Flexible versus Fixed Spatial Self-Ordered Response Sequencing: Effects of Inactivation and Neurochemical Modulation of Ventrolateral Prefrontal Cortex.

Previously, studies using human neuroimaging and excitotoxic lesions in non-human primate have demonstrated an important role of ventrolateral prefrontal cortex (vlPFC) in higher order cognitive functions such as cognitive flexibility and the planning of behavioral sequences. In the present experiments, we tested effects on performance of temporary inactivation (using GABA receptor agonists) and dopamine (DA) D2 and 5-HT2A-receptor (R) blockade of vlPFC via local intracerebral infusions in the marmoset. We trained common marmosets to perform spatial self-ordered sequencing tasks in which one cohort of animals performed two and three response sequences on a continuously varying spatial array of response options on a touch-sensitive screen. Inactivation of vlPFC produced a marked disruption of accuracy of sequencing which also exhibited significant error perseveration. There were somewhat contrasting effects of D2 and 5-HT2A-R blockade, with the former producing error perseveration on incorrect trials, though not significantly impairing accuracy overall, and the latter significantly impairing accuracy but not error perseveration. A second cohort of marmosets were directly compared on performance of fixed versus variable spatial arrays. Inactivation of vlPFC again impaired self-ordered sequencing, but only with varying, and not fixed spatial arrays, the latter leading to the consistent use of fewer, preferred sequences. These findings add to evidence that vlPFC is implicated in goal-directed behavior that requires higher-order response heuristics that can be applied flexibly over different (variable), as compared with fixed stimulus exemplars. They also show that dopaminergic and serotonergic chemomodulation has distinctive effects on such performance.SIGNIFICANCE STATEMENT This investigation employing local intracerebral infusions to inactivate the lateral prefrontal cortex (PFC) of the New World marmoset reveals the important role of this region in self-ordered response sequencing in variable but not fixed spatial arrays. These novel findings emphasize the higher order functions of this region, contributing to cognitive flexibility and planning of goal directed behavior. The investigation also reports for the first time somewhat contrasting neuromodulatory deficits produced by infusions of dopamine (DA) D2 and 5-HT2A receptor (R) antagonists into the same region, of possible significance for understanding cognitive deficits produced by anti-psychotic drugs.

Trait Anxiety Mediated by Amygdala Serotonin Transporter in the Common Marmoset.

High trait anxiety is associated with altered activity across emotion regulation circuitry and a higher risk of developing anxiety disorders and depression. This circuitry is extensively modulated by serotonin. Here, to understand why some people may be more vulnerable to developing affective disorders, we investigated whether serotonin-related gene expression across the brain's emotion regulation circuitry may underlie individual differences in trait anxiety using the common marmoset (Callithrix jacchus, mixed sexes) as a model. First, we assessed the association of region-specific expression of the serotonin transporter (SLC6A4) and serotonin receptor (HTR1A, HTR2A, HTR2C) genes with anxiety-like behavior; and second, we investigated their causal role in two key features of the high trait anxious phenotype: high responsivity to anxiety-provoking stimuli and an exaggerated conditioned threat response. While the expression of the serotonin receptors did not show a significant relationship with anxiety-like behavior in any of the targeted brain regions, serotonin transporter expression, specifically within the right ventrolateral prefrontal cortex (vlPFC) and most strongly in the right amygdala, was associated positively with anxiety-like behavior. The causal relationship between amygdala serotonin levels and an animal's sensitivity to threat was confirmed via direct amygdala infusions of a selective serotonin reuptake inhibitor (SSRI), citalopram. Both anxiety-like behaviors, and conditioned threat-induced responses were reduced by the blockade of serotonin reuptake in the amygdala. Together, these findings provide evidence that high amygdala serotonin transporter expression contributes to the high trait anxious phenotype and suggest that reduction of threat reactivity by SSRIs may be mediated by their actions in the amygdala.SIGNIFICANCE STATEMENT Findings here contribute to our understanding of how the serotonin system underlies an individual's expression of threat-elicited negative emotions such as anxiety and fear within nonhuman primates. Exploration of serotonergic gene expression across brain regions implicated in emotion regulation revealed that serotonin transporter gene expression in the ventrolateral prefrontal cortex (vlPFC) and most strongly in the amygdala, but none of the serotonin receptor genes, were predictive of interindividual differences in anxiety-like behavior. Targeting of amygdala serotonin reuptake with selective serotonin reuptake inhibitors (SSRIs) confirmed the causal relationship between amygdala serotonin transporter and an animal's sensitivity to threat by reversing expression of two key features of the high trait-like anxiety phenotype: high responsivity to anxiety-provoking uncertain threat and responsivity to certain conditioned threat.

