Examination of morphological and synaptic features of calbindin-immunoreactive neurons in deep layers of the rat olfactory bulb with correlative laser and volume electron microscopy.

The olfactory bulb (OB) contains various interneuron types that play key roles in processing olfactory information via synaptic contacts. Many previous studies have reported synaptic connections of heterogeneous interneurons in superficial OB layers. In contrast, few studies have examined synaptic connections in deep layers because of the lack of a selective marker for intrinsic neurons located in the deeper layers, including the mitral cell layer, internal plexiform layer (IPL) and granule cell layer. However, neural circuits in the deep layers are likely to have a strong effect on the output of the OB because of the cellular composition of these regions. Here, we analyzed the calbindin-immunoreactive neurons in the IPL, one of the clearly neurochemically defined interneuron types in the deep layers, using multiple immunolabeling and confocal laser scanning microscopy combined with electron microscopic three-dimensional serial-section reconstruction, enabling correlated laser and volume electron microscopy (EM). Despite a resemblance to the morphological features of deep short axon cells, IPL calbindin-immunoreactive (IPL-CB-ir) neurons lacked axons. Furthermore, multiple immunolabeling for plural neurochemicals indicated that IPL-CB-ir neurons differed from any interneuron types reported previously. We identified symmetrical synapses formed by IPL-CB-ir neurons on granule cells (GCs) using correlated laser and volume EM. These synapses might inhibit GCs and thus disinhibit mitral and tufted cells. Our present findings indicate, for the first time, that IPL-CB-ir neurons are involved in regulating the activities of projection neurons, further suggesting their involvement in synaptic circuitry for output from the deeper layers of the OB, which has not previously been clarified.