Neuroscience Letters, 110 (1990) 143-147 Elsevier Scientific Publishers Ireland Ltd.
143
NSL 06702
Tyr-MIF-1 binding in brain is not altered by ligands selective for the GABAA/benzodiazepine receptor James E. Zadina, Abba J. Kastin and Lin-Jun Ge Veterans Administration Medical Center and Tulane University School of Medicine, New Orleans, LA 70146 (U.S.A.) (Received 22 August 1989; Revised version received 25 October 1989; Accepted 6 November 1989) Key words:
Tyr-MIF-1; Benzodiazepine receptor; 7-Aminobutyric acid receptor; Chloride channel; Picrotoxinin; Clonazepam; Flunitrazepam; GABA
Binding of benzodiazepines to the benzodiazepine 7-aminobutyric acid (GABA) receptor-chloride channel complex has been shown to be altered by Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2). This raised the possibility of allosteric binding interactions between Tyr-MIF-I sites and the GABAA receptor complex. We tested this possibility in rat brain by examining the binding of Tyr-MIF-1 to brain membranes in the presence of clonazepam, GABA, a combination of clonazepam and GABA, RO15788, or picrotoxinin. None of the tested substances affected Tyr-MIF-1 binding. We also tested mouse cortex for changes in Tyr-MIFI binding in the presence of ligands that bind to the GABA/benzodiazepine/chloride channel complex. Clonazepam, flunitrazepam, RO15788, and picrotoxinin at concentrations ranging from I0 -~3 to 10 5 M, each in the absence or presence of GABA at concentrations ranging from 10 -9 to 10-6 M, did not significantly alter the binding of Tyr-MIF-1. The results indicate that simple bidirectional allosteric interactions between Tyr-MIF-I binding sites and benzodiazepine, GABA or chloride channel binding sites are not likely to be the mechanism by which Tyr-MIF-I affects binding at this complex.
Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), a tetrapeptide present in brain [2] that is capable of modulating opiate activity [1, 15] has recently been shown to affect binding of benzodiazepines to their binding sites in the benzodiazepine/GABA receptorchloride channel complex [3-5]. Tyr-MIF-1 augmented the effects of gamma-aminobutyric acid (GABA) on benzodiazepine receptor binding both in vitro [3] and in vivo [4]. In addition, Tyr-MIF-I augmented displacement of a chloride channel ligand (t-butylbicyclophosphorothionate, TBPS) by the y-aminobutyric acid (GABA) analog muscimol [5]. The authors of these earlier studies suggested that it is the coupling between the GABA and benzodiazepine sites that is affected by Tyr-MIF-1 rather than actions directly at the binding sites. This coupling is thought to involve allosteric interactions among binding sites on separate subunits of the receptor complex [6]. Correspondence." J.E. Zadina, Veterans Administration Medical Center and Tulane University School of Medicine, New Orleans, LA 70146, U.S.A. 0304-3940/90/$ 03.50 © 1990 Elsevier Scientific Publishers Ireland Ltd.
