Endogenous neurosteroids influence synaptic GABAA receptors during postnatal development.

GABA plays a key role in both embryonic and neonatal brain development. For example, during early neonatal nervous system maturation, synaptic transmission, mediated by GABAA receptors (GABAA Rs), undergoes a temporally specific form of synaptic plasticity to accommodate the changing requirements of maturing neural networks. Specifically, the duration of miniature inhibitory postsynaptic currents (mIPSCs), resulting from vesicular GABA activating synaptic GABAA Rs, is reduced, permitting neurones to appropriately influence the window for postsynaptic excitation. Conventionally, programmed expression changes to the subtype of synaptic GABAA R are primarily implicated in this plasticity. However, it is now evident that, in developing thalamic and cortical principal- and inter-neurones, an endogenous neurosteroid tone (eg, allopregnanolone) enhances synaptic GABAA R function. Furthermore, a cessation of steroidogenesis, as a result of a lack of substrate, or a co-factor, appears to be primarily responsible for early neonatal changes to GABAergic synaptic transmission, followed by further refinement, which results from subsequent alterations of the GABAA R subtype. The timing of this cessation of neurosteroid influence is neurone-specific, occurring by postnatal day (P)10 in the thalamus but approximately 1 week later in the cortex. Neurosteroid levels are not static and change dynamically in a variety of physiological and pathophysiological scenarios. Given that GABA plays an important role in brain development, abnormal perturbations of neonatal GABAA R-active neurosteroids may have not only a considerable immediate, but also a longer-term impact upon neural network activity. Here, we review recent evidence indicating that changes in neurosteroidogenesis substantially influence neonatal GABAergic synaptic transmission. We discuss the physiological relevance of these findings and how the interference of neurosteroid-GABAA R interaction early in life may contribute to psychiatric conditions later in life.

PDGF-BB induced chemotaxis is impaired in aged capillary endothelial cells.

The object of this study was to determine if the decreased angiogenesis in the healing wounds of the aged is due to the failure of endothelial cells to respond to locally produced growth factors. Endothelial cells isolated from wound sponges implanted in aged (24-month-old) and young (2-month-old) rats were tested for their chemotactic response to the BB isoform of platelet-derived growth factor (PDGF-BB). A similar number of cells isolated from both old and young rats stained positive (75-90%) for acetylated-LDL uptake, and the same number of viable cells was used in the chemotaxis assay. Endothelial cells from both old and young rats migrated in a dose-dependent (0.1-3.0 ng/ml) manner in response to PDGF-BB. At all concentrations tested, PDGF-BB elicited the migration of more endothelial cells from the young rats. The difference between the number of young and old cells that responded to PDGF-BB was statistically significant at the 1.0 ng/ml and 3.0 ng/ml concentrations. These results suggest that the impaired angiogenic response in the healing of wounds of the aged is due to altered endothelial cell reaction to the growth factors in the wound microenvironment.

Proliferation of wound derived capillary endothelial cells: young versus aged.

The objective of this study was to compare the proliferative potential of wound derived capillary endothelial cells (WCEC) from aged and young rats. Endothelial cells were isolated from subcutaneously implanted sponges in 2- and 24-month-old rats. The identity of the cells as endothelial was confirmed by staining for Ac-LDL uptake. Aged and young WCEC (20,000/well) were stimulated with increasing concentrations of fetal calf serum (0, 2.5, 5, 10 and 15%). The increase in cell number was determined with a Coulter counter. At all serum concentrations, the proliferative capacity of WCEC from aged rats was significantly higher than that of WCEC from young rats.

Age-related alterations in the morphology of femoral artery vasa vasorum in the rat.

The objective of this study was to explore any age-related morphological changes in the vasa vasorum of the rat femoral artery. Vascular corrosion casts were prepared from 2, 12 and 24-month-old rats. Examination of the casts with the scanning electron microscope revealed dramatic differences in the appearance of the vessels of young and aged rats. The vasa vasorum of 2-month-old rats consisted of a dense network of capillaries. These vessels were dramatically reduced in number by 12 months, and even fewer capillaries were present at 24 months. This reduction in capillary density is consistent with the observed age-related decreases in oxygen tension and may explain why the aged are more prone to atherosclerosis.

