Cerebral dopamine metabolism and stereotyped behaviour in early-weaned piglets

CEREBRAL DOPAMINE METABOLISM AND STEREOTYPED BEHAVIOUR IN EARLY-WEANED PIGLETS D. F. SHARMAN, S. P. MANN, J. P. FRY. H. BANNS and D. B. STEPHENS Agrlcultura] Research Council Institute of Animal Physiology, Babraham. Cambridge CB:! 4AT. UK. A&&act 7-k hehavioural response to apomorphine in cattle. sheep and pigs resembles some aberrant. oral behaviour patterns or vices that can occur as the result of restrictive environmental conditions and suggests that central dopaminergic neuronal systems might be involved. This study of the function of central dopaminergic mechanisms indicates that. in early-weaned piglets showing stereotyped snoutrubbing behavlour. there is a reduction in the metabolism of dopamine in parts of the brain receiving a dopaminergic neuronal input. The change in the metabolism of dopamine appears to be associated with the SepdratiOn of the piglets from the sow and provides further evidence for biochemical changes in the brain occurring as the result of early-weaning.

Animals often develop repetitive or stereotyped behaviour patterns when kept in a restrictive environment. The development of such behaviour patterns in zoo animals was described by Hediger18 and Holzapfel*O and further analyzed by Morris3’ Farm animals, although they are usually more amenable to being kept in restrictive environments. will also develop stereotyped behaviour patterns. Stereotyped behaviour involving the mouth is well known in farm animals and when the behaviour is disadvantageous to the economy of animal production it is referred to as a vice. Such vices include ‘licking sickness’ or pica in cattle. wool biting in sheep and tail-biting in pigs. It appears to be difficult to induce such behaviour patterns ex~rimentally in weaned animals in order to investigate them. However, snout-rubbing, which may develop into cross-sucking, can often be observed in early-weaned piglets. In 1875, Feser13 reported that apomorphine induces symptoms similar to those of the so-called ‘licking sickness’ in cattle and wool biting in sheep. This observation has been extended by Sharman & Stephen? who noted the similarity between the effects of apomorphine in cattle and sheep and the behaviour of some animals kept under intensive husbandry conditions. 45.46 In pigs. apomorphine induces intense snout-rubbing activity.’ 2.40 Snout-rubbing is not generally regarded as a vice in weaned pigs, but was reported as such by Allison.’ The snout-rubbing seen in early-weaned piglet? is comparable with the effect of apomorphine in these young animals.” Apomorphine is thought to induce stereotyped gnawing m ITitS by stimulating dopamine receptors in the brain,‘.“‘.“~‘4 Fesert3 concluded that apomorphine affected the same part of the brain that was Ahhrwiurions: DOPA. 3,4_dihydroxyphenylalanine: DOPAC, 3.4-dihydroxyphenyl-acetic acid; HVA. homovanilfic acid.

disturbed in ‘licking sickness’. The possibility that snout-rubbing in early-weaned piglets might be the result of an increased release of dopamine in the brain was investigated by Fry er ~1.‘~ who found evidence for a decreased release of dopamine from dopaminergic neurons in the putamen and nucleus accumbens in piglets in which stereotyped snout-rubbing was induced by reducing the oral stimulation associated with feeding. The present report presents the result of an investigation into the effects of early-weaning on dopaminergic neuronal mechanisms in the brain of the pig.

EXPER~~lENTAL, PROCEDURES

The piglets were from the Institute of Animal Physiology herd of Large White Pigs. The piglets were, routinely, given a subcutaneous injection of Vitavet (Beecham Animal Health: a preparation containing iron and vitamin B12 to prevent the development of Iron deficiency anaemia) on the first day after birth and a second subcutaneous injection at around 14 days of age. In the first experiment? half of the litter was removed from the sow at one day of age and hand-reared. The hand-reared piglets were fed a mixture of cow’s milk and water (I:1 v.v\ containing LO”,w:v giucose from a bottie fitted with a rubber teat. The early-weaned piglets were housed singly in wooden boxes (45 x 52 x 52cm) floored with rubber matting and having a wire mesh lid. The boxes were kept in normal daylight conditions and were warmed continuously by infra-red lamp. During the first three days the piglets were fed at intervals of 2-3 h. then the feeds were spaced to 3 4 h intervals. The volume of feed was initially Z&-30 ml per feed durmg ihe first 2 3 days increasing to 40 ml. The piglets were trained to drmk from the teat and acquired the sucking response withm 8 h. The feed was usually consumed in 30 40 s, after which the animals were allowed to nose at the teat until. usually within one minute. they became restless. Hand-reared piglets and their sow-reared litter-mates were killed at 8 days of age by an

