Convergence and interaction of neck and macular vestibular inputs on locus coeruleus and subcoeruleus neurons

Extracellular recordings were obtained in precollicular decerebrate cats from 90 neurons located in the noradrenergic area of the dorsal pontine tegmentum, namely in the dorsal (LCd, n = 24) and the ventral part (LC alpha, n = 40) of the locus coeruleus (LC) as well as in the locus subcoeruleus (SC, n = 26). Among these units of the LC complex, 13 were coerulospinal (CS) neurons antidromically identified following stimulation of the spinal cord at T12-L1. Some of these neurons showed the main physiological characteristics of the norepinephrine (NE)-containing LC neurons, i.e., a slow and regular resting discharge and a typical biphasic response to fore- and hindpaw compression consisting of a short burst of excitation followed by a period of quiescence, due, in part at least, to recurrent and/or lateral inhibition. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotation about the longitudinal axis at 0.15 Hz, +/- 10 degrees peak amplitude. Among the 90 LC-complex neurons, 60 (66.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 67 (74.4%) responded to neck rotation. Convergence of macular and neck inputs was found in 52/90 (57.8%) LC-complex neurons; in these units, the gain and the sensitivity of the first harmonic of the response corresponded on the average to 0.34 +/- 0.45, SD, impulses.s-1.deg-1 and 3.55 +/- 2.82, SD, %/deg for the neck responses and to 0.23 +/- 0.29, SD, impulses.s-1.deg-1 and 3.13 +/- 3.04, SD, %/deg for the macular responses. In addition to these convergent units, 8/90 (8.9%) and 15/90 (16.7%) LC-complex units responded to selective stimulation either of macular or of neck receptors only. These units displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent LC-complex units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of about +31.0 degrees with respect to neck position and +17.6 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (43/52, i.e., 82.7%) showed reciprocal ("out of phase") responses to the two inputs; moreover, most of these units were excited during side-down neck rotation, but inhibited during side-down animal tilt.(ABSTRACT TRUNCATED AT 400 WORDS)