Effects of microinjection of vasopressin in dorsal pontine reticular structures on the gain of vestibulospinal reflexes in decerebrate cats.

Abstract

1. The possibility that vasopressin (VP) acts on the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system to control posture as well as the vestibulospinal reflexes has been investigated by injecting small doses of VP in precollicular decerebrate cats. 2. Unilateral microinjection of VP (0.25 microliters at the concentration of 10(-11) micrograms/microliters saline) in the pRF decreased the extensor rigidity in the ipsilateral limbs, while that of the contralateral limbs either decreased or increased. The same injection also produced a moderate or a prominent increase in gain of the multiunit EMG responses of the ipsilateral triceps brachii to roll tilt of the animal (t-test, P less than 0.001 for either group of responses). In the first instance the response gain of the contralateral triceps brachii to animal tilt slightly increased, while the pattern of response remained always of the alpha-type, as shown for the ipsilateral responses (increased EMG activity during ipsilateral tilt and decreased activity during contralateral tilt). In the second instance, however, the response gain showed only slight changes, while the pattern of responses reversed from the alpha- to the beta-type. These findings occurred 5-20 min after the injection, fully developed within 30-60 min and disappeared in about 2-3 hours. 3. The structures responsible for the postural and reflex changes described above were located in the dorsal pontine tegmental region immediately ventral to the LC, and included the peri-LC alpha and the surrounding dorsal pRF. The induced effects depended upon the injected neuropeptide, since previous injection of an equal volume of saline stained by the pontamine sky blue dye into the same dorsal pontine area was ineffective. 4. We postulated that VP exerts an excitatory influence on ipsilateral dorsal pRF neurons. The increased discharge of these neurons and the related medullary inhibitory RS neurons would lead to a decreased postural activity in the ipsilateral limbs. However, since these inhibitory RS neurons fire out of phase with respect to the excitatory vestibulospinal neurons, it appears that the higher the firing rate of the RS neurons in the animal at rest, the greater the disinhibition that affects the limb extensor motoneurons during ipsilateral tilt. These motoneurons would then respond more efficiently to the same excitatory volleys elicited by given parameters of stimulation, thus leading to an increased gain of the EMG responses.(ABSTRACT TRUNCATED AT 400 WORDS)