Discrete mapping of brain Mu and delta opioid receptors using selective peptides: quantitative autoradiography, species differences and comparison with kappa receptors

The opioid peptides, [3H]DAGO and [3H]DPDPE, bound to rat and guinea pig brain homogenates with a high, nanomolar affinity and to a high density of mu and delta receptors, respectively. [3H]DAGO binding to mu receptors was competitively inhibited by unlabelled opioids with the following rank order of potency: DAGO greater than morphine greater than DADLE greater than naloxone greater than etorphine much greater than U50488 much greater than DPDPE. In contrast, [3H]DPDPE binding to delta receptors was inhibited by compounds with the following rank order of potency: DPDPE greater than DADLE greater than etorphine greater than dynorphin(1-8) greater than naloxone much greater than U50488 much greater than DAGO. These profiles were consistent with specific labelling of the mu and delta opioid receptors, respectively. In vitro autoradiographic techniques coupled with computer-assisted image analyses revealed a discrete but differential anatomical localization of mu and delta receptors in the rat and guinea pig brain. In general, mu and delta receptor density in the rat exceeded that in the guinea pig brain and differed markedly from that of kappa receptors in these species. However, while mu receptors were distributed throughout the brain with "hotspots" in the fore-, mid- and hindbrain of the two rodents, the delta sites were relatively diffusely distributed, and were mainly concentrated in the forebrain with particularly high levels within the olfactory bulb (OB), n. accumbens and striatum. Notable regions of high density of mu receptors in the rat and guinea pig brain were the accessory olfactory bulb, striatal "patches" and "streaks," amygdaloid nuclei, ventral hippocampal subiculum and dentate gyrus, numerous thalamic nuclei, geniculate bodies, central grey, superior and inferior colliculi, solitary and pontine nuclei and s. nigra. Tissues of high delta receptor concentration included, OB (external plexiform layer), striatum, n. accumbens, amygdala and cortex (layers I-II and V-VI). Delta receptors in the guinea pig were, in general, similarly distributed to the rat, but in contrast to the latter, the hindbrain regions such as the thalamus, geniculate bodies, central grey and superior and inferior colliculi of the guinea pig were apparently more enriched than the rat. These patterns of mu and delta site distribution differed dramatically from that of the kappa opioid sites in these species studied with the peptide [125I]dynorphin(1-8).     

A photoactivatable naltrexone derivative labels glycoproteins of different molecular weight corresponding to the mu- and kappa-opioid receptors.

The synthesis and characterization of a novel opioid receptor photoaffinity probe [3H]naltrexyl urea phenylazido derivative ([3H]NUPA) is described. In the absence of light, [3H]NUPA binds with high affinity in a reversible and saturable manner to rat brain and guinea pig cerebellum membranes. Dissociation constants and binding capacities (Scatchard plots) are 0.11 nM and 250 fmol/mg of protein for rat brain and 0.24 nM and 135 fmol/mg of protein for guinea pig cerebellum. Competition experiments indicate that this ligand interacts with high affinity at both mu- and kappa-opioid binding sites while exhibiting low affinity at delta sites (Ki = 21 nM). On irradiation, [3H]NUPA incorporates irreversibly into rat brain and guinea pig cerebellum membranes. SDS gel electrophoresis of rat brain membranes reveals specific photolabeling of a 67-kDa molecular mass band. Conversely, a major component of 58 kDa and a minor component of 36 kDa are obtained from [3H]NUPA-labeled guinea pig cerebellum membranes. Different photolabeling patterns are obtained in rat brain (mu/delta/kappa, 4/5/1) and guinea pig cerebellum (mu+delta/kappa, 1,5/8,5) membranes in the presence of selective opioid ligands indicating labeling of mu and kappa sites, respectively. Thus, [3H]NUPA behaves as an efficient photoaffinity probe of mu- and kappa-opioid receptors, which are probably represented by distinct glycoproteins of 67 and 58 kDa, respectively.     