Trajectories and Milestones of Cortical and Subcortical Development of the Marmoset Brain From Infancy to Adulthood.

With increasing attention on the developmental causes of neuropsychiatric disorders, appropriate animal models are crucial to identifying causes and assessing potential interventions. The common marmoset is an ideal model as it has sophisticated social/emotional behavior, reaching adulthood within 2 years of birth. Magnetic resonance imaging was used in an accelerated longitudinal cohort (n = 41; aged 3-27 months; scanned 2-7 times over 2 years). Splines were used to model nonlinear trajectories of grey matter volume development in 53 cortical areas and 16 subcortical nuclei. Generally, volumes increased before puberty, peaked, and declined into adulthood. We identified 3 milestones of grey matter development: I) age at peak volume; II) age at onset of volume decline; and III) age at maximum rate of volume decline. These milestones differentiated growth trajectories of primary sensory/motor cortical areas from those of association cortex but also revealed distinct trajectories between association cortices. Cluster analysis of trajectories showed that prefrontal cortex was the most heterogenous of association regions, comprising areas with distinct milestones and developmental trajectories. These results highlight the potential of high-field structural MRI to define the dynamics of primate brain development and importantly to identify when specific prefrontal circuits may be most vulnerable to environmental impact.

Orbitofrontal dopamine depletion upregulates caudate dopamine and alters behavior via changes in reinforcement sensitivity.

Schizophrenia is associated with upregulation of dopamine (DA) release in the caudate nucleus. The caudate has dense connections with the orbitofrontal cortex (OFC) via the frontostriatal loops, and both areas exhibit pathophysiological change in schizophrenia. Despite evidence that abnormalities in dopaminergic neurotransmission and prefrontal cortex function co-occur in schizophrenia, the influence of OFC DA on caudate DA and reinforcement processing is poorly understood. To test the hypothesis that OFC dopaminergic dysfunction disrupts caudate dopamine function, we selectively depleted dopamine from the OFC of marmoset monkeys and measured striatal extracellular dopamine levels (using microdialysis) and dopamine D2/D3 receptor binding (using positron emission tomography), while modeling reinforcement-related behavior in a discrimination learning paradigm. OFC dopamine depletion caused an increase in tonic dopamine levels in the caudate nucleus and a corresponding reduction in D2/D3 receptor binding. Computational modeling of behavior showed that the lesion increased response exploration, reducing the tendency to persist with a recently chosen response side. This effect is akin to increased response switching previously seen in schizophrenia and was correlated with striatal but not OFC D2/D3 receptor binding. These results demonstrate that OFC dopamine depletion is sufficient to induce striatal hyperdopaminergia and changes in reinforcement learning relevant to schizophrenia.

Opposing effects of 5,7-DHT infusions into the orbitofrontal cortex and amygdala on flexible responding.

Central serotonin is implicated in a variety of emotional and behavioral control processes. Serotonin depletion can lead to exaggerated aversive processing and deficient response inhibition, effects that have been linked to serotonin's actions in the amygdala and orbitofrontal cortex (OFC), respectively. However, a direct comparison of serotonin manipulations within the OFC and amygdala in the same experimental context has not been undertaken. This study compared the effects of infusing the serotonin neurotoxin, 5,7-dihydroxytryptamine into the OFC and amygdala of marmosets performing an appetitive test of response inhibition. Marmosets had to learn to inhibit a prepotent response tendency to choose a box containing high-incentive food and instead choose a box containing low-incentive food, to obtain reward. OFC infusions caused long-lasting reductions in serotonin tissue levels, as revealed at postmortem, and exaggerated prepotent responses. In contrast, the significantly reduced prepotent responses following amygdala infusions occurred at a time when serotonin tissue levels had undergone considerable recovery, but there remained residual reductions in extracellular serotonin, in vivo. These opposing behavioral effects of serotonin manipulations in the same experimental context may be understood in terms of the top-down regulatory control of the amygdala by the OFC.

