GABA(A) receptors in the basolateral amygdala are involved in mediating morphine reward

In the present study, the effects of intra-basolateral amygdala (BLA) injection of GABA(A) receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) in male Wistar rats have been investigated. Subcutaneous (s.c.) administration of different doses of morphine sulfate (1-9 mg/kg) produced a dose-dependent CPP. Using a 3-day schedule of conditioning, it was found that the GABA(A) receptor agonist, muscimol (0.125, 0.25 and 0.5 microg/rat) or the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 microg/rat), did not produce a significant place preference or place aversion. Intra-BLA administration of muscimol (0.25 and 0.5 microg/rat) decreased the acquisition of CPP induced by morphine (6 mg/kg). On the other hand, intra-BLA injection of bicuculline (0.25 and 0.5 microg/rat) in combination with an ineffective dose of morphine (1 mg/kg) elicited a significant CPP. The response of different doses of muscimol was attenuated by bicuculline (0.125 and 0.25 microg/rat). Furthermore, intra-BLA administration of bicuculline but not muscimol before testing significantly decreased the expression of morphine (6 mg/kg)-induced place preference. The administration of the higher doses of bicuculline (0.25 and 0.5 microg/rat) during acquisition and the higher dose of muscimol (2 microg/rat) on the test day decreased the locomotor activity of the animals on the testing phase. It can be concluded that GABA(A) receptors in the amygdala are involved in morphine reward.     

Activation of hippocampal postsynaptic muscarinic receptors is involved in long-term spatial memory formation

Spatial memory has been strongly associated with hippocampal function. There are several reports of the participation of this structure in acquisition and consolidation of spatial tasks. In this study, we evaluated the effects of selective and non-selective muscarinic antagonists in the dorsal hippocampus of rats during acquisition and encoding of a spatial task. Rats were trained in a Morris water maze for 4 days with identical daily sessions, and tested for long-term memory (LTM) 1 week after training. The animals were injected bilaterally in the dorsal hippocampus 20 min before the start of every day of training. The results showed that the non-selective muscarinic antagonist, scopolamine, disrupted acquisition of water maze memory formation. Moreover, microinjections of a selective postsynaptic muscarinic antagonist, pirenzepine, disrupted LTM, whereas it did not affect acquisition. Conversely, a selective presynaptic muscarinic antagonist, AFDX-116, did not disrupt either water maze acquisition or LTM formation. Combination of AFDX-116 and pirenzepine had similar effects as scopolamine, partially blocking acquisition and impairing long-term spatial memory. These results support the view that muscarinic receptors are involved in spatial learning and that postsynaptic muscarinic receptors in the dorsal hippocampus are particularly involved in long-term spatial memory formation.     

Altered hippocampal muscarinic receptors in acetylcholinesterase-deficient mice

A primary therapeutic strategy for Alzheimer's disease includes acetylcholinesterase (AChE) inhibitors with the goal of enhancing cholinergic transmission. Stimulation of muscarinic acetylcholine receptors (mAChRs) by elevated levels of ACh plays a role in the effects of AChE inhibitors on cognition and behavior. However, AChE inhibitors only demonstrate modest symptomatic improvements. Chronic treatment with these drugs may cause mAChR downregulation and consequently limit the treatment efficacy. AChE knockout (-/-) mice were utilized in this study as a model for investigating the effects of selective, complete, and chronic diminished AChE activity on mAChR expression and function. In AChE -/- mice, the M(1), M(2), and M(4) mAChRs showed strikingly 50 to 80% decreased expression in brain regions associated with memory. In addition, mAChRs showed decreased presynaptic, cell surface, and dendritic distributions and increased localization to intracellular puncta. Furthermore, mAChR agonist-induced activation of extracellular signal-regulated kinase, a signaling pathway associated with synaptic plasticity and amyloidogenesis, is diminished in the hippocampus and cortex of AChE -/- mice. Therefore, chronic diminished ACh metabolism produces profound effects on mAChR expression and function. The alterations of mAChRs in AChE -/- mice suggest that mAChR downregulation may contribute to the limited efficacy of AChE inhibitors in Alzheimer's disease treatment.     

