Vagal afferent input alters the discharge of osmotic and ANG II-responsive median preoptic neurons projecting to the hypothalamic paraventricular nucleus

Abstract

The goal of the present study was to determine the effect of activating vagal afferent fibers on the discharge of median preoptic (MnPO) neurons responsive to peripheral angiotensin II (ANG II) and osmotic inputs. Vagal afferents were activated by electrical stimulation of the proximal end of the transected cervical vagus nerve (3 pulses, 100 Hz, 1 ms, 100-500 muA). Of 21 MnPO neurons, 19 were antidromically activated from the hypothalamic paraventricular nucleus (PVH) (latency: 10.3+/-1.3 ms, threshold: 278+/-25 muA). MnPO-PVH cells had an average spontaneous discharge of 2.1+/-0.4 Hz. Injection of ANG II (150 ng) and/or hypertonic NaCl (1.5 Osm/L, 100 mul) through the internal carotid artery significantly (P<0.01) increased the firing rate of most MnPO-PVH neurons (16/19, 84%). Vagus nerve stimulation significantly (P<0.01) decreased discharge (-73+/-9%) in 10 of 16 (63%) neurons with an average onset latency of 108+/-19 ms. Among the remaining 6 MnPO-PVH neurons vagal activation either increased discharge (177+/-100%) with a latency of 115+/-15 ms (n=2) or had no effect (n=4). Pharmacological activation of chemosensitive vagal afferents with phenyl biguanide produced an increase (n=3), decrease (n=2), or no change (n=6) in discharge. These observations indicate that a significant proportion of ANG II- and/or osmo-sensitive MnPO neurons receive convergent vagal input. Although the sensory modalities transmitted by the vagal afferents to MnPO-PVH neurons are not presently known, the presence of inhibitory and excitatory vagal-evoked responses indicates that synaptic processing by these cells integrates humoral and visceral information to subserve potentially important cardiovascular and body fluid homeostatic functions.

Fig. 1. Activation of vagal afferent fibers decreased the discharge of MnPO-PVH neurons responsive to osmotic and ANG II stimulation

(A) ICA injection of hypertonic saline significantly increased cell discharge whereas isotonic saline had no effect. This cell was not barosensitive as a PE-evoked increase in ABP did not change neuronal firing rate. Activation of vagal afferents by intravenous injection of PBG did not alter cell discharge. (B) ICA injection of ANG II increased cell discharge. (C) A peristimulus time histogram (5 ms bins) shows that electrical stimulation of the vagus nerve with 3 pulses (100 Hz) at 500 μA significantly decreased cell discharge (*P<0.05) with a latency of 75 ms. (D) This neuron was antidromically activated from the left PVH with a constant latency of 9 ms (a) and stimulus intensity of 150 μA. Note that the antidromic spike was collided with a spontaneous action potential (b). ▾, spontaneous spike;▽, antidromic spike; s, antidromic stimulus artifact

Fig. 2. Activation of vagal afferent fibers decreased the cell discharge of a MnPO-PVH neuron responsive to ANG II stimulation

(A) ICA injection of hypertonic saline decreased cell discharge and increased MAP. However, this neuron was barosensitive as an increase in ABP produced by inflation of an aortic cuff or injection of PE produced a robust decrease in cell discharge. Chemical activation of vagal afferents by PBG rapidly silenced cell discharge. (B) ICA injection of ANG II did not alter cell discharge (a), but when the ANG II-evoked increase in ABP was attenuated with SNP, ICA injection of ANG II produced a clear increase in cell discharge (b). (C) A peristimulus time histogram (5 ms bins, 327 sweeps) shows that electrical stimulation of the vagus nerve with 3 pulses (100 Hz) at 500 μA significantly decreased cell discharge (*P<0.05) with a latency of 80 ms. (D) This neuron was antidromically activated from the left PVH with a constant latency of 14 ms (a) and stimulus threshold of 300 μA. Note that the antidromic spike was collided with a spontaneous spike (b). ▾, spontaneous spike; ▽, antidromic spike; s, antidromic stimulus artifact

