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Notes: Abstract In decerebrate cats, rotation about the longitudinal axis of the whole animal at 0.15 Hz,±10° produced an increased electromyogram (EMG) activity of the triceps brachii during side-down tilt and a decreased activity during side-up tilt. This vestibulospinal reflex (VSR) was tested before, during and after a sustained (3-h) period of roll tilt of the head at the parameters indicated above, associated with a synchronous roll tilt of the body at 0.15 Hz, but at the peak amplitude of either 12.5° or 7.5°. This additional stimulus led to 2.5° of neck rotation, which was respectively out of phase (condition A) or in-phase (condition B) with head rotation. In a few instances the peak amplitude of neck rotation was increased to 5°. In the first experimental condition A, the gain of the VSR (tested every 10–15 min) progressively increased, starting from the first hour of out of phase neck-vestibular stimulation to reach, on average, 241% of the control value at the end of the third hour of stimulation. On the other hand, in the second experimental condition B, the mean gain of the VSR first decreased to 82% during the first hour of in-phase neck-vestibular stimulation, but then increased to 165% of the corresponding control during the last hour of recording. In other experiments an adaptive increase in gain of the pure VSR occurred during a sustained (3-h) period of selective roll tilt of the whole animal, but it was less consistent and, on average, smaller in amplitude than that obtained during out of phase neck-vestibular stimulation. The adaptive changes in gain of the VSR described above were not associated with changes in the phase angle of the responses, and were also observed during the post-adaptation period. Further experiments indicated that the gain of the N-VSR, i. e. of the EMG responses to combined neck-vestibular stimulation, displayed a prominent adaptive increase during the sustained out of phase stimulation, but not during the inphase stimulation.

Notes: Abstract Vasopressin (VP) acts as a neurotransmitter or a neuromodulator on noradrenergic locus coeruleus (LC) neurons by exciting them. Experiments were performed in precollicular decerebrate cats to investigate whether direct infusion of VP into the LC complex of one side produced changes in posture as well as in the gain of vestibulospinal reflexes acting on forelimb extensors. Unilateral microinjection of 0.25 μl VP solution (10−11 μg/μl saline) into the LC complex increased the extensor rigidity in the ipsilateral limbs, while that of the contralateral limbs either remained unmodified or slightly decreased. The amplitude of modulation and thus the response gain of both the ipsilateral and the contralateral triceps brachii to roll tilt of the animal leading to stimulation of labyrinth receptors decreased (t-test, P〈0.001 for both the ipsilateral and the contralateral responses). Moreover, a slight decrease in phase lead of the responses was observed. These findings occurred 5–10 min after the injection, were fully developed within 30 min and disappeared in about 2 h. The changes in posture as well as in the gain of vestibulospinal reflexes described above were site specific and depended upon the injected neuropeptide. They were attributed to tonic activation of presumptive noradrenergic neurons, which exert a facilitatory influence on limb extensor motoneurons either directly, by utilizing the coeruleospinal pathway, or indirectly by inhibiting the dorsal pontine reticular formation and the related medullary inhibitory reticulospinal neurons.

Notes: Abstract 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 (LCα,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° 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, impulsed·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° with respect to neck position and +17.6° 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. The second group of units (9/52, i.e., 17.3%) showed parallel (“in phase”) responses to the two inputs and they were excited by side-down or side-up neck rotation and animal tilt. The response characteristics of LC-complex neurons to combined neck and macular inputs, elicited during head rotation, corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, “out of phase” units displayed small amplitudes and large phase shifts of their responses with respect to those obtained during individual neck or macular stimulation. In contrast, “in phase” units displayed large responses during head rotation. Some nonlinearities of the responses to combined stimulation of neck and macular receptors, however, were observed. The possibility that the CS neurons contributed, with the vestibulospinal (VS) neurons, to the postural adjustments of the limb musculature during labyrinth and neck reflexes was discussed.