Interaction between thyrotropinreleasing hormone and the mesolimbic dopamine system

Neuropharmocology Vol. 26, No. I, pp. 33-38, 1987 Printed in Great Britain. All rights reserved

Copyright 0

0028-3908/87 $3.00 + 0.00 1987 Pergamon Journals Ltd

INTERACTION BETWEEN THYROTROPINRELEASING HORMONE AND THE MESOLIMBIC DOPAMINE SYSTEM KALIVAS’,

P. W.

D.

STANLEY~

‘Veterinary and Comparative Anatomy, Pharmacology Pullman, WA 99614, U.S.A. 2Department of Psychiatry Center, University of North Carolina,

and A. J.

PRANGE JR~

and Physiology, Washington State University, and Neurobiology, Biological Sciences Research Chapel Hill, NC 27514, U.S.A.

(Accepted 20 February 1986) Summary-The involvement of the mesolimbic dopamine (DA) system in the excitatory behavioral effects of thyrotropin-releasing hormone (TRH) has been a controversial topic. In this study TRH was injected into the nucleus accumbens, lateral ventricles or ventral tegmental area and changes in spontaneous motor activity and metabolism of DA in the nucleus accumbens and striatum measured. Injection of TRH into all three areas of the brain produced an increase in photocell counts of locomotor activity and, in the nucleus accumbens, a significant decrease in photocell counts of rearing was measured. Injection of TRH into the nucleus accumbens caused a marked increase in metabolism of DA in both the nucleus accumbens and striatum. A smaller increase in metabolism of DA was also observed after injection of TRH into the lateral ventricles, but no significant change was found after intra-ventral tegmental administration of TRH. These data indicate that while TRH probably acts in the nucleus accumbens to enhance the metabolism of DA, and presumably release of DA, the excitatory behavioral effect of TRH is only partially mediated by this dopaminergic mechanism. Key words: dopamine, nucleus accumbens.

Thyrotropin-releasing characterized release

hormone

as a tripeptide

of thyroid-stimulating

thyrotropin-releasing

(TRH) which

can

hormone

was

hormone,

from

microinjection,

motor

behavior,

into the nucleus accumbens, a major mesolimbic terminal field for DA, was examined. Miyamoto and Nagawa (1977) first reported that administration of TRH into the accumbens produced an increase in locomotor activity in rats. This effect was later shown to be blocked by both DA receptor antagonists and depletion of DA with methyltyrosine (Miyamoto, Narumi, Nagai, Shima and Nagawa, 1979; Heal and Green, 1979; Heal, Sabbagh, Youdim and Green, 1981). However, certain notable exceptions have been reported in which the motor stimulant effect of TRH or its analogues was shown not to be localized to the nucleus accumbens, but also occurred after injection into other regions of the brain (Carino, Smith, Weick and Horita, 1976; Costall, Hui, Metcalf and Naylor, 1979; Sharp, Bennett, Marsden and Tulloch, 1984a). Concurrent with behavioral studies, examining the interaction between TRH and DA, in vitro and in vivo neurochemical studies have ensued which largely support a modulatory action by TRH on the release of DA (Kerwin and Pycock, 1979; Miyamoto et al., 1979; Sharp, Bennett and Marsden, 1982; Narumi and Nagawa, 1983; Bennett, Sharp, Braze11 and Marsden, 1983; Sharp, Brozell, Bennett and Marsden, 1984b; Nielsen and Moore, 1984). While numerous data appear to support an action by TRH in the nucleus accumbens to promote the release of DA in the nucleus accumbens, thereby producing an increase in locomotor activity, other

