Faculty Bibliography

Chronic food restriction enhances behavioral responsiveness to amphetamine and other abused drugs. Because this effect is evident when drugs are administered intracerebroventricularly (i.c.v.) as well as systemically, it would seem to reflect increased sensitivity of a neural substrate rather than a change in drug disposition. In the present study, c-Fos immunohistochemistry was used to evaluate whether the magnitude and pattern of cellular activation induced by i.c.v. amphetamine is altered by a regimen of food restriction previously shown to potentiate amphetamine reward. In the absence of amphetamine challenge, there was generally no difference in brain Fos-like immunoreactivity (FLI) between ad libitum fed and food-restricted rats. In response to amphetamine (50 microg), both groups displayed increased FLI in caudate-putamen, nucleus accumbens, bed nucleus of the stria terminalis, ventral pallidum, central nucleus of the amygdala, and cingulate cortex. With the exception of cingulate cortex and caudal caudate-putamen, a significantly greater response was observed in brain regions of food-restricted rats. These results indicate that food restriction augments a cellular immediate early gene (IEG) response to acute amphetamine in brain regions known to mediate rewarding and other behavioral effects of psychostimulants. The difference between these results and those produced by sensitizing regimens of psychostimulant exposure are discussed, as are possible endocrine factors that could be involved in the modulatory effect of food restriction on cellular and behavioral responses to amphetamine

In hippocampal membranes, the selective 5-hydroxytryptamine (5-HT(1A)) receptor agonists 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and N,N-dipropyl-5-carboxamidotryptamine (N,N-DP-5-CT) stimulated guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) binding by 130 to 140%; binding stimulated by nonselective agonists (5-HT and 5-CT) was approximately 30% greater. However, the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohex anecarboxamide (WAY100,635) completely abolished the increases produced by 8-OH-DPAT and N,N-DP-5-CT but only eliminated 70% of that elicited by 5-CT. The rank potency order of the tested agonists was identical with their rank order of affinity for 5-HT(1A) receptors [5-CT congruent with N,N-DP-5-CT > R-(+)-8-OH-DPAT > 5-HT > ipsapirone]. Racemic 8-OH-DPAT and the partial agonist ipsapirone exhibited lower intrinsic activity than R-(+)-8-OH-DPAT. R-(+)-8-OH-DPAT also stimulated [(35)S]GTPgammaS binding in cortex, but not in striatum, which lacks 5-HT(1A) receptors. Partial irreversible inactivation of 5-HT(1A) receptors, in vitro with phenoxybenzamine (0.3 or 1 microM) or in vivo with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (1 mg/kg), reduced the maximal response produced by R-(+)-8-OH-DPAT but did not alter its EC(50). In autoradiographic sections, R-(+)-8-OH-DPAT stimulated [(35)S]GTPgammaS binding in 5-HT(1A) receptor-rich regions (dorsal hippocampus, 123%; lateral septum, 111%; midhippocampus, 110%; dorsal raphe nucleus, 83%; medial prefrontal cortex, approximately 60%). The EC(50) of R-(+)-8-OH-DPAT did not vary significantly among brain regions (46-96 nM). Partial irreversible blockade of 5-HT(1A) receptors in brain sections (phenoxybenzamine, 10 microM) reduced the maximal response without altering the EC(50) in both the hippocampus and dorsal raphe. Despite prior evidence that dorsal raphe somatodendritic 5-HT(1A) autoreceptors exhibit high receptor/effector coupling efficiency (receptor reserve) compared with postsynaptic receptors in hippocampus, there was no evidence of a difference at the level of receptor/G protein coupling

