By V. Harek. Concordia University, Irvine California.
DISTRIBUTION AND MECHANISMS The potential value of the discovery order 50 mg naltrexone overnight delivery symptoms 7dp3dt, classification and subdivisions of any NT receptors rests on the knowledge that Figure 7 naltrexone 50mg visa medicine and science in sports and exercise. The figure attempts to highlight the major differences between extra- and intracellular loops, especially the intracellular loops between transmembrane sections 5 and 6 and the much longer C terminal of the D1 compared with the D2 receptor. The thickened length of the D2 receptor represents the amino-acid sequence missing in the short form of the receptor. No attempt has been made to show differences in amino-acid sequencing or transmembrane topography DOPAMINE 147 (1) Those receptors are linked to different cellular actions and/or are located in different brain regions or parts of the neuron so as to produce different functional effects. To some extent these requirements are cyclic since the establishment of different functions (1) depends on the availability of appropriate drugs (2). There is no shortage of drugs, especially antagonists, but since the main difference in structure between DA receptors is intracellular, rather than at the binding or recognition site, very specific drugs may be hard to produce. Since receptors can be expressed in cell lines the affinity of drugs for the different receptors can, however, be established, as can their cellular actions. Detection of appropriate mRNA also makes it possible to map the distribution of the receptors. The main characteristics of the DA receptors are summarised below and in Table 7. D1 receptor family D1 Highest expression in human striatum and nucleus accumbens and olfactory tubercle but also some in cortex and hypothalamus. In the striatum 50% of medium sized striato-nigral neurons, which also express substance P, express them. Antagonists [ [ Clozapine Number High Low High Low Low Distribution Striatum [ [ Nuc. See Sibley and Monsma (1997), Sokoloff and Schwartz (1995) and Strange (1996). D5 Highest concentration in hippocampus and hypothalamus but much lower expression overall. Also linked to stimulation of adenylate cyclase but higher submicromolar affinity for DA (K1$200 nM). D2 receptor family D2 Mostly in striatum, nucleus accumbens and olfactory tubercle but also on neuron cell bodies in substantia nigra and ventral tegmentum where they are the auto- receptors for locally (dendritic) released DA. The loss of specific D2 antagonist binding in the striatum after lesions of the afferent nigro-striatal tract indicates their presynaptic autoreceptor role on terminals there. Other lesion studies have also established D2 receptors on other inputs such as the cortico striatal tract. As with D1 receptors some 50% of striatal medium-sized cells contain them but they are different neurons as they co-express enkephalin rather than substance P. The importance of this difference in the therapy of Parkinsonism is taken up later (Chapter 15). Although linked to inhibition of adenylate cyclase (and IP3 turnover) this is not their primary action. They increase K conductance (hyperpolarise neurons) but also inhibit Ca2 entry through voltage-sensitive channels, probably directly. When functioning as autoreceptors, these effects would also reduce DA release. The affinity for DA is slightly higher for the D2 (K1$400 nM) than for D1 receptors. No pharmacological differences have been established between the long or short forms of the D2 receptor. Mainly in limbic regions (nucleus accumbens and olfactory tubercle) but also in hypothalamus. Some in caudate and cortex and also expressed on DA neurons in substantia nigra, presumably as autoreceptors. No effect on adenylate cyclase but inhibits Ca2 entry (autoreceptor role). D4 Again very few in number compared with D2 but located in frontal cortex, mid- brain and amygdala.
