Appetitive associative learning recruits a distinct network with cortical, striatal, and hypothalamic regions.

The amygdala, prefrontal cortex, striatum and other connected forebrain areas are important for reward-associated learning and subsequent behaviors. How these structurally and functionally dissociable regions are recruited during initial learning, however, is unclear. Recently, we showed amygdalar nuclei were differentially recruited across different stages of cue-food associations in a Pavlovian conditioning paradigm. Here, we systematically examined Fos induction in the forebrain, including areas associated with the amygdala, during early (day 1) and late (day 10) training sessions of cue-food conditioning. During training, rats in the conditioned group received tone-food pairings, while controls received presentations of the tone alone in the conditioning chamber followed by food delivery in their home cage. We found that a small subset of telencephalic and hypothalamic regions were differentially recruited during the early and late stages of training, suggesting evidence of learning-induced plasticity. Initial tone-food pairings recruited solely the amygdala, while late tone-food pairings came to induce Fos in distinct areas within the medial and lateral prefrontal cortex, the dorsal striatum, and the hypothalamus (lateral hypothalamus and paraventricular nucleus). Furthermore, within the perifornical lateral hypothalamus, tone-food pairings selectively recruited neurons that produce the orexigenic neuropeptide orexin/hypocretin. These data show a functional map of the forebrain areas recruited by appetitive associative learning and dependent on experience. These selectively activated regions include interconnected prefrontal, striatal, and hypothalamic regions that form a discrete but distributed network that is well placed to simultaneously inform cortical (cognitive) processing and behavioral (motivational) control during cue-food learning.

Selective Fos induction in hypothalamic orexin/hypocretin, but not melanin-concentrating hormone neurons, by a learned food-cue that stimulates feeding in sated rats.

Associative learning can enable cues from the environment to stimulate feeding in the absence of physiological hunger. How learned cues are integrated with the homeostatic regulatory system is unknown. Here we examined whether the underlying mechanism involves the hypothalamic orexigenic neuropeptide regulators orexin/hypocretin (ORX) and melanin-concentrating hormone (MCH). We used a Pavlovian conditioning procedure to train food-restricted rats to associate a discrete cue, a tone, with food pellets distinct from their regular lab chow diet. Rats in the conditioned group (Paired) received presentations of a tone immediately prior to food delivery, while the rats in the control group (Unpaired) received random presentations of the same number of tones and food pellets. After conditioning rats were allowed ad libitum access to lab chow for at least 10days before testing. At test sated rats were presented with the tones in their home cages, and then one group was allowed to consume food pellets, while another group was left undisturbed until sacrifice for Fos induction analysis. The tone cue stimulated food consumption in this setting; rats in the Paired group consumed larger amounts of food pellets than rats in the Unpaired group. To examine Fos induction we processed the brain tissue using fluorescent immunohistochemistry methods for combined detection of Fos and characterization of ORX and MCH neurons. We found a greater percentage of ORX and Fos double-labeled neurons in the Paired compared to the Unpaired condition, specifically in the perifornical area. In contrast, there were very few MCH neurons with Fos induction in both the Paired and Unpaired conditions. Thus, the food-cue selectively induced Fos in ORX but not in MCH neurons. These results suggest a role for ORX in cue-induced feeding that occurs in the absence of physiological hunger.

Combinatorial amygdalar inputs to hippocampal domains and hypothalamic behavior systems.

The expression of innate reproductive, defensive, and ingestive behaviors appears to be controlled by three sets of medial hypothalamic nuclei, which are modulated by cognitive influences from the cerebral hemispheres, including especially the amygdala and hippocampal formation. PHAL analysis of the rat amygdala indicates that a majority of its cell groups project topographically (a) to hypothalamic behavior systems via direct inputs, and (b) to partly overlapping sets of hypothalamic behavior control systems through inputs to ventral hippocampal functional domains that in turn project to the medial hypothalamus directly, and by way of the lateral septal nucleus. Amygdalar cell groups are in a position to help bias or prioritize the temporal order of instinctive behavior expression controlled by the medial hypothalamus, and the memory of associated events that include an emotional or affective component.

Basic organization of projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis in adult rat brain.

The organization of axonal projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis (BST) was characterized with the Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing method in adult male rats. Within the BST, the oval nucleus (BSTov) projects very densely to the fusiform nucleus (BSTfu) and also innervates the caudal anterolateral area, anterodorsal area, rhomboid nucleus, and subcommissural zone. Outside the BST, its heaviest inputs are to the caudal substantia innominata and adjacent central amygdalar nucleus, retrorubral area, and lateral parabrachial nucleus. It generates moderate inputs to the caudal nucleus accumbens, parasubthalamic nucleus, and medial and ventrolateral divisions of the periaqueductal gray, and it sends a light input to the anterior parvicellular part of the hypothalamic paraventricular nucleus and nucleus of the solitary tract. The BSTfu displays a much more complex projection pattern. Within the BST, it densely innervates the anterodorsal area, dorsomedial nucleus, and caudal anterolateral area, and it moderately innervates the BSTov, subcommissural zone, and rhomboid nucleus. Outside the BST, the BSTfu provides dense inputs to the nucleus accumbens, caudal substantia innominata and central amygdalar nucleus, thalamic paraventricular nucleus, hypothalamic paraventricular and periventricular nuclei, hypothalamic dorsomedial nucleus, perifornical lateral hypothalamic area, and lateral tegmental nucleus. Moderately dense inputs are found in the parastrial, tuberal, dorsal raphé, and parabrachial nuclei and in the retrorubral area, ventrolateral division of the periaqueductal gray, and pontine central gray. Light projections end in the olfactory tubercle, lateral septal nucleus, posterior basolateral amygdalar nucleus, supramammillary nucleus, and nucleus of the solitary tract. These and other results suggest that the BSTov and BSTfu are basal telencephalic parts of a circuit that coordinates autonomic, neuroendocrine, and ingestive behavioral responses during stress.