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site. [3H]Dynorphint-~ should prove useful in this respect, Note added in pred. Recent work suggests a differential distribution of opiate receptor sub-types within the cord. (Govard6res, C. and Cros. J. NeuropeFtides, in press). JOHN R. TRAYNOR Lecturer in Plmrmacology, Department of Chemistry, University of Technology, Loughborough, Leicestershire LEII 3TU, UK.

M. J. (1982) Life Sci. 31, 1377-1380 7 Czlonkowski, A., Costa, T., Przewlocki, R., Pasi, A. and Herz, A. (1983) Brain Res. 267, 392-396 8 Slater, P. and Patel, S. (1983) Fur. J. Pharmacol. 92, 159-160 9 Anali, B., Gouarderes, C., Mazarguil, H., Audigier, Y. and Cros. J. (1982) Neoropeptides 3, 53-64 10 Botticelli, L. J., Cox, B. M. and Golds'ein, A. (1981) Proc. Natl Acad. ScL USA. 78, 7783-7786

Reading list I Yaksh, T. L, (1981) Pain 11,293-346 2 Atweh, S. F. and Kuhar, M. J. (1977) Brain Res. 124, 53-67 3 Jessell, '~. M. and lversen, L. L. (1977) Nature (London) 268, 549-551 4 Cuello, A. C. (1983) Br. Med. Bull. 39, 11-16 5 Fields, H. L., Emson, P. C., Lei8h, B. K., Gilbert, R. F. T., and Iversen, L. L. (1980) Nature (London), 284, 351-353 6 Traynor, J. R., Kelly, P. D., and Rance,

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Computer Club 'l he world of p;~armacology computing simulation can be halted at any time to allow sketching ~f the trace. Hardcopy facilities are neither necessary nor desirable, since the act of sketching appears to assist learning.

Computer animations for teaching of introductory pharmacology Computer-aided instruction usually entail:; the presentation to the student of textual material, such as questions and paragraphs of information, sometimes accompanied by static pictorial displays. A more stimulating approach results from the use of microcomputers with a graphi~ capability. This article discusses topics that can be taught efft'ctively with the help of computer animations. One such application already developed in ~his department is in pharmacokinetic simulatie, ns. SIMPAK i is a set of interactive nicrocomputer programs written for the Apple II which plot plasma concentration-time curves for one- or two-compartment models, with linear or nonlinear kinetics. I have writgen several programs for the Apple II (PRACPAC) for use in practical classes. Some mimic the more tradi-'ional type of ptactical class experiment whereas others are best described as animations. Simulation of animal experiments Two computer programs model the recording of arterial blood pressure and the effects either of catecholamines and adrenoreceptor blockers, or of acetylcholine, atropine and ganglion blockers. The purpose of the programs is to provide acceptably realistic traces for teaching, not to model accurately the physiology of the cardiovascular system. In both programs the screen display mimics a chart recorder: the blood pressure trace is written at the right of -the screen and scrolls continuously to ~) ig84, Ehev~r Science~

the left (Fig. 1). Each heart beat is signalled by an audible tick. Heart rate, Systolic pressure and diastolic pressure are independently controlled according to the drugs present: To increase the clarity of display, the heart rate is purposely made low and the duration of agonist drug effects short. Drugs are 'administered' by single keystrokes. A useful feature lacking from real experiments is the ability to remove blocking drugs. The

Pharmaeekinetics In the early stages of a pharmacology course students often have difficulty in relating their knowledge of drug disposition to plasma concentration-time curves. We have found it helpful to show a stylized model of the circulation, into which drug molecules can be introduced. Drug distribution and elimination can be watched on the I . v . bolus Plosmo ,-oncentrotion of d r u 9 b, e r ~ u s time