Enhanced dopamine efflux in the amygdala by a predictive, but not a non-predictive, stimulus: facilitation by prior repeated D-amphetamine.

Extracellular levels of dopamine within the amygdala were monitored using in vivo microdialysis during performance of an appetitive Pavlovian conditioning task in sensitized rats and unsensitized controls. Animals received exposure either to D-amphetamine or to vehicle for seven consecutive days (2 mg/kg/day, i.p.) in the home cage. Training began following a further seven injection-free days. Animals were exposed to two session types: during conditioning sessions, a stimulus (tone or light) immediately preceded sucrose pellet delivery. During control sessions, the alternative stimulus was also presented, but not in temporal proximity to an otherwise identical schedule of pellet delivery. There was a total of three alternating presentations of each session type during training. Sensitization enhanced Pavlovian conditioned approach behaviour to the stimulus predictive of imminent pellet delivery, and was without effect upon approach behaviours either to the food pellets themselves or to the control stimulus. Extracellular levels of dopamine within the amygdala were assessed during the fourth conditioning and control sessions. Mesoamygdaloid dopamine efflux increased significantly during the conditioning test session, but not during the control session, and this dopaminergic response was more marked in rats with prior repeated D-amphetamine experience. Hence, these results add to evidence suggesting a role for amygdaloid dopamine in appetitive Pavlovian conditioning, and in the facilitation of associative learning following prior experience of D-amphetamine.

Immunohistochemical assessment of mesotelencephalic dopamine activity during the acquisition and expression of Pavlovian versus instrumental behaviours.

Dopaminergic activity during Pavlovian or instrumental learning in key target regions of the mesotelencephalic dopamine system was investigated immunohistochemically using antibodies raised against glutaraldehyde-conjugated dopamine. Experiment 1 examined dopamine immunoreactivity during acquisition of a Pavlovian conditioned-approach response. Observations were taken at three stages of learning: initial, intermediate and asymptotic; each with a conditioned stimulus+ (CS+) group for whom visual or auditory stimuli immediately preceded an unconditioned stimulus (sucrose), and a conditioned stimulus- (CS-) group for whom stimuli and the unconditioned stimulus were unpaired. Animals learned to approach the alcove during CS+ presentations, whilst approach behaviour of the CS- group remained low. In general, target regions exhibiting a dopaminergic reaction responded maximally during the intermediate stage of acquisition, and were less responsive initially, and not responsive at all at asymptote. Specifically, the pattern of dopaminergic response was: shell more than core of the nucleus accumbens; prefrontal cortex, central and basolateral nuclei of the amygdala also significantly responsive. Mediodorsal and laterodorsal striatal regions were reactive only very early in training. Experiment 2 examined dopaminergic reaction following acquisition of a novel conditioned instrumental response. The conditioned response+ (CR+) group responded at a much higher rate on the lever for which unconditioned stimulus-associated stimuli were presented, than on the control lever. The conditioned response- (CR-) group responded at a low rate on both levers. In contrast with experiment 1, the most responsive regions were the core of the nucleus accumbens, medial prefrontal cortex and basolateral area of the amygdala. Thus, the acquisition, but not expression of Pavlovian associations activated dopamine within several key target regions of the mesotelencephalic dopamine system, and preferentially within the shell rather than core of the nucleus accumbens. By contrast, acquisition of a novel instrumental response preferentially activated the core of the nucleus accumbens, and basolateral area of the amygdala. These data carry significant implications for the potential role of these regions in learning and memory.

An immunohistochemical examination of the effects of sensitisation on the mesotelencephalic dopaminergic response to d-amphetamine.

