WordNet sense 1 (openly distrustful and unwilling to confide):

leery, mistrustful, suspicious, wary

WordNet sense 2 (suspicious or unduly suspicious or fearful of being displaced by a rival):

jealous

WordNet sense 3 (marked by or given to doubt):

skeptical

WordNet sense 4 (believing the worst of human nature and motives; having a sneering disbelief in e.g. selflessness of others):

cynical

WordNet sense 5 (not disposed or willing to believe; unbelieving):

incredulous