Direct determination of dopamine D4 receptors in normal and schizophrenic postmortem brain tissue: a [3H]NGD-94-1 study

Using an indirect subtraction binding technique and human postmortem tissue, several laboratories reported finding increases in dopamine D4 receptors in caudate nuclei of schizophrenic patients, although others have not replicated these findings. NGD-94-1 is a selective D4 antagonist with low affinity for the D2 and D3 receptors. [3H]NGD-94-1 has been used in this study to directly determine the density of D4 receptors in normals (n = 13) and schizophrenic subjects (n = 7) off antipsychotic drugs for at least 3 months prior to death, or on antipsychotic (n = 7) drugs at the time of death. Human postmortem coronal brain sections were incubated with [3H]NGD-94-1 and autoradiograms developed; and binding in pertinent regions was quantified. In normals, the highest density of [3H]NGD-94-1 binding was in the hippocampus (68 fmol mg(-1), temporal (33), insular (30), and entorhinal cortices (24.9). Significant increases in [3H]NGD-94-1 density in schizophrenics (n = 14) vs normals (n = 13) were observed in the entorhinal cortex (46%) at both low and high magnifications. The increases observed in the schizophrenics were found in both schizophrenics off antipsychotic drugs for at least 3 months prior to death and those on antipsychotic drugs at the time of death. Thus, the changes may be disease-related and not a consequence of pharmacological treatment. No significant differences were found between the two schizophrenic groups in any brain area studied.

In vitro autoradiography of GTPgamma[35S] binding at activated NPY receptor subtypes in adult rat brain

Guanylyl 5'-[gamma[35S]thio]-triphosphate (GTPgamma[35S]) binding to NPY receptor-activated G-proteins was measured in adult rat brain sections in order to determine the neuroanatomical distribution of NPY receptor subtypes. Using the pharmacological specificity of the NPY receptor subtypes, differential stimulation of GTPgamma[] binding by subtype-specific agonists was used to demonstrate the differential distribution of these subtypes in rat brain. Treatment of rat brain slices with selective agonists for the NPY receptor subtypes in the presence of 2000 microM GDP was used to discriminate populations of NPY receptor subtypes. Activation of a NPY Y1 receptor subtype by human [Leu31Pro34]NPY stimulated GTPgamma[35S] binding in the rank order: frontal cortex>dentate gyrus>inferior colliculus>/=thalamus>hypothalamus. In contrast, NPY Y2/Y5 peptide agonist, human PYY(3-36), stimulated GTPgamma[35S] binding in the rank order: hypothalamus>substantia nigra>hippocampus>frontal cortex>/=inferior colliculus. Stimulation of NPY Y5 receptor subtypes by a NPY Y5 selective agonist, rat/human D-Trp, was shown to stimulate GTPgamma[35S] binding in the hypothalamus and discrete nuclei of the thalamus. Little GTPgamma[35S] binding in the dentate gyrus, frontal cortex, or inferior colliculus was measured following stimulation with D-Trp. Stimulation of GTPgamma[35S] binding by [Leu31Pro34]NPY, but not by the other NPY receptor agonists, was blocked by the selective NPY Y1 receptor antagonist, BIBP 3226. In conclusion, functional coupling at NPY receptor subtypes can be shown in rat brain and populations of NPY receptor subtypes can be anatomically discriminated by NPY agonist stimulation of GTPgamma[35S] binding in rat brain.

Autoradiographic localization of CRF1 and CRF2 binding sites in adult rat brain

The regional distribution of corticotropin-releasing factor1 (CRF1) and CRF2 binding sites was assessed autoradiographically in adult rat brain. The differential pharmacological profiles of the CRF1 and CRF2 receptor subtypes were used for the discrimination of the CRF1 and CRF2 receptor subtypes in rat brain. Pharmacological characterization at the human CRF1 receptor subtype, expressed in baculovirus-infected Sf9 cells, showed high affinity binding (Ki < or = 10.0 nM) for the peptide agonists sauvagine, urotensin I, rat/human CRF, and ovine CRF. Pharmacological characterization at the rat CRF2 receptor subtype expressed in CHO cells showed a rank order affinity for the peptide agonists such that sauvagine, urotensin I and rat/human CRF showed high affinity binding whereas ovine CRF had a Ki value of 300 nM. Based on this differential binding affinity for ovine CRF, [125I]sauvagine binding in the presence of increasing concentrations of ovine CRF was used to discriminate CRF1 from CRF2 receptor subtypes in rat brain. The CRF1 receptor subtype was found to be localized to various regions of the cerebellum, as well as to several cortical areas. The CRF2 receptor subtype was shown to be localized to the lateral septal nucleus, entorhinal cortex, and to amygdaloid and hypothalamic regions. The present autoradiographic findings provide evidence that each subtype has a distinct regional distribution, thus strengthening the suggestion that CRF1 and CRF2 receptors serve different roles in mediating CRF function. Such data suggest that the development of CRF receptor subtype selective antagonists should help to delineate the role of CRF1 and CRF2 receptor subtypes in central nervous system function.

I. NGD 94-1: identification of a novel, high-affinity antagonist at the human dopamine D4 receptor

NGD 94-1 was evaluated for selectivity and in vitro functional activity at the recombinant human D4.2 receptor stably expressed in Chinese hamster ovary cells. NGD 94-1 showed high affinity for the cloned human D4.2 receptor (Ki = 3.6 +/- 0.6 nM) and had greater than 600-fold selectivity for the D4.2 receptor subtype compared with a wide variety of monoamine or other neurotransmitter receptor or modulatory sites except for 5-HT1A and 5-HT3 receptors, in which NGD 94-1 was approximately 50- and 200-fold selective, respectively, for the D4.2 receptor. In measures of in vitro functional activity, NGD 94-1 showed an antagonist profile at the cloned human D4.2 receptor subtype. NGD 94-1 completely reversed the decrease in forskolin-stimulated cAMP levels produced by the dopamine receptor full agonist quinpirole. Furthermore, NGD 94-1 produced a complete reversal of GTPgamma35S binding induced by quinpirole, but was unable on its own to affect GTPgamma35S binding. These data suggest that NGD 94-1 functions as an antagonist rather than a full or partial agonist at the human D4.2 receptor. In addition, NGD 94-1 binding affinity at the D4.2 receptor subtype was unaffected by G-protein activation by GTP, consistent with the binding affinity seen for other antagonists at the D4 receptor. The binding of tritiated NGD 94-1 was saturable and of high affinity at cloned human D4.2 receptors. Furthermore, the binding of [3H]NGD 94-1 to cloned human D4.2 receptors expressed in Chinese hamster ovary cells displayed a pharmacological profile similar to that observed with the nonselective dopamine receptor ligand [3H]YM 09151-2. Saturation and pharmacological analyses of [3H]NGD 94-1 binding at cloned human D4.2, D4.4 and D4.7 receptor variants showed no difference between the three variants. NGD 94-1 is a novel, high-affinity, D4 receptor-selective antagonist. The clinical use of this subtype-specific compound should permit direct evaluation of the role of D4 receptors in psychiatric disorders.

