Involvement of the supramammillary nucleus in aversive conditioning

Hypothalamic Supramammillary Control of Cognition and Motivation

The supramammillary nucleus (SuM) is a small region in the ventromedial posterior hypothalamus. The SuM has been relatively understudied with much of the prior focus being on its connection with septo-hippocampal circuitry. Thus, most studies conducted until the 21st century examined its role in hippocampal processes, such as theta rhythm and learning/memory. In recent years, the SuM has been "rediscovered" as a crucial hub for several behavioral and cognitive processes, including reward-seeking, exploration, and social memory. Additionally, it has been shown to play significant roles in hippocampal plasticity and adult neurogenesis. This review highlights findings from recent studies using cutting-edge systems neuroscience tools that have shed light on these fascinating roles for the SuM.

Seeking motivation and reward: roles of dopamine, hippocampus and supramammillo-septal pathway

Reinforcement learning and goal-seeking behavior are thought to be mediated by midbrain dopamine neurons. However, little is known about neural substrates of curiosity and exploratory behavior, which occur in the absence of clear goal or reward. This is despite behavioral scientists having long suggested that curiosity and exploratory behaviors are regulated by an innate drive. We refer to such behavior as information-seeking behavior and propose 1) key neural substrates and 2) the concept of environment prediction error as a framework to understand information-seeking processes. The cognitive aspect of information-seeking behavior, including the perception of salience and uncertainty, involves, in part, the pathways from the posterior hypothalamic supramammillary region to the hippocampal formation. The vigor of such behavior is modulated by the following: supramammillary glutamatergic neurons; their projections to medial septal glutamatergic neurons; and the projections of medial septal glutamatergic neurons to ventral tegmental dopaminergic neurons. Phasic responses of dopaminergic neurons are characterized as signaling potentially important stimuli rather than rewards. This paper describes how novel stimuli and uncertainty trigger seeking motivation and how these neural substrates modulate information-seeking behavior.

Supramammillary neurons projecting to the septum regulate dopamine and motivation for environmental interaction in mice

The supramammillary region (SuM) is a posterior hypothalamic structure, known to regulate hippocampal theta oscillations and arousal. However, recent studies reported that the stimulation of SuM neurons with neuroactive chemicals, including substances of abuse, is reinforcing. We conducted experiments to elucidate how SuM neurons mediate such effects. Using optogenetics, we found that the excitation of SuM glutamatergic (GLU) neurons was reinforcing in mice; this effect was relayed by their projections to septal GLU neurons. SuM neurons were active during exploration and approach behavior and diminished activity during sucrose consumption. Consistently, inhibition of SuM neurons disrupted approach responses, but not sucrose consumption. Such functions are similar to those of mesolimbic dopamine neurons. Indeed, the stimulation of SuM-to-septum GLU neurons and septum-to-ventral tegmental area (VTA) GLU neurons activated mesolimbic dopamine neurons. We propose that the supramammillo-septo-VTA pathway regulates arousal that reinforces and energizes behavioral interaction with the environment.

Whole-brain mapping of afferent projections to the bed nucleus of the stria terminalis in tree shrews

The bed nucleus of the stria terminalis (BST) plays an important role in integrating and relaying input information to other brain regions in response to stress. The cytoarchitecture of the BST in tree shrews (Tupaia belangeri chinensis) has been comprehensively described in our previous publications. However, the inputs to the BST have not been described in previous reports. The aim of the present study was to investigate the sources of afferent projections to the BST throughout the brain of tree shrews using the retrograde tracer Fluoro-Gold (FG). The present results provide the first detailed whole-brain mapping of BST-projecting neurons in the tree shrew brain. The BST was densely innervated by the prefrontal cortex, entorhinal cortex, ventral subiculum, amygdala, ventral tegmental area, and parabrachial nucleus. Moreover, moderate projections to the BST originated from the medial preoptic area, supramammillary nucleus, paraventricular thalamic nucleus, pedunculopontine tegmental nucleus, dorsal raphe nucleus, locus coeruleus, and nucleus of the solitary tract. Afferent projections to the BST are identified in the ventral pallidum, nucleus of the diagonal band, ventral posteromedial thalamic nucleus, posterior complex of the thalamus, interfascicular nucleus, retrorubral field, rhabdoid nucleus, intermediate reticular nucleus, and parvicellular reticular nucleus. In addition, the different densities of BST-projecting neurons in various regions were analyzed in the tree shrew brains. In summary, whole-brain mapping of direct inputs to the BST is delineated in tree shrews. These brain circuits are implicated in the regulation of numerous physiological and behavioral processes including stress, reward, food intake, and arousal.

