The role of dorsal vs ventral striatal pathways in cocaine-seeking behavior after prolonged abstinence in rats.

RATIONALE: Recent studies have implicated an important role for the dorsal striatum during craving for cocaine and in cocaine-seeking after abstinence in rats. OBJECTIVES: We compared the effects of pharmacological inactivation of mesencephalic dopamine (DA) cell body regions and dorsal vs ventral striatal terminal fields in an animal model of relapse after chronic cocaine self-administration. MATERIALS AND METHODS: Rats self-administered cocaine for 2 h/day for ten sessions, followed by 2 weeks of abstinence (i.e., no extinction training). Immediately before being returned to the self-administration chamber, we assessed the effects of gamma-aminobutyric acid agonist inhibition of midbrain DA regions (substantia nigra [SN] and ventral tegmental area [VTA]) and striatum (dorsolateral caudate-putamen, nucleus accumbens core, and nucleus accumbens shell) on relapse to cocaine-seeking in the absence of reinforcement. Further testing examined daily extinction responding subsequent to the initial relapse test. RESULTS: Inactivation of the dorsal caudate-putamen and midbrain regions attenuated cocaine seeking, while inactivation of the ventral striatum had no such effects. However, subsequent sessions under extinction conditions revealed a rebound in cocaine seeking in animals that had undergone inactivation in all regions except the dorsolateral caudate-putamen. CONCLUSIONS: The dorsal but not ventral striatum plays a critical role in cocaine seeking immediately after abstinence. These data support the theory that chronic cocaine may shift activity from the ventral to dorsal striatum during drug seeking under certain conditions. While not necessary at the time of relapse, the ventral striatum appears to be involved in processing critical information of the relapse event.

Demineralisation of permeable hydroxyapatite with alternating water and acidic buffer: scanning microradiographic study of effect of switching period.

Permeable hydroxyapatite (HAP) blocks were exposed for equal times alternately to pH 4.0 buffer and water for 237 h. Rates of HAP loss with time (determined from changes in X-ray attenuation) were measured as a function of switching period tau (the time for a complete cycle) from 0.5 to 6 h and with a continuous buffer flow. The mean rate of HAP loss decreased markedly as tau increased, and for large tau was about half the rate for continuous buffer flow. We propose that demineralising conditions through the depth of the HAP are influenced by the extent of retention of buffer within its pores which will depend on tau. A mathematical model with parameters R(0) and Deltat was developed, where R(0) is the rate of demineralisation for continuous flow, and Deltat a time added to each tau/2 buffer exposure to account for its retention in the HAP pores. Experimental data fitted the model with Deltat approximately 8 to approximately 10 min and with R(0) close to the rate observed for continuous buffer flow. The model predicts that the rate decreases and approaches R(0)/2 as tau --> infinity, as was found experimentally to be the case. This type of study could potentially give information about subsurface porosity and transport processes during acidic dissolution of permeable solids, for example in dental caries and dental erosion.

Synchrotron x-ray microtomographic investigation of mineral concentrations at micrometre scale in sound and carious enamel.

Synchrotron X-ray microtomography (XMT) was used to measure the linear attenuation coefficient (LAC) for 1.9-microm sidelength voxels within approximal brown spot lesions and sound human enamel. XMT demonstrated three-dimensional features, notably sheets with approximately 30 microm periodicity having low LAC, identified as regions of demineralization corresponding to Retzius lines. Quantitative three-dimensional measurements of mineral concentration, derived from LAC with assumption of a single model composition, were consistent with previous measurements of sound and carious enamel from microradiographic projections. The uncertainty in measurements of mineral concentration and mineral fraction volume was investigated by modelling enamel with a range of composition and component densities. This analysis showed that, although mineral concentration can be determined from LAC with an error of <0.2 g cm(-3), the variation in pore fraction volume within caries lesions cannot be reliably determined from X-ray attenuation measurements alone.

Demineralization in enamel and hydroxyapatite aggregates at increasing ionic strengths.