It is continuous with the ven- heart rate generic 50 mg naltrexone fast delivery medications prolonged qt, breathing rate safe naltrexone 50 mg treatment whooping cough, blood pressure, and digestive ac- tricles of the brain and is filled with cerebrospinal fluid. Spinal nerve pathways are also involved in swal- lowing, coughing, sneezing, and vomiting. Spinal Cord Tracts Impulses are conducted through the ascending and descending Structure of the Spinal Cord tracts of the spinal cord within the columns of white matter. The spinal cord extends inferiorly from the position of the fora- men magnum of the occipital bone to the level of the first lum- bar vertebra (L1). It is somewhat flattened posteroventrally, making it oval in cross section. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Chapter 11 Nervous Tissue and the Central Nervous System 385 Medulla oblongata C1 C2 Cervical spinal nerve C3 Cervical plexus C4 Cervical spinal C5 enlargement C6 C7 C8 T1 Brachial plexus T2 Arachnoid T3 T4 T5 Dura mater T6 Cervical enlargement T7 T8 Thoracic spinal nerve T9 Dura mater T10 Lumbar enlargement Lumbar spinal T11 enlargement Dura mater Conus medullaris T12 Filum terminale Conus medullaris L1 Cauda equina Lumbar spinal L2 Cauda equina nerve L3 Filum terminale L4 L5 Lumbar plexus S1 S2 S3 (b) S4 Sacral plexus S5 (a) FIGURE 11. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 386 Unit 5 Integration and Coordination Posterior horn Posterior funiculus Posterior median Lateral funiculus sulcus Gray commissure Posterior root of spinal nerve Central canal Lateral horn Spinal ganglion Paras Anterior horn Anterior root Anterior median Anterior funiculus fissure Spinal nerve of spinal nerve (a) Posterior horn Central canal Anterior horn Spinal ganglion Spinal nerve (b) FIGURE 11. The spinal cord has six columns of white matter called funiculi ated and are named according to their origin and termination. The two anterior funiculi are lo- brain and spinal cord or cross over within the medulla oblongata cated between the two anterior horns of gray matter, to either or the spinal cord. The crossing over of nerve tracts is referred to side of the anterior median fissure (fig. Each funiculus consists of both ascending and descending Descending tracts are grouped according to place of origin as tracts. The nerve fibers within the tracts are generally myelin- either corticospinal or extrapyramidal. Corticospinal (pyramidal) tracts descend directly, without synaptic interruption, from the cerebral cortex to the lower motor neurons. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Chapter 11 Nervous Tissue and the Central Nervous System 387 Cerebrum Sensory cortex Motor cortex of cerebrum of cerebrum Cerebrum Corticospinal tract Thalamus Midbrain Sections of brain Midbrain Sections of brain Pons Pons Sensory fibers decussate Medulla oblongata Medulla Motor fibers oblongata decussate Section of cervical spinal cord Section of cervical spinal cord Motor impulse to skeletal muscle Sensory impulse from skin receptors FIGURE 11. Most (about 85%) of The corticospinal tracts appear to be particularly important in the corticospinal fibers decussate in the pyramids of the medulla voluntary movements that require complex interactions between the motor cortex and sensory input. The remaining 15% do not cross from ample, is impaired when the corticospinal tracts are damaged one side to the other. The fibers that cross compose the lateral in the thoracic region of the spinal cord, whereas involuntary breath- corticospinal tracts, and the remaining uncrossed fibers compose ing continues. Damage to the pyramidal motor system can be de- the anterior corticospinal tracts. Because of the crossing of fibers tected clinically by the presence of Babinski’s reflex, in which stimulation of the sole of the foot causes extension (upward move- from higher motor neurons in the pyramids, the right hemisphere ment) of the great toe and fanning out of the other toes. Babinski’s re- primarily controls the musculature on the left side of the body, flex is normally present in infants because neural control is not yet whereas the left hemisphere controls the right musculature. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 388 Unit 5 Integration and Coordination TABLE 11. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Chapter 11 Nervous Tissue and the Central Nervous System 389 FIGURE 11. The remaining descending tracts are extrapyramidal tracts that originate in the brain stem region. Electrical stimulation of the cerebral cortex, the cerebellum, and the basal nuclei indi- rectly evokes movements because of synaptic connections within extrapyramidal tracts. Neurostimulation of the reticular formation by the cerebrum or cerebellum either facilitates or inhibits the ac- tivity of lower motor neurons (depending on the area stimu- lated) (fig. The cerebellum can influence motor activity only indirectly, through the vestibular nuclei, red nucleus, and basal nuclei. These struc- tures, in turn, affect lower motor neurons via the vestibulospinal tracts, rubrospinal tracts, and reticulospinal tracts. Damage to the cerebellum disrupts the coordination of movements with spa- tial judgment. Underreaching or overreaching for an object may occur, followed by intention tremor, in which the limb moves back and forth in a pendulum-like motion. The basal nuclei, acting through synapses in the reticular formation in particular, appear normally to exert an inhibitory FIGURE 11. People with shown in red and the extrapyramidal tracts are shown in black. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Developmental Exposition lateral walls thicken to form a groove called the sulcus limitans The Spinal Cord along each lateral wall of the central canal.