Allosteric uncoupling after chronic benzodiazepine exposure of recombinant gamma-aminobutyric acid(A) receptors expressed in Sf9 cells: ligand efficacy and subtype selectivity

By using the baculovirus expression system, we report decreases in allosteric coupling at individual gamma-aminobutyric acid (GABA)(A) receptor subtypes (alpha-1, beta-2 and gamma-2, alpha-2, beta-3 and gamma-2 and alpha-5, beta-3 and gamma-2) after chronic benzodiazepine exposure that replicate coupling changes measured in rat cortical membranes after in vivo benzodiazepine exposure. The appearance of uncoupling was time-dependent and the magnitude of uncoupling at expressed GABA(A) receptor subtypes after chronic exposure was dependent upon the efficacy of the ligand in a subtype-specific manner. In addition, the expression of uncoupling was not accompanied by changes in benzodiazepine receptor number or affinity at any expressed GABA(A) subtype examined. The specificity of the coupling change was further shown by the ability of a brief exposure to the benzodiazepine receptor antagonist, Ro15-1788, to reverse the uncoupling induced by chronic benzodiazepine exposure. These findings suggest that alterations at the GABA(A) receptor complex after chronic benzodiazepine exposure are mediated directly by agonist effects at the GABA(A) receptor complex and are not the product of the changes in the surrounding neuronal environment. Furthermore, the present study shows that drug efficacy, and not simply affinity, plays a critical role in determining the degree of uncoupling, and perhaps, in the development of tolerance and dependence.

Effect of subunit composition on GABAA receptor complex characteristics in a baculovirus expression system

A baculovirus expression system was used to produce functional human recombinant GABAA receptors in Sf-9 insect cells in order to study the biochemistry, pharmacology and functional characteristics of this receptor complex. We have identified and characterized various factors which influence the level of receptor expression in multiple virus infections. We have shown that the level of expression of the GABAA receptor complex varies with the levels of expression of the individual subunits. We have also shown that the assembly process has a defined timecourse, and it is dependent upon the ratio of the number of infectious virus particles (MOI ratio) of each subunit in multi-virus infections. In multiple infections, the capacity for expression of the infected cell is shared proportionally by entering virus particles and, there is a direct correlation between the amounts of subunit mRNA and levels of subunit protein expression, and the amount of ligand binding to expressed protein. Finally, reinfection of previously infected cells does not result in subsequent protein expression. Knowledge of these various factors allows us to construct recombinant GABAA receptor complexes with reproducibility and flexibility with regard to subunit composition. By co-expression of alpha, beta, and gamma subunits, both the recognition site for GABA and the allosteric (benzodiazepine) modulatory site are formed and appear to reproduce the pharmacology of endogenously expressed receptors as measured in mammalian CNS. Only a single receptor is produced irrespective of the expression levels of the subunits, showing that GABAA receptor assembly is highly regulated.

Effect of amygdala kindling on the in vivo release of GABA and 5-HT in the dorsal raphe nucleus in freely moving rats

Our laboratory has previously reported a significant subsensitivity to iontophoretically applied GABA (gamma-aminobutyric acid) in dorsal raphe neurons of amygdala-kindled rats. This subsensitivity was selective for GABA and persisted at least 3 months after the last kindled seizure. In the present series of experiments, we explored mechanisms by which kindling could result in persistent GABA sensitivity changes, using in vivo microdialysis to quantitate neurotransmitter [including GABA and 5-hydroxytryptamine (5-HT)] release in the dorsal raphe nucleus of awake, unrestrained amygdala-kindled rats. Depolarization-induced release of GABA is markedly increased in the dorsal raphe nucleus in amygdala-kindled animals. This change in depolarization-induced GABA release appeared to be graded, dependent upon the stage to which the animal is kindled. Thus GABA release is increased in animals kindled to Stage 2 and even greater in animals kindled to Stage 5 seizures. The change in GABA release is also selective, since no consistent change in the release of other putative amino acid neurotransmitters or 5-HT was observed in these same animals. We hypothesize that this increase in depolarization-induced release of GABA in the amygdala-kindled animal underlies the development of subsensitivity to GABA in dorsal raphe neurons.

Developmental profile of polyadenylated and non-polyadenylated GABAA receptor subunit mRNAs

The ratio of mRNA not selected for polyadenylation (non-poly(A)+ selected) to mRNA selected for polyadenylation (poly(A)+) for the beta 1, alpha 1 and gamma 2 subunits of the GABAA receptor complex was examined in rats as a function of age. RNA was extracted from whole brain of rats that were either 0, 1, 3, 5 or over 60 days of postnatal age. Poly(A)+ mRNA was purified by oligo(dT)-cellulose chromatography. Non-poly(A)+ selected mRNA and poly(A)+ mRNA for the GABAA receptor beta 1, alpha 1 and gamma 2 subunits were examined by Northern blot analysis using cDNA probes specific for these subunits. Levels of GABAA receptor beta 1 subunit mRNA were also examined by solution hybridization analysis with a beta 1 riboprobe. Analysis of Northern blots revealed that levels of poly(A)+ beta 1 subunit mRNA were highest at 0 days of age, but decreased and reached adult levels by 5 days of postnatal age. However, levels of the beta 1 subunit message extracted from non-poly(A)+ selected mRNA were not significantly different at any of the ages examined, suggesting the existence of a population of beta 1 subunit mRNA that is not polyadenylated. The age-related discrepancy between beta 1 subunit levels measured in non-poly(A)+ selected mRNA and poly(A)+ mRNA was also observed using solution hybridization analysis. In contrast, levels of both non-poly(A)+ selected mRNA and poly(A)+ mRNA for the alpha 1 subunit of the GABAA complex increased from 0 days of age to adulthood. Similarly, levels of both non-poly(A)+ selected mRNA and poly(A)+ mRNA for the GABAA receptor gamma 2 subunit increased with age.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of long-term subsensitivity to GABA in dorsal raphe neurons of amygdala-kindled rats

Previously we reported a long-term change in neuronal sensitivity to GABA following amygdala kindling. Dorsal raphe neurons of amygdala-kindled rats exhibited significant subsensitivity to GABA 4 weeks after the last fully generalized (Stage 5) seizure. We hypothesized that this alteration in GABA sensitivity might reflect neuronal changes corresponding to kindled seizure susceptibility and subsequent experiments have investigated this hypothesis. The progression towards neuronal subsensitivity to GABA during amygdala kindling can be correlated with the Stage to which an animal has been kindled. That is, when measured 4 weeks after the last kindled seizure, dorsal raphe neurons are supersensitive to GABA following a Stage 2 seizure, not different from controls following a Stage 3 seizure and subsensitive to GABA following a Stage 5 seizure. In addition, subsensitivity to GABA appears to be permanent in that it is still measurable 3 months after the last Stage 5 seizure. Thus, amygdala kindling produces long-term, perhaps permanent, changes in neuronal sensitivity to GABA and these changes reflect the Stage to which an animal has been kindled.

