Chronic diazepam administration and appetitive discrimination learning: acquisition versus steady-state performance in pigeons

Tolerance to allopregnanolone with focus on the GABA-A receptor

Many studies have suggested a relationship between stress, sex steroids, and negative mental and mood changes in humans. The progesterone metabolite allopregnanolone is a potent endogenous ligand of the γ-amino butyric acid -A (GABA-A) receptor, and the most discussed neuroactive steroid. Variations in the levels of neuroactive steroids that influence the activity of the GABA-A receptor cause a vulnerability to mental and emotional pathology. There are physiological conditions in which allopregnanolone production increases acutely (e.g. stress) or chronically (e.g. menstrual cycle, pregnancy), thus exposing the GABA-A receptor to high and continuous allopregnanolone concentrations. In such conditions, tolerance to allopregnanolone may develop. We have shown that both acute and chronic tolerances can develop to the effects of allopregnanolone. Following the development of acute allopregnanolone tolerance, there is a decrease in the abundance of the GABA-A receptor α4 subunit and the expression of the α4 subunit mRNA in the ventral-posteriomedial nucleus of the thalamus. Little is known about the mechanism behind allopregnanolone tolerance and its effects on assembly of the GABA-A receptor composition. The exact mechanism of the allopregnanolone tolerance phenomena remains unclear. The purpose of this review is to summarize certain aspects of current knowledge concerning allopregnanolone tolerance and changes in the GABA-A receptors.

Tolerance development to Morris water maze test impairments induced by acute allopregnanolone

The progesterone metabolite allopregnanolone, like benzodiazepines, reduces learning and impairs memory in rats. Both substances act as GABA agonists at the GABA-A receptor and impair the performance in the Morris water maze test. Women are during the menstrual cycle, pregnancy, and during hormone replacement therapy exposed to allopregnanolone or allopregnanolone-like substances for extended periods. Long-term benzodiazepine treatment can cause tolerance against benzodiazepine-induced learning impairments. In this study we evaluated whether a corresponding allopregnanolone tolerance develops in rats. Adult male Wistar rats were pretreated for 3 days with i.v. allopregnanolone injections (2 mg/kg) one or two times a day, or for 7 days with allopregnanolone injections 20 mg/kg intraperitoneally, twice a day. Thereafter the rats were tested in the Morris water maze for 5 days and compared with relevant controls. Rats pretreated with allopregnanolone twice a day had decreased escape latency, path length and thigmotaxis compared with the acute allopregnanolone group that was pretreated with vehicle. Pretreatment for 7 days resulted in learning of the platform position. However, the memory of the platform position was in these tolerant rats not as strong as in controls only given vehicle. Allopregnanolone treatment was therefore seen to induce a partial tolerance against acute allopregnanolone effects in the Morris water maze.

Sensitivity to the effects of sedative-hypnotics on motor performance: influence of task difficulty and chronic phenobarbital administration

The present investigation examined sensitivity to the effects of various sedative-hypnotics on motor performance in rats treated chronically with phenobarbital. Eight rats were trained to walk on a rotorod treadmill at low (8 r.p.m.) and high (24 r.p.m.) rotational speeds. Prior to the chronic regimen, phenobarbital, pentobarbital, amobarbital, diazepam and clonazepam produced dose-dependent impairments in motor performance at both speeds. During chronic treatment with phenobarbital (100 mg/kg/day), tolerance was conferred to the effects of all the drugs examined, as evidenced by rightward shifts in their dose-effect curves. For all drugs, the magnitude of this tolerance was generally consistent across the two speeds. Following a 6-week washout period, during which no drugs were administered, dose-effect curves for each drug shifted back toward their original (i.e. pre-chronic) positions. Under all conditions, the doses required for each drug to impair motor performance at the low speed were higher than those required to impair motor performance at the high speed. These data suggest that sensitivity to the motor-impairing effects of sedative-hypnotics is influenced by the difficulty of the behavioral task, but that task difficulty does not modulate the maximal extent to which tolerance and cross-tolerance are expressed.

Tolerance develops to the spatial learning deficit produced by diazepam in rats

The present experiment sought to determine in rats if 1) tolerance develops to the amnesic effect of diazepam after chronic treatment, 2) the sedative and amnesic effects of diazepam can be dissociated via differential rates of tolerance development, and 3) withdrawal from long-term diazepam treatment affects mnemonic processes. Rats were given diazepam (3 mg/kg) acutely or chronically for 5, 15, or 30 d prior to behavioral testing. Sedation was assessed as exploratory activity in an open field and amnesia was assessed as spatial learning in the Morris water maze. Tolerance to the sedative effect of diazepam was exhibited after 5 d pretreatment whereas tolerance to the amnesic effect of diazepam was exhibited only after 30 d pretreatment. Withdrawal from diazepam produced a transitory and mild disruption of spatial learning. The data demonstrate 1) tolerance can develop to the amnesic effect of diazepam with extended treatment, 2) the sedative and amnesic effects of diazepam are largely independent, and 3) withdrawal from chronic diazepam treatment can retard optimal learning.