Chronic food restriction (FR) augments the locomotor and rewarding effects of abused drugs (Cabeza de Vaca and Carr, J. Neurosci. 18:7502,1998). FR also augments the induction of c-fos by amphetamine in nucleus accumbens, central nucleus of the amygdala (CEA) and bed nucleus of the stria terminalis (BNST) (Carr and Kutchukhidze, Brain Res. 861:88, 2000). Because all drugs tested to date, whose rewarding effects are augmented by FR, directly or indirectly stimulate dopamine (DA) release, the c-fos findings suggest that FR may augment behavioral responses by enhancing DA releasability in one or more DA terminal regions. The purpose of this study was to test whether direct DA receptor agonists are exempt from the effect of FR. In Exp. 1, the D-2 agonist quinpirole (0.2 and 0.5 mg/kg, i.p.) produced a dramatic locomotor response in FR rats and no response in ad libitum fed controls. The D-1 agonist A77636 (1.0 and 2.5 mg/kg, i.p.) did not differentially affect the two groups. In Exp. 2, the rewarding effect of A77636 (1.0 and 2.5 mg/kg, i.p.), assessed in a self-stimulation rate-frequency paradigm, was greater in FR rats than controls. The rewarding effect of quinpirole (0.2 and 0.5 mg/kg, i.p.) displayed only a trend in that direction. In Exp. 3, quinpirole (0.5 mg/kg, i.p.) induced fos-like immunoreactivity (FLI) in globus pallidus, BNST, and CEA; the response in CEA was augmented by FR. The behavioral results indicate that augmentation of rewarding/stimulant drug effects by FR may be mediated, at least in part, by postsynaptic DA mechanisms. The immunohistochemical results suggest that augmented transsynaptic effects of quinpirole on cellular activity in CEA may be involved in the locomotor response to this drug in FR rats

It was previously reported that systemic administration of the nonselective opioid antagonist, naltrexone, induces Fos-like immunoreactivity (FLI) within the central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (lateral-dorsal division; BSTLD), nucleus accumbens shell (NACshell) and ventral tegmental area (VTA) of free-feeding rats. These findings suggest that cellular activity in these brain regions is subject to opioid-mediated inhibitory control under basal conditions. Considering the involvement of mesoaccumbens dopamine neurons and components of the 'extended amygdala' in motivated behavior and reward, it was hypothesized that the induction of c-Fos by naltrexone accounts for the motivational-affective consequences of opioid antagonism. In Experiment 1, naltrexone was administered intracerebroventricularly (i.c.v.; 100 microg) to determine whether results obtained in the prior immunohistochemical studies could be attributed to blockade of opioid receptors in brain as opposed to peripheral tissues that convey visceral sensory inputs to the CeA and BSTLD. Naltrexone produced a marked increase in FLI within the CeA and BSTLD, and a moderate increase in NACshell. In Experiment 2, the kappa opioid antagonist, nor-binaltorphimine (Nor-BNI; 20.0 microg, i.c.v.) reproduced the effect of naltrexone in BSTLD and CeA, suggesting that the induction of c-Fos in these two structures is a consequence of kappa receptor blockade. The selective mu antagonist, CTAP (2.0 microg, i.c.v.), reproduced the effect of naltrexone in NACshell, suggesting that the induction of c-Fos in this structure is a consequence of mu receptor blockade. The functional implications of these results are discussed in terms of the known functions of these brain regions and opioid receptor types, and the prior observation that chronic food restriction eliminates the FLI induced by naltrexone in CeA and BSTLD. It is suggested that tonic mu opioid-mediated inhibition in NACshell has a predisposing effect on goal-approach behavior in general while kappa opioid-mediated inhibition in CeA and BSTLD has a predisposing effect on palatability-driven feeding in particular. Finally, a possible relationship between food restriction-induced suppression of the kappa opioid mechanism in CeA/BSTLD, local CRH function, and sensitization of the neural substrate for incentive-motivating effects of abused drugs is discussed.

