Faculty Bibliography

Wistar rat pups, aged Postnatal Day 5, were trained in an olfactory associative learning task with citral odor as the conditioned stimulus (CS) and intraoral infusions of milk as the unconditioned stimulus (US). Following a 30-min training session, pups were injected with either the norepinephrine beta-receptor antagonist propranolol or the beta-receptor agonist isoproterenol. Pups were tested 24 hr later for an acquired relative odor preference for the CS. Propranolol injected immediately following training impaired memory for the CS in a dose-dependent manner. This posttraining effect lasted less than 4 hr. Isoproterenol injected immediately after training also impaired memory performance, even at very low doses. These results suggest that posttraining levels of norepinephrine play a critical role in memory consolidation in the newborn, with elevations or decrements in noradrenergic activity resulting in impaired memory

The role of the serotonergic innervation of the olfactory bulb was examined in neonate rat pups (Sprague-Dawley) by utilizing an olfactory learning paradigm (Sullivan, McGaugh, & Leon, 1991; Sullivan, Wilson, & Leon, 1989). One hundred fifty nanoliters of the neurotoxin 5,7-dihydroxytryptamine (5,7-dHT, 10 micrograms/microliters) was injected into each anterior olfactory nucleus of Postnatal Day 2 (PND 2) pups in order to selectively deplete the serotonergic (5-HT) innervation of the olfactory bulb. On PND 8, control, sham-operated, or 5-HT-depleted pups were presented with stroke (unconditioned stimulus) paired with peppermint odor (conditioned stimulus) or were presented with peppermint alone. Other pups remained naive, i.e., they were not presented with either stroke or odor. The following day, the pups were placed on a mesh screen above two boxes containing either fresh bedding or bedding with peppermint odor for five 1-min trials. Control and sham-operated pups that were previously trained by the odor/stroke paradigm spent significantly more time over the peppermint odor than similarly trained 5-HT-depleted pups, pups trained using the odor only paradigm, or naive pups. Immunocytochemistry verified that the 5,7-dHT injections specifically depleted the 5-HT innervation of the olfactory bulb and left the 5-HT innervation of more caudal levels of the neuraxis (e.g., piriform cortex) intact. The locomotor behavior of the pups was not affected by the 5-HT depletions. This study is the initial investigation to suggest that 5-HT innervation to the olfactory bulb is important in the acquisition or expression of olfactory-based learned behavior in the neonate rat.

Although olfactory associative conditioning in newborn rats produces marked structural and functional changes in the olfactory bulb, recent evidence suggests that extrabulbar circuits must be involved in storing these early memories. The present experiments examined the role of the amygdala complex on early olfactory learning. Bilateral amygdala lesions or sham lesions were performed on Postnatal Day (PN) 5. On PN6, pups were trained in a standard classical conditioning paradigm associating odor with tactile stimulation. Behavioral testing on PN7 revealed that amygdala lesions blocked odor preferences but had no effect on conditioned behavioral activation. Similar sized neocortical lesions did not impair odor preferences. Importantly, amygdala lesion effects on learned odor preferences could be reversed by extensive overtraining. These results suggest that the amygdala complex plays a critical role in modulating associative learning as early as the first postnatal week in the rat

Adult male Wistar rats were trained in a simple odor detection task, with peppermint odor serving as either an S+, S- or as a randomly presented odor. Twenty-four hours after the last training session, rats were injected with [14C]2-deoxyglucose and exposed to the odor. Mean relative 2-deoxyglucose uptake to the odor was enhanced in the pars dorsalis of the anterior olfactory nucleus of S+ and S- trained rats compared to controls. In contrast, no differences in uptake were detected in either odor-specific focal regions of the olfactory bulb glomerular layer, the pyriform cortex, or the hippocampus

