INHIBITION BY TOPICALLY APPLIED AND ON THE PRESSOR RESPONSE TO STIMULATION OF THE LOCUS COERULEUS IN CATS

Shoji MARUYAMA

Department of Neurophysiology, Brain Research Institute, Niigata University, Niigata 951, Japan

Accepted June 10, 1981

There is considerable evidence that a lation of the LC. mechanisms within medulla and Cats of either sex weighing 2.2-3.5 kg hypothalamus play a role in cardiovascular were anesthetized with a-chloralose-ure regulation (1-4). Apart from the action on thane. After tracheotomy, the left carotid medullary mechanisms, clonidine has been artery was catheterized for measurement reported to elicit by an action of the blood pressure with a pressure on hypothalamic a-adrenergic receptors and transducer. A coaxial electrode was stereo the possible mechanism of action of the drug taxically inserted into right or left LC (P2, applied into the hypothalamus has hitherto L2, H-2) according to the atlas of Berman been discussed in relation to the descending (14) for electrical stimulation with square pathways from the hypothalamus to the wave pulses of 1 msec duration at 250 Hz, medullary cardiovascular control system (3, 3-5 V for 10 sec. Before the drug application, 5-8). Contrary to this, there has been no the LC was stimulated electrically three or available report referring to an effect of four times every 10 min to elicit nearly a clonidine on the ascending noradrenergic constant pressor response (a rise of the mean pathway from the locus coeruleus (LC) to arterial blood pressure), and three pressor the posterior hypothalamus (HPA), which is responses were averaged for the control. assumed to be involved in regulation of Then, a drug solution was injected into HPA arterial blood pressure (9, 10). In addition, (F 9.5, L 0.8, H -2.5) at a rate of 1 /d/30 sec it is not clear whether clonidine acts either with a microsyringe of 1 al introduced as an a- at the presynaptic through the guide cannula which was receptor or as an a- cemented to the skull, and the LC was at the postsynaptic receptor (3, 11). stimulated to evoke a pressor response 10, Guanfacine, N-amidino-2-(2,6-dichloro 30, 60 and 90 min after . The drug phenyl) acetamide HCI, which resembles effect was estimated by comparing the clonidine in chemical structure, appears to pressor responses before and after injection produce its hypotensive effects as a result of and statistical significance was calculated presynaptic a-adrenoceptor stimulation at by using Student's t-test. Drugs used were central sympathetic control systems, though noradrenaline (NA), phentolamine, clonidine the possible site of central action of the drug and guanfacine. has not yet been defined (12, 13). Thus, it In preliminary experiments, NA, phentol was of interest to investigate an effect of amine, clonidine and guanfacine each in a clonidine and guanfacine injected into HPA variety of doses were stereotaxically injected on the pressor response to electrical stimu into HPA and their effects on the mean arterial blood pressure were observed for (Table 1 and Fig. 1). Next an attempt was 2 hours to determine the dose of each drug made to determine the effects of 99.5% to be applied in stimulation experiments as ethanol and 32 nM of clonidine injected follows: 32 nM for NA, 71 nM for phentol locally into HPA on the pressor response to amine, 32 nM for clonidine and 100 nM for electrical stimulation of the ipsilateral and guanfacine. the contralateral LC. A hundred icl of 99.5% Prior to every experiment, electrical ethanol reduced the pressor response to stimulation of the LC of the cat was adjusted electrical stimulation of the ipsilateral LC to evoke a rise of the mean arterial blood much more significantly (from 46.4+1.6 to pressure by 40-50 mmHg, and then the 30.6+2.0 mmHg in an average of 6 experi control pressor responses were recorded three ments) than that to stimulation of the con or four times. Lesion of HPA with 99.5% tralateral LC (from 46.5+1.3 to 40.5+1.4 ethanol significantly diminished the pressor mmHg). Clonidine in a dose of 32 nM also response to electrical stimulation of the reduced the pressor response to the ipsilateral ipsilateral IC. Microinjection of 0.9% NaCl LC stimulation more significantly (from solution into HPA did not produce any 45.0+1.2 to 32.9+1.5 mmHg in an average of significant changes in the pressor responses 7 experiments) than that to the contralateral to stimulation of the LC. After application of LC stimulation (from 43.4+0.8 to 39.1 +0.7 32 nM of NA into the HPA, the pressor mmHg). A statistically significant difference response to the LC stimulation was increased existed between inhibition of the pressor significantly from the control 42.8±2.9 mmHg response to the ipsilateral LC stimulation and to 55.4±2.4 mmHg (29.1 %) at the maximum. that to the contralateral one by ethanol and Phentolamine in a dose of 71 nM reduced the clonidine applied into HPA, respectively. pressor response to the LC stimulation from Previously Philippu et al. showed that 40.6±2.3 to 36.8+1.5 mmHg (9.3%), but superfusion of the HPA with high concen not significantly. Clonidine in a dose of trations of clonidine reduced the pressor 32 nM reduced the pressor response to the response to electrical stimulation of the area, LC stimulation significantly from 45.0±1.2 to and concluded that inhibition of the pressor 32.9±1.5 mmHg (26.8%). Guanfacine in a response with clonidine might be in part dose of 100 nM also reduced the pressor due to inhibition of NA release through some response to the LC stimulation significantly negative feedback mechanism in the hypo from 44.4+1.7 to 38.0+2.1 mmHg (14.4%) thalamus (5). In the present experiments NA