M4 muscarinic receptors are involved in modulation of neurotransmission at synapses of Schaffer collaterals on CA1 hippocampal neurons in rats

All five subtypes of muscarinic acetylcholine receptors (mAChR; M(1)-M(5)) are expressed in the hippocampus, where they are involved both in cognitive functions and in synaptic plasticity, such as long-term potentiation (LTP). Muscarinic toxins (MTs) are small proteins from mamba snake venoms that display exquisite discrimination between mAChRs. MT1 acts as an agonist at M(1) and an antagonist at M(4) receptors, with similar affinities for both. MT3, the most selective antagonist available for M(4) receptors, infused into the CA1 region immediately after training caused amnesia in the rat, indicating the participation of M(4) receptors in memory consolidation. Our goal was to investigate the participation of M(4) receptor in neurotransmission at the hippocampal Schaffer collaterals-CA1 synapses. Two different preparations were used: 1) field potential recordings in freshly prepared rat hippocampal slices with high-frequency stimulation to induce potentiation and 2) whole-cell voltage clamp in cultured hippocampal organotypic slices with paired stimuli. In preparation 1, a dose of MT3 that was previously shown to cause amnesia blocked LTP; the nonselective antagonist scopolamine blocked LTP without affecting basal transmission, although it was depressed with higher concentration. In preparation 2, basal transmission was decreased and LTP induction was prevented by an MT3 concentration that would bind mainly to M(4) receptors. Although M(1) receptors appeared to modulate transmission positively at these excitatory synapses, M(1) activation concomitant with M(4) blockade (by MT1) only allowed a brief, short-term potentiation. Accordingly, M(4) blockade by MT3 strongly supports a permissive role of M(4) receptors and suggests their necessary participation in synaptic plasticity at these synapses.     

Lateral hypothalamic orexin neurons are critically involved in learning to associate an environment with morphine reward

Previously, we reported that lateral hypothalamic (LH) orexin neurons are stimulated in proportion to the preference shown for reward-associated cues during conditioned place preference (CPP) testing. Here, we examine for the first time the role of these neurons in the acquisition of morphine CPP. Results show that LH orexin neurons, but not those in the perifornical area (PFA), are stimulated during conditioning when morphine is given in a novel drug-paired environment (CPP compartment) but not when given in the home cage, nor when saline was given in the CPP environment. Furthermore, bilateral excitotoxic lesions of the LH orexin area completely blocked the acquisition of morphine CPP. Lesions that spared LH orexin neurons had no effect. Orexin neurons in the LH project to the ventral tegmental area (VTA), an area important in the acquisition of morphine CPP. Therefore, we investigated the importance of the LH orexin connection to the VTA in the acquisition of a morphine CPP using a disconnection technique involving a unilateral excitotoxic lesion of LH orexin neurons and contralateral blockade of VTA orexin receptors. Results indicated that a unilateral LH orexin lesion together with a microinjection of the orexin A antagonist (SB 334867) into the contralateral VTA prior to each morphine-pairing session was sufficient to block the development of a morphine CPP. Either of these treatments by themselves was not sufficient to block CPP development. These results demonstrate the importance of LH orexin neurons and their projections to the VTA in the formation of associations between environmental cues and drug reward.     

Seizures down-regulate muscarinic cholinergic receptors in hippocampal formation

Muscarinic cholinergic receptors (MCR) have been previously shown to decline in the hippocampal formation (HPF) of amygdala-kindled rats. Seizures have been proposed as the process responsible for this down-regulation. We now demonstrate similar down-regulation of MCR within HPF in 3 additional methods of inducing seizures: electroconvulsive shock, entorhinal kindling and entorhinal lesion. Two key parameters which causally link the MCR declines with seizures are their time course and reversal with anticonvulsants. The transient decline of MCR induced by entorhinal lesion-induced seizures parallels the time course established in amygdala kindling. Further, phenobarbital could block both these seizures and the MCR declines. Together, this supports the relationship of seizures causing the declines. We postulate that the MCR down-regulation represents an endogenous inhibitory response of neurons that are intensely and repeatedly depolarized during the seizures. neurons that are intensely and repeatedly depolarized during the seizures.     