Subsurface demineralization of dental enamel is a curious feature of both in vivo and in vitro lesion formation. Numerous explanations have been proposed to explain this. One general hypothesis is that subsurface demineralization in enamel and synthetic hydroxyapatite (HAP) aggregates may result from the phenomenon of coupled diffusion between the inward transport of acid and the outward transport of dissolution products. The aim of this study was to test the validity of this explanation. Inert electrolyte was added to demineralizing solutions in order to reduce electrostatic coupling between the diffusive flows that occur during lesion formation. Scanning microradiography (SMR) was used to examine surface layer formation, and to measure the rate of mineral loss at increasing ionic strengths. It was found that surface layer formation was significantly reduced as the concentration of inert electrolyte was increased. Further, the rate of mineral loss from the developing lesion increased as the concentration of inert electrolyte (and therefore the ionic strength) in the demineralizing solution increased. It is concluded that electrostatic coupling between counter diffusing acid and dissolution products during lesion formation can significantly influence the mineral concentration within the surface layer.

Longitudinal study of the three-dimensional development of subsurface enamel lesions during in vitro demineralisation.

A longitudinal study was made of the 3D development of subsurface enamel lesions in whole human molars. X-ray microtomography (XMT) was used to measure the 3D distribution of linear attenuation coefficients in the tissue at 8-15 stages during cumulative times of 36-107 days demineralisation through approximately 1-mm-wide windows. Although lesion morphology was consistent with preferential anisotropic dissolution parallel to enamel prisms at the advancing front, detail (at a scale of approximately 100 microm) varied in relation to exposed surface sites separated by <1 mm. The distribution of mineral in the most superficial region varied across the exposed face of each lesion. Within lesions, localised foci of low mineral concentration (at a scale of approximately 200 microm) retained their general form through successive stages of demineralisation before coalescing. The most advanced regions within a lesion seemed to correspond with surface regions with lowest mineral concentration. These findings indicate that local variations in fractional pore volume of partially demineralised enamel influence the subsequent spatial development of lesions.

Affinity binding phenomena of DNA onto apatite crystals.

The effect of DNA on the crystal growth of hydroxyapatite (HAp) and its morphology was examined. X-ray diffraction patterns of DNA-containing apatites showed typical apatitic features. However, crystal growth was greatly inhibited in the presence of DNA during synthesis; particularly, the crystallinity in the a-axis direction decreased dramatically at low concentrations of DNA. The a- and c-axis dimensions of each precipitate were almost the same, which implies that DNA molecules affect only the crystal surface. CHN analysis clearly showed the presence of these elements, which increased with an increase in DNA concentration in the solution. Scanning electron micrographs of the precipitates formed in the presence of DNA showed typical needle-like crystals, with a decreased crystal size, especially width. Infrared absorption spectroscopy of the DNA-containing apatites showed that the 1630-1700 cm(-1) absorption band due to C=C and C=N stretching increased with an increase in DNA concentration during precipitation. The ESCA spectrum of HAp(DNA 1.0) shows N 1s and C 1s peaks that are absent and weak, respectively in HAp. 31P NMR spectroscopy revealed a weak peak at the base of the 31P peak from the PO4(3-) ions in the HAp crystals. This weak 31P peak had a small positive shift from the position found in native DNA which may be due to the phosphate backbone of adsorbed DNA. The apparent solubility of the HAps increased with an increase in DNA concentration. These results suggest that there is an affinity binding between apatite crystal and DNA molecules.

Acute cocaine alters oxytocin levels in the medial preoptic area and amygdala in lactating rat dams: implications for cocaine-induced changes in maternal behavior and maternal aggression.

Acute cocaine administration has been correlated with disruptions in the onset and maintenance of maternal behavior as well as decreases in maternal aggressive behavior in rat dams. A growing body of evidence suggests that cocaine may alter oxytocin levels leading to impairments in maternal behavior and aggression. The current study assessed whether acute cocaine injections alter oxytocin (OT) levels in the medial preoptic area (MPOA), ventral tegmental area (VTA), amygdala (AMY), and hippocampus (HIP) on postpartum day (PPD) 1 or PPD 6. On PPD 1, 30 mg/kg cocaine reduced OT levels by approximately 26.9% (picograms/milligram) in the MPOA (t (18) = 3.44, P<.01) compared to saline. On PPD 6, 30 mg/kg cocaine significantly increased OT levels by approximately 20.9% (picograms/brain area) in the AMY (F (2,25) = 3.44, P=.05) relative to saline. These findings suggest that acute cocaine may disrupt maternal behavior and maternal aggression at least in part through its action on the oxytocinergic system.

X-ray microtomographic study of mineral distribution in enamel of mandibular rat incisors.

