Role of ionotropic GABA, glutamate and glycine receptors in the tonic and reflex control of cardiac vagal outflow in the rat

Abstract

Background: Cardiac vagal preganglionic neurons (CVPN) are responsible for the tonic, reflex and respiratory modulation of heart rate (HR). Although CVPN receive GABAergic and glutamatergic inputs, likely involved in respiratory and reflex modulation of HR respectively, little else is known regarding the functions controlled by ionotropic inputs. Activation of g-protein coupled receptors (GPCR) alters these inputs, but the functional consequence is largely unknown. The present study aimed to delineate how ionotropic GABAergic, glycinergic and glutamatergic inputs contribute to the tonic and reflex control of HR and in particular determine which receptor subtypes were involved. Furthermore, we wished to establish how activation of the 5-HT1A GPCR affects tonic and reflex control of HR and what ionotropic interactions this might involve.

Results: Microinjection of the GABAA antagonist picrotoxin into CVPN decreased HR but did not affect baroreflex bradycardia. The glycine antagonist strychnine did not alter HR or baroreflex bradycardia. Combined microinjection of the NMDA antagonist, MK801, and AMPA antagonist, CNQX, into CVPN evoked a small bradycardia and abolished baroreflex bradycardia. MK801 attenuated whereas CNQX abolished baroreceptor bradycardia. Control intravenous injections of the 5-HT1A agonist 8-OH-DPAT evoked a small bradycardia and potentiated baroreflex bradycardia. These effects were still observed following microinjection of picrotoxin but not strychnine into CVPN.

Conclusions: We conclude that activation of GABAA receptors set the level of HR whereas AMPA to a greater extent than NMDA receptors elicit baroreflex changes in HR. Furthermore, activation of 5-HT1A receptors evokes bradycardia and enhances baroreflex changes in HR due to interactions with glycinergic neurons involving strychnine receptors. This study provides reference for future studies investigating how diseases alter neurochemical inputs to CVPN.

Figure 1

Verification that dual bilateral microinjections in the ventral medulla inhibits cardiac vagal outflow. A representative trace is provided in panel A showing that microinjection of muscimol (100 mM, 100 nl) into four regions of the medulla where bradycardia greater than 50 bpm was evoked by prior microinjection of L-glutamate (100 mM, 50 nl) increases resting heart rate (HR) and prevents baroreflex mediated bradycardia. Grouped data (n = 4) shows that microinjection of muscimol into these four sites evokes a large increase in HR (panel B) and dramatically reduces baroreflex sensitivity (BRS, panel C). * P < 0.05.

Figure 2

Role of ionotropic GABA_A and strychnine sensitive glycine receptors in areas of the medulla containing cardiac vagal preganglionic neurons in the tonic and baroreflex control of HR. Panel A and B show the effects of bilateral microinjection of the GABA_A antagonists bicuculline (0.4 mM, n = 4) and picrotoxin (2 mM, n = 6) respectively on resting heart rate (HR) and baroreflex evoked bradycardia induced using phenylephrine (PE). Both bicuculline and picrotoxin evoked large and similar decreases in resting HR. Bicuculline, however, evoked arrhythmia and the effects on baroreflex bradycardia could not be quantified. Panel C shows the effects of bilateral microinjection of strychnine (3 mM, n = 4) on resting HR and baroreflex bradycardia. Group data (panels D and E) shows that picrotoxin evoked a large decrease in resting HR whereas strychnine had no effect (panel D). Neither picrotoxin nor strychnine affected baroreflex bradycardia or baroreflex sensitivity (BRS, panel E). *** P < 0.001.

Figure 3

Role of ionotropic glutamate receptors in areas of the medulla containing CVPN in the tonic and baroreflex control of HR. Panel A illustrates the effects of bilateral microinjection of the NMDA antagonist MK801 (5 mM, n = 4), panel B illustrates the effects of bilateral microinjection of the AMPA antagonist CNQX (2 mM, n = 3) and panel C illustrates the effects of bilateral microinjection of CNQX following MK801 (n = 4) on resting heart rate (HR) and baroreflex bradycardia. Panel D illustrates the effects of these antagonists on resting HR. Neither MK801 nor CNQX alone altered resting HR; however, combined microinjection of these antagonists evoked a small but significant decrease in resting HR. In panel E the effect of these antagonists on baroreflex function are shown. Microinjection of MK801 evoked a modest decrease in baroreflex sensitivity (BRS) whereas microinjection of CNQX dramatically reduced BRS and abolished all baroreflex function. Combined microinjection of MK801 and CNQX reduced BRS to levels similar to that observed following microinjection of CNQX alone. * P < 0.05, ** P < 0.01.

Figure 4

Role of GABAergic and glycinergic neurotransmission in regions of the medulla containing cardiac vagal preganglionic neurons in cardio-vagal responses to 5-HT_1A receptor activation. Panel A illustrates the effects of intravenous injection of the 5-HT_1A agonist 8-OH-DPAT (0.1 mg/kg, n = 9) on heart rate (HR) and baroreflex bradycardia. Panels B and C shows the intravenous injection of 8-OH-DPAT on HR and baroreflex bradycardia following prior microinjection of the GABA_A antagonist picrotoxin (2 mM, n = 6) and glycine antagonist strychnine (3 mM, n = 4) into regions of the medulla containing cardiac vagal preganglionic neurons respectively. Panel D shows that an injection of 8-OH-DPAT alone evokes a small bradycardia which is not altered by prior microinjection of picrotoxin. Prior microinjection of strychnine prevents this bradycardia. Panel E shows that injections of 8-OH-DPAT alone increase baroreflex sensitivity (BRS). This increase in BRS is still seen following prior microinjection of picrotoxin but not strychnine. * P < 0.05, ** P < 0.01.