193x

D. F. Sharman YI JI.

intracardiac injection of pentobarbitone (5 ml of a 60 mg/ml solution). The brains were rapidly removed (within 2min) and frozen in solid carbon dioxide. The brains were cut into coronal sections while still frozen and the required brain regions removed by means of a small knife, with reference to the description of the anatomy of the pig brain provided by Yoshikawa.” in the remaining experiments, the piglets were earlyweaned under conditions more comparable with those employed in commercial practice. Half of each litter was removed from the sow at 12-14 days after birth. The earlyweaned piglets were then housed singly in wooden boxes floored with rubber matting and having a wire mesh lid in a thermally insulated room maintained at a temperature of 27’. The animals were kept in dim lighting conditions. The room was lit by a single 60 watt bulb in a waterproof safety housing placed at one end of the room so that approximately 66”, of the floor of the box was illuminated with an intensity of 20 lx. the remainder with 45 Ix. The light was left on continuously to permit observaGons to be made without changing the lighting conditions but the piglets had the choice of two subdued lighting conditions. No preference for either was noticed. The early-weaned piglets were kept under these conditions for IO or 11days. The remaining litter-mates were left with the sow. Both sets of animals were given a supply of Super Kwik Start feed pellets (Rank Hovis McDougall). These pellets are specially formulated for the early weaning of piglets. The early-weaned piglets and their controls were killed at the same time by an intraperitoneal injection of pentobarbitone (100 mg/kg body wtB The brains were rapidly removed and the required regions of the brain were dissected out. The right caudate nucleus was used for measurements of tyrosine hydroxylase activity and of spiperone binding. The left caudate nucleus was used for the estimation of dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid. HVA). Where other regions of the brain were analyzed for DOPAC and HVA. tissues from both sides of the brain were pooled. The

estimution

droxyphenylucetic

of dopuminr,

homouanillic

ucid

and

3,4-dihy

acid

The brain tissues from the piglets weaned at one day old were analyzed for HVA and DOPAC by the fluorimetric method of Murphy, Robinson & Sharman.32 Brain tissues from pigiets weaned at 12 -14 days were analyzed using a modification of the method of Westerink & Korf3’ which permitted the separation of dopamine, HVA and DOPAC from each other on a column of Sephadex GlO (10 cm x 0.5 cm). After adsorption of the compounds from the tissue extract, the column was washed with 3 ml 0.01 M formic acid; dopamine was then eluted with 2 ml 0.01 M formic acid. HVA was then eluted with 3 ml 0,Ol M formic acid. The column was then washed with 0.5 ml 0.01 M formic acid and DOPAC eluted with 3 ml 0.01 M formic acid. In later experiments, the last wash with formic acid was omitted and the DOPAC eluted with 3 ml 0.005 M phosphate buffer pH 7.1 into a tube containing 0.t ml 0.1 M hydrochloric acid. This procedure reduced the time taken for the extraction. Dopamine was estimated fluorimetritally by the method of Laverty & SharmanZh except that sodium metaborate was used instead of sodium hydrogen carbonate in the acetylation step. HVA was estimated by the automated fluorimetric procedure described by Westerink & Kortd9 DOPAC was converted to a fluorescent de-