Systemic mechanism of taste, flavour and palatability in brain

Taste is considered as one of the five traditional senses and has the ability to detect the flavour of food and certain minerals. Information of taste is transferred to the cortical gustatory area for identification and discrimination of taste quality. Animals have memory recognition power to maintain the familiar foods which are already encountered. Animal shows neophobic response when it encounters novel taste and shows no hesitation when the food is known to be safe. Palatability is the hedonic reward provided by foods and fluids. Palatability is closely related to neurochemicals, and this chemical influences the consumption of food and fluid. Even though, the food is palatable that can become aversive and avoided as a consequence of postingestional unpleasant experience such as malaise. This review presents the overall view on brain mechanisms of taste, flavour and palatability.

Effects of treadmill exercise on the LiCl-induced conditioned taste aversion in rats

Studies have shown that exercise can enhance learning and memory. Conditioned taste aversion (CTA) is an avoidance behavior induced by associative memory of the taste sensation for something pleasant or neutral with a negative visceral reaction caused by the coincident action of a toxic substance that is tasteless or administered systemically. We sought to measure the effects of treadmill exercise on CTA in rats by investigating the effects of exercise on acquisition, extinction and spontaneous recovery of CTA. We made two groups of rats: an exercise group that ran on a treadmill, and a control group that did not have structured exercise periods. To condition rats to disfavor a sweet taste, consumption of a 0.1% saccharin solution in place of drinking water was paired with 0.15M LiCl (2% body weight, i.p.) to induce visceral discomfort. We measured changes of saccharin consumption during acquisition and extinction of CTA. The exercise and no-exercise groups both acquired CTA to similar levels and showed maximum extinction of CTA around 6 days after acquisition. This result indicates that exercise affects neither acquisition nor extinction of CTA. However, in testing for preservation of CTA after much longer extinction periods that included exercise or not during the intervening period, exercising animals showed a significantly lower saccharin intake, irrespective of having exercised or not during the conditioning phase of the trial. This result suggests that exercise may help to preserve aversive memory (taste aversion in this example) as evidence by the significant spontaneous recovery of aversion in exercising animals.

Brain mechanisms of flavor learning

Once the flavor of the ingested food (conditioned stimulus, CS) is associated with a preferable (e.g., good taste or nutritive satisfaction) or aversive (e.g., malaise with displeasure) signal (unconditioned stimulus, US), animals react to its subsequent exposure by increasing or decreasing ingestion to the food. These two types of association learning (preference learning vs. aversion learning) are known as classical conditioned reactions which are basic learning and memory phenomena, leading selection of food and proper food intake. Since the perception of flavor is generated by interaction of taste and odor during food intake, taste and/or odor are mainly associated with bodily signals in the flavor learning. After briefly reviewing flavor learning in general, brain mechanisms of conditioned taste aversion is described in more detail. The CS-US association leading to long-term potentiation in the amygdala, especially in its basolateral nucleus, is the basis of establishment of conditioned taste aversion. The novelty of the CS detected by the cortical gustatory area may be supportive in CS-US association. After the association, CS input is conveyed through the amygdala to different brain regions including the hippocampus for contextual fear formation, to the supramammillary and thalamic paraventricular nuclei for stressful anxiety or memory dependent fearful or stressful emotion, to the reward system to induce aversive expression to the CS, or hedonic shift from positive to negative, and to the CS-responsive neurons in the gustatory system to enhance the responsiveness to facilitate to detect the harmful stimulus.

A postpartum separation induces c-Fos expression in the supramammillary nucleus of lactating rats

Aim

Elucidation of the neural mechanism of maternal behaviors is a medically and biologically important research task. The rat is the laboratory animal most extensively analyzed for maternal behaviors. However, the neural mechanism that maintains the motivation of postpartum rats for maternal behaviors has not yet been elucidated. In this study, we aimed to identify brain regions involved in the maintenance of motivation for maternal behaviors by detecting brain regions that exhibit changes in nerve activity when the mother rat is separated from her pups.

Methods

Lactating mother rats were separated from their pups on postpartum day 3 and kept away from the pups for a certain period of time, and brain regions that exhibited changes in nerve activity when the rats were separated from their pups and those that exhibited changes in nerve activity when the pups are returned were detected by immunohistochemistry using anti-c-Fos antibody, a marker for increased nerve activity.

Results

Rats that were separated from their pups and with the pups returned later showed increases in the number of c-Fos immunoreactive (c-Fos-IR) cells in the medial preoptic area (MPA), the bed nucleus of the stria terminalis (BST), the caudal portion of posterior hypothalamic area (PH) and the supramamillary nucleus (SUM). In mother rats permanently separated from their pups, only the PH and SUM exhibited an increase in the number of c-Fos-IR cells.