Characterization of nicotinic receptors in immortalized hippocampal neurons

Nicotinic acetylcholine receptors, particularly nicotinic a-bungarotoxin (a-BGT) receptors, are present in relatively high concentrations in rat hippocampus. Because of the difficulties encountered in studying receptors using primary cells in culture, especially for biochemical work, we investigated the possibility of using an immortalized cell line from embryonic rat hippocampus (H19-7). RNase protection assays show that α4, α7 and β2 neuronal nicotinic receptor subunit mRNAs are present in differentiated but not undifferentiated H19-7 cells, while α2, α3, α5 and β3 subunit mRNAs were not detectable under either condition. In line with these results, the present data demonstrate that the H19-7 cells express cell surface nicotinic a-BGT binding sites, which were maximal after seven days of differentiation in culture. The receptors were saturable, of high affinity (K_d = 1.30 nM and B_max = 11.70 fmol/10⁵ cells) and had a pharmacological profile similar to that observed for CNS a-BGT receptors. On the other hand, although α4 and β2 neuronal nicotinic subunit mRNAs were present in differentiated H19-7 cells, no [3H]cytisine binding was observed. Because immortalized cell lines have the advantage that they provide a limitless supply of cells as compared to primary cell cultures, but yet are not malignant in origin, the present results may suggest that the H19-7 immortalized hippocampul cell line represent a useful CNS model system for examining α-BGT nicotinic receptors.     

Brain opioid-receptors are involved in mediating peripheral electric stimulation-induced inhibition of morphine conditioned place preference in rats

Conditioned place preference (CPP) paradigm has been suggested as one of the animal models for drug craving. The present study was performed to examine the effect of 100 Hz peripheral electric stimulation (PES) on the expression of morphine-induced CPP. Rats were trained with morphine for 4 days to establish the CPP paradigm in a three-chamber "unbiased" apparatus. Morphine-induced CPP was maintained up to 4 weeks when tests were given once a week. PES of 100 Hz administered 30 min a day for 3 days significantly attenuated morphine-induced CPP (P<0.01). I.c.v. injection of the delta-opioid receptor antagonist naltrindole (NTI) or the kappa-antagonist norbinaltorphimine (nor-BNI) but not the mu-antagonist cyclic D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), completely blocked the inhibitory effect of 100 Hz PES on the expression of morphine-induced CPP (P<0.05-0.01). These results indicate that the anti-craving effects induced by repeated PES of 100 Hz is mediated by the activation of supra-segmental delta- and kappa-opioid receptors in the central nervous system.     

Joint modulation of neocortical electrical activity by nicotinic and muscarinic receptors

A combination of subthreshold doses of nicotinic and muscarinic antagonist induced an increase in all the spectral components. The spectral amplitude change induced by a large scopolamine dose could not be augmented by either mecamylamine or an additional injection of scopolamine. NB lesions increased frontal slow wave activity (delta and theta amplitudes). Single or combined injections of scopolamine and mecamylamine produced a significantly smaller change in the frontal EEG amplitude values of NB-lesioned rats than in controls.     

Are postsynaptic nicotinic end-plate receptors involved in lead toxicity?

The effects of Pb2+ on the postsynaptic nicotinic end-plate receptor were examined in the perfused mouse hemidiaphragm preparation. Postsynaptic nicotinic responses, evoked by pressure ejection of ACh, were blocked by Pb2+ in a transient way. After 9-12 min of exposure to 1 microM Pb2+ the amplitude of the depolarization induced by 1 mM ACh was reduced to 39.5 +/- 11% of the control value. During continued exposure to Pb2+ this blocking effect was reversed and after 30 min of exposure to 1 microM Pb2+ the amplitude of the ACh-induced depolarization had returned to the control value. The amplitude and the frequency of miniature end-plate potentials were not altered in the presence of 1 microM Pb2+. Under voltage clamp conditions the effects of Pb2+ on the ACh-induced inward current were similar to those of Pb2+ on the ACh-induced depolarization. After 12 min of exposure to 1 microM Pb2+ the inward current induced by 1 mM ACh was reduced to 44% of the control value and after 30 min the ACh-induced inward current had recovered to 94% of the control value. It is concluded that, in addition to the generally established mechanism of action of Pb2+ at the muscle end-plate, Pb2+ blocks the postsynaptic nicotinic receptor-mediated response at a relative low concentration. The contribution of these postsynaptic effects to the neurotoxic symptoms of Pb2+ remains to be established.     