rivative by the method of Murphy it ‘I! ‘! ;III~ the tluorescence read m an automated fluorimeter usmg H”,,H L ammonium chloride solution as the wash ~&tlon. The mean recoveries of different am
The rtght caudate nucleus of each plglet was h~,mogenized rn ice-cold. 0 I M dimcthylglutaratc hutfer. pH 6.0 (9 ml:g tissue) usmg a Polytronx homogentlsr A portion of the homogenate was taken for the measurement of ~)rosine hydroxylase activny. The remamder ~;IF centrifuged ;~t 50,000 9 for IO min. The pellet was rehomogenized in 10 ml 0.05 M tris HCI buffer (pH 7.6) and centrifuged at SO.tltX1 d for 10 min. This washing procedure &as rcpcatcd t,ncc tind the pellet was tinally suspended m a volume’. corresponding to 100 300 times the original tlssuc Hclght. of tris HCI buffer (0.05 M: pH 7.1) containing Na<‘l. I20 mhl; KCI. 5 mM ; CaCi2. 2 mM: MgCI,, I mM: pargylme. IO PM and ascorbic acid 0. l”,,.2- In other experiments. the initial homogenisation wa\ made in 0.05 M tris HCI buffer pH 7 6. Incubations were carried out wnh I ml portlons of the membrane suspension. 0.05 ml [“Hlsplperons solution and 0.02 ml tris HCI buffer. Specific binding uas determined by subtracting the non-specific bmding of [ ‘Hlspiperone that occurred in the presence of IO ’ \I dopammc which wa:, added in a volume of 0.02 ml In place of the tris HCI buffer. The duration of the Incubation was IOmrli tit 3’ and dcterminations with membranes prepared from a control and an early-weaned animal were carried out II the hame time. At the end of the incubation. the ml\ture P;I\ filtered through a Whatman GFB filter using a Qldllpore’ tiltration apparatus The filter ~3s washed IUIC‘C\\lth Ice-cold tris HCI buffer and the radloactivlt!, retamed hy the tiltcr measured by hquid scintillatmn sprctromctr\

The activity of the enzyme tyrosme h>drorytase [t-tyrosine. tetrahydropteridine, oxygen oxidoreductasc 13-h)droxylating), EC 1.14.16.2.] in homogenates of pig caudate nucleus was estimated by the method of Mann.” The incubation mixture consisted of IO0 pi of a dlmethylgtutarate NaOH butTer. 4OmM. pH 6.0: NaCI. IOOmM: FeSOo. 20011M; bovine serum alhumm. 4 mg,ml drthlothrcltol. 2.5 mM and catalase. 2500 units ml. The enzyme activtty was measured m the presence of high cq>ncentrations of substrates ([‘H]I -tyrosine. 260 JIM: tetrahydrobiopterm. 2.5 mM) to obtain an index of the total amount of emyme present and with low substrate concentratmns ([‘H]t.-tyro. sine, 65 PM: tetrahydrobiopterin. 31? PM LOohtam an UImate of the enLyme actlbity under conditions .ipproximating more closely IO those occurring in I tro. The incuhations, at 37 for 30 min. were started b> the addition of 25 JJ of caudare nucleus homogenate and stopped by the addition of 1001tl of 0.15 M HCIO, The isolation and measurement of the [JH-J1.-3.4-dihydroxyphenylalanme (L_DOPA). formed during the incubation. by means ol alumina columns was as previously tiescribed 2L The

Cerebral Table

1. Concentrations

of DOPAC

metabolism

in early-weaned

and HVA in brains regions sow-reared litter-mates DOPAC (fig/g tissue) Hand-reared Sow-reared

Brain region Caudate nucleus Putamen Nucleus accumbens Olfactory tubercle

dopamine

septi

0.75 0.75 0.86 0.71

* + & *

0.13 0.05 0.05 0.03

0.47 0.47 0.54 0.39

i i * k

0.06* 0.06* 0.09* 0.06*

lY3Y

piglets

of early-weaned

piglets

and

their

HVA (pg;g tissue) Hand-reared Sow-reared _ 1.20 146 1.33 0.58

f f * *

0.09 0.12 0.14 0.02

0.57 0 69 0.62 0.26

+ f ) f

(x08** O.OY** 0.08** O.O’*** _

Values are mean i S.E.M. 01 = 3). Difference between results from hand-reared and sow-reared animals: Student’s I-test *P c 0.05. **p < 0.01, ***p < 0.001. Hand-reared piglets were removed from the sow at one day of age and kept until 8-days-old: sowreared piglets were litter-mates left with the sow for the same period.

measurements of enzyme activity were made in the presence, and absence. of Triton X-100 (I”, v ~1.“)

RESULTS

Hand-rewed piglets weuned ut one day of qr. Nonnutritive oral activity was observed with all handreared piglets. This consisted of rubbing of the snout against the wall or floor of the home box and was often associated with licking or chewing movements. Snout-rubbing was observed in all hand-reared piglets immediately before feeding. After feeding, the animals usually lay down and slept until the next feeding time. Piylers wrunrd ut l-7- 14 days of‘ugr. During the first day after weaning, the piglets vocalized and tried to climb out of their individual pens. Snout-rubbing and chewing was particularly intense during this period. After the first day. the piglets became quieter but snout-rubbing continued. In nearly all cases, the piglets ate well and scourmg (diarrhoea) was infrequent. There was a check in the growth rate of the earlyweaned piglets when compared with that of the controls: 30 piglets were weaned at a mean body weight of 3.98 + 0.12 kg. At the end of the experiment, they weighed 4.27 & 0.22 kg whereas the mean body weight of the litter-mate control animals left with the sow was 5.78 f 0.28 kg. The &ecr dopumine.

