RESUMEN
A fully reusable electrochemical device is proposed for the first time made from laser cutting and a homemade conductive ink composed of carbon and nail polish. As a sensor substrate, we applied polymethyl methacrylate, which allows the surface to be renewed by simply removing and reapplying a new layer of ink. In addition to the ease of renewing the sensor's conductive surface, the design of the device has allowed for the integration of different forms of analysis. The determination of L-Dopa was performed using DPV, which presented a linear response range between 5.0 and 1000.0 µmol L-1, and a LOD of 0.11 µmol L-1. For dopamine, a flow injection analysis system was employed, and using the amperometric technique measurements were performed with a linear ranging from 2.0 to 100.0 µmol L-1 and a LOD of 0.26 µmol L-1. To demonstrate its applicability, the device was used in the quantification of analytes in pharmaceutical drug and synthetic urine samples.
Asunto(s)
Grafito , Levodopa , Levodopa/análisis , Dopamina/análisis , Técnicas Electroquímicas/métodos , Electrodos , Reproducibilidad de los ResultadosRESUMEN
In this work the Successive Projection Algorithm is presented for intervals selection in N-PLS for three-way data modeling. The proposed algorithm combines noise-reduction properties of PLS with the possibility of discarding uninformative variables in SPA. In addition, second-order advantage can be achieved by the residual bilinearization (RBL) procedure when an unexpected constituent is present in a test sample. For this purpose, SPA was modified in order to select intervals for use in trilinear PLS. The ability of the proposed algorithm, namely iSPA-N-PLS, was evaluated on one simulated and two experimental data sets, comparing the results to those obtained by N-PLS. In the simulated system, two analytes were quantitated in two test sets, with and without unexpected constituent. In the first experimental system, the determination of the four fluorophores (l-phenylalanine; l-3,4-dihydroxyphenylalanine; 1,4-dihydroxybenzene and l-tryptophan) was conducted with excitation-emission data matrices. In the second experimental system, quantitation of ofloxacin was performed in water samples containing two other uncalibrated quinolones (ciprofloxacin and danofloxacin) by high performance liquid chromatography with UV-vis diode array detector. For comparison purpose, a GA algorithm coupled with N-PLS/RBL was also used in this work. In most of the studied cases iSPA-N-PLS proved to be a promising tool for selection of variables in second-order calibration, generating models with smaller RMSEP, when compared to both the global model using all of the sensors in two dimensions and GA-NPLS/RBL.
Asunto(s)
Algoritmos , Cromatografía Líquida de Alta Presión , Ciprofloxacina/análisis , Fluoroquinolonas/análisis , Hidroquinonas/análisis , Análisis de los Mínimos Cuadrados , Levodopa/análisis , Ofloxacino/análisis , Fenilalanina/análisis , Programas Informáticos , Espectrofotometría Ultravioleta , Triptófano/análisis , Agua/químicaRESUMEN
L-Dopa is the immediate precursor of the neurotransmitter dopamine, being the most widely prescribed drug in the treatment of Parkinson's disease. A sensitive and selective method is presented for the voltammetric determination of L-Dopa in pharmaceutical formulations using a basal plane pyrolytic graphite (BPPG) electrode modified with chloro(pyridine)bis(dimethylglyoximato)cobalt(III) (Co(DMG)(2)ClPy) absorbed in a multi-walled carbon nanotube (MWCNT). Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy were used to characterize the materials. The electrocatalytical oxidation of L-Dopa using the Co(DMG)(2)ClPy/MWCNT/BPPG electrode was investigated by cyclic voltammetry and square wave voltammetry. The parameters that influence the electrode response (the amount of Co(DMG)(2)ClPy and of MWCNT, buffer solution, buffer concentration, buffer pH, frequency and potential pulse amplitude) were investigated. Voltammetric peak currents showed a linear response for L-Dopa concentration in the range of 3 to 100 µM, with a sensitivity of 4.43 µAcm(-2)/µM and a detection limit of 0.86 µM. The related standard deviation for 10 determinations of 50 µM L-Dopa was 1.6%. The results obtained for L-Dopa determination in pharmaceutical formulations (tablets) were in agreement with the compared official method. The sensor was successfully applied for L-Dopa selective determination in pharmaceutical formulations.
