Characterization of rat cerebral cortical beta adrenoceptor subtypes using (-)-[125I]-iodocyanopindolol

β-Adrenoceptors and synaptic plasticity in the perirhinal cortex

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Bath application of isoprenaline induces LTP in the perirhinal cortex.

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LTP at amygdala to perirhinal synapses requires β1- but not β2-ADRs.

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Combination of β1-ADRs, NMDARs, and VGCC required for LTP.

Experiences with a high degree of emotional salience are better remembered than events that have little emotional context and the amygdala is thought to play an important role in this enhancement of memory. Visual recognition memory relies on synaptic plasticity in the perirhinal cortex but little is known about the mechanisms that may underlie emotional enhancement of this form of memory. There is good evidence that noradrenaline acting via β-adrenoceptors (β-ADRs) can enhance memory consolidation. In the present study we examine the role of β-ADRs in synaptic plasticity at the amygdala–perirhinal pathway (LA–PRh) and compare this to mechanisms of intra-perirhinal (PRh–PRh) synaptic plasticity. We demonstrate that activity-dependent PRh–PRh long-term potentiation (LTP) does not rely on β1- or β2-ADRs and that LA–PRh LTP relies on β1-ADRs but not β2-ADRs. We further demonstrate that application of the β-ADR agonist isoprenaline produces lasting PRh–PRh potentiation but only transient potentiation at the LA–PRh input. However, at the LA–PRh input, combining stimulation that is subthreshold for LTP induction with isoprenaline results in long-lasting potentiation. Isoprenaline-induced and isoprenaline plus subthreshold stimulation-induced potentiation in the PRh–PRh and LA–PRh inputs, respectively were both dependent on activation of NMDARs (N-methyl-D-aspartate receptors), voltage-gated calcium channels and PKA (protein kinase A). Understanding the mechanisms of amygdala–perirhinal cortex plasticity will allow a greater understanding of how emotionally-charged events are remembered.

Validation of S-1'-[18F]fluorocarazolol for in vivo imaging and quantification of cerebral beta-adrenoceptors

S-1'-[18F]fluorocarazolol (S-(-)-4-(2-hydroxy-3-(1'-[18F]fluoroisopropyl)-aminopropoxy)carba zole, a non-subtype-selective beta-adrenoceptor antagonist) has been investigated for in vivo studies of beta-adrenoceptors. Previous results indicated that uptake of this radioligand in heart and lung can be inhibited by beta-adrenoceptor agonists and antagonists. In the present study, blocking, displacement and saturation experiments were performed in rats, in combination with metabolite analysis to investigate the suitability of this radioligand for in vivo positron emission tomography (PET) imaging and quantification of beta-adrenoceptors in the brain. The results demonstrate that, (i) the uptake of S-1'-[18F]fluorocarazolol reflects specific binding to beta-adrenoceptors, (ii) binding of S-1'-[18F]fluorocarazolol to atypical or non-beta-adrenergic sites is negligible, (iii) uptake of radioactive metabolites in the brain is less than 25% of total radioactivity, 60 min after injection, (iv) in vivo measurements of receptor densities (Bmax) in cortex, cerebellum, heart, lung and erythrocytes are within range of densities determined from in vitro assays, (v) binding of S-1'-[18F]fluorocarazolol can be displaced. In conclusion, S-1'-[18F]fluorocarazolol seems to possess the appropriate characteristics to visualize and quantify beta-adrenoceptors in vivo in the central nervous system using PET.

Myocardial and pulmonary uptake of S-1'-[18F]fluorocarazolol in intact rats reflects radioligand binding to beta-adrenoceptors

The biodistribution of S-(-)-4-(2-hydroxy-3-(1'-[18F]fluoroisopropyl)- aminopropoxy)carbazole ([18F]S-fluorocarazolol, a non-selective beta-adrenoceptor antagonist) was studied in rats (60 min after 18F injection when specific binding in peripheral organs was maximal). 18F uptake in brain, erythrocytes, heart and lung appeared to be linked to beta-adrenoceptors. CGP-20712A and ICI-89,406 inhibited 18F uptake in heart (predominantly beta 1-adrenoceptors) more potently than in lungs (predominantly beta 2-adrenoceptors). In contrast, ICI-118,551 and procaterol were more potent in the lungs than in the heart. ICI-118,551 inhibited 18F uptake in cerebellum (predominantly beta 2-adrenoceptors) more potently than in cerebral cortex (predominantly beta 1-adrenoceptors). Stereoselectivity of the in vivo binding was demonstrated since S-(-)-propranolol inhibited uptake in target tissues more effectively than R-(+)-propranolol. Myocardial and cerebral imaging may be hampered by poor heart-to-lung contrast and low signal-to-noise ratios, but [18F]S-fluorocarazolol seems suitable for positron emission tomography (PET) of pulmonary beta-adrenoceptors.