Wistar rat pups were implanted with bilateral olfactory bulb cannulas on postnatal day 5 (PN5). On PN6, pups were trained in an olfactory classical conditioning task with peppermint odor as the CS and tactile stimulation/stroking as the UCS. Pups were randomly assigned to either PAIRED, BACKWARD or ODOR-only conditions. Half the pups in each group received intrabulbar infusions of 100 microM propranolol and half received intrabulbar infusions of saline during the training session. Propranolol infusions blocked acquisition of the learned odor preference expressed by PAIRED saline-infused pups. Diffusion of the infusate was checked in additional pups by infusing [3H]NE and performing LSC analysis. Infusate concentration did not significantly differ between the anterior and posterior halves of the bulb, but were sharply lower in the olfactory peduncle and more posterior areas. The results suggest that olfactory bulb NE is critical for early olfactory learning

Association of odor and reward during the early postnatal period modifies rat pup behavioral responses and olfactory bulb neural responses to subsequent presentations of that odor. Recent evidence has shown that olfactory bulb output neurons, mitral/tufted cells, receive convergent odor and reward inputs. The present report demonstrates that contiguous odor-reward pairings prevent mitral/tufted cell habituation to the odor that normally occurs to repeated odor-only stimulation. It is hypothesized that the maintenance of olfactory bulb responses to conditioned odors during training may allow for activation of long-term memory mechanisms

The influence of norepinephrine (NE) on the acquisition of a conditioned odor preference and enhanced focal uptake of [14C]2-deoxyglucose (2-DG) within the olfactory bulb was assessed in neonatal rat pups. On postnatal day (PN) 6, pups were injected with either an NE receptor agonist (isoproterenol), NE receptor antagonist (propranolol or timolol), or saline before one-trial odor conditioning. The experimental conditioning group received a 10-min exposure to an odor (peppermint) and reinforcing tactile stimulation similar to that received from the dam. Control groups received only the odor, only the tactile stimulation, backwards presentation of the odor and tactile stimulation or neither of these stimuli. The next day, pups were either tested for an olfactory preference (Expts. 1 and 2) or assessed for differential olfactory bulb activity using the 2-DG technique (Expt. 3). The results indicate that early odor experience with either tactile stimulation or isoproterenol is sufficient to produce a learned behavioral preference and enhanced focal 2-DG uptake within the olfactory bulb. Moreover, an NE receptor blocker injected prior to training with odor and tactile stimulation blocks the acquisition of both behavioral preference and the enhanced 2-DG uptake. In Expt. 4, the effects of tactile stimulation and isoproterenol were further assessed. An odor paired with a moderate level of either of these stimuli produces learning. However, the simultaneous presentation of a moderate level of these stimuli paired with an odor does not result in an odor preference. An odor preference may be reinstated by simultaneous presentation of these stimuli, provided the level of each of these stimuli is too low to produce an odor preference when presented alone with an odor. These data suggest that exogenous NE and tactile stimulation are additive in their effect on learning. These results are discussed in terms of the neural mechanisms underlying reinforcement in infant rats.

Newborn rat pups can learn to either approach or avoid odor cues through associative conditioning. The present results demonstrate that preference conditioning and avoidance conditioning both modify olfactory bulb responses (focal 2-deoxyglucose uptake and mitral-tufted cell single unit responses) to the conditioned odor. Despite opposing behavioral responses to the conditioned odor, however, olfactory bulb neural responses did not detectably differ between learned odor cues signaling approach and those signaling avoidance. Control pups exhibited neither the behavioral nor neural changes. Furthermore, both the behavioral and neural changes to these odor cues could be extinguished. These results suggest that the olfactory bulb in neonates may code learned odor importance, but specific information attached to that importance may require processing in other brain regions

One of the circuits modified by early olfactory learning is in the olfactory bulb. Specifically, response patterns of mitral-tufted cells are modified by associative conditioning during the early postnatal period. In addition, previous work has demonstrated that mitral-tufted cell single units respond to both olfactory conditioned stimuli and rewarding stimulation of the medial forebrain bundle-lateral hypothalamus (MFB-LH). The present study suggests that norepinephrine beta-receptor activation is required for early olfactory learning using MFB-LH stimulation as reward. Propranolol injected before odor-MFB-LH pairings blocks the acquisition of conditioned behavioral responses and their neural correlates to the conditioned odor. Furthermore, propranolol blocks a specific class of the mitral-tufted cell responses to MFB-LH reward stimulation. The relationship of this response to reward and early learning is discussed