Uncoupling of behavioral and autonomic responses after lesions of the primate orbitofrontal cortex.

Successful adaptation to changes in an animal's emotional and motivational environment depends on behavioral flexibility accompanied by changes in bodily responses, e.g., autonomic and endocrine, which support the change in behavior. Here, we identify the orbitofrontal cortex (OFC) as pivotal in the flexible regulation and coordination of behavioral and autonomic responses during adaptation. Using an appetitive Pavlovian task, we demonstrate that OFC lesions in the marmoset (i) impair an animal's ability to rapidly suppress its appetitive cardiovascular arousal upon termination of a conditioned stimulus and (ii) cause an uncoupling of the behavioral and autonomic components of the adaptive response after reversal of the reward contingencies. These findings highlight the role of the OFC in emotional regulation and are highly relevant to our understanding of disorders such as schizophrenia and autism in which uncoupling of emotional responses may contribute to the experiential distress and disadvantageous behavior associated with these disorders.

The role of the orbitofrontal cortex and medial striatum in the regulation of prepotent responses to food rewards.

An impairment in learning to inhibit prepotent responses to positive stimuli is associated with damage to the orbitofrontal cortex (OFC) in rats, monkeys, and humans performing discrimination reversal, extinction, and detour reaching tasks. In contrast, a recent study showed that OFC-lesioned rhesus monkeys could learn to select the smaller of 2 quantities of food reward in order to receive the larger reward, at an equivalent rate to controls, despite the requirement to inhibit a prepotent response. Given this result, the aim of the present study was to further specify the contexts under which the OFC regulates responding and to identify additional components of limbic circuitry that contribute to such regulation. Marmosets with lesions of the OFC and medial striatum (MS), but not the amygdala, made more prepotent responses to a clear Perspex box containing high incentive food before learning to choose the box containing low incentive food, to obtain reward. However, having learned the incongruent incentive discrimination OFC- and MS-lesioned monkeys were impaired upon reversal of the reward contingencies, repeatedly selecting the previously rewarded low incentive object. These findings identify the critical contribution of the OFC and MS in the regulation of responding by affective cues.

Differential contributions of dopamine and serotonin to orbitofrontal cortex function in the marmoset.

We have shown previously that the inhibitory control functions of the orbitofrontal cortex (OFC) are disrupted by serotonin, but not dopamine depletions. However, both dopamine and serotonin terminals and receptors are present within the OFC and thus the aim of the present study was to determine the differential contributions of these neurotransmitters to orbitofrontal function. OFC and dopamine are involved in the process by which neutral stimuli take on reinforcing properties, by virtue of their prior association with reward, and guide behavior. Thus, we compared the performance of marmosets with dopaminergic or serotoninergic OFC depletions on a test of conditioned reinforcement. To further our understanding of serotonin in behavioral flexibility, the effect of these depletions was also compared on the extinction of a visual discrimination. Monkeys with serotonin depletions of the OFC displayed stimulus-bound responding on both tests of conditioned reinforcement and discrimination extinction suggesting that orbitofrontal serotonin plays a specific role in preventing competing, task irrelevant, salient stimuli from biasing responding. In contrast, monkeys with dopamine depletion were insensitive to conditioned reinforcers and displayed persistent responding in the absence of reward in extinction, a pattern of deficits that may reflect basic deficits in the associative processing of reward.

Primate orbitofrontal cortex and adaptive behaviour.

