Catherine A. Cornwell-Jones

Neonatal 6-hydroxydopamine alters olfactory development.

The odor preferences and brain catecholamine content of Sprague-Dawley rat pups 5–16 days of age were monitored following systemic injections of 6-hydroxydopamine (6-OHDA) on Days 0–3 postpartum. During the first postnatal week, 6-OHDA produced an alteration in the development of olfactory-guided behavior which was confined to conspecific odor. This deficit did not reflect changes in general sensory or motor capabilities, as both control and treated pups reliably avoided the nonconspecific scents of lemon and cedar. The emergence of preference for nest shaving odor on Day 8 represented an underlying developmental change in the pup rather than changes in the concentration of the odorant in nest shavings. Changes in norepinephrine (NE) concentrations in rostral olfactory structures which paralleled changes in responses to conspecific odorant may underlie the observed behavioral recovery. The involvement of other brain areas innervated by the lateral olfactory tract in the development of odor-guided behavior is also discussed.

Development of odor-guided behavior in Wistar and Spraque-Dawley rat pups

The odor of home-cage shavings attracts Sprague-Dawley and Wistar pups 3-4 days old. Attraction increases during postnatal week one in both strains. Lemon odor repels Sprague-Dawley pups 3 and 8 days old equally, while Wistar aversion to lemon increases during the first postnatal week. Shavings from the nests of strange litters 4 or 8 days old attract Sprague-Dawley pups 4 but not 8 days old equally. Sensory changes appear to underlie Sprague-Dawley response development, while locomotor and sensory changes may mediate Wistar response ontogeny.

6-Hydroxydopamine reduces preference for conspecific but not other familiar odors in rat pups

Rat pups injected subcutaneously with 6-hydroxydopamine (6-OHDA) showed reduced preference for a familiar combination of conspecific and botanical odors when subsequently tested in a two-choice situation. However, drug treatment did not influence preference for the simple botanical odor. 6-OHDA also reduced norepinephrine (NE) concentrations in the forebrain and the olfactory bulbs. The data implicate NE in the ontogeny of the acquired responses to conspecific odors.

Castration decreases olfactory bulb norepinephrine in male rats but not hamsters.

Norepinephrine (NE) concentrations were measured in the olfactory bulbs and olfactory cortex of male albino rats and golden hamsters sacrificed 13-16 days after castration or sham surgery. Castration significantly decreased NE concentrations in the olfactory bulbs of rats but not hamsters. Castration had no significant influence on NE levels in the olfactory cortex of either species. Regional brain weights were not influenced by surgery, and previous olfactory exposure had no significant effect on any variable measured. The decrease in rat olfactory bulb NE levels may be involved in castration-induced changes in preferences for conspecific odors.

Early odor preferences of rats are preserved by neonatal 6-hydroxydopamine

Infant rats treated with 6-hydroxydopamine (6-OHDA) showed normal experience-dependent odor preferences. Adult rats that had been neonatally drug treated retained infantile odor preferences while controls did not. Exposing juveniles to a new odor for 10 days changed the preferences of controls but not of neonatally drug-injected animals. Juveniles in the two treatment groups habituated similarly to a novel object, indicating that drug treatment did not produce a general inability to ignore irrelevant stimuli. Neonatal 6-OHDA treatment reduced concentrations of norepinephrine but not dopamine in the adult olfactory cortex. Norepinephrine may be a substrate for infantile amnesia.

Conspecific odor preferences of male albino rats are reversed by intracerebral 6-hydroxydopamine.

The odor of pine shavings from the nest of a female and her litter attracted sexually experienced male Sprague-Dawley rats tested in a two-choice situation. The preference persisted after surgery in controls treated with bilateral intracerebral injections of saline-ascorbic vehicle into the vicinity of the ascending noradrenergic bundle. In contrast, 7 out of 8 animals receiving bilateral injections of the neurotoxin 6-hydroxydopamine (6-OHDA), preferred the odor of clean pine to pine nest odor after surgery, indicating preference reversal. 6-hydroxydopamine also reduced olfactory cortex norepinephrine (NE) concentrations by 85%. Pretreatment with intracerebral injection of amphetamine, a catecholamine uptake inhibitor and releaser, prevented 6-OHDA-induced preference reversal in 7 out of 8 animals and limited NE reduction to 38% of concentrations measured in amphetamine-pretreated vehicle-injected controls. The data implicate central NE in the modulation of responses to conspecific odors.

Neonatal 6-hydroxydopa alters conspecific odor investigation by male rats.

Odor-guided behavior was examined in male rats injected at birth and 48 h later with either the catecholaminergic neurotoxin 6-hydroxydopamine (60 micrograms/g, i.p.) or vehicle. In odor preference tests administered 8 or 74 days postnatally, drug-treated animals avoided novel odors which were neutral for controls and showed reduced preference for conspecific nest odors. In emergence tests administered 73-75 days postnatally, odors from an anesthetized female reduced approach latency and increased investigation of familiar conspecific odors for control but not drug-treated males. Neonatal drug treatment decreased adult olfactory cortex norepinephrine (NE) levels by 50%, but did not significantly influence either cardiac NE or olfactory cortex dopamine. The results imply that brain NE facilitates attraction to familiar conspecific odors.

Testosterone reduces olfactory neophobia in male golden hamsters

Abstract Juvenile hamsters housed in pine shavings and castrated or sham-operated 10 days before testing, prefer familiar pine odor to an unfamiliar botanical odor, cedar. The preference persists in adult castrates, but adult sham-operates show no preference. Tolerance for cedar odor is restored by implanting adult castrates with constant-release testosterone-filled capsules measuring 16 mm but not 4 mm. Making cedar odor familiar by exposing adults to it for 3 days before testing, induces similar preference for the odor in both castrates and sham-operates. The data suggest that testosterone reduces aversion to unfamiliar nonsexual odors, but does not influence preference when such odors are familiar. In the wild, this mechanism may prevent unfamiliar odors from disrupting the search for a mate outside a males home territory.

Early olfactory learning is influenced by sex in hamsters but not rats.

Abstract The odor of cedar shavings repels infant and juvenile rats and hamsters reared in pine shavings when animals are tested in a two-choice situation. Housing male and female infants of either species in cedar shavings for 3 days before testing increases preference for both cedar odor and for a mixture of cedar and conspecific odors. Similar exposure of juvenile male and female rats and male hamsters, has little or no significant impact on odor preferences. However, housing female hamsters in cedar at either developmental stage equally influences odor preferences. The data imply that sex differences influence the development of early odor-guided behavior in hamsters but not rats. These species differences may have adaptive significance in that the birth of a litter produces a greater environmental change demanding a larger preference adjustment of solitary female hamsters than colonial female rats.