CARDIOVASCULAR EFFECTS AND CHANGES IN MIDBRAIN PERIAQUEDUCTAL GRAY NEURONAL-ACTIVITY INDUCED BY ELECTRICAL-STIMULATION OF THE HYPOTHALAMUS IN THE RAT

The effects of low-intensity electrical stimulation of sites in the hy pothalamus and zona incerta (ZI) on mean blood pressure (MBP), heart r ate (HR), and neuronal activity in the midbrain periaqueductal gray (F AG) were investigated in rats. Longlasting depressor responses were el icited from 67 sites in the hypothalamus and ZI. Effects on HR were va riable, except for the ZI where bradycardic responses were evoked. The amplitude of the depressor responses was significantly correlated wit h baseline MBP on stimulation of the dorsomedial hypothalamic nucleus (DMH) or the dorsal hypothalamic area. Extracellular single-unit recor dings were made from 94 FAG neurons. Most units were located in the ve ntral half of the FAG (62/94), where spontaneous firing rates were sig nificantly higher than in the dorsal half: 12.5 +/- 1.4 spikes/s as co mpared to 6.0 +/- 0.9 spikes/s. Changes in FAG neuronal activity to bo th ipsi- and contralateral hypothalamic stimulation were observed. Mos t neurons were inhibited or unresponsive. There was no site specificit y: responsive and unresponsive neurons were scattered throughout the F AG. Inhibition was most effectively evoked by stimulation of the DMH ( 25 out of 39 neurons) and the ZI (9 out of 15 neurons). In most neuron s, the inhibition strictly followed the time course of hypothalamic st imulation. The results suggest that FAG as well as nonPAG pathways par ticipate in the hypothalamic control of cardiovascular function.