Orbitofrontal cortex contributes to behavioural adaptation in response to changes in the contingent relationship and incentive value of positive affective stimuli in the environment. This article integrates early descriptions of the effects of orbitofrontal ablation in monkeys, on object discrimination reversal and extinction, with contemporary theories of animal learning. Studies of incentive devaluation, conditioned reinforcement and changes in reward contingency are reviewed, highlighting the role of the orbitofrontal cortex in processing the affective and non-affective properties of rewarding stimuli, in reward expectation, and in goal selection. It is argued that future studies should focus on the interaction of the orbitofrontal cortex with peripheral arousal systems and the ascending monoamine systems in order to understand fully the role of the orbitofrontal cortex in behavioural adaptation.

Synergistic and regulatory effects of orbitofrontal cortex on amygdala-dependent appetitive behavior.

This paper will review two avenues of our research in marmosets that have focused on the role of the orbitofrontal cortex (OFC) in amygdala-dependent appetitive behavior. The first demonstrates the important contribution of both the OFC and the amygdala to conditioned reinforcement (CRF). The second reveals the regulatory effects of the OFC on amygdala-dependent autonomic and behavioral arousal in appetitive conditioning. The process of CRF is one way in which an environmental cue can guide emotional behavior. As a consequence of its previous relationship with reward, a cue can take on affective value and reinforce behavior. Lesion studies in marmosets are described that show that CRF is dependent upon both the amygdala and OFC. The synergistic interactions between these structures that have been shown to underlie other aspects of reward processing are then considered with respect to CRF. The results are contrasted with those that show the importance of the OFC in suppressing positive affective responses elicited by the amygdala in response to a conditioned stimulus (CS). Specifically, it will be shown that the OFC is involved in the rapid suppression of conditioned autonomic arousal upon CS withdrawal and in the co-ordination of conditioned autonomic and behavioral responses when adapting to changing reward contingencies. It will be argued that, overall, the OFC plays a critical role in the context-dependent regulation of positive affective responding governed by external cues, in keeping with a role in executive control.

Prefrontal serotonin depletion affects reversal learning but not attentional set shifting.

Recently, we have shown that serotonin (5-HT) depletion from the prefrontal cortex (PFC) of the marmoset monkey impairs performance on a serial discrimination reversal (SDR) task, resulting in perseverative responding to the previously correct stimulus (Clarke et al., 2004). This pattern of impairment is just one example of inflexible responding seen after damage to the PFC, with performance on the SDR task being dependent on the integrity of the orbitofrontal cortex. However, the contribution of 5-HT to other forms of flexible responding, such as attentional set shifting, an ability dependent on lateral PFC (Dias et al., 1996a), is unknown. The present study addresses this issue by examining the effects of 5,7-dihydroxytryptamine-induced PFC 5-HT depletions on the ability to shift attention between two perceptual dimensions of a compound visual stimulus (extradimensional shift). Monkeys with selective PFC 5-HT lesions, despite being impaired in their ability to reverse a stimulus-reward association, were unimpaired in their ability to make an extradimensional shift when compared with sham-operated controls. These findings suggest that 5-HT is critical for flexible responding at the level of changing stimulus-reward contingencies but is not essential for the higher-order shifting of attentional set. Thus, psychological functions dependent on different loci within the PFC are differentially sensitive to serotonergic modulation, a finding of relevance to our understanding of cognitive inflexibility apparent in disorders such as obsessive-compulsive disorder and schizophrenia.

The role of the primate amygdala in conditioned reinforcement.

Conditioned reinforcement refers to the capacity of a conditioned stimulus to support instrumental behavior by acquiring affective properties of the primary reinforcer with which it is associated. Conditioned reinforcers maintain behavior over protracted periods of time in the absence of, and potentially in conflict with, primary reinforcers and as such may play a fundamental role in complex social behavior. A relatively large body of evidence supports the view that the amygdala (and in particular the basolateral area) contributes to conditioned reinforcement by maintaining a representation of the affective value of conditioned stimuli. However, a recent study in primates (Malkova et al., 1997), using a second-order visual discrimination task, suggests that the amygdala is not critical for the conditioned reinforcement process. In the present study, excitotoxic lesions of the amygdala in a new world primate, the common marmoset, resulted in a progressive impairment in responding under a second-order schedule of food reinforcement. In addition, the responding of amygdala-lesioned animals was insensitive to the omission of the conditioned reinforcer, unlike that of control animals, for which responding was markedly reduced. In contrast, lesioned animals were unimpaired when responding on a progression of fixed-ratio schedules of primary reinforcement. These data confirm that the amygdala is critical for the conditioned reinforcement process in primates, and taken together with other recent work in monkeys, these results suggest that the contribution of the amygdala is to provide the affective value of specific reinforcers as accessed by associated conditioned stimuli.