X-ray microtomography was used to study the mineral concentrations in sequential slices of enamel of 5 mandibular incisors which showed an increase from approximately 1.0 to approximately 2.7 g cm(-3) from the apex towards the incisal end. For points at the same distance from the apex, there were differences up to 0.6 g cm(-3) between the teeth. The change of mean concentrations in the slices with distance could be modelled as (different) saturating exponentials. Under the assumption of a uniform growth rate of a mandibular incisor of 0.6 mm per day and a common time origin for the start of maturation (taken as a mineral concentration of 1 g cm(-3)), the distances were transformed to a common time frame to give a pooled data set. A single saturating exponential could be fitted to this pooled transformed data; this was: Cm = 2.84-1.94exp (-0.18d) where Cm is the mean mineral concentration (g cm(-3)) and d the time (days) from the start of maturation. This gives an asymptotic concentration of 2.84 g cm(-3) towards the incisal end, with a time constant of 7.7 days. The mineral concentration distribution functions were found to be more positively skewed closer to the apex, but more negatively skewed towards the incisal end. The difference between the higher mineral concentration in the outer enamel and the enamel near the amelodentinal junction (ADJ) was approximately 3%. The direction of maximum increase in concentration from the outer enamel surface to the ADJ meets the boundary of the ADJ at approximately 80 degrees. Three dimensional surface rendering of isodensity contours showed that the previously described C-shaped pattern of mineralisation is not solely a surface phenomenon, but extends through the depth of the enamel.

Optical profilometric study of changes in surface roughness of enamel during in vitro demineralization.

The application of non-contact optical profilometry for non-destructive study of changes in the surface roughness of natural enamel surfaces during in vitro demineralization was investigated. Repeated measurement of profiles of the same enamel surfaces after successive demineralization episodes was achieved by a kinematic specimen mount that could be removed, then accurately repositioned. Surface roughness parameters and reflectivity of natural enamel surfaces exposed to either a demineralizing solution (test) or deionized water (control) for up to 6 days were measured at 6- to 24-hour intervals. The results prior to demineralization showed that surface roughness varies with position on the enamel surface. During demineralization there was an approximately linear increase in surface roughness for the first 70 h followed by somewhat erratic behaviour, and a saturating exponential with time increase in reflectivity for the first 70 h which subsequently remained constant. Changes in enamel surface during in vitro demineralization were also observed using scanning electron microscopy.

Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate.

Five 100 g batches of a carbonate apatite (the intermediate) were produced by heating an aqueous slurry of CaCO3 and CaHPO4 with an overall Ca/P mole ratio of 5/3 with vigorous stirring. Each intermediate produced by boiling off water was heated in vacuum at 1100 degrees C to remove carbonate, then steamed at 900 degrees C to ensure complete hydroxylation. Comparison of calculated and observed X-ray diffraction patterns showed final products containing 50-100 wt% monoclinic hydroxyapatite (remainder hexagonal). Rietveld refinements in P6(3)/m gave structures similar to several hydroxyapatite standards, including NIST SRM 2910, although there was no evidence from X-ray diffraction that the latter was in the monoclinic form. Refinements from standards and final products were slightly different from published single crystal data for Holly Springs hydroxyapatite. This is attributed to known impurities in mineral hydroxyapatite and indicates that parameters from the Rietveld refinements are closer to the true values for pure hydroxyapatite. Rietveld refinements for intermediates showed small, but significant differences from the final product, the largest being in O1x, O2x and O(H)z. All P-O bond lengths were shorter than in the final product, resulting in a 3.2% lower PO4 tetrahedron volume. The occupancies of P and Ca(2) were reduced. These differences are attributed to partial replacement of PO4(3) by CO3(2-) ions.

Rates of mineral loss in human enamel during in vitro demineralization perpendicular and parallel to the natural surface.

Human enamel is a structurally anisotropic material. The aim of this study was to investigate whether this structural anisotropy is reflected in the demineralization behaviour of enamel. Kinetics of demineralization of in vitro caries lesions with the direction of acid attack perpendicular to the natural surface of dental enamel from human premolar teeth were compared with kinetics when demineralized parallel to this surface. Pairs of enamel samples from the same tooth were demineralized under identical conditions. Loss of mineral with time was very nearly linear for both directions (consistent with the rate-controlling step being reaction at the advancing front rather than transport processes), but the perpendicular rate was, on average, about 14% higher than the parallel rate. The rate of demineralization parallel to the surface increased from the natural surface to the enamel-dentine junction by 10-25%, depending on sample. The origin of fine structure and slight departures from linearity in the loss of mineral with time plots are discussed. Mineral masses per unit area were determined from absorption of a 15-microm diameter X-ray beam using photon (AgKalpha) counting methods.