The dopaminergic response to d-amphetamine with or without prior repeated experience with the drug was investigated immunohistochemically in key target regions of the mesotelencephalic dopamine system using antibodies raised against glutaraldehyde-conjugated dopamine. This methodology permitted the unambiguous determination of dopaminergic activity within specific subregions of structures implicated in the behavioural effects of psychomotor stimulants drugs, and in the expression of behavioural sensitisation. Experiment 1 examined dopamine immunoreactivity in central or basolateral amygdala, shell or core of the nucleus accumbens, medial and lateral caudate-putamen and medial prefrontal cortex following the administration of various doses of d-amphetamine to drug-naïve rats. Whilst dose-related increases in dopaminergic activity were detected in all regions examined, a regional heterogeneity was clearly evident. For example, d-amphetamine enhanced dopaminergic activity preferentially within the shell subregion of the nucleus accumbens both with respect to the core subregion and to other striatal and non-striatal areas. Experiment 2 examined changes in dopaminergic activity following the administration of a low dose of d-amphetamine to d-amphetamine-sensitised rats and saline-pretreated control animals. Regional heterogeneity both between and within terminal areas was again detected. Thus, there was evidence of a preferential increase in dopaminergic activity within the shell of the nucleus accumbens of sensitised rats. Moreover, sensitisation to d-amphetamine increased the dopaminergic response to acute administration of d-amphetamine within all striatal and non-striatal areas examined. Comparison of this effect across subterritories of the areas under investigation revealed that in sensitised rats, acute administration of d-amphetamine elevated dopaminergic activity within the shell of the nucleus accumbens to a greater extent than within the core. These data therefore indicate that systemic administration of d-amphetamine is associated with regionally heterogeneous changes in dopaminergic activity within terminal regions of the mesotelencephalic dopamine system in both sensitised and unsensitised rats. Moreover, the present methodology permitted resolution of these changes at an anatomical level beyond that of conventional approaches.

Parenteral nutrition: current status and concepts.

Parenteral nutrition is regarded as a form of nutrition in some countries and as an extension of intravenous fluid therapy in others. The optimum clinical application of parenteral nutrition as a form of therapy requires detailed knowledge of the nutrient solutions themselves, including the commonly used solutions such as dextrose, soybean oil emulsion, synthetic crystalline L-amino acid solutions; older solutions such as xylitol, protein hydrolysates; and newer solutions such as glycerides and special purpose amino acid solutions. Additionally, information has accumulated over the past 10 years, leading to the rational use of vitamins and trace elements in parenteral nutrition. Metabolism of the substrates has been correlated with known pathways of intermediary metabolism in normal, starved and stressed subjects. Several new concepts have arisen: a) Infusion of excessive quantities of dextrose results in lipogenesis and increased carbon dioxide production. Hyperalimentation of this type is being replaced by infusion of lesser quantities of dextrose, supplemented by intravenous infusion of lipid as a calorie source. b) Protein hydrolysates and racemic synthetic crystalline amino acid solutions have been replaced by synthetic crystalline L-amino acid solutions. c) A new fat emulsion based on safflower oil is competing successfully with the traditional soybean oil emulsion. d) Newer substrates are being explored. These include branched chain amino acids, keto analogues of amino acids, synthetic glycerides and maltose. e) Deficiencies of essential fatty acids, trace elements and vitamins have been studied in patients on long term parenteral nutrition and their mechanisms elucidated. Official recommendations for intravenous administration of these nutrients have been made. f) Several techniques have been applied in several circumstances, including protein sparing therapy, cyclic nutrition, home therapy, and parenteral nutrition in liver and renal failure. Parenteral nutrition is now used extensively, not only in major hospitals where the resources of a team approach with physician, nurse, pharmacist and dietitian are available, but also in smaller hospitals where all of these facilities may not be at hand. However, whatever the setting, the principles behind the clinical application of parenteral nutrition should be well understood by those involved, including current approaches to safe preparation and infusion of parenteral nutrition solutions.

Role of CT in the management of pneumothorax in patients with complex cystic lung disease.

The diagnosis and treatment of pneumothorax in patients with complex cystic lung disease may be difficult when relying on plain chest radiography alone. We report four cases in which management was greatly facilitated by the use of CT scanning of the chest.

Isolation rearing enhances the locomotor stimulant properties of intra-perifornical sulpiride, but impairs the acquisition of a conditioned place preference.

