Intrinsic membrane properties of pre-oromotor neurons in the intermediate zone of the medullary reticular formation

Regulation of Rostral Nucleus of the Solitary Tract Responses to Afferent Input by A-type K+ Current

Responses in the rostral (gustatory) nucleus of the solitary tract (rNST) are modified by synaptic interactions within the nucleus and the constitutive membrane properties of the neurons themselves. The potassium current IA is one potential source of modulation. In the caudal NST, projection neurons with IA show lower fidelity to afferent stimulation compared to cells without. We explored the role of an A-type K+ current (IA) in modulating the response to afferent stimulation and GABA-mediated inhibition in the rNST using whole cell patch clamp recording in transgenic mice that expressed channelrhodopsin (ChR2 H134R) in GABAergic neurons. The presence of IA was determined in current clamp and the response to electrical stimulation of afferent fibers in the solitary tract was assessed before and after treatment with the specific Kv4 channel blocker AmmTX3. Blocking IA significantly increased the response to afferent stimulation by 53%. Using dynamic clamp to create a synthetic IA conductance, we demonstrated a significant 14% decrease in responsiveness to afferent stimulation in cells lacking IA. Because IA reduced excitability and is hyperpolarization-sensitive, we examined whether IA contributed to the inhibition resulting from optogenetic release of GABA. Although blocking IA decreased the percent suppression induced by GABA, this effect was attributable to the increased responsiveness resulting from AmmTX3, not to a change in the absolute magnitude of suppression. We conclude that rNST responses to afferent input are regulated independently by IA and GABA.

Inhibitory modulation of optogenetically identified neuron subtypes in the rostral solitary nucleus

Inhibition is presumed to play an important role in gustatory processing in the rostral nucleus of the solitary tract (rNST). One source of inhibition, GABA, is abundant within the nucleus and comes both from local, intrasolitary sources and from outside the nucleus. In addition to the receptor-mediated effects of GABA on rNST neurons, the hyperpolarization-sensitive currents, Ih and IA, have the potential to further modulate afferent signals. To elucidate the effects of GABAergic modulation on solitary tract (ST)-evoked responses in phenotypically defined rNST neurons and to define the presence of IA and Ih in the same cells, we combined in vitro recording and optogenetics in a transgenic mouse model. This mouse expresses channelrhodopsin 2 (ChR2) in GAD65-expressing GABAergic neurons throughout the rNST. GABA positive (GABA+) neurons differed from GABA negative (GABA-) neurons in their response to membrane depolarization and ST stimulation. GABA+ neurons had lower thresholds to direct membrane depolarization compared with GABA- neurons, but GABA- neurons responded more faithfully to ST stimulation. Both IA and Ih were present in subsets of GABA+ and GABA- neurons. Interestingly, GABA+ neurons with Ih were more responsive to afferent stimulation than inhibitory neurons devoid of these currents, whereas GABA- neurons with IA were more subject to inhibitory modulation. These results suggest that the voltage-gated channels underlying IA and Ih play an important role in modulating rNST output through a circuit of feedforward inhibition.

A physiological perspective on the neuroscience of eating

I present the thesis that 'being physiological,' i.e., analyzing eating under conditions that do not perturb, or minimally perturb, the organism's endogenous processes, should be a central goal of the neuroscience of eating. I describe my understanding of 'being physiological' based on [i] the central neural-network heuristic of CNS function that traces back to Cajal and Sherrington, [ii] research on one of the simpler problems in the neuroscience of eating, identification of endocrine signals that control eating. In this context I consider natural meals, physiological doses and ranges, and antagonist studies. Several examples involve CCK. Next I describe my view of the cutting edge in the molecular neuroscience of eating as it has evolved from the discovery of leptin signaling through the application of optogenetic and pharmacogenetic methods. Finally I describe some novel approaches that may advance the neuroscience of eating in the foreseeable future. I conclude that [i] the neuroscience of eating may soon be able to discern 'physiological' function in the operation of CNS networks mediating eating, [ii] the neuroscience of eating should capitalize on methods developed in other areas of neuroscience, e.g., improved methods to record and manipulate CNS function in behaving animals, identification of canonical regional circuits, use of population electrophysiology, etc., and [iii] subjective aspects of eating are crucial aspects of eating science, but remain beyond mechanistic understanding.

The pronociceptive dorsal reticular nucleus contains mostly tonic neurons and shows a high prevalence of spontaneous activity in block preparation

Despite the importance and significant clinical impact of understanding information processing in the nociceptive system, the functional properties of neurons in many parts of this system are still unknown. In this work we performed whole cell patch-clamp recording in rat brain stem blocks to characterize the electrophysiological properties of neurons in the dorsal reticular nucleus (DRt), a region known to be involved in pronociceptive modulation. We also compared properties of DRt neurons with those in the adjacent parvicellular reticular nucleus and in neighboring regions outside the reticular formation. We found that neurons in the DRt and parvicellular reticular nucleus had similar electrophysiological properties and exhibited mostly toniclike firing patterns, whereas neurons outside the reticular formation showed a larger diversity of firing patterns. Interestingly, more than one-half of the neurons also showed spontaneous activity. While the general view of the reticular formation, being a loosely associated mesh of groups of neurons with diverse function, and earlier reports suggests more electrophysiological heterogeneity, we showed that this is indeed not the case. Our results indicate that functional difference of neurons in the reticular formation may mostly be determined by their connectivity profiles and not by their intrinsic electrophysiological properties. The dominance of tonic neurons in the DRt supports previous conclusions that these neurons encode stimulus intensity through their firing frequency, while the high prevalence of spontaneous activity most likely shapes nociceptive modulation by this brain stem region.

