Drug-protein binding and blood-brain barrier permeability.
(1/2027)
The permeability surface area (PS) product, an index of permeability of the blood-brain barrier (BBB), was measured by using the in situ perfusion method. In the cerebral circulation, the fraction of drug that permeates into the brain through the BBB is not only the unbound fraction but also the fraction dissociated from the protein in the perfusate. The sum of these two fractions, the apparent exchangeable fraction, was estimated by fitting the parameters of the BBB permeability under the condition of varying BSA concentrations in the perfusate. The unbound fraction of drugs in a buffer containing 0.5 mM BSA was measured by using the ultrafiltration method in vitro, and the apparent exchangeable fraction was measured in vivo by using the intracarotid artery injection method. The apparent exchange fraction was 100% for S-8510, 96.5% for diazepam, 90.9% for caffeine, 38.3% for S-312-d, 33.1% for propranolol, and 6.68% for (+)-S-145 Na, and each of these was higher than the corresponding unbound fraction in vitro in all drugs. The apparent exchangeable fractions, for example, were 8 times higher for diazepam and 38 times for S-312-d than the unbound fractions in vitro. The apparent exchangeable fraction of drugs was also estimated from the parameters obtained with the perfusion method. Because drugs can be infused for an arbitrary length of time in the perfusion method, substances with low permeability can be measured. The apparent exchangeable fractions obtained with this method were almost the same as those obtained with the intracarotid artery injection method. (+info)
Alpha-2 adrenergic receptor functional coupling to G proteins in rat brain during postnatal development.
(2/2027)
During postnatal development, alpha-2 adrenergic receptors (A2AR) change in both density and distribution. In forebrain, receptor density increases about 4-fold over neonatal levels, reaching adult levels before postnatal day (P) 28, whereas in hindbrain, including cerebellum, there is a decrease in overall receptor density. We examined the coupling of A2AR to G proteins using agonist-stimulated [35S]GTPgammaS binding as a functional assay. In forebrain the A2AR agonist-stimulated [35S]GTPgammaS binding increases rapidly after P7, reaching its highest levels at P21 and then declining slightly to adult levels. This binding increases more slowly than receptor number, suggesting that the appearance of G proteins, rather than the A2AR, determines the developmental appearance of functional A2AR-G protein interactions in forebrain. Basal [35S]GTPgammaS binding and [35S]GTPgammaS binding stimulated by other neurotransmitter receptor systems (GABA-B, mu opiate, and muscarinic) increase with a time course similar to A2AR-stimulated [35S]GTPgammaS binding. In contrast, in hindbrain, A2AR-stimulated [35S]GTPgammaS binding decreases during postnatal development in parallel with the decrease in A2AR levels, whereas [35S]GTPgammaS binding stimulated by other neurotransmitter receptor systems increases in parallel with basal [35S]GTPgammaS binding. Functional receptor-G protein coupling in hindbrain appears to be dependent on the developmental appearance of G proteins for most neurotransmitter systems. However, for A2AR the decrease in receptor density is the overriding factor. These studies 1) demonstrate the functional measurement of A2AR-G protein coupling in native tissue for the first time, 2) demonstrate that A2AR are coupled to G proteins throughout postnatal development, and 3) describe developmental increases and decreases in functional A2AR in brain. (+info)
Ethanol exposure differentially alters central monoamine neurotransmission in alcohol-preferring versus -nonpreferring rats.
(3/2027)
Individual differences in ethanol preference may be linked to differences in the functional activity of forebrain monoamine systems or their sensitivity to modification by ethanol. To test this hypothesis, basal extracellular concentrations of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens as well as the effects of repeated ethanol pretreatment on the basal release of these transmitters were examined in alcohol-preferring (P), alcohol-nonpreferring (NP), and genetically heterogeneous Wistar rats. All animals received i.p. injections of ethanol (1.0 g/kg) or saline for 5 consecutive days. Fifteen hours after the final pretreatment, basal extracellular concentrations and "in vivo extraction fraction" values for DA and 5-HT were determined by no-net-flux in vivo microdialysis. In ethanol-naive rats, significant line differences were observed with high basal 5-HT release in P rats, low 5-HT release in NP rats, and intermediate 5-HT levels in Wistar rats. No differences among groups were noted in basal DA release. Ethanol pretreatment decreased basal extracellular 5-HT levels in P rats whereas increasing 5-HT efflux was seen in the Wistar and NP lines. In addition, ethanol pretreatment increased extracellular DA concentrations in Wistar and P rats, but not in NP rats. The results confirm a relationship between the functional status of forebrain DA and 5-HT systems and ethanol preference or aversion. Moreover, the data suggest that ethanol exposure can alter basal DA and 5-HT in the nucleus accumbens and that vulnerability to ethanol-induced changes in monoamine neurotransmission may be a factor in genetically determined ethanol preference. (+info)
Comparative synteny cloning of zebrafish you-too: mutations in the Hedgehog target gli2 affect ventral forebrain patterning.
(4/2027)
Zebrafish you-too (yot) mutations interfere with Hedgehog (Hh) signaling during embryogenesis. Using a comparative synteny approach, we isolated yot as a zinc finger transcription factor homologous to the Hh target gli2. Two alleles of yot contain nonsense mutations resulting in carboxy-terminally truncated proteins. In addition to causing defects in midline development, muscle differentiation, and retinal axon guidance, yot mutations disrupt anterior pituitary and ventral forebrain differentiation. yot mutations also cause ectopic lens formation in the ventral diencephalon. These findings reveal that truncated zebrafish Gli2 proteins interfere with Hh signaling necessary for differentiation and axon guidance in the ventral forebrain. (+info)
Ectopic bone morphogenetic proteins 5 and 4 in the chicken forebrain lead to cyclopia and holoprosencephaly.