Sphingomyelinase selectively reduces M1 muscarinic receptors in rat hippocampal membranes

Although there is evidence that nicotinic acetylcholine (Ach) receptors are influenced by ceramides, we do not currently know whether or not these sphingolipids can also regulate the muscarinic subtypes of Ach receptors. Using the whole-cell patch technique, we demonstrated that the effectiveness of the muscarinic receptor agonist pilocarpine, in enhancing spontaneous inhibitory postsynaptic currents in CA1 pyramidal cells, was completely abolished in hippocampal slices pre-exposed to the ceramide-generating enzyme sphingomyelinase (SMase). Western blot experiments, performed with biotinylated hippocampal membranes, showed that this electrophysiological defect possibly relies on the loss of M1 muscarinic Ach receptors at the cell surface. However, the effect appears to be relatively specific as the cell-surface expression of M4 muscarinic receptors was not found to be impacted by SMase treatment. Interestingly, we observed that G protein-coupled receptor kinases 2 and β-arrestin1/2 interactions with M1-immunoprecipitated proteins were substantially augmented in SMase-treated slices and that the reduction of cell-surface M1 muscarinic receptor expression generated was completely suppressed by the muscarinic antagonist atropine. Collectively, our data suggest that selective internalization of M1 muscarinic receptors can be accentuated in neurons subjected to high ceramide levels. The potential physiopathological implications of this finding are presented.     

Hippocampal sympathetic ingrowth and cholinergic denervation alter hippocampal muscarinic cholinergic receptors

Cholinergic denervation of the hippocampus by medial septal (MS) lesions results in the ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglia, into the hippocampus. To determine the effect of hippocampal sympathetic ingrowth (HSI) [³H]-QNB (L-quinuclidinyl [benzilic-4, 4(n)] benzilate) binding was assessed in the dorsal and ventral hippocampus four weeks after MS lesions. In dorsal hippocampus, HSI was found to signignificantly increase the number (B_max) of [³H]-QNB binding sites and to normalize the decrease in affinity found in animals with MS lesions plus ganglionectomy (i. e., no ingrowth). In ventral hippocampus, HSI was found to normalize the increased number of binding sites and decreased affinity found in animals with MS lesions without ingrowth. No effect on either K_d or B_max was found in animals that had undergone ganglionectomy with sham MS lesions. These results suggest that HSI can induce changes in hippocampal muscarinic cholinergic receptors. © 1994 Wiley-Liss, Inc.     

High-affinity nicotinic acetylcholine receptors are required for antidepressant effects of amitriptyline on behavior and hippocampal cell proliferation.

BACKGROUND: A wide variety of antidepressants act as noncompetitive antagonists of nicotinic acetylcholine receptors (nAChRs), but the relationship between this antagonism and the therapeutic effects of antidepressants is unknown. METHODS: Antidepressant properties of the noncompetitive nAChR antagonist mecamylamine in the forced swim test were tested alone and in combination with the tricyclic antidepressant amitriptyline. Mice lacking high-affinity nAChRs were tested in three behavioral models to determine whether these receptors are required for behavioral effects of amitriptyline in common models of antidepressant action. Finally, the brains of wild-type and knockout animals treated with amitriptyline were examined to determine whether high-affinity nAChRs are required for antidepressant-induced increases in hippocampal cell proliferation. RESULTS: Inhibition of nAChRs by mecamylamine had antidepressant-like effects in the forced swim test and potentiated the antidepressant activity of amitriptyline when the two drugs were used in combination. Mice lacking high-affinity nAChRs showed no behavioral response to amitriptyline. Finally, after chronic treatment with amitriptyline, nAChR knockout mice did not show the increase in hippocampal cell proliferation seen in wild-type mice. CONCLUSIONS: These data support the hypothesis that antagonism of nAChRs is an essential component of the therapeutic action of antidepressants.     

NMDA receptors, mGluR5, and endocannabinoids are involved in a cascade leading to hippocampal long-term depression

In the CA1 region of the rat hippocampus, metabotropic glutamate receptor-5 (mGluR5) and cannabinoid-1 receptors (CB1Rs) are believed to participate in long-term synaptic depression (LTD). How mGluRs and CB1Rs interact to promote LTD remains uncertain. In this study, we examined LTD induced by CB1R agonists, mGluR5 agonists, and low-frequency electrical stimulation (LFS) of the Schaffer collateral pathway. Synthetic CB1R agonists induced robust LTD that was mimicked by the endocannabinoid (EC), noladin ether (NLDE), but not by anandamide. 2-Arachidonylglycerol (2AG) produced only a small degree of LTD. The selective mGluR5 agonist, namely (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), also induced robust LTD, and CHPG and NLDE occluded each other's effects. Similarly, CHPG and NLDE occluded LFS-induced LTD, and LTD resulting from all three treatments was blocked by a CB1R antagonist. CHPG-LTD and NLDE-LTD were insensitive to N-methyl-D-aspartate receptor (NMDAR) block, even though LFS-LTD requires NMDARs. LTD induced by LFS or CHPG, but not NLDE-LTD, was blocked by a selective mGluR5 antagonist. (RS)-3,5-dihydroxyphenylglycine (DHPG), a less selective group I mGluR agonist, also induced LTD, but its effects were not blocked by mGluR5 or CB1R antagonists. Furthermore, DHPG-LTD was additive with LFS-LTD and CHGP-LTD. These results suggest that NMDARs, mGluR5, and CB1Rs participate in a cascade that underlies LFS-LTD and that release of an EC and CB1R activation occur downstream of NMDARs and mGluR5. Furthermore, DHPG induces a form of LTD that differs mechanistically from LFS-induced depression.     