Previous data indicated that infusions of the D2/D3 dopamine receptor antagonist sulpiride within the perifornical region of the lateral hypothalamus may engage neural circuitry relevant to activation of the mesoaccumbens dopamine projection. The present work examined this proposition further. Experiment 1 examined the ability of intra-perifornical sulpiride to induce a conditioned place preference, using an unbiased conditioning procedure. Thus, bilateral guide cannulae were implanted to gain access to the perifornical region of the lateral hypothalamus. Following recovery, animals were subjected to an initial exposure to the place preference apparatus. The apparatus consisted of three distinctive compartments, the central compartment allowing access to the two outermost compartments. Initial exposure indicated equal preference for each. Then, in alternating sessions, animals received infusions of sulpiride (5, 10 or 20 micrograms) before being placed in one of the two outermost compartments, and infusions of vehicle before being placed in the alternate compartment. Compartment-drug pairings were counterbalanced across animals. Four drug, and four saline sessions were completed, each being separated by at least 2 full days. On the final test day, animals were allowed free access to compartments, and the time spent in each was compared with that of initial exposure. Results showed that intra-perifornical sulpiride increased activity during drug-conditioning sessions in an incremental fashion, and supported dose-dependently the acquisition of a conditioned place preference. Experiment 2 examined the effects of isolation rearing upon the locomotor stimulant properties of intra-perifornical sulpiride, and the acquisition of a conditioned place preference. Rats were raised from weaning either alone (isolation-reared) or in groups of five (socially-reared controls) until 4 months of age. Consistent with previous reports of the effects of isolation rearing upon psychomotor stimulant responsivity, here isolates were found to be more responsive to the locomotor stimulant properties of intra-perifornical sulpiride, but were less responsive to the ability of intra-perifornical sulpiride to support the acquisition of a conditioned place preference. These data were suggested to provide further support for the proposition that blockade of dopamine receptors of the D2 family within the perifornical region of the lateral hypothalamus results in the activation of the mesoaccumbens dopamine projection, via the ventral tegmental area.

Amygdala and hippocampus control dissociable aspects of drug-associated conditioned rewards.

Limbic innervation of the nucleus accumbens via the ventral subiculum/hippocampus and basolateral area of the amygdala has been shown to determine dissociable aspects of behaviour controlled by stimuli associated with natural rewards. However, the respective contributions of the ventral subiculum and amygdala to behaviour governed by drug-associated stimuli remain to be determined. Experiments consisted of two phases: drug-stimulus training, and subsequent stimulus-only testing. Initial training sessions were of two alternating forms. During drug sessions, responding upon one lever resulted in an infusion of 1 microgram d-amphetamine into the nucleus accumbens, whilst during saline sessions d-amphetamine was replaced with saline. Each infusion (drug or saline) was preceded with either a light, or tone. Responding upon a control lever had no programmed consequences. Following training, the levers were retracted, and instead two novel vertical bars were extended from the chamber ceiling. Movement of one bar produced the drug-associated stimulus, whilst the alternative bar produced the saline-associated stimulus. Infusions of the AMPA receptor antagonist CNQX into the ventral subiculum or basolateral area of the amygdala (0, 0.2, 2.0 nmol) were made immediately before the start of each session. Intra-basolateral area of the amygdala CNQX reduced responding upon the drug-associated stimulus bar, but at the same time increased responding upon the saline-associated stimulus bar. By contrast, intra-ventral subiculum CNQX reduced drug-associated stimulus responding selectively. Neither manipulation affected levels of activity within the operant chamber extraneous to the bar-pushing response. Hence, the basolateral area of the amygdala appeared to have determined the degree of discriminative control exerted over behaviour by the drug-associated stimulus, whilst the ventral subiculum is suggested to have determined the efficacy of the conditioned reward.

Cross-sensitisation with d-amphetamine following repeated intra-perifornical sulpiride infusions.

