en Experimental research article Experimental research article <span style="FONT-SIZE: 12pt FONT-FAMILY: "Palatino Linotype" mso-fareast-font-family: 'Times New Roman' mso-bidi-font-family: 'Times New Roman' mso-ansi-language: EN-US mso-fareast-language: EN-US mso-bidi-language: AR-SA">The effects of chronic morphine administration on the development of long-term potentiation (LTP) were investigated at the Schaffer collateral-CA1 pyramidal cell synapses of the rat hippocampal slices using primed-bursts tetanic stimulation. Significant enhancement of orthodromic population spike (OPS) was found for all stimulus intensities after tetanic stimulation. OPS enhancement was greatest when tested with low to mid-range stimulus intensities (25, 50 and 100 </span><span style="FONT-SIZE: 12pt FONT-FAMILY: "Palatino Linotype" mso-fareast-font-family: 'Times New Roman' mso-bidi-font-family: 'Levenim MT' mso-ansi-language: EN-US mso-fareast-language: EN-US mso-bidi-language: AR-SA">µ</span><span style="FONT-SIZE: 12pt FONT-FAMILY: "Palatino Linotype" mso-fareast-font-family: 'Times New Roman' mso-bidi-font-family: 'Times New Roman' mso-ansi-language: EN-US mso-fareast-language: EN-US mso-bidi-language: AR-SA">A<i>). </i>There was also significant decrease in OPS delay. These responses were similar in slices from both control and morphine dependent rats. At all delivered stimulus intensities, the amount of LTP of OPS in slices from dependent rats was larger than that of control slices. However, these differences in LTP of OPS were significant at low stimulus intensities. These findings suggest that chronic morphine administration had induced changes in CA1 neurocircuitry which modulated synaptic plasticity during high frequency stimulation and appeared as augmented LTP and also inhibition of LTP decay. </span> Hippocampus; Chronic morphine; Long-term potentiation 45 53 http://ppj.phypha.ir/browse.php?a_code=A-10-107-1&slc_lang=en&sid=1 Farshad Alizadeh Mansouri 320031947532846001660 320031947532846001660 No Fereshteh Motamedi 320031947532846001661 320031947532846001661 Yes Fereshteh Fathollahi 320031947532846001662 320031947532846001662 No Nafiseh Atapour 320031947532846001663 320031947532846001663 No Saeed Semnanian 320031947532846001664 320031947532846001664 No