Since the molecular cut-off of the dialysis membrane is in the region of 6±20 kDa (depending on the type of membrane used) discount naltrexone 50mg mastercard symptoms 0f gallbladder problems, this technique can also be used to measure release of some small neuropeptides (e cheap 50 mg naltrexone with amex medications not to take after gastric bypass. One advantage of microdialysis is that it enables the study of transmitter release in specific brain areas or nuclei. To ensure its correct placement, the probe is implanted, under anaesthesia, by sterotaxic surgery. Another advantage is that the probe can be anchored in place with dental cement and experiments carried out later, in conscious freely moving animals once they have recovered from the anaesthetic. Indeed, comparison of results from studies carried out on both anaesthetised and freely moving subjects has revealed drug interactions with anaesthetics that can affect transmitter release: anaesthetic-induced changes in the regulation of noradrenaline release by a2-adrenoceptors is a case in point. It is also possible to carry out long-term 88 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION Figure 4. The length of membrane below the probe support can be altered (1±10 mm)to suit the size of the animal and the brain area being studied. Flow rates are normally below 2 ml/min or repeated studies on the same animals but this requires a slight modification of the technique. Unfortunately, for a variety of reasons, each microdialysis probe can be used for only a few hours and so it has to be replaced each day. However, the presence of the guide cannula makes this a relatively straightforward process that requires only light sedation of the animal. A further advantage of microdialysis is that, unlike the push±pull cannula or the cortical cup, the perfusion medium does not come into direct contact with the tissue being studied. This reduces damage caused by turbulence as well as enzymic degradation of the transmitter. For instance, acetylcholine, but not cholinesterase, will penetrate the probe membrane. Finally, because solutes will pass out of the probe, as well as into it, the probe can also be used for infusing ions (Fig. This avoids many of the problems that arise when trying to determine the synaptic actions of drugs when these are administered systemically. The rate at which the probes are perfused with aCSF is a compromise between the time required for the solutes in the CSF to reach equilibrium with those in the probe (the slower, the better)versus the ideal time-frame for studying changes in transmitter release (the shorter, the better). In general, flow rates of around 1±2 ml/min are used and the time which elapses between taking samples is determined by how much transmitter NEUROTRANSMITTER RELEASE 89 Figure 4. The graph shows efflux of noradrenaline in the frontal cortex of anaesthetised rats. Increasing the concentration of K in the medium infused via the probe increases noradrenaline efflux whereas removing Ca2 reduces it is needed for analysis: i. It is acknowledged that the solutes are not in equilibrium with the CSF outside the probe. In any case, the efficiency of the probe membrane limits the net influx (or efflux)of solutes to about 10± 20% of the theoretical maximum. It should also be borne in mind that the microdialysis probe is not sampling the transmitter in the synapse: only that transmitter which escapes metabolism in, or reuptake from, synapses will migrate towards the probe. In the drug-free state, any change in the transmitter concentration in the dialysis samples is usually assumed to indicate a change in its rate of release from nerve terminals; this is supported by the spontaneous efflux of transmitters being Ca2-dependent and K- sensitive (Fig. However, efflux does not always reflect release rate, especially if experimental interventions (e. Voltammetry This can be carried out in vitro (in brain slices, cultured cell preparations)or in vivo and involves penetrating the experimental tissue with a carbon-fibre electrode of 5±30 mmin diameter (Fig. This serves as an oxidising electrode and the Faradaic current generated by the oxidation of solutes on the surface of the electrode is proportional to their concentration. Obviously, only neurotransmitters which can be oxidised can be measured in this way so the technique is mainly limited to the study of monoamines and their metabolites. The amplitude of each peak on the ensuing voltammogram is a measure of solute concentration and individual peaks can be identified because different 90 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION Figure 4. Changes in the concentration of transmitters are monitored by rapid cycles of voltage scans (e. Since a complete scan takes only about 20 ms, the time resolution with voltammetry is much better than with microdialysis and is suitable for studying rapid, transient changes in transmitter release.
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