In the present study, stereological techniques applied to electron micrographs of the molecular layer of the rat cerebellum have been used to estimate the number of parallel fiber synapses on the dendritic tree of a single Purkinje cell. Quantitative features of the parallel fiber to Purkinje cell dendritic spine synapses and of the parallel fibers were investigated as a preliminary to estimating the number of synapses. Parallel fiber to Purkinje cell synapses are flattened disclike structures with a mean axial ratio of 14.7 and a mean diameter of 319 microns in fixed tissue. The density of synapses in our fixed material was 8.17 x 10(8) per microliters of molecular layer. Determination of the length density of the synapses per unit area of micrograph indicated a synapse density of 8.03 x 10(8) per microliters. These densities give a total number of synapses per Purkinje cell of 1.74 x 10(5) and 1.71 x 10(5), respectively. Estimation of the number of parallel fiber varicosities and of varicosity length gave a density of 9.31 x 10(8) varicosities per microliters of molecular layer and determining the mean number of parallel fiber to Purkinje cell synapses per varicosity gave a synapse density of 9.82 x 10(8) per microliters, equivalent to 2.09 x 10(5) per Purkinje cell. The reasons why this estimate is likely to be too high are discussed. We conclude that there are some 175,000 parallel fiber synapses on an individual Purkinje cell dendritic tree in the cerebellar cortex of the rat, considerably more than previously reported.

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Adult worker honey bees alter their behaviour with age but retain a strong reliance on sensory information from the antennae. The antennae house a diverse array of receptors, including mechanoreceptors, hygroreceptors, olfactory receptors, and contact chemoreceptors, which relay information to the brain. Antennal sensory neurons that project to the antennal lobes of the brain converge onto second-order interneurones to form discrete spheres of neuropil, called glomeruli. The spatial organisation of glomeruli in the antennal lobes of the honey bee is constant, but the central distribution of information from receptors tuned to different sensory modalities is unknown. Here we show that the glomerular neuropil of the antennal lobes undergoes constant modification during the lifetime of the adult worker bee. Changes in morphology are site specific and highly predictable. The total volume of the glomerular neuropil of the antennal lobe increased significantly during the first 4 days of adult life. Each of the five readily identifiable glomeruli examined in this study exhibited a unique pattern of growth. The growth of two of the five glomeruli changed dramatically with the shift to foraging duties. Furthermore, significant differences were identified between the antennal lobes of bees performing nectar- and pollen-foraging tasks. The highly compartmentalized nature of the antennal lobes, the ease with which specific glomeruli can be identified, and the predictability of changes to the antennal lobe neuropil make this an ideal system for examining the mechanisms and behavioural consequences of structural plasticity in primary sensory centres of the brain.

This study demonstrates that exposure to alcohol during a period of rapid brain growth can lead to severe and permanent deficits in the number of granule cells and mitral cells in the main olfactory bulb. Sprague-Dawley rat pups were reared artificially and were administered alcohol over postnatal days (PD) 4 through 9, a period of brain development comparable to part of the human third trimester. The daily alcohol dose of 6.6 g/kg was concentrated into two of the twelve daily feedings, producing high peak blood alcohol concentrations followed by near total clearance. Pups were either sacrificed on PD10 or were allowed to grow to adulthood and sacrificed on PD115. The total number of granule cells and mitral cells in the main olfactory bulb were estimated with the aid of unbiased stereological principles and systematic sampling techniques. Exposure to alcohol resulted in significant reductions in the number of both granule cells and mitral cells on PD10. Significant deficits in both neuronal populations remained on PD115. The results support the hypothesis that alcohol exposure can kill developing neurons and lead to permanent neuronal deficits. Substantial developmental changes also occurred in the total number of mitral cells and granule cells between PD10 and PD115 in the control groups. In untreated rats, the number of granule cells increased from 2.20 x 10(6) on PD10 to 5.06 x 10(6) on PD115, while the number of mitral cells decreased from 5.30 x 10(4) to 4.33 x 10(4) over the same time period. These results demonstrate that there is a natural loss of mitral cells during postnatal development at the same time that granule cell number is increasing.