Elective, Non-urgent Procedures and Aesthetic Surgery in the Wake of SARS-COVID-19: Considerations Regarding Safety, Feasibility and Impact on Clinical Management.

BACKGROUND: The worldwide spread of a novel coronavirus disease (COVID-19) has led to a near total stop of non-urgent, elective surgeries across all specialties in most affected countries. In the field of aesthetic surgery, the self-imposed moratorium for all aesthetic surgery procedures recommended by most international scientific societies has been adopted by many surgeons worldwide and resulted in a huge socioeconomic impact for most private practices and clinics. An important question still unanswered in most countries is when and how should elective/aesthetic procedures be scheduled again and what kind of organizational changes are necessary to protect patients and healthcare workers when clinics and practices reopen. Defining manageable, evidence-based protocols for testing, surgical/procedural risk mitigation and clinical flow management/contamination management will be paramount for the safety of non-urgent surgical procedures. METHODS: We conducted a MEDLINE/PubMed research for all available publications on COVID-19 and surgery and COVID-19 and anesthesia. Articles and referenced literature describing possible procedural impact factors leading to exacerbation of the clinical evolution of COVID-19-positive patients were identified to perform risk stratification for elective surgery. Based on these impact factors, considerations for patient selection, choice of procedural complexity, duration of procedure, type of anesthesia, etc., are discussed in this article and translated into algorithms for surgical/anesthesia risk management and clinical management. Current recommendations and published protocols on contamination control, avoidance of cross-contamination and procedural patient flow are reviewed. A COVID-19 testing guideline protocol for patients planning to undergo elective aesthetic surgery is presented and recommendations are made regarding adaptation of current patient information/informed consent forms and patient health questionnaires. CONCLUSION: The COVID-19 crisis has led to unprecedented challenges in the acute management of the crisis, and the wave only recently seems to flatten out in some countries. The adaptation of surgical and procedural steps for a risk-minimizing management of potential COVID-19-positive patients seeking to undergo elective aesthetic procedures in the wake of that wave will present the next big challenge for the aesthetic surgery community. We propose a clinical algorithm to enhance patient safety in elective surgery in the context of COVID-19 and to minimize cross-contamination between healthcare workers and patients. New evidence-based guidelines regarding surgical risk stratification, testing, and clinical flow management/contamination management are proposed. We believe that only the continuous development and broad implementation of guidelines like the ones proposed in this paper will allow an early reintegration of all aesthetic procedures into the scope of surgical care currently performed and to prepare the elective surgical specialties better for a possible second wave of the pandemic. LEVEL OF EVIDENCE V: This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266.

Metabotropic glutamate receptors: a new target for the therapy of neurodegenerative disorders?

Metabotropic glutamate (mGlu) receptors are a large, heterogeneous family of G-protein coupled receptors, which modulate excitatory synaptic transmission through various transduction pathways. Evidence is now accumulating that individual mGlu-receptor subtypes mediate distinct, facilitatory (group I subtypes) or inhibitory (group II and group III subtypes), actions on neurodegenerative processes. Drugs interacting with mGlu receptors are expected to influence both the induction and progression of neuronal degeneration without hampering the efficiency of fast excitatory synaptic transmission. For these reasons, mGlu receptors can be considered as promising drug targets in the experimental therapy of acute or chronic neurodegenerative diseases.

Fear conditioning-induced time- and subregion-specific increase in expression of mGlu5 receptor protein in rat hippocampus.

Memory formation involves encoding, consolidation and retention. These processes have been the subjects of considerable research, but physiological mechanisms underlying consolidation have proved difficult to dissociate experimentally. Previous reports have indicated a role for metabotropic glutamate receptors (mGluRs) in memory formation, and we here examined the specific role of mGluRs in the consolidation phase of memory formation. Particular weight was given to the hippocampus due to a high expression level for group I mGluRs and its outstanding role in spatial learning. Rats were first trained in a combined context and cue conditioning paradigm. Then, ex vivo analysis of neuronal tissue taken from hippocampal CA1, CA3 or dentate gyrus of behaviourally trained animals showed a 3-fold hyper-expression of mGluR5 protein in CA3 one day after acquisition training. This increase was transient and greatly diminished within ten days. The decline was paralleled by an increase in mGluR5 protein expression in CA1 and, to a lesser extent, in dentate gyrus, ten days posttraining. Overexpression in CA1 was also obtained after 9 days of extinction training. These data provide new insight into the role of the hippocampus and its subregions in memory consolidation. They support the notion that mGluRs in CA3 may play a part in short-term, and those in CA1 may play a part in long-term consolidation of memory.