144
A variety of binding sites appear to influence these allosteric interactions, including those for anxiolytic (benzodiazepine), anxiogenic (fl-carboline), convulsant (picrotoxin), and anticonvulsant (barbiturate) drugs. Since Tyr-MIF-1 apparently does not act directly at the benzodiazepine, GABA or chloride channel sites, a second mechanism by which it could potentiate GABAinduced changes in benzodiazepine and chloride channel (TBPS) binding would be if the Tyr-MIF-1 binding site [12, 13, 16] is functionally connected to the GABAA receptor complex and is capable of modulating the allosteric interactions known to occur among the various binding sites in this complex. A simple test of this possibility is to measure whether benzodiazepines, GABA or picrotoxin alter binding of TyrMIF-I to its binding site. Indeed, some of the earliest evidence that the benzodiazepine binding site was part of a GABA receptor complex was the demonstration that GABA produced a small but significant increase in benzodiazepine binding [10]. Tyr-MIF-I was obtained from Bachem Switzerland, iodinated by the chloramine T method, and purified by high-performance liquid chromatography (HPLC) to isolate the monoiodinated fraction (1800-2100 Ci/mmol). Male Sprague-Dawley-derived adult rats were obtained from Zivic-Miller (Allison Park, PA). Male CD-1 mice, 6-8 weeks of age, were obtained from Charles River Labs (Wilmington, MA). All animals were housed in a 12:12 h dark-light cycle at 21 _ I°C. Crude synaptic plasma membranes (SPMs) were prepared according to previously described methods [13, 16]. Rat brains minus cerebelli or mouse cortices were homogenized in 20 vols. of ice-cold 0.32 M sucrose with a Brinkman polytron at setting 6 for 20 s. Homogenates were centrifuged at 1000 g for l0 rain at 4°C to separate the crude nuclear pellet. Centrifugation of the supernatant and all subsequent centrifugations were at 30,000 g for 15 min. The resulting pellet was reconstituted in 50 mM Tris (pH 7.5 at 23°C) and incubated for 1 h in a water bath at room temperature to remove endogenous ligands. Homogenates were again centrifuged and pellets reconstituted in STEM buffer (0.25 M sucrose, 5 mM Tris, 0.5 mM EDTA, and 1 mM MgSO4) and recentrifuged. The resulting pellets were either reconstituted in 5 vols of STEM, stored at - 70'~C for a maximum of 3 weeks, and then thawed with 15 vols. of phosphate buffer (100 mM potassium phosphate, pH 7.4) or were immediately reconstituted with a 5:15 STEM:phosphate mixture. The tissue was recentrifuged, and the pellet brought to 20 vols. of the phosphate buffer for incubation. Protein concentrations (500-600/~g/ml final incubation concentration) were determined with the Lowry method with bovine serum albumin (BSA) as the standard. A fixed concentration (100 pM) of 125I-Tyr-MIF-1 was incubated with varying concentrations of the following test compounds: (a) the benzodiazepine agonist clonazepam; (b) GABA; (c) clonazepam in the presence of I / t M GABA; (d) the benzodiazepine receptor antagonist RO15788; or (e) the chloride channel ligand picrotoxinin, the toxic component of picrotoxin. Non-specific binding was defined as that observed in the presence of 10/~M Tyr-MIF-I. At concentrations ranging from 1 pM to l0 or 100 ~M, none of the test compounds significantly affected binding of 125I-Tyr-MIF-I to its binding site in rat brain. Figure 1 shows the specific binding of 125I-Tyr-MIF-I expressed as a percent
145
of binding in the absence of test compounds (B0). The first panel shows the results when clonazepam was included in the incubation at concentrations ranging from 1 pM to 10 /~M. This compound was not fully soluble at 100/IM. No significant changes were observed at any of the tested concentrations. Similarly negative results were observed with varying concentrations of GABA as shown in the second panel. GABA at a concentration of 1/~M is known to enhance binding of benzodiazepines to their binding sites, a process that is further augmented by Tyr-MIF-1 [3, 4]. As shown in the third panel of Fig. 1, however, the combination of 1/~M GABA with varying concentrations of clonazepam did not alter binding of Tyr-MIF-1 to its binding site. The benzodiazepine receptor antagonist RO15788 and the chloride channel ligand picrotoxinin were also without effect on Tyr-MIF-1 binding, as shown in the fourth and fifth panels of Fig. 1. The in vitro studies demonstrating that Tyr-MIF-I augments benzodiazepine binding in the presence of GABA were conducted with cortical tissue from male CD1 mice from Charles River Labs. [3, 5]. Accordingly, we tested cortical membranes from this same strain for changes in Tyr-MIF-1 binding. The benzodiazepine agonists clonazepam and flunitrazepam, the antagonist RO 15788, and the chloride channel ligand picrotoxinin were tested at a wide range of concentrations (10-13 to 10 -5 M) in the absence or presence of 10 -8, 10 - 7 , o r 10 -6 M GABA. Fig. 2 shows the results of the experiment with flunitrazepam, the ligand used for in vitro labeling of the benzodiazepam receptor in the study of Miller et al. [3]. This figure shows that
I 10o
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0
80
80
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60
60
40
40
20
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0
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I
-12 -11 -10
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Log
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-7
-6
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"-----O"~O'"O
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-12
t
-11
-10
-9
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-7
-6
-5
-4
Log Dose o f I r l d l c s t e d CoEpound
Fig. I. Effect of varying concentrations of test compounds on binding of 100 pM 125I-Tyr-MIF-1 to rat brain membranes.