Infusions of the dopamine D2/D3 receptor antagonist sulpiride within the perifornical region of the lateral hypothalamus have been reported to increase locomotor activity. The current investigation examined the effect of repeated lateral hypothalamic sulpiride infusions. In experiment la, rats were placed repeatedly in an activity chamber either prior to, or following an infusion of 10 micrograms sulpiride or vehicle. Repeated infusions of sulpiride prior to, but not following exposure to the activity chamber increased locomotor activity during subsequent sessions. In experiment 1b, repeated pretreatment with intra-perifornical sulpiride prior to placement within the activity chamber was found to engender a significant increase in conditioned activity when placed subsequently within the same chamber drug-free. Alternatively, pretreatment with sulpiride in the home cage was found subsequently to engender a significant increase in locomotor activity during a test session with intra-perifornical sulpiride. In experiment 2, repeated pretreatment with intra-perifornical sulpiride significantly increased the locomotor response to a subsequent systemic challenge with d-amphetamine. Animals pretreated in the home cage exhibited a moderate increase in activity over vehicle controls, while animals repeatedly pretreated immediately prior to placement in the activity chamber exhibited the largest response subsequently to d-amphetamine of any group. Experiment 3 showed that repeated sulpiride infusions either 1 mm anterior or 1 mm posterior to the perifornical region were without effect upon locomotor activity. These data are suggested to reflect an indirect action of intra-perifornical sulpiride upon the mesoaccumbens dopamine projection, via the level of the ventral tegmental area. Precise neural mechanisms are under current investigation.

Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning. I. Pavlovian conditioning.

We have previously obtained evidence that the mesoamygdaloid dopamine projection modulates the acquisition of a conditioned response (CR) elicited by presentation of a conditioned stimulus (CS) predicting the availability of a natural (sucrose) reward. This property was found to be dependent upon D3, but not D1 or D2, dopamine receptor activation. The aim of the present study was to determine whether the mesoamygdaloid dopamine projection is similarly involved in the acquisition of a drug-associated CR. Thus, two groups of rats with guide cannulae aimed at the nucleus accumbens and amygdala were trained using a Pavlovian conditioning procedure in which an initially neutral CS was paired with a computer-controlled, bilateral intraaccumbens infusion of d-amphetamine (the unconditioned stimulus: US). Conditioning sessions were conducted in standard operant chambers, with each session consisting of a single CS-US trial. For one group of rats, CS presentation was positively correlated with the drug US (Paired group), while for the second group CS and US presentations were negatively correlated (Unpaired group). During training, locomotor activity was recorded and was utilised as the measure both of the unconditioned (UR) and conditioned response (CR). A within-subjects design was utilised to investigate the effect of post-session bilateral intraamygdala administration of R(+) 7-OH-DPAT on the development of the drug-associated CR. Hence, both Paired and Unpaired groups were exposed to two different CSs which were presented on alternate sessions. Post-session bilateral intra-amygdala administration of R(+) 7-OH-DPAT (10 nmol) followed sessions in which one CS was presented, while intra-amygdala vehicle followed sessions in which the alternate CS was presented. The development of a CR occurred only in the presence of a CS that had been positively correlated with presentation of the drug US. Post-session, intra-amygdala administration of R(+) 7-OH-DPAT enhanced the acquisition of this CR. However, R(+) 7-OH-DPAT was without effect upon the unconditioned response to intra-accumbens d-amphetamine. Our previous data indicate a comparable effect of R(+) 7-OH-DPAT on conditioning to a CS associated with a non-drug, natural reward. Therefore, taken together, these findings suggest that D3 dopamine receptors within the amygdala modulate specifically the acquisition of Pavlovian conditioned responses, regardless of whether drug or natural rewards constitute the US.

Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning. II. Instrumental conditioning.