Fig. 3. Activation of vagal afferent fibers decreased the neuronal discharge of an osmotically-responsive MnPO-PVH neuron

(A) ICA injection of hypertonic NaCl increased cell discharge whereas isotonic saline had no effect. Raising ABP with PE was also without affect. (B) ICA injection of ANG II did not alter cell discharge. (C) Peristimulus time histograms (5 ms bins) were constructed during electrical stimulation of the cervical vagus with 3 pulses (100 Hz) at 500 μA (a, 300 sweeps), 1 pulse at 500 μA (b, 305 sweeps), and 1 pulse at 100 μA (c, 331 sweeps). Note that electrical stimulation of the vagus nerve produced a biphasic response (a, b) with an initial decrease in cell discharge (*, P<0.05; latency: 70 ms) followed by an increase in discharge (†, P<0.05, latency 340 ms). No change in cell discharge was observed with 1 pulse at 100 μA (c). (D) This neuron was antidromically activated from the left PVN with a constant onset latency of 9 ms and activation threshold of 150 μA (a). The antidromic spike collided with a spontaneous action potential (b), and the neuron followed high frequency stimulation (>333 Hz, trace not shown). This neuron could not be antidromically activated from the right PVN (stimulus intensity: 1 mA, traces not shown). ▾, spontaneous spike; ▽, antidromic spike; s, antidromic stimulus artifact

Fig. 4. Activation of vagal afferent fibers increased the discharge of an osmotically- responsive MnPO-PVH neuron

(A) ICA injection of hypertonic NaCl increased cell discharge whereas isotonic saline had no effect. Although stimulating and unloading arterial baroreceptors with PE and SNP, respectively, did not alter cell discharge, activation of vagal afferent fibers with PBG produced a rapid and transient increase in cell discharge. (B) ICA injection of ANG II did not alter the neuronal firing rate of this neuron. (C) Peristimulus time histograms (5 ms bins) triggered by stimulation of the cervical vagus with 3 pulses (100 Hz) at 500 μA (a, 460 sweeps) and 1 pulse at 500 μA (b, 354 sweeps) reveal a biphasic response with an initial increase in cell discharge (*, P<0.05; latency: 85 ms) followed by an immediate decrease (†, P<0.05; latency: 140 ms). The magnitude of the response was smaller with 1 pulse. The effect was not observed when the stimulus intensity was 100 μA. (D) This neuron was antidromically activated from the left PVH with a constant latency of 6 ms (a) and stimulus threshold of 250 μA. The antidromic spike was collided with a spontaneous spike thereby confirming its antidromic nature (b). ▾, spontaneous spike; ▽, antidromic spike; s, antidromic stimulus artifact

Fig. 5. Location of osmotic and/or ANG II-responsive MnPO-PVH neurons that displayed an increase or decrease in cell discharge during activation of vagal afferent fibers

(A) As previously reported (Stocker and Toney, 2005), MnPO-PVH neurons responsive to osmotic and ANG II stimulation were located throughout the rostral-caudal distribution of the nucleus (not shown). There was no observable difference between the location of MnPO-PVH neurons that increased (□) or decreased (•) discharge to activation of vagal afferent fibers. (B, C) Examples of juxtacellular-labeled neurons. The neurons are located in the precommisural MnPO (B) and the dorsal MnPO (C) nucleus. Note that the morphology of both neurons is similar – both have a simple bipolar cytoarchitecture with limited dendritic branching. Where sufficiently resolved, presumptive axons appear to emerge from a proximal dendrite, not from the cell soma. dMnPO, dorsal median preoptic nucleus; vMnPO, ventral median preoptic nucleus; AC, anterior commissure; f, fornix; 3V, 3^rd ventricle, OC, optic chiasm.