Modulation of the piriform cortex activity by the basal forebrain: an optical recording study in the rat

Neuronal activation by stimuli that predict sexual reward in female rats

Conditioned stimuli (CSs) associated with paced copulation induce a conditioned partner preference for males bearing the CS. Here we examined the activation of Fos immunoreactivity (Fos-IR) following exposure to a CS previously paired with either paced or nonpaced copulation. Ovariectomized, hormone-primed rats received 10 sequential conditioning trials at 4-day intervals. In experiment 1, females in the odor-paired group learned to associate an almond odor on a male with paced copulation and an unscented male with nonpaced copulation. In the odor-unpaired group, females received the opposite association. In experiment 2, females associated two different strains of male, Long-Evans or Wistar, with paced or nonpaced copulation, respectively. A preference test indicated that females in both experiments developed a conditioned preference for the pacing-related males, as indicated by significantly more solicitations toward the male and a preference to copulate with the pacing-related male. Subsequently, females were exposed to the CS (odor or strain) alone for 1 h prior to kill and preparation of their brains for immunocytochemistry. In both experiments, the CS associated with paced copulation produced significantly more Fos-IR in the piriform cortex, medial preoptic area, and ventral tegmental area, relative to the same odor or strain cues associated with nonpaced copulation. These findings provide evidence that the state associated with paced copulation can be conditioned to environmental stimuli such as neutral odors or strain cues, which earn an incentive value via classical conditioning. The significance of the brain areas activated is discussed with regard to their role in sexual and other motivated behaviors.

Transmitter balances in the olfactory cortex: adaptations to early methamphetamine trauma and rearing environment

The olfactory cortex, comprising the anterior olfactory cortex (AOC) and the anterior piriform cortex (PirC), is a model system for the study of neural plasticity. We investigated the structural imbalances of different transmitter systems induced in this area by an early traumatisation (methamphetamine [MA] intoxication) and/or environmental deprivation (isolated rearing [IR]), with the working hypothesis that such alterations will not occur in an isolated fashion, but in mutual interaction. Indeed, acetylcholine fibre density is increased by IR in both hemispheres of the PirC (left: +22%, p<0.01, right: +21%, p<0.05) and the left hemisphere of the AOC (+13%, p<0.05), while an early MA intoxication increases it in afterwards enriched-reared animals in the PirC (+14%/+17%, p<0.05), but decreases it in the AOC (-18%/-22%, p<0.001). The serotonin fibre density is increased by IR in the right PirC of saline-treated (+13%, p<0.01), but not of MA-traumatised gerbils. GABA and dopamine in the AOC show an inverse correlation, with dopamine innervation density being increased by IR (+30%, p<0.001) and MA (+26%, p<0.01), and GABA neuropil density being reduced. Furthermore, switches in hemispheric laterality occur in the AOC. These results demonstrate the complex recursive interactions in structural cortical plasticity.

Altered odor-induced expression of c-fos and arg 3.1 immediate early genes in the olfactory system after familiarization with an odor

In adult rats, repeated exposure to an odorant, in absence of any experimentally delivered reinforcement, leads to a drastic decrease in mitral/tufted (M/T) cell responsiveness, not only for the familiar odor but also for other novel odors. In the present study, using two different and complementary in situ hybridization methods, we analyzed the effect of familiarization with an odorant on c-fos and arg 3.1 mRNA expression levels, and we examined the odor specificity of this effect. Odor exposure induces a specific increase in c-fos and arg 3.1 expression in some particular olfactory bulb quadrants. Previous familiarization with the test odor results in a decreased expression of both IEGs in these quadrants, leading to the alteration of the odor-specific pattern of c-fos and arg 3.1 expression. In contrast, this odor-specific pattern is not affected when different odors are used for familiarization and test. Similarly, an odor-specific familiarization effect leading to a reduced c-fos and arg 3.1 expression was also detected in the cingulate cortex and in the anterior piriform cortex. These results support our hypothesis that the decrease in M/T cell responsiveness following a preceding familiarization with an odorant may be related to a particular form of synaptic plasticity involving changes at the genomic level, and reveals further insight in olfactory information processing and the cellular mechanisms underlying familiarization in the olfactory system.

Scopolamine enhances generalization between odor representations in rat olfactory cortex

Acetylcholine (ACh) has a critical, modulatory role in plasticity in many sensory systems. In the rat olfactory system, both behavioral and physiological data indicate that ACh may be required for normal odor memory and synaptic plasticity. Based on these data, neural network models have hypothesized that ACh muscarinic receptors reduce interference between learned cortical representations of odors within the piriform cortex. In this study, odor receptive fields of rat anterior piriform cortex (aPCX) single-units for alkane odors were mapped before and after either a systemic injection of the muscarinic receptor antagonist scopolamine (0.5 mg/kg) or aPCX surface application of 500 microM scopolamine (or saline/ACSF controls). Cross-habituation between alkanes differing by two to four carbons was then examined following a 50-sec habituating stimulus. The results demonstrate that neither aPCX spontaneous activity nor odor-evoked activity (receptive field) was affected by scopolamine, but that cross-habituation in aPCX neurons was enhanced significantly by either systemic or cortical scopolamine. These results indicate that scopolamine selectively enhances generalization between odor representations in aPCX in a simple memory task. Given that ACh primarily affects intracortical association fibers in the aPCX, the results support a role for the association system in odor memory and discrimination and indicate an important ACh modulatory control over this basic sensory process.