146
100 90 8O v
70
.3 D 0 m
60
,~
so
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GABA
~,
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0 M
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i
H
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i
i
i
i
i
-13
-12
-11
-10
-9
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Dose
i
-8
i
i
i
-7
-6
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Flunitrazepam
Fig. 2. Effectof varyingconcentrationsofflunitrazepam, in the absence or presenceof I0 ~, 10 7 or 10 ~' GABA, on binding of 100 pM J251-Tyr-MIF-Ito mouse cortex membranes. flunitrazepam had little effect on Tyr-MIF-! binding, either in the absence of GABA or at any of the 3 concentrations of GABA. Similarly negative results were observed for clonazepam, RO 15788, and picrotoxinin. Tyr-MIF-1 potentiated GABA-induced changes in benzodiazepine and chloride channel binding in studies indicating that Tyr-MIF-1 did not act directly at the binding sites for these substances [3-5]. In the current studies, we tested an alternative possibility that Tyr-MIF-1 is involved in allosteric changes in the GABAA receptor complex. We used ligands most likely to affect Tyr-MIF-1 binding, based on current knowledge of the GABAA receptor complex and the effects of Tyr-MIF-1 on it. Our results indicate that changes in either the binding of ligands to benzodiazepine and GABA receptors or the coupling of these sites do not alter the binding of TyrMIF-1 to its own site. In addition, binding of picrotoxinin to the chloride channel does not alter Tyr-MIF-1 binding. These results indicate that the interactions between Tyr-MIF-1 binding sites and GABA, benzodiazepine, and chloride channel binding sites are more complex than simple bidirectional allosteric effects, as is the coupling of GABA and benzodiazepine receptors: the enhancement of benzodiazepine binding by GABA appears to be a robust phenomenon while the reverse (benzodiazepine-induced changes in GABA binding) has proven more elusive and may be under greater influence by endogenous modulators [8]. Tyr-MIF-1 could be one of these modulators. Mechanisms not precluded by these data include: unidirectional allosteric changes induced by Tyr-MIF-I in GABA, benzodiazepine, or chloride
147
channel sites that are not reflected by changes in Tyr-MIF-1 binding; allosteric interactions with other binding sites on the complex such as those for barbiturates or GABA-modulin [11]; and mechanisms 'downstream' from the Tyr-MIF-I binding site. These studies were supported by the VA. 1 Galina, Z.H. and Kastin, A.J., Existence of antiopiate systems as illustrated by MIF-I/Tyr-MIF-1, Life Sci., 39 (1986) 2153-2159. 2 Horvath, A. and Kastin, A.J., Isolation of Tyr-melanocyte-stimulating hormone release-inhibiting factor 1 from bovine brain tissue, J. Biol. Chem., 264 (1989) 217~2179. 3 Miller, L.G. and Kastin, A.J., MIF-1 and Tyr-MIF-1 augment GABA-stimulated benzodiazepine receptor binding, Peptides, 8 (1987) 751 755. 4 Miller, L., Kastin, A.J. and Greenblatt, D.J., Tyr-MIF-I augments benzodiazepine receptor binding in vivo, Pharmacol. Biochem. Behav., 28 (1987) 521-524. 5 Miller, L., Kastin, A.J. and Roy, R.B., Effects of Tyr-MIF-1 and MIF-1 at the GABAA receptor chloride channel site, Brain Res. Bull., 19 (1987) 743-745. 6 Schofield, P.R., Darlison, M.G., Fujita, N., Burt,D.R., Stephenson, F.A., Rodriguez, H., Rhee, L.M., Ramachandran, J., Reale, V., Glencorse, T.A., Seeburg, P.H. and Barnard, E.A., Sequence and functional expression of the GABAA receptor shows a ligand-gated receptor super-family, Nature (Lond.), 328 (1987) 221-227. 