Cholinergic nicotinic and muscarinic receptors in dementia of Alzheimer, Parkinson and Lewy body types

Summary Cholinergic nicotinic and muscarinic receptor binding were measured in post mortem human brain tissue, using low (nM) concentrations of (³H)-nicotine to detect predominately the high affinity nicotinic site and (³H)-N-methylscopolamine in the presence and absence of 3×10^–4 M carbachol to measure both the low and high affinity agonist subtypes of the muscarinic receptor group. Consistent with most previous reports, the nicotinic but not muscarinic binding was reduced in the different forms of dementia associated with cortical cholinergic deficits, including Alzheimer's and Parkinson's disease, senile dementia of Lewy body type (SDLT) and Down's syndrome (over 50 years). Analysis of (³H)-nicotine binding displaced by a range of carbachol concentrations (10^–9–10^–3 M) indicated 2 binding sites for nicotine and that the high affinity rather than low affinity site was reduced in Alzheimer's disease. In all 3 cortical areas investigated (temporal, parietal and occipital) there were increases in the low affinity muscarinic site in Parkinson's disease and SDLT but not Alzheimer's disease or middle-aged Down's syndrome. This observation raised the question of whether the presence of neurofibrilalry tangles (evident in the latter but not former 2 disorders) is incompatible with denervation-induced muscarinic supersensitivity in cholinoceptive neurons which include cortical pyramids generally affeced by tangle formation.     

Differential regulation of muscarinic and nicotinic receptors by cholinergic stimulation in cultured avian retina cells

Sustained cholinergic stimulation of retina cells grown in primary aggregate and monolayer cultures regulated the concentration of muscarinic but not nicotinic receptors. Muscarinic receptor sites, quantified by the binding of [³H]quinuclidinyl benzilate to membranes and the binding of [³H]N-methyl-scopolamine to intact cells, decreased up to 84% following long-term incubation of cultures in muscarinic agonists. This decrease was blocked by atropine and was not induced by chronic nicotine treatment. The rate of the muscarinic response was biphasic. A rapid binding decrease of 30% occurred within 15 min. The slower phase was half-maximal by 6 h and was complete by 24 h. Neither the fast nor the slow receptor loss was reversed by the guanine nucleotide GppNp. Three different depolarizing agents (gramicidin D, protoveratrine, and ouabain) blocked the cholinergic-induced receptor loss, but the hyperpolarizing ionophore valinomycin had no effect. In contrast to the muscarinic response, nicotinic receptor binding was not altered by chronic receptor stimulation. Exposure to receptor-saturating doses of carbamylcholine or nicotine for 48 h did not change [¹²⁵I]α-bungarotoxin or [³H]bromoacetylcholine binding. Differential regulation of acetylcholine receptors is discussed in relation to the possible physiological role of receptor regulation by receptor activity.     

Cingulate cell discharge patterns related to hippocampal EEG and their modulation by muscarinic and nicotinic agents

Surface to depth recordings of slow wave θ activity were made through the posterior cingulate cortex (area 29). In other experiments the discharge patterns of neurons located throughout this region were recorded during the occurrence of large amplitude irregular activity (LIA) and slow wave θ activity (type 2) in the hippocampal formation of the urethane-anesthetized rat. The response of these neurons to the systemic administration of the cholinergic agonists, eserine and nicotine, was also tested. The majority of cells (80%) related to hippocampal EEG states were classified as tonic θ-on cells (non-rhythmic). These cells increased their discharge rates significantly, when the slow wave activity in the hippocampus changed from LIA to θ. The administration of eserine and nicotine induced slow wave θ in the hippocampus accompanied by the increased discharge rate of tonic θ-on cingulate cells. The excitatory action of nicotine on the discharges of tonic θ-on cingulate cortical cells is in direct contrast to its inhibitory action on phasic and tonic hippocampal θ-on cell discharges. The observation that phase reversals did not occur in area 29, together with the low incidence of phasic (rhythmic) θ-on cells, suggests that the posterior cingulate cortex does not independently generate type 2 θ. Possible significance of these findings for the interactive functioning of the hippocampal formation and posterior cingulate cortex is discussed.     