Activation of metabotropic glutamate receptors coupled to inositol phospholipid hydrolysis amplifies NMDA-induced neuronal degeneration in cultured cortical cells.

We have studied the influence of class I metabotropic glutamate receptors (mGluRs) on excitotoxic neuronal degeneration in cultured murine cortical neurons grown on a monolayer of astrocytes. These cultures expressed high levels of mGluR5 mRNA, which were comparable to those found in RNA extracts from cerebral cortex. Cortical neurons in mixed cultures were heavily stained with antibodies raised against mGluR5 and were also stained--albeit to a much lower extent--with mGluR1a but not with mGluR1b or c antibodies. Preferential agonists of class I mGluRs, such as quisqualate, 3,5-dihydroxyphenylglycine (DHPG), and trans-azetidine-2,4-dicarboxylic acid (t-ADA), as well as the mixed mGluR agonist, 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) all stimulated PPI hydrolysis in cultured cortical cells. The potency of N-methyl-D-aspartate (NMDA) in inducing neuronal degeneration was substantially enhanced when the drug was coincubated with quisqualate, DHPG or t-ADA during a 10-min pulse (paradigm of "fast" toxicity). None of the mGluR agonists influenced neuronal viability by itself. The amplification of NMDA toxicity by quisqualate or DHPG was attenuated by a series of protein kinase C (PKC) inhibitors, suggesting that class I mGluRs operate, at least in part, through activation of PKC. Quisqualate and, in particular, DHPG enhanced excitoxic neuronal degeneration even when applied after the toxic pulse with NMDA. This action is likely to occur early in the maturation of excitotoxic damage, because the functional activity of class I mGluRs was substantially reduced at 2 or 3 hr after the NMDA pulse. These results suggest that activation of class I mGluRs enhances NMDA-receptor mediated neuronal toxicity and encourage the search for selective antagonists for the experimental therapy of acute or chronic neurodegenerative diseases.

Melittin enhances excitatory amino acid release and AMPA-stimulated 45Ca2+ influx in cultured neurons.

Melittin, a potent activator of phospholipase A2, enhanced both spontaneous and depolarization-induced release of D-[3H]aspartate in primary cultures of cerebellar granule cells. The action of melittin was concentration-dependent (EC50 value = 300 ng/ml) and did not require the presence of extracellular Ca2+. Melittin also stimulated the release of glutamate and aspartate, in addition to other endogenous amino acids (taurine, alanine and gamma-aminobutyric acid). These effects were accompanied by an enhanced influx of 45Ca2+, which was in part mediated by the activation of excitatory amino acid receptors by endogenous agonists. Low concentrations of melittin (50 ng/ml) potentiated the efficacy of AMPA in stimulating 45Ca2+ influx without affecting stimulation by kainate or by glutamate added in the absence of extracellular Mg2+ (a condition that favors the activation of NMDA receptors). These results indicate that activation of phospholipase A2 evokes both an enhanced glutamate release and an increased sensitivity of AMPA receptors, two events that may support synaptic facilitation and LTP formation.

Amyloid beta protein does not interact with tachykinin receptors coupled to inositol phospholipid hydrolysis in human astrocytoma cells.

We have tested the interaction between amyloid beta protein (A beta P) and tachykinin receptors in cultured UC-11MG astrocytoma cells, which express high affinity substance P receptors and respond to substance P with an unusually large stimulation of polyphosphoinositide hydrolysis. Both the full-length A beta P (A beta P1-40) and the fragment 25-35 (A beta P25-35) did not affect the stimulation of [3H]inositolmonophosphate (InsP) formation by substance P. A beta P25-35 was also inactive when applied to the cultures 18 or 72 h prior to the assay. In addition, A beta P25-35 did not displace specifically bound [3H]SarMet substance P from its recognition sites in intact UC-11MG cells. These results suggest that, at least in this specific cell type, amyloid peptides do not interact with substance P receptors.

