Differentiation of autonomic reflex control begins with cellular mechanisms at the first synapse within the nucleus tractus solitarius

Visceral afferents send information via cranial nerves to the nucleus tractus solitarius (NTS). The NTS is the initial step of information processing that culminates in homeostatic reflex responses. Recent evidence suggests that strong afferent synaptic responses in the NTS are most often modulated by depression and this forms a basic principle of central integration of these autonomic pathways. The visceral afferent synapse is uncommonly powerful at the NTS with large unitary response amplitudes and depression rather than facilitation at moderate to high frequencies of activation. Substantial signal depression occurs through multiple mechanisms at this very first brainstem synapse onto second order NTS neurons. This review highlights new approaches to the study of these basic processes featuring patch clamp recordings in NTS brain slices and optical techniques with fluorescent tracers. The vanilloid receptor agonist, capsaicin, distinguishes two classes of second order neurons (capsaicin sensitive or capsaicin resistant) that appear to reflect unmyelinated and myelinated afferent pathways. The differences in cellular properties of these two classes of NTS neurons indicate clear functional differentiation at both the pre- and postsynaptic portions of these first synapses. By virtue of their position at the earliest stage of these pathways, such mechanistic differences probably impart important differentiation in the performance over the entire reflex pathways.

Reliability of monosynaptic sensory transmission in brain stem neurons in vitro

The timing of events within the nervous system is a critical feature of signal processing and integration. In neurotransmission, the synaptic latency, the time between stimulus delivery and appearance of the synaptic event, is generally thought to be directly related to the complexity of that pathway. In horizontal brain stem slices, we examined synaptic latency and its shock-to-shock variability (synaptic jitter) in medial nucleus tractus solitarius (NTS) neurons in response to solitary tract (ST) electrical activation. Using a visualized patch recording approach, we activated ST 1-3 mm from the recorded neuron with short trains (50-200 Hz) and measured synaptic currents under voltage clamp. Latencies ranged from 1.5 to 8.6 ms, and jitter values (SD of intraneuronal latency) ranged from 26 to 764 micros (n = 49). Surprisingly, frequency of synaptic failure was not correlated with either latency or jitter (P > 0.147; n = 49). Despite conventional expectations, no clear divisions in latency were found from the earliest arriving excitatory postsynaptic currents (EPSCs) to late pharmacologically polysynaptic responses. Shortest latency EPSCs (<3 ms) were mediated by non-N-methyl-D-aspartate (non-NMDA) glutamate receptors. Longer latency responses were a mix of excitatory and inhibitory currents including non-NMDA EPSCs and GABAa receptor-mediated currents (IPSC). All synaptic responses exhibited prominent frequency-dependent depression. In a subset of neurons, we labeled sensory boutons by the anterograde fluorescent tracer, DiA, from aortic nerve baroreceptors and then recorded from anatomically identified second-order neurons. In identified second-order NTS neurons, ST activation evoked EPSCs with short to moderate latency (1.9-4.8 ms) but uniformly minimal jitter (31 to 61 micros) that were mediated by non-NMDA receptors but had failure rates as high as 39%. These monosynaptic EPSCs in identified second-order neurons were significantly different in latency and jitter than GABAergic IPSCs (latency, 2.95 +/- 0.71 vs. 5.56 +/- 0.74 ms, mean +/- SE, P = 0.027; jitter, 42.3 +/- 6.5 vs. 416.3 +/- 94.4 micros, P = 0.013, n = 4, 6, respectively), but failure rates were similar (27.8 +/- 9.0 vs. 9.7 +/- 4.4%, P = 0.08, respectively). Such results suggest that jitter and not absolute latency or failure rate is the most reliable discriminator of mono- versus polysynaptic pathways. The results suggest that brain stem sensory pathways may differ in their principles of integration compared with cortical models and that this importantly impacts synaptic performance. The unique performance properties of the sensory-NTS pathway may reflect stronger axosomatic synaptic processing in brain stem compared with dendritically weighted models typical in cortical structures and thus may reflect very different strategies of spatio-temporal integration in this NTS region and for autonomic regulation.

Cellular mechanisms of baroreceptor integration at the nucleus tractus solitarius

The autonomic nervous system makes important contributions to the homeostatic regulation of the heart and blood vessels through arterial baroreflexes, and yet our understanding of the central nervous system mechanisms is limited. The sensory synapse of baroreceptors in the nucleus tractus solitarius (NTS) is unique because its participation is obligatory in the baroreflex. Here we describe experiments targeting this synapse to provide greater understanding of the cellular mechanisms at the earliest stages of the baroreflex. Our approach utilizes electrophysiology, pharmacology, and anatomical tracers to identify and evaluate key elements of the sensory information processing in NTS.

Expression of the orphan receptor steroidogenic factor-1 mRNA in the rat medial basal hypothalamus

Steroidogenic factor-1 (SF-1), an orphan receptor of the nuclear hormone receptor family, binds to the AAGGTCA motif in the promoter elements of several diverse target genes, including some that mediate steroidogenesis and sexual differentiation. In addition, SF-1 is expressed in embryonic forebrain, suggesting that it plays a role in neural development. This study was undertaken to study the distribution and regulation of SF-1 mRNA expression in the rat brain. SF-1 mRNA levels were measured in tissue dissections by ribonuclease protection assay. A 452 nt 32P-labeled cRNA probe, complementary to the putative ligand-binding domain of the rat SF-1 mRNA, was synthesized from the rat SF-1 cDNA inserted into pBluescript II KS, using a Sty 1 fragment and T3 polymerase. The probe protected a single 390 nt transcript in the medial basal hypothalamus (MBH) and peripheral steroidogenic tissues of the male rat. The size of this protected band corresponded to that of the protected sense RNA standard (HindIII fragment of the SF-1 cDNA transcribed with T7 polymerase). No SF-1 mRNA was detected in the preoptic area, amygdala or cingulate cortex. The levels of SF-1 mRNA in MBH were not affected by gonadectomy or androgen treatment, nor was there a sex difference in its expression in adults. In situ hybridization histochemistry revealed that SF-1 was localized to the ventromedial nucleus of the adult hypothalamus. The levels of SF-1 mRNA were high on gestational day 18 after which they fell by approximately 30% and remained constant throughout gestation, the first week of neonatal life, and into adulthood. These results demonstrate that the gene encoding SF-1 is expressed in a discrete region of the rat hypothalamus and appears to be developmentally regulated, but not affected by gonadal hormones in adults.

Effect of physiological factors on proximal flow convergence upstream of an incompetent valve: an in-vitro study

The flow (Q) through regurgitant valves may be quantified by multiplying the area of an isovelocity contour (isovel) by its velocity. This was tested computationally and experimentally (using MRI). Q = 14 to 141 ml/s, using flat and conical orifice plates. Plotting Q versus isovelocity radius, a plateau was found which, for low flow, corresponded to the true Q. At higher flow or large confinement, Q was overestimated. For conical plates, angle correction worked at low Q but not at higher values due to the formation of separation regions. These converted the cone plate into a flat plate. MRI produced similar results at 57 ml/s in that Q was correct with no angle correction. At low flow, MRI was too noisy to produce a clear plateau consistently.