Gregory P. Mark

Properties and Opioid Inhibition of Mesolimbic Dopamine Neurons Vary according to Target Location

The mesolimbic dopamine system, which mediates the rewarding properties of nearly all drugs of abuse, originates in the ventral tegmental area (VTA) and sends major projections to both the nucleus accumbens (NAc) and the basolateral amygdala (BLA). To address whether differences occur between neurons that project to these separate areas, retrograde microspheres were injected to either the BLA or the NAc of DBA/2J mice. Whole-cell recordings were made from labeled VTA dopamine neurons. We found that identified neurons that projected to the BLA and NAc originated within different quadrants of the VTA with neither group exhibiting large-amplitude h-currents. Neurons that projected to the NAc exhibited a greater outward current in response to the κ-opioid agonist (5α,7α,8α)-(+)-N-methyl-N-[7-(pyrrolidinyl)-1-oxaspiro [4,5]dec-8-yl]-benzeneacetamide (U69593; 200 nm), whereas neurons that projected to the BLA exhibited greater inhibition to the μ/δ opioid agonist [Met5] enkephalin (ME; 3 μm). In addition, we found that the presynaptic inhibition of GABAergic transmission at both GABAA and GABAB receptors was differentially regulated by U69593 between the two groups. When dopamine IPSCs were examined, U69593 caused a greater inhibition in NAc- than BLA-projecting neurons. ME had no effect on either. Finally, the regulation of extracellular dopamine by dopamine uptake transporters was equal across the VTA. These results suggest that opioids differentially inhibit mesolimbic neurons depending on their target projections. Identifying the properties of projecting mesolimbic VTA dopamine neurons is crucial to understanding the action of drugs of abuse.

Microdialysis Studies of Brain Norepinephrine, Serotonin, and Dopamine Release During Ingestive Behavior Theoretical and Clinical Implications

This minireview deals with the possible roles of monoamines in feeding and feeding disorders. The introduction sketches the results of earlier studies with local drug injections and selective neurotoxins which provided pharmacological evidence that monoamines can influence food intake and body weight. A table summarizing this evidence is used to list monoamine changes that could underlie anorexia or hyperphagia. It is apparent that abnormalities in the monoamines, along with their cotransmitters, could cause many forms of feeding disorder. It is proposed as a working hypothesis that several varieties of hyperphagia leading to obesity have a common element. This common factor is a change in excitability of a lateral hypothalamic reinforcement system as manifested in self-stimulation at a stimulation-bound feeding site. Understanding this feeding reward-aversion system helps us understand hyperphagia and anorexia. The neurochemistry of reward and aversion involves the monoamines. This paper focuses on dopamine and serotonin. The data support the hypothesis that dopamine systems projecting to the nucleus accumbens and other forebrain areas from the mid-brain ventral tegmental area (VTA) are important for approach and positive reinforcement in ingestive behavior and self-stimulation. Serotonin is hypothesized to facilitate satiety and inhibition of feeding reward in the hypothalamus. The next section abstracts our recent experiments that measured pharmacological and physiological release of the monoamines in the hypothalamus and nucleus accumbens during ingestive behavior and self-stimulation. In vivo microdialysis in freely moving rats suggested the following: (1) Norepinephrine was released in the paraventricular nucleus during the active, feeding period of the circadian cycle. (2) The serotonin metabolite 5-HIAA also increased in the PVN at the same time if there was food to eat. (3) Amphetamine infused into the lateral hypothalamus (LH) by reverse dialysis increased synaptic dopamine, norepinephrine, and serotonin. (4) The anorectic drug d-fenfluramine increased synaptic serotonin in the LH and also increased the dopamine metabolite DOPAC, suggesting that serotonin and dopamine in the LH might contribute to fenfluramine-induced satiety. Local d-fenfluramine injection into the LH or local infusion by reverse dialysis again increased serotonin and decreased 5-HIAA and interfered with local dopamine metabolism as reflected in decreased DOPAC and HVA. (5) Tryptophan, a serotonin precursor, given systemically at an anorectic dose, increased extracellular serotonin in the LH, but this effect was only detectable in food-deprived rats. This was seemingly pH independent (between 5.8 and 8). The passage other cations through CFo is strictly suppressed (even at pH 8 and with 300 mM NaCl in the medium).(ABSTRACT TRUNCATED AT 400 WORDS)

