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 hypodalamus and zona incerta (ZI) on mean blood pressure (MBP), heart rate (HR), and neuronal activity in the midbrain periaqueductal gray (PAC) were investigated in rats. Long-lasting depressor responses were elicited from 67 sites in the hypothalamus and ZI. Effects on HR were variable, except for the ZI where bradycardic responses were evoked. The amplitude of the depressor responses was significantly correlated with baseline MBP on stimulation of the dorsomedial hypothalamic nucleus (DMH) or the dorsal hypothalamic area. Extracellular single-unit recordings were made from 94 PAG neurons. Most units were boated in the ventral half of the PAG (62/94), where spontaneous firing rates were significantly higher than in the dorsal half: 12.5 ± 1.4 spikes/s as compared to 6.0 ± 0.9 spikes/s. Changes in PAG neuronal activity to both ipsi- and contralaterel hypothalamic stimulation were observed. Most neurons were inhibited or unresponsive. There was no site specificity: responsive and unresponsive neurons were scattered throughout the PAG. Inhibition was most effectively evoked by stimulation of the DMH (25 out of 39 neurons) and the Zl (9 out of 15 neurons). In most neurons, the inhibition strictly followed the time course of hypothalamic stimulation. The results suggest that PAG as well as nonPAG pathways participate in the hypothalamic control of cardiovascular function.