Quantitative receptor autoradiography using [3H]ultrofilm: application to multiple benzodiazepine receptors

Autoradiographic mapping of spirodecanone binding-sites in the hippocampal region of the rat. Evidence for a localization on intrinsic neurons

The method of in vitro receptor autoradiography was used to map the distribution of spirodecanone binding-sites in the rat hippocampal region. Incubations of horizontal sections through the hippocampus with [3H]spiperone (1 nM) resulted in dense labeling restricted to the pyramid cell layer in CA1, the parasubiculum and layers I and II of the entorhinal area (EA), while the other hippocampal subfields contained moderate to low binding. ADTN, serotonin, ketanserin and mianserin (1-100 microM) all failed to displace the [3H]spiperone in the layers of densest binding, while displacement of the [3H]spiperone binding occurred with high concentrations of spiperone (1 microM) and haloperidol (100 microM). Intra-entorhinal injections of the neurotoxin ibotenic acid prevented [3H]spiperone binding to layers I and II of the EA, while transections of septal and commissural afferents or the degeneration of serotonin and noradrenaline terminals appeared not to reduce the [3H]spiperone binding in any part of the hippocampal region. These findings suggest that spirodecanone binding-sites are located on intrinsic neurons in restricted laminae of the hippocampal region.

Quantitative autoradiography of muscarinic cholinergic receptors in the rat brain

Using the in vitro autoradiographic technique with tritium-sensitive LKB sheet film and the liquid scintillation counting method, the distribution and the binding parameters of the muscarinic cholinergic receptors (MChR) were determined in various discrete regions of the rat brain. The results obtained in the present study were as follows: (1) Specific binding of [3H]QNB to the slide-mounted tissue sections increased slowly when incubated at room temperature; saturation occurred 2 h after incubation. Only 23% of [3H]QNB bound to the tissue section was dissociated 5 h after the addition of 20 microM atropine to the medium. These findings were very different from those obtained in the study using the tissue homogenates. (2) The regional distribution of MChR was determined using both autoradiographic and liquid scintillation counting methods. The distribution of MChR was heterogeneous, with highest densities in the striatum and nucleus accumbens and lowest in the globus pallidus, nucleus interpeduncularis and nucleus septi. Moreover, MChR were unevenly distributed within the subfields of each region. (3) In saturation binding studies using the slide-mounted tissue sections of 20 micron thickness the (Kd)app-values were similar but not exactly identical in 5 discrete regions, i.e. the striatum, somatosensory cortex, hippocampus (the subiculum + CA1 field), nucleus accumbens and gyrus dentatus, determined in the present study. The (Kd)app-value of each region was about 700 pM which was about 20 times higher than that obtained in the study using the tissue homogenates. However (Kd)app-values obtained with 5 and 10 micron tissue sections were approximately 3-fold lower.

Computer-assisted video analysis of [3H]spiroperidol binding autoradiographs

A computer-based video image processing system is described that quantifies the binding of the neuroleptic drug, [3H]spiroperidol, to rat forebrain sections. Adjacent sections were incubated in buffer containing [3H]spiroperidol or [3H]spiroperidol plus 1 microM (+)-butaclamol and exposed to tritium-sensitive film to produce autoradiographs of total binding or non-butaclamol-displaced ('non-specific') binding. The image processing system digitized each autoradiograph, attenuated geometric distortion and unevenness of background illumination, and reassigned the digitized image intensities (gray values) to be a linear function of fmol [3H]spiroperidol bound per mg protein by using a calibration curve generated from 3H-containing tissue standards. An image of butaclamol-displaced ('specific') [3H]spiroperidol binding was produced by subtracting the linearized image of non-specific binding from the superimposed image of total binding. An image of percent specific [3H]spiroperidol binding was obtained by dividing the image of specific binding by the superimposed image of total binding. The computer-derived images, which could be displayed in gray tones or pseudocolor-coded, revealed that the greatest amounts of specific [3H]spiroperidol binding (1000-3000 fmol/mg protein; 60-80% specifically bound) were located in layers 5A and 5C of the neocortex, the claustrum, and in the caudate-putamen, where a lateral-to-medial binding gradient occurred. [3H]Spiroperidol was bound to a lesser extent (400-1000 fmol/mg protein; 31-51% specifically bound) to the medial nucleus accumbens septi or olfactory tubercle and without measurable specificity to the lateral septum, anterior commissure, corpus callosum, or superficial neocortex. These procedures are particularly useful for the quantitative and visual analysis of autoradiographs in which [3H]ligand binding is associated with more than a single site.

