Functional Brain Circuitry Related to Arousal and Learning in Rats

The findings reviewed provide the first demonstration of how arousing and learning experiences modify the metabolic activity of the rat brain, and point to discrete functional pathways that seem to play key roles in the brain circuitry related to arousal and learning in mammals Autoradiographic 2-deoxyglucose (2DG) techniques were used to map the functional brain circuitry influenced by arousing electrical stimulation of the midbrain reticular formation (RET) in behaving rats. RET stimulation produced selective patterns of metabolic activation and suppression in discrete regions. The findings point to an integrative role of ascending and descending RET pathways in alerting and defensive reactions. During classical conditioning,involving an auditory stimulus and RET activation, metabolic changes were clearly different from those seen after RET stimulation alone. For example, opposite effects of learning and arousing conditions on the hippocampus were clearly discriminated. It is suggested that the structures affected by RET form part of the ascending reticular activating system and that some of these structures are also engaged in classical conditioning. Besides subserving arousal, the structures metabolically activated during conditioning may play different roles in signal learning, such as: Selective perception (auditory system), directed attention (prefrontal and posterior parietal cortex), motivation (thalamic and limbic structures), and temporal association of the conditioned and unconditioned CS-US stimulus pair (auditory nuclei, hippocampal formation). Finally, there are descending systems mediating the conditioned response made of mixed sensorimotor (flocculus, vestibular nuclei, central grey, spinal cord) and autonomic (medullary nuclei) components involved in the defensive “freezing” reaction to RET stimuli.