Asunto(s)
Electroquímica/métodos , Electrodos , Levodopa/análisis , Nanotubos de Carbono/química , Tampones (Química) , Cobalto , Grafito/química , Concentración de Iones de Hidrógeno , Levodopa/química , Oxidación-Reducción , Sensibilidad y Especificidad , Comprimidos/análisisRESUMEN
An analytical methodology based on differential pulse voltammetry (DPV) on a glassy carbon electrode and the partial least-squares (PLS-1) algorithm for the simultaneous determination of levodopa, carbidopa and benserazide in pharmaceutical formulations was developed and validated. Some sources of bi-linearity deviation for electrochemical data are discussed and analyzed. The multivariate model was developed as a ternary calibration model and it was built and validated with an independent set of drug mixtures in presence of excipients, according with manufacturer specifications. The proposed method was applied to both the assay and the uniformity content of two commercial formulations containing mixtures of levodopa-carbidopa (10:1) and levodopa-benserazide (4:1). The results were satisfactory and statistically comparable to those obtained by applying the reference Pharmacopoeia method based on high performance liquid chromatography. In conclusion, the methodology proposed based on DPV data processed with the PLS-1 algorithm was able to quantify simultaneously levodopa, carbidopa and benserazide in its pharmaceuticals formulations using a ternary calibration model for these drugs in presence of excipients. Furthermore, the model appears to be successful even in the presence of slight potential shifts in the processed data, which have been taken into account by the flexible chemometric PLS-1 approach.
Asunto(s)
Dopaminérgicos/análisis , Técnicas Electroquímicas/métodos , Algoritmos , Benserazida/análisis , Calibración , Carbidopa/análisis , Combinación de Medicamentos , Técnicas Electroquímicas/normas , Electrodos , Excipientes , Levodopa/análisisRESUMEN
A combination of kinetic spectroscopic monitoring and multivariate curve resolution-alternating least squares (MCR-ALS) was proposed for the enzymatic determination of levodopa (LVD) and carbidopa (CBD) in pharmaceuticals. The enzymatic reaction process was carried out in a reverse stopped-flow injection system and monitored by UV-vis spectroscopy. The spectra (292-600 nm) were recorded throughout the reaction and were analyzed by multivariate curve resolution-alternating least squares. A small calibration matrix containing nine mixtures was used in the model construction. Additionally, to evaluate the prediction ability of the model, a set with six validation mixtures was used. The lack of fit obtained was 4.3%, the explained variance 99.8% and the overall prediction error 5.5%. Tablets of commercial samples were analyzed and the results were validated by pharmacopeia method (high performance liquid chromatography). No significant differences were found (alpha=0.05) between the reference values and the ones obtained with the proposed method. It is important to note that a unique chemometric model made it possible to determine both analytes simultaneously.
Asunto(s)
Carbidopa/análisis , Levodopa/análisis , Preparaciones Farmacéuticas/química , Calibración , Carbidopa/metabolismo , Catecol Oxidasa/metabolismo , Diseño de Equipo , Ipomoea batatas/enzimología , Cinética , Análisis de los Mínimos Cuadrados , Levodopa/metabolismo , Análisis Multivariante , Valores de Referencia , Espectrofotometría/economía , Espectrofotometría/instrumentación , Espectrofotometría/métodosRESUMEN
A simple and sensitive methodology for the simultaneous determination of levodopa and carbidopa in pharmaceutical samples is described. The method combines the advantages of fluorescence with partial least-squares (PLS) analysis, and requires no previous separation steps. The developed method is based on the oxidation of levodopa and carbidopa by cerium(IV) in a sulfuric acid medium and monitoring the fluorescence of the formed Ce(III) at lambda(exc) = 255 nm and lambda(em) = 355 nm. PLS uses differences in the reaction rates as a discriminatory parameter, and regresses the data of fluorescence vs. time onto the concentrations of the standards. Eight validation samples and seven commercial tablets were studied, using a nine-sample aqueous calibration set. The analyte recoveries from pharmaceuticals ranged from 98 to 101% for levodopa and from 100 to 108% for carbidopa. The results obtained by the developed method were statistically comparable to those obtained with high-performance liquid chromatography.