Predominant expression of group-II metabotropic glutamate receptors in the goldfish brain.

Group-II metabotropic glutamate (mGlu) receptors (mGlu2/3 receptors) were highly expressed in various regions (telencephalon, optic tectum, and cerebellum, but not vagal lobe) of the goldfish brain. In the goldfish telencephalon, expression of mGlu2/3 receptors was even higher than in the rat cerebral cortex. In contrast, mGlu5 receptors showed low levels of expression in all goldfish brain regions, whereas mGlu1a receptors were only expressed in the goldfish cerebellum. Pharmacological activation of group-II mGlu receptors with the selective agonists, 2R,4R-4-aminopyrrolidine-2, 4-dicarboxylic acid and (2S,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine, reduced the evoked release of glutamate from goldfish brain synaptosomes, whereas agonists of group-I and -III mGlu receptors (3, 5-dihydroxyphenylglycine and L-2-amino-4-phosphonobutanoate) were inactive. The predominance of group-II over group-I mGlu receptors in the goldfish brain may provide a natural defense against excitotoxic neuronal death and contribute to the unusually high resistance of goldfish against hypoxic brain damage.

(2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine positively modulates metabotropic glutamate receptors coupled to polyphosphoinositide hydrolysis in rat hippocampal slices.

In rat hippocampal slices, the novel metabotropic glutamate receptor (mGluR) ligand, (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) enhanced the stimulation of polyphosphoinositide (PPI) hydrolysis elicited by quisqualate or by submaximal concentrations of ibotenate or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The enhancing effect of DCG-IV was (i) specific for mGluR agonists, (ii) restricted to hippocampal slice preparation, (iii) reversible, and (iv) not subject to homologous desensitization, in addition, DCG-IV did not interact with L-2-amino-4-phosphonobutanoate (AP4), a noncompetitive antagonist of mGluRs coupled to PPI hydrolysis in brain slices [32]. The action of DCG-IV on quisqualate-stimulated PPI hydrolysis was insensitive to antagonists of ionotropic glutamate receptors and did not appear to be a consequence of a reduction in the intracellular levels of cAMP [14]. When the stimulation of PPI hydrolysis was measured as a function of the incubation time, DCG-IV potentiated quisqualate-stimulated PPI hydrolysis after 60 min of incubation, when quisqualate had already reached its maximal effect. Knowing that activation of protein kinase C (PKC) limits the extent of mGluR agonist-stimulated PPI hydrolysis over time, we have studied the enhancing effect of DCG-IV in the presence of the PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA). As expected [9], TPA reduced quisqualate-stimulated PPI hydrolysis in control slices, but was inactive in slices incubated in the presence of DCG-IV. Taken collectively, these results suggest that DCG-IV positively modulates the activity of mGluRs coupled to PPI hydrolysis through a mechanism, which involves PKC-mediated phosphorylation processes.

Interaction between metabotropic receptors and purinergic transmission in rat hippocampal slices.

Inhibition of forskolin-stimulated cAMP formation by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices was partially obliterated by the adenosine-depleting enzyme, adenosine deaminase, or by the adenosine receptor agonist, 5'-(N-ethylcarboxamido)-adenosine, suggesting that activation of metabotropic glutamate receptors (mGluRs) modulates the release of endogenous adenosine. Consistent with this hypothesis, forskolin stimulated the release of purines from rat hippocampal slices, and this effect was reduced by 1S,3R-ACPD. To establish which transduction pathway is involved in the modulation of forskolin-stimulated purine release, we have tested the novel mGluR2 agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), which reduced forskolin-stimulated cAMP formation but, as opposed to 1S,3R-ACPD, did not stimulate polyphosphoinositide hydrolysis. DCG-IV was highly potent and more efficacious than 1S,3R-ACPD in inhibiting forskolin-stimulated purine release. Neither DCG-IV nor 1S,3R-ACPD reduced the release of purines stimulated by depolarizing concentrations of K+, suggesting that their effect was stimulus-specific. These results indicate that, in rat hippocampal slices, activation of mGluR2 receptors attenuates the release of purines induced by forskolin, a process that amplifies the final effect of forskolin on cAMP formation as a result of A2 purinergic receptor activation. Thus, the final effect of mGluR agonists on forskolin-stimulated cAMP formation in hippocampal slices depends on both a direct inhibition of adenylyl cyclase and the inhibition of adenosine release.

Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices.

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR decreases cAMP) in brain slices. Among the putative mGluR agonists, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, L-2-amino-3-phosphonopropionate (AP3), quisqualate, L-glutamate and beta-N-methylamino-L-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR decreases cAMP subtype. Both L-AP4 and L-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their D-isomers, indicating that interaction of these drugs with the mGluR decreases cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR decreases cAMPS, as estimated by using 1S,3R-ACPD as an agonist, did not correlate with the steady-state levels of mGluR2 mRNA. Thus, 1S,3R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1S,3R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR decreases cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Reducing conditions differentially affect the functional and structural properties of group-I and -II metabotropic glutamate receptors.

We have examined the influence of reducing conditions on the activity of group-I or -II metabotropic glutamate receptors. In cultured cerebellar granule cells or in hippocampal slices, the reducing agent dithiothreitol (DTT) inhibited the stimulation of polyphosphoinositide (PPI) hydrolysis elicited by group-I mGlu receptor agonists without affecting responses to norepinephrine or carbamylcholine. Similarly, DTT reduced the increase in intracellular free Ca(2+) induced by glutamate in HEK-293 cells expressing mGlu5 receptors. In adult hippocampal slices, the selective group-II mGlu receptor agonist, (2S,1'R,2'R,3'R)-2-(2, 3-dicarboxycyclopropyl)glycine (DCG-IV) had no effect per se on PPI hydrolysis, but potentiated the response to quisqualate. Although DTT substantially attenuated the action of quisqualate, it did not affect the potentiation by DCG-IV, suggesting that group-II mGlu receptors are resistant to extracellular reduction. Accordingly, DTT did not affect the inhibition of forskolin-stimulated cAMP formation induced by maximally effective concentrations of group-II mGlu receptor agonists in hippocampal slices or in CHO cells expressing mGlu2 receptors. At structural level, DTT differentially affected the aggregation state of mGlu1a, -2/3 or -5 receptors. In immunoblots performed under non-reducing conditions, mGlu1a, -2/3 or -5 antibodies labeled exclusively a high-molecular weight band, corresponding to receptor dimers. Under reducing conditions, mGlu1a or -5 receptors were detected as monomers, whereas a large proportion of mGlu2/3 receptors was still present in a dimeric form. We conclude that reducing conditions differentially influence the aggregation state of group-I and -II mGlu receptors and suggest that dimerization affects the functional activity of native mGlu receptors.

(2s,1'R,2'R,3'R)-2-(2,3-Dicarboxycyclopropyl) glycine enhances quisqualate-stimulated inositol phospholipid hydrolysis in hippocampal slices.

In adult rat hippocampal slices, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) did not affect the basal hydrolysis of polyphosphoinositides but dramatically enhanced (EC50 value = 30 nM) the stimulation of [3H]inositol monophosphate (InsP) formation by quisqualate, without substantially affecting the stimulation produced by maximal concentrations of 1S,3R-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) or carbamylcholine. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) was virtually devoid of activity on [3H]InsP formation, either in the absence or presence of DCG-IV. These results suggest that DCG-IV acts, directly or indirectly, as a positive modulator of metabotropic glutamate receptors in the rat hippocampus.

Thyrotropin releasing hormone (TRH) and its analog, RGH-2202, accelerate maturation of cerebellar neurons in vitro.

