HT innervation of the MOB

The cells upon which serotonergic axons synapse in the olfactory bulb are becoming elucidated. Most of the earlier information is based on location of receptor binding which may lead one to infer that certain cell types receive serotonergic input. More recently, the cells which express mRNA for serotonergic receptor subtypes have been identified which has helped identify the cells receiving serotonergic input even further. In the case of the olfactory bulb, there are mainly two subtypes present 5-HT,a and 

a receptor is found mainly in the external plexiform layer. On the other hand, the mRNA for the S-HTja receptor has been shown by in situ hybridization to be in the mitral cell and external plexiform layers (Pompeiano et al. 1994) and more precisely, in mitral and tufted cells (McLean et al. 1994). This leads one to speculate that the dendrites or cell bodies of olfactory bulb output cells receive serotonergic input via 

S-HTja receptors. It is interesting that 5-HT,a and S-HTja receptor binding are both relatively low in the glomerular layer where there is substantial 5-HT innervation and possibly, synaptic input onto output cells (Liu and Shipley, in preparation). This could mean that a 5-HT receptor subtype other than the 5-HT, a or 5-HT2A is responsible for receiving 5-HT input in the glomeruli or it could mean there is a receptor/axon mismatch for 5-HT input in the bulb. Another 5-HT receptor subtype, S-HTjc is in low quantities in the olfactory bulb. The accessory olfactory bulb has not been analyzed adequately for the 5-HT receptor subtypes but it is clear that the mRNA for the S-HTja receptor is low to negligible in AOB (McLean et al. 1994). 

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