Asunto(s)
Antiparkinsonianos/análisis , Carbidopa/análisis , Levodopa/análisis , Espectrometría de Fluorescencia/métodos , Cerio/química , Fluorescencia , Concentración de Iones de Hidrógeno , Cinética , Análisis de los Mínimos Cuadrados , Oxidación-Reducción , Preparaciones Farmacéuticas/química , Ácidos Sulfúricos/química , TemperaturaRESUMEN
An enzymatic flow-batch system with spectrophotometric detection was developed for simultaneous determination of levodopa [(S)-2 amino-3-(3,4-dihydroxyphenyl)propionic acid] and carbidopa [(S)-3-(3,4-dihydroxyphenyl)-2-hydrazino-2-methylpropionic acid] in pharmaceutical preparations. The data were analysed by univariate method, partial least squares (PLS) and a novel variable selection for multiple lineal regression (MLR), the successive projections algorithm (SPA). The enzyme polyphenol oxidase (PPO; EC 1.14.18.1) obtained from Ipomoea batatas (L.) Lam. was used to oxidize both analytes to their respective dopaquinones, which presented a strong absorption between 295 and 540 nm. The statistical parameters (RMSE and correlation coefficient) calculated after the PLS in the spectral region between 295 and 540 nm and MLR-SPA application were appropriate for levodopa and carbidopa. A comparative study of univariate, PLS, in different ranges, and MLR-SPA chemometrics models, was carried out by applying the elliptical joint confidence region (EJCR) test. The results were satisfactory for PLS in the spectral region between 295 and 540 nm and for MLR-SPA. Tablets of commercial samples were analysed and the results obtained are in close agreement with both, spectrophotometric and HPLC pharmacopeia methods. The sample throughput was 18 h(-1).
Asunto(s)
Algoritmos , Carbidopa/análisis , Catecol Oxidasa/química , Ipomoea batatas/enzimología , Levodopa/análisis , Preparaciones Farmacéuticas/química , Espectrofotometría/métodos , Tampones (Química) , Carbidopa/química , Catecol Oxidasa/metabolismo , Simulación por Computador , Análisis de los Mínimos Cuadrados , Levodopa/química , Modelos Lineales , Oxidación-Reducción , Fosfatos/química , Factores de TiempoRESUMEN
The anionic complexes [Cu(L(1-))3](1-), L(-)=dopasemiquinone or L-dopasemiquinone, were prepared and characterized. The complexes are stable in aqueous solution showing intense absorption bands at ca. 605 nm for Cu(II)-L-dopasemiquinone and at ca. 595 nm for Cu(II)-dopasemiquinone in the UV-vis spectra, that can be assigned to intraligand transitions. Noradrenaline and adrenaline, under the same reaction conditions, did not yield Cu-complexes, despite the bands in the UV region showing that noradrenaline and adrenaline were oxidized during the process. The complexes display a resonance Raman effect, and the most enhanced bands involve ring modes and particularly the nuCC+nuCO stretching mode at ca. 1384 cm(-1). The free radical nature of the ligands and the oxidation state of the Cu(II) were confirmed by the EPR spectra that display absorptions assigned to organic radicals with g=2.0005 and g=2.0923, and for Cu(II) with g=2.008 and g=2.0897 for L-dopasemiquinone and dopasemiquinone, respectively. The possibility that dopamine and L-dopa can form stable and aqueous-soluble copper complexes at neutral pH, whereas noradrenaline and adrenaline cannot, may be important in understanding how Cu(II)-dopamine crosses the cellular membrane as proposed in the literature to explain the role of copper in Wilson disease.
Asunto(s)
Cobre/química , Dopamina/química , Espectroscopía de Resonancia por Spin del Electrón/métodos , Levodopa/análisis , Espectrofotometría Ultravioleta/métodos , Espectrometría Raman/métodos , Cloroformo/química , Epinefrina/análisis , Radicales Libres , Concentración de Iones de Hidrógeno , Ligandos , Metales/química , Norepinefrina/análisis , Oxidación-Reducción , Oxígeno/químicaRESUMEN
This paper presents the use of elastomeric polyurethane (PU), derived from castor oil (CO) biosource, as a new material for fabrication of microfluidic devices by rapid prototyping. Including the irreversible sealing step, PU microchips were fabricated in less than 1h by casting PU resin directly on the positive high-relief molds fabricated by standard photolithography and nickel electrodeposition. Physical characterization of microchannels was performed by scanning electron microscopy (SEM) and profilometry. Polymer surface was characterized using contact angle measurements and the results showed that the hydrophilicity of the PU surface increases after oxygen plasma treatment. The polymer surface demonstrated the capability of generating an electroosmotic flow (EOF) of 2.6 x 10(-4)cm(2)V(-1)s(-1) at pH 7 in the cathode direction, which was characterized by current monitoring method at different pH values. The compatibility of PU with a wide range of solvents and electrolytes was tested by determining its degree of swelling over a 24h period of contact. The performance of microfluidic systems fabricated using this new material was evaluated by fabricating miniaturized capillary electrophoresis systems. Epinephrine and l-DOPA, as model analytes, were separated in aqueous solutions and detected with end-channel amperometric detection.
