Consequences of early postnatal benzodiazepines exposure in rats. I. Cognitive-like behavior

Modulatory effect of long-term treatment with escitalopram and clonazepam on the expression of anxiety-related neuropeptides: neuromedin U, neuropeptide S and their receptors in the rat brain

BACKGROUND

Newly identified multifunctional peptidergic modulators of stress responses: neuromedin U (NMU) and neuropeptide S (NPS) are involved in the wide spectrum of brain functions. However, there are no reports dealing with potential molecular relationships between the action of diverse anxiolytic or antidepressant drugs and NMU and NPS signaling in the brain. The present work was therefore focused on local expression of the aforementioned stress-related neuropeptides in the rat brain after long-term treatment with escitalopram and clonazepam.

METHODS

Studies were carried out on adult, male Sprague-Dawley rats that were divided into 3 groups: animals injected with saline (control) and experimental individuals treated with escitalopram (at single dose 5 mg/kg daily), and clonazepam (at single dose 0.5 mg/kg). All individuals were sacrificed under anaesthesia and the whole brain excised. Total mRNA was isolated from homogenized samples of amygdala, hippocampus, hypothalamus, thalamus, cerebellum and brainstem. Real time-PCR method was used for estimation of related NPS, NPS receptor (NPSR), NMU, NMU and receptor 2 (NMUR2) mRNA expression. The whole brains were also sliced for general immunohistochemical assessment of the neuropeptides expression.

RESULTS

Chronic administration of clonazepam resulted in an increase of NMU mRNA expression and formation of NMU-expressing fibers in the amygdala, while escitalopram produced a significant decrease in NPSR mRNA level in hypothalamus. Long-term escitalopram administration affects the local expression of examined neuropeptides mRNA in a varied manner depending on the brain structure.

CONCLUSIONS

Pharmacological effects of escitalopram may be connected with local at least partially NPSR-related alterations in the NPS/NMU/NMUR2 gene expression at the level selected rat brain regions. A novel alternative mode of SSRI action can be therefore cautiously proposed.

The GluN2B-Selective Antagonist Ro 25-6981 Is Effective against PTZ-Induced Seizures and Safe for Further Development in Infantile Rats

The GluN2B subunit of NMDA receptors represents a perspective therapeutic target in various CNS pathologies, including epilepsy. Because of its predominant expression in the immature brain, selective GluN2B antagonists are expected to be more effective early in postnatal development. The aim of this study was to identify age-dependent differences in the anticonvulsant activity of the GluN2B-selective antagonist Ro 25-6981 and assess the safety of this drug for the developing brain. Anticonvulsant activity of Ro 25-6981 (1, 3, and 10 mg/kg) was tested in a pentylenetetrazol (PTZ) model in infantile (12-day-old, P12) and juvenile (25-day-old, P25) rats. Ro 25-6981 (1 or 3 mg/kg/day) was administered from P7 till P11 to assess safety for the developing brain. Animals were then tested repeatedly in a battery of behavioral tests focusing on sensorimotor development, cognition, and emotionality till adulthood. Effects of early exposure to Ro 25-6981 on later seizure susceptibility were tested in the PTZ model. Ro 25-6981 was effective against PTZ-induced seizures in infantile rats, specifically suppressing the tonic phase of the generalized tonic-clonic seizures, but it failed in juveniles. Neither sensorimotor development nor cognitive abilities and emotionality were affected by early-life exposure to Ro 25-6981. Treatment cessation did not affect later seizure susceptibility. Our data are in line with the maturational gradient of the GluN2B-subunit of NMDA receptors and demonstrate developmental differences in the anti-seizure activity of the GluN2B-selective antagonist and its safety for the developing brain.

