1. The intracellular reaction mechanism underlying electrophysiological responses evoked by neurotensin (NT) was studied using Xenopus laevis oocytes injected with poly (A)+ messenger ribonucleic acid (mRNA) isolated from rat brains. 2. A few days after the injection of mRNA, oocytes were found to acquire sensitivity to NT and substance P. 3. Under voltage‐clamp conditions (‐60 mV), application of NT to mRNA‐injected oocytes produced transient and oscillatory inward currents which began after a delay of several tens of seconds. These inward currents were accompanied by an increase in membrane conductance. 4. NT receptors on mRNA‐injected oocytes showed essentially the same pharmacological properties as those of native NT receptors. 5. The NT response showed desensitization and was not readily recovered even after extensive washing of cells for more than 30 min. 6. NT response was suppressed when the muscarinic acetylcholine (ACh) response of the same cell, which was also induced by the same mRNA, was desensitized by a large dose of ACh. 7. NT response and ACh response showed many similarities: they were both inhibited by pertussis toxin and intracellular ethyleneglycol‐bis‐(beta‐aminoethylether) N, N'‐tetraacetic acid (EGTA), mimicked by intracellularly injected inositol 1, 4, 5‐trisphosphate (InsP3), and suppressed when cell response to InsP3 was desensitized by a large dose of InsP3. Reversal‐potential analyses indicated that both responses were mediated by an increase in membrane permeability to Cl‐. 8. It is concluded that NT responses and muscarinic ACh responses of Xenopus oocytes induced by rat brain mRNA may most likely share a common reaction mechanism. The reaction sequence includes the activation of receptors, activation of inhibitory guanine nucleotide‐binding regulatory protein, production of InsP3, intracellular Ca2+ mobilization, and increased membrane permeability to Cl‐.
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