A conditioned stimulus decreases extracellular dopamine in the nucleus accumbens after the development of a learned taste aversion

The conditioned taste aversion (CTA) paradigm and microdialysis were used to determine if extracellular dopamine in the nucleus accumbens is related to the reward value of a stimulus. Intraorally applied saccharin caused a 37% increase in DA in naive rats and a 40% decrease in subjects with a CTA to this taste. These results suggest that accumbens DA is not just a function of arousal but is related to stimulus reward.

Effects of Feeding and Drinking on Acetylcholine Release in the Nucleus Accumbens, Striatum, and Hippocampus of Freely Behaving Rats

Abstract: Extracellular levels of acetylcholine (ACh) were measured in the nucleus accumbens (NAC), striatum (STR), and hippocampus (HIPP) using microdialysis in 30‐min intervals before, during, and after free‐feeding in 20‐h food‐deprived rats. The effects on ACh in the NAC and STR were also observed in response to water intake in 20‐h water‐deprived animals. Neostigmine was used in the perfusate to improve ACh recovery. Basal ACh was sensitive to tetrodotoxin and low calcium, and therefore largely neuronal in origin. Feeding caused a 38% increase in extracellular ACh in the NAC and no change in the STR or HIPP. Dopamine was also increased in the NAC (48%) and to a lesser extent in the STR (21%) following feeding. Drinking caused 18–20% increases in ACh release in both the NAC and STR. In a separate experiment, ACh release in the NAC was monitored in 10‐min intervals during free‐feeding; ACh increased in the interval immediately following maximal food intake. These results suggest a site‐specific increase in ACh release following feeding that cannot be solely attributed to the activation associated with this behavior.

Interleukin-1β decreases acetylcholine measured by microdialysis in the hippocampus of freely moving rats

Interleukin (IL-1) is a cytokine which plays an important role in the modulation of the acute response in host defense. This cytokine is also increased in patients with Alzheimers disease. In the present experiment systemic injection of IL-1 beta (7.5-50 micrograms/kg) decreased extracellular acetylcholine in the hippocampus. This effect could not be attributed entirely to general malaise since lithium chloride (130 mg/kg) had the opposite effect. Heat-inactivation of the cytokine eliminated the reduction of extracellular ACh. The results give further evidence of a relationship between the immune system and the central nervous system and suggest a possible relationship between IL-1 and cholinergic function or dysfunction in the hippocampus.

Inescapable stress enhances extracellular acetylcholine in the rat hippocampus and prefrontal cortex but not the nucleus accumbens or amygdala

A number of experimental results has pointed to a cholinergic involvement in the stress response. Recently, analytical techniques have become available to measure acetylcholine release in vivo during exposure to various stressors. In these experiments, microdialysis was used to monitor acetylcholine output every 15 min in the dorsal hippocampus, amygdala, nucleus accumbens and prefrontal cortex before, during and after 1 h of restraint, including a 15-min session of intermittent tail-shock (1/min, 1 mA, 1-s duration) in rats. In response to the stressful event, acetylcholine release was significantly increased in the prefrontal cortex (186%; p < 0.01) and hippocampus (168%; P < 0.01) but not in the amygdala or nucleus accumbens. The sole effects observed in the amygdala and nucleus accumbens occurred upon release from the restrainer, at which point acetylcholine levels were significantly elevated in both areas (amygdala: 150%; P < 0.05; nucleus accumbens: 13%; P < 0.05). An enhanced acetylcholine release was also evident during this sample period in the hippocampus and prefrontal cortex. These data demonstrate an enhancement of cholinergic activity in response to stress in two acetylcholine projection systems (hippocampus and prefrontal cortex) but not in the intrinsic acetylcholine system of the nucleus accumbens or the extrinsic innervation of the amygdala. Moreover, the data showed that relief from stress was accompanied by a more ubiquitous acetylcholine response that extended to each site tested.

