calcium imaging ͉ mitral cell ͉ synchronous activity T he glomeruli of the olfactory bulb (OB) constitute an important interface for the coding of odor information. Every individual glomerulus collects convergent inputs from olfactory sensory neurons (OSN) expressing the same olfactory receptor (OR) (1), and it delivers output information to higher brain regions via a number of M/T cells connected to it (2). This modular architecture is thought to play a fundamental role in odor coding (3).M/T cells of the same glomerulus receive primary inputs from OSNs of the same type, but each M/T cell also extends additional dendrites and gets complex secondary inputs from local interneurons (4, 5), top-down projections (6, 7), and from other M/T cells (8, 9). M/T cells are also connected through gap junctions within glomeruli (9-11). While the intraglomerular gap junctions could produce a certain degree of synchronization (9,11,12), the synaptic interactions could transform the initial OSN inputs into spatiotemporal patterns of M/T cell output activity (13-15). As the secondary inputs presumably differ from M/T cell to M/T cell, every M/T cell could process its primary and secondary inputs differentially, which would allow for a large information capacity of M/T cell activity patterns.Three major signal sources thus determine the M/T cell activity patterns conveyed to higher brain centers: primary OSN inputs, currents through intraglomerular gap junctions, and complex secondary synaptic inputs of various origins. As all three input sources of M/T cells depend, directly or indirectly, on odor stimulation, the physiological M/T cell activity patterns must be measured while applying odorants to the olfactory epithelium.Here, we use a nose-brain preparation of the Xenopus tadpole to analyze M/T cell activity pattern following natural odor stimulation. Using a method for identifying neurons that are connected to the same glomerulus, we can observe how natural odorants are represented by modules of glomerulus-specific M/T cells.
Results
Identification of Glomerulus-specific M/T Cells Using [Ca 2؉ ] Imaging.We investigated the neuronal network of the Xenopus tadpole OB using a combination of [Ca 2ϩ ] imaging and the patch clamp technique. Recordings were obtained from a nose-brain preparation that preserves the connections between the olfactory epithelium and the OB (16, 17) (Fig. S1). This allows us to analyze the circuitry and the response of the OB network to natural odor inputs. In the absence of stimulation, neurons in the M/T cell layer show a rich pattern of spontaneous [Ca 2ϩ ] activities ( Fig. 1A and B) that reflect their spontaneous firing patterns (Fig. S2) (18). The correlation of these activities was low in randomly chosen cell pairs (activity correlation index r ϭ 0.0069 Ϯ 0.0003; n ϭ 50325 pairs). However, a complete search in a correlation matrix (Fig. 1C) revealed specific pairs of neurons that showed strongly correlated activities (r Ͼ 0.6, Fig. 1D, E, and G). These neurons were synchronized for almost every ...