Abstract. Our work with both sheep and mouse models has revealed many of the neural substrates and signalling pathways involved in olfactory recognition memory in the main olfactory system. A distributed neural system is required for initial memory formation and its short-term retention-the olfactory bulb, piriform and entorhinal cortices and hippocampus. Following memory consolidation, after 8 h or so, only the olfactory bulb and piriform cortex appear to be important for effective recall. Similarly, whereas the glutamate-NMDA/AMPA receptor-nitric oxide (NO)-cyclic GMP signalling pathway is important for memory formation it is not involved in recall post-consolidation. Here, within the olfactory bulb, up-regulation of class 1 metabotropic glutamate receptors appears to maintain the enhanced sensitivity at the mitral to granule cell synapses required for effective memory recall. Recently we have investigated whether fluctuating sex hormone levels during the oestrous cycle modulate olfactory recognition memory and the different neural substrates and signalling pathways involved. These studies have used two robust models of social olfactory memory in the mouse which either involve social or non social odours (habituation-dishabituation and social transmission of food preference tasks). In both cases significant improvement of learning retention occurs when original learning takes place during the proestrus phase of the ovarian cycle. This is probably the result of oestrogen changes at this time since transgenic mice lacking functional expression of oestrogen receptors (ERα and ERβ, the two main oestrogen receptor sub-types) have shown problems in social recognition. Therefore, oestrogen appears to act at the level of the olfactory bulb by modulating both noradrenaline and the glutamate/NO signalling pathway. Key words: Olfactory bulb, Olfactory memory, Olfactory recognition, Social learning, Nitric oxide (NO), Oestrous cycle, Sheep, Mouse, Social transmission of food preference (J. Reprod. Dev. 51: [547][548][549][550][551][552][553][554][555][556][557][558] 2005) or several decades, olfactory learning has been used as a model for the study of memory. This is not only because mammalian olfactory learning shows a number of features common to the human declarative memory system one-trial learning, rapid encoding, long lasting memories and a high capacity for information storage-but also because olfactory memory models represent a relatively simple form of learning and the olfactory system has well characterised pathways and structural organisation which offers many advantages in defining how processing occurs at a systems level. This makes it feasible to study plasticity at all levels in the system, from the initial sensory detection of odourant molecules at the level of the olfactory receptors righ t thr ough to high er cortical processing.There are many different forms of olfactory learning and memory in diverse behavioural contexts involving different mechanisms within the olfactory system. Olfactory learning ...

: A resistance management programme comparing rotations, mosaics and single use of insecticides for residual house-spraying against the insect vectors of malaria is being carried out in Southern Mexico. The area was chosen because of its prior history of insecticide use, relatively sedentary vector, and physical features of the area which limit inward migration of insects to the study area.A high level of resistance to DDT and low levels of organophosphorus (OP), carbamate and pyrethroid resistance were detected by WHO discriminating-dose assays in Ðeld populations of Anopheles albimanus in the pre-spray period in the region where this resistance management project is being undertaken. After the Ðrst year of spraying, resistance, as measured by a discriminating-dose assay, was still at a high level for DDT and had risen for all the other insecticides.Biochemical assays showed that DDT resistance was primarily caused by elevated levels of glutathione S-transferase (GST) activity leading to increased rates of metabolism of DDT to DDE. The numbers of individuals with elevated GST and DDT resistance were well correlated, suggesting that this is the only major DDT resistance mechanism in this population.The carbamate resistance in this population was conferred by an altered acetylcholinesterase (AChE) mechanism. The level of resistance in bioassays correlated well with the frequency of individuals homozygous for the altered AChE allele. This suggests that the level of resistance conferred by this mechanism in its heterozygous state is below the level of detection of the bioassay.The low levels of OP and pyrethroid resistance could be conferred by either the elevated esterase or monooxygenase enzymes. The esterases, however, are elevated only with p-nitrophenyl acetate (PNPA), and are unlikely to be causing broad-spectrum OP resistance. The altered AChE mechanism may also be contributing to the OP but not the pyrethroid resistance.There were signiÐcant di †erences in some resistance gene frequencies for insects obtained by di †erent indoor and outdoor trapping methods. To determine whether the di †erent sampling methods were e †ectively sampling the same interbreeding population, RAPD analysis of insects obtained by di †erent collection methods in di †erent villages was undertaken. There was no observed variability in the RAPD patterns for the di †erent mosquito samples with a number of primers.

There is evidence for both neurotoxic and neuroprotective roles of nitric oxide (NO) in the brain and changes in the expression of the neuronal isoform of NO synthase (nNOS) gene occur during aging. The current studies have investigated potential support for either a neurotoxic or neuroprotective role of NO derived from nNOS in the context of aging by comparing olfactory learning and locomotor function in young compared to old nNOS knockout (nNOS−/−) and wildtype control mice. Tasks involving social recognition and olfactory conditioning paradigms showed that old nNOS−/− animals had improved retention of learning compared to similar aged wildtype controls. Young nNOS−/− animals showed superior reversal learning to wildtypes in a conditioned learning task, although their performance was weakened with age. Interestingly, whereas young nNOS−/− animals were impaired in long term memory for social odors compared to wildtype controls, in old animals this pattern was reversed, possibly indicating beneficial compensatory changes influencing olfactory memory may occur during aging in nNOS−/− animals. Possibly such compensatory changes may have involved increased NO from other NOS isoforms since the memory deficit in young nNOS−/− animals could be rescued by the NO-donor, molsidomine. Both nNOS−/− and wildtype animals showed an age-associated decline in locomotor activity although young nNOS−/− animals were significantly more active than wildtypes, possibly due to an increased interest in novelty. Overall our findings suggest that lack of NO release via nNOS may protect animals to some extent against age-associated cognitive decline in memory tasks typically involving olfactory and hippocampal regions, but not against declines in reversal learning or locomotor activity.