GABAA receptor subunit mRNA levels are differentially influenced by chronic FG 7142 and diazepam exposure

Levels of mRNA for the alpha 1, gamma 2 and beta 1 subunits of the GABAA receptor complex were examined in rats maintained on a chronic, continuous schedule of exposure to the benzodiazepine inverse agonist FG 7142. The effect of chronic exposure to the benzodiazepine agonist diazepam was also examined on levels of gamma 2 subunit mRNA. FG 7142 (2 mg/ml of 100% dimethyl sulfoxide (DMSO) or vehicle (100% DMSO) was administered continuously for 8 days in the right ventricle via an osmotic minipump. At the end of the eighth day of exposure, the brain was removed and cerebral cortex, cerebellum and hippocampus were dissected and mRNA prepared from each region. Levels of GABAA alpha 1 and gamma 2 subunit mRNA were examined by Northern blot analysis with cDNA probes specific for these subunits. A significant increase in alpha 1 mRNA was measured in both cortex and hippocampus, but not in cerebellum, of rats chronically exposed to FG 7142 relative to vehicle-treated rats. A significant increase in gamma 2 subunit mRNA in cortex was also evident in drug-treated rats; however, no change in gamma 2 subunit mRNA was observed in either the hippocampus or cerebellum. Examination of GABAA beta 1 subunit mRNA by solution hybridization using a beta 1 riboprobe revealed no effect of chronic FG 7142 treatment on this subunit in either cortex, hippocampus or cerebellum. In rats chronically exposed to diazepam (21 days via silastic implants), levels of gamma 2 subunit mRNA were significantly decreased in cortex, but not changed in either hippocampus or cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of beta-carboline-3-carboxylic acid methylamide by continuous intraventricular infusion increases GABAergic function

The repeated, intraperitoneal administration of the benzodiazepine receptor inverse agonist, FG 7142 (beta-carboline-3-carboxylic acid methylamide), leads to pharmacological kindling and an associated decrease in GABA-stimulated influx of 36Cl- into cortical membrane preparations. The chronic administration of benzodiazepine agonists results in the development of tolerance and also results in a decrease in GABA-stimulated uptake of 36Cl-. The present study was designed to evaluate further the paradoxical reports that both chronic treatment with benzodiazepine receptor agonists and inverse agonists results in a decreased ability of GABA to stimulate uptake of 36Cl- into cortical membrane preparations. The effects of continuous administration of FG 7142 on GABA-stimulated uptake of 36Cl-, the threshold for bicuculline-induced seizures and the proconvulsant actions of acute administration FG 7142 were evaluated. The continuous administration of FG 7142 resulted in an increased capacity of GABA to stimulate the uptake of 36Cl- into cortical membrane preparations and a significant increase in the seizure threshold for bicuculline following the acute administration of FG 7142. These data, therefore, indicate that changes in GABAergic function following chronic administration of GF 7142 are dependent on the regimen of administration of drug. The results also suggest that the GABA receptor homeostatically responds to continuous occupation by inverse agonists by an upregulation of its functional response to GABA.

Distribution of dopaminergic receptors in the primate cerebral cortex: quantitative autoradiographic analysis using [3H]raclopride, [3H]spiperone and [3H]SCH23390

A widespread distribution of dopamine D1 receptors in the neocortex is well recognized. However, the presence of dopamine D2 receptors in this structure has only recently been established [Martres et al. (1985) Eur. J. Pharmac. 118, 211-219; Lidow et al. (1989) Proc. natn. Acad. Sci. U.S.A. 86, 6412-6416]. In the present paper, a highly specific antagonist, [3H]raclopride, was used for autoradiographic determination of the distribution of D2 receptors in 12 cytoarchitectonic areas of the frontal, parietal, and occipital lobes of the rhesus monkey. A low density of D2-specific [3H]raclopride binding (1.5-4.0 fmol/mg tissue) was detected in all layers of all cortical areas studied. Throughout the entire cortex, the highest density of binding was consistently found in layer V. This is a unique distribution not observed so far for any other neurotransmitter receptor subtype in monkey cerebral cortex, including D1 receptor. In addition, a comparison was made of the distribution of [3H]raclopride and [3H]spiperone, which has been commonly used in previous attempts to label cortical D2 receptors. We found marked differences in the distribution of these two radioligands. In the prefrontal cortex, the pattern of [3H]spiperone binding in the presence of ketanserin resembled the combined distribution of 5-HT1C serotoninergic and alpha 2-adrenergic sites as well as D2 receptors. Thus, [3H]raclopride provides a better estimation of the D2 receptor distribution than does [3H]spiperone. The distribution of D2-specific binding of [3H]raclopride was also compared with the D1-specific binding of [3H]SCH23390 in the presence of mianserin to block labeling to 5-HT2 and 5-HT1C sites. The density of D1-specific [3H]SCH23390 binding was 10-20 times higher than that of D2-specific [3H]raclopride binding throughout the cortex. The densities of both [3H]raclopride and [3H]SCH23390 binding sites display a rostral-caudal gradient with the highest concentrations in prefrontal and the lowest concentrations in the occipital cortex. However, the binding sites of these two ligands had different laminar distributions in all areas examined. In contrast to preferential [3H]raclopride binding in layer V, a bilaminar pattern of [3H]SCH23390 labeling was observed in most cytoarchitectonic areas, with the highest concentrations in supragranular layers I, II and IIIa and infragranular layers V and VI. Whereas [3H]raclopride binding was similar in all cytoarchitectonic areas, [3H]SCH23390 exhibited some region-specific variations in the primary visual and motor cortex. The different regional and laminar distributions of D1 and D2 dopaminergic receptors indicates that they may subserve different aspects of dopamine function in the cerebral cortex.

Autoradiographic comparison of D1-specific binding of [3H]SCH39166 and [3H]SCH23390 in the primate cerebral cortex

Quantitative autoradiography was used to compare the binding of the novel dopamine D1 receptor antagonist, [3H]SCH39166, with that of the widely used radioligand, [3H]SCH23390 (in the presence of ritanserin), in the primate cerebral cortex. Specific binding of both radioligands, determined using SCH23390 or cis-flupentixol as displacing agents, had very similar densities and distributions throughout the cortex. However, the specific binding of [3H]SCH39166 obtained with SCH39166 as a blank was significantly higher than that obtained using SCH23390 or cis-flupentixol as displacing agents in some layers of motor, somatosensory and occipital cortices. In addition, the non-specific binding of [3H]SCH39166 obtained in the presence of an excess of SCH23390 of cis-flupentixol displayed a complex laminar pattern very different from that of the specific binding. These observations suggest that [3H]SCH39166 may have a high affinity to more than the D1 receptor subtype bound by SCH23390 or cis-flupentixol. Also, these additional sites are likely to be different from 5-HT2 or 5-HT1C receptors since the latter sites were not displaced by 1 microM SCH23390.