Distribution of alpha 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents

Using [3H]para-aminoclonidine, alpha 2 adrenergic binding sites have been mapped in the rat and human CNS using in vitro labeling autoradiographic techniques. In both the rat and human thoracic spinal cord, high densities of alpha 2 binding sites were associated with the substantia gelatinosa and the intermediolateral cell column. In the rat medulla, high binding site density was observed in the medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, raphe pallidus and the substantia gelatinosa of the trigeminal nucleus, while lower levels of specific binding were found in the lateral and ventrolateral medulla. In the human, a similar distribution was observed. However, significantly lower levels of specific binding were seen in the medial nts as opposed to the dmv. In the rat, high levels of specific binding were seen at pontine and midbrain levels in the locus coeruleus, parabrachial nucleus and periaqueductal gray. In the forebrain, several hypothalmic and limbic regions, including the paraventricular and arcuate nuclei of the hypothalamus, the central, medial and basal nuclei of the amygdala, lateral septum and bed nucleus of the stria terminalis and pyriform, entorhinal and insular cortex were labeled. Each of these regions are involved in either modulating autonomic functions directly or integrating somatosensory and/or affective function with autonomic mechanisms. Further, these regions are interrelated by reciprocal connections, and neurons that utilize noradrenaline or adrenaline as their neurotransmitter form a vital part of these connections. Thus, these functional, anatomical and neurochemical correlates of the alpha 2 binding site distribution establish a neurological basis for the complex pharmacological effects of centrally acting alpha 2 agonists.

Autoradiographic visualization of angiotensin-converting enzyme in rat brain with [3H]captopril: localization to a striatonigral pathway

We have visualized angiotensin-converting enzyme (ACE; dipeptidyl carboxypeptidase, peptidylpeptide hydrolase, EC 3.4.15.1) in rat brain by in vitro [3H]captopril autoradiography. [3H]Captopril binding to brain slices displays a high affinity (Kd = 1.8 X 10(-9) M) and a pharmacological profile similar to that of ACE activity. Very high densities of [3H]captopril binding were found in the choroid plexus and the subfornical organ. High densities were present in the caudate putamen and substantia nigra, zona reticulata. Moderate levels were found in the entopeduncular nucleus, globus pallidus, and median eminence of the hypothalamus. Lower levels were detectable in the supraoptic and paraventricular nuclei of the hypothalamus, the medial habenula, the median preoptic area, and the locus coeruleus. Injection of ibotenic acid or colchicine into the caudate putamen decreased [3H]captopril-associated autoradiographic grains by 85% in the ipsilateral caudate putamen and by greater than 50% in the ipsilateral substantia nigra. Thus, ACE in the substantia nigra is located on presynaptic terminals of axons originating from the caudate putamen, and ACE in the caudate putamen is situated in neuronal perikarya or at the terminals of striatal interneurons. The lack of effect of similar injections into the substantia nigra confirmed that the caudate putamen injections did not cause trans-synaptic changes. The presence of [3H]captopril binding is consistent with an ACE-mediated production of angiotensin II in some brain regions. Although [3H]captopril autoradiography reveals ACE in a striatonigral pathway, there is no evidence for angiotensin II involvement in such a neuronal pathway.

Quantitative autoradiography of beta 1- and beta 2-adrenergic receptors in rat brain

We have used quantitative autoradiography to localize in rat brain beta 1- and beta 2-adrenergic receptors. These receptors were labeled in vitro with 125I-labeled pindolol, an antagonist of beta-adrenergic receptors that binds nonselectively to both beta 1 and beta 2 subtypes. The selective inhibition of 125I-labeled pindolol binding with specific antagonists of beta 1 and beta 2 receptors allowed the visualization of beta-adrenergic receptor subtypes. High levels of beta 1 receptors were observed in the cingulate cortex, layers I and II of the cerebral cortex, the hippocampus, the Islands of Calleja, and the gelatinosus, mediodorsal, and ventral nuclei of the thalamus. High levels of beta 2 receptors were found in the molecular layer of the cerebellum, over pia mater, and in the central, paraventricular, and caudal lateral posterior thalamic nuclei. Approximately equal levels of beta 1 and beta 2 receptors occurred in the substantia nigra, the olfactory tubercle, layer IV of the cerebral cortex, the medial preoptic nucleus, and all nuclei of the medulla. The pronounced differences in the ratio of beta 1 to beta 2 receptors among brain regions suggests that the subtypes of beta-adrenergic receptors may play different roles in neuronal function.