initially

provoke

mesolimbic,

the

the ad-

(Boler, Enzmann, Folders, Bowers, Wakabayski, Kastin, Redding, Mittler, Nair, Pissolata and Segal, 1969; Burgus, Dunn, Desiderio and Gullemin, 1969). However, it is now clear that TRH probably has a neurotransmitter-like function in the central nervous system (CNS) outside the hypothalamic-hypophyseal axis. Among the first indications of an extrahypothalamic action by TRH were the observations by Plotnikoff, Prange, Breese and Wilson (1972) and Plotnikoff and Kastin (1974) that TRH potentiates the behavioral stimulant effects of L-DOPA in mice and rats treated with pargyline. After these studies, other investigators demonstrated that intravenous or intracerebroventricular (i.c.v.) administration of TRH alone produced behavioral excitation in rodents and rabbits (Schenkel-Hulliger, Koella, Hartman and Maitre, 1981; Horita, Carino and Smith, 1975; Andrews and Sahgal, 1983). Taken together, these data pointed to the possibility that TRH may function in the CNS as an endogenous modulator of behavioral arousal and that central dopamine (DA) systems may be involved in this action. Considering the data supporting a role for the mesolimbic DA system in modulating locomotor behavior (Koob, Stinus and LeMoal, 1981; Fink and Smith, 1980), the effect of injection of TRH directly enohypophysis

33

P. W.

34

KALWAS et al.

studies argue against this possibility. The present study was undertaken in an effort to clarify this putative TRH-DA interaction by injection various doses of TRH into either the nucleus accumbens, lateral ventricles or ventral tegmental area (location of mesolimbic dopamingergic perikarya) and measuring changes in both motor activity and metabolism of DA in the nucleus accumbens of rats. ME~ODS

Male Sprague-Dawley rats were housed individually, with food and water made available ad lib&m. When rats attained a weight of 280-350 g, they were anesthetized with ketamine (75 mg/kg, i.p.) and pentobarbital (10 mg/kg, i.p.) and mounted in a stereotaxic apparatus. Chronic bilateral cannulae (26 gauge stainless steel tubing) implantations were made according to the atlas of Pellegrino, Pelligrino and Cushman (1979) into the nucleus accumbens (A/P 9.0 mm; M/L 1.7 mm; D/V 0.0 mm, relative to the interaural line), lateral ventricle (A/P 5.2mm; M/L 1.5 mm; D/V 2.5 mm) or ventral tegmental area (A/P 2.3 mm; M/L 0.7 mm; D/V -2.5 mm). The cannulae were lowered to a depth of I mm above the desired injection site, secured to the skull with dental acrylic and stainless steel screws, and the rats allowed a minimum 7 day period of recovery in their home cage. Measurement of locomotor activity and rearing was made in a photocell apparatus described in detail elsewhere (Kalivas, Nemeroff and Prange, 1984). Following a 90min adaption period, the rats were removed from the photocell cage and simultaneous bilateral infusions of TRH (Sigma Chemical Co., St. Louis, Missouri, U.S.A.) or 0.9% sterile saline vehicle were made by inserting a 33 gauge stainless steel needle exactly 1 mm below the tip of the guide cannula. The injection needles were connected via PE-10 tubing to 1 ~1 Hamilton syringes which were mounted in a Sage infusion pump. The infusions were made over 60 set in a volume of 0.5 PI/side for the nucleus accumbens and ventral tegmental area and 1.OPI/side into the lateral ventricles. Immediately after removal of the injection needles, the rats were returned to the same photocell cage and motor activity monitored for 120 min. All rats were injected with vehicle and 1 or 2 doses of TRH, using an intertrial interval of 72 hr. Levels of DA and its major metabolites, 3,4_dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured using reversed-phase high pressure liquid chromatography (HPLC) and electrochemical detection (Kilts, Breese and Mailman, 1981). The rats were injected with TRH or saline as described above and 30min after injection, they were decapitated and the nucleus accumbens and dorsolateral anterior striatum dissected on an ice-cooled plate. The caudal portion of the brain was placed in 10% formalin for 1 week in