Overlap of dopaminergic, adrenergic, and serotoninergic receptors and complementarity of their subtypes in primate prefrontal cortex

Quantitative in vitro autoradiography was used to determine and compare the areal and laminar distribution of the major dopaminergic, adrenergic, and serotonergic neurotransmitter receptors in 4 cytoarchitectonic regions of the prefrontal cortex (Walker's areas 12, 46, 9, and 25) in adult rhesus monkeys. The selective ligands, 3H-SCH-23390, 3H-raclopride, 3H-prazosin, and 3H-clonidine were used to label the D1 and D2 dopamine receptor subtypes and the alpha 1- and alpha 2-adrenergic receptors, respectively, while 125I-iodopindolol was used to detect beta-adrenergic receptors. The radioligands, 3H-5-hydroxytryptamine and 3H-ketanserin labeled, respectively, the 5-HT1 and 5-HT2 receptors. Densitometry was performed on all cortical layers and sublayers for each of the 7 ligands to allow quantitative as well as qualitative comparison among them in each cytoarchitectonic area. Although each monoamine receptor was distributed in a distinctive laminar-specific pattern that was remarkably similar from area to area, there was considerable overlap among the dopaminergic, adrenergic, and serotoninergic receptors, while subtypes of the same receptor class tended to have complementary laminar profiles and different concentrations. Thus, the D1 dopamine, the alpha 1- and alpha 2-adrenergic, and the 5-HT1 receptors were present in highest relative concentration in superficial layers I, II, and IIIa (the "S" group). In contrast, the beta 1- and beta 2-adrenergic subtypes and the 5-HT2 receptor had their highest concentrations in the intermediate layers, IIIb and IV (the "I" group), while the D2 receptor was distinguished by relatively high concentrations in the deep layer V compared to all other layers (the "D" class). Consequently, clear laminar differences were observed in the D1 vs D2 dopaminergic, the alpha- vs beta-adrenergic, and the 5-HT1 vs 5-HT2 serotoninergic receptor subtypes in all 4 areas examined. The anatomical overlap of different monoaminergic receptors in the same cortical strata suggests that there may be families of receptors linked by localization on common targets, while the complementary laminar distribution of the D1 vs D2, the 5-HT1 vs 5-HT2 and the alpha- vs beta-adrenergic receptors raises the possibility that different subtypes within a given class may have distinctive actions in cortex by virtue of their localization on different cells or possibly different portions of the same cell. Understanding the anatomical arrangement of receptors within the cortical layers may aid in the analysis of monoaminergic modulation of higher cortical function.

Effects of continuous diazepam administration on GABAA subunit mRNA in rat brain

Rats treated chronically with diazepam develop tolerance to diazepam effects and show changes in sensitivity of GABAergic systems. In order to investigate possible molecular mechanisms associated with these changes, we have evaluated the effects of acute and chronic diazepam treatment on levels of mRNA for the alpha 1 and beta 1 subunits of the GABAA receptor. Northern blots were hybridized with 32P-labeled GABA alpha 1 and beta 1 cDNA probes, and resulting bands were quantified by autoradiography and densitometry. Levels of alpha 1 mRNA were significantly decreased in cerebral cortex but not in cerebellum or hippocampus of chronic diazepam-treated rats. Acute diazepam treatment did not change levels of alpha 1 mRNA in any of the brain regions. Levels of beta 1 mRNA were examined by Northern blot analysis and also by solution hybridization analysis using a 32P-labeled riboprobe. Both methods showed that beta 1 mRNA was not significantly changed by chronic diazepam treatment. These results demonstrate a specific change in alpha 1 subunit that is associated with a state of altered GABA sensitivity and provide further support for the regional heterogeneity of chronic diazepam effects.

Long-term changes in sensitivity to GABA in dorsal raphe neurons following amygdala kindling

The present experiments were undertaken to evaluate GABA sensitivity in dorsal raphe neurons following amygdala-kindled seizures. Dorsal raphe neurons of amygdala-kindled rats exhibited significant subsensitivity to GABA as measured electrophysiologically 3 or 4 weeks after the last stage 5 seizure. Amygdala stimulation with currents which did not produce kindled seizures did not produce subsensitivity to GABA. The subsensitivity observed after kindling was equivalent in magnitude to that observed following chronic diazepam treatment. However, exposure of fully kindled rats to chronic diazepam did not further decrease the sensitivity of dorsal raphe neurons to GABA. Additionally, while subsensitivity to GABA was reversed by bath application of the benzodiazepine antagonist, Ro 15-1788, in chronic diazepam-treated rats, it had no effect on GABA subsensitivity in fully kindled rats. These findings suggest a decrease in GABA sensitivity within the dorsal raphe might reflect long-term neuronal changes associated with kindled seizures. These data also suggest that the decrease in GABA sensitivity of dorsal raphe neurons following chronic diazepam may involve different mechanisms from those observed after amygdala kindling.

Relationship of GABAa receptor heterogeneity to regional differences in drug response

Molecular biological approaches to the GABAa receptor have resulted in new insights into the structure and pharmacology of this complex. It is known that the GABAa complex is a hetero-oligomer composed of multiple subunits which contain binding sites for the GABA, benzodiazepines and barbiturates. These subunits also contain regulatory sites for phosphorylation by intracellular kinases. There appear to be regional differences in the expression of the various subunits for the GABAa receptor complex. The functional significance of molecular heterogeneity is not yet known but it is expected that regional differences may result in pharmacologically diverse responses. Studies on the effects of chronic administration of diazepam have clearly delineated such regional differences. Chronic benzodiazepine administration results in the development of subsensitivity to the electrophysiological actions of GABA in the dorsal raphe, but not in GABA receptive neurons of the substantia nigra pars reticulata. Such data is consistent with regional heterogeneity in response to chronic benzodiazepine exposure. It is hoped that by understanding GABAa receptor heterogeneity, and its molecular basis, we can improve the existing receptor subtype specificity and pharmacology of the benzodiazepines.

Regional differences in the distribution of muscarinic cholinergic receptors in the macaque cerebral cortex

The in vitro autoradiographic technique was used to characterize the density and laminar distribution of muscarinic cholinergic receptors in 12 cytoarchitectonic areas in the frontal, parietal, and occipital lobes of the rhesus monkey. The entire population of muscarinic receptors was labeled with [3H]quinuclidinyl-benzilate; the M1 receptor subtype was labeled with [3H]pirenzepine; and the density of the M2 receptor subtype was estimated by subtracting the density of M1 receptors from the total population. The overall density of M1 and M2 receptor subtypes was similar throughout the cerebral cortex. However, their laminar distribution varied regionally. In cortical regions of the parietal and occipital lobes and in the primary motor cortex of the frontal lobe, both M1 and M2 receptor subtypes were concentrated in the supragranular layers. By contrast, in prefrontal cortical areas, the combined population of M1 and M2 receptors was evenly distributed across the cortical layers, though M1 receptors were most dense and M2 receptors least dense in layer IV. The difference in the distribution of cholinergic receptors in the prefrontal cortex compared to other neocortical areas reveals a degree of chemoarchitectural specificity of this region with respect to cholinergic markers that has escaped immunohistochemical and other anatomical and functional techniques.

Relationship of agonist efficacy to changes in GABA sensitivity and anticonvulsant tolerance following chronic benzodiazepine ligand exposure

Benzodiazepine ligands of differing efficacy including the agonist, diazepam, the partial agonist, Ro 16-6028 and the antagonist, Ro 15-1788, were administered in vivo to rats continuously for 3 weeks. The magnitude of change in various measures of GABA sensitivity could be correlated directly with increasing agonist efficacy: maximal changes were seen following chronic treatment with diazepam, intermediate changes were seen following the partial agonist Ro 16-6028 and no changes were observed following chronic Ro 15-1788 administration. The magnitude of change could also be correlated with increasing potential for tolerance development to anticonvulsant efficacy following chronic exposure to these benzodiazepine ligands.

