Showing papers by "Michela Gallagher published in 2002"

Amygdalo-hypothalamic circuit allows learned cues to override satiety and promote eating.

Abstract: Organisms eat not only in a response to signals related to energy balance. Eating also occurs in response to "extrinsic," or environmental, signals, including learned cues. Such cues can modify feeding based on motivational value acquired through association with either rewarding or aversive events. We provide evidence that a specific brain system, involving connections between basolateral amygdala and the lateral hypothalamus, is crucial for allowing learned cues (signals that were paired with food delivery when the animal was hungry) to override satiety and promote eating in sated rats. In an assessment of second-order conditioning, we also found that disconnection of this circuitry had no effect on the ability of a conditioned cue to support new learning. Knowledge about neural systems through which food-associated cues specifically control feeding behavior provides a defined model for the study of learning that may be informative for understanding mechanisms that contribute to eating disorders and more moderate forms of overeating.

Disconnection of the basolateral amygdala complex and nucleus accumbens impairs appetitive Pavlovian second-order conditioned responses.

Abstract: There is considerable evidence that the basolateral complex of the amygdala (ABL) is involved in learning about the motivational value of otherwise neutral stimuli. The authors examined the role in this function of the ABL and one of its major efferent structures, the nucleus accumbens. Male Long-Evans rats received either sham, ipsilaterally, or contralaterally placed unilateral lesions of the ABL and accumbens and were trained in an appetitive Pavlovian second-order conditioning task. Sham-lesioned and ipsilaterally lesioned rats acquired the task normally, but contralaterally lesioned rats, in which the ABL and accumbens were functionally disconnected, failed to acquire second-order conditioned responses (although they did acquire second-order conditioned orienting responses). The results suggest that the ABL and accumbens are part of a system critical for processing information about learned motivational value.

The basolateral complex of the amygdala is necessary for acquisition but not expression of CS motivational value in appetitive Pavlovian second-order conditioning.

Abstract: The basolateral complex of the amygdala (ABL) is involved in processing information about stimulus motivational value. However, it is not clear whether the ABL is critical for acquisition, maintenance, or expression of this information. Our previous work has shown that ABL lesions made prior to training, block acquisition of an appetitive Pavlovian second-order conditioning task, in which performance is thought to depend on the acquisition of motivational (conditioned reinforcement) value by the first-order conditioned stimulus (CS). The present experiments examined the effects of ABL lesions made after first-order conditioning, when the CS acquires motivational value, but before second-order conditioning, the test for acquired value of that CS. Rats received pairings of a visual CS with a food reinforcer. They then received bilateral sham or excitotoxic lesions of the ABL. After recovery, they received pairings of a second-order auditory CS with the previously conditioned visual CS. In two experiments, both sham and lesioned rats acquired normal second-order conditioned behaviours. Some of the same rats were then given another round of second-order conditioning with novel CSs. In this case, when first-order training occurred after surgery, some second-order conditioned behaviours were impaired in lesioned rats. Tests of the associative underpinnings of second-order conditioned behaviours showed that those behaviours impaired by ABL lesions were based on stimulus-response associations. The results show that although the ABL is critical for second-order conditioning, this role is limited to acquisition of information about the motivational value of the first-order CS, and it is not critical for maintenance of this information or for its use in forming second-order associations.

Cholinergic system regulation of spatial representation by the hippocampus.

Abstract: The role of the basal forebrain cholinergic system in hippocampal spatial representation was explored by examining the effects of immunotoxic lesions of the septo-hippocampal cholinergic neurons on the firing patterns of hippocampal place cells as rats explored familiar and novel environments In a highly familiar environment, the basic qualities and stability of place fields were unaffected by the lesion When first exposed to a set of novel environmental cues without otherwise disorienting the animals, place cells in both normal and lesioned animals responded with similar alterations in their firing patterns Upon subsequent repetitive exposures to the new environment, place cells of normal rats developed a spatial representation distinct from that of the familiar environment By contrast, place cells of lesioned animals reconverged in the direction of the representation associated with the familiar environment These results suggest that cholinergic input may determine whether new visual information or a stored representation of the current environment will be actively processed in the hippocampal network

Individual differences in spatial memory among aged rats are related to hippocampal PKCγ immunoreactivity

Abstract: We reported previously that the extent of spatial memory impairment among aged rats was correlated positively with levels of protein kinase Cγ in hippocampal homogenates measured by quantitative Western blotting (Colombo et al., 1997). In the current study, immunocytochemistry was used to test whether the relationship between elevated PKCγ and memory impairment among aged rats could be localized further within regions of the hippocampus. Six- and 24-month-old male Long-Evans rats were first trained in the water maze on a standard place-learning task and then trained 2 weeks later on a transfer task designed for rapid acquisition. In comparison with young rats, aged rats with impaired spatial memory had increased PKCγ-immunoreactivity (PKCγ-ir) in CA1 of the hippocampus, but not the dentate gyrus. In addition, PKCγ-ir in CA1 was correlated positively with spatial memory impairment among aged rats on the standard place-learning and the transfer training tasks. The current results are consistent with our previous report of PKCγ in hippocampal homogenates, and show further that the relationships between PKCγ-ir and memory impairments among aged rats are most evident in area CA1. Thus age-related impairments of spatial memory, as well as deficits in the flexible use of previously acquired information, may result from dysregulation of PKCγ. Hippocampus 2002;12:285–289. © 2002 Wiley-Liss, Inc.

Decreased glucocorticoid receptor mRNA and dysfunction of HPA axis in rats after removal of the cholinergic innervation to hippocampus.

Abstract: Excess exposure to glucocorticoids can have deleterious effects on physiology and cognition. Glucocorticoids acting via receptors located in hippocampal neurons contribute to negative feedback after stress by terminating the further release of glucocorticoids. The current study investigated the effects of selective immunolesions of septo-hippocampal cholinergic neurons on hippocampal corticosterone receptor mRNA and on hypothalamic-pituitary-adrenal (HPA) activity. As evaluated by in situ hybridization, hippocampal glucocorticoid receptor (GR) mRNA, but not mineralocorticoid receptor (MR) mRNA, was significantly decreased in lesioned rats compared to controls. In a companion study, the peak corticosterone response to one hour of restraint stress did not differ between lesion and control groups but the post-stress decline of corticosterone was more protracted in the lesioned rats. These findings are discussed in terms of their possible relevance to ageing as age-related degeneration of the basal forebrain cholinergic system may contribute to the commonly observed dysfunction of the HPA axis in older animals.