delta-Opioid receptor-mediated regulation of central dopaminergic neurons in the rat

Acute effects of capsaicin on proopioimelanocortin mRNA levels in the arcuate nucleus of Sprague-Dawley rats

OBJECTIVE

Capsaicin, a noxious stimulant and main component of the hot flavor of red peppers, has an analgesic effect when administered to humans. We investigated the expression of proopioimelanocortin (POMC) mRNA in the arcuate nucleus of Sprague-Dawley (SD) rats after administering capsaicin, hypothesizing that administering capsaicin activates the central opioid system.

METHODS

SD rats were divided randomly into two groups; one group received a saline injection and the other received a capsaicin injection. The POMC mRNA level in the arcuate nucleus of the hypothalamus was measured by the reverse transcription-polymerase chain reaction at 0, 20, 40, 60, and 120 minutes after capsaicin administration.

RESULTS

Capsaicin administration resulted in a significantly increased POMC mRNA level, compared to that in saline-treated rats at the 20-minute time point (t=-4.445, p=0.001). However, no significant group differences were observed at other times (t=-1.886, p=0.089; t= -0.973, p=0.353; t=-2.193, p=0.053 for 40, 60, and 120 minutes, respectively).

CONCLUSION

The analgesic effect of capsaicin might be associated with increased activity of the cerebral opioid system. This finding suggests that capsaicin acted for nociception and analgesia and could affect alcohol-intake behavior, which might further imply that a food culture could affect drinking behavior.

The selective delta opioid agonist SNC80 enhances amphetamine-mediated efflux of dopamine from rat striatum

The highly selective delta opioid agonist, SNC80, elicits dopamine-related behaviors including locomotor stimulation and conditioned place-preference. In contrast, it has been reported that SNC80 fails to promote dopamine efflux from the striatum of freely moving rats. However, SNC80 does enhance behavioral responses to the stimulants, amphetamine and cocaine, suggesting an interaction between delta opioids and psychostimulants. Since the increase in locomotor activity elicited by amphetamine and related stimulants acting at the dopamine transporter is associated with increases in extracellular concentrations of dopamine within the striatum, we hypothesized that SNC80 enhances this activity by potentiating the overflow of dopamine through the transporter. To test this hypothesis, striatal preparations from Sprague Dawley rats were assayed for dopamine efflux in response to amphetamine challenge. SNC80 was given either in vivo or in vitro directly to rat striatal tissue, prior to in vitro amphetamine challenge. Both in vivo and in vitro administration of SNC80 enhanced amphetamine-mediated dopamine efflux in a concentration- and time-dependent manner. However, SNC80 in either treatment paradigm produced no stimulation of dopamine efflux in the absence of amphetamine. The effect of SNC80 on amphetamine-mediated dopamine overflow, but not the effect of amphetamine alone, was blocked by the delta selective antagonist, naltrindole and was also observed with other delta agonists. The results of this study demonstrate that even though SNC80 does not stimulate dopamine efflux alone, it is able to augment amphetamine-mediated dopamine efflux through a delta opioid receptor mediated action locally in the striatum.

Microinjection of the delta-opioid receptor selective antagonist naltrindole 5'-isothiocyanate site specifically affects cocaine self-administration in rats responding under a progressive ratio schedule of reinforcement

Whether the delta-opioid receptor (DOR) system can modulate behavioral effects of cocaine remains equivocal. We examined whether site- and subtype-selective blockade of DORs within the rat mesocorticolimbic system affects cocaine self-administration. The DOR antagonist naltrindole 5'-isothiocyanate (5'-NTII; 5nmol) was microinjected into the nucleus accumbens (NAcc), ventral tegmental area (VTA), or amygdala (AMYG) in rats self-administering 1.5mg/kg cocaine under a progressive ratio (PR) schedule. Intra-NAcc 5'-NTII significantly decreased cocaine self-administration, while 5'-NTII administration into the VTA significantly increased cocaine-maintained responding. 5'-NTII administration into the AMYG produced no effect. These data support a site-specific role of DORs in cocaine's behavioral effects.