Comparison of the endogenous heptapeptide Met-enkephalin-Arg6-Phe7 binding in amphibian and mammalian brain

In previous communications [4, 38] we published that [3H]Met-enkephalin-Arg6-Phe7 (MERF) binds to opioid (kappa2 and delta) and sigma2 sites in frog and rat brain membrane preparations, however no binding to kappa1 sites could be established. In the present paper we compare the frog, rat and guinea pig brain membrane fractions with respect to their MERF binding data. No qualitative differences were found between the three species but specific binding of labelled MERF was maximal in frog brain and lowest in guinea pig brain, which corresponds to their kappa2 opioid receptor distribution. The naloxone resistant binding was also present in all investigated species and varied from 25% in frog and guinea pig cerebrum, to 50% in rat cerebrum and cerebellum, but no naloxone inhibition was found in guinea pig cerebellum where no kappa2 opioid receptors have been found. The presence of sigma2-like receptor was demonstrated in each investigated membrane fraction with displacement experiments using (-)N-allyl-normetazocine as competitor of tritiated MERF. It was shown that this site was responsible for 60-80% of [3H]MERF binding. The remaining part of the naloxone resistant labelled MERF binding could be displaced only with endogenous opioid peptides as met-enkephalin, dynorphin and beta-endorphin. The eventual physiological role of multiple MERF receptors is discussed.     

Characterization of kappa1-opioid receptor binding in human insular cortex.

Mesolimbic dopaminergic neurotransmission is modulated by dynorphin peptides binding to kappa-opioid receptors. The interaction between dynorphin and dopamine systems makes the kappa-opioid receptor a potential drug discovery target for the development of therapeutic agents for schizophrenia and drug abuse. This study reports the specificity and parameters of [3H]U69593 binding in the insular cortex, a representative corticolimbic area of the human brain. The results demonstrate that the radioligand [3H]U69593 labels a single population of receptors in human insular cortex with an affinity in the low nanomolar range. The pharmacological profile for inhibition of [3H]U69593 binding was determined in this brain region using drugs known to bind to mu, kappa and delta opioid receptors. The results show that kappa-opioid selective agonists and antagonists inhibit binding of this ligand in human brain with comparable affinities and rank order as previously described for rat and guinea pig brain and the cloned kappa1-opioid receptor subtype.     

Focal Stimulation of the Mossy Fibers Releases Endogenous Dynorphins That Bind K1-Opioid Receptors in Guinea Pig Hippocampus

Physiological release of endogenous opioids in guinea pig hippocampal slices was detected in an in vitro competition binding assay using [3H]U69,593, a kappa 1-selective radioligand. Veratridine-induced opioid release caused a decrease in [3H]U69,593 binding that was blocked by either tetrodotoxin addition or the removal of calcium from the incubation buffer. Focal electrical stimulation of opioid peptide-containing afferent pathways resulted in a decrease in [3H]U69,593 binding, whereas stimulation of a major afferent lacking endogenous opioid immunoreactivity had no effect. The addition of 6-cyano-7-nitroquinoxaline-2,3-dione blocked the reduction in [3H]U69,593 binding caused by perforant path stimulation, but not the reduction caused by mossy fiber stimulation, suggesting that the primary source of endogenous kappa ligands was likely to be the dentate granule cells. Antisera against dynorphin A(1-8) or dynorphin B peptides inhibited the effects of mossy fiber stimulation in the [3H]U69,593 displacement assay. Antisera against other prodynorphin- and proenkephalin-derived opioid peptides had no effect. As shown by receptor autoradiography, the distribution of kappa 1 binding sites was limited to the molecular layer of the dentate gyrus and the presubiculum region of temporal hippocampal slices. These results indicate that prodynorphin-derived opioids released under physiological conditions from the mossy fibers act at kappa 1 receptors in the guinea pig dentate gyrus.     

Beta-endorphin-like and adrenocorticotropin-like materials in heart tissues of the rat, gerbil, hamster and guinea pig