Real-time measurement of in vitro enamel demineralization in the vicinity of the restoration-tooth interface.

An X-ray attenuation method using photon counting (scanning microradiography) is presented for the real-time study of in vitro demineralization of dental tissues in the vicinity of the restoration-tooth interface. By repeated measurement of mineral content profiles during the course of demineralization, the pattern of lesion development and the rate of mineral loss can be studied. The method is illustrated by comparison of enamel demineralization near a polyacid-modified composite resin restoration, near a bis-GMA/TEGMA composite resin restoration, and in an unrestored control. The method has potential for study of the influence of restorative materials on susceptibility of tooth tissue to demineralization.

Cells for the study of acidic dissolution in packed apatite powders as model systems for dental caries.

A number of different designs of cells have been developed for the study of dissolution processes in packed apatite powders. Basic design requirements were that no deleterious processes, such as high temperature sintering or binding agents, were involved, and that there was maximum opportunity for experimental study. One design used alternating filter paper discs and apatite layers (typically 42 mg). At the end of an experiment, the cell could be disassembled and infrared spectra and X-ray powder diffraction patterns made of individual layers. Cells without filter paper discs were also made, but terminal sampling at well-defined depths was more difficult or impossible. The cells were constructed from poly(methylmethacrylate) so the course of dissolution could be monitored by radiography. Subsurface loss of apatite was almost always seen after from 2 weeks to 8 months exposure to buffer solutions at pH 3.0-5.5. The greatest loss of apatite was typically 0.4-2 mm below the surface, which is at a larger distance than usually seen for dental enamel. This may be attributable to the low packing density (typically 35-60 vol%) found in the present systems compared with enamel.

Determination of mineral concentration in dental enamel from X-ray attenuation measurements.

The mineral content of dental enamel is commonly measured by X-ray attenuation experiments. Most studies have used contact microradiography in which intensities are measured with photographic film which is convenient and gives high spatial resolution. However photon counting intensity measurements are to be preferred in many experiments (longitudinal and scanning microradiography, and microtomography), as illustrated here, because they have a larger dynamic range and greater sensitivity to small intensity changes. Additionally, the detector and specimen are well separated which allows the pseudo-continuous study of de- and remineralization. The mineral content is often quoted as 95 wt% or 87 vol% hydroxyapatite for permanent human enamel. This determination from attenuation experiments requires accurate values of elemental mass attenuation coefficients and a number of assumptions. The effects of possible choices of these are considered and it is shown that the most important is the density of enamel mineral used in conversion of wt% to vol%. If the density is taken as 2.99 g cm(-3), as recently suggested (J.C. Elliott, Dental Enamel, Ciba Foundation Symposium 205, Wiley, Chichester, pp. 54-72, 1997), instead of 3.15 g cm(-3) as for hydroxyapatite, the calculated vol% is approximately 93 instead of approximately 87.

X-ray microtomography: 3-dimensional imaging of teeth for computer-assisted learning.

X-ray microtomography (XMT), a miniaturised form of computed tomography, has been used to generate 3-dimensional volume data sets of the X-ray absorption of human teeth in vitro, with a resolution of approximately 40 microns (cubic voxel sidelength). Examples are presented of images relevant to dental morphology, cariology, and cavity preparation and restoration. Applications of XMT imaging to dental education are discussed in the context of new approaches to visual learning through computer-assisted methods.

X-ray microtomography: nondestructive three-dimensional imaging for in vitro endodontic studies.

Article shows the application of a laboratory x-ray microtomography system, a miniaturized form of conventional computerized axial tomography, to the study of root canal morphologic characteristics and changes in the course of root canal treatment in extracted teeth. After reconstruction of the three-dimensional images, the IDL software package (Research Systems, Inc., Colorado) was used to obtain cross-sectional slices of the tooth and three-dimensional views of rendered surfaces of constant mineral density. The root canal systems and changes in these were imaged at a resolution (cubic voxel side-length) of approximately 40 microns.

