George Fouriezos

Strain and gender specific effects in the forced swim test: Effects of previous stress exposure

The chronic mild stress (CMS) procedure was developed in rodents to target anhedonia, the core symptom of depressive melancholia. Stress exposure has been shown to induce a variety of physiological, biochemical and behavioral alterations associated with depression, although its anhedonic consequences as indexed by either sucrose intake and preference or thresholds for brain stimulation reward are less reliably observed. In the present study, we assessed the effects of six weeks of CMS on the latter measure in two strains of male and female rats subsequently challenged with an acute psychophysical stressor, forced swimming; their behavior in the swimming cylinder was evaluated on two consecutive days. While brain stimulation reward thresholds and response rates were unchanged by CMS exposure, significant differences in forced swim behaviors were observed between male control and CMS groups. In particular, male Long Evans rats with a history of CMS showed the largest decrease in the duration of active behaviors on the second test day, a pattern less evident in the Sprague-Dawley strain of rats, or in any of the female groups. The results suggest that the effects of depressogenic manipulations are strain and gender dependent, with male Long Evans rats most susceptible, as demonstrated by the selective reduction of struggling behaviors. Inclusion of multiple measures, including the forced swim test, would provide a better understanding of the psychopathological profile engendered by chronic exposure to mild stressors and its genetic specificity.

Task difficulty increases thresholds of rewarding brain stimulation

The effect of increasing task difficulty on the threshold of rewarding, electrical brain stimulation was evaluated. Rats were trained to press a lever to obtain a brief burst of pulses to the lateral hypothalamus. The threshold was psychophysically scaled using a descending method of limits in which the pulse frequency was varied to yield a maximum to minimum range of self-stimulation rates. As expected, weighting the lever with 0, 15, 30, or 45 g produced progressive decreases in maximal rates, but it also caused a weight-related shift to the right of the rate-frequency ogives in each of the 7 rats. Although the degree of shift varied from rat to rat, it did not matter whether criterion performance was defined as half-maximum rate or as a constant rate of 5 responses. These results suggest that the effort required to make the operant response contributes to the position of rate-frequency curves and, further, that shifts in rate-frequency functions must be interpreted with caution when such shifts are obtained by CNS lesions or drug injections.

Apomorphine and electrical self-stimulation of rat brain

The participation of dopamine neurons in reward produced by electrical stimulation of the brain was examined by measuring self-stimulation thresholds after injections of apomorphine, a direct agonist of dopamine receptors. Rats were trained to press a lever to obtain 0.3-s trains of electrical stimulation applied to lateral hypothalamic electrodes in a paradigm where the pulse frequency was decreased every eight stimulations by approximately 20%. The pulse frequency interpolated at 50% of maximum rate was taken as threshold. In a completely within-subject design, five doses of apomorphine from 0.01 to 1.00 mg/kg and the ascorbic acid vehicle were injected in a random order and thresholds were tracked at intervals of 5 min for 2 h postinjection. Low doses from 0.01 to 0.10 mg/kg caused thresholds to increase while the two higher doses, 0.30 and 1.00 mg/kg, caused thresholds to drop; the switch in the direction of the behavioural effect is thought to parallel the shift in apomorphines action from presynaptic to predominantly postsynaptic activation of dopamine receptors as the concentration of apomorphine increases.

Refractoriness of neurons mediating intracranial self-stimulation in the anterior basal forebrain

The post-stimulation excitability of neurons mediating electrical self-stimulation of the anterior basal forebrain was evaluated psychophysically in the rat. Rats with electrodes in the nucleus accumbens, caudate nucleus, lateral preoptic area, diagonal band, or anterior medial forebrain bundle pressed a lever to earn 0.5-s trains of conditioning (C) and test (T) pulse pairs. The C-T interval was systematically varied and the effectiveness of the T-pulse was estimated by measuring the frequency of pulse pairs required to sustain criterion responding. All sites tested demonstrated similar recovery; T-pulse effectiveness, normalized against the effect of the C-pulse, was lowest at delays of 0.4-0.8 ms and it rose monotonically until 5 ms when it achieved an effectiveness plateau of one. Increasing the current of the T-pulse by 50 or 60% failed to hasten recovery, suggesting that the recovery profiles primarily reflect the activation of neurons very soon after emergence from absolute refractoriness. Compared to lateral hypothalamic and ventral tegmental self-stimulation, the neurons that support self-stimulation in the ventral basal forebrain recover more slowly; recovery here is only about half done by the time lateral hypothalamic placements demonstrate complete recovery.

Visual statistical decisions

To identify variables that underlie intuitive judgments about the sizes of groups of similar objects, we asked people to judge the relative heights of vertical bars briefly shown, two groups at a time, on a computer display. Randomly selected normal deviates determined individual bar height. Average differences in height and group sizes were also randomly varied. Twenty-eight participants judged 250 differences each, which were then submitted to multiple regression analysis and psychophysical inspection. The total number of bars sharpened discrimination, whereas variance dulled it. Critical ratio (CR), the forerunner to the modern t test, emerged as the most important predictor; little additional variance was explained by other factors. The difference in the number of bars was a reliable factor, favoring the greater number of bars. Confidence limits around thresholds, defined as CRs needed to say “possibly greater,” surrounded 1.65; as a z value, this corresponded to a one-tailed probability of .05. Judgments about noisy stimuli thus seem to be based on a statistical process and to employ a probability criterion similar to that used in the formal statistical evaluation of experimental findings—namely, p<.05.

