Cortical Hub for Flavor Sensation in Rodents

The experience of eating is inherently multimodal, combining intraoral gustatory, olfactory, and somatosensory signals into a single percept called flavor. As foods and beverages enter the mouth, movements associated with chewing and swallowing activate somatosensory receptors in the oral cavity, dissolve tastants in the saliva to activate taste receptors, and release volatile odorant molecules to retronasally activate olfactory receptors in the nasal epithelium. Human studies indicate that sensory cortical areas are important for intraoral multimodal processing, yet their circuit-level mechanisms remain unclear. Animal models allow for detailed analyses of neural circuits due to the large number of molecular tools available for tracing and neuronal manipulations. In this review, we concentrate on the anatomical and neurophysiological evidence from rodent models toward a better understanding of the circuit-level mechanisms underlying the cortical processing of flavor. While more work is needed, the emerging view pertaining to the multimodal processing of food and beverages is that the piriform, gustatory, and somatosensory cortical regions do not function solely as independent areas. Rather they act as an intraoral cortical hub, simultaneously receiving and processing multimodal sensory information from the mouth to produce the rich and complex flavor experience that guides consummatory behavior.

Coherent theta activity in the medial and orbital frontal cortices encodes reward value

This study examined how the medial frontal (MFC) and orbital frontal (OFC) cortices process reward information. We simultaneously recorded local field potentials in the two areas as rats consumed liquid sucrose rewards. Both areas exhibited a 4–8 Hz ‘theta’ rhythm that was phase-locked to the lick cycle. The rhythm tracked shifts in sucrose concentrations and fluid volumes, demonstrating that it is sensitive to differences in reward magnitude. The coupling between the rhythm and licking was stronger in MFC than OFC and varied with response vigor and absolute reward value in the MFC. Spectral analysis revealed zero-lag coherence between the cortical areas, and found evidence for a directionality of the rhythm, with MFC leading OFC. Our findings suggest that consummatory behavior generates simultaneous theta range activity in the MFC and OFC that encodes the value of consumed fluids, with the MFC having a top-down role in the control of consumption.

138 Randel Lecture: Reproductive Behavior – the basis of reproductive success

In domesticated, promiscuous species like sheep, mate choice is constrained by breeding strategies with limited mate choice. The proportion of rams with limited sexual interest in ewes is nearly 30% of the total number of breeding rams. Rams with low sexual interest detect putative sexual signals from ewes but fail to act in a suitable manner. While a tempered reward mechanism with decreased dopamine synthesis in the mid-brain of low sexually performing rams may partially account for this behavior, how the ewe responds to low-sexually performing rams remained unknown. Ewes are relatively undemonstrative in the expression of sexual behavior. The most widely expressed behavior from the ewe is the increased ambulatory behavior to seek out and affiliation with a ram. The expression of this behavior is estrogen dependent and likely increases the sexual interest of the ram and the reproductive success of the ewe. Rams with low sexual interest do mate and sire lambs. It might be expected that ewe affiliation behavior would increase when the ratio of ewes to rams is high (ie. breeding intensity) or sexual interest is low. This does not appear to be the case since ewes express more proceptive behavior toward rams with greater sexual interest. Ewes spent more time in affiliation with high sexually performing rams (P < 0.001) 57.7% (±3.3) than those expressing low sexual interest (23.8% [± 3.4]). Rams expressing high sexually interest garner more solicitous looks from ewes than do low performing rams. Time in affiliation highly correlated with expression of ram behaviors including number of mounts, total anticipatory, and consummatory behavior. While ewes exhibited increased number of purposeful moves toward high performing rams, movement away from the ram to did not differ by expression of male behavior. In conclusion, ewe behavior does not compensate for low sexual interest from the ram and would not be expected to contribute to the reproductive success of rams expressing low sexual interest.

Rhythmic activity in the medial and orbital frontal cortices tracks reward value and the vigor of consummatory behavior

ABSTRACTThis study examined how the medial frontal (MFC) and orbital frontal (OFC) cortices process reward information to guide behavior. We simultaneously recorded local field potentials in the two areas as rats consumed liquid sucrose rewards and examined how the areas collectively process reward information. Both areas exhibited a 4-8 Hz “theta” rhythm that was phase locked to the lick cycle. The rhythm similarly tracked shifts in sucrose concentrations and fluid volumes, suggesting that it is sensitive to general differences in reward magnitude. Differences between the MFC and OFC were noted, specifically that the rhythm varied with response vigor and absolute reward value in the MFC, but not the OFC. Our findings suggest that the MFC and OFC concurrently process reward information but have distinct roles in the control of consummatory behavior.

A ventral striatal-orexin/hypocretin circuit modulates approach but not consumption of food

SummaryFeeding is at once both a basic biological need and a function set in a complex system of competing motivational drivers. Orexin/hypocretin neurons are located exclusively within the lateral hypothalamus (LH) and are commonly implicated in feeding, arousal, and motivated behavior, although largely based on studies employing long-term systemic manipulations. Here we show how orexin neurons in freely behaving mice respond in real time to food presentations, and how this response is modulated by differences in metabolic state and salience. Orexin neurons increased activity during approach to food, and this activity declined to baseline at the start of consummatory behavior. Furthermore, the activity of orexin neurons on approach was enhanced by manipulations of metabolic state, and increased food salience. We investigated the nucleus accumbens shell (NAcSh) as a candidate afferent region to inhibit LH orexin neurons following approach, and using projection and cell type-specific electrophysiology, demonstrated that the NAcSh forms both direct and indirect inhibitory projections to LH orexin cells. Together these findings reveal that the activity of orexin neurons is associated with food approach rather than consumption, is modulated by motivationally relevant factors, and that the NAcSh-LH pathway is capable of suppressing orexin cell recruitment.

Effects of Immediate or Delayed Estradiol on Behavior in Old Menopausal Macaques on Obesogenic Diet

Macaques have served as effective models of human disease, including pathological processes associated with obesity and the metabolic syndrome. This study approached several questions: (1) does a western-style diet (WSD) contribute to sedentary behavior or is sedentary behavior a consequence of obesity and (2) does estradiol (E) hormone therapy offset WSD or ameliorate sedentary behavior? We further questioned whether the timing of E administration (immediately following hysterectomy, ImE; or after a 2-year delay, DE) would impact behavior. Focal observations were taken on the animals in social housing over a period of 2.5 years before and after initiation of the WSD and hysterectomy. In addition, anxiety was assessed through the Human Intruder and Novel Object Tests. All animals gained weight, but ImE delayed the time to maximum weight achieved at 18 months. Over the course of the study, ImE-treated monkeys spent more time “alone” and less time in “close social” contact than placebo-controls. The DE-treated monkeys were not different from placebo-controls in these 2 outcomes. The placebo-control group exhibited more “self-groom” behavior, an indicator of anxiety, than did the ImE-treated group, and DE-treated animals approached levels observed in the ImE-treated animals. All animals exhibited an increase in “consume” behavior over time with no statistical difference between the groups. By the end of the protocol, the placebo-control group exhibited less activity compared to ImE + DE-treated animals combined. Animals also showed increased anxiety after starting on the WSD in the Human Intruder Test and the Novel Object Test. In summary, the data indicated that WSD per se promoted increased consummatory behavior, sedentary behavior, and anxiety-type behaviors, whereas ImE promoted activity. Thus, WSD may precipitate the behaviors observed in humans who then become obese, sedentary, anxious, and socially isolated. ImE replacement ameliorates some of these behaviors, but not all.