1. Heart tissues of several rodent species including the rat, gerbil (Meriones unguiculatus), hamster (Mesocricetus auratus) and guinea pig (Cavia porcellus) were extracted with an acetone-water-HCl mixture. An acid acetone powder was obtained by adding a copious volume of acetone to the extract. 2. Rat heart acid acetone powder was subjected to ion exchange chromatography on CM-cellulose. Gerbil heart acid acetone powder was subjected to salt fractionation, gel filtration on Sephadex G-10 and then ion exchange chromatography on CM-cellulose. Hamster and guinea pig heart acid acetone powders were subjected to gel filtration on Sephadex G-25. 3. The fractions were assayed for the ability to stimulate corticosterone production in isolated rat adrenal decapsular (zona fasciculata, zona reticularis and medulla) cells, to displace D-ala2-D-leu5-(tyrosyl-3,5-3H) enkephalin from binding to rat brain membranes, and to inhibit 125I-human beta-endorphin from binding to its antibodies. 4. The widespread occurrence of beta-endorphin-like immunoreactivity among the rat heart CM-cellulose fractions may reflect different species of beta-endorphin. The fraction with the highest beta-endorphin-like immunoreactivity and opiate receptor binding activity was strongly adsorbed on CM-cellulose. 5. In hamster and guinea pig hearts, beta-endorphin-like immunoreactivity and opiate receptor binding activity were distributed among high molecular weight and low molecular weight fractions. 6. In gerbil hearts, opiate receptor binding activity was present in fractions unretarded on Sephadex G-10 (i.e. with a molecular weight greater than 700) as well as in the retarded fractions (i.e. with a molecular weight smaller than 700).(ABSTRACT TRUNCATED AT 250 WORDS)     

A Monoclonal Antibody Recognizing K- but Not γ- and δ-Opioid Receptors

A monoclonal antibody (mAb), KA8 that interacts with the kappa-opioid receptor binding site was generated. BALB/c female mice were immunized with a partially purified kappa-opioid receptor preparation from frog brain. Spleen cells were hybridized with SP2/0AG8 myeloma cells. The antibody-producing hybridomas were screened for competition with opioid ligands in a modified enzyme-linked immunosorbent assay. The cell line KA8 secretes an IgG1 (kappa-light chain) immunoglobulin. The mAb KA8 purified by affinity chromatography on protein A-Sepharose CL4B was able to precipitate the antigen from a solubilized and affinity-purified frog brain kappa-opioid receptor preparation. In competition studies, the mAb KA8 decreased specific [3H]ethylketocyclazocine ([3H]EKC) binding to the frog brain membrane fraction in a concentration-dependent manner to a maximum to 72%. The degree of the inhibition was increased to 86% when mu- and delta-opioid binding was suppressed by 100 nM [D-Ala2,NMe-Phe4,Gly-ol]-enkephalin (DAGO) and 100 nM [D-Ala2,L-Leu5]-enkephalin (DADLE), respectively, and to 100% when mu-, delta-, and kappa 2-sites were blocked by 5 microM DADLE. However, the mu-specific [3H]DAGO and the delta-preferring [3H]DADLE binding to frog brain membranes cannot be inhibited by mAb KA8. These data suggest that this mAb is recognizing the kappa- but not the mu- and delta-subtype of opioid receptors. The mAb KA8 also inhibits specific [3H]naloxone and [3H]EKC binding to chick brain cultured neurons and rat brain membranes, whereas it has only a slight effect on [3H]EKC binding to guinea pig cerebellar membranes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of mu and delta but not kappa opioid binding to membranes by Fab fragments from a monoclonal antibody directed against the opioid receptor

Fab fragments from a monoclonal antibody, OR-689.2.4, directed against the opioid receptor, selectively inhibited opioid binding to rat and guinea pig neural membranes. In a titratable manner, the Fab fragments noncompetitively inhibited the binding of the mu selective peptide [D-Ala2,(Me)Phe4,Gly(OH)5][3H] enkephalin and the delta selective peptide [D-Pen2,D-Pen5] [3H]enkephalin (where Pen represents penicillamine) to neural membranes. In contrast, kappa opioid binding, as measured by the binding of [3H]bremazocine to rat neural membranes and guinea pig cerebellum in the presence of mu and delta blockers, was not significantly altered by the Fab fragments. In addition to blocking the binding of mu and delta ligands, the Fab fragments displaced bound opioids from the membranes. When mu sites were blocked with [D-Ala2,(Me)Phe4,Gly(OH)5]enkephalin, the Fab fragments suppressed the binding of [D-Pen2,D-Pen5][3H]enkephalin to the same degree as when the mu binding site was not blocked. The Fab fragments also inhibited binding to the mu site regardless of whether or not the delta site was blocked with [D-Pen2,D-Pen5]enkephalin. This monoclonal antibody is directed against a 35,000-dalton protein. Since the antibody is able to inhibit mu and delta binding but not kappa opioid binding, it appears that this 35,000-dalton protein is an integral component of mu and delta opioid receptors but not kappa receptors.     