Electrophysiological and biochemical characterization of the development of alpha 1-adrenergic and 5-HT1 receptors associated with dorsal raphe neurons

Electrophysiological and quantitative autoradiographic receptor binding approaches were used to characterize the postnatal development of alpha 1-adrenergic and serotonergic receptors in the midbrain dorsal raphe nucleus (DRN) in rats. Dose-response data were obtained from extracellular records of the firing of DRN neurons recorded in midbrain slices perfused with artificial cerebral spinal fluid containing varying concentrations of the drugs investigated. These data indicated that DRN neurons, at all postnatal ages, are equally sensitive to the excitatory effects of phenylephrine (an alpha 1-norepinephrine agonist) and the inhibitory effects of serotonin (5-HT). In contrast, LSD (a 5-HT agonist) was considerably more potent in neonates as compared to adults. With one exception, the autoradiographic binding data was found to be in agreement with the electrophysiological findings. There were no observed changes in the number of alpha 1-adrenergic ([3H]prazosin) binding sites at different ages suggesting that cells in this region exhibit functionally mature alpha 1-adrenergic responses at birth. Similarly, the number of [3H]LSD binding sites was found to increase in older animals consistent with the increased potency of LSD in older animals. Paradoxically, the number of [3H]5-HT sites was observed to increase with age, a finding inconsistent with the functional data obtained using 5-HT in the slice. Hypotheses for the observed discrepancy are examined.

Quantitative autoradiographic mapping of serotonin 5-HT1 and 5-HT2 receptors and uptake sites in the neocortex of the rhesus monkey

The in vitro autoradiographic technique was used to characterize the distribution of serotonin 5-HT1 and 5-HT2 receptors and uptake sites in 11 cortical areas of frontal, parietal, and occipital lobes in the rhesus monkey; 5-HT1 receptors were labeled with [3H]5-HT; 5-HT2 receptors were labeled with [3H]ketanserin; and 5-HT uptake sites were labeled with [3H]citalopram. Five-HT1 and 5-HT2 receptors and 5-HT uptake sites were found in every cortical area examined with the absolute concentration of 5-HT1 receptors higher than that of 5-HT2 receptors in all areas. In eight regions of prefrontal and parietal as well as in prestriate cortex, 5-HT1 and 5-HT2 receptors had complementary distribution profiles: 5-HT1 receptors were concentrated in layers I and II and the upper strata of layer III, while 5-HT2 receptors had their highest concentration throughout layers III and IV. Only the primary motor and visual cortex had receptor distributions different from that described above. Thus, in the primary visual cortex, both 5-HT1 and 5-HT2 receptors were found in high concentration in sublayer IVc beta, though the density of 5-HT1 receptor was also high in other subdivisions of layer IV and in layers III, V, and VI. In the primary motor cortex, both receptor subtypes were concentrated in layers I and II and the upper strata of layer III. The pattern of distribution of serotonin uptake sites did not match the patterns of distribution of either 5-HT1 or 5-HT2 receptors alone; rather it approximated the combined patterns of distribution of both receptor subtypes. The complementary patterns of distribution of 5-HT1 and 5-HT2 receptors in most areas of the monkey cerebral cortex suggest that these two receptor subtypes may make differential contributions to cortical functions.

Responses of substantia nigra pars reticulata neurons to benzodiazepine ligands after acute and prolonged diazepam exposure. II. Modulation of firing rate

We have examined the effects of chronic (3 week) and acute (1 hr) exposure to diazepam on firing rate responses of reticulata neurons to the inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) and the benzodiazepine antagonist Ro15-1788. The ability of DMCM and Ro15-1788 to alter firing rate were tested in control, chronic diazepam and acute diazepam-treated rats. In controls, DMCM increased firing rate of reticulata neurons. This effect was reversed by Ro15-1788, which had no effect on firing rate when administered alone. In acute and chronic diazepam-treated groups, low doses of DMCM were less effective in increasing firing than in controls. However, the administration of high doses of DMCM to diazepam-treated animals increased firing rates above the rates observed in control animals after the same compounds. Administration of Ro15-1788 also increased reticulata firing rates above control levels in both acute and chronic diazepam-treated groups. The similarity in reticulata responses after chronic (3 week) and acute (1 hr) diazepam treatments suggests that the altered properties of the benzodiazepine ligands observed after chronic exposure could reflect the presence of diazepam. This may indicate that reticulata neurons themselves do not show changes in sensitivity to benzodiazepine ligands after chronic benzodiazepine exposure. We propose that the altered responses to DMCM and Ro15-1788 in diazepam-treated rats reflect a compensatory change which occurs by 1 hr and maintains reticulata activity at a constant level throughout the period of prolonged exposure to the agonist.

Responses of substantia nigra pars reticulata neurons to benzodiazepine ligands after acute and prolonged diazepam exposure. I. Modulation of gamma-aminobutyric acid sensitivity

We have examined the ability of the benzodiazepine inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), and the antagonist, Ro15-1788, to modulate gamma-amino-butyric acid (GABA) sensitivity of reticulata neurons after chronic (3 week) and acute (1 hr) diazepam exposure. Sensitivity to GABA was analyzed by monitoring single unit responses to microiontophoretically applied GABA. The inverse agonist DMCM decreased GABA sensitivity of reticulata neurons to similar levels in control, chronic diazepam and acute diazepam-treated groups. Ro15-1788 administration reversed the effects of DMCM on GABA sensitivity and also resulted in comparable levels of GABA sensitivity in all treatment groups. Thus, the ability of the inverse agonist DMCM and the antagonist Ro15-1788 to alter GABA sensitivity of reticulata neurons was not altered by diazepam treatment. However, reticulata GABA sensitivity in the absence of additional benzodiazepine ligands was enhanced in both acute and chronic diazepam groups when compared to control values. The similarity in reticulata responses after both chronic (3 week) and acute (1 hr) diazepam treatments suggests that the enhanced sensitivity to GABA merely reflects the presence of diazepam. Thus, the ability of diazepam to potentiate reticulata GABA responses does not show time-dependent changes with prolonged benzodiazepine exposure.

Chronic diazepam treatment produces regionally specific changes in GABA-stimulated chloride influx

GABA-stimulated 36Cl- influx was used to investigate regional differences in response to chronic diazepam treatment by comparing cortical and cerebellar tissue from rats chronically treated with diazepam for 3 weeks. Using a treatment protocol which has previously been shown to produce behavioral tolerance and physical dependence, cortical membrane preparations from chronic diazepam-treated rats were found to exhibit a decreased responsiveness to the stimulation of 36Cl- influx by GABA and a corresponding decrease in the ability of flunitrazepam to enhance GABA-stimulated 36Cl-influx. This decrease in sensitivity to flunitrazepam, however, appears to reflect the underlying decrease in sensitivity to GABA. In contrast, in membrane vesicles prepared from cerebella of chronic diazepam-treated rats, there was no measurable effect on GABA-stimulated 36Cl--influx or on the enhancement of GABA-stimulated 36Cl- influx by flunitrazepam. These results support the suggestion that there is a regionally specific reduction in GABA/benzodiazepine receptor function following chronic benzodiazepine treatment.

