Cholera toxin-induced Gsα down-regulation in neural tissue: studies on the pineal gland

Cholera toxin (CT) treatment (50 μg/ ml) was used to down regulate the α subunit of the stimulatory guanine nucleotide binding protein (Gsα) in pineal glands in organ culture, as has been seen in non-neural tissue. A 15 h treatment reduces G_sα by ≈ 75% as measured using semi-quantitative Western blot technology. In contrast, this treatment does not alter the abundance of Gβ, G_iα or G_oα. This effect on G_sα was still apparent following a 36-h washout period. The 48-h CT treatment increased cyclic AMP accumulation 10- to 17-fold but blocked the norepinephrine (NE)-induced increase in cyclic AMP accumulation, presumably reflecting the loss of G_sα. This treatment did not, however, inhibit protein synthesis or stimulation of arylalkylamine N-acetyltransferase (NAT) activity produced by treatment with either DB-cyclic AMP (N⁶,2′-O-dibutyryl adenosine 3′,5′monophosphate) or 8 Br-cyclic AMP, stable cyclic AMP derivatives. This indicates that a 48-h CT treatment was not generally toxic. In contrast, this treatment blocked subsequent CT stimulation of NAT. The effects of CT treatment on the adrenergic stimulation of NAT was examined using treatments which selectively produced α- or β-adrenergic stimulation. α₁-Adrenergic activation of the pineal gland elevates Ca²⁺]_i, which potentiates effects of cyclic AMP; in these studies the response to α-adrenergic activation was markedly increased in 48-h CT-treated glands, reflecting Ca²⁺ potentiation of the effects of elevated levels of cyclic AMP. In contrast, the effects of the selective β-adrenergic agonist isoproterenol was reduced by ≈ 75%. These studies not only establish CT-induced G_sα down-regulation as a new tool for the study of adrenergic signal transduction in the pineal gland, but indicate that this paradigm is probably useful in all neural tissue.