Neonatal administration of a subanaesthetic dose of JM-1232(-) in mice results in no behavioural deficits in adulthood

In animal models, neonatal exposure of general anaesthetics significantly increases apoptosis in the brain, resulting in persistent behavioural deficits later in adulthood. Consequently, there is growing concern about the use of general anaesthetics in obstetric and paediatric practice. JM-1232(−) has been developed as a novel intravenous anaesthetic, but the effects of JM-1232(−) on the developing brain are not understood. Here we show that neonatal administration of JM-1232(−) does not lead to detectable behavioural deficits in adulthood, contrarily to other widely-used intravenous anaesthetics. At postnatal day 6 (P6), mice were injected intraperitoneally with a sedative-equivalent dose of JM-1232(−), propofol, or midazolam. Western blot analysis of forebrain extracts using cleaved poly-(adenosine diphosphate-ribose) polymerase antibody showed that JM-1232(−) is accompanied by slight but measurable apoptosis 6 h after administration, but it was relatively small compared to those of propofol and midazolam. Behavioural studies were performed in adulthood, long after the neonatal anaesthesia, to evaluate the long-term effects on cognitive, social, and affective functions. P6 administration to JM-1232(−) was not accompanied by detectable long-term behavioural deficits in adulthood. However, animals receiving propofol or midazolam had impaired social and/or cognitive functions. These data suggest that JM-1232(−) has prospects for use in obstetric and paediatric practice.

Neonatal Clonazepam Administration Induced Long-Lasting Changes in GABAA and GABAB Receptors

Benzodiazepines (BZDs) are widely used in patients of all ages. Unlike adults, neonatal animals treated with BZDs exhibit a variety of behavioral deficits later in life; however, the mechanisms underlying these deficits are poorly understood. This study aims to examine whether administration of clonazepam (CZP; 1 mg/kg/day) in 7-11-day-old rats affects Gama aminobutyric acid (GABA)ergic receptors in both the short and long terms. Using RT-PCR and quantitative autoradiography, we examined the expression of the selected GABA_A receptor subunits (α1, α2, α4, γ2, and δ) and the GABA_B B2 subunit, and GABA_A, benzodiazepine, and GABA_B receptor binding 48 h, 1 week, and 2 months after treatment discontinuation. Within one week after CZP cessation, the expression of the α2 subunit was upregulated, whereas that of the δ subunit was downregulated in both the hippocampus and cortex. In the hippocampus, the α4 subunit was downregulated after the 2-month interval. Changes in receptor binding were highly dependent on the receptor type, the interval after treatment cessation, and the brain structure. GABA_A receptor binding was increased in almost all of the brain structures after the 48-h interval. BZD-binding was decreased in many brain structures involved in the neuronal networks associated with emotional behavior, anxiety, and cognitive functions after the 2-month interval. Binding of the GABA_B receptors changed depending on the interval and brain structure. Overall, the described changes may affect both synaptic development and functioning and may potentially cause behavioral impairment.

Neonatal Clonazepam Administration Induces Long-Lasting Changes in Glutamate Receptors

γ-aminobutyric acid (GABA) pathways play an important role in neuronal circuitry formation during early postnatal development. Our previous studies revealed an increased risk for adverse neurodevelopmental consequences in animals exposed to benzodiazepines, which enhance GABA inhibition via GABA_A receptors. We reported that administration of the benzodiazepine clonazepam (CZP) during postnatal days 7-11 resulted in permanent behavioral alterations. However, the mechanisms underlying these changes are unknown. We hypothesized that early CZP exposure modifies development of glutamatergic receptors and their composition due to the tight developmental link between GABAergic functions and maturation of glutamatergic signaling. These changes may alter excitatory synapses, as well as neuronal connectivity and function of the neural network. We used quantitative real-time PCR and quantitative autoradiography to examine changes in NMDA and AMPA receptor composition and binding in response to CZP (1 mg/kg/day) administration for five consecutive days, beginning on P7. Brains were collected 48 h, 1 week, or 60 days after treatment cessation, and mRNA subunit expression was assessed in the hippocampus and sensorimotor cortex. A separate group of animals was used to determine binding to NMDA in different brain regions. Patterns of CZP-induced alterations in subunit mRNA expression were dependent on brain structure, interval after CZP cessation, and receptor subunit type. In the hippocampus, upregulation of GluN1, GluN3, and GluR2 subunit mRNA was observed at the 48-h interval, and GluN2A and GluR1 mRNA expression levels were higher 1 week after CZP cessation compared to controls, while GluN2B was downregulated. CZP exposure increased GluN3 and GluR2 subunit mRNA expression levels in the sensorimotor cortex 48 h after treatment cessation. GluA3 was higher 1 week after the CZP exposure, and GluN2A and GluA4 mRNA were significantly upregulated 2 months later. Expression of other subunits was not significantly different from that of the controls. NMDA receptor binding increased 1 week after the end of exposure in most hippocampal and cortical areas, including the sensorimotor cortex at the 48-h interval. CZP exposure decreased NMDA receptor binding in most evaluated hippocampal and cortical areas 2 months after the end of administration. Overall, early CZP exposure likely results in long-term glutamatergic receptor modulation that may affect synaptic development and function, potentially causing behavioral impairment.