preparation for histological verrification of cannulae placement in the ventral tegmental area or lateral ventricles. If the rats were cannulated in the nucleus accumbens, placement was considered correct if the cannula tract penetrated the dorsal edge of the dissected nucleus accumbens, but not the ventral surface (Kalivas et al., 1984). Using these criteria, 20 of 24 rats implanted in the nucleus accumbens were determined to have the cannulae correctly placed. The bilateral tissue samples were sonicated in 0.5 ml of mobile phase (IOOmM citrate, 75mM Na,HPO,, 0.75 mM sodium heptane sulfonate, l&7 M epinine as an internal standard and 18% methanol, v/v; pH = 4.0) centrifuged and 100 ~1 of the supernatant injected into the HPLC system. The pellet was assayed for protein content with Folin reagent. Oxidation of the biogenic amines was performed at +0.7V and sample peak height compared with a standard curve for each amine (5 x 10~8-10-6M). The minimum detectable level with this HPLC method was 0.2 pmol for HVA and 0.1 pmol for the remainder of the amines. To minimize interassay variability, while the data from each region of the brain was obtained from 2 or 3 separate groups of rats, all the neurochemical samples in each group were analyzed consecutively. Coronal sections (100 pm thick) from the brains, stored in formalin, were made of the cannulae tracks using a vibratome. The sections were mounted on gelatin-coated slides, stained with cresyl violet and the placement of the cannulae tip in the ventral tegmental area and lateral ventricles verified by a trained individual who was ignorant of the behavioral or neurochemical response of the animal. In the ventral tegmental area, placement was considered to be correct if the cannulae were in the nucleus paranigralis, nucleus parabrachialis pigmentosus, nucleus linearis or nucleus interfascicularis (Kalivas, 1984). Sixteen of 16 rats had both cannulae placed correctly in the lateral ventricles and 12 of 16 rats were implanted correctly in the ventral tegmental area Unless otherwise stated, the behavioral and neurochemical data were evaluated statistically with a two-tailed Student’s t-test for comparing a single treatment with control or with a one-way analysis of variance, followed by a Dunnett’s test for multiple comparison with a control treatment. RESULTS I.

Behavioral effects

Figure 1 shows that injection with the largest dose of TRH (15.0 nmol; 5.1 +ug/side) elicited a modest increase in locomotor activity, regardless of the site of injection. After injection with this dose of TRH into either the ventral tegmental area or lateral ventricles, a significant increase in photocell counts for rearing was also observed. However, in the nucleus accumbens both doses of TRH caused a significant decrease in photocell counts for rearing.

TRH

80 -

Locomotion

.E

E

:: : * E a 0

r

35

and dopamine

Rearing

; 60 4O20.

n

ICV

D N. Accumbens El VTA *

P CO.05

1.5 nmol

15 “InO,

Dose of TRHlside Fig. 1. Effect of TRH on cumulative photocell counts for locomotor activity and rearing after injection into the nucleus accumbens, lateral ventricles or ventral tegmental area. All rats were given one behavioral trial with an injection of saline, and the data are presented, normalized to the effect of saline (mean& SEM). The number of determinations at each dose are shown in parenthesis in Fig. 2. 1.5 nmol = 0.5 pg TRH/side; 15.0 nmol = 5. I pg TRHjside. *P < 0.05, compared with saline using a twotailed paired Student’s t-test.

By examining the time-course of the motor response (see Fig. 2), it can be seen that the locomotorstimulant effect appeared at 20-30min after the injection and continued for 30min, except in the lateral ventricles where TRH produced an inconsistent increase in photocell counts for locomotor activity up to 100 min after the injection. In contrast, the inhibitory effect on photocell counts for rearing produced by injection of TRH into the nucleus

?

Fig. 2. The time-course of changes induced by TRH in photocell counts for locomotor activity and rearing after injection into the nucleus accumbens, lateral ventricles and ventral tegmental area. Time zero shows the number of photocell counts observed during the last IOmin of the adaptation period. Data are shown as the mean number of photocell counts/IO min and the number of determinations in each group is shown in parenthesis. *P -c0.05. compared with saline, using a one-way analysis of variance, followed by a Dunnett’s test.