Post-stimulation excitability of mediodorsal thalamic self-stimulation.

The post-stimulation excitability of the substrate for brain stimulation reward in the mediodorsal thalamus was assessed using equal- and unequal-pulse procedures. In 3 rats, refractory periods were found to begin no earlier than 1 ms and to end as late as 10 ms. Using test (T) pulses 1.5 times the amplitude of condition (C) pulses, the contribution of absolute and relative refractory periods was determined in one subject. No change in the slope of the recovery function was obtained in this condition, suggesting that several populations of neurons with different absolute refractory periods compose the behaviorally relevant substrate. A large supernormal contribution, evaluated by increasing the C amplitude to 1.5T, occurred between 3 and 10 ms with a peak at 7.5 ms. These results suggest that mediodorsal thalamic self-stimulation is mediated by a wide range of small, probably unmyelinated fibers.

The effect of pulse duration on refractory periods of neurons mediating brain-stimulation reward

The post-stimulation excitability characteristics of neurons mediating the rewarding effects of electrical stimulation of the medial forebrain bundle were behaviorally assessed at 5 different pulse durations. Recovery from refractoriness was inferred from the results of double-pulse tests in which the interval between conditioning (C) and test (T) pulses of equal amplitude was varied. Pulses of 0.1-0.5 ms had little effect on the time course of recovery which ranged from 0.4 to about 1.5 ms in each animal. In some subjects, however, complete recovery from refractoriness was significantly delayed with 1- and 2-ms pulses, with as much as a tripling in the C-T interval at which recovery approached asymptotic levels. This effect appeared to be placement-specific and was well correlated with the threshold charge, which ranged from 0.5 to 1.3 microC. The highest charge values were obtained in those subjects in which longer pulses displaced the refractory period profile towards a slower recovery; the electrode tips in this group were located in more anterior positions than those subjects in which pulse duration had no effect on recovery from refractoriness. It appears that at some placements slowly recovering elements can contribute to the circuitry underlying brain-stimulation reward when suitable stimulation parameters are employed.

Short- and long-term effects of interleukin-2 on weight, food intake, and hedonic mechanisms in the rat

In the present work, we investigated the short- and long-term effects of a single systemic injection of rat recombinant interleukin-2 on weight, food intake, and brain stimulation reward thresholds elicited from the ventral tegmental area. An inverted U-shaped dose-function was obtained with 0.5 microg producing the greatest increases in the threshold for rewarding brain stimulation which were sustained during the month long tests. No differences between groups in terms of maximum response rates, a measure of performance, were observed. Although all injected groups showed a minor decline in the rate of weight gain over time, percent efficiency of food utilization (percent weight gain/food intake) was the same across groups, suggesting that metabolic function was not affected by the cytokine. In animals with bilateral ventral tegmental area implants, there was no consistent correspondence between the threshold change obtained from ipsilateral stimulation and that associated with the contralateral site; side-to-side differences ranged from 0 to 100%, suggesting a specific interaction between cytokine activity and the locus of rewarding brain stimulation. These data suggest that peripheral IL-2 significantly modifies hedonic processes arising from medial forebrain bundle stimulation in a long-term manner. We further suggest that since this modulation appears to be notably site-specific, IL-2 receptors or its metabolites may not be evenly distributed within the medial forebrain bundle.

The cost of delaying rewarding brain stimulation

Six rats trained to press a lever to obtain rewarding electrical stimulation of the brain through chronically implanted, lateral hypothalamic electrodes were used to estimate the rate at which short delays between the response and the reward degraded the rewarding effect of the stimulation. Frequency thresholds rose steadily with delays through to 2.2 s at a rate of 10% per second.

Interhemispheric involvement of the anterior cortical nuclei of the amygdala in rewarding brain stimulation.

The amygdaloid complex is one of the structures thought to modulate brain stimulation reward (BSR) elicited from the median forebrain bundle (MFB). Previous metabolic and behavioral data from our laboratory point to the amygdaloid cortical nuclei as key to this process. In this study, thresholds for rewarding stimulation of the MFB were determined for 42 days, 21 days following an electrolytic lesion to amygdaloid nuclei ipsilateral to the stimulation electrode, and 21 days following one applied to the contralateral amygdala. A subset of animals showed post-lesion changes in MFB frequency thresholds that were maintained if not augmented after the second lesion. These ranged from 26% to 150% compared to baseline values, among the largest ever reported to our knowledge. Interestingly, damage to anterior sites within the cortical nuclei was the most effective in producing modifications to the rewarding value of the stimulation. Equally singular was the finding that contralateral lesions tended to alter thresholds more than ipsilateral ones, confirming our earlier finding of interhemispheric connectivity in amygdaloid modulation of MFB reward signals. This interpretation was substantiated by tracking long-term metabolic activity in the amygdala using cytochrome oxidase histochemistry. The density of reaction product at damaged amygdala sites was negatively correlated (r=-0.90) with the increases in thresholds obtained at contralateral MFB loci. Together with the fact that such large lesion effects are seldom obtained, our metabolic results point to the existence of a relationship between these nuclei and reward signals generated at the MFB. Moreover, our data suggest that this communication takes place interhemispherically.