Allan J. Tobin

Autoimmunity to two forms of glutamate decarboxylase in insulin-dependent diabetes mellitus.

Insulin-dependent diabetes mellitus (IDDM) is thought to result from the autoimmune destruction of the insulin-producing beta cells of the pancreas. Years before IDDM symptoms appear, we can detect autoantibodies to one or both forms of glutamate decarboxylase (GAD65 and GAD67), synthesized from their respective cDNAs in a bacterial expression system. Individual IDDM sera show distinctive profiles of epitope recognition, suggesting different humoral immune responses. Although the level of GAD autoantibodies generally decline after IDDM onset, patients with IDDM-associated neuropathies have high levels of antibodies to GAD, years after the appearance of clinical IDDM. We note a striking sequence similarity between the two GADs and Coxsackievirus, a virus that has been associated with IDDM both in humans and in experimental animals. This similarity suggests that molecular mimicry may play a role in the pathogenesis of IDDM.

The GABA Receptor-Chloride Ion Channel Protein Complex

The majority of inhibitory synaptic transmission in the central nervous system involves γ-aminobutyric acid (GABA) as the neurotransmitter (1). The signal transduction mechanism at the majority of GABA synapses involves a ligand-gated chloride channel; binding of GABA to its receptor increases postsynaptic membrane chloride conductance and inhibits the target cell (2). This GABA receptor, called GABAA, is defined pharmacologically by sensitivity to the agonist muscimol ana the antagonist bicuculline (3). At least one other type of GABA receptor exists, GABAB, defined as insensitive to bicuculline and sensitive to baclofen; GKBAB receptors are coupled to GTP-binding proteins for a variety of signal transduction mechanisms (4). GABAA receptor function is also modulated by at least three classes of centrally active drugs, the picrotoxin-like convulsants, that inhibit GABA function, and the benzodiazepines and the barbiturates, both of which enhance GABA function (5).

Structural, Developmental and Functional Heterogeneity of Rat GABAA Receptors

My laboratory has had a long interest in multigene families, starting with our earlier work on the developmental regulation of hemoglobin switching. I have especially been impressed both by the number of multigene families in the vertebrate genome and by their widespread developmental regulation, as exemplified by switching within the globin gene families. Even the lamprey, whose adult hemoglobin, unlike those of higher vertebrates, contains only one type of globin chain, has different forms in its larval and adult stages.

Expression of mRNAs that encode D2 dopamine receptor subtypes: Anatomical, developmental, and pharmacological studies.

We used oligonucleotide probes to isolate cDNAs that encode two subtypes of D2 dopamine receptors (D2(long) and D2(short)). In situ hybridization histochemistry with cRNA probes derived from these cDNAs revealed relatively high concentrations of D2 receptor mRNA in brain regions associated with the nigrostriatal and mesolimbic dopamine systems. D2 receptor mRNA was also present in several other regions including the oculomotor nucleus and areas of the pons and medulla. Hybridizations with a probe specifically recognizing D2(long) mRNA indicated that the fraction of total D2 receptor mRNA that encodes D2(long) receptors is higher in the nigrostriatal and mesolimbic dopamine systems than in the dorsal tegmental nucleus (of Gudden). Northern blot analysis revealed that developmental changes in D2 receptor mRNA concentrations in whole brain roughly parallel previously documented changes in D2 binding site concentrations. In contrast, Northern blot analysis indicated that D2 mRNA concentrations are unaffected by subchronic administration of the D2 receptor antagonist, haloperidol.

GABAA -Benzodiazepine Receptors: Demonstration of Pharmacological Subtypes in the Brain

The GABAA receptor is a ligand-gated chloride ion channel that mediates the majority of rapid-acting inhibitory synapses in the central nervous system (Olsen and Venter, 1986). The GABAA receptors are also the target of numerous clinically important depressant and excitatory drugs (Olsen, 1981; Tallman and Gallager, 1985; Biggio and Costa, 1988). The convulsant drug bicuculline acts as a competitive antagonist at the GABA recognition site, beta-carbolines block GABA function as ’ inverse agonists’ at the benzodiazepine recognition site, and picrotoxin and cage convulsants inhibit the chloride channel function at a site on the receptor complex distinct from the GABA and benzodiazepine receptor sites. Clinically important depressant benzodiazepines enhance GABA-mediated inhibition via their own binding sites on the receptor complex. Still additional sites on the receptor-ion channel complex mediate the action of barbiturates, steroid anesthetics, and possibly ethanol to enhance GABAA receptor function at the membrane level (Olsen and Venter, 1986; Biggio and Costa, 1988).

Glutamate Decarboxylase, GABA and Autoimmunity

GABA is a widely utilized neurotransmitter throughout the central and peripheral nervous systems (reviewed in 1–4). Its synthesis is largely controlled by the activity of glutamate decarboxylase which converts the amino acid glutamate directly into GABA. Alterations in GAD and GABA have been associated with a number of neurogenetic diseases and pharmacological interference with GAD or GABA rapidly induces seizures in both man and animals. Growing evidence also suggests that GABA acts as a neurotrophic factor during development.