Fear conditioning is associated with synaptogenesis in the lateral amygdala.

Fear conditioning in the rat typically involves pairing a conditioned stimulus (tone) with an aversive unconditioned stimulus (foot shock) which elicits a freeze response. Although the circuitry that underlies this form of learning is well defined, potential synaptic changes associated with this form of learning have not been fully investigated. This experiment examined synaptic structural plasticity in the lateral amygdala which is critical for the acquisition of the conditioned fear response. Adult male rats were randomly allocated to either a paired, unpaired or tone only condition. One day after the initial fear conditioning session and 1 h after a probe trial confirmation of a conditioned fear response, the rats were perfused and the relevant tissue was embedded for electron microscopic analysis. Synaptic changes were quantified in the lateral amygdala using a stereological approach. The results showed a significant increase in the number of synapses in the conditioned animals compared to controls. This finding suggests that an increase in synaptic compliment in the amygdala may underlie the acquisition of the conditioned fear response.

Neonatal 6-OHDA lesions and rearing in complex environments: regional effects on adult brain 14C-2-deoxyglucose uptake revealed by exposure to novel stimulation.

Behavioral and neuromorphological data have suggested at least a partial interaction between the effects of norepinephrine-depleting neonatal 6-OHDA lesions and the effects of rearing in enriched environments. The present study examined the impact of both of these early manipulations upon regional brain uptake of 14C-2-deoxyglucose (14C-2DG) in adulthood. Newborn rats received 6-OHDA (50 mg/kg s.c.) or vehicle and, after weaning at 25 days, were reared in isolated versus enriched conditions. Regional brain 14C-2DG uptake was then examined at 70-80 days of age--either in the home cage or while animals were being exposed to novel, presumably arousing, stimulation. Ninety-seven brain regions were examined in eight separate groups. Results indicated that (1) Under baseline conditions, neither neonatal 6-OHDA nor differential rearing conditions produced widespread alterations in regional brain 14C-2DG uptake profiles. An overall enrichment effect was seen on only five brain areas, with rats reared in enriched environments showing lower levels of 14C-2DG uptake (-20% to -30%) than isolated rats. Neonatal 6-OHDA produced no main effect on 14C-2DG uptake in any brain region. (2) In contrast, when 14C-2DG uptake was assessed during exposure to a novel environment, five brain areas showed differential 14C-2DG uptake in 6-OHDA-treated rats, and 20 brain areas showed differential uptake in rats reared in enriched conditions. (3) No significant interaction effect on brain regional 14C-2DG uptake was observed between neonatal 6-OHDA and environmental complexity factors. These results are consistent with the notion that enduring effects of rearing and early 6-OHDA treatment may, independently, relate to a general reactivity factor. They also indicate that some effects of early neurochemical injury and subsequent experiential factors may not be apparent under normal resting conditions, but only become evident in the presence of appropriate "activating" stimulation.

The postweaning housing environment determines expression of learning deficit associated with neonatal monosodium glutamate (M.S.G.).

Perinatal M.S.G. treatment causes a syndrome characterized by damage to the hypothalamic arcuate nucleus, other circumventricular areas, parts of the visual system and the dentate gyrus of the hippocampus. The resulting hormonal dysfunction may be responsible for developmental anomalies of organ systems, obesity, and alterations in sensory/motor performance. We have shown that some behavioral indicators of M.S.G. toxicity in rats can be masked by rearing them in enriched housing conditions. Here, we evaluated the impact of six housing conditions on M.S.G.-induced alterations of organ systems and behavior. Perinatal M.S.G. treatment reduced adrenal, heart and testes weights, as well as total white blood cell (WBC) counts, and increased tail flick latencies. These measures were unaffected by the housing condition. M.S.G.-induced reductions in body weight, grip strength, water maze and dominance task performance varied as a function of housing. Deficits in water maze performance were most evident following social and isolated single-cage housing. We propose that deficits in water maze performance following perinatal M.S.G. may be attributable to hippocampal damage that can be alleviated by rearing the rats in stimulating environments.

Neonatal 6-hydroxydopamine alters the behavior of enriched-impoverished rats in a novel test environment.

The hypothesis that neonatal norepinephrine (NE) depletion lessens the behavioral consequences of differential housing was tested. Male Wistar rats were injected with 6-hydroxydopamine (6-OHDA) or vehicle twice within 24 hr of birth, weaned at 25 days, and reared under either impoverished (IC) or enriched conditions (EC) for 30 days. In 3 experiments, rats were tested in the Morris water maze, the colony-intruder test, and 2 tests of dominance. 6-OHDA treatment reduced cortical and hypothalamic NE concentrations and increased brainstem NE concentrations. EC housing increased cortical dopamine (DA). Behavioral differences caused by postweaning enrichment-isolation were reduced by neonatal NE depletion, primarily in early test trials. The authors conclude that forebrain NE afferents from the locus coeruleus are important for housing-related behavioral changes and responsivity to novel testing environments.

Enriched housing masks deficits in place navigation induced by neonatal monosodium glutamate.

Monosodium-L-glutamate (MSG)-induced behavioural deficits were examined under two housing conditions. Male Wistar rat pups were cross-fostered at birth and injected with MSG on days 2 to 11 postpartum. Control pups were injected with saline (Sal) of equivalent volume. Following weaning at day 25 the rats were allocated to either "enriched" (EC) or "impoverished" (IC) housing, thus forming four treatment groups: MSG-EC, MSG-IC, Sal-EC, and Sal-IC. Thirty-five days later all rats were tested in the open field and in the place navigation task. Multivariate analysis of variance (MANOVA) revealed a significant two-way interaction of the housing x drug variables. While the MSG-IC rats were deficient in the water maze, the performance of the MSG-EC approached that of saline-injected controls. Although the Sal-IC rats developed higher body weights than the Sal-EC rats the MSG-EC and MSG-IC rats did not differ in body weight. The housing by drug treatment interaction was not apparent in the open field.

Cardiac cycle phase and movement and reaction times.

The EKG was recorded while Ss differentially responded to auditory or visual stimuli in a reaction time task. The EKG record was analyzed by dividing each R-R interval encompassing a stimulus presentation into 9 equal phases. Reaction times were determined as a function of the phase encompassing stimulus onset while movement times were determined for the phase in which the response was initiated. Only reaction time significantly varied with cardiac cycle, with reactions during the second phase being slower than later phases.