Asunto(s)
Electroforesis por Microchip/métodos , Técnicas Analíticas Microfluídicas/métodos , Microfluídica/métodos , Poliuretanos/química , Electroforesis Capilar/métodos , Epinefrina/análisis , Epinefrina/química , Levodopa/análisis , Levodopa/química , Microscopía Electrónica de Rastreo , Reproducibilidad de los ResultadosRESUMEN
A densitometric high performance thin-layer chromatographic (HPTLC) method was developed and validated for quantitative analysis of L-DOPA in tablets. Chromatographic separation was achieved on precoated silica gel F 254 HPTLC plates using a mixture of acetone-chloroform-n-butanol-acetic acid glacial-water (60:40:40:40:35 v/v/v/v/v) as mobile phase. Quantitative analysis was carried out at a wavelength of 497 nm. The method was linear between 100 and 500 ng/microL, with a correlation coefficient of 0.999. The intra-assay variation was between 0.26 and 0.65% and the interassay was between 0.52 and 2.04%. The detection limit was 1.12 ng/microL, and the quantification limit was 3.29 ng/microL. The accuracy ranged from 100.40 to 101.09%, with a CV not higher than 1.40%. The method was successfully applied to quantify L-DOPA in real pharmaceutical samples, including the comparison with HPLC measurements. The method was fast, specific, with a good precision, and accurate for the quantitative determination of L-DOPA in tablets.
Asunto(s)
Antiparkinsonianos/análisis , Química Farmacéutica/métodos , Cromatografía Líquida de Alta Presión/métodos , Cromatografía en Capa Delgada/métodos , Levodopa/análisis , 1-Butanol/análisis , Ácido Acético/análisis , Acetona/análisis , Antiparkinsonianos/química , Antiparkinsonianos/aislamiento & purificación , Cloroformo/análisis , Densitometría , Levodopa/química , Levodopa/aislamiento & purificación , Preparaciones Farmacéuticas/análisis , Preparaciones Farmacéuticas/química , Reproducibilidad de los Resultados , Comprimidos , Factores de Tiempo , Agua/químicaRESUMEN
The main limiting factor in the consumption by humans of the velvet bean (Mucuna) is its relatively high content of L-Dihydroxyphenylalanine (L-Dopa), with levels as high as 9%. Conventional cooking methods used to transform raw velvet bean into an edible product are not sufficiently effective in reducing the levels of L-Dopa in adequate processing time. In this report, Mucuna beans were cooked by microwave, utilizing vapor and in water solutions at pH 3, 6, 7, 9 and 11. Cooking alkaline solutions were achieved using sodium hydroxide, potassium hydroxide, and calcium hydroxide. The acid pH was achieved through the use of HCl. The initial cooking time was fixed at 6 hrs. The processed bean samples were dried, ground and analyzed for L-Dopa and protein. The ground samples were further washed with boiling water for 0, 3 and 6 minutes, them dried and analyzed. None of the procedures evaluated was capable of eliminating L-Dopa from Mucuna beans. The Ca(OH)2 treatment at pH 9 which was washed with hot water produce a reduction of L-Dopa of 80.4%. There was not effect attributed to the alkaline ions. Reducing particle size appears to be most effective as it has been shown by other workers.