Species differences in the stereoselectivity of kappa opioid binding sites for [3H]U-69593 and [3H]ethylketocyclazocine.

Stereoselectivity of the binding sites for the specific kappa-opioid agonist [3H]U-69593, a benzeneacetamido based ligand was investigated in membrane suspension prepared from frog and rat brain, as well as guinea pig cerebellum, using the pure chiral forms of different unlabelled opiates. The ligand binding sites showed stereospecificity with at least three orders of magnitude differences in the affinities (measured as Ki values) of the opioid stereoisomer pairs both in rat and guinea pig membrane fractions. However, in frog brain membranes there was no substantial difference in potencies of the (-) and (+) isomers competing for the [3H]U-69593 binding sites. Another type of the kappa-site preferring opioid ligand, [3H]ethylketocyclazocine, a benzomorphan derivative was able to discriminate between (-) and (+) forms of the same compounds even in frog brain membrane preparation. Our data concerning binding profile of [3H]U-69593 in frog brain membranes are consistent with the observation that kappa opioid binding sites in frog (Rana esculenta) brain differ from those kappa-sites found in mammalian brains.     

Distribution of serotonin immunoreactive neurons in the brainstem of the hamster, guinea pig, rabbit, and rat

Immunohistochemical techniques were employed to study the distribution of serotonin (5-HT) immunoreactive neurons in the brainstem of the hamster, guinea pig, rabbit and rat. 5-HT neurons were principally found to be concentrated in the midline raphe nuclei, particularly, the raphe pallidus, raphe obscurus, raphe magnus, raphe median, raphe pontis and raphe dorsalis nuclei. Characteristically, these cell bodies are displayed in bands or wing-like patterns which extend laterally from the raphe into reticular formations. The formations often appear to blend with the catecholamine system. They are particularly evident in the brainstems of the rabbit and hamster which contain wider and more lateral extensions of the serotonergic (5-HT) neurons than those observed in the brainstems of the rat and guinea pig. The widespread distribution of 5-HT immunoreacted cell bodies in the brainstem shows that there are significant prospects of 5-HT in neuronal activities.     

Non-specific prostaglandin antagonism by 8-ethoxycarbonyl-5-methyl-10, 11-dihydro-PGA1-ethyl ester (HR 546) on some smooth muscle preparations in vitro

The ability of 8-ethoxycarbonyl-10, 11 dihydro-A-prostaglandin(HR 546) to antagonise smooth muscle contracting effect of prostaglandins E2 and F2alpha on isolated preparations of rat and hamster stomach fundus, guinea pig ileum and gerbil colon has been studied. HR 546 was found to be a potent, non-specific, probably competitive, prostaglandin antagonist on these four smooth muscle preparations.     

Comparative mapping of opioid receptors and enkephalin immunoreactive nerve terminals in the rat hippocampus. A radiohistochemical and immunocytochemical study

Opioid receptors can be localized to the hippocampal formation of the rat by autoradiography. The binding of 3H-enkephalinamide to fixed and mounted tissue sections has all the characteristics associated with binding to opioid receptors. It is saturable, of high affinity and displays stereospecificity. The opioid receptor distribution shows striking regional variation throughout the hippocampal formation. Areas with high density include the pyramidal cell layer of both regio superior (CA1) and regio inferior (CA3), stratum moleculare of the hippocampus, the cell layer of subiculum, the superficial part of presubiculum and the deep layer (VI) of the medial and lateral entorhinal cortices. Areas with low to medium densities include regions corresponding to the dendritic field of the pyramidal cells (str. oriens, str. radiatum and the mossy fiber zone), the dentate granule cell layer and the molecular layer of the dentate area. Enkephalin-like immunoreactivity is detected in both intrinsic neuronal systems: 1) the mossy fibers which terminate on the proximal part of the CA3 pyramidal cell dendrites and on CA4 pyramidal cells, 2) cell bodies with multiple short processes, probably interneurons, dispersed throughout the hilus of the dentate area, the pyramidal cell layer of hippocampus, the str. radiatum, and occasionally in the str. moleculare and in the str. oriens, and extrinsic neuronal systems: 1) the lateral perforant path and 2) the lateral temporo-ammonic tract. Thus, the hippocampus contains intrinsic systems of enkephalin-like immunoreactive nerve terminals which may exert their effect on the opioid receptors with a localization corresponding to the pyramidal cells and their apical dendrites. Extrinsic enkephalinergic systems corresponding to the terminal fields of the lateral perforant path and the temporoammonic tract, both of entorhinal origin, may influence the opioid receptors located in the molecular layer of the dentate area, and in the molecular layer of the hippocampus and the subiculum. Thus, the enkephalin-like immunoreactive nerve terminals are all located in areas which contain opioid binding sites. This suggests that the "opioid peptide-opioid receptor" systems may regulate hippocampal neuronal activity via neurotransmission or neuromodulation. However, a high or medium number of opioid binding sites occur over the pyramidal cell bodies and the dentate granule cell bodies, and these opioid binding sites are not in close contact with the major enkephalinergic systems.(ABSTRACT TRUNCATED AT 400 WORDS)     