Localization of cysteine sulfinic acid uptake sites in rat brain by quantitative autoradiography

In vitro autoradiography was employed to localize and quantify Na-dependent binding sites of [35S]cysteic acid (CA), an analog of cysteine sulfinic acid (CSA). The heterogeneous anatomical distribution and pharmacological specificity of [35S]CA differs from that of the glutamate/aspartate marker D-[3H]aspartate, and appears to represent a specific uptake site for CSA. These results suggest that CSA may act as an excitatory transmitter in the central nervous system.

Autoradiographic evidence for the heterogeneity of 5-HT1 sites in the rat brain

The distribution of the binding sites of a new, potent agonist of serotonin (5-HT), 8-OH-N,N-dipropyl-2-aminotetralin (PAT), was studied in the rat brain with the quantitative autoradiographic technique utilizing tritium-sensitive LKB film. The localization of [3H]PAT binding sites was very similar to that of [3H]5-HT binding sites, except in some discrete regions (choroid plexus, striatum, area preoptica lateralis, subiculum, and substantia nigra), which exhibited very low levels of labeling with [3H]PAT and high levels with [3H]5-HT. These results indicate that 5-HT1 receptors are heterogeneous, and that [3H]PAT recognizes only a 5-HT1 subclass (called 5-HT1A).

Mapping of beta-adrenergic receptors in rat lung: effect of isoproterenol

beta-Adrenergic receptors were mapped in the rat lung using a light microscopic technique that identifies binding sites for 3H-labeled dihydroalprenolol in intact slide-mounted tissue sections, which were also used for biochemical analysis of ligand binding. Grain density measurements were combined with morphometric data. These results indicate that, although over 97% of the specific binding is to cells of alveolar walls, beta-adrenergic receptors are present and can be quantified in tissues that represent a small fraction of total lung mass, such as bronchial epithelium and smooth muscle of bronchial walls and pulmonary vessels. Repeated administration of isoproterenol decreased the receptor number, as determined biochemically and from grain density measurements, in all anatomic regions studied, but did not alter the distribution of binding.

Receptor autoradiography: the last ten years

The development of techniques for the microscopic visualization of receptor binding and other binding sites using autoradiography is reviewed. The application of these techniques to a number of pharmacological problems, particularly in the field of neuropharmacology is illustrated. The limitations and future trends in the field of receptor visualization are commented upon.

Comparative in vitro/in vivo autoradiography using the opiate ligand 3H-(-)-bremazocine

Autoradiograms of rat brain sections were compared obtained from animals receiving a tritiated drug through intravenous injection or from precut sections incubated in vitro. The benzomorphan analogue 3H-(-)-bremazocine was used as ligand and its distribution to all different opioid binding sites was followed. Although the general distribution of opioid binding sites visualized on 3H-LKB ultrofilms was independent of the methodological approach used, the maximal number of such sites (Bmax) was greater in brain sections incubated in vitro than after in vivo drug application. Since the number of binding sites is highly dependent on the particular incubation condition used, this finding has no further relevance.

Quantitative receptor autoradiography: application to the characterization of multiple receptor subtypes

In vitro autoradiographic techniques combined with computer assisted microdensitometry were used to analyze the characteristics and distribution of multiple recognition sites for the neurotransmitters acetylcholine (M1 and M2) and serotonin (5-HT1A and 5-HT1B). For this purpose, binding competition experiments were performed using non-subtype selective 3H-labeled ligands and selective unlabeled compounds. Consecutive tissue sections were incubated in the presence of increasing concentrations of displacers. By using this approach, maximal densities of binding sites, as well as competition profiles of several drugs could be analyzed and quantified in microscopic brain areas. Our results reveal the presence of brain structures enriched in one class of muscarinic or serotonergic-1 recognition sites. This provides a tool for better characterization of the proposed "subtype-selective" ligands and suggests physiological functions for these receptor subtypes. It is concluded that quantitative autoradiographic techniques provide a level of anatomical and pharmacological information on neurotransmitter receptor subtypes, which is difficult to attain using membrane binding studies.