Rats were trained to associate an initially neutral conditioned stimulus (CS) with a response-independent, intra-accumbens infusion of d-amphetamine (the unconditioned stimulus; US). Elsewhere, we have reported that as a result of this training, presentations of the CS alone elicited a conditioned response consisting of increased locomotor activity and that acquisition of this conditioned response was enhanced by post-session, intra-amygdala infusion of the dopamine D3 receptor preferring agonist, R(+) 7-OH-DPAT. Here, in this same group of animals, we have examined the conditioned rewarding properties of the drug-associated CS by determining its ability to support the acquisition of a novel instrumental response in the absence of drug reward. Thus, rats were presented with two novel levers. Presentation of the drug-associated CS was made contingent upon depression of one of the levers (CR lever), while responding upon the other lever (NCR lever) had no programmed consequences. Preferential responding upon the lever delivering the drug-associated CS was observed despite a 6-week interval between CS-US training and the conditioned reward test. Intra-accumbens administration of d-amphetamine (0-20 microg) increased the control over behaviour exerted by the CS, increasing CR. but not NCR lever responding. In contrast, rats that received three pairings of an intra-accumbens infusion of d-amphetamine in combination with intra-amygdala infusion of R(+) 7-OH-DPAT, 3 weeks prior to testing, displayed similar rates of response upon both levers and were insensitive to the potentiation of responding for conditioned reward following intra-accumbens d-amphetamine. However, intra-accumbens d-amphetamine stimulated locomotor activity in a similar, dose-related manner in both groups. In this way, rats that had received intra-accumbens infusion of d-amphetamine in combination with intra-amygdala infusion of R(+) 7-OH-DPAT appeared exactly like control group rats, for which the CS had been paired with intra-accumbens d-amphetamine on a negative basis only. A locomotor activity test indicated that one behavioural consequence of intra-amygdala administration of R(+) 7-OH-DPAT was the reduction of the unconditioned locomotor response resulting from intra-accumbens administration of d-amphetamine. Hence, the present data demonstrate that the conditioned rewarding properties of a drug-associated CS are specific to the CS-US association and are relatively insensitive to decay over time. However, the rewarding properties of a drug-associated CS were selectively abolished following activation of amygdala D3 receptors during presentation of the drug reward. Potential explanations for this effect are discussed, including the possibility that intra-amygdala R(+) 7-OH-DPAT reduced the incentive value of the US.

Post-session sulpiride infusions within the perifornical region of the lateral hypothalamus enhance consolidation of associative learning.

Whilst neurons within the lateral hypothalamus are well known to be responsive to the presentation of previously learned associative stimuli, the consolidation of a Pavlovian association is thought to depend in large part upon other brain regions, including the amygdala. The present study addressed this assumption directly, by examining the effect of post-session infusions of sulpiride within the lateral hypothalamus upon the acquisition of a conditioned approach response in an appetitive differential conditioning task. Subjects were exposed to an initially neutral stimulus (CS+), which immediately preceded the availability of a 10% sucrose reward (US). A second, control stimulus (CS ) was also presented. but never in close temporal proximity to the US. The number and duration of alcove approaches were recorded. Immediately following each training session, subjects were infused bilaterally with sulpiride (0, 0.5, 5 microg) in the vicinity of the perifornical region of the lateral hypothalamus. Sulpiride dose-dependently enhanced the rate of acquisition of a conditioned approach response to presentation of the CS+, but was without affect upon approach behaviour during CS(-) or US presentations. Thus, 0.5 microg sulpiride facilitated at an early stage (session 2 onwards) the number of alcove approaches to the CS+, while 5 microg sulpiride enhanced to a greater extent the duration of conditioned approach, particularly during later sessions. A subsequent locomotor test using 0.5 mg/kg d-amphetamine indicated that repeated infusions of the higher dose sulpiride (5 microg), but not the lower dose (0.5 microg), resulted in behavioural sensitisation to administration of the psychomotor stimulant. Acquisition of a novel conditioned instrumental response was not affected by previous exposure to sulpiride. These data suggest that dopamine-sensitive neurons within the lateral hypothalamus may play a significant role in the acquisition of appetitive Pavlovian associations.

Double dissociation of the behavioural effects of R(+) 7-OH-DPAT infusions in the central and basolateral amygdala nuclei upon Pavlovian and instrumental conditioned appetitive behaviours.

