Both α1- and α2-adrenoceptors in the insular cortex are involved in the cardiovascular responses to acute restraint stress in rats

Abstract

The insular cortex (IC) is a limbic structure involved in cardiovascular responses observed during aversive threats. However, the specific neurotransmitter mediating IC control of cardiovascular adjustments to stress is yet unknown. Therefore, in the present study we investigated the role of local IC adrenoceptors in the cardiovascular responses elicited by acute restraint stress in rats. Bilateral microinjection of different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α1-adrenoceptor antagonist WB4101 into the IC reduced both the arterial pressure and heart rate increases elicited by restraint stress. However, local IC treatment with different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α2-adrenoceptor antagonist RX821002 reduced restraint-evoked tachycardia without affecting the pressor response. The present findings are the first direct evidence showing the involvement of IC adrenoceptors in cardiovascular adjustments observed during aversive threats. Our findings indicate that IC noradrenergic neurotransmission acting through activation of both α1- and α2-adrenoceptors has a facilitatory influence on pressor response to acute restraint stress. Moreover, IC α1-adrenoceptors also play a facilitatory role on restraint-evoked tachycardiac response.

Figure 1. A photomicrograph of a coronal brain section, from one representative rat, which shows bilateral injection sites in the insular cortex.

Diagrammatic representation based on the rat brain atlas of Paxinos and Watson (1997) indicating the microinjection sites of vehicle (white circles), WB4101 (black circles) and RX821002 (black squares) into the IC as well as WB4101 (gray circles) and RX821002 (gray squares) into structures surrounding the IC. Cg1 – cingulate cortex, area; PrL – prelimbic cortex, M1 – primary motor cortex; insular cortex – insular cortex, cc – corpus callosum, forceps minor of the corpus callosum (fmi).

Figure 2. Representative recordings of mean arterial pressure (MAP), pulsatile arterial pressure (PAP) and heart rate (HR) of representative rats submitted to acute restraint stress.

A) Recordings of MAP, PAP and HR of one representative rat treated with vehicle (ACSF) into the insular cortex and submitted to acute restraint stress. The onset of restraint is at t = 0. B) Recordings of MAP, PAP and HR of one representative rat treated with the selective α₁-adrenoceptor antagonist WB4101 into the insular cortex and submitted to acute restraint stress. The onset of restraint is at t = 0. Note the decrease in the MAP, PAP and HR responses to restraint stress in animals treated with WB4101 into the insular cortex. C) Recordings of MAP, PAP and HR of one representative rat treated with the selective α₂-adrenoceptor antagonist RX821002 into the insular cortex and submitted to acute restraint stress. The onset of restraint is at t = 0. Note the decrease in the MAP and PAP responses to restraint stress in animals treated with RX821002 into the insular cortex.

Figure 3. Changes in mean arterial pressure (ΔMAP) and heart rate (ΔHR) evoked by acute restraint stress in animals treated with different doses of the selective α₁-adrenoceptor antagonist WB4101 into the insular cortex.

(Left) Time-course of ΔMAP and ΔHR during acute restraint stress in rats treated with vehicle (ACSF, 100 nl, n = 7) or different doses (0.3, 5, 10 and 15 nmol/100 nl, n = 5/group) of WB4101 into the insular cortex. The onset of exercise was at t = 0. Circles represent the mean and bars the S.E.M. # P<0.05, indicates a significant difference over the whole restraint stress period compared to vehicle treated animals; ANOVA followed by Bonferroni's post test. (Right) ΔMAP and ΔHR during acute restraint stress in rats treated with increasing doses of WB4101 (0.3, 5, 10 and 15 nmol/100 nl). V: vehicle (ACSF, 100 nl). Dose–effect curves were generated by nonlinear regression analysis. Data shown represent the means±S.E.M. of the variation of MAP and HR during the 60 min of restraint. # P<0.05, significantly different from vehicle group; one-way ANOVA followed by Bonferroni's post test.

Figure 4. Changes in mean arterial pressure (ΔMAP) and heart rate (ΔHR) evoked by acute restraint stress in animals treated with different doses of the selective α₂-adrenoceptor antagonist RX821002 into the insular cortex.

(Left) Time-course of ΔMAP and ΔHR during acute restraint stress in rats treated with vehicle (ACSF, 100 nl, n = 6) or different doses (0.3, 5, 10 and 15 nmol/100 nl, n = 5/group) of the selective α₂-adrenoceptor antagonist RX821002 into the insular cortex. The onset of exercise was at t = 0. Circles represent the mean and bars the S.E.M. # P<0.05, indicates a significant difference over the whole restraint stress period compared to vehicle treated animals; ANOVA followed by Bonferroni's post test. (Right) ΔMAP and ΔHR during restraint stress in rats treated with increasing doses of RX821002 (0.3, 5, 10 and 15 nmol/100 nl). V: vehicle (ACSF, 100 nl). Dose–effect curves were generated by nonlinear regression analysis. Data shown represent the means±S.E.M. of the variation of MAP and HR during the 60 min of restraint. # P<0.05, significantly different from vehicle group; one-way ANOVA followed by Bonferroni's post test.