We have studied the "trophic" action of thyrotropin releasing hormone (TRH) in cultured cerebellar granule cells, a pure and homogeneous population of glutamatergic neurons. As an index of neuronal maturation, we have measured the uptake of D-[3H]aspartate (a non-metabolizable analog of glutamate) at different days of maturation in vitro (DIV). In control cultures, D-[3H]Aspartate increased linearly during maturation reaching plateau values between 7 and 9 DIV; daily addition of TRH tartrate (TRH-t) or RGH-2202 (a TRH analog) accelerated in a concentration-dependent manner the maturation profile of D-[3H]aspartate uptake. This effect was more pronounced for RGH-2202: in cultures treated daily with RGH-2202, D-[3H]aspartate uptake was fully expressed after 3 DIV. Neither TRH-t nor RGH-2202 significantly increased D-[3H]aspartate uptake in mature cells, excluding a direct action on the glutamate transport system. Both compounds specifically potentiated the increase in [3H]inositol monophosphate formation (but not the stimulation of 45Ca2+ influx) induced by N-methyl-D-aspartate (NMDA) receptor agonists, without affecting the stimulation of inositol phospholipid hydrolysis by quisqualate or carbamylcholine. We suggest that, in cultured cerebellar granule cells, TRH and RGH-2202 enhance the trophic action of endogenous glutamate by amplifying some of the intracellular events that follow the influx of extracellular Ca2+ through NMDA-gated ion channels.

Intracellular signal modulation: a pivotal role for protein kinase C.

1. Protein Kinase C represents a family of Ca(2+)- and phospholipid-dependent enzymes which catalyzes the covalent transfer of phosphate from ATP to serine and threonine residues on proteins. Phosphorylation of the substrate protein induces a conformational change and thereby a modification of its functional properties. 2. PKC family consists of at least twelve members, divided in three subgroups:classical PKCs, (alpha, beta I, beta II, gamma), new PKCs, (delta, epsilon, eta, theta, mu) and atypical PKCs, (zeta, lambda, iota). The three subgroups differ in cofactors requirements and tissue expression; these differences in co-activators dependency and regional distribution account for the differential activation profile of the various PKC isoenzymes. 3. Different molecules involved in the intracellular signaling network are phosphorylated "in vitro" and "in vivo" by PKC. Many target proteins show a preferential pattern of phosphorylation by the different PKC isotypes. 4. Through phosphorylation PKC modulates the functional activity of many different intracellular signaling systems which transport extracellular messages from the membrane to the nucleus. 5. The induction of apoptotic processes by the protein kinase inhibitor staurosporine indicates a possible role for PKC in the modulation of the intracellular mechanisms leading to Programmed Cell Death. 6. Abnormalities in both levels and activity of PKC, recently found in some chronic neurodegenerative syndromes, lead to the possibility that PKC dysfunction could be involved in the pathogenetic mechanisms of disease.

HPLC analysis of cyclic adenosine diphosphate ribose and adenosine diphosphate ribose: determination of NAD+ metabolites in hippocampal membranes.

Cyclic adenosine diphosphate-ribose (cADPR) and ADPR were separated by high-performance liquid chromatography (HPLC) on a CarboPac PA-1 column at strong basic pH and quantitated by a pulsed amperometric detector. Although this HPLC method was quite sensitive and highly reproducible, it did not allow the separation of cADPR from guanosine monophosphate (GMP) which, when present, could be removed by ion-affinity chromatography, using gel-immobilized Fe3+ columns. Crude synaptic membranes from rat hippocampi were incubated with nicotinamide adenine dinucleotide (NAD) and acidic extracts were subject to HPLC analysis after neutralization. Incubation led to a time-dependent formation of ADPR, which was amplified when membranes were incubated in the presence of guanosine trisphosphate (GTP), guanosine-5'-0-(3-thiotrisphosphate) (GTP-gamma-S) or AlF3. cADPR did not accumulate in detectable amounts and only a minimal proportion (< 5%) of radioactivity originating from [3H]NAD co-eluted with authentic cADPR in extracts from hippocampal membranes. The simultaneous detection of cADPR and ADPR we have described may help the search for inhibitors of cADPR metabolism, which will allow to measure the cADPR that accumulates under basal conditions or in response to extracellular signals.

