Plugin and page called by Jquery

Dear All,
could you help me?

I’m developing my first plugin. I have wordpress 3.2.1 with buddypress theme.
Basically the plugin has two php files. The main file is.php and a second file into of folder /includes/is_result.php

is.php calls /includes/is_result.php by jquery through .POST method. is_result.php gets information from my DB and provide back the information to is.php into a div.

The plugin works properly. My problem is that I need to call some stantard method/function/constant of WordPress but is_result.php does not know any of them.
I got always “undefined function…” bla bla..

Here below my the code:

is.php
—–

<input id=”ricerca” type=”text” onkeyup=”instantsearch(this.value,'<?php echo $pluginpath.”/includes/is_result.php?funzione=1″;?>’ ,’#risultato’)” />

<ul id=”risultato” class=”item-list2″ style=”width:100px;”>

<?php
}
?>

<script type=”text/javascript”>
function instantsearch (value,fun,out)
{
jq.post(fun, {valore:value}, function(data) {jq(out).html(data);});
}
</script>

——-

Please, could you help me?