Distribution of major neurotransmitter receptors in the motor and somatosensory cortex of the rhesus monkey

The in vitro quantitative autoradiographic technique was used to characterize the distributions of alpha 1, alpha 2, beta 1 and beta 2 adrenergic, D1 and D2 dopaminergic, 5-HT1 and 5-HT2 serotonergic, M1 and M2 cholinergic, GABAA and benzodiazepine receptors in the motor (Brodmann's area 4) and somatosensory (Brodmann's areas 3, 1 and 2) cortex of the adult rhesus monkey. All receptor subtypes studied were present throughout all layers of both areas. In the somatosensory cortex, each receptor had its own laminar distribution. Some subtypes of the same receptor (5-HT1 and 5-HT2; alpha 1 and alpha 2) had complementary distributions while others (beta 1 and beta 2; D1 and D2; M1 and M2) had largely overlapping distributions. In contrast, different receptors had remarkably coincidental distributions in the motor cortex. In this area, they all tended to concentrate in layers I, II and the upper part of layer III. However, such coextensive distribution of many types of neurotransmitter receptors is not observed in motor cortex of rats and humans and therefore may be a distinctive feature of motor cortex in the rhesus monkey. The findings described in this paper indicate that somatosensory and motor areas are distinct in their receptor architecture and that receptor autoradiography provides a useful complement to classical histological techniques in elucidating areal differences in the cortex.

GABAergic subsensitivity of dorsal raphe neurons in vitro after chronic benzodiazepine treatment in vivo

Previous studies have shown that chronic benzodiazepine treatment reduces the in vivo sensitivity of dorsal raphe neurons (DRN) to microiontophoretically applied gamma-aminobutyric acid (GABA). We have examined sensitivity of DRN in vitro using a modified midbrain slice technique which allows side-by-side analysis of slices from control and chronic diazepam-treated rats. GABA sensitivity of raphe neurons was reduced in slices from rats treated for 3 weeks with diazepam, compared to control sensitivity. Thus, GABA subsensitivity following chronic diazepam treatment appears to be dependent on changes intrinsic to the midbrain area.

Altered gamma-aminobutyric acid/benzodiazepine interaction after chronic diazepam exposure

Binding to components of the GABA/benzodiazepine receptor complex was examined in cortical membranes from rats treated for 3 weeks with continuously releasing diazepam pellet implants. Chronic diazepam treatment resulted in a decrease in the ability of GABA to inhibit benzodiazepine inverse agonist binding. The amount of binding of benzodiazepine agonist, antagonist, and inverse agonist to benzodiazepine recognition sites was unaltered by the chronic treatment, as were the potencies of these benzodiazepine ligands in inhibiting agonist (3H-flunitrazepam) binding. Chloride channel binding (35S-TBPS) was also unchanged by chronic diazepam treatment. A change in GABA/benzodiazepine coupling and/or a decreased effectiveness of GABA may be responsible for the desensitization of GABA/benzodiazepine interaction.

Phosphorylation of the GABAa/benzodiazepine receptor alpha subunit by a receptor-associated protein kinase

Partially purified preparations of GABAa/benzodiazepine receptor from rat brain were found to contain high levels of a protein kinase activity that phosphorylated a small number of proteins in the receptor preparations, including a 50-kilodalton (kD) phosphoprotein that comigrated on two-dimensional electrophoresis with purified, immunolabeled, and photolabeled receptor alpha subunit. Further evidence that the comigrating 50-kD phosphoprotein was, in fact, the receptor alpha subunit was obtained by peptide mapping analysis: the 50-kD phosphoprotein yielded one-dimensional peptide maps identical to those obtained from iodinated, purified alpha subunit. Phosphoamino acid analysis revealed that the receptor alpha subunit is phosphorylated on serine residues by the protein kinase activity present in receptor preparations. Preliminary characterization of the receptor-associated protein kinase activity suggested that it may be a second messenger-independent protein kinase. Protein kinase activity was unaltered by cyclic AMP, cyclic GMP, calcium plus calmodulin, calcium plus phosphatidylserine, and various inhibitors of these protein kinases. Examination of the substrate specificity of the receptor-associated protein kinase indicated that the enzyme preferred basic proteins as substrates. Endogenous phosphorylation experiments indicated that the receptor alpha subunit may also be phosphorylated in crude membranes by a protein kinase activity present in those membranes. As with phosphorylation of the receptor in purified preparations, its phosphorylation in crude membranes also appeared to be unaffected by activators and inhibitors of second messenger-dependent protein kinases. These findings raise the possibility that the phosphorylation of the alpha subunit of the GABAa/benzodiazepine receptor by a receptor-associated protein kinase plays a role in modulating the physiological activity of the receptor in vivo.

Differential quenching and limits of resolution in autoradiograms of brain tissue labeled with 3H-, 125I- and 14C-compounds

Problems in interpretation of autoradiograms generated by ligand binding in brain tissue may be caused by two types of technical limitations: the differential absorption of 3H-generated emissions within the tissue (differential quenching) and the reduced resolution when 125I and 14C are used as isotopes. In the course of our ongoing receptor binding studies in primate brain, we have examined these methodological problems using neocortex of adult rhesus monkey as an example of a complex multilayered brain structure. We have compared: (1) film images produced by brain sections mounted on 3H- and 14C-labeled plastic; (2) autoradiograms of sections labeled with pairs of similar compounds containing 3H, 125I or 14C; and (3) autoradiograms of normal and defatted brain sections. The results indicate that differential absorption of 3H-generated emissions presents a genuine problem for film autoradiography of neocortex of adult monkey when 3H-compounds are used. Particularly significant attenuations of 3H-generated emissions are associated with sublayer IVb of primary visual cortex (Brodmann's are 17) and layers III (deep strata), V and VI of primary motor cortex (Brodmann's area 4). This study provides the necessary corrections for autoradiographic measurements. We also found a loss of resolution associated with use of 125I and 14C, a result that poses a significant problem for analysis of fine laminar patterns of the neocortex in adult monkeys. The use of isotopes with high energy emissions tends to decrease the variations in optical densities within the autoradiograms of cortical sections. Thus, the variations in optical density of autoradiograms of cortices labeled with 125I- and 14C-compounds may not represent the true distribution of these compounds.

Continuous slow release of low levels of diazepam produces tolerance to its depressant and anxiolytic effects on the startle reflex

Development of tolerance to the depressant effects of diazepam on the acoustic startle reflex and to the blockade of fear-potentiated startle, a measure of fear or anxiety in rodents, was evaluated after chronic administration via continuous release from implanted diazepam-filled silastic capsules or daily intraperitoneal (i.p.) injections. After continuous exposure to diazepam via capsule implants, complete tolerance occurred to the depressant effects of diazepam on startle and partial tolerance occurred to the antifear effects. In contrast, no tolerance was observed after daily i.p. injection with comparable amounts of diazepam (5 mg/kg) although tolerance could be produced by daily i.p. injections of a much higher dose of diazepam (20 mg/kg). These data suggest that tolerance to at least some behavioral effects may be much easier to produce with continuous rather than intermittent occupation of benzodiazepine receptors.

Ro 15-1788-induced seizures in rats continually exposed to diazepam for prolonged periods

Previous reports using rats have failed to demonstrate convulsions upon withdrawal from chronic benzodiazepine (BZ) treatment. We have shown earlier that rats develop tolerance to benzodiazepines following continuous exposure to low levels of diazepam (DZ) obtained by treatment with s.c. diazepam-filled silastic capsules. This report shows that intravenous infusion of the benzodiazepine antagonist Ro 15-1788 can induce seizures in rats treated for prolonged periods (4 weeks) with such diazepam-filled capsules. Precipitated seizures are also seen in rats 24 h after the cessation of chronic diazepam exposure. This procedure may provide a useful paradigm with which to study in rats the neural changes which are related to the physical dependence and withdrawal associated with prolonged exposure to the benzodiazepines.