Excitatory amino acids and neuronal plasticity: modulation of AMPA receptors as a novel substrate for the action of nootropic drugs.

The nootropic drug, aniracetam, behaves as a positive modulator of AMPA-sensitive glutamate receptors in a variety of systems, including intact brain tissue, amphibian oocytes injected with rat brain mRNA, and cultured neurons. In electrophysiological studies, aniracetam both increases the peak amplitude and reduces the rate of decay of the ion current generated by AMPA or quisqualate. In cultured neurons, aniracetam (as well as oxiracetam and piracetam) enhances the stimulation of 45Ca2+ influx produced by AMPA but not that produced by kainate or NMDA. In addition, aniracetam (as other nootropic drugs) increases the maximal density of low affinity binding sites for [3H]AMPA in crude synaptic membranes. Positive modulation of AMPA receptors by aniracetam provides a novel molecular substrate which explains the clinical efficacy of nootropic drugs as memory and cognition enhancers.

Repeated injections of piracetam improve spatial learning and increase the stimulation of inositol phospholipid hydrolysis by excitatory amino acids in aged rats.

Repeated injections of piracetam (400 mg/kg, i.p. once a day for 15 days) to 16-month old rats led to an improved performance on an 8-arm radial maze, used as a test for spatial learning. This effect was accompanied by a greater ability of excitatory amino acids (ibotenate and glutamate) to stimulate [3H]inositol-monophosphate formation in hippocampal slices incubated in the presence of 10 mM Li+. Repeated administration of piracetam did not induce changes in excitatory amino acid-stimulated polyphosphoinositide hydrolysis in hippocampal slices prepared from 2-month old animals.

Inhibition of long-term potentiation in the dentate gyrus of freely moving rats by the metabotropic glutamate receptor antagonist MCPG.

Metabotropic glutamate receptors (mGluRs) are critically involved in the maintenance of long-term potentiation (LTP) (Reymann and Matthies, 1989; Behnisch et al., 1991; Izumi et al., 1991; Bashir et al., 1993). In order to assess further the physiological role of MGluRs in LTP, we injected freely moving rats with the recently available, competitive mGluR antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) intraventricularly and recorded extracellularly the population spike (PS) as well as the field excitatory postsynaptic potential (fEPSP) of the granule cells of the dentate gyrus in response to stimulation of fibers of the perforant path. MCPG was administered in two concentrations (A = 20 mM/5 microliters; B = 200 mM/5 microliters) either 30 min prior to or 5 min after LTP induction. Sodium chloride infusion served as a control. Normal synaptic transmission was not altered by MCPG. However, the mGluR antagonist inhibited LTP in a concentration-dependent manner. Concentration A did not influence the potentiation shortly after the tetanus. In the PS, short-term potentiation (STP), which is decremental in its time course, occurred normally, but in contrast to controls the potentiation declined back to baseline values after 2-3 hr. This dose also reduced the posttetanic increase in the slope function of the fEPSP, and led to a time course of potentiation similar to that for the PS. Concentration B completely abolished the tetanus-induced potentiation. This block was similar to that obtained for the NMDA antagonist 2-amino-5-phosphonopentanoate (AP5). Both MCPG concentrations had no influence on the time course of preestablished LTP. These effects seem to be due to the action of the (+)-isomer of MCPG, since intracerebroventricular application of the (-)-isomer was without effect on the duration and magnitude of LTP. In addition, we were interested in the mGluR subtypes involved in the blocking mechanism of MCPG. 1S,3R-aminocyclopentane-1,3-dicarboxic acid (ACPD)-activated PPI hydrolysis in hippocampal slices was competitively inhibited by MCPG at a concentration of 1 mM or higher. In contrast, this concentration of MCPG did not affect the reduction of forskolin-stimulated cAMP formation by ACPD. These results corroborate recent findings that mGluRs are required for the induction of LTP in CA1 and CA3 in vitro (Bashir et al., 1993; Sergueeva et al., 1993) and in vivo (Riedel and Reymann, 1993). The process of STP is found to be independent of mGluR activation.(ABSTRACT TRUNCATED AT 400 WORDS)