of rurlyweuning DOPAC

on rhr

concentrations

of

und H VA in rhr piglet bruin

Table 1 shows the concentrations of DOPAC and HVA in the brains of &day-old hand-reared piglets weaned at one day of age compared with results obtained from litter-mate controls left with the sow, In all of the regions of the brain that were analyzed. the concentrations of DOPAC and HVA were lower in the hand-reared animals than in the controls. The concentration of dopamine, DOPAC and HVA in the brains of piglets weaned at 12-14 days and kept for l@l 1 days under the conditions described above and their litter-mate controls is shown in Table 2. The concentration of dopamine in the caudate nucleus was unaltered by early weaning but the concentration of

DOPAC in the caudate nucleus of early-weaned piglets was lower than that seen in litter-mate control animals. There was also some evidence for a reduction in the concentration of HVA but this ~21s not as consistent as that of DOPAC. Table 3 shows the results of a detailed statistical analysis of the concentrations of DOPAC and HVA in the caudate nucleus in early-weaned and sow-reared piglets. In one litter other parts of the brain were analyzed. There was a reduction in the concentration of both DOPAC and HVA in the nucleus accumbens septi.

T/W
nucleus qf’the

binding

pig

Figure 1 shows Scatchard plots34 for the binding of [3H]spiperone to membrane fragments from caudate nuclei of new-born and adult pigs. The Hill plot” for binding of [3H]spiperone to membranes from adult pig caudate nuclei gave a line with a slope 0.99 (results not shown). There was no clear evidence for more than one binding site for spiperone on membrane fragments derived from the pig caudate nucleus. From the Scatchard analysis, it was calculated that the new-born pig caudate nucleus has spiperone binding sites equivalent to 13.3 pmoles.‘g tissue and that in the adult this value has increased to 24.9 pmoles,g tissue. Scatchard analyses of [3H]spiperone binding to membrane fragments from caudate nuclei from early-weaned and sow-reared piglets (litters I, 2 and 3) were made. The results are given in Table 4 which shows that there is a higher concentration of r3H]spiperone binding sites in the caudate nuclei of earlyweaned piglets than in their litter-mate, sow-reared controls. There was some evidence for a small reduction in the affinity of the binding site for [3H]spiperone in membranes from early-weaned piglets (Table 4).

Table 5 shows estimates of the activity of tyrosine hydroxylase under different analytical conditions in caudate nuclei from early-weaned and sow-reared piglets. The tissues used were the same as those used for [3H]spiperone binding studies (litters I. 2 and 3). The

1940

D. F. Sharman et ul

results show that the estimates

of actlvlty of tyrosme hydroxylase were lower in the caudatc nuclei of earlyweaned piglets than in the caudate nuclei of SOWreared litter-mate controls in three ,)ut of the four analyses. A detailed analysis of the kinetic changes associated with this difference will be published separately 6 P. Mann & D. F. Sharman. !:I preparationj. DIS(‘I:SSIOF\