Intracerebroventricular d-Pen2, d-Pen5-enkephalin administration soon after stressor imposition influences behavioral responsivity to a subsequent stressor encounter in CD-1 mice

Endogenous opioid peptide systems diminish stress-induced autonomic nervous system, neuroendocrine (hypothalamic-pituitary-adrenal axis) and behavioral responses, attenuating a collection of physiological symptoms basic to emotional and affective states. Neurogenic stressors may incite specific central changes in opioid peptide availability as well as changes in mu and delta-opioid receptor function. The present investigation evaluated the proactive influence of an intracerebroventricular injection of the opioid receptor agonist D-Pen2, D-Pen5-enkephalin (DPDPE) (0 microg, 0.005 microg, 1.0 microg or 2.5 microg) on locomotor behavior of mice following uncontrollable footshock (Shock) or novel shock chamber exposure (No Shock). It was expected that DPDPE administration following Shock on Day 1 would restore locomotor activity up to 1 week and prevent shock-associated behavior of mice encountering a brief session of footshock 18 days later. Exposure to Shock reduced horizontal locomotor and vertical locomotor (rearing) activity of mice while 2.5 microg DPDPE restored behavior. Eighteen days following Shock and DPDPE challenge, mice were exposed to either an abbreviated session of footshock (Mild Stress) or the shock chamber (Cues). Mice in the No Shock and Shock groups administered 2.5 microg DPDPE on Day 1 did not exhibit any locomotor deficits in response to Mild Stress on Day 18. Mice in the Shock group administered 0.005 microg DPDPE on Day 1, did not exhibit exaggerated rearing deficits following ensuing Mild Stressor encounter relative to mice reexposed to Cues on Day 18. Taken together, these data show that (a) footshock differentially affects rearing and locomotor activity, (b) DPDPE administration increases locomotor activity for up to 1 week following footshock and DPDPE administration, (c) reexposure to Mild Stress affects rearing and locomotor performance differently depending on previous stressor history and DPDPE dose, (d) DPDPE affords long-lasting protection to previously non-stressed mice against the deleterious effects of subsequent mild stress on locomotor activity, while a low dose of DPDE is sufficient to prevent shock-induced sensitization of rearing deficits, 18 days following original stressor and drug presentation. Finally, our investigation demonstrates that DPDPE administration alters the behavioral impact of future stressful encounters and emphasizes the importance of investigating opioid mechanisms in chronic stress disorders.

Effect of protein kinases on lactate dehydrogenase activity in cortical neurons during hypoxia

Our previous work shows that delta-opioid receptor (DOR) protects cortical neurons from hypoxic insults. Since an enhanced anaerobic capacity is important for neurons to adapt to the reduction of oxidative phosphorylation, we asked whether DOR plays a role in neuronal regulation of anaerobic capacity, thus protecting neurons from O(2) deprivation. Indeed, there is evidence suggesting that DOR may regulate protein kinase A (PKA) and C (PKC), which are involved in regulation of lactate dehydrogenase (LDH). However, little is known regarding the role of DOR and protein kinases in the regulation of glycolytic and related enzymes. As a first step, the present studies were performed in primary cultures of rat cortical neurons to clarify two issues: (1) Are protein kinases involved in the regulation of LDH activity in hypoxia? and (2) Does DOR affect LDH activity in hypoxic neurons? The results showed that PKC activation yielded substantial increases in normoxic LDH activity and significantly augmented LDH activity in hypoxic neurons, while PKC inhibition decreased LDH activity in both normoxic and hypoxic neurons. PKA activation significantly increased LDH activity in normoxic neurons and further elevated LDH activity in hypoxic neurons. However, PKA inhibition did not decrease in LDH activity in either normoxic or hypoxic neurons. Although DOR inhibition slightly reduced LDH activity in normoxia, DOR activation or inactivation did not alter LDH activity in hypoxic neurons. These data suggest that in cortical neurons, (i) PKC up-regulates LDH activity and plays an important role in its up-regulation during hypoxia; (ii) PKA is less likely involved in the regulation of LDH activity during hypoxia although its stimulation may slightly increase LDH activity and (iii) DOR does not contribute to LDH activity up-regulation during hypoxia.