An immunological study of δ-aminolevulinic acid dehydratase specificity consistent with the phylogeny of species

Rabbit antibody directed to homogeneously purified mouse liver δ-aminolevulinic acid dehydratase cross-reacted with the enzyme in erythrocytes, spleen, kidney and brain in the mouse. The antibody also cross-reacted with the enzyme in the rat, hamster and gerbil, but not in the rabbit, guinea pig, cattle, chick embryo, and human. In contrast, rabbit antibody against the human enzyme partially recognized the monkey enzyme, but not the enzyme in the other species. The species specificity of δ-aminolevulinic acid dehydratase in this study was consistent with the phylogenetic evolution of the species examined.     

Comparative histochemical investigation of the organ of Corti in the kangaroo rat, gerbil, and guinea pig

The distribution of carbohydrates, proteins, and lipids was studied in the cochleae of kangaroo rat, gerbil, and guinea pig using both fixed paraffin sections and fresh-frozen cryostat sections. Enzyme distribution in the cochleae of the three speices was studied with both EDTA-decalcified and undecalcified fresh-frozen cryostat sections. Although the cochleae of the three species are morphologically different, their distributions of proteins, carbohydrates, and lipids are similar. The zona pectinata of the basilar membrane—which is hypertrophied in the kangaroo rat and gerbil but normal in the guinea pig—stains the same in all three species. The unique, flaskshaped Hensen cells of the kangaroo rat contain more protein than do the normal Hensen cells of the gerbil and guinea pig. At least some of the protein in the kangaroo rat Hensen cells is in the form of carboxylic esterases which are not affected by 10^?4 M eserine, but are inhibited by 10^?2 M eserine and 10^?6 M E600. More than one population of carboxylic esterases is indicated by this reaction to inhibitors and by the results of enzyme distribution tests which used different substrates. A high concentration of malate dehydrogenase in the kangaroo rat Hensen cells may be related to the synthesis of carboxylic esterases. The possible role of these esterases in cochlear functioning is discussed.     

Kappa-receptor selective binding of opioid ligands with a heterocyclic bicyclo[3.3.1]nonan-9-one structure

Previous pharmacological results have suggested that members of the heterocyclic bicyclo[3.3.1]nonan-9-one-like compounds are potent kappa-opioid receptor specific agonists. One lead molecule of this series. called compound 1 (dimethyl 7-methyl-2,4-di-2-pyridyl-3.7-diazabicyclo[3.3.1]nonan-9-one-1,5-dicarboxylate) exhibited high affinity for [3H]ethylketocyclazocine and [3H]U-69.593 binding sites in guinea pig cerebellar membranes which known to be a good source for kappa1 receptors. It was shown by molecular modelling that heterocyclic bicyclo[3.3.1]nonan-9-ones fit very well with the structure of ketazocine, a prototypic kappa-selective benzomorphan compound; when compared to the arylacetamide structure of U-69.593, a specific kappa1-receptor agonist, a similar geometry was found with a slightly different distribution of the charges. It is postulated, that the essential structural skeleton involved in the opioid activity is an aryl-propyl-amine element distributed along the N7-C6-C5-C4-aryl bonds.     