Normalization of spiroperidol binding in the denervated rat striatum by homologous grafts of substantia nigra

Transplantation of embryonic substantia nigra into the adult rat brain decreases the motor asymmetry that is produced by dopamine receptor supersensitivity after a unilateral lesion of the substantia nigra. The authors report that this effect of transplantation is specific to grafts of substantia nigra. They also report that, in conjunction with the decrease in motor asymmetry, these grafts cause postsynaptic dopaminergic binding sites to return to normal density as measured by tritiated spiroperidol autoradiography. Thus, in animals with brain lesions, grafts of substantia nigra produce a long-term alteration in the functional status of host brain cell receptors that is associated with a reduction in the behavioral deficit.

Anatomical distributions of four pharmacologically distinct 3H-L-glutamate binding sites

Glutamate is thought to serve as a major excitatory neurotransmitter throughout the central nervous system (CNS); electrophysiological studies indicate that its action is mediated by multiple receptors. Four receptors have been characterized by their selective sensitivity to N-methyl-D-aspartate (NMDA), kainic acid (KA), quisqualic acid (QA) and 2-amino-4-phosphonobutyric acid (APB). Electrophysiological evidence indicates that these receptors are all present in the rat hippocampus and that the anatomically discrete synaptic fields within the hippocampus exhibit differential sensitivity to the selective excitatory amino acid agents. Thus, we have used the hippocampus as a model system to investigate possible subpopulations of 3H-L-glutamate binding sites. By using quantitative autoradiography, the pharmacological specificity of 3H-L-glutamate binding in discrete terminal fields was determined. We report here that there are at least four distinct classes of 3H-L-glutamate binding sites which differ in their anatomical distribution, pharmacological profile and regulation by ions. Two of these sites seem to correspond to the KA and NMDA receptor classes, and a third site may represent the QA receptor. The fourth binding site does not conform to present receptor classifications. None of these binding sites corresponds to the major glutamate binding site observed in biochemical studies.

Differential localization of type I and type II benzodiazepine binding sites in substantia nigra

A number of studies have suggested the existence of multiple benzodiazepine binding sites in the brain. We have recently reported the physical separation of two apparent benzodiazepine binding site subtypes, the pharmacological properties, and distribution in tissue sections of which correspond to the putative type I and type II sites. Benzodiazepine and gamma-aminobutyric acid (GABA) receptors have been shown to interact, and lesions of the GABAergic striatonigral pathway, which lead to GABA supersensitivity, both increase the numbers of GABA binding sites and enhance GABA-stimulated benzodiazepine binding. We demonstrate here that degeneration of striatonigral fibres increases the density of putative type I benzodiazepine binding sites in the substantia nigra and decreases the density of the putative type II sites. This suggests that type I sites that increase after denervation are postsynaptic, whereas the type II sites reduced by the lesion may be localized to axons or terminals of the striatonigral pathways.

Quantitative autoradiography of [3H]prazosin binding sites in rat forebrain

We have used the LKB Ultrofilm method of autoradiography to localize and quantify in rat forebrain the binding sites for [3H]prazosin, a highly-selective antagonist for the alpha 1 adrenoreceptor subtype. Frozen 32 microns thick brain sections were labeled in vitro with 1 nM [3H]prazosin and applied against LKB Ultrofilm for 60 days to generate autoradiograms. Non-specific binding was defined as the labeling in the presence of 10 microM phentolamine. The highest levels of prazosin binding were found in layer V of the motor portion of the frontoparietal cortex and in all nuclei of the thalamus. Moderate levels of alpha 1 receptors were observed in the remaining layers of the cerebral cortex and in most regions of the limbic system. Low levels of prazosin binding occurred in the caudate-putamen and the accumbens nucleus. Our results indicate that alpha 1 adrenoceptors are distributed heterogeneously throughout the rat forebrain.

Binding of radiolabeled opiates to slide-mounted sections of molded minced rat brain: a novel method for conducting radioreceptor assays

To facilitate the quantitative study of the opiate receptor, we have developed a novel variation of the ligand binding technique. The binding of 3H-opiates to rat brain membranes is compared with the binding to slide-mounted sections of molded minced rat brain. The latter method is characterized by a high signal and a superior signal-to-noise ratio over a wide range of ligand concentrations. The results are discussed in reference to other ligand binding methodologies.

