Abuse has to do with it as well. Fast acting Barbiturates are C-II or C-III, which is a big step up from C-IV. Yeah, Butalbital is C-III, but it doesn't behave the same way, has a slightly longer half - life (only slightly longer, probably the reason for the C-III regulation (which mean fuck all for living in Canada - ya, I remember you, from TC, how you been (I'm Doug
). Barbiturates also inhibit a glutamate receptor (AMPA, but that doesn't matter) which illicit a Sodium ion influx which is excitatory for the cell as opposed to inhibitory, as well as blocks Calcium ion influxes at high doses, which also has an excitatory effect. Barbiturates also antagonize NMDA receptors and have anti-cholinergic effects - Nicotinic or Musacarinic, I'm not sure - I'd have to do more research - leading to increased intoxication; thus, leading to a higher potential for abuse.
As I stated in my last post, Barbiturates increase the maximal effect of GABA, while Benzodiazepines increase its potency (more influxes of Chloride as opposed to LARGER influxes of Chloride ions, so a much larger effect happening every time the channel opens.
And while Benzodiazepines don't have much recreational potential to the healthy individual, to someone with severe anxiety issues/PTSD, they have very high abuse potential (from a pseudo - addict's perspective - one who abuses drugs to a clinical point). They're also HIGHLY dependence forming (I believe all Benzodiazepines and Barbiturates, as well as Carisoprodol should be classified as C-II in the US for dependence reasons) - much more so than opioid drugs do (in my opinion having detoxed from all of these on their own as well as simultaneously).
Edit: Barbiturates also have greater analgesic effects than Benzodiazepines.