Characterization of [3H]Met-enkephalin-Arg6-Phe7 binding to multiple sites in rat and guinea pig cerebellum

[3H]Met-enkephalin-Arg6-Phe7 (MERF) has been shown to label opioid (kappa2 and delta) and sigma2 sites in rat and frog brain membrane preparations, and no specific binding to kappa1 opioid receptors could be established (refs. 6 and 8). In this study the binding was examined in rat cerebellar membranes which are relatively rich in kappa2-sites, and in guinea pig cerebellar preparations where kappa1 opioid receptors are almost exclusively present. In accordance with our previous results, [3H]MERF binding could not be displaced in guinea pig cerebellar membranes neither with U-69,593 nor with naloxone or levorphanol suggesting no interaction with opioid sites, nevertheless a Kd of 2.8 nM was calculated in cold saturation experiments. In rat cerebellar membrane fractions about the half of the specific [3H]MERF binding sites was inhibited by opiate alkaloids such as naloxone, ethylketocyclazocine, or bremazocine. This portion of the heptapeptide binding sites was stereoselective as demonstrated by the difference in the affinities of the enantiomeric compounds levorphanol and dextrorphan, therefore it would represent an opioid site. In both tissues (-)N-allyl-normetazocine (SKF-10,047), which is also considered as sigma2 ligand, displayed the highest affinities. Among opioid peptides beta-endorphin and dynorphin(1-13) showed the highest potencies, displacing [3H]MERF also from its non-opioid sites. It was concluded therefore that [3H]MERF does not bind to kappa1 sites, and besides kappa2-opioid sites substantial binding to peptide preferring non-opioid sites, and/or sigma2 receptors also occurs.     

Orally administered kappa as well as mu opiate agonists delay gastrointestinal transit time in the guinea pig

When an orally administered opiate agonist is systemically bioavailable, the relative activity of that opioid in delaying gastrointestinal transit (GIT) depends on its relative action at central and peripheral sites. This in turn depends on the density of opioid receptor specific subtypes at those sites of action in the species under study. In rats the kappa selective agonist U-50,488H has no effect on GIT. We have found that this same agonist is equipotent to mu agonists morphine and 1-methadone in delaying the orocecal transit of a charcoal meal when administered orally to guinea pigs. Thus, both kappa as well as mu receptor subtypes are involved in the mechanisms of opiate induced slowing of GIT in the guinea pig in contrast to the rat. Interspecies differences must be considered when determining the contribution of opiate receptor subtypes to the mechanisms of opiate-induced constipation.     

A novel opioid peptide, leumorphin, acts as an agonist at the kappa opiate receptor

The primary structure of the common precursor of porcine beta-neo-endorphin and dynorphin (preproenkephalin B) has shown the existence of a third leucine-enkephalin (leu-enkephalin) sequence with a C-terminal extension of 24 amino acids. This nonacosapeptide, named leumorphin, was approximately 70 times more potent than leu-enkephalin in inhibiting the contraction of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum. This action of leumorphin, like those of beta-neo-endorphin and dynorphin, was antagonized less effectively by naloxone than that of leu-enkephalin, but more effectively by Mr2266, an antagonist relatively specific for the kappa type opiate receptor. The inhibitory action of leumorphin or beta-neo-endorphin on the contraction of the guinea pig ileum muscle strip was reduced in a dose-dependent manner by pretreatment with dynorphin and vice versa. Leumorphin as well as beta-neo-endorphin and dynorphin inhibits the contraction of the rabbit vas deferens which is known to have only the kappa type opiate receptor. This action was also effectively antagonized by Mr2266. It is concluded that leumorphin has potent opioid activity and acts at the kappa receptor, like other opioid peptides derived from preproenkephalin B.     

Cloning and characterization of the hamster and guinea pig nicotinic acid receptors

In this study, we present the identification and characterization of hamster and guinea pig nicotinic acid receptors. The hamster receptor shares approximately 80-90% identity with the nucleotide and amino acid sequences of human, mouse, and rat receptors. The guinea pig receptor shares 76-80% identity with the nucleotide and amino acid sequences of these other species. [(3)H]nicotinic acid binding affinity at guinea pig and hamster receptors is similar to that in human (dissociation constant = 121 nM for guinea pig, 72 nM for hamster, and 74 nM for human), as are potencies of nicotinic acid analogs in competition binding studies. Inhibition of forskolin-stimulated cAMP production by nicotinic acid and related analogs is also similar to the activity in the human receptor. Analysis of mRNA tissue distribution for the hamster and guinea pig nicotinic acid receptors shows expression across a number of tissues, with higher expression in adipose, lung, skeletal muscle, spleen, testis, and ovary.