Systemic morphine simultaneously decreases extracellular acetylcholine and increases dopamine in the nucleus accumbens of freely moving rats

Microdialysis was used to measure extracellular levels of acetylcholine (ACh) and dopamine (DA) simultaneously in the nucleus accumbens (NAC) of freely moving rats. Systemic injection of morphine (20 mg/kg) significantly decreased ACh (30%, p less than .01) while it increased DA (55%, p less than .01). The effects of morphine were eliminated by naloxone. The results confirm that morphine increases DA and in addition, demonstrate an inhibitory influence of this opiate on extracellular levels of ACh in the NAC.

Self-administration of cocaine increases the release of acetylcholine to a greater extent than response-independent cocaine in the nucleus accumbens of rats.

Abstract  Rationale: The neurochemical effects of psychostimulant exposure may depend on how these drugs are encountered. A useful method for examining this issue is to compare neurotransmitter release following response-dependent, or self-administered, drug exposure and response-independent exposure. Objectives: This experiment examined the effect of active and passive cocaine administration on acetylcholine (ACh) efflux in the shell region of the nucleus accumbens (NAc) in rats. Methods: One group of rats (CSA: cocaine self-administration) was trained to lever-press for intravenous infusions of cocaine (0.42 mg/kg per infusion) on a fixed-ratio-1 schedule of reinforcement. Cocaine infusions were accompanied by the onset of a stimulus light that signaled a 20-s time-out period. Control rats received intravenous cocaine (cocaine non-contingent: CNC) or saline (SAL) in a manner that was not contingent upon their behavior. Drug infusions in these groups were determined by the lever-press behavior of the animals in the CSA group, i.e. they were yoked to rats in the self-administration group such that CNC animals received equal amounts of cocaine as CSA rats. Animals received cocaine or saline in 3-h sessions for 13 consecutive days before testing. On day 14, extracellular ACh was measured in 15-min intervals before, during and after a 3-h session of cocaine exposure using unilateral microdialysis probes located in the NAc shell coupled with HPLC. Results: ACh efflux was significantly increased above baseline in both groups of rats that received cocaine but CSA rats had significantly higher ACh levels during the self-administration period compared to their yoked counterparts. In addition, ACh efflux remained elevated longer in CSA animals relative to CNC rats following cessation of cocaine exposure. Conclusions: These results demonstrate that ACh interneurons in the NAc shell are responsive to cocaine exposure. In addition, these findings suggest that the manner in which the drug is administered (i.e. either by active self-administration or passive exposure) may be relevant to the magnitude of the neural response.

Self-administration enhances excitatory synaptic transmission in the bed nucleus of the stria terminalis

Understanding the neurobiology of motivation might help in reducing compulsive behaviors such as drug addiction or eating disorders. This study shows that excitatory synaptic transmission was enhanced in the bed nucleus of the stria terminalis of rats that performed an operant task to obtain cocaine or palatable food. There was no effect when cocaine or food was delivered passively, suggesting that synaptic plasticity in this area is involved in reward-seeking behaviors.

An appetitively conditioned taste elicits a preferential increase in mesolimbic dopamine release

Rats were prepared with intragastric (IG) cannulae for infusing a nutrient into the stomach and microdialysis guide shafts in the nucleus accumbens (NAC) and striatum (STR) for measuring changes in extracellular dopamine. Prior to dialysis, subjects were trained to prefer the mildly bitter taste of sucrose octaacetate (SOA; CS+) by pairing voluntary intake with automatic IG infusions of nutritive polycose. The mildly sour taste of citric acid (CS-) was paired with IG water infusions as a control. Unconditioned animals received four exposures to SOA and citric acid on counterbalanced, alternating days. After training, dialysis samples were collected every 30 min before, during, and after intake of the CS+ or CS- in response to 14 h water deprivation on counterbalanced, consecutive days. Voluntary intake of the CS+ for 30 min significantly increased extracellular DA in the NAC but not in the STR of conditioned subjects. Intake of the CS- did not alter DA efflux at either site. Unconditioned, control rats also showed no DA response to either taste. These results show selective activation of the mesolimbic dopaminergic projection system as a consequence of a conditioned taste stimulus paired with a nutritive gastric load. This suggests that conditioned DA release may play a role in learned ingestive behavior based on the postingestive effects of food.