These all stem from a PIE syllabic nasal privative *n̥-, the zero ablaut grade of the negation *ne, i.e. "n" used as a vowel, as in some English pronunciations of "button". This is the source of the 'n' in 'an-' privative prefixed nouns deriving from the Greek, which had both. For this reason, it appears as an- before vowel, e.g. anorexia, anesthesia.
The same prefix appears in Sanskrit, also as a-, an-. In North Germanic languages, the -n- has disappeared and Old Norse has ú- (e.g. ú-dáins-akr), Danish and Norwegian have u-, whereas Swedish uses o, and Icelandic uses the etymologically related ó.