Diazepam-induced impairment of a go-no go successive discrimination

Diazepam (2.0 and 4.0 mg/kg, but not 1.0 mg/kg) administered in eight acquisition sessions significantly impaired the light-cued successive discrimination of male Sprague-Dawley rats. In two postdrug (vehicle) sessions, groups previously treated with the drug demonstrated good recovery in discrimination. An analysis of response components indicated that the impairment was due to the failure of drugged subjects to inhibit or withhold responses during the no go periods of the task. These findings are consistent with a "disinhibitory hypothesis" of drug impairment. The similarity of the present findings to those previously reported with chlordiazepoxide suggests that such effects are a generalized characteristic of the benzodiazepine class of drugs.

Multiple behavioral effects of diazepam in rhesus monkeys

The acute effects of diazepam (Valium) were assessed using a battery of complex food-reinforced operant tasks that included responding in delayed matching to sample (DMTS, n = 5), conditioned position response (CPR, n = 7) progressive ratio (PR, n = 8), temporal response differentiation (TRD, n = 4), and incremental repeated acquisition (IRA, n = 9) tests. Diazepam (0.25-4.0 mg/kg IV) produced significant dose-dependent decreases in the number of reinforcers obtained in the TRD and IRA tasks only. TRD accuracy was significantly decreased at doses of 0.25, 1.0, 2.0, and 4.0 mg/kg when compared to vehicle injections. Significant decreases in IRA accuracy generally did not occur at doses below 1.0 mg/kg. DMTS accuracy was decreased at 0.5 mg/kg for some time delays but showed no clear dose-delay interaction. Performance in the CPR and PR tests showed no significant effects of diazepam exposure over the dose range tested. These results indicate that diazepam selectively disrupts performance of operant tasks in monkeys designed to model human correlates of time perception, learning ability and visual attention/short-term memory while not affecting tasks designed to model motivation and position/color discrimination.

Repeated diazepam administration: effects on the acquisition and performance of response chains in humans

The effects of repeated diazepam administration (80 mg) were assessed across a 12-hr time course with humans responding under a two-component multiple schedule of repeated acquisition and performance of response sequences. Subjects resided in an inpatient clinical research ward for the duration of the study. In each component of the multiple schedule, subjects completed sequences of 10 responses in a predetermined order using three keys of a numeric keypad. In the acquisition component, a new response sequence was to be acquired each session. In the performance component, the response sequence always remained the same. After stable responding was obtained and the effects of the placebo assessed, diazepam was administered for 3 consecutive days. The effects of repeated diazepam administration on overall percentage of errors across the two components of the multiple schedule were selective. In the acquisition component, the first dose of diazepam increased percentage errors with the magnitude of effects decreasing across the second and third days of diazepam administration. In the performance component, the percentage of errors was either minimally affected across all 3 days of diazepam administration or substantively increased on Day 1 with subsequent diazepam administrations having minimal effects. Effects on response rate were not selective. Diazepam decreased rates of responding in both schedule components, with the magnitude of effects decreasing across successive administrations. These results replicate previous findings in humans and nonhumans on the selective effects of diazepam on acquisition versus performance baselines. Also, the results suggest that the selective effects do not result from differences in reinforcement rate. Finally, the present results demonstrate that the selective recovery from repeated drug administration previously demonstrated in nonhumans using a repeated acquisition arrangement has generality to human behavior.

Effects of benzodiazepines on acquisition and performance: a critical assessment

Evidence related to the effects of benzodiazepines on learning and memory is reviewed in the contexts of human verbal learning studies and animal studies using both aversive and non-aversive paradigms. While the impairment of acquisition by benzodiazepines appears to be a robust phenomenon generalizing across species and experimental conditions, the impairment in the performance of an already-learned task by such drugs appears to be more restrictive and highly dependent upon experimental contingencies. Thus far, performance impairment appears to be found mainly in animal studies using non-aversive, food-motivated tasks, with such tasks being particularly well suited for investigating such a phenomenon. At present, there is a noticeable lack of knowledge regarding the neurochemical substrates underlying BDZ-induced impairment. Finally, some issues that may contribute to the presence or absence of a BDZ-induced performance impairment in published studies are briefly considered.

Effect of LSD on acquisition, maintenance, extinction and differentiation of conditioned responses

Three experiments were conducted to compare the effects of LSD (30 nmol/kg) on the acquisition, maintenance, extinction and differentiation of the rabbit's classically conditioned nictitating membrane response. LSD significantly enhanced the acquisition of conditioned responses to tone and light conditioned stimuli as compared with vehicle injected controls (Experiments 1 and 2), but had no detectable effect on differential conditioning in Experiment 3. The conditioned responses acquired under LSD in Experiments 1 and 2 exhibited some unusual features in that: they were more rapidly extinguished under continued injections of LSD; they demonstrated a significant decrement when animals were switched from LSD to vehicle during maintenance; and they were virtually eliminated when animals were switched from LSD to vehicle during extinction. In contrast, conditioned responses acquired under saline injections in Experiments 1 and 2 were not affected when animals were switched to LSD injections during either maintenance or extinction. These results of Experiments 1 and 2 were interpreted as indicating that LSD produces an asymmetrical state-dependent learning.