7 Sivam, S.P. and Ho, I.K., Influence of morphine dependence on GABA-stimulated benzodiazepine binding to mouse brain synaptic membranes, Eur. J. Pharmacol., 79 (1982) 335-336. 8 Skerrin, J.H., Chow, S.C. and Johnston, G.A.R., Differences in the interactions between GABA and benzodiazepine binding sites, Neurosci. Lett., 33 (1982) 173-178. 9 Smith, J.E., Co, C. and Lane, J.D., Limbic muscarinic cholinergic and benzodiazepine receptor changes with chronic intravenous morphine and self-administration, Pharmacol. Biochem. Behav., 20 (1984) 443-450. l0 Tallman, J.F., Thomas, J.W. and Gallagher, D.W., GABAergic modulation ofbenzodiazepine binding site sensitivity, Nature (Lond.), 274 (1979) 383-385. 11 Vaccarino, F.M., Alho, H., Santi, M.R. and Guidotti, A., Coexistence of GABA receptors and GABA modulin in primary cultures of rat cerebellar granule cells, J. Neurosci., 7 (1987) 65-76. 12 Zadina, J.E., Kastin, A.J., Krieg, Jr., E.F. and Coy, D.H., Characterization of binding sites for N-TyrMIF-1 (Tyr-Pro-Leu-Gly-NH2) in rat brain, Pharmacol. Biochem. Behav., 17 (1982) 1193-1198. 13 Zadina, J.E. and Kastin, A.J., Interactions between the antiopiate Tyr-MIF-I and the mu opiate morphiceptin at their respective binding sites in brain, Peptides, 6 (1985) 965-970. 14 Zadina, J.E. and Kastin, A.J., Interactions of Tyr-MIF-I at opiate receptor sites, Pharmacol. Biochem. Behav., 25 (1986) 1303 1305. 15 Zadina, J.E., Kastin, A.J., Manasco, P.K., Pignatiello, M.F. and Nastiuk, K.L., Long-term hyperalgesia induced by neonatal fl-endorphin and morphiceptin is blocked by neonatal Tyr-MIF-1, Brain Res., 409 (1987) 10-18. 16 Zadina, J.E., Kastin, A.J., Ge, L.-J., Gulden, H. and Bungart, K.J., Chronic, but not acute, administration of morphine alters antiopiate (Tyr-MIF-1) binding sites in rat brain, Life Sci., 44 (1989) 555-561.
143
NSL 06702
Tyr-MIF-1 binding in brain is not altered by ligands selective for the GABAA/benzodiazepine receptor James E. Zadina, Abba J. Kastin and Lin-Jun Ge Veterans Administration Medical Center and Tulane University School of Medicine, New Orleans, LA 70146 (U.S.A.) (Received 22 August 1989; Revised version received 25 October 1989; Accepted 6 November 1989) Key words:
Tyr-MIF-1; Benzodiazepine receptor; 7-Aminobutyric acid receptor; Chloride channel; Picrotoxinin; Clonazepam; Flunitrazepam; GABA
Binding of benzodiazepines to the benzodiazepine 7-aminobutyric acid (GABA) receptor-chloride channel complex has been shown to be altered by Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2). This raised the possibility of allosteric binding interactions between Tyr-MIF-I sites and the GABAA receptor complex. We tested this possibility in rat brain by examining the binding of Tyr-MIF-1 to brain membranes in the presence of clonazepam, GABA, a combination of clonazepam and GABA, RO15788, or picrotoxinin. None of the tested substances affected Tyr-MIF-1 binding. We also tested mouse cortex for changes in Tyr-MIFI binding in the presence of ligands that bind to the GABA/benzodiazepine/chloride channel complex. Clonazepam, flunitrazepam, RO15788, and picrotoxinin at concentrations ranging from I0 -~3 to 10 5 M, each in the absence or presence of GABA at concentrations ranging from 10 -9 to 10-6 M, did not significantly alter the binding of Tyr-MIF-1. The results indicate that simple bidirectional allosteric interactions between Tyr-MIF-I binding sites and benzodiazepine, GABA or chloride channel binding sites are not likely to be the mechanism by which Tyr-MIF-I affects binding at this complex.
Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), a tetrapeptide present in brain [2] that is capable of modulating opiate activity [1, 15] has recently been shown to affect binding of benzodiazepines to their binding sites in the benzodiazepine/GABA receptorchloride channel complex [3-5]. Tyr-MIF-1 augmented the effects of gamma-aminobutyric acid (GABA) on benzodiazepine receptor binding both in vitro [3] and in vivo [4]. In addition, Tyr-MIF-I augmented displacement of a chloride channel ligand (t-butylbicyclophosphorothionate, TBPS) by the y-aminobutyric acid (GABA) analog muscimol [5]. The authors of these earlier studies suggested that it is the coupling between the GABA and benzodiazepine sites that is affected by Tyr-MIF-1 rather than actions directly at the binding sites. This coupling is thought to involve allosteric interactions among binding sites on separate subunits of the receptor complex [6]. Correspondence." J.E. Zadina, Veterans Administration Medical Center and Tulane University School of Medicine, New Orleans, LA 70146, U.S.A. 0304-3940/90/$ 03.50 © 1990 Elsevier Scientific Publishers Ireland Ltd.
144
A variety of binding sites appear to influence these allosteric interactions, including those for anxiolytic (benzodiazepine), anxiogenic (fl-carboline), convulsant (picrotoxin), and anticonvulsant (barbiturate) drugs. Since Tyr-MIF-1 apparently does not act directly at the benzodiazepine, GABA or chloride channel sites, a second mechanism by which it could potentiate GABAinduced changes in benzodiazepine and chloride channel (TBPS) binding would be if the Tyr-MIF-1 binding site [12, 13, 16] is functionally connected to the GABAA receptor complex and is capable of modulating the allosteric interactions known to occur among the various binding sites in this complex. A simple test of this possibility is to measure whether benzodiazepines, GABA or picrotoxin alter binding of TyrMIF-I to its binding site. Indeed, some of the earliest evidence that the benzodiazepine binding site was part of a GABA receptor complex was the demonstration that GABA produced a small but significant increase in benzodiazepine binding [10]. Tyr-MIF-I was obtained from Bachem Switzerland, iodinated by the chloramine T method, and purified by high-performance liquid chromatography (HPLC) to isolate the monoiodinated fraction (1800-2100 Ci/mmol). Male Sprague-Dawley-derived adult rats were obtained from Zivic-Miller (Allison Park, PA). Male CD-1 mice, 6-8 weeks of age, were obtained from Charles River Labs (Wilmington, MA). All animals were housed in a 12:12 h dark-light cycle at 21 _ I°C. Crude synaptic plasma membranes (SPMs) were prepared according to previously described methods [13, 16]. Rat brains minus cerebelli or mouse cortices were homogenized in 20 vols. of ice-cold 0.32 M sucrose with a Brinkman polytron at setting 6 for 20 s. Homogenates were centrifuged at 1000 g for l0 rain at 4°C to separate the crude nuclear pellet. Centrifugation of the supernatant and all subsequent centrifugations were at 30,000 g for 15 min. The resulting pellet was reconstituted in 50 mM Tris (pH 7.5 at 23°C) and incubated for 1 h in a water bath at room temperature to remove endogenous ligands. Homogenates were again centrifuged and pellets reconstituted in STEM buffer (0.25 M sucrose, 5 mM Tris, 0.5 mM EDTA, and 1 mM MgSO4) and recentrifuged. The resulting pellets were either reconstituted in 5 vols of STEM, stored at - 70'~C for a maximum of 3 weeks, and then thawed with 15 vols. of phosphate buffer (100 mM potassium phosphate, pH 7.4) or were immediately reconstituted with a 5:15 STEM:phosphate mixture. The tissue was recentrifuged, and the pellet brought to 20 vols. of the phosphate buffer for incubation. Protein concentrations (500-600/~g/ml final incubation concentration) were determined with the Lowry method with bovine serum albumin (BSA) as the standard. A fixed concentration (100 pM) of 125I-Tyr-MIF-1 was incubated with varying concentrations of the following test compounds: (a) the benzodiazepine agonist clonazepam; (b) GABA; (c) clonazepam in the presence of I / t M GABA; (d) the benzodiazepine receptor antagonist RO15788; or (e) the chloride channel ligand picrotoxinin, the toxic component of picrotoxin. Non-specific binding was defined as that observed in the presence of 10/~M Tyr-MIF-I. At concentrations ranging from 1 pM to l0 or 100 ~M, none of the test compounds significantly affected binding of 125I-Tyr-MIF-I to its binding site in rat brain. Figure 1 shows the specific binding of 125I-Tyr-MIF-I expressed as a percent