The group of symptoms to which the name ‘pm I:, applied has been called ‘licking sickness’ HI cattle. ‘wool-biting’ in sheep and ‘gnawing sickness’ in pigs ‘3*44 and appears to be associated with a wide range of conditions. These include deficiencies of vitamins or phosphorus and deficlensies or imbalances in dietary sodium. patasslm~. and chloride.36 However, stereotyped licking and chewing can occur in the absence of dietar! deficiencies ;;nd appears to be related to cnvlronmental conditions (see Kiley-Worthingtonz3 for rcvicw). Thu< the syndromes involving stereotyped oral behavlour in farm animals might be general behavioural responses to an environment that is inadequate because of spatial or dietary limitations. The close similarity between the behavloural effects of apomorphine in sheep, cattle and pigs and some behavioural disorders or vices that occur m these species13,‘* and the present knowledge of the mode of action of apomorphine In stimulating cerebral dopamine receptors suggested that such behavioural disorders might be accompamed b) dn increased release of dopamine from dopaminergic neurones m the brain. The present results and those of Fry (‘I 01.‘~ mdicate that there is a reduced metabolism :md. by mference, a reduced release of dopamine in the brains of early-weaned piglets showing stereotyped snoutrubbing. Fry et a1.16 found that hand-reared piglets showing increased snout-rubbing because of reduced oral stimulation associated with feeding, had a lower concentration of HVA in the putamen and the nucleus accumbens when compared with hand-reared piglets that were allowed to suck ;~t the teat of ;L feeding bottle until sated. The present observations show that there is an additional and more generalized reduction in the concentrations of both DOPAC and HVA in the brains of earl>-aeaned piglets that appears to be associated with the separation of the piglet from the sow. Fry’” has considered the possrbillty that these changes in the concentrations of the metabolites of dopamine might be related to the change from the high fat diet of sow’s milk to a diet containing less fat. Such a relation is indicated by the finding that in the rat brain, the concentrations of DOPAC and HVA increase to 3-4 times the adult values during the suckling period and decline after weaning.‘.” Evidence for an association between ketone body metabolism and the function of cerebral dopdmmergic neUrOneS

1941

Cerebral dopamine metabolism in early-weaned piglets Table 3. Statistical analysis of the concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-bydroxy-3-methoxyphenylacet~c acid (HVA) in the caudate nuclei of early-weaned and sow-reared piglets DOPAC Sow-reared piglets Early-weaned piglets Mean difference for litters HVA Sow-reared piglets Early-weaned piglets Mean difference for litters

Concentration @gig tissue) 0.956 k 0.045 (25) o.694 rt o,027 (25) P < 0.001 (student’s ‘t’ test) 0.262 k 0.062 (5)

P < 0.02 (paired ‘t’ test)

1.808 t LO65 (25) ,,499 ; o,065 (251 P < 0.2 (student’s 7’ test) 0.308 i 0.126(5)

P < 0.I (paired ‘t‘ test)

Analysis of variance showed that differences between litters contributed significantly to the variance for both DOPAC (F = 10.95, P < 0.01) and HVA (F = 1127, P < 0.05). The variance ratios for treatment were for DOPAC, F = 48.33, P i 0.01 and for HVA, F = It .27. P < 0.05. There was evidence for a negative correlation between the concentrations of DOPAC and HVA: r = -0.412, P < 0.02.

Kt,= 1.08nM

V=

3Hpiperone bindii , pmda/g tirsue.

C- &I IpiperoneI. nM

Fig. t. The characteristics of [3H]spiperone binding to membrane fragments derived from the caudate nucleus in newborn and adult pigs. Each point is the mean value obtained from triplicate observations with membrane fragments obtained from 3 newborn and 4 adult pigs, can be found in the reports of Sahakian & Robbins”’ and Fuenmayor.” The former authors observed that rats which had been fasted for 48 h, a procedure that causes the cerebral concentrations of 3-hydroxybutyrate and acetoacetate to increase,34 showed an increased locomotor activity and an increased response to apomorphine. Fuenmayor’7 reported that mice when fasted for 24 h showed an altered metabolism of dopamine in the striatum. Recently, Sailer & Chiodo3’ have demonstrated in the rat that the administration of glucose, sufficient to raise the blood glucose to values similar to those seen during stress or following a meal. will suppress the firing of dopamine neurones in the substantia nigra. Changes in two other mechanisms involved in the function of cerebral dopaminergic neurons were observed in the early-weaned piglet: the activity of the enzyme tyrosine hydroxyIase in the caudate nucleus

was lower and there was a small increase in the number of spiperone binding sites on membrane fragments derived from this tissue. The increase in the

Table 4. [“Hlspiperone binding to membranes derived from the caudate nuclei of early-weaned and sow-reared litter-mate piglets [3H]spiperone binding Early-weaned Sow-reared pm01 ‘g tissue pmobg protein K, InM)

20.50 f 0.92** 0.22 + 0.01** 0.72 f 0.09*

18.26 $: 0.72 0.18 i 0.01 0.63 2 0.07

Paned r-test, ,r = 14 pairs **P < 0.01, *P -c 0.05. Analyses were carried out with membrane preparations from one early-weaned piglet and one sow-reared piglet m parallel. The paired ‘t’ test was based on the differences observed between such pairs of samples.

