| The ventral partition of medial geniculate body (MGBv) has been known as the special nucleus of relaying auditory information. In auditory processing, a primary function of thalamocortical relay neurons in MGBv is to encode sound signals as different sequences of action potential. However, during postnatal development, the membrane properties of the MGBv neurons and how to process the auditory information are not clear. To data, no report has been published as to how the single neuron on the MGBv integrates the sound signals during postnatal development. In order to investigate the electrophysiological and morphological properties of the MGBv in rats during postnatal 3-30 days and relationship between them, we recorded the electrophysiological properties of membrane including passive and positive membrane properties, the influence of voltage-dependent ion channel, GABA receptor on the action potential waveforms and firing patterns in rat's MGBv during postnatal development with the use of patch clamp whole cell recording technique, and we observed the changes of morphological properties in rat's MGBv neurons during postnatal development with intracellular labeling technique injecting biocytin into a MGBv neuron during whole-cell patch clamp recording and Nissl' staining. The results were as follows: 1. From P3 to P11, RMP, Ri and Tau of the MGBv neurons changed dramatically, but no change after P12. During the period of cell growth from P3 to P11 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 P3 to P11, the amplitude and duration of action potential of the MGBv neurons changed dramatically.3. During postnatal development, the change of voltage-dependent ion channel dramatically influenced the waveform of action potential in rat's MGBv neurons. Under current clamp, 4-AP(10μmol/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' MGBv neurons after birth. TTX(300nmol/L) blocking voltage-dependent Na+-channel, had the similar effect on the MGBv neurons at P6 and P16 reducing or abolishing action potential firing. This indicates that voltage-dependent Na+-channel has been matured in rat' MGBv neurons after birth. CdCl2(50μmol/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 PD6, however, only diminished the action potential at P16. This shows that voltage-dependent Ca2+-channel generate and maintain the action potential of immatured MGBv neurons.4. GABAA receptor agonist, muscimol(5μmol/L), and GABAB receptor agonist, baclofen(40μmol/L) had the effect on the responses evoked by depolarizing cerrent injection into the rat' MGBv neurons at P6 and P15. Muscimol(5μmol/L) and baclofen(40μmol/L) reduced the action potential firing, but not the action potential amplitude of the MGBv neurons at P6. Baclofen(40μmol/L) had little effect on the action potential firing produced by positive or negative current injection at P15. Muscimol(5μmol/L) greatly reduced the size of voltage shift produced by positive or negative current injection at P15. The above results suggest that GABA receptor play a excitatory role to rat's MGBv neurons at P6. However, following the development of excitatory amino acid, such as glutamate, the effect of GABA receptor changed to inhibition. 5. The Nissl' staining results revealed that the cell formed cluster in rat's MGBv at P3-7, but l...
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