Metabotropic glutamate receptors in the hippocampus and nucleus accumbens are involved in generating seizure-induced hippocampal gamma waves and behavioral hyperactivity

The involvement of metabotropic glutamate receptor (mGluR) subtypes in the generation of hippocampal EEG (30-100 Hz) and behaviors induced by a hippocampal afterdischarge (AD) was examined in freely behaving rats. A hippocampal AD induced an increase in gamma waves (30-100 Hz) for 20 min, accompanied by behavioral hyperactivity. Bilateral intracerebroventricular (i.c.v.) infusion of (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), a group I and II mGluR antagonist, 30 min before a hippocampal AD, significantly suppressed both the increase in gamma waves and the behavioral hyperactivity. The hippocampal theta rhythm, the spontaneous hippocampal gamma waves, and evoked field potential oscillations of approximately 40 Hz were not affected by MCPG. Pre-infusion (i.c.v.) of (2S)-alpha-ethylglutamic acid (EGLU; a group II mGluR antagonist), but not (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; a group I mGluR antagonist), suppressed the postictal increase of both hippocampal gamma waves and behaviors. MCPG was infused locally into different brain structures in order to specify its target sites. Intra-hippocampal infusion of MCPG, or EGLU, blocked the increase in both gamma waves and behaviors. Infusion of MCPG into the nucleus accumbens suppressed the postictal behavioral hyperactivity without affecting the increase in hippocampal gamma waves. MCPG injected into the medial septum blocked neither postictal gamma activity nor behavioral hyperactivity. It is suggested that the group II mGluRs in the hippocampus are involved in generation of the postictal hippocampal gamma waves, while behavioral hyperactivity is partly mediated by mGluRs in the nucleus accumbens. However, spontaneous gamma and theta waves in the normal hippocampus are not mediated by mGluRs.     

Metabotropic glutamate receptors are involved in amygdaloid plasticity

The amygdala plays an important role in emotional learning. Synaptic plasticity underlying the acquisition of conditioned fear occurs in the lateral nucleus of the amygdala: long-term potentiation (LTP) of synapses in the pathway of the conditioned stimulus (CS) has shown to be a neural correlate of this kind of emotional learning. The present study is based on previous results of our laboratory showing an important role of the metabotropic glutamate receptor subtype 5 (mGluR5) in fear conditioning. Here, we explored whether mGlu5 receptors within the lateral nucleus of the amygdala are involved in the plasticity underlying fear conditioning. We used an in vivo approach investigating the acquisition, consolidation and expression of conditioned fear by the fear-potentiated startle paradigm and by the inhibition of motor activity during CS presentation. Additionally, we used an in vitro approach inducing LTP in the lateral amygdala by patch-clamp recordings in rat brain slices. Acquisition of conditioned fear, but not consolidation and expression, was blocked by intra-amygdaloid injections of the specific mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) in vivo. Furthermore, induction of amygdaloid LTP but not synaptic transmission was disrupted by MPEP application in vitro. These experiments show for the first time in vivo and in vitro that mGluR5 receptors are necessary for plasticity in the amygdala.     

Differential regulation of muscarinic and nicotinic cholinergic receptors and their mRNAs in cultured sympathetic neurons.

Mechanisms regulating expression of neuronal muscarinic and nicotinic receptors were examined in cultures of neonatal rat sympathetic neurons. Two factors known to stimulate cholinergic transmitter development in sympathetic neurons were examined for their effects on cholinergic receptor expression. A membrane associated factor (MANS46) and a diffusible factor produced by cultured rat fibroblasts (RFCM) each decreased muscarinic receptor number. By contrast, neither treatment altered levels of nicotinic receptors. Levels of muscarinic (m2) receptor mRNA were decreased by MANS but not by RFCM, indicating that effects of the two treatments were mediated by different mechanisms. Neither MANS nor RFCM altered levels of nicotinic alpha 3 or beta 2 mRNAs, consistent with the lack of change in numbers of nicotinic receptors. These observations indicate that receptor phenotype in developing neurons is subject to regulation by multiple epigenetic factors. Further, the same signals which regulate transmitter development may also regulate receptor expression in sympathetic neurons.