| The medial geniculate body (MGB) is the last obligatory synaptic site for relaying ascending auditory information to the cerebral cortex. The auditory thalamocortical pathway begins in the MG and courses anteriorly through the thalamus in the superior thalamic radiation before it turns laterally to join the internal capsule. Fibers then fan out and traverse the caudate nucleus before reaching auditory cortex, where they turn posteriorly and course in deep layers before branching off to terminate primarily in layer IV. The projections abide by the tonotopic rules. Those functional fibers consists of the most important neurons loop in central auditory pathway.Thalamic axons were capable of conducting action potentials to the cortex and some of them had already formed functional synapses there at embryonic 18 day. This study demonstrated that thalamic input may promote the development and mature of the auditory cortex. Although many anatomy and electrophysiology in vivo study have focused on thalamocortical synaptic transmission, the cellular mechanism how they deal with auditory information and how they affect each other still not clear. Now thalamocortical brain slice preparations contain functional neuclus in thalamus and MGBv and intact input fibers between them, so they have greatly facilitated the study of cellular and synaptic mechanisms in electrophysiology and pharmacology.Therefore, we applied the whole-cell patch clamp techniques to investigate the electrophysiological and morphological properties of the A1 in rats during postnatal 3-30 days and relationship between them.Part â… : with the use of whole-cell patch clamp recording technique, we recorded the electrophysiological properties of membrane including passive and positive membrane properties, the influence of voltage-dependent ion channel, intracellular labeling techniquein rat's Al during postnatal development. The results were as follows:1. From P3d to PI Id, RMP, Ri and Tau of the Al neurons changed dramatically, but no change after P12d. During the period of cell growth from P3d to PI Id the resting membrane potential hyperpolarized from -40mV to -67mV . This, rather gradual hyperpolarization has been attribute to the increasing of Na+/K+-pump activity, and K+-current or the decreasing of the Cl" reversal potential. An increase in ion channel density together with an expanding membrane area may explain the increase in input conductance during maturation.2. From P3d to Plld, the amplitude and duration of action potential of the Al neurons changed obviously..3. During postnatal development , the change of voltage-dependent ion channel dramatically influenced the waveform of action potential in rat's Al neurons . Under current clamp, 4-AP(lO u mol/L) blocking voltage-dependent K+-channel, reduced the amplitude of action potentials evoked by depolarizing pulse injections into P6 neurons, and resulted in an action potential that inactivated progressively reaching a plateau potential at the end of the pulse. This result suggested that voltage-dependent K+-channel has not been matured in rat's Al neurons after birth. TTX(300nmol/L) blocking voltage-dependent Na+-channel, had the similar effect on the MGBv neurons at P6d and P16d reducing or abolishing action potential firing. This indicates that voltage-dependent Na+-channel has been matured in rat' Al neurons after birth. CdCl2(50 U mol/L) blocking voltage-dependent Ca2+-channel, resulted in a train of action potential that inactivated gradually reaching a plateau potential at the end of the pulse at P6d, however, only diminished the action potential at P16d. This shows that voltage-dependent Ca2+-channel generate and maintain the action potential of immatured Al neurons.4. With intracellular labeling technique injecting biocytin into a Al neuron during whole-cell patch clamp recording, we observed the changes of morphological properties in rat's Al neurons during postnatal development. The Nissl' staining results revealed that the cell formed cluster in rat's Al at P3-7d, but laminae at Pll-17d. During postnatal development, cell area and density dramatically changed, for example, cell density decreased significantly, but not after P20d, and cell area increased to matured Al at P30d. Microstructure showed that glial cell increased. The biocytin staining results revealed that immature Al cell had smaller somata and short, simple dendritic aborization, and duringpostnatal development, the mature neurons had oval somata and 3-4 primary dendrities that radiated irregularly and projected outside the section and appeared spines. The results revealed that the morphology of rat' Al neurons had not been matured at birth, and need a period to mature.Part II: Changed the angle in slice, making it contain MGBv> Al and connection fibers between them, stimulated MGBv or projection fibers from thalamus to Al, to study the electrophysiology changes in Al recording neurons and know more about mechanism of central auditory information processing.1. According to published reports (Ryugo and Killackey 1974; Winer 1992), the auditory thalamocortical slices contain primary thalamocortical pathway and non-primary thalamocortical pathway, it is difficult to distinguish them although they have distinct functions. Because the thalamocortical pathway turns at several points, 400-um-thick slices rarely contained a fully intact thalamocortical system, and stimulation of the MG per se evoked cortical responses in only a few cases. However, stimulation of the str reliably elicited cortical responses in about one slice per mouse. Since the str is a known part of the auditory thalamocortical pathway, it seemed likely that responses evoked in layer IV of auditory cortex resulted from activation of thalamocortical fibers. Later experiments slice thickness was increased to 600 um in an attempt to maintain the entire thalamocortical pathway. In these experiments, usually one slice per animal contained a fully connected thalamocortical pathway, such that stimulation of the MG evoked clear, short-latency cortical responses.2. Subcortical stimulation elicits fast and slow PSCs. Recording electrode was placed in layer IItoIV, use subcortical stimulation currents(amppulitute 0.5m A,duration IOOjj. s ) can induce a response which contain fast and slow PSCs. Fast and slow PSCs increased in amplitude with membrane hyperpolarization and decreased with membrane depolarization.Selective antagonists were used to assess the involvement of glutamate receptors in the subcortically evoked fast and slow PSCs. Bath application of the iV-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV, 25-50 uM) slightly reduced the fast currents, but completely and reversibly blocked the slow currents in whole cell recordings. The -amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/ kainite (AMPA/KA) glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione(CNQX, 10-20 uM) initially reduced the fast potential gradually over a 3- to 10-min period, and APV subsequently reduced the residual fast component gradually over a similar time period. Thus, the putative monosynaptic fast potential contained components mediated by both AMPA/KA and NMD A glutamate receptors.
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