Connections between hypothalamus and medullary reticular formation mediate coronary vasoconstriction

We have recently identified discrete sites within the lateral hypothalamus and medullary reticular formation that, when stimulated electrically, produce neurally mediated coronary vasoconstriction. This study examined whether these sites are part of the same coronary vasomotor pathway. The neuronal tracing dye fast blue was injected in cats into the coronary vasoconstrictor site within medullary reticular formation. Fluorescence microscopy revealed major afferent projections originating from within the same region of midbrain ventrolateral periaqueductal gray that receives projections from lateral hypothalamus. To determine the functional importance of the proposed connections between the hypothalamic and medullary sites, anesthetized cats were prepared for continuous hemodynamic measurements. Constant current electrical stimulation within lateral hypothalamus produced significant increases in heart rate (21 +/- 6%), arterial pressure (11 +/- 3%), and femoral (36 +/- 18%) and coronary resistances (14 +/- 9%) with no change in coronary flow velocity (-1.1 +/- 2.5%). After beta-adrenoreceptor blockade, significantly greater increases in arterial pressure (35 +/- 8%) and coronary resistance (39 +/- 5%) with transient decreases in coronary flow velocity (21 +/- 6%) were seen. Microinjections of lidocaine into the medullary site blocked coronary constriction produced by lateral hypothalamic stimulation (39 +/- 5% increase in coronary resistance to electrical stimulation before and 2.4 +/- 2% increase after lidocaine in medullary reticular formation). These data provide evidence that specific regions of lateral hypothalamus and medullary reticular formation are part of a common central vasomotor projection that mediates coronary vasoconstriction in addition to other hemodynamic effects.