145
of binding in the absence of test compounds (B0). The first panel shows the results when clonazepam was included in the incubation at concentrations ranging from 1 pM to 10 /~M. This compound was not fully soluble at 100/IM. No significant changes were observed at any of the tested concentrations. Similarly negative results were observed with varying concentrations of GABA as shown in the second panel. GABA at a concentration of 1/~M is known to enhance binding of benzodiazepines to their binding sites, a process that is further augmented by Tyr-MIF-1 [3, 4]. As shown in the third panel of Fig. 1, however, the combination of 1/~M GABA with varying concentrations of clonazepam did not alter binding of Tyr-MIF-1 to its binding site. The benzodiazepine receptor antagonist RO15788 and the chloride channel ligand picrotoxinin were also without effect on Tyr-MIF-1 binding, as shown in the fourth and fifth panels of Fig. 1. The in vitro studies demonstrating that Tyr-MIF-I augments benzodiazepine binding in the presence of GABA were conducted with cortical tissue from male CD1 mice from Charles River Labs. [3, 5]. Accordingly, we tested cortical membranes from this same strain for changes in Tyr-MIF-1 binding. The benzodiazepine agonists clonazepam and flunitrazepam, the antagonist RO 15788, and the chloride channel ligand picrotoxinin were tested at a wide range of concentrations (10-13 to 10 -5 M) in the absence or presence of 10 -8, 10 - 7 , o r 10 -6 M GABA. Fig. 2 shows the results of the experiment with flunitrazepam, the ligand used for in vitro labeling of the benzodiazepam receptor in the study of Miller et al. [3]. This figure shows that
I 10o
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loo
80 ~
Clonazepam
°---o-._ °
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60
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40
40
20
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0
,n
7 I
0
80
80
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60
60
40
40
20
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0
0 0
I
-12 -11 -10
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Log
Dose
-7
-6
-5
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I-4 100
"-----O"~O'"O
~
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80
of
C l o a a z e p a m + GABA
Indicated
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60 40 20 0
I
-12
t
-11
-10
-9
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-7
-6
-5
-4
Log Dose o f I r l d l c s t e d CoEpound
Fig. I. Effect of varying concentrations of test compounds on binding of 100 pM 125I-Tyr-MIF-1 to rat brain membranes.
146
100 90 8O v
70
.3 D 0 m
60
,~
so
O
10-sM
GABA
~,
~o
•
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0 M
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i
H
~
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i
i
i
i
i
-13
-12
-11
-10
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Dose
i
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i
i
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Fig. 2. Effectof varyingconcentrationsofflunitrazepam, in the absence or presenceof I0 ~, 10 7 or 10 ~' GABA, on binding of 100 pM J251-Tyr-MIF-Ito mouse cortex membranes. flunitrazepam had little effect on Tyr-MIF-! binding, either in the absence of GABA or at any of the 3 concentrations of GABA. Similarly negative results were observed for clonazepam, RO 15788, and picrotoxinin. Tyr-MIF-1 potentiated GABA-induced changes in benzodiazepine and chloride channel binding in studies indicating that Tyr-MIF-1 did not act directly at the binding sites for these substances [3-5]. In the current studies, we tested an alternative possibility that Tyr-MIF-1 is involved in allosteric changes in the GABAA receptor complex. We used ligands most likely to affect Tyr-MIF-1 binding, based on current knowledge