Differential effects of gamma-hexachlorocyclohexane (lindane) on pharmacologically-induced seizures

Gamma-hexachlorocyclohexane (gamma-HCH), the active ingredient of the insecticide lindane, has been shown to decrease seizure threshold to pentylenetrazol (PTZ) 3 h after exposure to gamma-HCH and conversely increase threshold to PTZ-induced seizures 24 h after exposure to gamma-HCH (Vohland et al. 1981). In this study, the severity of response to other seizure-inducing agents was tested in mice 1 and 24 h after intraperitoneal administration of 80 mg/kg gamma-HCH. One hour after the administration of gamma-HCH, the activity of seizure-inducing agents was increased, regardless of their mechanism, while 24 h after gamma-HCH a differential response was observed. Seizure activity due to PTZ and picrotoxin (PTX) was significantly decreased; however, seizure activity due to 3-mercaptopropionic acid (MPA), bicuculline (BCC), methyl 6,7-dimethoxy-4-ethyl-B-carboline-3-carboxylate (DMCM), or strychnine (STR) was not different from control. In vitro, gamma-HCH, pentylenetetrazol and picrotoxin were shown to inhibit 3H-TBOB binding in mouse whole brain, with IC₅₀ values of 4.6, 404 and 9.4 M, respectively. MPA, BCC, DMCM, and STR showed no inhibition of 3H-TBOB (t-butyl bicyclo-orthobenzoate) binding at concentrations of 100 m. The pharmacological challenge data suggest that tolerance may occur to seizure activity induced by PTZ and PTX 24 h after gamma-HCH, since the response to only these two seizure-inducing agents is decreased. The in vitro data suggest that the site responsible for the decrease in seizure activity 24 h after gamma-HCH may be the GABA-A receptor-linked chloride channel.