The unsilent majority-TRPV1 drives "spontaneous" transmission of unmyelinated primary afferents within cardiorespiratory NTS

Abstract

Cranial primary afferent sensory neurons figure importantly in homeostatic control of visceral organ systems. Of the two broad classes of visceral afferents, the role of unmyelinated or C-type class remains poorly understood. This review contrasts key aspects of peripheral discharge properties of C-fiber afferents and their glutamate transmission mechanisms within the solitary tract nucleus (NTS). During normal prevailing conditions, most information arrives at the NTS through myelinated A-type nerves. However, most of visceral afferent axons (75-90%) in NTS are unmyelinated, C-type axons. Centrally, C-type solitary tract (ST) afferent terminals have presynaptic transient receptor potential vanilloid type 1 (TRPV1) receptors. Capsaicin activation of TRPV1 blocks phasic or synchronous release of glutamate but facilitates release of glutamate from a separate pool of vesicles. This TRPV1-operated pool of vesicles is active at normal temperatures and is responsible for actively driving a 10-fold higher release of glutamate at TRPV1 compared with TRPV1- terminals even in the absence of afferent action potentials. This novel TRPV1 mechanism is responsible for an additional asynchronous release of glutamate that is not present in myelinated terminals. The NTS is rich with presynaptic G protein-coupled receptors, and the implications of TRPV1-operated glutamate offer unique targets for signaling in C-type sensory afferent terminals from neuropeptides, inflammatory mediators, lipid metabolites, cytokines, and cannabinoids. From a homeostatic view, this combination could have broad implications for integration in chronic pathological disturbances in which the numeric dominance of C-type endings and TRPV1 would broadly disturb multisystem control mechanisms.

Fig. 1.

Whole nerve activity and single fiber nerve activity from the aortic depressor nerve (ADN). Top: a whole ADN activity recorded for two cardiac cycles from a conscious rabbit while occlusion of the descending aorta forced arterial pressure higher. Bottom: recorded activity from a thin, split fiber divided from the ADN of an anesthetized rabbit. Whole nerve recording registers activity from a nerve trunk containing thousands of axons but reflects electrical signal interactions from an unknown number of these axons (14). The single fiber recording shows the phasic activation of a regularly discharging aortic baroreceptor whose instantaneous frequency encodes and reports details of the arterial pressure typical of myelinated, A-type baroreceptors. A cluster of small amplitude spikes likely from C-type baroreceptors fires sparsely only at the peak of systole.

Fig. 2.

Myelinated aortic baroreceptors have highly reproducible and detailed pressure responses, but C-fiber aortic baroreceptors have quite sparse firing patterns with variable discharge. Recordings and data modified with permission from Yao and Thoren (114). Left: Five consecutive responses from a representative A-fiber baroreceptor (top) and C-fiber baroreceptor (bottom) were R-wave averaged overlays of action potential time of occurrence. The solid line is arterial pressure in each case. Right: averaged data from this study was replotted to display the distribution of pressure thresholds for 35 myelinated and 28 unmyelinated fibers. Note that nearly all unmyelinated fibers required greater pressures than the highest myelinated baroreceptor threshold.

Fig. 3.

Central transmission for transient receptor potential vanilloid 1 (TRPV1)+ and TRPV1− solitary tract (ST) afferents. Recordings are from a rat horizontal solitary tract nucleus (NTS) slice. Panels show responses to ST activation in two different representative second-order NTS neurons with TRPV1+ ST input (top) and TRPV1− ST input (bottom). Labels mark the separate regions of the record analyzed to measure the specified features. Traces for five stimulation trials are overlaid in each case. Basal activity is measured for 1 s before ST activation (Sync, expanded in inset). ST activation delivered five 100 ms shocks at 50 Hz. Sampling continued for 6 s before repeating (note the broken axis). The 1 s following the ST synced responses is the asynchronous period (Async). The typical TRPV1+ neuron has high Basal spontaneous excitatory postsynaptic current (sEPSC) rate before ST activation and the Async period has elevated EPSC activity that decays in frequency back to the Basal rate by the end of 6 s. ST-EPSC activity was blocked from the TRPV1+ afferent during capsaicin exposure (not shown). TRPV1− afferents do not have additional EPSC activity following identical ST shocks and capsaicin does not block the ST-EPSC activity.

Fig. 4.

For neurons with TRPV1+ solitary tract afferents, the rate of sEPSCs increases when bath temperature is raised. The histogram of sEPSC rate over time is depicted in grey and black outline. Recording from a second-order TRPV1+ neuron in a horizontal slice of rat NTS identified as outlined in Fig. 3. Red line and axis indicates bath temperature controlled by an in-line heater. Without electrical stimulation, the spontaneous EPSC rate was high and reproducibly tracked increases in bath temperature (red) between 31°C and 37°C. Such temperature responses persist in tetrodotoxin (TTX) or calcium channel blockers such as cadmium but are attenuated by TRPV1 antagonists such as SB-366791. Similar recordings in TRPV1− second-order neurons showed low sEPSC rates and little change with similar temperature shifts (not shown).