Rodríguez-Sierra OE, Turesson HK, Pare D. Contrasting distribution of physiological cell types in different regions of the bed nucleus of the stria terminalis. J Neurophysiol 110: 2037-2049. First published August 7, 2013 doi:10.1152/jn.00408.2013.-We characterized the electroresponsive and morphological properties of neurons in the bed nucleus of the stria terminalis (BNST). Previously, Rainnie and colleagues distinguished three cell types in the anterolateral region of BNST (BNST-AL): low-threshold bursting cells (LTB; type II) and regular spiking neurons that display time-dependent (RS; type I) or fast (fIR; type III) inward rectification in the hyperpolarizing direction (Hammack SE, Mania I, Rainnie DG. J Neurophysiol 98: 638 -56, 2007). We report that the same neuronal types exist in the anteromedial (AM) and anteroventral (AV) regions of BNST. In addition, we observed two hitherto unreported cell types: late-firing (LF) cells, only seen in BNST-AL, that display a conspicuous delay to firing, and spontaneously active (SA) neurons, only present in BNST-AV, firing continuously at rest. However, the feature that most clearly distinguished the three BNST regions was the incidence of LTB cells (approximately 40 -70%) and the strength of their bursting behavior (both higher in BNST-AM and AV relative to AL). The incidence of RS cells was similar in the three regions (ϳ25%), whereas that of fIR cells was higher in BNST-AL (ϳ25%) than AV or AM (Յ8%). With the use of biocytin, two dominant morphological cell classes were identified but they were not consistently related to particular physiological phenotypes. One neuronal class had highly branched and spiny dendrites; the second had longer but poorly branched and sparsely spiny dendrites. Both often exhibited dendritic varicosities. Since LTB cells prevail in BNST, it will be important to determine what inputs set their firing mode (tonic vs. bursting) and in what behavioral states.bed nucleus of the stria terminalis; anxiety; fear; intrinsic properties; morphology DESPITE ANATOMICAL SIMILARITIES (Alheid and Heimer 1988; deOlmos and Heimer 1999;McDonald 2003), dense interconnections (Krettek and Price 1978a,b; Dong et al. 2001a), and functional kinship (Walker et al. 2003) between the amygdala and bed nucleus of the stria terminalis (BNST), there is a stark contrast between our understanding of these two structures. For instance, numerous in vitro studies have examined the physiological properties of amygdala neurons, mechanisms of synaptic transmission, neuromodulation, and activity-dependent plasticity (reviewed in Sah et al. 2003;Pape and Pare 2010). In contrast, relatively few reports on these themes are available for the BNST (reviewed in McElligott and Winder 2009; Hammack et al. 2009). As a result, the operations carried out by the BNST remain poorly understood.