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.

delta-, but not mu- and kappa-, opioid receptor activation protects neocortical neurons from glutamate-induced excitotoxic injury

Recent observations from our laboratory have led us to hypothesize that delta-opioid receptors may play a role in neuronal protection against hypoxic/ischemic or glutamate excitotocity. To test our hypothesis in this work, we used two independent methods, i.e., "same field quantification" of morphologic criteria and a biochemical assay of lactate dehydrogenase (LDH) release (an index of cellular injury). We used neuronal cultures from rat neocortex and studied whether (1) glutamate induces neuronal injury as a function of age and (2) activation of opioid receptors (delta, mu and kappa subtypes) protects neurons from glutamate-induced injury. Our results show that glutamate induced neuronal injury and cell death and this was dependent on glutamate concentration, exposure period and days in culture. At 4 days, glutamate (up to 10 mM, 4 h-exposure) did not cause apparent injury. After 8-10 days in culture, neurons exposed to a much lower dose of glutamate (100 microM, 4 h) showed substantial neuronal injury as assessed by morphologic criteria (>65%, n=23, P<0.01) and LDH release (n=16, P<0. 001). Activation of delta-opioid receptors with 10 microM DADLE reduced glutamate-induced injury by almost half as assessed by the same criteria (morphologic criteria, n=21, P<0.01; LDH release, n=16, P<0.01). Naltrindole (10 microM), a delta-opioid receptor antagonist, completely blocked the DADLE protective effect. Administration of mu- and kappa-opioid receptor agonists (DAMGO and U50488H respectively, 5-10 microM) did not induce appreciable neuroprotection. Also, mu- or kappa-opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronal delta-opioid receptors, but not mu- and kappa-opioid receptors, protect neocortical neurons from glutamate excitotoxicity.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Immunoneutralization of endogenous opioid peptides prevents the suckling-induced prolactin increase and the inhibition of tuberoinfundibular dopaminergic neurons

Previous studies have shown that the endogenous opioid peptides, acting at specific opiate receptor subtypes, are involved in the suckling-induced prolactin secretory response. The prolactin increase elicited by suckling is due, at least in part, to an inhibition of tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We investigated the effects of immunoneutralization of dynorphin, leu-enkephalin and met-enkephalin on the suckling-induced prolactin increase and on the activity of the TIDA neurons in lactating female rats between days 7 and 12 postpartum. Rats were injected into the right lateral ventricle with antiserum specific for one of these three peptides. Control rats were administered equal amounts of immunoglobulin proteins. Suckling produced a profound and significant increase in prolactin levels, as well as a decrease in DOPA accumulation in the median eminence of lactating rats. Administration of immunoglobulin concentrations of up to 3.6 microg did not inhibit the prolactin secretory response to the suckling stimulus and did not prevent the suckling-induced inhibition of TIDA neurons. Antisera to all three endogenous opioid peptides abolished the suckling-induced prolactin increase and prevented the inhibition in DOPA accumulation in the median eminence. Thus, the endogenous opioid peptides, dynorphin, leu-enkephalin and met-enkephalin, are essential for the prolactin secretory response to suckling and inhibition of TIDA neuronal activity is at least one of the mechanisms of action utilized by these peptides.

Localization of the delta-opioid receptor and dopamine transporter in the nucleus accumbens shell: implications for opiate and psychostimulant cross-sensitization