Dopaminergic cell bodies located within the ventral mesencephalon innervate the amygdaloid complex, a region critically involved in the attribution of affective significance to environmental stimuli. Recently, we have shown that post-session intra-amygdala administration of a D3 dopamine receptor agonist enhances selectively the acquisition of an appetitive conditioned response. In the present study, we have investigated the potential involvement of the central nucleus and the basolateral nuclei of the amygdala in mediating this effect. Thus, rats were trained to associate an arbitrary stimulus (CS+) with the availability of 10% sucrose reward. Post-session infusions of the D3 receptor-preferring agonist, R(+) 7-OH-DPAT, were made into either the central nucleus or basolateral nuclei. Acquisition of a conditioned approach response was enhanced by R(+) 7-OH-DPAT infusions within the central nucleus, but not within the basolateral nuclei. Drug infusions into either region failed to affect approach behaviour elicited by presentation of a control stimulus (CS-), explicitly unpaired with sucrose reward. The effects of pre-test infusions of R(+) 7-OH-DPAT on the instrumental properties of the stimuli were then determined. Rats were presented with two novel levers, depression of one lever resulted in presentation of the CS+, while presentation of the CS- was contingent upon depression of the other lever. Rates of response upon each lever as well as the ability of the conditioned stimuli subsequently to elicit conditioned approach behaviour were recorded. Data revealed a double dissociation of the effects of R(+) 7-OH-DPAT on the expression of the Pavlovian and instrumental properties of the reward-related stimulus. Thus, within the central nucleus R(+) 7-OH-DPAT dose-dependently attenuated expression of the conditioned approach response, but had no effect upon instrumental responding maintained by the conditioned reward. In contrast, within the basolateral nuclei, R(+) 7-OH-DPAT had no effect upon expression of conditioned approach behaviour, but abolished selectively the ability of the reward-associated stimulus to support the acquisition of a novel instrumental response. Hence, these data indicate that distinct regions of the amygdaloid complex process distinct aspects of conditioned appetitive behaviours.

Interactions between intra-perifornical region sulpiride and intra-ventral tegmental area AP5 on measures of locomotor activity and conditioned place preference.

Infusions of sulpiride, a dopamine D2/D3 receptor antagonist within the perifornical region of the lateral hypothalamus have been shown previously to exhibit a behavioural profile generally attributed specifically to activation of the mesoaccumbens dopamine projection. Experiment 1 confirmed previous work showing that repeated homecage pretreatment with sulpiride (5 microg) in the perifornical region of the lateral hypothalamus resulted subsequently in an enhanced locomotor response to a d-amphetamine challenge. Experiment 2 examined the possibility that the observed behavioural changes were due to stimulation of the mesoaccumbens dopamine projection via the ventral tegmental area. Thus, repeated intra-perifornical infusions with sulpiride were without effect initially, but resulted in a gradual increase in locomotor activity during subsequent sessions. Intra-ventral tegmental area infusions of the NMDA receptor antagonist AP5 (0.3, 1.0 nmol) were without intrinsic effect upon locomotor activity at any time. However, AP5 blocked the ability of repeated sulpiride infusions to increase locomotor activity, and the ability of intra-perifornical sulpiride to support the acquisition of a conditioned place preference. AP5-sulpiride co-infusions also increased locomotor activity in a non-incremental manner. These data suggest there to be a functionally significant projection from the perifornical region of the lateral hypothalamus to the ventral tegmental area in the control over locomotor activity and rewarded behaviour.

Enhanced conditioned inhibition following repeated pretreatment with d-amphetamine.

We have shown that prior repeated exposure to d-amphetamine facilitates appetitive Pavlovian conditioning. However, animals sensitised in this manner also show elevated levels of stimulated activity. To investigate whether enhanced conditioning was dependent upon increased activity, a conditioned inhibition task was employed in the present study. Rats received d-amphetamine (2 mg/kg, i.p.) or vehicle once per day for 7 days. After a 7-day drug-free period, an activity assay confirmed that repeated d-amphetamine treatment markedly elevated the locomotor response to a subsequent challenge with 0.5 mg/kg d-amphetamine. Conditioning began 6 days later. In phase 1, stimulus A+ (light or tone) immediately preceded sucrose availability (excitatory conditioning). In phase 2, sucrose again was presented after A+ alone, but not after presentation of a compound of A+ with a second stimulus (AB-). Sensitisation enhanced the acquisition of conditioned approach behaviour to the excitatory stimulus A+ in phase 1. Furthermore, acquisition of conditioned inhibition to the stimulus compound, AB-, was also facilitated. Thus, sensitised rats showed reduced levels of responding to the stimulus compound far sooner than controls. Finally, a retardation test was carried out in stage 3, in which the inhibitory stimulus B- was paired alone with sucrose reward. Sensitised rats initially showed retarded acquisition of excitatory conditioned responding relative to controls, suggesting that B possessed stronger inhibitory associations in these animals. However, sensitised animals again exhibited higher levels of responding in later sessions, consistent with the enhanced excitatory conditioning shown in phase 1. These findings suggest that prior repeated d-amphetamine enhanced the acquisition of inhibitory and excitatory Pavlovian associations; a propensity not readily attributable to stimulated locomotor hyperactivity.