Table 1. The pressor responses to electrical stimulation of the locus coeruleus after micro injection of a-adrenergic drugs into the ipsilateral posterior hypothalamus of cats Fig. 1. Effects of a-adrenergic drugs injected into the HPA on the pressor response to electrical stimulation of the ipsilateral LC. Ordinate: blood pressure in mmHg. Abscissa: time after drug injection in min. C expresses the control pressor response before drug injection.

given into the HPA significantly increased LC was much more significantly reduced by the pressor response to electrical stimulation ethanol and clonidine injected into the of LC, while clonidine, as well as guanfacine ipsilateral HPA than by these drugs applied which is considered to be a presynaptic into the contralateral HPA. Therefore, it a-adrenergic agonist (13), reduced it sig seems very probable that the ipsilateral nificantly. Therefore, it seems probable that control of noradrenergic neurons ascending clonidine and guanfacine act at least as an from LC to HPA, which may be involved in a-adrenergic agonist at the presynaptic the hypothalamic regulation of the arterial receptor to induce a diminished release of blood pressure, predominates over the con NA from the nerve endings of the ascending tralateral control. This view is supported by noradrenergic fibers from LC to HPA and the findings of Philippu et al. (15) that the to elicit inhibition of the pressor response release of NA in HPA was increased to a to stimulation of the LC. On the other hand, higher extent on stimulation of the ipsilateral the pressor response to the LC stimulation LC than on stimulation of the contralateral LC. remained inhibited, not significantly, but slightly, after pretreatment with phentolamine. REFERENCES The failure of phentolamine to inhibit the 1) Buccafusco, J.J. and Brezenoff, H.E.: Mecha pressor response significantly remains nisms involved in the cardiovascular response to inexplicable until more detailed experiments intracerebroventricular injection of nor and phentolamine. Neurophar can be carried out. macology 16, 775-780 (1977) The pressor response to stimulation of the 2) Palkovitz, M. and Zaborszky, L.: Progress in Brain Research, Edited by de Jong, W., thalamic alpha-adrenergic receptors in cardio Provoost, A.P. and Shapiro, A.P., Vol. 47, vascular regulation. Neuropharmacology 13, p. 9-34, Elsevier, Amsterdam (1977) 837-846 (1974) 3) Philippu, A., Roensberg, W. and Przuntek, H.: 9) Maeda, T. and Shimizu, N.: Projections ascen Effects of adrenergic drugs on pressor responses dantes du locus coeruleus et d'autres neurones to hypothalamic stimulation. Naunyn aminergiques pontiques au niveau du pro Schmiedeberg's Arch. Pharmacol. 278,- 373 sencephale du rat. Brain Res. 36, 19-35 (1972) 386 (1973) - 10) Przuntek, H. and Philippu, A.: Reduced pressor 4) Zandberg, P., de Jong, W. and de Wied, D.: responses to stimulation of the locus coeruleus Effect of catecholamine-receptor stimulating after lesion of the posterior hypothalamus. agents on blood pressure after local application Naunyn-Schmiedeberg's Arch. Pharmacol. 276, in the nucleus tractus solitarii of the medulla 119-122 (1973) oblongata. Europ. J. Pharmacol. 55, 43-56 11) Kobinger, K.: Central alpha-adrenergic systems (1979) as targets for hypotensive drugs. Rev. Physiol. 5) Philippu, A., Demmeler, R. and Roensberg, G.: Biochem. Pharmacol. 81, 40-100 (1978) Effects of centrally applied drugs on pressor 12) Pacha, W., Salzmann, R. and Scholtysik, G.: responses to hypothalamic stimulation. Naunyn Inhibitory effects of clonidine and BS 100-41 Schmiedeberg's Arch. Pharmacol. 282,- 389 on responses to sympathetic nerve stimulation in 400 (1974) - cats and rabbits. Brit. J. Pharmacol. 53, 513 6) Shaw, J., Hunyor, S. and Korner, P.I.: Sites of 516 (1975) - central nervous system action of clonidine on 13) Scholtysik, G., Jerie, P. and Picard, C.W.: reflex autonomic function in the unanesthetized Pharmacology of Antihypertensive Drugs, Edited rabbit. Europ. J. Pharmacol. 15, 66-78 (1971) by Scriabine, A., p. 79-98, Raven Press, New 7) Struyker Boudier, H.A.J. and van Rossum, York (1980) J.M.: Clonidine-induced cardiovascular effects 14) Berman, A.L.: The Brain Stem of the Cat, The after stereotaxic application in the hypothalamus University of Wisconsin Press, Madison (1968) of rats. J. Pharm. Pharmacol. 24, 410-411 15) Philippu, A., Dieti, H. and Sinha, J.N.: In vivo (1972) release of endogenous catecholamines in the 8) Struyker Boudier, H.A.J., Smeets, G.W.M., hypothalamus. Naunyn-Schmiedeberg's Arch. Brouwer, G.M. and van Rossum, J.M.: Hypo Pharmacol. 308, 137-142 (1979)