(5/2027)
Proper dorsal-ventral patterning in the developing central nervous system requires signals from both the dorsal and ventral portions of the neural tube. Data from multiple studies have demonstrated that bone morphogenetic proteins (BMPs) and Sonic hedgehog protein are secreted factors that regulate dorsal and ventral specification, respectively, within the caudal neural tube. In the developing rostral central nervous system Sonic hedgehog protein also participates in ventral regionalization; however, the roles of BMPs in the developing brain are less clear. We hypothesized that BMPs also play a role in dorsal specification of the vertebrate forebrain. To test our hypothesis we implanted beads soaked in recombinant BMP5 or BMP4 into the neural tube of the chicken forebrain. Experimental embryos showed a loss of the basal telencephalon that resulted in holoprosencephaly (a single cerebral hemisphere), cyclopia (a single midline eye), and loss of ventral midline structures. In situ hybridization using a panel of probes to genes expressed in the dorsal and ventral forebrain revealed the loss of ventral markers with the maintenance of dorsal markers. Furthermore, we found that the loss of the basal telencephalon was the result of excessive cell death and not a change in cell fates. These data provide evidence that BMP signaling participates in dorsal-ventral patterning of the developing brain in vivo, and disturbances in dorsal-ventral signaling result in specific malformations of the forebrain. (+info)
Blockade of N-methyl-D-aspartate receptor activation suppresses learning-induced synaptic elimination.
(6/2027)
Auditory filial imprinting in the domestic chicken is accompanied by a dramatic loss of spine synapses in two higher associative forebrain areas, the mediorostral neostriatum/hyperstriatum ventrale (MNH) and the dorsocaudal neostriatum (Ndc). The cellular mechanisms that underlie this learning-induced synaptic reorganization are unclear. We found that local pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors in the MNH, a manipulation that has been shown previously to impair auditory imprinting, suppresses the learning-induced spine reduction in this region. Chicks treated with the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV) during the behavioral training for imprinting (postnatal day 0-2) displayed similar spine frequencies at postnatal day 7 as naive control animals, which, in both groups, were significantly higher than in imprinted animals. Because the average dendritic length did not differ between the experimental groups, the reduced spine frequency can be interpreted as a reduction of the total number of spine synapses per neuron. In the Ndc, which is reciprocally connected with the MNH and not directly influenced by the injected drug, learning-induced spine elimination was partly suppressed. Spine frequencies of the APV-treated, behaviorally trained but nonimprinted animals were higher than in the imprinted animals but lower than in the naive animals. These results provide evidence that NMDA receptor activation is required for the learning-induced selective reduction of spine synapses, which may serve as a mechanism of information storage specific for juvenile emotional learning events. (+info)
The Ca2+ channel blockade changes the behavioral and biochemical effects of immobilization stress.
(7/2027)
We investigated how the effects of chronic immobilization stress in rats are modified by Ca2+ channel blockade preceding restraint sessions. The application of nifedipine (5 mg/kg) shortly before each of seven daily 2 h restraint sessions prevented the development of sensitized response to amphetamine as well as the stress-induced elevation of the densities of L-type Ca2+ channel in the hippocampus and significantly reduced the elevation of the densities of [3H]nitrendipine binding sites in the cortex and D1 dopamine receptors in the limbic forebrain. Neither stress, nor nifedipine affected the density of alpha 1-adrenoceptors and D1 receptors in the cerebral cortex nor D2 dopamine receptors in the striatum. A single restraint session caused an elevation of blood corticosterone level that remained unaffected by nifedipine pretreatment, but the reduction of this response during the eighth session was significantly less expressed in nifedipine-treated rats. We conclude that L-type calcium channel blockade prevents development of several stress-induced adaptive responses. (+info)
Early visual experience shapes the representation of auditory space in the forebrain gaze fields of the barn owl.
(8/2027)
Auditory spatial information is processed in parallel forebrain and midbrain pathways. Sensory experience early in life has been shown to exert a powerful influence on the representation of auditory space in the midbrain space-processing pathway. The goal of this study was to determine whether early experience also shapes the representation of auditory space in the forebrain. Owls were raised wearing prismatic spectacles that shifted the visual field in the horizontal plane. This manipulation altered the relationship between interaural time differences (ITDs), the principal cue used for azimuthal localization, and locations of auditory stimuli in the visual field. Extracellular recordings were used to characterize ITD tuning in the auditory archistriatum (AAr), a subdivision of the forebrain gaze fields, in normal and prism-reared owls. Prism rearing altered the representation of ITD in the AAr. In prism-reared owls, unit tuning for ITD was shifted in the adaptive direction, according to the direction of the optical displacement imposed by the spectacles. Changes in ITD tuning involved the acquisition of unit responses to adaptive ITD values and, to a lesser extent, the elimination of responses to nonadaptive (previously normal) ITD values. Shifts in ITD tuning in the AAr were similar to shifts in ITD tuning observed in the optic tectum of the same owls. This experience-based adjustment of binaural tuning in the AAr helps to maintain mutual registry between the forebrain and midbrain representations of auditory space and may help to ensure consistent behavioral responses to auditory stimuli. (+info)