Autoradiographic visualization of [3H]kainic acid receptor subtypes in the rat hippocampus

Unfixed, slide-mounted tissue sections from the rat forebrain have been incubated in the presence of 20 and 100 nM [3H]kainic acid ([3H]KA). For the last 2 min of incubation, 10 micrometers unlabelled KA was added to displace [3H]KA from binding sites with high on-off rate. Washed and dried slices were exposed on [3H]Ultrofilm for 178 days. Our results confirm the high density of KA receptors in the terminal field of the hippocampal mossy fibre system which is shown to be due to receptors with slow dissociation rate. Furthermore, the concentration dependency of the specific labelling, as quantified by microdensitometry, allows some suggestions concerning the local binding affinities involved.

The distribution of serotonin receptors in the human brain: high density of [3H]LSD binding sites in the raphe nuclei of the brainstem

Serotonin receptors were localized autoradiographically in the human brainstem after in vitro labeling using [3H]lysergic acid diethylamide (LSD). Very high concentration of [3H]LSD binding sites, apparently belonging to the 5-HT1 class were localized in the raphe nuclei. Other areas of the brainstem presented only moderate or low receptor densities. Labeled areas were the nucleus interpeduncularis, periaqueductal gray matter, locus coeruleus and nucleus tractus solitarius. The choroid plexus was also labeled by [3H]LSD. The use of [3H]LSD binding as a marker for serotonin cells in the brainstem is suggested.

Benzodiazepine receptors: preferential stimulation of Type 1 receptors by pentobarbital

The binding of [3H]flunitrazepam (FLU) to Type 1 and Type 2 benzodiazepine receptors in rat cerebellum and cerebral cortex was differentiated by the addition of 200 nM CL218,872 which preferentially displaces [3H]FLU from Type 1 receptors. Type 1 but not Type 2 receptor binding was significantly stimulated by 1 mM sodium pentobarbital.

Autoradiographic differentiation of multiple benzodiazepine receptors by detergent solubilization and pharmacologic specificity

We have used light microscopic autoradiography to localize differentially Type 1 and 2 benzodiazepine receptors using pharmacologic and detergent treatment techniques. The triazolopyridazine CL218872 has preferential affinity for Type 1 receptors from which it displaces [3H]flunitrazepam selectivity. In biochemical experiments [3H]flunitrazepam binding sites with pharmacologic properties of Type 2 receptors are preferentially solubilized from brain membranes by detergents. We have now treated rat brain slices either with CL218872 or 2% sodium cholate and evaluated the autoradiographic distribution of [3H]flunitrazepam-labeled receptors. Whether calculated from the drug or detergent treated preparations, the relative densities of apparent Type 1 and 2 receptors are the same. Thus differential sensitivity to determine solubilization distinguishes the same two populations of receptors as Type 1 and 2 specific drugs.

Quantitative autoradiography of [3H]corticosterone receptors in rat brain

We have quantified corticosterone receptors in rat brain by optical density measurements of tritium-film autoradiograms. Rats were injected i.v. with 500 microCi [3H]corticosterone to label brain receptors. Frozen sections of brain were cut with a cryostat and exposed for 2 months against tritium-sensitive sheet film (LKB Ultrofilm). Tritium standards were used to convert optical density readings into molar concentrations of receptor. High levels of corticosterone receptors were present throughout the pyramidal and granule cell layers of the hippocampus. Moderate levels of receptors were found in the neuropil of the hippocampus, the lateral septum, the cortical nucleus of the amygdala and the entorhinal cortex. All other brain regions had low levels of receptors. These results extend previous non-quantitative autoradiographic studies of corticosterone receptors and provide a general procedure for the quantitative autoradiography of steroid hormone receptors in brain tissue.