Neonatal phenobarbital exposure disrupts GABAergic synaptic maturation in rat CA1 neurons

OBJECTIVE

Phenobarbital is the most commonly utilized drug for the treatment of neonatal seizures. The use of phenobarbital continues despite growing evidence that it exerts suboptimal seizure control and is associated with long-term alterations in brain structure, function, and behavior. Alterations following neonatal phenobarbital exposure include acute induction of neuronal apoptosis, disruption of synaptic development in the striatum, and a host of behavioral deficits. These behavioral deficits include those in learning and memory mediated by the hippocampus. However, the synaptic changes caused by acute exposure to phenobarbital that lead to lasting effects on brain function and behavior remain understudied.

METHODS

Postnatal day (P)7 rat pups were treated with phenobarbital (75 mg/kg) or saline. On P13-14 or P29-37, acute slices were prepared and whole-cell patch-clamp recordings were made from CA1 pyramidal neurons.

RESULTS

At P14 we found an increase in miniature inhibitory postsynaptic current (mIPSC) frequency in the phenobarbital-exposed as compared to the saline-exposed group. In addition to this change in mIPSC frequency, the phenobarbital group displayed larger bicuculline-sensitive tonic currents, decreased capacitance and membrane time constant, and a surprising persistence of giant depolarizing potentials. At P29+, the frequency of mIPSCs in the saline-exposed group had increased significantly from the frequency at P14, typical of normal synaptic development; at this age the phenobarbital-exposed group displayed a lower mIPSC frequency than did the control group. Spontaneous inhibitory postsynaptic current (sIPSC) frequency was unaffected at either P14 or P29+.

SIGNIFICANCE

These neurophysiological alterations following phenobarbital exposure provide a potential mechanism by which acute phenobarbital exposure can have a long-lasting impact on brain development and behavior.

Preferential Inhibition of Tonically over Phasically Activated NMDA Receptors by Pregnane Derivatives

Postsynaptic N-methyl-d-aspartate receptors (NMDARs) phasically activated by presynaptically released glutamate are critical for synaptic transmission and plasticity. However, under pathological conditions, excessive activation of NMDARs by tonically increased ambient glutamate contributes to excitotoxicity associated with various acute and chronic neurological disorders. Here, using heterologously expressed GluN1/GluN2A and GluN1/GluN2B receptors and rat autaptic hippocampal microisland cultures, we show that pregnanolone sulfate inhibits NMDAR currents induced by a prolonged glutamate application with a higher potency than the NMDAR component of EPSCs. For synthetic pregnanolone derivatives substituted with a carboxylic acid moiety at the end of an aliphatic chain of varying length and attached to the steroid skeleton at C3, the difference in potency between tonic and phasic inhibition increased with the length of the residue. The steroid with the longest substituent, pregnanolone hemipimelate, had no effect on phasically activated receptors while inhibiting tonically activated receptors. In behavioral tests, pregnanolone hemipimelate showed neuroprotective activity without psychomimetic symptoms. These results provide insight into the influence of steroids on neuronal function and stress their potential use in the development of novel therapeutics with neuroprotective action.