Activation of striatal dopamine (DA) neurons by neuroleptic treatment or by electrical stimulation of the nigrostriatal pathway increases the activity of tyrosine hydroxylase (TH). The increase is mediated by phosphorylation of the enzyme. However, abolition of DA neuronal activity [by gamma-butyrolactone (GBL) treatment or transection of the nigrostriatal pathway] also increases TH activity. Quantitative blot immunolabeling experiments using site- and phosphorylation state-specific antibodies to TH demonstrated that GBL treatment (750 mg/kg, 35 min) significantly increased phosphorylation at Ser19 (+40%) and Ser40 (+217%) without altering Ser31 phosphorylation. Concomitantly, GBL treatment [along with the 3,4-dihydroxyphenylalanine (dopa) decarboxylase inhibitor NSD-1015, 100 mg/kg, 30 min] increased in vivo striatal dopa accumulation and in vitro TH activity 3-fold. Likewise, cerebral hemitransection of the nigrostriatal pathway significantly increased phosphorylation of TH at Ser19 (+89%) and Ser40 (+158%) but not at Ser31; dopa levels were increased accordingly (+191%). Kinetic analysis of TH activity established that GBL treatment and hemitransection primarily decreased the Km for the cofactor tetrahydrobiopterin (3-fold). The effects of GBL and hemitransection were abolished or attenuated by pretreatment with the DA agonist R-(-)-N-n-propylnorapomorphine (NPA; 30 microgram/kg, 40 min), presumably via stimulation of inhibitory presynaptic DA autoreceptors. NPA dose-response curves for reversal of GBL-induced dopa accumulation and Ser40 phosphorylation were identical; however, only the highest dose of NPA reversed the small and variable increase in Ser19 phosphorylation. Thus, TH activity seems to be regulated by phosphorylation in both hyper- and hypoactive striatal DA neurons; in the latter case, activation seems to be caused by selective phosphorylation of Ser40

Chronic food restriction increases the systemic self-administration and locomotor-stimulating effect of abused drugs. However, it is not clear whether these behavioral changes reflect enhanced rewarding potency or a CNS-based modulatory process. The purpose of this study was to determine whether food restriction specifically increases the rewarding potency of drugs, as indexed by their threshold-lowering effect on lateral hypothalamic self-stimulation, and whether any such effect can be attributed to an enhanced central response rather than changes in drug disposition. When drugs were administered systemically, food restriction potentiated the threshold-lowering effect of amphetamine (0.125, 0.25, and 0.5 mg/kg, i.p.), phencyclidine (1.0, 2.0, and 3.0 mg/kg, i.p.), and dizocilpine (MK-801) (0.0125, 0.05, and 0.1 mg/kg, i.p.) but not nicotine (0.15, 0.3, 0.45 mg/kg, s.c.). When amphetamine (25.0, 50.0, and 100.0 microgram) and MK-801 (5.0, 10.0, and 20.0 microgram) were administered via the intracerebroventricular route, food restriction again potentiated the threshold-lowering effects and increased the locomotor-stimulating effects of both drugs. These results indicate that food restriction increases the sensitivity of neural substrates for rewarding and stimulant effects of drugs. In light of work that attributes rewarding effects of MK-801 to blockade of NMDA receptors on medium spiny neurons in nucleus accumbens, the elements affected by food restriction may lie downstream from the mesoaccumbens dopamine neurons whose terminals are the site of amphetamine-rewarding action. Possible metabolic-endocrine triggers of this effect are discussed, as is the likelihood that mechanisms mediating the modulatory effect of food restriction differ from those mediating sensitization by intermittent drug exposure

Opioid antagonists block the positive hedonic response to food taste and are potent inhibitors of palatability-driven feeding. However, the specific brain regions within which opioid peptide secretion contributes to the maintenance of palatability-driven feeding have not been clearly established. In the present study, c-Fos immunohistochemistry was used to identify regions rostral to the hindbrain that display cellular activation in response to a palatable meal and the meal-paired environment. Further, it was determined whether any of the cellular responses could be prevented by pretreating animals with naltrexone. Twenty brain regions known to be involved in gustation, appetite and reward functions were examined. Ingestion of the palatable meal (3.0 g of 30% shortening, 20% sucrose and 50% powdered Purina rat chow) increased Fos-like immunoreactivity (FLI) in lateral hypothalamus (LH), ventral tegmentum (VTA) and medial preoptic area (MPOA), and decreased FLI in the habenula (Hab). The meal-paired environment increased FLI in the VTA and nucleus accumbens shell (NAC shell). Naltrexone (1.0 mg/kg, i.p.) did not block consumption of the small meal but did prevent all of the distinctive increases in FLI induced by the meal and meal-paired environment. Since naltrexone, alone, increased FLI in VTA, NAC shell, central amygdala (ceA) and laterodorsal bed nucleus of the stria terminalis (BSTLD), the blunting of ingestion reward by naltrexone may result from direct or transsynaptic activating effects on opponent neuronal activity within this highly interconnected set of structures that mediate and modulate reward.