Hindbrain neurons as an essential hub in the neuroanatomically distributed control of energy balance

This Review highlights the processing and integration performed by hindbrain nuclei, focusing on the inputs received by nucleus tractus solitarius (NTS) neurons. These inputs include vagally mediated gastrointestinal satiation signals, blood-borne energy-related hormonal and nutrient signals, and descending neural signals from the forebrain. We propose that NTS (and hindbrain neurons, more broadly) integrate these multiple energy status signals and issue-output commands controlling the behavioral, autonomic, and endocrine responses that collectively govern energy balance. These hindbrain-mediated controls are neuroanatomically distributed; they involve endemic hindbrain neurons and circuits, hindbrain projections to peripheral circuits, and projections to and from midbrain and forebrain nuclei.

Activation of NPY receptors suppresses excitatory synaptic transmission in a taste-feeding network in the lower brain stem

Consummatory responses to taste stimuli are modulated by visceral signals processed in the caudal nucleus of the solitary tract (cNST) and ventrolateral medulla. On the basis of decerebrate preparations, this modulation can occur through local brain stem pathways. Among the large number of neuropeptides and neuromodulators implicated in these visceral pathways is neuropeptide Y (NPY), which is oftentimes colocalized in catecholaminergic neurons themselves implicated in glucoprivic-induced feeding and satiety. In addition to the cNST and ventrolateral medulla, noradrenergic and NPY receptors are found in circumscribed regions of the medullary reticular formation rich in preoromotor neurons. To test the hypothesis that NPY may act as a neuromodulator on preoromotor neurons, we recorded the effects of bath application of NPY and specific Y1 and Y2 agonists on currents elicited from electrical stimulation of the rostral (taste) NST in prehypoglossal neurons in a brain stem slice preparation. A high proportion of NST-driven responses were suppressed by NPY, as well as Y1 and Y2 agonists. On the basis of paired pulse ratios and changes in membrane resistance, we concluded that Y1 receptors influence these neurons both presynaptically and postsynaptically and that Y2 receptors have a presynaptic locus. To test the hypothesis that NPY may act in concert with norepinephrine (NE), we examined neurons showing suppressed responses in the presence of a Y2 agonist and demonstrated a greater degree of suppression to a Y2 agonist/NE cocktail. These suppressive effects on preoromotoneurons may reflect a satiety pathway originating from A2 neurons in the caudal brain stem.

Genetic control of a central pattern generator: rhythmic oromotor movement in mice is controlled by a major locus near Atp1a2

Fluid licking in mice is a rhythmic behavior that is controlled by a central pattern generator (CPG) located in a complex of brainstem nuclei. C57BL/6J (B6) and DBA/2J (D2) strains differ significantly in water-restricted licking, with a highly heritable difference in rates (h(2)≥0.62) and a corresponding 20% difference in interlick interval (mean ± SEM = 116.3±1 vs 95.4±1.1 ms). We systematically quantified motor output in these strains, their F(1) hybrids, and a set of 64 BXD progeny strains. The mean primary interlick interval (MPI) varied continuously among progeny strains. We detected a significant quantitative trait locus (QTL) for a CPG controlling lick rate on Chr 1 (Lick1), and a suggestive locus on Chr 10 (Lick10). Linkage was verified by testing of B6.D2-1D congenic stock in which a segment of Chr 1 of the D2 strain was introgressed onto the B6 parent. The Lick1 interval on distal Chr 1 contains several strong candidate genes. One of these is a sodium/potassium pump subunit (Atp1a2) with widespread expression in astrocytes, as well as in a restricted population of neurons. Both this subunit and the entire Na(+)/K(+)-ATPase molecule have been implicated in rhythmogenesis for respiration and locomotion. Sequence variants in or near Apt1a2 strongly modulate expression of the cognate mRNA in multiple brain regions. This gene region has recently been sequenced exhaustively and we have cataloged over 300 non-coding and synonymous mutations segregating among BXD strains, one or more of which is likely to contribute to differences in central pattern generator tempo.

Temporal coding of taste in the parabrachial nucleus of the pons of the rat

Recent studies have provided evidence that temporal coding contributes significantly to encoding taste stimuli at the first central relay for taste, the nucleus of the solitary tract (NTS). However, it is not known whether this coding mechanism is also used at the next synapse in the central taste pathway, the parabrachial nucleus of the pons (PbN). In the present study, electrophysiological responses to taste stimuli (sucrose, NaCl, HCl, and quinine) were recorded from 44 cells in the PbN of anesthetized rats. In 29 cells, the contribution of the temporal characteristics of the response to the discrimination of various taste qualities was assessed. A family of metrics that quantifies the similarity of two spike trains in terms of spike count and spike timing was used. Results showed that spike timing in 14 PbN cells (48%) conveyed a significant amount of information about taste quality, beyond what could be conveyed by spike count alone. In another 14 cells (48%), the rate envelope (time course) of the response contributed significantly more information than spike count alone. Across cells there was a significant correlation (r = 0.51; P < 0.01) between breadth of tuning and the proportion of information conveyed by temporal dynamics. Comparison with previous data from the NTS (Di Lorenzo PM and Victor JD. J Neurophysiol 90: 1418-31, 2003 and J Neurophysiol 97: 1857-1861, 2007) showed that temporal coding in the NTS occurred in a similar proportion of cells and contributed a similar fraction of the total information at the same average level of temporal precision, even though trial-to-trial variability was higher in the PbN than in the NTS. These data suggest that information about taste quality conveyed by the temporal characteristics of evoked responses is transmitted with high fidelity from the NTS to the PbN.