Opiate- and psychostimulant-induced modulation of dopamine transmission in the nucleus accumbens shell (AcbSh) is thought to play a key role in their potent reinforcing and locomotor effects. To investigate the cellular basis for potential functional interactions involving opiates active at the delta-opioid receptor (DOR) and psychostimulants that bind selectively to the dopamine transporter (DAT), we examined the electron microscopic localization of their respective antisera in rat AcbSh. DOR immunoperoxidase labeling was seen primarily, and DAT immunogold particles exclusively, in axon terminals. In these terminals, DOR immunoreactivity was prominently associated with discrete segments of the plasma membrane and the membranes of nearby small synaptic and large dense core vesicles. DAT immunogold particles were almost exclusively distributed along nonsynaptic axonal plasma membranes. Thirty-nine percent DOR-labeled profiles (221/566) either apposed DAT-immunoreactive terminals or also contained DAT. Of these 221 DOR-labeled profiles, 13% were axon terminals containing DAT and 15% were dendritic spines apposed to DAT-immunoreactive terminals. In contrast, 70% were morphologically heterogeneous axon terminals and small axons apposed to DAT-immunoreactive terminals. Our results indicate that DOR agonists in the AcbSh can directly modulate the release of dopamine, as well as postsynaptic responses in spiny neurons that receive dopaminergic input, but act principally to control the presynaptic secretion of other neurotransmitters whose release may influence or be influenced by extracellular dopamine. Thus, while opiates and psychostimulants mainly have differential sites of action, cross-sensitization of their addictive properties may occur through common neuronal targets.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Comparative analysis of L-DOPA actions on nociceptive and non-nociceptive spinal reflex pathways in the cat

The actions of L-DOPA (40-100 mg/kg i.v.) on nociceptive and non-nociceptive spinal reflex pathways were investigated in anaemically decapitated high spinal cats. The results revealed a differential pattern of effects of L-DOPA on monosynaptic and oligo-orpolysynaptic nociceptive and non-nociceptive reflexes. (1) L-DOPA depressed monosynaptic reflexes of flexors without affecting those of the extensors. (2) Excitatory pathways from flexor reflex afferents (FRA) were distinctly depressed by L-DOPA, pathways from group II muscle afferents reacted with greater sensitivity than pathways from non-nociceptive cutaneous and joint afferents. (3) Inhibitory FRA pathways were distinctly less affected by L-DOPA than excitatory ones. (4) Transmission in nociceptive excitatory FRA pathways was depressed to the same high degree as that in pathways from group II muscle afferents. (5) Effects on transmission in non-FRA pathways such as the group Ib inhibitory pathway and the excitatory nociceptive pathway from the foot pad to plantaris and intrinsic foot extensors were either minor or absent. (6) L-DOPA increased the delay in the reaction to noxious stimulation. (7) The effects of L-DOPA could not be specifically antagonised by naloxone. Thus, mainly excitatory FRA pathways, irrespective of a nociceptive or non-nociceptive origin, are under strong depressive dopaminergic influences. These effects are similar to those evoked by opioids.

Endogenous opioid peptides contribute to suckling-induced prolactin release by suppressing tyrosine hydroxylase activity and messenger ribonucleic acid levels in tuberoinfundibular dopaminergic neurons

The endogenous opioid peptides have been implicated in the control of the suckling-induced PRL rise during lactation. This study examined the role of the endogenous opioid peptides in suppressing tuberoinfundibular dopaminergic neuronal activity during lactation. In the first experiment, lactating rats were constantly exposed to pups. Naloxone (NAL; 60 mg/kg x h; i.v.), an opioid receptor antagonist, or saline was infused for 12 h. Blood was collected before and at 2-h intervals during the infusion. NAL suppressed circulating PRL levels to less than 36% of control values at 4, 6, 8, and 12 h after the onset of the infusion. Tyrosine hydroxylase (TH) activity in the stalk-median eminence and TH messenger RNA signal levels in the arcuate nucleus were determined at the end of the NAL infusion. TH activity and TH messenger RNA signal levels were increased 2.5- and 2.7-fold, respectively, after the 12-h NAL infusion. Even though the time spent with their pups was similar between the two groups, the pups in the NAL-treated group failed to gain weight during the 12-h NAL infusion period, whereas the control litters (8 pups) gained 5 g. In a second experiment, pups were removed from the dams before the 12-h NAL infusion and were returned after 11 h. Blood was collected before the infusion, at 3-h intervals during the pup separation period, and at 15-min intervals after reunion with the pups. Plasma PRL in control and NAL-treated rats was low (1-15 ng/ml) and similar during the separation period. The suckling-induced PRL surge in NAL-treated rats was markedly attenuated to 9-25% of control levels (350-650 ng/ml). After a 1-h suckling episode, TH activity in the stalk-median eminence of NAL-treated rats was 4.5-fold greater than controls. Litter weight gains were significantly less in NAL-treated rats during the 1-h suckling episode. These data indicate that the endogenous opioid peptides are an integral component for increasing PRL release in response to suckling and they act to decrease tuberoinfundibular dopaminergic neuronal activity during lactation, in part, by suppressing TH gene expression.