Changes in photoperiod alter the daily rhythms of pineal melatonin content and hypothalamic beta-endorphin content and the luteinizing hormone response to naloxone in the male Syrian hamster.

This study examines the possible involvement of beta-endorphin in the photoperiodic control of reproduction in the Syrian hamster. beta-Endorphin and LHRH concentrations in the medial basal hypothalamus (MBH), anterior hypothalamus (AHA), and the preoptic area (POA) as well as pineal melatonin content were determined by RIA in male Syrian hamsters exposed to either a long day [(LD) 16-h light; 8-h dark; lights on 0700-2300] or short day [(SD) 8-h light, 16-h dark; lights on 0700-1500] for 8 weeks. Groups of eight animals from each photoperiod were killed by decapitation at 4-h intervals over 24 h. Twenty minutes before death half the animals from each photoperiod were given naloxone (5 mg/kg, sc), the other half saline. Exposure to a long photoperiod maintained testicular activity while a short photoperiod induced testicular regression. Pineal melatonin content in both photoperiods was maximal at 0500 h, i.e. 2 h before the onset of light (SD, 435.58 +/- 82.7 pg/pineal; LD, 276.78 +/- 56.8 pg/pineal). However, the duration of the nighttime rise in pineal melatonin content was increased in SD animals with elevated melatonin levels at 2100 h (157.10 +/- 41.8 pg/pineal) and 0100 h (199.11 +/- 58.9 pg/pineal). In contrast pineal melatonin content in LD animals was only higher than daytime values at 0500 h. A daily rhythm of beta-endorphin content within both the AHA and MBH of animals exposed to a short photoperiod coincided with this prolonged nighttime rise in pineal melatonin content, although a causal relationship between the two was not established. Peak levels of beta-endorphin occurred at 2100 h (AHA, 6.569 +/- 1.2 pmol/mg protein; MBH, 4.877 +/- 0.45 pmol/mg protein) and at 0100 h (AHA, 6.107 +/- 0.66 pmol/mg protein; MBH, 4.49 +/- 00.79 pmol/mg protein) which was 6 h and 10 h into the dark phase, respectively, with lowest levels in the middle of the light phase (AHA, 3.561 +/- 0.56 pmol/mg protein; MBH, 2.688 +/- 0.3 pmol/mg protein). This rhythm was absent in animals exposed to a long photoperiod. There was no effect of photoperiod or time of day on the content of beta-endorphin in the POA. LHRH levels were not altered by changes in photoperiod in all three brain regions studied. In the AHA and MBH, concentrations of LHRH were similar at all times of day whereas, in the POA, LHRH levels varied with time in both photoperiods. Peak levels occurred in the middle of the dark phase at 0100 h (LD, 2.774 +/- 0.24 pmol LHRH/mg protein; SD, 3.206 +/- 0.48 pmol LHRH/mg protein) with lowest levels during the light phase (LD, 1.664 pmol LHRH/mg protein; SD, 1.775 pmol LHRH/mg protein).(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution and some projections of cholinergic neurons in the brain of the common marmoset, Callithrix jacchus.

