LETTERS

Guanidinosuccinic Acid Inhibits Excitatory Synaptic Transmission in CA1 Region of Rat Hippocampal Slices Rudi DHooge, MA, MSc,*S Jacqueline Manil, MD, PhD,’ Fernand Colin, MD, PhD,t and Peter P. De Deyn, MD, PhDS In a recent article in AnnaL, De Deyn and Macdonald El) reported the effect of four guanidino compounds on the inhibitory responses of mouse spinal cord neurons in cell culture. These authors suggested that the inhibitory effect of these substances on gamma aminobutyric acid (GABA) and glycine responses might account for some of the neurological complications seen in uremia, since these guanidino compounds are indeed highly increased in uremic patients with neurological symptomatology f27. W e set out to investigate the effect of one of these compounds, namely guanidinosuccinic acid (GSA), on excitatory responses of hippocampal slice neurons. Parasagittal rat hippocampal slices, 300 p m thick, were cut with a vibrarome and perfused and gassed in a modified Oslo chamber using standard methods. Every 4.1 seconds, a bipolar nickel-chrome electrode stimulated the Schaffer collaterals of the slices with a maximal intensity of 2 mA. Multiple tetanic bursts (100 Hz) were delivered at fixed intervals. Glass micropipettes recorded the resulting field potentials in the CA 1 region of the slices (Fig, A). After due amplification (conventional AC coupled high impedance amplifier), typical field potentials, comprising excitatory postsynaptic potentials (EPSPs) and population spikes (PSs), were averaged and recorded. GSA was dissolved in perfusion fluid and injected in the perfusate. Most slices submitted to our experimental paradigm (n > 100) showed both posttetanic and long-term potentiation (PTP and LTP, respectively) of synaptic responses. I”appeared as a sharp transient increase in EPSP and PS surfaces and amplitudes. LTP appeared as a steady and persistent rise of baseline surfaces and amplitudes. One thousand, 100, and 50 p M GSA in the perfusion fluid clearly disrupted these “standard” patterns in all slices tested (see Figure, B for an example). Under the influence of GSA, EPSP and PS surfaces and amplitudes remained the same or decreased; the usual increase was never noticed. Twenty-five or 10 p M GSA did not produce such effects (no example shown). Thus, probit analysis yields a tentative IC,, (inhibitory concentration in 50% of the animals) of 35.4 pM GSA (total n = 20). De Deyn and Macdonald El) suggested that the antagonistic effect of guanidino compounds on GABA and glycine responses, alone or in combinatiion with other effects exerted by these compounds or other toxins, might underlie the pathogenesis of myoclonus, epilepsy, and encephalopathy presenting in uremia. As far as uremic encephalopathy is concerned, the demonstrated inhibitory effect of GSA on excitatory synaptic transmission in hippocampus again sug-

gests an involvement of guanidino compounds. Indeed, the neurons in the CA1 field of hippocampus were shown to be essentially important in mentation and memory 131, and impairment in memory processes is recognized as one of the typical clinical features of uremia {47. Some as yet unknown mode of action causes GSA to inhibit excitatory transmission in CA1 region of rat hippocampal slices. GSA concentrations of more than 30 pM were found in cerebrospinal fluid of uremic patients with neurological symptomatology {2]. These concentrations are comparable to the concentrations we found to be effective in inhibiting hippocampal transmission. Therefore, we postulate that the latter inhibition might be another possible cause of uremic encephalopathy. Postulated involvement of guanidino compounds in uremic encephalopathy might include multiple transmitter systems. GSA was shown to be the most potent GABA and glycine antagonist of the guanidino compounds De Deyn and Macdonald tested on cultured spinal cord neurons {I]. In

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(A) Depiction of a parasagittal slice of rat hippocampus with enkargement of the studied excitatoy synapse. The stimulation electrode ( S ) was pkmd on the Schafier colkaterals of the slices. The field potentials that resulted from the stimulation were recorded in the CAI region of the slices (R). (B) Typical experiment of GSA application. Consecutive averaged responses are shown: three before (PRE), three during (100 pM GSA), and three after (POST) injection of GSA in the perfusion fluid. Every averaged response is the result of 4field potentials and is recorded 100 s afer a tetanus. A concentration of 100 pM GSA inhibited the popuhtzon spike (ps) almost completely and reduced in the presented example the surface of the excitatoy s p aptic potential (epsp) to a minimum of 69.4% the value before injection.

622 Copyright 0 1991 by the American Neurological Association

contrast, excitatory synaptic transmission in CA1 region of rat hippocampus depends on three kinds of excitatory amino acid receptors: quisqualate, kainate, and N-methy1-Daspartate (NMDA) receptors (see, e.g., [ 5 3 . Nevertheless, a common underlying mechanism could still be feasible, involvement of different transmitter systems notwithstanding. Indeed, D e Deyn and Macdonald [l] suggested the blocking of the GABA and glycine receptor-associated ion channel as the cause of guanidino compound GABA and glycine antagonism. GSA could disturb the function of excitatory amino acid receptors in rat hippocampus in a similar fashion. Thus, blockage of receptor associated ion channels could be a candidate common mechanism. Certainly, further work will be needed to test this hypothesis.