Effects of DSP-4 on monoamine and monoamine metabolite levels and on beta adrenoceptor binding kinetics in rat brain at different times after administration

Effects of DSP-4 on noradrenaline (NA), 3-methoxy-4-hydroxyphenyl glycol (MHPG), serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) levels and on beta adrenoceptor binding kinetics (Bmax and KD) in rat hippocampus, cortex and hypothalamus were studied between 24 hours and 14 days after systemic administration. Beta adrenoceptor numbers in hippocampus and cortex, but not in hypothalamus, were significantly increased after DSP-4. No significant changes in KD values were observed in hypothalamus, but significant increases in this parameter were measured in hippocampus and cortex. NA and MHPG levels were significantly decreased in all three brain regions, but MHPG/NA ratios were increased in hippocampus, decreased in cortex and unchanged in hypothalamus. Very prominent increases in 5-HIAA levels were observed in all three brain regions, but only at one day after DSP-4. The greatest increases in 5-HIAA levels occurred in the hippocampus, but this effect of DSP-4 appeared to be slightly diminished by pre-treatment with fluoxetine. In cortex and hippocampus 5-HT levels were slightly, but significantly decreased after DSP-4.

Differential effects of olfactory bulbectomy on beta-adrenoceptors in rat amygdala, hippocampus and cerebral cortex

Rats were bilaterally olfactory bulbectomized. At 15 days post-lesion, olfactory bulbectomized (OB) rats exhibited significant deficits in the acquisition of passive avoidance learning compared to sham lesioned rats. beta-Adrenoceptor binding in the amygdala, hippocampus and cerebral cortex was assayed with (-)-[125I]iodocyanopindolol (ICYP). Scatchard analyses revealed no difference between OB and sham rats in maximal binding density (Bmax) in any of the three tissues. However, in the OB rats, the affinity of the beta-adrenoceptor for the ligand was significantly increased in the amygdala and hippocampus but not in the cortex. Bulbectomy did not affect the ratio of beta 1- to beta 2-adrenoceptor subtypes in the three brain tissues. In amygdala and hippocampus but not cerebral cortex, bulbectomy resulted in an increase in the proportion and the affinity of the high-affinity beta-adrenoceptor binding sites for isoproterenol. The affinity of the low-affinity sites in the hippocampus was also increased in the OB rats. The results suggest that olfactory bulbectomy causes supersensitivity of the amygdaloid and hippocampal beta-adrenoceptor by increasing the degree of coupling of the receptor with the stimulatory guanine nucleotide binding protein (Gs protein).

The characterization of beta adrenoceptor subtypes in the rat amygdala and hippocampus

(-)-[125I]Iodocyanopindolol [-]ICYP), is a ligand with high specific activity and nearly equal affinity for beta 1 and beta 2 adrenoceptors in a variety of tissues. Unfortunately, (-)ICYP also has affinity for 5HT1B serotonin receptors. To get an accurate estimate of beta adrenoceptors in the rat amygdala and hippocampus, (-)ICYP binding studies were done with membranes from these limbic structures in the presence of 10 microM serotonin to prevent the binding of (-)ICYP to serotonin receptors. Under these conditions. (-)ICYP binding to amygdaloid and hippocampus membrane preparations is saturable and reversible. Scatchard analyses revealed in both regions a single class of binding sites with an equilibrium dissociation constant (KD) of 18.5 pM for the amygdala and 19.6 pM for the hippocampus. The hippocampus has a significantly lower density of binding sites (Bmax) than amygdala (51.6 vs 62.3 fmol/mg membrane protein, p less than .05). The two brain regions do not differ with respect to kinetic reactions in that both show comparable slow association and dissociation rates. However, the dissociation reactions do reveal two affinity states for the binding sites in both areas. Detailed competition analyses with beta adrenoceptor subtype selective drugs (ICI-89406 and ICI-118551) show that in both regions about 70% of the beta adrenoceptor population is of the beta 1 subtype with the remainder being beta 2 subtype.