Structure, crystal chemistry and density of enamel apatites.

The apatitic calcium phosphate crystals in dental enamel are too small for single crystal diffraction studies so the only possible direct structure determination must use whole-pattern-fitting Rietveld analysis of X-ray and neutron powder diffraction patterns. As a result, aspects of the structure are not known in detail. Further structural information can be obtained by consideration of published chemical analyses and infrared studies, taking into account studies of the crystal chemistry of synthetic apatitic analogues of enamel apatite. The apatitic constitutional water and total water content of enamel are particularly important, but there are difficulties in their determination. Making reasonable assumptions, a number of models of the unit cell can be derived. The weight per cent (including constitutional water) and density of the enamel apatite crystals for the most probable model are about 98 wt.% and 3.0 g cm-3, respectively. The apatite volume per cent calculated from these values is about 96%. The weight per cent and volume per cent of enamel apatite are higher than normally accepted values because of inclusion of constitutional water and use of a density for enamel apatite that takes into account its known lattice expansion over hydroxyapatite and probable lattice vacancies.

Theoretical explanation of the relationship between backscattered electron and x-ray linear attenuation coefficients in calcified tissues.

X-ray absorption and backscattered electron (BSE) microscopies are two commonly used techniques for estimating mineral contents in calcified tissues. The resolution in BSE images is usually higher than in x-ray images, but due to the previous lack of good standards to quantify the grey levels in BSE images of bones and teeth, x-ray microtomography (XMT) images of the same specimens have been used for calibration. However, the physics of these two techniques is different: for a specimen with a given composition, the x-ray linear attenuation coefficient is proportional to density, but there is no such relation with the BSE coefficient. To understand the reason that this calibration appears to be valid, the behaviour of simulated bone samples was investigated. In this, the bone samples were modelled as having three phases: hydroxyapatite (Ca10(PO4)6(OH)2), protein, and void (either empty or completely filled with polymethylmethacrylate (PMMA), a resin which is usually used for embedding bones and teeth in microscopic studies). The x-ray linear attenuation coefficients (calculated using published data) and the BSE coefficients (calculated using Monte Carlo simulation) were compared for samples of various phase proportions. It was found that the BSE coefficient correlated only with the x-ray attenuation coefficient for samples with PMMA infiltration. This was attributed to the properties of PMMA (density and mean atomic number) being very similar to those of the protein; therefore, the sample behaves like a two-phase system which allows the establishment of a monotonic relation between density and BSE coefficient. With the newly developed standards (brominated and iodinated dimethacrylate esters) for BSE microscopy of bone, grey levels can be converted to absolute BSE coefficients by linear interpolation, from which equivalent densities can be determined.

A comparison of the mineral content of enamel and dentine in human premolars and enamel pearls measured by X-ray microtomography.

Mineral content gradients in two composite enamel pearls from permanent human upper molars were measured by X-ray microtomography (XMT) at a resolution of 15-30 microns. This non-destructive microscopic technique was used to make 15-microns thick XMT slices with 100-microns separation through one pearl and 250-microns separation through the other. Average mineral contents were calculated from the linear absorption coefficients determined from regions of the XMT slices assuming the inorganic component to be calcium hydroxyapatite. These values were compared with similar XMT studies of coronal enamel and dentine of upper permanent premolars. A mineral content gradient in the pearls, reducing from the enamel surface to the amelodentinal junction, was found; this was similar to that observed in the coronal enamel of the upper premolar. The mineral contents in the surface and deeper enamel regions of the pearl were similar to those observed in premolar enamel. In contrast, the mineral content for the dentine of the pearl was greatest at the amelodentinal junction, i.e. the gradient was in the opposite direction to that observed in premolar dentine. These results suggest that the process of mineralization of the pearl dentine differs from that in permanent control dentine. In addition, gradients in enamel and dentine mineral contents reducing from the tip of the pearl to the base of the pearl were found.

An analysis of biological hard tissues using the tomographic reconstruction error formula.

A variety of teeth and bone specimens have been scanned using the constant statistics tomographic scanner developed at the London Hospital Medical College. Each data set has been reconstructed with several Hamming filters, and for a geometric resolution ranging from 20 to 50 micrometers. The reconstructions have been studied using the approximate error formula for the convolution backprojection algorithm. Some inferences are made on the relative structure of the cross sections.