SUBSTANCE P-SENSITIVE NEURONS IN THE RAT AUDITORY BRAIN-STEM - POSSIBLE RELATIONSHIP TO MEDIAL OLIVOCOCHLEAR NEURONS

Using in vitro techniques, intracellular microelectrode recordings wer e made from the regions of origin of medial olivocochlear neurones in the rat auditory brainstem. Cells were characterised according to thei r action potential shape and their excitatory response to bath applica tion of micromolar concentrations of the peptide neurotransmitter subs tance P and were filled with biocytin by ii!lection through the intrac ellular microelectrode for subsequent morphological reconstruction. Ce lls with a rapid component to the after-hyperpolarising phase of their action potentials (AHP2 cells) were most likely to show excitatory ef fects of substance P. Histological reconstruction showed that these ce lls were stellate with numerous large, slowly tapering dendrites exhib iting small, scattered spines. III examples in which the major axon wa s not cut near the cell body-the axons ascended dorsally out of the su periord olivary complex, in a manner that was consistent with the traj ectory of axons of medial olivocochlear neurones. These features diffe red from other cells in the ventral nucleus of the trapezoid body that were unresponsive to substance P. In a further series of experiments, medial olivocochlear cells in the same region were retrogradely label ed by prior intracochlear injection with fast blue and recordings were made under direct visual observation using either microelectrode impa lement or whole-cell patch methods. These data support the view that m edial olivocochlear neurones are substance P-sensitive and exhibit a c haracteristic spike shape. These data strongly suggest that medial oli vocochlear neurones possess receptors for substance P and may therefor e receive excitatory input from a substance P-utilising neural pathway . (C) 1998 Elsevier Science B.V.