, 1987). The complex circuitry of the hilus contains neurons covered with spines and thorny excrescences, spiny mossy cells (SMCs), and neurons totally devoid of spines, aspiny hilar interneurons (AHIs). Axon collaterals from en passant mossy fibers arising from dentate granule cells provide excitatory synaptic input to SMCs and some AHls as the main mossy fiber branches trans- versethe hilustoCA3pyramidal neuronsofthe hippo- campus (Amaral, 1978; Ribak and Seress, 1983; Ribak et al., 1985). Additional excitatory inputs to neurons of both classes have recently been reported in adult rats, from the entorhinal cortex via the perforant path (Scharfman, 1991). AHls and SMCs in turn project to a variety of targets in the ipsilateral and contralateral fascia dentata (Blackstad, 1956; Zimmer, 1971; Swan- son et al., 1978; Laurberg and Sorensen, 1981). It is well known that the mossy fibers provide a primary excitatory input to CA3 pyramidal cells, and neurotransmission at these synapses is thought to be mediated primarily through glutamate acting on non-N-methyl-o-aspartate (NMDA) receptor-gated ion channels (Storm-Mathisen and Ottersen, 1989). Anatomically similar synapses occur on the thorny excrescences (complex spines) located on the proxi- mal dendrites and cell bodies of SMCs (Amaral, 1978; Ribak et al., 1985), and large mossy fiber-like synaptic expansions have been observed in apposition to smooth cell bodies and dendrites in the hilus (Amaral, 1978; Ribakand Seress, 1983). However, the physiolog- ical and pharmacological properties of hilar synapses have not been examined as thoroughly those in CA3. Stimulation and intracellular recording from pairs of hilar neurons have confirmed connections from granule cells to SMCs, granule cells to AHls, AHlstogranulecells,AHlstoSMCs,andAHlstoAHls (Scharfman et al., 1990). The output of granule cells was found to be excitatory, whereas that of AHls inhibitory. The output of SMCs remains unclear, since these neurons, at least in some studies, display immu- noreactivity for glutamate (Fischer et al., 1986; Storm- Mathisen et al., 1983) and are thought to provide sig- nificant commisural input to the dentate, which is primarily excitatory (Deadwyler et al., 1974), but were found to make symmetric synapses (which generally indicates inhibitory action) with interneuron den- drites (Scharfman et al., 1990). Thus, given the current understanding of hilar circuitry, the excitatory synap- tic connections to the AHls and SMCs in hilar region seem quite similar. Interestingly, intense and continuous spontaneous excitatory synaptic activity was also reported in both neuronal types (Scharfman et al., 1990; Scharfman, 1991), similar to observations made in CA3 neurons (Brown et al., 1988). In the present paper, we investigate thecharacteris- tics of excitatory postsynaptic currents (EPSCs) in SMCs and AHls using the high resolution whole-cell patch-clamp technique (Hamill et al., 1981) from brain slices (Edwards et al., 1989). Spontaneous EPSCs (sEPSCs) occur with high frequency in both SMCs and AHls, thus providing large samples of comparable synaptic events from a wide range of membrane loca- tions from spiny and aspiny neurons. We demonstrate that fast or slow kinetics of non-NMDA receptor-me- diated sEPSCs correlate well with the absence or pres- ence of postsynaptic spines, respectively. We provide evidence that these differences in kinetics do not arise simply from dendritic filtering of the synaptic cur- rents, but are likely due to filtering by the dendritic spine and/or differences in non-NMDA glutamate re- ceptor subtypes in the two neuronal populations. Electrophysiological and Morphological Characterization of Hilar Neurons At least nine morphologically distinct neuronal types with a large variety of shapes and sizes are present in the selected region of rat hippocampal hilus