293 - Sullivan

The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. During the first postnatal days, infant rats show a sensitive period for learning and are particularly susceptible to learning an attraction to their mother’s odor. Classical conditioning appears to underlie this learning that is expressed behaviorally as an increased ability to acquire odor preferences and a decreased ability to acquire odor aversions. Specifically, in neonatal rats, pairing an odor with moderately painful shock (0.5mA) or milk produces a subsequent relative preference for that odor. The neural circuitry supporting the increased ability to acquire odor preferences appears to be the heightened functioning of the noradrenergic pontine nucleus locus coeruleus. Indeed, norepinephrine from the locus coeruleus appears to be both necessary and sufficient for learning during the sensitive period. On the other hand, the decreased ability to acquire odor aversions seems to be due to the lack of participation of the amygdala in at least some aversive learning situations. The site of plasticity in the pup’s brain appears to be limited to the olfactory bulb. This neonatal sensitive period for learning ends around postnatal day 9– 10, at which time pups make the transition from crawling to walking and classical conditioning becomes “adultlike.” The neonatal behavioral and neural induced changes are retained into adulthood where it modifies sexual behavior. Key Words—classical conditioning, olfactory learning, imprinting, sensitive period, neonatal learning, odor conditioning, attachment, abuse, amygdala, fear conditioning, locus coeruleus, norepinephrine IN ALTRICIAL SPECIES, such as humans and rats, rapid formation of attachment is critical for survival. While some prenatal experiences contribute to attachment formation, most species dependent on parental care exhibit some form of rapid, specialized postnatal learning, such as imprinting in avian species (Salzen, 1970). We have recently developed a mammalian model of imprinting in the rat which has enabled us to explore the neurobiology of attachment using a classical conditioning paradigm (for a review of other maternalinfant attachment models see Brennen & Keverne, 1997; Hudson, 1993). According to Bowlby (1965), attachment is characterized by the infant seeking proximity to the caregiver and the infant’s endurance of considerable abuse to maintain contact with the caregiver. These attachment characteristics are not unique to mammals and appears to occur throughout the animal kingdom. Specifically, attachment to an abusive caregiver has been clinically documented in both human infants (review, Helfer, Kempe & Krugman, 1997), and nonhuman infant primates (Harlow & Harlow, 1965). These results are consistent with experimental manipulations; chicks shocked during imprinting show Address for correspondence: Regina Sullivan, Department of Zoology, University of Oklahoma, 730 VanVleet Oval, Norman OK 73019; email: [email protected].; fax: 405–325–2699 phone: 405–325–5653.