Persistent reversal of tolerance to anticonvulsant effects and GABAergic subsensitivity by a single exposure to benzodiazepine antagonist during chronic benzodiazepine administration

The persistence of benzodiazepine antagonists in reversing neuronal and behavioral tolerance during chronic diazepam exposure was examined in rodents by investigating the time course for antagonist-induced alterations in iontophoretic sensitivity to gamma-aminobutyric acid on dorsal raphe neurons and the re-emergence of anticonvulsant efficacy to bicuculline-induced seizures. In these studies, exposure to Ro15-1788 resulted in the persistent reversal of GABAergic subsensitivity and restoration of anticonvulsant actions of diazepam despite the continued presence of diazepam in the rats. Reversal of tolerance appears to persist for up to 7 days after a single exposure to benzodiazepine antagonists.

Effects of chronic diazepam exposure on GABA sensitivity and on benzodiazepine potentiation of GABA-mediated responses of substantia nigra pars reticulata neurons of rats

This study examined the effects of chronic diazepam treatment on GABA sensitivity of substantia nigra pars reticulata neurons and on the ability of benzodiazepines to enhance GABAergic responses of these neurons in rats. Chronic diazepam exposure failed to significantly alter the sensitivity of reticulata neurons to microiontophoretically applied GABA. However, following chronic diazepam treatment for 1 day, or 1, 3 or 7-11 weeks, reticulata neurons showed tolerance to additional systemically or iontophoretically applied benzodiazepines and displayed an increased firing rate following injection of Ro 15-1788. These changes were not apparent 24 h after cessation of chronic treatment. Thus, tolerance to the effects of benzodiazepines on reticulata neurons appeared to develop after a single day of diazepam exposure and to dissipate by 24 h after cessation of treatment. When compared to our previous studies on dorsal raphe neurons, these results demonstrate regional differences in neuronal responses to chronic diazepam exposure, which may help elucidate neural systems which are involved in tolerance to the various functional aspects of benzodiazepines.

Time course for development of anticonvulsant tolerance and GABAergic subsensitivity after chronic diazepam

The time courses for development of neuronal and behavioral tolerance to diazepam (DZ) were estimated in rats continuously exposed to low levels of DZ for 3, 7, 14 or 21 days. Microiontophoretic sensitivity of dorsal raphe neurons to gamma-aminobutyric acid (GABA) was initially facilitated after short-term exposure to DZ released from implanted capsules for up to 3 days but returned to control levels by 7 days postimplantation and continued to decrease thereafter. GABAergic sensitivity remained depressed for a minimum of 5 days following removal of DZ capsules. To obtain a behavioral measure of tolerance, the anticonvulsant activity of DZ against bicuculline-induced seizures was also assessed. Rats studied 3 days after capsule implantation showed a significant elevation in seizure threshold. Seizure liability returned to control levels ca. 7 days after chronic treatment was initiated. These results indicate that tolerance to anticonvulsant efficacy against bicuculline seizures are temporally related to the onset of reduced GABA sensitivity on dorsal raphe neurons during prolonged exposure to DZ.

Periodic benzodiazepine antagonist administration prevents benzodiazepine withdrawal symptoms in primates

Daily administration of diazepam (1.5 or 6 mg/kg) in Rhesus monkeys results in the progressive development of physical dependence, as evidenced by Ro15-1788 (5 mg/kg i.m.) precipitated withdrawal symptoms including retching, vomiting, face and limb tremors. Every third day administration of the benzodiazepine antagonist, Ro15-1788, during a similar period of continuous diazepam exposure, significantly decreases withdrawal behaviors. During the course of diazepam exposure (with or without periodic Ro15-1788 administration) effects of chronic diazepam on spontaneously elicited sedative and active behaviors were not altered. It is postulated that physical dependence reverts to a drug naive state after each exposure to the benzodiazepine antagonist. This treatment may represent a possible therapeutic approach for preventing the (time dependent) development of physical dependence and the accompanying severe withdrawal symptoms.

Tolerance to anti-pentylenetetrazol effects following chronic diazepam

Continuous release of diazepam from subcutaneously implanted silastic capsules provided significant protection against pentylenetetrazol seizures in rats for up to 3 weeks. However, the degree of protection seen after 3 weeks of exposure to diazepam was significantly less than after 1 h. These data suggest that continuous exposure to constant low levels of diazepam results in the development of partial tolerance which is not sufficient to eliminate significant anticonvulsant effects.

Continuous release of diazepam: electrophysiological, biochemical and behavioral consequences

Neuronal GABAergic sensitivity was assessed using electrophysiological, biochemical and behavioral techniques following the continuous release and maintenance of relatively constant brain levels of diazepam for greater than or equal to 21 days. Our studies indicate that long-term exposure to diazepam results in: (1) a decrease in iontophoretic sensitivity to GABA in the dorsal raphe nucleus, (2) an increase in the affinity of the GABA recognition site in brain tissue and (3) an increase in susceptibility to bicuculline-induced seizures in the intact animal. Since the decrease in GABAergic responsiveness was observed in the presence of measurable levels of diazepam, it was concluded that this subsensitivity phenomenon is associated with tolerance and not with withdrawal effects of the benzodiazepines.

Spinalization unmasks clonidine's alpha 1-adrenergic mediated excitation of the flexor reflex in rats

Clonidine exerts alpha 2-adrenergic mediated depressant effects on most behaviors measured in a normal animal. However, in the spinal-transected (spinalized) animal, clonidine apparently facilitates the flexor reflex through a stimulation of spinal alpha 1-adrenoceptors. The purpose of the present study was to determine if spinalization per se causes the shift in clonidine's profile from an alpha 2- to an alpha 1-adrenergic agonist. The hindlimb flexor reflex was elicited by electrical pulses delivered through electrodes implanted subcutaneously in the hindpaw and was measured with a force transducer and polygraph. In contrast to an alpha 2-adrenergic mediated inhibition of the flexor reflex in intact rats, clonidine produced an alpha 1-adrenergic mediated increase in flexor reflex amplitude in spinalized rats. Because decerebration did not alter the depression due to clonidine, and intraventricular (but not intrathecal) administration of oxymetazoline mimicked the effect of clonidine, the depressant effects of alpha 2-adrenergic agonists are mediated through alpha 2-adrenergic receptors localized in the brainstem. Alternate methods for inducing a functional spinal transection (spinal block with intrathecal procaine; spinal ligation) indicated that the shift in clonidine's effect from inhibition of the flexor reflex to excitation occurred immediately following spinalization. Spinal ligation did not produce alpha 1-adrenergic supersensitivity at 15 min or 2 hr after transection, as measured by alterations in [3H]prazosin receptor binding or behavioral responses to clonidine. Thus, the shift in clonidine's effects from alpha 2-adrenergic mediated inhibition of the flexor reflex in intact rats to alpha 1-adrenergic mediated excitation in spinalized rats results because spinal transection unmasks clonidine's alpha 1-adrenergic stimulatory effect. Other conditions under which clonidine exerts alpha 1-adrenoceptor mediated excitatory effects on behavior are discussed.