Conclusion

In rats, the SUM is involved in aversive memory and changes in the postpartum anxiety level. The observed increase in the number of c-Fos-IR cells in the SUM of mother rats separated from their pups suggests that the nerve activity change in the SUM, which is involved in aversive memory and anxiety, is involved in the maintenance of maternal behaviors.

Central mechanisms of roles of taste in reward and eating

Taste is unique among sensory systems in its innate association with mechanisms of reward and aversion in addition to its recognition of quality, e.g., sucrose is sweet and preferable, and quinine is bitter and aversive. Taste information is sent to the reward system and feeding center via the prefrontal cortices such as the mediodorsal and ventrolateral prefrontal cortices in rodents and the orbitofrontal cortex in primates. The amygdala, which receives taste inputs, also influences reward and feeding. In terms of neuroactive substances, palatability is closely related to benzodiazepine derivatives and beta-endorphin, both of which facilitate consumption of food and fluid. The reward system contains the ventral tegmental area, nucleus accumbens and ventral pallidum and finally sends information to the lateral hypothalamic area, the feeding center. The dopaminergic system originating from the ventral tegmental area mediates the motivation to consume palatable food. The actual ingestive behavior is promoted by the orexigenic neuropeptides from the hypothalamus. Even palatable food can become aversive and avoided as a consequence of a postingestional unpleasant experience such as malaise. The neural mechanisms of this conditioned taste aversion will also be elucidated.

Differential activation of anterior and midline thalamic nuclei following retrieval of aversively motivated learning tasks

Two thalamic nuclear groups, the anterior thalamic nuclei (ATN) and midline and intralaminar thalamic complex (MITC) have connections to the prefrontal cortex, amygdala, hippocampus and accumbens that are important for learning and memory. However, the anatomical proximity between the ATN and MITC makes it difficult to reveal their roles in memory retrieval of aversive conditioned behavior. To address the issue, we explored the activation of the ATN and MITC, as represented by the expression of the immediate early gene c-fos, following either the retrieval of a conditioned taste aversion (CTA) induced by taste-LiCl pairing (visceral aversion) or of inhibitory avoidance (IA) induced by context-foot shock pairing (somatic aversion) in rats. The anterodorsal (AD) nucleus in the ATN was activated by foot shock and the recall of IA, but not by i.p. injection of LiCl or the recall of CTA. No significant elevation was observed in the other ATN following these treatments. Among nuclei of the MITC, the paraventricular thalamic nucleus (PVT) was activated by the delivery of shock or LiCl and by the recall of both CTA and IA, while the mediodorsal thalamus (MD) and central medial and intermediate thalamus (CM/IMD) were not. The innately aversive taste of quinine did not elevate c-fos expression in either the ATN or MITC. These results suggest that the PVT in the MITC is involved in the processing and retrieval of both taste-malaise and context-shock association tasks, while the AD in the ATN is involved in those of context-shock association only. The difference of the activity between the ATN and MITC demonstrates their functional and anatomical heterogeneity in neural substrates for aversive learning tasks.

Neural substrates for the processing of cognitive and affective aspects of taste in the brain

Taste is unique among the sensory systems in that, besides its recognition of quality, it is innately associated with hedonic aspects of reward and aversion. This review of the literature will show how taste information is conveyed through the central gustatory pathways to the cortical gustatory area and is processed in terms of qualitative and quantitative aspects. Taste information is also sent to the reward system and feeding center via several brain sites including the prefrontal cortex, insular cortex, and amygdala. The reward system contains the ventral tegmental area, nucleus accumbens, and ventral pallidum; it finally sends information to the lateral hypothalamic area, the feeding center. The dopamine system originating from the ventral tegmental area mediates the motivation to consume palatable food. The actual ingestive behavior is promoted by the orexigenic neuropeptides from the hypothalamus. In the last section, the neural substrate of learning and memory of taste is introduced and the biological mechanisms are elucidated.

Brain Regions Responsible for the Expression of Conditioned Taste Aversion in Rats

Conditioned taste aversion (CTA) is acquired when the ingestion of a food is followed by malaise. CTA is a kind of fear learning making animals avoid subsequent intake of the food and show aversive behavior to the taste of the food. To elucidate the brain regions responsible for the expression of CTA, our previous electrophysiological and recent c-fos immunohistochemical studies have been reviewed. Among a variety of brain regions including the parabrachial nucleus, amygdala, insular cortex, supramammillary nucleus, nucleus accumbens, and ventral pallidum that are involved in different phases of CTA expression, the enhanced taste sensitivity to facilitate detection of the conditioned stimulus may originate in the central nucleus of the amygdala and the hedonic shift, from positive to negative, may originate in the basolateral nucleus of the amygdala.

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.