Motor effects of two sigma ligands mediated by nigrostriatal dopamine neurons

Regional distribution and kinetics of haloperidol binding in human brain: a PET study with [18F]haloperidol

The regional distribution and the kinetics of haloperidol uptake in human brain were examined using [18F]haloperidol and PET in 9 controls and 5 schizophrenics while on haloperidol medication and after haloperidol washout. The regional distribution of [18F]N-methylspiroperidol, a tracer for D2 receptors, was measured in 1 normal subject for comparison. The uptake of [18F]haloperidol in the whole brain in normals was high (6.6% of the injected dose at 2 hr), and regional distribution was much more extensive than could be accounted for by the distribution of dopamine D2 receptors. In normals, the cerebellum, basal ganglia, and thalamus showed a greater concentration than the cortex, and there was minimal clearance of 18F from the brain during the 10-hr period of the study. Medicated schizophrenics showed a total brain uptake of 4.0% and had a significant clearance of [18F]haloperidol from brain and a higher concentration of [18F]haloperidol in plasma. After withdrawal from medication, [18F]haloperidol clearance from brain became slower than while on medication. These results are discussed in terms of the pharmacokinetics of haloperidol in the human brain and its binding to dopamine D2 receptors and to sigma receptors.

Sigma receptors. Putative links between nervous, endocrine and immune systems

The sigma receptor is a neuronal substrate that binds several psychoactive compounds. These include cocaine, some steroids, dextromethorphan, phencyclidine (PCP), and benzomorphans such as pentazocine and N-allyl-normatezocine (SKF-10047). Many newer atypical antipsychotic drugs also bind to the sigma receptor. The sigma receptor, however, is not the PCP receptor. The sigma receptor exists in the central nervous system, endocrine, immune and certain peripheral tissues. Progesterone and certain steroids have been shown to represent endogenous ligands for the sigma receptor. The sigma receptor resides likely in the nonsynaptic region of the plasma membrane. The sigma receptor exists in two forms: high-affinity and low-affinity. The solubilized sigma receptor retains all of the pharmacological characteristics of a membrane-bound receptor. A major physiological role of the sigma receptor may involve the modulation of a tonic potassium channel.

A sigma-like binding site in rat pheochromocytoma (PC12) cells: decreased affinity for (+)-benzomorphans and lower molecular weight suggest a different sigma receptor form from that of guinea pig brain

Two highly selective radiolabeled probes for sigma receptors were found to bind with high affinity and capacity to membranes from undifferentiated PC12 cells. [3H]1,3-di-o-tolylguanidine [( 3H]DTG) bound with Kd = 23.7 +/- 4.6 nM and Bmax = 2025 +/- 660 fmol/mg protein. The Kd and Bmax for [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)-3-PPP) were 86.3 +/- 21.6 nM and 1539 +/- 242 fmol/mg protein, respectively. These binding parameters were comparable to those observed in guinea pig brain, although the Kd for [3H](+)-3-PPP was 3-fold higher in the PC12 membranes. Both the PC12 and guinea pig brain sites exhibited high affinity for haloperidol, moderate affinity for phencyclidine (PCP), and negligible affinity for MK-801, apomorphine, and (-)-sulpiride. These data suggest a relationship of the PC12 site to sigma receptors. However, all (+)-opiates [+)-benzomorphans and (+)-morphinans) tested bound with markedly lower affinity to the PC12 site compared to guinea pig brain. These include (+)-N-allylnormetazocine [+)-SKF 10,047), (+)-pentazocine, and dextrallorphan. In fact, PC12 sites exhibited preference for (-)-benzomorphans, the reverse stereoselectivity of guinea pig brain sites. Binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) could not be detected, demonstrating absence of PCP receptors on this cell line. Differentiation of cells by treatment with nerve growth factor had no effect on sigma binding parameters. Membranes from guinea pig brain and PC12 cells were photoaffinity-labeled using [3H]azido-di-o-tolylguanidine. In guinea pig brain, a polypeptide of 25 kDa was specifically labeled. However, label was incorporated into polypeptides of 18 kDa and 21 kDa in membranes from PC12 cells. In view of the otherwise similar binding characteristics, the marked differences in affinity for (+)-benzomorphans and molecular weight suggest that PC12 cells contain a molecular form of sigma receptor distinct from that predominant in guinea pig brain. This raises the possibility of multiple sigma receptor types.