Electrophysiological and receptor studies in rat brain: effects of clorgyline

Acute high doses of clorgyline produce a rapid inhibition of monoamine oxidase type A (MAO A) in the rat brain, together with an increase in norepinephrine and a decrease in the firing rate of locus coeruleus (LC) neurones: this decrease is reversed by piperoxane, an alpha 2 antagonist. In control animals, piperoxane increases LC neuronal firing showing that these noradrenergic neurones are under alpha 2-adrenoceptor-mediated tonic inhibition. Chronic administration of clorgyline, like acute doses of this MAO A inhibitor, significantly decreases cell firing in the LC and the effect is partially reversed by piperoxane. Chronic clorgyline treatment also produces significant decreases in [3H]clonidine and [3H]dihydroalprenolol binding in cerebral cortex, receptor changes which are slightly greater in animals showing greater inhibition of LC neuronal firing: such receptor changes do not occur following a single exposure to clorgyline. Electrophysiological studies in hippocampal pyramidal cells show that the chronic clorgyline treatment does not significantly induced subsensitivity to NE in these adrenoreceptive cells.

Spontaneous and RO 15-1788-induced reversal of subsensitivity to GABA following chronic benzodiazepines

Chronic daily injections of diazepam (5 mg/kg i.p.) for 21-30 days have previously been shown to cause a selective subsensitivity to microiontophoretically applied GABA in serotonergic dorsal raphe neurons in the rat. Following termination of chronic benzodiazepine treatment, GABAergic sensitivity remained depressed for up to 96 h even though pharmacologically active concentrations of diazepam and its active metabolites could no longer be detected in brain tissue. In contrast, a single injection of the specific benzodiazepine antagonist, RO 15-1788, given 22 h prior to electrophysiological recording, restored GABAergic sensitivity to the control range. Sensitivity of dorsal raphe neurons to serotonin was not altered by either chronic treatment with or withdrawal from diazepam. These results indicate that (1) RO 15-1788 can accelerate the time course of withdrawal and (2) administration of this benzodiazepine antagonist can induce a persistent change in GABAergic responsiveness.

Histamine-induced depression of serotoninergic dorsal raphe neurons: antagonism by cimetidine, a reevaluation

Microiontophoretic application of the histamine H2-receptor antagonist cimetidine selectively attenuated the histamine-induced, but not the gamma-aminobutyric acid (GABA)-induced, depression of serotoninergic cells recorded in the dorsal raphe nucleus of the rat. These data support the view that an H2-receptor mediates the effects of histamine on these serotoninergic neurons. We also now ascribe a previous report of GABA-like properties of cimetidine and metiamide to an impurity in the lots of these compounds used originally.

Chronic benzodiazepine treatment decreases postsynaptic GABA sensitivity

Benzodiazepines exert most of their pharmacological effects by a selective facilitation of the postsynaptic actions of GABA. Clinical, behavioural and electrophysiological studies have shown reduced drug response following chronic benzodiazepine administration. We present here electrophysiological evidence for decreased postsynaptic sensitivity to GABA following chronic benzodiazepine administration as measured by the direct iontophoretic application of GABA and serotonin onto serotonergic cells in the midbrain dorsal raphe nucleus (DRN), known to receive GABAergic input. The subsensitivity to GABA was found to be dose dependent and was seen when diazepam administration was three weeks or longer. Further, acute injection of the specific benzodiazepine antagonist, Ro15-1788, was found to reverse rapidly the decrease in GABA sensitivity observed in chronically diazepam-treated animals without altering GABA sensitivity in vehicle-treated rats. Decreased response to chronic benzodiazepines does not appear to be consistently related to alterations in the number or affinity of receptors for benzodiazepines. Our studies of radioligand-binding showed a decrease in the ability of GABA to enhance benzodiazepine binding in cerebral cortical membranes from chronic diazepam-treated animals without significant changes in benzodiazepine binding site density or affinity.

Consequences of benzodiazepine receptor occupancy

The pharmacological consequences of the occupancy of benzodiazepine receptors have been a recent area of active research. There is good agreement between the electrophysiological effects of benzodiazepines and their binding to benzodiazepine receptors when both are studied in vitro under identical conditions. Compounds of different structure from the benzodiazepines can occupy the receptor in a way, which produces little overt effect (imidazodiazepines) or actually causes actions opposite to the benzodiazepines (beta-carbolines, inverse agonists). Several biochemical tests (GABA-shift, photo-shift) for distinguishing these different behavioral properties are described. A model is described for the interactions at membranes of agonists, antagonists and inverse agonists with benzodiazepine receptors in the GABA receptor complex.

Behavior and binding: desensitization to alpha 1-adrenergic stimulation of acoustic startle is associated with a decrease in alpha 1-adrenoceptor binding sites

Desensitization of the excitatory effects of alpha 1-adrenergic agonists on acoustic startle occurred 6 h after intrathecal administration of the alpha 1-adrenergic agonist, phenylephrine. This desensitization was associated with a decrease in alpha 1-adrenoceptor sites in the lumbar spinal cord.

Alpha 1-adrenoceptor denervation supersensitivity in brain: physiological and receptor binding studies

Electrophysiological and radioligand binding methods were used to characterize noradrenergic denervation supersensitivity at alpha 1-adrenoceptors in rat thalamus. Denervation was accomplished either by intraventricular or intracerebral injection of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA). In the physiological studies, the sensitivity of single lateral geniculate neurons to norepinephrine, carbachol, and serotonin was compared in sham and lesioned animals various times after 6-OHDA. Conducted in parallel were radioligand binding studies in which the density and affinity of thalamic alpha 1-adrenoceptors were measured with the specific antagonist [3H]prazosin. The results indicate that denervation produces a selective increase in the sensitivity of geniculate neurons to alpha 1-adrenergic stimulation and a concomitant increase in alpha 1-adrenoceptor density and agonist affinity.

Functional supersensitivity of CNS alpha 1-adrenoceptors following chronic antidepressant treatment

Chronic but not acute administration (21 days) of desipramine (10 mg/kg), amitriptyline (10 mg/kg) or iprindole (5 mg/kg) enhanced the stimulatory effect of the alpha 1-adrenergic agonist phenylephrine on the acoustic startle reflex when phenylephrine was infused into the subarachnoid space of the spinal cord. Comparable supersensitivity to phenylephrine also occurred 1 week after selective depletion of norepinephrine in the spinal cord via intrathecal administration of 6-hydroxydopamine. Behavioral supersensitivity to phenylephrine was associated with an increase in the number of 3H-prazosin binding sites following denervation but not following chronic antidepressant treatments. The results indicate that chronic antidepressant treatments may enhance functional alpha 1-adrenergic transmission through mechanisms different than those following denervation.

Interaction of histamine H2-receptor antagonists with GABA and benzodiazepine binding sites in the CNS

The histamine H2-receptor antagonist cimetidine potently inhibited [3H]muscimol and enhanced [3H]flunitrazepam binding in membranes prepared from several brain regions in the rat, including the dorsal raphe nucleus. As further examined in cortical membranes, this effect on both GABA and benzodiazepine binding sites was specific for imidazole-derived H2-receptor antagonists (potency: cimetidine greater than metiamide greater than tiotidine) and not observed with either several H1-receptor antagonists or histamine. These data indicate a striking similarity between the actions of cimetidine (and other imidazole-derived H2-receptor antagonists) and GABA on binding parameters at the GABA receptor complex.