Table 1. Effect of injection of TRH into various regions of the brain on the levels of dopamine and its metabolites in dopamine-containing terminal regions Treatment

N

DA

Saline TRH (1s)

10 6

367 * 22 415i_20

Saline TRH (1.5) TRH (IS)

8 6 6

421+ 42 384 f 47 385 f 20

Saline TRH (15)

6 6

341 + 33 339 f 44

Saline TRH (15)

IO 6

817&33 a32 i 39

Saline TRH (1.5) TRH (IS)

8 6 6

808 * 70 862 * a4 871 * 55

&tine TRH (15)

6 6

833 & 79 at7+74

DOPAC Lateral

HVA

DOPACiDA

40 i: 3 46 i: 5

0.186~0.0l1 0.229 & 0.009*

37 & 3 37i4 57 i 5*

0.199 + 0.009 0.195 f 0.021 0.387 k 0.095*

4228 4s*a

0.229 iO.017 0.256 t 0.036

58&6 56 _f 4

0.128~0.012 0.159 + 0.006

61 17 74 i a 89+_9*

0.134~0.013 0.152 F0.17 0.204 i. 0.023*

75 i 8 7a+ 10

0.136 & 0.021 0.167-io.018

ventricles

76+8 103 + lo* Nucleus accumbens 85 + 8 75 + 1 149 * 24** Venlral tegmental area 78 + 8 s7+9

Striatum Lateral

ventricles

105 + 3 132 f 5’ Nucleus occumbens 105 * 13 121 i: 14 178 -i-25* ventrat tegmentai anx 114il3 137 + 19

All rats were decapitated 30min after injection. The dose of TRH (nmol/side) is shown in parenthesis and all data are expressed a pmole/mg protein 5 SEM. *P ~0.05, compared to injection of saline, using a two-tailed Student’s t-test for the lateral ventricles and ventral tegmental area, and a one-way analysis of variance followed by a Dunnett’s test for the nucleus accumbens. **P < 0.005.

N.P.

m--c

P. W. KArrvXi et al.

36

accumbens occurred in the first 10 min after injection. A similar initial decrease in rearing behavior was also observed after injection of TRH into the lateral ventricles. However, a subs~uent increase in photocell counts for rearing between 30 and 120 min after the injection resulted in a significant increase in cumulative photocell counts for rearing by intraventricular injection of TRH. While not quantified in this investigation, as reported elsewhere (Wei, Siger, Loh and Way, 1975; Webster, Grifliths and Sla:er, 1982), both doses of TRH produced numerous “wetdog” shakes after injection into all 3 regions of the brain. 2. Ne~rochemical effects Table 1 shows the effect of injection of TRH on the levels of DA, DOPAC and HVA in the nucleus accumbens and striatum. With injection into the nucleus accumbens, the largest dose of TRH produced a significant increase in DOPAC, HVA and the DOPACjDA ratio in the nucleus accumbens and striatum. Likewise, intraventricular admjnistration of TRH produced a significant increase in DOPAC in both the nucleus accumbens and striatum. However, intraventricular injection of TRH did not significantly change the levels of HVA in either area of the brain nor did it significantly alter the DOPAC/DA ratio in the striatum. No significant change in any neurochemical measurements was observed after injection with TRH into the ventral tegmental area.

