Down-regulation of muscarinic receptors in the striatum of organophosphate-treated swine

Subacute (daily) administration of diisopropylfluorophosphate (DFP) to male swine (Yorkshire white) resulted in a 97% inhibition of cholinesterase and a decrease of [³H]quinuclidinyl benzilate ([³H]QNB) binding sites in homogenates of striata by ∼ 50% after 14 days. The maximal density of receptors (B_max) decreased from 2.1 ± 0.3 to 1.0 ± 0.2 pmole/mg protein. There was no significant change in the dissociation constant (K_d) for [³H]QNB binding (control: 52.6 ± 10.7 pm; 7-day: 57 ± 2.8 pm). Carbachol displacement of [³H]QNB binding yielded data best fit by a two-binding site model. The dissociation constants were K_iL = 115 ± 62 μm (55 ± 3%) and K_iH = 1.8 ± 0.7 μm (45 ± 3%), respectively, for the low- and high-affinity states. Seven-Day treatment with DFP reduced the percentage of high-affinity receptors to 22 ± 8.6%, but affected neither the low- nor the high-affinity K_d (100 ± 20 and 2 ± 0.6 μm). With the addition of Mg²⁺, striatal homogenates had low- and high-affinity receptors in the proportion of approximately 1 to 1. In the presence of Gpp(NH)p + Mg²⁺ the ratio of high- to low-affinity receptors was 3:1 in homogenates of control tissue (to 26 ± 5%). This treatment had no effect on this ratio in homogenates of tissue from 7-day DFP-treated swine (3:1) since it was already 3:1. Pirenzepine displacement of [³H]QNB binding was best described by a two-binding site model, with K_i values of 38 ± 14 and 201 ± 78 nm, which represent 74 and 26% of the binding sites, respectively. The high affinity K_d value was unchanged following 7 days of DFP treatment (24 ± 5 nm). There appears to be little change in the displacement curves for pirenzepine inhibition of [³H]QNB binding. This suggests that about 75% of the receptors are of the M₁ subtype. Thus, subacute administration of DFP causes not only a decrease in the number of receptors, but also a change in the proportion of agonist affinity states which is related to the interaction of the guanine nucleotide binding protein and the muscarinic receptor.