Asunto(s)
Calor , Levodopa/análisis , Mucuna/química , Semillas/química , Análisis de Varianza , Hidróxido de Calcio/administración & dosificación , Cáusticos/administración & dosificación , Concentración de Iones de Hidrógeno , Hidróxidos/administración & dosificación , Indicadores y Reactivos/administración & dosificación , Levodopa/toxicidad , Modelos Lineales , Microondas , Proteínas de Plantas/análisis , Compuestos de Potasio/administración & dosificación , Hidróxido de Sodio/administración & dosificaciónRESUMEN
Levodopa (L-dopa), the biological precursor of catecholamines, is the most widely prescribed drug in the treatment of Parkinson's disease. The present work presents a proposal for the application of a gold screen-printed electrode an electrochemical sensor for monitoring L-dopa in stationary solution and a flow system. Using the electrooxidation of L-dopa at +0.63 V in acetate buffer pH 3.0 on a gold screen-printed electrode it is possible to obtain a linear calibration curve from 9.9 x 10(-5) to 1.2 x 10(-3) mol L(-1) and a detection limit of 6.8 x 10(-5) mol L(-1). Under amperometric conditions (E(app) = 0.8 V; flow rate = 14.1 mL min(-1); pH 3.0), an analytical calibration graph for l-dopa was obtained from 1.0 x 10(-6) mol L(-1) 6.6 x 10(-4) mol L(-1) with a detection limit of 9.9 x 10(-7) mol L(-1). The method was successfully applied to the determination of L-dopa in commercial dosage forms without any pre-treatment.
Asunto(s)
Dopaminérgicos/análisis , Electroquímica/instrumentación , Levodopa/análisis , Preparaciones Farmacéuticas/químicaRESUMEN
The simultaneous determination of levodopa and benserazide in pharmaceutical formulations is described, based on the application of multidimensional partial least-squares regression to the kinetic-spectrophotometric data provided by diode-array detection within a stopped-flow injection method where analytes react with periodate. Flow injection parameters were adequately optimized. Accurate analysis is performed with no sample pre-treatment steps, and with minimum experimental effort. Satisfactory recovery results were obtained on a number of synthetic and commercial samples, in the latter case including the comparison with liquid chromatography measurements.
Asunto(s)
Benserazida/análisis , Levodopa/análisis , Cromatografía Líquida de Alta Presión/métodos , Análisis de Inyección de Flujo/métodos , Análisis Multivariante , Espectrofotometría Ultravioleta/métodosRESUMEN
A flow injection spectrophotometric procedure was developed for determining levodopa in tablets. The determination of this drug was carried out by reacting it with lead(IV) dioxide immobilized in polyester resin packed in a solid-phase reactor and the dopachrome yielded was monitored at 520 nm. The analytical curve for levodopa was linear in the concentration range from 1.0x10(-4) to 1.0x10(-3) mol l(-1) with a detection limit of 8.0x10(-5) mol l(-1). The relative standard deviation (R.S.D.) was 0.2% for a solution containing 4.0x10(-4) mol l(-1) levodopa (n=10), and 90 determinations per hour were obtained.
Asunto(s)
Levodopa/análisis , Análisis de Inyección de Flujo , Plomo , ComprimidosAsunto(s)
Contaminación de Medicamentos , Levodopa/efectos adversos , Metoclopramida/efectos adversos , Trastornos del Movimiento/diagnóstico , Trastornos del Movimiento/etiología , Adulto , Anciano , Anciano de 80 o más Años , Argentina , Femenino , Humanos , Levodopa/análisis , Masculino , Metoclopramida/análisis , Persona de Mediana Edad , Trastornos del Movimiento/terapia , Perú , Vigilancia de Productos ComercializadosRESUMEN
The human dopamine D4 receptor (D4R) has received considerable attention because of its high affinity for the atypical antipsychotic clozapine and the unusually polymorphic nature of its gene. To clarify the in vivo role of the D4R, we produced and analyzed mutant mice (D4R-/-) lacking this protein. Although less active in open field tests, D4R-/- mice outperformed wild-type mice on the rotarod and displayed locomotor supersensitivity to ethanol, cocaine, and methamphetamine. Biochemical analyses revealed that dopamine synthesis and its conversion to DOPAC were elevated in the dorsal striatum from D4R-/- mice. Based on these findings, we propose that the D4R modulates normal, coordinated and drug-stimulated motor behaviors as well as the activity of nigrostriatal dopamine neurons.