‘Laboratoy of Physiology and Pathophysiology flaborat0y of Electrophysiology Free University of Brussels Brussels, Belgium jLaboratoy of Neurochemistry Born-Bunge Foundation University of Antwerp Antwerp, Belgium

References 1. De Deyn PP, Macdonald RL. Guanidino compounds that are increased in uremia inhibit GABA and glycine responses o n mouse neurons in cell culture. Ann Neurol 1990;28:627-633 2. De Deyn PP, Marescau B, Cuykens JJ, et al. Guanidino compounds in serum and cerebrospinal fluid of non-dialysed patients with renal insufficiency. Clin Chim Acta 1987;167:81-88 3. Squire LR, Zola-Morgan S . Memory: brain systems and behavior. Trends Neurosci 1988;11:170-175 4. Osberg JW,Meares GJ, McKee DC, Burnett GB. Intellectual hccioning in renal failure and chronic dialysis. J Chron Dis 1982;35:445-457 5. Collingridge GL, Bliss TVP. NMDA receptors-their role in long-term potentiation. Trends Neurosci 1987;10:288-293

Reply

Robert L. Macdonald, MD, PhD

DHooge and colleagues have demonstrated that guanidinosuccinic acid (GSA) disrupts excitatory synaptic transmission between Schaffer collateral axons and CA1 hippocampal pyramidal cells in the rat hippocampal slice. These results complement our observation that GSA reduces GABAergic inhibition and suggest that GSA may have multiple effects on neurotransmitter receptor systems. Further investigation of the interactions of these metabolites with neurotransmitter systems may clarify the bases for the neurological deficits seen in uremic encephalopathy.

Department d Neurology The University of Michigan Medical Center Ann Arbor, M I

T-cell Receptor Biology and Multiple Sclerosis Klaus Lauer, MD

In their recent paper on the Vp-gene usage in multiple sclerosis (MS) patients {l], Lee and coworkers interpreted their interesting findings of oligoclonal T cells and of “recurrent” T-cell receptor (TcR) idiotypes as indicative of the operation of a specific antigen, probably located in the myelin sheath. Knowledge of clonality and idiotypic interactions is much more limited for the TcR than for immunoglobulins (Ig). This justifies drawing analogies between the two arms of the immune system. O n the humoral level, restricted heterogeneity resulting in an “oligoclonal I g G pattern” is, in fact, characteristic of specific immune responses to defined infectious agents like mumps virus, herpes simplex virus, and subacute sclerosing panencephalitis virus. It is also a main characteristic of early, germ-line encoded “natural” antibodies [2, 31, the typical features of which are multispecificity toward foreign and self-antigens, low avidity to the different antigens, and a high degree of idiotypic interconnectivity possibly allowing selfaggregation [4, 51. Recurrent or “public” Ig idiotypes are found predominantly on antibodies directed against highly conserved autoantigens (e.g., D N A , Ig, M H C products, cytoskeletal proteins, and so on) [GI, and these are also the main targets of natural antibodies 14, 51. Assuming analogies between Ig and TcR, the findings of Lee and associates [l} might be compatible with the activation of some type of “natural” T cells having more immunoregulatory properties than being specifically targeted against a defined (myelin) autoantigen and thus a typical part of a mature immune response. The occurrence of natural antibodies [7) and elevated numbers of CD5 + B cells (the probable source of natural antibodies) 181 in the CSF of MS patients might indicate regression of the intrathecal immune system to an immature state that normally characterizes only fetal and neontal life [3-51 and lower vertebrates [2]. The heterogeneity of the intrathecal antibody response with respect to antigen specificities [9f, along with the low avidity of such antibodies [lo] and the lack of any reactivity with a number of viruses and MBP by the oligoclonal T cells reported by Lee and colleagues [l], is in line with this interpretation. When assuming an important immunoregulatory role of such putative ‘‘natural” T cells in the CNS of MS patients, the search for antiHLA-class 2 activity of these clones, as demonstrated, for example, in the autologous mixed lymphocyte reaction, might be an interesting tool, all the more because an anti-Ia T-cell response resulted in a polyclonal activation in virro [ll], and evidence of polyclonal background B-cell activation exists in MS 191.

Department of Neurology Academic Teaching Hospital Darmstadt, Germany

References 1. Lee SJ, Wucherpfennig KW, Brod SA, et al. Common T-cell receptor V, usage in oligoclonal T lymphocytes derived from

Annals of Neurology

Vol 30 No 4 October 1991 623

Guanidinosuccinic acid inhibits excitatory synaptic transmission in CA1 region of rat hippocampal slices.

LETTERS Guanidinosuccinic Acid Inhibits Excitatory Synaptic Transmission in CA1 Region of Rat Hippocampal Slices Rudi DHooge, MA, MSc,*S Jacqueline M...
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