Amyotrophic lateral sclerosis: alterations in neurotransmitter receptors

Loss of motor neurons is the primary pathological hallmark of amyotrophic lateral sclerosis. Drug and neurotransmitter receptors are neuronal markers and can be indicators of neuronal connectivity. Knowledge of alterations in receptors in amyotrophic lateral sclerosis should contribute to our understanding of normal spinal cord neurotransmitter systems as well as of the pathophysiology of amyotrophic lateral sclerosis. We therefore used a sensitive, light microscopic in vitro labeling receptor autoradiographic technique to map and quantitate muscarinic cholinergic, glycinergic, and benzodiazepine receptors in three levels of spinal cord from six patients with amyotrophic lateral sclerosis and six age- and sex-matched control patients. In control tissues, the receptor distributions were similar in the three levels of spinal cord and also similar to those found in previous studies with animals. In amyotrophic lateral sclerosis, major reductions in receptor densities were noted in Rexed layer IX, the region containing motor neurons. Reductions were noted in other laminae as well, particularly for muscarinic receptors. The changes in muscarinic receptors were caused solely by changes in high-affinity agonist sites. Reductions in glycine and muscarinic receptors were highly correlated with the degree of motor neuron loss found in the amyotrophic lateral sclerosis patients. The findings in this study point out the usefulness of this receptor mapping technique in understanding the changes in neuronal populations that occur in the degenerative neurological diseases.

Quantitative autoradiographic localization of [3H]imipramine binding sites in the brain of the rat: relationship to ascending 5-hydroxytryptamine neuron systems

Quantitative autoradiography shows that there is a close relationship between [3H]imipramine binding sites and the distribution of 5-hydroxytryptamine (5-HT) neurons in the rat brain. High labeling is observed in the midbrain raphe nuclei, the areas of the dopamine cell groups of the substantia nigra and of the ventral tegmental area of Tsai, the ventral amygdaloid nucleus, the midline thalamic area, and parts of the hypothalmus. Thus, antidepressant drugs that have high affinity for [3H]imipramine binding sites can exert an influence at the 5-HT cell body as well as at the 5-HT nerve terminal level. The present results underline the possibility that the 5-HT and dopamine hypotheses for the mechanism of action of antidepressant drugs are not mutually exclusive, because both 5-HT and dopamine neurons can be regulated by large numbers of [3H]imipramine binding sites.

Autoradiographic localization of high-affinity [3H]kainic acid binding sites in the rat forebrain

Utilizing in vitro autoradiographic techniques, we have studied the distribution of high affinity [3H]kainic acid ([3H]KA) binding sites in intact sections of the rat forebrain. These sites have the same kinetic and pharmacological characteristics as the [3H]KA site described in tissue homogenates. Moderate to high levels of specific binding were observed in several discrete brain regions. These include lamina I, V and VI of the neo- and cingulate cortex, superficial layers of the pyriform cortex, striatum, external plexiform and granule cell layers of the olfactory bulb, olfactory tubercle, the stratum lucidum of CA3 of the hippocampus, molecular layer of the dentate gyrus, reticular nucleus of the thalamus, the hypothalamic median eminence, and the granule cell layer of the cerebellum. Low levels of specific binding were associated with other discrete regions such as the lateral septum, bed nucleus of the stria terminalis, medial geniculate, superficial layers of the superior colliculus, nuclei of the central grey, interpeduncular nucleus and the molecular layer of the cerebellum. Moderate uniform levels of specific binding were observed over the hypothalamus, zona incerta and the amygdala. One of the important factors in KA neurotoxicity seems to be the presence of KA receptors, and regions that are susceptible to the toxic effects of KA after local administration, such as the striatum, hippocampus, amygdala and pyriform cortex, have moderate to high levels of binding. Thus, these data provide a useful map for studying the relationship between receptor-mediated and seizure-induced neuronal damage following KA administration.

Kappa opiate binding sites in the substantia nigra and bulbus olfactorius of the guinea pig as shown by in vitro autoradiography

The purpose of the present study was to further map the distribution of the kappa subtype of opiate binding sites in the guinea pig brain. Cryostat sections of brain were incubated in 2 nM [3H] (-)-bremazocine with or without the addition of D-Ala2,MePhe4,Gly-ol5-enkephalin and/or D-Ala2,D-Leu5-enkephalin at 1 - 3000 nM. A sheet of [3H] Ultrofilm R (LKB) was exposed to the dry sections for up to 207 days. A computer coupled to a microdensitometer and a television camera was used to convert grey areas of the film to femtomoles of binding sites per mg protein. The substantia nigra and lamina plexiformis externa of the bulbus olfactorius remained densely labeled even in the presence of 3000 nM of both enkephalins. The guinea pig is the only species studied thus far which exhibits this distribution, but the physiological significance is not yet understood.