The distribution of choline acetyltransferase-immunoreactive (ChAT-IR) neurons was studied in the brain of the common marmoset by using immunohistochemistry. ChAT-IR neurons were found in the medial septal nucleus, vertical and horizontal limb nuclei of the diagonal band, the nucleus basalis of Meynert, pedunculopontine nucleus and laterodorsal tegmental nucleus, and also in the striatum, habenula, and brainstem cranial nerve motor nuclei. The organization of ChAT-IR neurons in the basal forebrain, midbrain, and pons is consistent with the Ch1-Ch6 nomenclature introduced by Mesulam et al. ('83). The combination of the retrograde transport of HRP-WGA with ChAT immunohistochemistry revealed the distribution of neurons in the Ch4 cell group projecting to the dorsolateral prefrontal cortex. The activity of ChAT was highest in limbic cortical structures, such as the hippocampus, and lowest in association areas of the neocortex. Lesions at various loci in the basal forebrain resulted in differential patterns of ChAT loss in the cortex, which suggests some degree of topographical organization of Ch4 projections to the cortical mantle.

Extra-dimensional versus intra-dimensional set shifting performance following frontal lobe excisions, temporal lobe excisions or amygdalo-hippocampectomy in man.

Attentional "set" shifting was assessed in a group of 20 neurosurgical patients with localized excisions of the frontal lobes, a group of 20 patients with unilateral temporal lobe lesions and a group of 11 patients who had undergone amygdalo-hippocampus removal. These three patient groups were compared with groups of both young (age-matched) and elderly normal control volunteers on a computerized test of visual discrimination learning involving both an intra- and an extra-dimensional shift. The frontal lobe group were selectively impaired in their ability to shift response set to a previously irrelevant dimension but not to shift attention to new exemplars of a previously relevant dimension. A similar pattern was observed in the elderly group of normal control volunteers. By comparison, both the temporal lobe patients and the amygdalo-hippocampectomy patients were unimpaired in their ability to perform either shift, although both groups had significantly prolonged selection latencies at the extra-dimensional shift stage of the task. These data are compared to previous findings from patients with idiopathic Parkinson's disease and are discussed in terms of a specific attentional set shifting deficit following frontal lobe damage.

Impaired extra-dimensional shift performance in medicated and unmedicated Parkinson's disease: evidence for a specific attentional dysfunction.

Groups of patients with Parkinson's disease, either medicated, or unmedicated and early in the course, together with age- and IQ-matched control subjects were tested in two paradigms measuring different aspects of selective attention. The first set of tests compared visual discrimination learning following intra- and extra-dimensional shifts, using a "total change" design in which each shift was made in the presence of novel exemplars of the compound stimuli used as discriminanda. The second test consisted of a visual search task in which the number of alternatives was varied. The results of the first experiment showed a selective deficit in both groups of Parkinsonian subjects in their ability to perform an extra-dimensional shift. In the visual search task, the patients were less accurate, but responded with equivalent choice reaction times to those of controls. The results are discussed in terms of the nature of the attentional dysfunction that occurs in Parkinson's disease.

Sparing of attentional relative to mnemonic function in a subgroup of patients with dementia of the Alzheimer type.

Patients with dementia of the Alzheimer type (DAT) received two tests of visual selective attention, together with tests of spatial and visual recognition memory and visuospatial conditional learning previously used to show deficits early in the course of DAT. One set of attentional tests compared visual discrimination learning along intra- and extra-dimensional shifts, using a "total change" design. In the 12 DAT patients capable of attempting the extra-dimensional shift (subgroup 1), performance was equivalent to that of controls. This subgroup was also unimpaired at simple and compound discrimination learning and reversal and an intra-dimensional shift. They were as accurate as controls on a visual search task requiring matching of stimuli on two dimensions with variable numbers of alternatives, but were significantly impaired in the tests of recognition memory and learning. By contrast, the other 13 patients showed marked impairments in the attentional tasks. This subgroup was also significantly worse than subgroup 1 in performance on the visual recognition and conditional learning tasks, and showed greater severity on most of the clinical ratings of dementia. The sparing of attentional shifting in patients early in the course of DAT is contrasted with the impairments previously described in patients with Parkinson's disease with only mild or absent memory loss. The implications of this double dissociation of deficits for understanding the neural bases of the cognitive deficits in these two neurodegenerative diseases are discussed and their significance for the staging of DAT is considered.