Asunto(s)
Depresores del Sistema Nervioso Central/farmacología , Cocaína/farmacología , Dopaminérgicos/farmacología , Etanol/farmacología , Metanfetamina/farmacología , Narcóticos/farmacología , Receptores de Dopamina D2/genética , Ácido 3,4-Dihidroxifenilacético/metabolismo , Secuencia de Aminoácidos , Animales , Antipsicóticos/farmacología , Conducta Animal/efectos de los fármacos , Clozapina/farmacología , Cuerpo Estriado/anatomía & histología , Cuerpo Estriado/química , Cuerpo Estriado/metabolismo , Dopamina/metabolismo , Genotipo , Humanos , Levodopa/análisis , Levodopa/farmacocinética , Locomoción/efectos de los fármacos , Conducta Materna/efectos de los fármacos , Ratones , Ratones Noqueados , Datos de Secuencia Molecular , Actividad Motora/efectos de los fármacos , Mutagénesis Sitio-Dirigida/fisiología , Núcleo Accumbens/química , Núcleo Accumbens/metabolismo , Receptores de Dopamina D2/deficiencia , Receptores de Dopamina D4 , Sensibilidad y Especificidad , Sustancia Negra/anatomía & histología , Sustancia Negra/química , Sustancia Negra/metabolismo , Transcripción Genética/genéticaRESUMEN
A flow injection (FI) spectrophotometric method is proposed for the determination of L-dopa and carbidopa in pharmaceutical formulations. After selection of the extraction medium (e.g., buffer-to-tissue ratio, pH, buffer concentration, protective agents and/or stabilizers) and storage conditions, crude extract of sweet potato root [Ipomoea batatas (L.) Lam.] was used as an enzymatic source of polyphenol oxidase (Tyrosinase; catechol oxidase; EC.1.14.18.1) directly in the carrier. This enzyme catalyses the oxidation of these catecholamines to the corresponding dopaquinone. Further, dopaquinone undergoes a rapid spontaneous auto-oxidation to leucodopachrome, which is in turn oxidized to dopachrome; this last compound has a strong absorption at 480 and 360 nm for L-dopa and carbidopa, respectively. For the optimum extraction conditions found the enzyme activity of the crude extract did not vary for at least 5 months when stored at 4 degrees C and decreased by only 4-5% during an 8 h working period at 25 degrees C. The results obtained for L-dopa and carbidopa by the proposed enzymatic FI method were in close agreement with the label values (r1 = 0.9699 and r2 = 0.9999) and also with those obtained using a pharmacopeial method (r3 = 0.9675). The throughput was 26 samples h-1, and 2.30 ml of crude extract were consumed in each determination, corresponding to only 72 mg of the original sweet potato root. The detection limit (three times the signal blank/slope) was 1.5 x 10(-5) and 2.0 x 10(-5) mol l-1 for L-dopa and carbidopa, respectively; the recovery of L-dopa and carbidopa from three samples ranged from 98.6 to 106.3% of the added amount.
Asunto(s)
Dihidroxifenilalanina/análisis , Tecnología Farmacéutica , Carbidopa/análisis , Levodopa/análisis , Espectrofotometría/métodos , VerdurasAsunto(s)
Ratones , Animales , Masculino , Apomorfina/farmacología , Catalepsia/efectos de los fármacos , Haloperidol/farmacología , Levodopa/farmacología , Receptores Dopaminérgicos/efectos de los fármacos , Ácido Homovanílico/análisis , Apomorfina/uso terapéutico , Benserazida/uso terapéutico , Quimioterapia Combinada , Haloperidol/análisis , Levodopa/análisis , Levodopa/uso terapéutico , Ratones Endogámicos , Química EncefálicaRESUMEN
Mice were treated acutely or chronically with apomorphine (APO) (0.1 and 1.0 mg/kg) or L-DOPA (150 mg/kg) plus benserazide (50 mg/kg). The decrease of homovanillic acid (HVA) caused by APO was greater in acutely treated animals. After APO or L-DOPA pretreatment the animals received single doses of haloperidol (HALO) (1.5 mg/kg) or saline and were submitted to behavioral and biochemical analysis in order to determine changes in dopaminergic receptor sensitivity. When the acutely and chronically treated groups were compared, we found no difference in the cataleptogenic effect of HALO nor any difference in the effect of HALO in increasing brain HVA levels. The data suggest that chronic treatment with dopaminergic agonists leads to changes in receptor sensitivity for the agonist but not for the antagonist effect.