DISCLiSSION

These experiments indicate that TRH can act in the nucleus accumbens to promote metabolism of DA, and presumably release of DA (Roth, Murrin and Walters, 1976). While this increase in release of DA in the nucleus accumbens may play a role in the excitatory behavioral syndrome produced by TRH, injection of TRH into the ventral tegmental area elicited an increase in photocell counts for locmotor activity equivalent to that observed in the nucleus accumbens, but did not significantly alter the metabolism of DA. Thus, while these data support previous studies indicating that injection of TRH or its analogues into the accumbens increases release of DA and spontaneous motor activity (Miyamoto and Nagawa, 1977; Miyamoto et al., 1979; Heal and Green, 1979; Kerwin and Pycock, 1979; Sharp et al., 1982; Sharp et al., 1984a, b), they also support investigators who claim that the behavioral excitatory response of peripheral or intraventricular administration of TRH is not an effect which is mediated by DA (Costa11 et al., 1979; Ervin, Schmitz, Nemeroff and Prange, 1981). In addition to the nucleus accumbens, an increase in metabolism of DA in the dorosolateral striatum

was produced by both intra-accumbens and intraventricular injection with TRH. While these data indicate that TRH has an effect on the release of DA in the striatum, it is likely that this is the indirect consequence of an action by TRH elsewhere in the brain and does not result from an action by TRH directly in the striatum. This conclusion is supported by the observations that TRH does not enhance the release of dopamine from striatal synaptosomes or tissue slices (Kerwin and Pycock, 1979; Sharp et al., 1982, 1984b). However, contradictory data have been published (Horst and Spirt, 1974; Narumi and Nagawa, 1983). It is interesting that injection of TRH into the accumbens decreased photocell counts for rearing. This is in contrast to the increase in rearing behavior that would be predicted to occur after enhanced release of DA in the nucleus accumbens (Fray, Sahakian, Robbins, Koob and Iversen, 1980; Kelley, Seviour and Iversen, 1975). Thus, in addition to an effect on DA terminals, TRH may also act in the nucleus accumbens, postsynaptic to the DA terminals, to directly or indirectly counteract the behavioral consequence of enhanced release of DA. However, the observations by Pinnock, Woodruff and Turnbill (1983) that the iontophoretic application of TRH onto neurons in the nucleus accumbens had no effect on spontaneous firing frequency, nor altered the inhibitory effect of iontophoreticaIly-applied DA, argues against an action by TRH postsynaptic to the DA terminals. Alternatively, the excitatory behavioral syndrome produced by TRH may involve behavior that competes with the manifestation of rearing behavior (Ervin et al., 1981; Costa11 et al., 1979). The physiological relevance of an action by TRH in the nucleus accumbens is supported by the presence of immunoreactive TRH and a high density of TRH binding sites in membrane homogenates from the nucleus accumbens (Brownstein, Palkovits, Saavedra, Bassis and Utiger, 1974; Hokfelt, Fuxe, Johdnsson, Jeffcoate and White, 1975; Taylor and Burt, 1982). The more recent reports by Manaker, Winokur, Rostene and Rainbow (1985) and Mantyh and Hunt (1985), using light microscopic autoradiographic localization, demonstrate that TRH receptors are heterogenously distributed in the nucleus accumbens. Thus, while the medial edge of the nucleus accumbens and adjacent diagonal band of Broca contain a high density of binding sites for TRH, the majority of the nucleus accumbens possesses a relatively low density. However, it should be noted that while the density of autoradiographicallydefined TRH receptors in the rat nucleus accumbens is low, in the mouse the density of TRH receptors is among the highest in the brain (Shariff and Burt, 1985). Based upon the present study and the experimental observations by others regarding the excitatory behavioral effects of TRH, the following proposals are suggested for the CNS mechanisms mediating the

TRH

excitatory

behavioral

and dopamine

effects of TRH:

(I) TRH can produce behavioral excitation, specifically an increase in locomotor activity, through an action in the nucleus accumbens to release DA from mesolimbic DA nerve terminals and (2) TRH also acts on non-dopaminergic neuronal systems to produce behavioral excitation. Acknowledgements-This research was supported in part by USPHS grants, MH-32316, MH-34121, MH-33127, MH22536, MH-08435

and MH-40817.

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