Importância da região AV3V e de mecanismos colinérgicos e angiotensinérgicos centrais para os efeitos cardiovasculares produzidos pela ativação da área rostroventrolateral do bulbo

Abstract

Cardiovascular responses are integrated at different levels of the central nervous system (CNS). In the brainstem, there are different areas related to the cardiovascular control such as the rostral ventrolateral medulla (RVLM) that activates sympathetic pre-ganglionic neurons in the spinal cord (IML) inducing pressor response. Like glutamatergic activation, central cholinergic and angiotensinergic activation modulates sympathetic activity and increases arterial pressure. The RVLM receives inhibitory and excitatory projections from different areas of the central nervous system that are important to modulate cardiovascular responses. One of the areas that send projection to the RVLM is the anteroventral third ventricle (AV3V) region. AV3V lesion impairs many forms of hypertension and reduces pressor responses like those produced by central cholinergic and angiotensinergic activation. Recent study in unanesthetized rats has shown that the AV3V lesion attenuates the pressor response to glutamatergic activation into the RVLM. Besides glutamate, injections of angiotensin II (ANG II) or carbachol (cholinergic agonist) into the RVLM evoke increase in the sympathetic activity and blood pressure. For this reason, in the present study, we investigated the effects of acute (1 day) or chronic (15 days) AV3V lesions on pressor responses produced by ANG II (200 ng/100 nl) or carbachol (1 nmol/100 nl) into the RVLM. Male Holtzman rats (280 a 320 g) with sham or electrolytic AV3V lesions and a stainless steel cannula implanted into the RVLM were used. Mean arterial pressure (MAP) and heart rate (HR) were recorded in unanesthetized rats that had polyethylene tubing (PE 10) implanted into the abdominal aorta through the femoral artery on day before the experiments. A second polyethylene tubing was inserted in the femoral vein for baroreflex and chemoreflex tests. Central injections were made using 5 ml Hamilton syringes. The volume of the central injections into the RVLM was 100 nl. The results have shown that both acute and chronic AV3V lesion attenuate the pressor responses to ANG II (12 ± 3 and 12 ± 5 vs. control: 26 ± 4 mmHg) but not the pressor responses to carbachol (38 ± 4 and 29 ± 3 vs. control: 34 ± 4 mmHg) suggesting that some mechanisms belonging to the RVLM are affected and other are not affected by AV3V lesions. Besides, AV3V lesion does not alter baro and chemoreflex responses produced by endovenous (i.v) injections of phenylephrine, sodium nitroprusside and potassium cyanide. After these results, another question arose: which would be the possible mechanisms impaired by the AV3V lesion that attenuate the pressor responses to RVLM activation? The AV3V region is important for the activation of pressor mechanism like sympathetic activity and vasopressin secretion produced by central cholinergic and angiotensinergic activation. Therefore, the pressor response to glutamate into the RVLM was tested in rats with central cholinergic blockade produced by the injection of atropine (4 nmol/1 ml) or angiotensinergic blockade produced by injection of losartan (100 mg/1 ml) or ZD 7155 (50 mg/1 ml) into the lateral ventricle (LV). Male Holtzman rats with stainless steel cannula implanted into the LV and unilaterally into the RVLM were used. MAP and HR were recorded in unanesthetized rats with polyetylene tubing inserted into the abdominal aorta through the femoral artery and vein. The volume of the central injections into the LV was 1 ml. The results showed that the pressor response to glutamate injected into the RVLM (51 ± 4 mmHg) in control condition was attenuated after injection of atropine (36 ± 5 mmHg), losartan (22 ± 5 mmHg) or ZD 7155 (26 ± 7 mmHg) into the LV. However, a question that remained was about the possible spreading of these antagonists into the brain blocking the receptors directly into the RVLM. For this reason, we tested the pressor responses to glutamate into the RVLM after cholinergic or angiotensinergic blockade into the own RVLM. Mean arterial pressure (MAP) and heart rate (HR) were recorded in unanesthetized rats with cannula only into the RVLM and polyethylene tubing implanted into the abdominal aorta through the femoral artery. The results showed that atropine (4 nmol/100 nl) injected into the RVLM did not alter the pressor response to glutamate into the same site (49± 4 vs. control: 50 ± 4 mmHg). So, only cholinergic mechanisms belonging to forebrain or areas outside the RVLM are important for the pressor response to glutamate into the RVLM. However, the pressor response to glutamate into the RVLM was almost abolished and attenuated after injection of the losartan (5 ± 3 mmHg) or ZD 7155 (33 ± 4 mmHg), respectively, into the RVLM, which did not solve the question about the possible spread of losartan or ZD 7155. With the same purpose we tested the pressor response to ANG II into the RVLM after losartan or ZD 7155 into the LV. In this experiment, the pressor response to ANG II into the RVLM in control condition (26 ± 3 mmHg) was not impaired after losartan or ZD 7155 into the LV (20 ± 3 e 23 ± 1 mmHg, respectively) suggesting that these antagonists injected into the LV do not reach the RVLM. Therefore, angiotensinergic mechanisms belonging in the RVLM or from outside the RVLM (probably forebrain) are important for the pressor response to glutamate into the RVLM. However, it is important to consider that AV3V lesion reduces the pressor response to ANG II into the RVLM, while icv injection of the angiotensinergic antagonists showed no effect in this response, which suggests that the effects of AV3V lesion reducing the pressor response to ANG II into the RVLM are not related to any impairment of forebrain angiotensinergic mechanisms by the lesion. Another aim was to study if glutamatergic receptor activation in the RVLM was necessary for the pressor response to central cholinergic or angiotensinergic activation. For this, male Holtzman rats with stainless steel cannulas implanted into the LV and bilaterally into the RVLM were used. The results showed no difference between the cardiovascular responses produced by carbachol into the LV after kynurenic acid (1 nmol/100 nl) bilaterally into the RVLM (36 ± 1 mmHg and 22 ± 10 bpm) when compared with control condition (39 ± 2 mmHg and 23 ± 14 bpm). Also, there was not difference between the cardiovascular responses produced by ANG II into the LV after kynurenic acid (1 nmol/100 nl) into the RVLM (28 ± 3 mmHg and 10 ± 13 bpm) when compared with control condition (26 ± 2 mmHg and -12 ± 5 bpm). This dose of kynurenic acid almost abolished the pressor response to glutamate (2 nmol/100 nl) injected into the RVLM but not the cardiovascular reflexes produced by baro and chemoreflex activation. Therefore, the pressor response to forebrain cholinergic or angiotensinergic activation does not depend on glutamatergic synapses in the RVLM. Besides, the results also showed that the pressor response to chemoreflex activation does not depend on release of glutamate into the RVLM