Drug specificity of pharmacological dystonia

Three (+)-benzomorphans that bind to sigma receptors produced dystonia in a dose-related manner when microinjected into the red nucleus of rats. Two lines of evidence suggest that these effects were related to the sigma-binding properties of the compounds. First, the behavioral potency of the (+)-benzomorphans and other active sigma compounds correlated highly with their affinities for [3H]1,3-di-o-tolylguanidine-labelled sigma receptors in the rat brain (r = .94). Second, similar intrarubral injections of non-sigma ligands were without effect: various vehicles, a structurally related (+)-opiate with no affinity for sigma receptors, and selective dopaminergic and serotonergic compounds failed to significantly alter the normal posture of rats. The only ligand in this study that binds with high affinity to sigma receptors, but failed to elicit torsional head movements was (+)-[3-(3-hydroxyphenyl)-N-(1-propyl)piperidine] [(+)-3PPP], a ligand with mixed activity at sigma and dopamine receptors. Since (+)-3PPP failed to produce an effect on its own and also failed to attenuate the dystonia produced by another sigma ligand (DTG), it may interact with a non-sigma mechanism or with a different sigma receptor type from the other compounds.

Metabolites of haloperidol display preferential activity at sigma receptors compared to dopamine D-2 receptors

Haloperidol bound with equal affinity to sigma and dopamine D-2 receptors (KI = 2.8 nM). Compared to haloperidol, its carbonyl-reduced metabolite bound to sigma receptors with nearly equal affinity. However, reduced haloperidol bound to dopamine receptors with 85-fold lower affinity compared to haloperidol (KI = 239 nM). The chlorophenyl-hydroxy-piperidine metabolite of haloperidol lacked affinity for dopamine receptors, but bound with moderate affinity to sigma receptors (KI = 326 nM). The carboxylic acid metabolite lacked affinity for either receptor. Like haloperidol, (+)-pentazocine, and 1,3-di-o-tolylguanidine, reduced haloperidol potently inhibited the phosphoinositide response to muscarinic agonists in rat brain synaptoneurosomes, an assay which monitors sigma agonist activity. This metabolite also produced a dystonic alteration of head position in rats when microinjected into the red nucleus. However, unlike observations with haloperidol and other sigma ligands, this effect was associated with pathological changes in the red nucleus. Therefore, it cannot be attributed to sigma receptor interactions with certainty. These findings suggest that administration of haloperidol results initially in effects mediated through both dopamine and sigma receptors, but as metabolism proceeds the sigma actions would be expected to decline at a significantly slower rate than the dopaminergic actions.

Age-related differences in the sensitivity of rats to a selective sigma ligand

Differences in sigma binding parameters and behavioral responses to injections of a selective sigma ligand were found in adult rats of different ages. Middle-aged rats (5-6 months old) had fewer sigma binding sites and sites with lower affinity for [3H]di-o-tolylguanidine than young adult animals (2-3 months old). The older animals also exhibited a decreased behavioral response to the selective sigma ligand, di-o-tolylguanidine (DTG). Unilateral microinjection of DTG into the substantia nigra of rats produced fewer net contralateral turns in middle-aged animals, compared to younger rats. Likewise, the postural changes produced by unilateral microinjection of DTG into the red nucleus were less pronounced in the older animals. These data suggest that changes in the number and affinity of sigma binding sites may affect movement and posture as an organism ages.