Enhanced stimulus-reward learning by intra-amygdala administration of a D3 dopamine receptor agonist.

The amygdala is considered to be a critical neural substrate underlying the formation of stimulus-reward associations, and is known to receive substantial innervation from dopaminergic neurons located within the ventral mesencephalon. However, relatively little is known about the function of the mesoamygdaloid dopamine projection in stimulus-reward learning. Recently, we have found post-session intra-amygdala microinjections of d-amphetamine to enhance appetitive Pavlovian conditioning as assessed in a discriminative approach task. In the present study, we have examined the effects of dopamine receptor agonists possessing relative selectivity for the D1, D2 and D3 receptor subtypes in order to examine more fully the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Thus, subjects were trained to associated an initially neutral stimulus (CS+) with 10% sucrose reward (US). A second, control stimulus (CS-) was also presented but never paired with sucrose reward. In order to measure specifically the conditioned response to CS+/CS- presentation, responding during CS and US presentations was measured separately. Immediately following each training session, subjects received bilateral intra-amygdala infusion of 0.1, 1 or 10 nmol/side of SKF-38393, quinpirole or 7-OH-DPAT. Infusions of SKF-38393 or quinpirole were without effect on CS+ approach. However, post-session intra-amygdala infusions of 7-OH-DPAT enhanced selectively CS+ approach in a dose-dependent fashion. No dose of any drug affected CS- approach, US behaviours, or measures of extraneous behaviour. Subsequent acquisition of a novel conditioned instrumental response was also unaffected. Thus, the present data indicate a selective involvement of the D3 dopamine receptor subtype in the modulation of stimulus-reward learning by the mesoamygdaloid dopamine projection.

Enhanced acquisition of discriminative approach following intra-amygdala d-amphetamine.

This study examined the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Bilateral post-session intra-amygdala microinjections of d-amphetamine were carried out in rats during training in a discriminative approach task known to be sensitive to experimental manipulations of the amygdala. The experiment consisted of two phases: discriminative approach training, and a subsequent assessment of instrumental conditioned reward efficacy. During discriminative approach training, subjects were trained to associate a neutral stimulus with 10% w/v sucrose reward. Each trial consisted of a 1-s light stimulus followed by a 5-s presentation of the sucrose reward. Approach behaviour into the recess housing sucrose reward was measured during each trial. Inappropriate approach behaviour (approach outside of the trial periods) was punished by delaying the next trial. Intra-amygdala d-amphetamine (10 microg/side) enhanced the rate of acquisition of discriminative approach behaviour. This effect was most evident early during training (sessions 2-4) and by the tenth session both groups had reached similar asymptotic performance. Horizontal and vertical activity increased slightly across sessions, but there was no indication of a differential effect of d-amphetamine. Thus, intra-amygdala microinjections of d-amphetamine enhanced selectively the acquisition of the stimulus-reward association. During a subsequent test of instrumental conditioned reward, presentation of the conditioned light stimulus was made contingent upon performance of a novel lever-pressing response (probability 0.5). Responding on a second, control lever was without programmed consequences. Sucrose reward was not available at any point, and subjects were tested drug-free. In both groups the conditioned stimulus was found to possess significant conditioned rewarding efficacy. Extraneous behaviour was increased in the d-amphetamine group but the rewarding properties of the conditioned stimulus were unaltered. These findings demonstrate that the mesoamygdaloid dopamine projection modulates the acquisition of a stimulus-reward association, but is apparently without subsequent effect on the rewarding efficacy of a conditioned stimulus.