of the GABAA receptor complex and the effects of Tyr-MIF-1 on it. Our results indicate that changes in either the binding of ligands to benzodiazepine and GABA receptors or the coupling of these sites do not alter the binding of TyrMIF-1 to its own site. In addition, binding of picrotoxinin to the chloride channel does not alter Tyr-MIF-1 binding. These results indicate that the interactions between Tyr-MIF-1 binding sites and GABA, benzodiazepine, and chloride channel binding sites are more complex than simple bidirectional allosteric effects, as is the coupling of GABA and benzodiazepine receptors: the enhancement of benzodiazepine binding by GABA appears to be a robust phenomenon while the reverse (benzodiazepine-induced changes in GABA binding) has proven more elusive and may be under greater influence by endogenous modulators [8]. Tyr-MIF-1 could be one of these modulators. Mechanisms not precluded by these data include: unidirectional allosteric changes induced by Tyr-MIF-I in GABA, benzodiazepine, or chloride
147
channel sites that are not reflected by changes in Tyr-MIF-1 binding; allosteric interactions with other binding sites on the complex such as those for barbiturates or GABA-modulin [11]; and mechanisms 'downstream' from the Tyr-MIF-I binding site. These studies were supported by the VA. 1 Galina, Z.H. and Kastin, A.J., Existence of antiopiate systems as illustrated by MIF-I/Tyr-MIF-1, Life Sci., 39 (1986) 2153-2159. 2 Horvath, A. and Kastin, A.J., Isolation of Tyr-melanocyte-stimulating hormone release-inhibiting factor 1 from bovine brain tissue, J. Biol. Chem., 264 (1989) 217~2179. 3 Miller, L.G. and Kastin, A.J., MIF-1 and Tyr-MIF-1 augment GABA-stimulated benzodiazepine receptor binding, Peptides, 8 (1987) 751 755. 4 Miller, L., Kastin, A.J. and Greenblatt, D.J., Tyr-MIF-I augments benzodiazepine receptor binding in vivo, Pharmacol. Biochem. Behav., 28 (1987) 521-524. 5 Miller, L., Kastin, A.J. and Roy, R.B., Effects of Tyr-MIF-1 and MIF-1 at the GABAA receptor chloride channel site, Brain Res. Bull., 19 (1987) 743-745. 6 Schofield, P.R., Darlison, M.G., Fujita, N., Burt,D.R., Stephenson, F.A., Rodriguez, H., Rhee, L.M., Ramachandran, J., Reale, V., Glencorse, T.A., Seeburg, P.H. and Barnard, E.A., Sequence and functional expression of the GABAA receptor shows a ligand-gated receptor super-family, Nature (Lond.), 328 (1987) 221-227. 7 Sivam, S.P. and Ho, I.K., Influence of morphine dependence on GABA-stimulated benzodiazepine binding to mouse brain synaptic membranes, Eur. J. Pharmacol., 79 (1982) 335-336. 8 Skerrin, J.H., Chow, S.C. and Johnston, G.A.R., Differences in the interactions between GABA and benzodiazepine binding sites, Neurosci. Lett., 33 (1982) 173-178. 9 Smith, J.E., Co, C. and Lane, J.D., Limbic muscarinic cholinergic and benzodiazepine receptor changes with chronic intravenous morphine and self-administration, Pharmacol. Biochem. Behav., 20 (1984) 443-450. l0 Tallman, J.F., Thomas, J.W. and Gallagher, D.W., GABAergic modulation ofbenzodiazepine binding site sensitivity, Nature (Lond.), 274 (1979) 383-385. 11 Vaccarino, F.M., Alho, H., Santi, M.R. and Guidotti, A., Coexistence of GABA receptors and GABA modulin in primary cultures of rat cerebellar granule cells, J. Neurosci., 7 (1987) 65-76. 12 Zadina, J.E., Kastin, A.J., Krieg, Jr., E.F. and Coy, D.H., Characterization of binding sites for N-TyrMIF-1 (Tyr-Pro-Leu-Gly-NH2) in rat brain, Pharmacol. Biochem. Behav., 17 (1982) 1193-1198. 13 Zadina, J.E. and Kastin, A.J., Interactions between the antiopiate Tyr-MIF-I and the mu opiate morphiceptin at their respective binding sites in brain, Peptides, 6 (1985) 965-970. 14 Zadina, J.E. and Kastin, A.J., Interactions of Tyr-MIF-I at opiate receptor sites, Pharmacol. Biochem. Behav., 25 (1986) 1303 1305. 15 Zadina, J.E., Kastin, A.J., Manasco, P.K., Pignatiello, M.F. and Nastiuk, K.L., Long-term hyperalgesia induced by neonatal fl-endorphin and morphiceptin is blocked by neonatal Tyr-MIF-1, Brain Res., 409 (1987) 10-18. 16 Zadina, J.E., Kastin, A.J., Ge, L.-J., Gulden, H. and Bungart, K.J., Chronic, but not acute, administration of morphine alters antiopiate (Tyr-MIF-1) binding sites in rat brain, Life Sci., 44 (1989) 555-561.