The effects of castration, testosterone replacement and photoperiod upon hypothalamic beta-endorphin levels in the male Syrian hamster.

Syrian hamsters kept in long day-lengths have active gonads and high circulating levels of gonadal steroids. Under the influence of the pineal gland, animals exposed to short photoperiods undergo testicular regression, have low circulating levels of testosterone and gonadotrophins and elevated levels of beta-endorphin within the hypothalamus. This paper describes the interaction between testosterone and photoperiod in the regulation of beta-endorphin levels in three regions of the hypothalamus. Hypothalamic beta-endorphin levels were measured by a combination of high-performance liquid chromatography and radioimmunoassay techniques that allows separation of the beta-endorphin (1-31) peptide from its metabolites and precursors. All of the beta-endorphin-like immunoreactivity in the hypothalamus of the male hamster, in both photoinhibited and photostimulated conditions, was found to represent the 31-amino-acid peptide. In photostimulated hamsters, chronic castration was associated with a significant increase of beta-endorphin levels in the anterior hypothalamus and mediobasal hypothalamus, which was reversed by treatment with exogenous testosterone. Castration prevented the ability of naloxone, an opiate receptor antagonist, to release luteinizing hormone, and this effect was also reversed by exogenous steroid. In photoinhibited hamsters, however, castration had no effect upon beta-endorphin levels in the preoptic area or mediobasal hypothalamus, and there was only a small increment in the anterior hypothalamus. Significantly, beta-endorphin levels in all areas of the hypothalamus of photoinhibited castrates were not decreased by testosterone treatment. In addition, administration of exogenous testosterone did not restore sensitivity to naloxone in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Intra-hypothalamic melatonin blocks photoperiodic responsiveness in the male Syrian hamster.

Exposure of male Syrian hamsters to a short daylength of 8L:16D leads to gonadal regression. This effect of photoperiod was prevented by pinealectomy or chronic exposure of the brain to exogenous melatonin delivered from in-dwelling cannulae. However, the effect of melatonin was dependent on the neural site of application. Melatonin delivered into the mid-brain, lateral hypothalamus or amygdala was ineffective. In contrast, bilateral administration of melatonin to the medial or amygdala was ineffective. In contrast, bilateral administration of melatonin to the medial hypothalamus prevented testicular regression and maintained high circulating levels of luteinizing hormone and prolactin. These findings suggest that the medial hypothalamus contains target sites for melatonin involved in pineal-mediated photoperiodic responses.

A specific form of cognitive rigidity following excitotoxic lesions of the basal forebrain in marmosets.

The effects of N-methyl-D-aspartate-induced lesions of the basal forebrain were studied on performance of a series of visual discrimination tests that examined a range of cognitive functions in the marmoset. These included the ability to attend to the various dimensional properties of stimuli and to use just one of these properties in order to solve a discrimination (intra-dimensional shift); to switch attention from one dimension to another (extra-dimensional shift); to learn the reinforcement value of specific exemplars within a dimension (new learning); and to relearn their reinforcement value following reversal of the reward contingencies (serial reversals). Lesions of the basal forebrain did not impair the ability either to attend selectively to the dimensional properties of the stimuli or to switch attention from one dimension to the other. However, the lesion did affect various aspects of associative learning including a transient impairment of new learning and a marked disruption of serial reversal learning. The reversal deficit could be characterised as a tendency to perseverate on the previously correct stimulus and as a failure to to show the formation of a reversal learning set. In addition, the lesion prevented disruption of performance of a well-learned discrimination when novel exemplars from the irrelevant dimension were introduced (probe test). It is suggested that the functional effects of the basal forebrain lesion reflect impaired learning of stimulus-reward associations and behavioural rigidity. The finding, however, that there was no effect of the lesion on attentional set-shifting suggests that any loss of inhibitory control was specific to the level of stimulus-response or stimulus-reward associations, inhibitory control at the level of attentional selection remaining intact. The similarity of the effects of damage to the basal forebrain to those seen following damage to the orbitofrontal cortex and the amygdala are discussed in the context of the close anatomical and functional relationships that exist among these three structures.