Keiji Hara

Differential regional hemodynamic changes produced by L-glutamate stimulation of the locus coeruleus

The locus coeruleus (LC) exerts an inhibitory influence on the cardiovascular system. Microinjection of the excitatory amino acid, L-glutamate, into the LC elicits a decrease in arterial pressure as a result of a decrease in total peripheral resistance (TPR). The aim of the present study was to examine the role of the LC in the regulation of the regional hemodynamics. Employing anesthetized rats, the blood flow to the renal, mesenteric and hind-limb vascular beds was measured with an electromagnetic flowmeter. The changes in regional blood flow and vascular resistance evoked by chemical stimulation of the LC were examined separately in each region. During the depressor response elicited by LC stimulation, the hind-limb and renal vascular resistance was significantly decreased, while the mesenteric resistance was unchanged. The vasodilatation appeared to be more prominent in the hind-limb muscle than in the systemic circulation. Renal nerve denervation attenuated the decrease in renal vascular resistance elicited by LC stimulation. However, a small part of this response still remained in the denervated kidney. The present results suggest therefore that: (1) LC neurons exert differential hemodynamic effects on the hind-limb muscle, renal and mesenteric vascular beds; (2) the largest contribution to the decrease in TPR is related to vasodilatation in the hind-limb muscles; and (3) the renal vasodilation elicited by LC stimulation is not mediated solely by the renal innervation.

Diminished sympathetic excitation of locus coeruleus noradrenergic neurons in the conscious mature spontaneously hypertensive rat.

We undertook a study to determine whether the activation of the nucleus locus coeruleus might be responsible for the sympathetic hyperactivity in the spontaneously hypertensive rat (SHR). Conscious mature SHR showed increased arterial pressure and plasma catecholamines with electrical stimulation of the locus coeruleus. However, SHR showed smaller increases in arterial pressure and plasma noradrenaline than Wistar-Kyoto rats (WKY). The spontaneous unit discharge in locus coeruleus neurons responded reciprocally to peripherally induced changes in arterial pressure and blood volume. However, the unit discharge in the SHR locus coeruleus is less responsive than that in WKY. Therefore, the locus coeruleus in mature SHR does not seem to be involved in the hyperactivity of the sympathetic nervous system. However, this may not be the case in young SHR.