Imaging evolutionarily conserved neural networks: preferential activation of the olfactory system by food-related odor.

Rodents routinely forge and rely on hippocampal-dependent spatial memory to guide them to sources of caloric rich food in their environment. Has evolution affected the olfactory system and its connections to the hippocampus and limbic cortex, so rodents have an innate sensitivity to energy rich food and their location? To test this notion, we used functional magnetic resonance imaging in awake rats to observe changes in brain activity in response to four odors: benzaldehyde (almond odor), isoamyl acetate (banana odor), methyl benzoate (rosy odor), and limonene (citrus odor). We chose the almond odor because nuts are high in calories and would be expected to convey greater valance as compared to the other odors. Moreover, the standard food chow is devoid of nuts, so laboratory bred rats would not have any previous exposure to this food. Activation maps derived from computational analysis using a 3D segmented rat MRI atlas were dramatically different between odors. Animals exposed to banana, rosy and citrus odors showed modest activation of the primary olfactory system, hippocampus and limbic cortex. However, animals exposed to almond showed a robust increase in brain activity in the primary olfactory system particularly the main olfactory bulb, anterior olfactory nucleus and tenia tecta. The most significant difference in brain activation between odors was observed in the hippocampus and limbic cortex. These findings show that fMRI can be used to identify neural circuits that have an innate sensitivity to environmental stimuli that may help in an animal's survival.

Functional magnetic resonance imaging in awake animals.

Awake animal imaging is becoming an important tool in behavioral neuroscience and preclinical drug discovery. Non-invasive ultra-high-field, functional magnetic resonance imaging (fMRI) provides a window to the mind, making it possible to image changes in brain activity across distributed, integrated neural circuits with high temporal and spatial resolution. In theory, changes in brain function, anatomy, and chemistry can be recorded in the same animal from early life into old age under stable or changing environmental conditions. This prospective capability of animal imaging to follow changes in brain neurobiology after genetic or environmental insult has great value to the fields of psychiatry and neurology and probably stands as the key advantage of MRI over other methods in the neuroscience toolbox. In addition, awake animal imaging offers the ability to record signal changes across the entire brain in seconds. When combined with the use of 3D segmented, annotated, brain atlases, and computational analysis, it is possible to reconstruct distributed, integrated neural circuits or 'fingerprints' of brain activity. These fingerprints can be used to characterize the activity and function of new psychotherapeutics in preclinical development and to study the neurobiology of integrated neural circuits controlling cognition and emotion. In this review, we describe the methods used to image awake animals and the recent advances in the radiofrequency electronics, pulse sequences, and the development of 3D segmented atlases and software for image analysis. Results from pharmacological MRI studies and from studies using provocation paradigms to elicit emotional responses are provided as a small sample of the number of different applications possible with awake animal imaging.

BOLD signal response to cocaine varies with sexual receptivity in female rats.

Blood oxygen level dependent MRI was used to test whether cocaine-stimulated mesolimbic activity varied with sexual receptivity. Rats were randomly screened for lordotic responses and were then imaged for their responses to centrally administered cocaine. We observed that female rats expressing no lordosis showed a greater activation of mesolimbic and nigrostriatal structures than lordotic female rats. Our data suggest that the differential sensitivity to cocaine occurs not only as a result of hormonal changes of the estrous cycle, but also in association with changes in sexual receptivity.

Imaging the immediate non-genomic effects of stress hormone on brain activity.

The stress hormones, glucocorticoids, bind to intracellular receptor proteins and act as transcription factors affecting gene activity. These genomic effects occur over hours and even days producing long-term changes in synaptic plasticity and neural transmission. In addition to this classic genomic pathway, there is evidence that stress hormones can have immediate, non-genomic effects on brain function. Using non-invasive functional magnetic resonance imaging, awake, adrenalectomized rats were given intravenous doses of corticosterone mimicking blood levels of hormone achieved with modest and intense stress. The dose of corticosterone mimicking high stress caused a significant increase in functional activity in the hippocampus, forebrain cortex and lateral hypothalamus within minutes of administration. This finding shows that stress hormones can have non-genomic effects on brain activity potentially affecting the immediate cognitive and behavioral response to a highly emotional experience.

Imaging the neural circuitry and chemical control of aggressive motivation.

BACKGROUND: With the advent of functional magnetic resonance imaging (fMRI) in awake animals it is possible to resolve patterns of neuronal activity across the entire brain with high spatial and temporal resolution. Synchronized changes in neuronal activity across multiple brain areas can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories and emotions for particular behaviors. To this end, fMRI in conscious rats combined with 3D computational analysis was used to identifying the putative distributed neural circuit involved in aggressive motivation and how this circuit is affected by drugs that block aggressive behavior. RESULTS: To trigger aggressive motivation, male rats were presented with their female cage mate plus a novel male intruder in the bore of the magnet during image acquisition. As expected, brain areas previously identified as critical in the organization and expression of aggressive behavior were activated, e.g., lateral hypothalamus, medial basal amygdala. Unexpected was the intense activation of the forebrain cortex and anterior thalamic nuclei. Oral administration of a selective vasopressin V1a receptor antagonist SRX251 or the selective serotonin reuptake inhibitor fluoxetine, drugs that block aggressive behavior, both caused a general suppression of the distributed neural circuit involved in aggressive motivation. However, the effect of SRX251, but not fluoxetine, was specific to aggression as brain activation in response to a novel sexually receptive female was unaffected. CONCLUSION: The putative neural circuit of aggressive motivation identified with fMRI includes neural substrates contributing to emotional expression (i.e. cortical and medial amygdala, BNST, lateral hypothalamus), emotional experience (i.e. hippocampus, forebrain cortex, anterior cingulate, retrosplenial cortex) and the anterior thalamic nuclei that bridge the motor and cognitive components of aggressive responding. Drugs that block vasopressin neurotransmission or enhance serotonin activity suppress activity in this putative neural circuit of aggressive motivation, particularly the anterior thalamic nuclei.

Nursing stimulation is more than tactile sensation: It is a multisensory experience.

Novel sensory experiences, particularly those associated with epochal developmental events like nursing alter cortical representation, affecting memory, perception and behavior. Functional MRI was used here to test whether the sensoricortical map of the ventrum is modified during lactation. Three stimuli were used to drive cortical activation in primiparous rats: natural, artificial suckling stimulation and general mechanical rubbing of the skin of the ventrum. These stimuli significantly activated the somatosensory cortex of dams. Of the three stimuli, artificial and pup suckling robustly activated much of the cerebrum, most notably the visual, auditory and olfactory cortices. Surprisingly, activation occurred even in the absence of pups, with artificial suckling. This finding suggests that incoming information from a single modality was sufficient to drive activity of others. Enhanced sensitivity across the cortical mantle during nursing may help the dam to perceive, process, and remember stimuli critical to the care and protection of her young.