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shows the fraction of time during which the receptor was z:ctivated. By m m H ._=i varying the number of agonist molecules, the student can construct a concentration-occupancy curve. Saturation non-linearity in the binding curve is easily seen to arise because the higher the drug concentration, the more likely is the recept,~r to be already occupied when a drug molecule collides with it. If the average fractional timc-ta:cupancy is 0.6 then A I 14 60% of collisions are ineffective. The concentration-occupancy relation has H' H L ' I I t R: r - e m c ...... ~ b l , : , , = l . ' . ~ n 9 dr.uq_--.-" the same form as the mass population g~: , . ' l d r . , . - . - n o l i n e t.l: nc, r - o , z t r . , ~ r , o l i r , e I : i-_--:,:,pt-er,,.-ilin0~ binding curve, illustrating an interestB: b e t , ~ blocker. [_: o l p l - , o blockering principle: the average fractional Fig. 2. Pharmacokinetic animations: (a) early stage of a one-compartment simulation sho~'ing i. ~'. number-occupahcy of a population of receptors is equal to the average injection of drug. Circulation is clockwise. The syringe is nearly empty and will be remored when the drug has completed its first circulation; (b) absorption kinetics in one-compartment model. hactional time-occupancy of a single receptor in that population. screen as time progresses; simultan- binding to a population of receptors Antagonist molecules can also be eously a plot of the plasma concentra- and does not emphasize the dynamic included in the animation: they occupy tion time-course is drawn (Fig. 2). No nature of the interaction. Without the receptor without activating it. The mathematical understanding is ,,:- careful tuition, few students under- mechanism of competitive antagonism quired of the student. Four simulation stand that equilibrium is a balance is made clear, along with the reason options are provided: between the rates of binding and for its surmountability by increased (1) i.v. bolus; one-compartment unbinding. The alternative used here agonist concentrations, since the stumodel. A syringe injects a stream of is to show an animated display of dent can see agonist and antagonist drug molecules for a period of one binding of individual molecules to a molecules competing for the receptor circulation time. The syringe is then single receptor. site whenever it becomes vacant. withdrawn. The drug molecules circuThe animation (Fig. 3) represents a Finally, the animation provides a late and as they pass the site of small enclosed volume with a receptor convenient talking-point for a range of elimination a small fraction is choser~ on one wall. Any desired number of questions which extend the abilit) of randomly for removal. drug molecules can be released into the teaching staff as much as that of (2) i.v. infusion; one-compartment the volume at the start of a simulation the students. behaviour. Infusion continue~ run. They then move randomly, rethroughout the simulation. bounding from the walls. A molecule Organ bath experiment~ (3) Oral dose; one-compartment be- colliding with the receptor when it is The last program of the set reprehaviour. A depot representing the vacant remains bound for several se- sents a classical organ-bath experiment intestine is initially stocked with drug. conds; if the receptor ;g already occu- on smooth muscle. Muscle contraction Absorption occurs at an exponentially pied the molecule is merely reflected. in response to various doses of an decreasing rate. Agonist molecules 'activate' the recep- ag.onist is visible on the scr_~en; "pul(4) i.v. bolus; two-compartment be- tor (indicated by a flashing sign). leys" and "cotton thread" link the haviour, A peripheral compartment A number at the right of the screen muscle to a "chart recorder'. In the accumulates drug molecules chosen ••[••••••••••••n•••••••••I•••••••••••••••••••••••I••••••••••••••••••••••••II••••.I.I randomly from the circulation and subsequently releases them. .,-,. I • Students can view any of the options as often as they wish and can stop and start the simulation. They are asked to sketch each completed curve a~xl label -" ! "the various phases with the dominant = pharmacokinetic process occurring at that time. This program has been used _= _= successfully with students before they = I =: had received lectures on pharmacoI = kinetics. m

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Receptor binding The binding of drugs to receptors is conventionally discussed in relation to 4 A I.'~0 H 1[ ':"...,T .;,. " 2 At-ITAGOH1[':,_:T MOLECULE"-' the binding equation (Langmuir adsorption isotherm). This approach Fig.3. Animation of binding of drug molecules to a receptor site. 4.. =m.gonist molecule occupies r',e focusses attention on equilibrium receptor. Four agonist and two antagonist molecules are present.

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self-contained, requiring 20-25 min. There seems no need for students to work individually, since small groups (3-4) encourage discussion and reduce the number of computers needed. Ethical, logistic and other constraints may combine to discourage the free use of animal experiments in teaching. The replacement of selected experiments by simulations has many advantages, not the least of which is that the experiment always works! Preparation time is negligible,, and students can repeat the experiment as Discussion We have used these simulations in often they wish. Nevertheless the approach must be practical classes for medical and pharmacy students a~.ld have received used with forethought. It is important favorable reaction:i. Each exercise is to make clear exactly what is being

simplest usage of this program students are asked to examine the doseresponse relation for the agonist and to suggest possible reasorts for it,~ shape, in a later class, a competitive or a non-competitive antagon~,st can be added to the bath; the students determine from the stirmountability or otherwise which ot the antagonists is which. The competitive antagonist's affinity constant can be calculated from the dose ratio.