Involvement of dopamine-dependent and -independent mechanisms in the rewarding effects mediated by delta opioid receptor subtypes in mice

The rewarding effects of the delta 1 opioid receptor agonist [D-Pen2, Pen5]enkephalin (DPDPE) and the delta 2 opioid receptor agonist [D-Ala2]deltorphin II (DELT) on the activity of mesolimbic and nigrostriatal dopamine (DA) neurons were examined in mice. Both DPDPE (15 nmol, i.c.v.) and DELT (5 nmol, i.c.v.) produced a significant place preference in mice. The DPDPE (15 mol, i.c.v.)-induced place preference was abolished by 7-benzylidenenaltrexone (BNTX; 0.5 mg/kg, s.c.), a delta1 opioid receptor antagonist, but not by naltriben (NTB; 0.5 mg/kg, s.c.), a delta 2 opioid receptor antagonist. In contrast, the DELT (5 nmol, i.c.v.)-induced place preference was antagonized by NTB, but not BNTX. I.c.v.. injection of DPDPE, but not DELT, at a dose that produced a significant place preference produced a significant elevation of DA turnover in the mouse limbic forebrain, and this effect of DPDPE was antagonized by BNTX but not by NTB. In addition, i.c.v. injection of DPDPE or DELT not affect DA turnover in the mouse striatum. These results suggest that the rewarding effects produced by the activation of central delta 1, but not delta 2, opioid receptors may be caused through the enhancement of the mesolimbic DA neurotransmission, and confirm our previous hypothesis that the DA-dependent and -independent mechanisms may exist in the rewarding effects produced by the activation of central delta opioid receptor subtypes.

Evidence that D2 receptor-mediated activation of hypothalamic tuberoinfundibular dopaminergic neurons in the male rat occurs via inhibition of tonically active afferent dynorphinergic neurons

The purpose of the present study was to determine if D2 receptor-mediated activation of hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons occurs via afferent neuronal inhibition of tonically active inhibitory dynorphinergic neurons in the male rat. To this end, the effects of either surgical deafferentation of the mediobasal hypothalamus or administration of a kappa opioid receptor agonist (U-50,488) or antagonist (nor-binaltorphimine (NOR-BNI)) on D2 receptor-mediated activation of TIDA neurons were assessed. For comparison, the activity of mesolimbic DA neurons was also determined in these studies. TIDA and mesolimbic DA neuronal activities were estimated by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)) in terminals of these neurons in the median eminence and nucleus accumbens, respectively. Intraperitoneal administration of the D2 receptor agonist quinelorane caused a dose-dependent increase in DOPAC in the median eminence and a decrease in DOPAC in the nucleus accumbens; surgical deafferentation of the mediobasal hypothalamus prevented the effect of quinelorane in the median eminence, but not the nucleus accumbens. Activation of kappa opioid receptors with U-50,488 had no effect per se, but blocked quinelorane-induced increases in median eminence DOPA. In contrast, U-50,488 had no effect on DOPA in the nucleus accumbens of either vehicle- or quinelorane-treated rats. Blockade of kappa opioid receptors with NOR-BNI increased median eminence DOPA, and prevented the stimulatory effects of quinelorane on dopamine synthesis. Administration of prolactin also increased median eminence DOPA, but did not alter the ability of quinelorane to stimulate dopamine synthesis. Neither NOR-BNI nor prolactin had any effect on DOPA in the nucleus accumbens of vehicle- or quinelorane-treated rats. These results suggest that D2 receptor-mediated activation of TIDA neurons occurs via an afferent neuronal mechanism involving, at least in part, inhibition of tonically active inhibitory dynorphinergic neurons in the male rat.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.