1942

D. F. Sharman

Table

5. The activity of tyrosme hydroxylase

et u/.

in caudate

and sou-reared

nuclei from early-ueaned

piglets

llmoles L-DOPA

Analytical conditions High substrate concentrations + 1% Triton X-100 Low substrate concentrations + l”, Triton X-100 High substrate concentrations no Triton X-100 Low substrate concentrations no Triton X-100 Student’s f-lest *P < 0.05: **P

g II~~UC II

Sow-reared 01 _ 141

0.524 f 0.013*

0.6XX : 0.07 I

0.237 rt_0.015**

0.3x0 ? 0.04c

0382 If: 0.024

0.467 : (J.&W

0.115 +- 0.006***

0. 189I

< 0.01 : ***P

of spiperone binding sites can be interpreted as an indication of increased sensitivity of dopaminergic receptor mechanisms. These changes are consistent with a maintained reduction in the release of dopamine from dopaminergic nerve endings but it is not possible, from the present evidence, to suggest which of the observed changes is responsible for the other two. The environmental changes experienced by the early-weaned piglets included separation from the sow and from each other. The separation of rat pups from the mother has been shown to induce biochemical changes in the brain. Butler, Suskind & Schanberg5 have demonstrated a decrease in the activity of the enzyme ornithine decarboxylase (EC 4.1.1.17). an effect that is accompanied by a depression of growth hormone in the blood.24 Slotkin has suggested that there is a direct relation between a delay in the postnatal development pattern of ornithine decarboxylase activity in the brain and a subsequent deficit in the activity of tyrosine hydroxylase. It has also been found that when male mice, that have been kept in isolation, are exposed to a fresh environment, there is an increase in the concentration of DOPAC in the brain.” Thus the early-weaned piglets in the present experiments were exposed to several environmental and dietary changes that could, on the evidence obtained with other species, modify the function of the cerebral dopaminergic neurones. acts on the Feser ’ 3 concluded that apomorphine same parts of the brain that are affected in some aberrant oral behaviour syndromes in farm animals. There is little recently published literature on the drug treatment of oral vices in farm animals, so that further pharmacological evidence that might support the involvement of cerebral dopaminergic neuronal systems in such behavioural problems is sparse. Nickelman’” suggested the use of chlorpromazine or propiomazine in persistent cases of cross- or milk-sucking. Reinhecke135 did not support this because the symptoms returned two days after the administration of chlornumber

byntheslrcd

Early-weaned (n = 14)

0.01 h

_.-_

< 0.001

promazine. Some neuroleptic drugs. v.hlch are kno\sn to interact with cerebral dopammergic neuronal mechanisms. have been reported I(\ he of use III ameliorating the effects of handling procedures in the pig. ” 9 25 Of such drugs. azaperone-I” is probably the most widely used in pigs. Howc\er. some neuroleptic drugs induce aberrant behavloural responses in the pig4’ that resemble those following apomorphine. 4”.41 Dantzer and Mormede‘ state that tailbiting in pigs does not appear to bc affected by neuroleptic drugs. The present results suggest that the persistent snout-rubbing that can be seen in early-weaned puglets and which resembles the behavloural response to apomorphine in piglets. is not associated with an increased release of dopaminc from the dopaminergic neurons in the brain. It would not be possible to distinguish a small number of neurons having an increased release of dopamine in the presence of the more generalized decreases in the metabolism ol dopamine that appear to be related to the process ol early-weaning. However. the cells on which dopamine and apomorphine act will have other neuronal inputs. alterations in which could have the same effect as stimulation of the dopamine receptors by apomorphine. Changes in neuronal function associated with the onset of oral vices in farm animals may be found among these other neuronal inputs. Because similar behavioural patterns occur m farm animals in response to a wide variety of environmental restriction (confined living space. dietary deficlcnces. etc.) It is possible that the central dopaminergic neuronal systems are involved in some but not all of these conditions.

Ackno\~ledye,nrrlr\ Walters

We

of the A.R.C.

would

detailed

statistlcal

Godden

and MISS A. Carlqle

analytical

analysir

procedures

like

IL> thank

Statcstics Group. of the

Mr

(‘ambridge.

resulls.

and

D.

E

for the Mrs

t:

for their assistance with the

Cerebral dopamine metabolism in early-weaned piglets

I943

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10, 3 $6-323.

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