Brainstem mediation of prefrontal stimulus-produced hypotension.

Electrical stimulation of the prefrontal cortex (PFC) in rats produces transient hypotension. It has been suggested that this stimulus-produced hypotension (SPH) may be mediated by direct PFC projections to either the posterolateral hypothalamus or the nucleus tractus solitarius (NTS). To initially test these hypotheses, microinjections (0.5 microliters) of 4% lidocaine were made into various brainstem sites, including the posterolateral hypothalamus, the PFC-NTS pathway and the NTS itself. Most injections made into the posterolateral hypothalamus, rostral portion of the PFC-NTS pathway or NTS were successful in blocking prefrontal SPH. In comparison, the majority of injections made into numerous other brainstem sites including the caudal portion of the PFC-NTS pathway did not block prefrontal SPH. These findings support the concept that prefrontal SPH may be mediated via both the posterolateral hypothalamus and NTS. However, these findings do not support the hypothesis that prefrontal SPH is mediated by direct projections from the PFC to the NTS. To eliminate the possibility that the effects of the lidocaine injections made into the hypothalamus were due to the incapacitation of fibers alone, a series of experiments was conducted in which microinjections (0.5 microliters) of ibotenic acid, a neurotoxin that destroys perikarya but spares axons, were made into the posterolateral hypothalamus. Each of these injections resulted in the blockade of prefrontal SPH. These findings further support the role of hypothalamic involvement in prefrontal SPH.