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Department of Pharmacology, Medical School, University of Otago, Box 913, Dunedin, New Zealand.

Reading list I Blackman, J.G. (1983) Br. J. Pharmacol. 80, 589P

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Receptor binding studies: yet more cause for caution One of the main drawbacks of radioreceptor ligand binding assays is that the criteria fo~.~receptor activity is binding affinity, not alteration or production of a response. Thus: 'half of the definition of a rec¢:ptor is lost in ligand binding experiments' (quote from Ref. 1). This problem has led to considerable debate concerning the value of receptor nomenclature ba;~ed solely on radioligand binding data (for a critical review, see Ref. 2). One way of avoiding this problem has been to combine a functional invitro biochemical test with receptor binding data. O~e such example was recently reported by Daum er a/.3, who found that histamine produced an increase in the accumulation of [3H]inositol-l-phosphate in guinea-pig cortical slices pretreated with myo(2pH])inositol and lithium chloride (the latter to prevent breakdown of the inositol-l-phosphate). The increase in the inositol-l-phosphate accumulation was antagonized by H~- but not by H2-histamim, antagonists, and was also induced by 2-thiazolylethylamine (an H~-selective agonist) but not by diamprit (an H2-selective agonist) 3. The authors also demonstrated a signiticant correlation between the histamine-induced inositoi-l-phosphate accumulation (expressed as % of control) and the density of histamine Hi-receptors (as determined by pH]mepyramine specific binding) across the guinea-pig brain 3. Similarly,

simulated; for example a labelled diagram should be provided with the simulation of blood pressure recording, to show the placeraent of arterial and venous cannulae ,~.td the connect~ons to the recording apparatus in a real experiment. Carefully chosen written questions should be supplied along with the microcomputer to guide the students and discourage aimless playing.

the IC50 values for the displacement of [3H]mepyramine binding to Hi-histamine receptors in the rat brain by antidepressant drugs correlate well both with their inhibitory potencies towards histamine-induced ileal contractions in the guinea-pig in vitro and with their sedative potencies in mice in vivo 4. Thus, in intact animals, receptor binding and functional response data may often mirror one another. Such an intimate coupling between functional response and in-vitro binding may not, however, be found in lesioned animals. Janowsky et al. 5, for example, studied the effect of 5,7-dihydroxytryptamine-induced serotoninergic cortical lesions upon the downregulation of I~-adrenergic receptors induced by repeated administration of the antidepressant desipramine. The authors found that the lesion produced a 31% increase in cortical[3H]dihydroaiprenolol binding to [~-adrenoceptors and blocked the down-regulation of the binding found in intact animals s. In the same animals, however, a completely different result was found when ~-adrenoceptor function was assessed by the cAMP response to either 100 p~ noradrenaline or 10 p~ isoprenaline. In this case, the lesion did not increase the cortical cAMP r~sponse to either noradrenaline or isoprenaline, and did not prevent the down-regulation induced by repeated desipramine treatment -~. It may be that differences between results from binding and func-

tional correlate experiments can be explained in terms of antagonist v. agonist binding: for examples Menkes et al.6 have reported that whilst repeated antidepressant treatment of intact rats does not change the specific binding of the cq-antagonist [3H]prazosin to rat thalamic membranes, the Kt value for competition of the binding by the atagonist phenylephrine is significantly reduced. Discrepancies between binding and functional correlate experiments may also be due to the use of unspecific ligands: thus methysergide-displaceable [3H]spiperone binding to mouse cortical membranes contains both 5-IT1"2 and cq-adrenoceptor componq,-nts7, and desipramine-displaceable pH]imipramine binding to rat cortical membranes contains both high affinity protease-sensitive and low affinity protease-resistant sitess. Lesion experiments of this sort have also been used to demonstrate the absence or presence of presynaptic receptor binding sites, the assumption being made that loss of presynaptic receptor sites upon lesion will be seen as a decrease in the Bmax of binding of the appropriate ligand (see e.g. Refs 9,10). Such an assumption can only be made either if the radioligand binds selectively to the presynaptic receptor (see e.g. Ref. 11), or if the number of presynaptic receptor binding sites is similar or larger than the number of postsynaptic sites. If the number of presynaptic sites is small with respect to the number of postsynaptic sites, then small changes in receptor number will not be seen upon lesion, especially if