Download THE JOURNAL OF COMPARATIVE NEUROLOGY 460:80–93 (2003)

THE JOURNAL OF COMPARATIVE NEUROLOGY 460:80–93 (2003)
Direct Input from Cochlear Root Neurons to Pontine
Reticulospinal Neurons in Albino Rat
FERNANDO R. NODAL1* AND DOLORES E. LOPEZ2
The cochlear root neurons (CRNs) are thought to mediate the auditory startle reflex (ASR) in the rat, which is widely used as a
behavioral model for the investigation of the sensorimotor integration. CRNs project, among other targets, to the nucleus reticularis
pontis caudalis (PnC), a major component of the ASR circuit, but little is known about the organization of this projection. Thus, we
injected biotinylated dextran amine (BDA) in CRNs to study their projections with light and electron microscopy. Also, we
performed doublelabeling experiments, injecting BDA in the CRNs and subunit B of the cholera toxin or Fluorogold in the spinal
cord to verify that CRNs project onto reticulospinal neurons. Electron microscopy of the labeled CRNs axons and terminals showed
that even their most central and thinnest processes are myelinated. Most of the terminals are axodendritic, with multiple asymmetric
synapses, and contain round vesicles (50 nm diameter). Double-labeling experiments demonstrated that CRN terminals are apposed
to retrogradely labeled reticulospinal neurons in the contralateral nucleus reticularis PnC and bilaterally in the lateral
paragigantocellular nucleus. Analyses of serial sections revealed that multiple CRNs synapse on single reticulospinal neurons in
PnC, suggesting a convergence of auditory information. The morphometric features of these neurons classify them as giant neurons.
This study confirms that CRNs project directly onto reticulospinal neurons and presents other anatomical features of the CRNs that
contribute to a better understanding of the circuitry of the ASR in the rat.
Fig. 7. Photomicrographs (left column, bar 25 _m) and camera lucida drawings (right column, scale bar _ 30 _m) of coronal sections from case 97086 showing
several retrogradely labeled PnC neurons (in brown) with appositions from anterogradely labeled CRN axons (in black) on them (arrows). Neurons in A, C,
and D were located in the PnC contralateral to the injection site in the cochlear root nerve. Neuron in B is located in the ipsilateral PnC.