SIGNIFICANCE STATEMENT

Synaptic activation of N-methyl-d-aspartate receptors (NMDARs) plays a key role in synaptic plasticity, but excessive tonic NMDAR activation mediates excitotoxicity associated with many neurological disorders. Therefore, there is much interest in pharmacological agents capable of selectively blocking tonically activated NMDARs while leaving synaptically activated NMDARs intact. Here, we show that an endogenous neurosteroid pregnanolone sulfate is more potent at inhibiting tonically than synaptically activated NMDARs. Further, we report that a novel synthetic analog of pregnanolone sulfate, pregnanolone hemipimelate, inhibits tonic NMDAR currents without inhibiting the NMDAR component of the EPSC and shows neuroprotective activity in vivo without inducing psychomimetic side effects. These results suggest steroids may have a clinical advantage over other known classes of NMDAR inhibitors.

Comparison of the long-term behavioral effects of neonatal exposure to retigabine or phenobarbital in rats

Anticonvulsant drugs, when given during vulnerable periods of brain development, can have long-lasting consequences on nervous system function. In rats, the second postnatal week approximately corresponds to the late third trimester of gestation/early infancy in humans. Exposure to phenobarbital during this period has been associated with deficits in learning and memory, anxiety-like behavior, and social behavior, among other domains. Phenobarbital is the most common anticonvulsant drug used in neonatology. Several other drugs, such as lamotrigine, phenytoin, and clonazepam, have also been reported to trigger behavioral changes. A new generation anticonvulsant drug, retigabine, has not previously been evaluated for long-term effects on behavior. Retigabine acts as an activator of KCNQ channels, a mechanism that is unique among anticonvulsants. Here, we examined the effects retigabine exposure from postnatal day (P)7 to P14 on behavior in adult rats. We compared these effects with those produced by phenobarbital (as a positive control) and saline (as a negative control). Motor behavior was assessed by using the open field and rotarod, anxiety-like behavior by the open field, elevated plus maze, and light-dark transition task, and learning/memory by the passive avoidance task; social interactions were assessed in same-treatment pairs, and nociceptive sensitivity was assessed via the tail-flick assay. Motor behavior was unaltered by exposure to either drug. We found that retigabine exposure and phenobarbital exposure both induced increased anxiety-like behavior in adult animals. Phenobarbital, but not retigabine, exposure impaired learning and memory. These drugs also differed in their effects on social behavior, with retigabine-exposed animals displaying greater social interaction than phenobarbital-exposed animals. These results indicate that neonatal retigabine induces a subset of behavioral alterations previously described for other anticonvulsant drugs and extend our knowledge of drug-induced behavioral teratogenesis to a new mechanism of anticonvulsant action.

Anticonvulsant drug-induced cell death in the developing white matter of the rodent brain

OBJECTIVE

During critical periods of brain development, both seizures and anticonvulsant medications can affect neurodevelopmental outcomes. In rodent models, many anticonvulsants trigger neuronal apoptosis. However, white matter apoptosis (WMA) has not been examined after anticonvulsant drug treatment. Herein, we sought to determine if anticonvulsant drugs induced apoptosis in the developing white matter (WM) in a rodent model.

METHODS

Postnatal day (P)7 rats were treated with phenobarbital (PB-75), MK-801 (dizocilpine, 0.5), lamotrigine (LTG-20), carbamazepine (CBZ-100), phenytoin (PHT-50), levetiracetam (LEV-250), or saline; all doses are mg/kg. Brain tissue collected 24 h after treatment was stained using the terminal deoxynucleotidyl transferase dUTP nick end labeling method. The number of degenerating cells within WM, that is, anterior commissure (AC), corpus callosum, cingulum, and hippocampus-associated WM tracts, was quantified.

RESULTS

Saline-treated rats showed low baseline level of apoptosis in developing WM on P8 in all the areas examined. PB, PHT, and MK-801 significantly increased apoptosis in all four brain areas examined. Exposure to CBZ, LTG, or LEV failed to increase apoptosis in all regions.

SIGNIFICANCE

Commonly used anticonvulsants (PB, PHT) cause apoptosis in the developing WM in a rat model; the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 has a similar effect. These results are consistent with reports of anesthesia-induced WMA during brain development. Consistent with the lack of neuronal apoptosis caused by LTG, LEV, and CBZ, these drugs did not cause WMA. Many infants treated with anticonvulsant drugs have underlying neurologic injury, including WM damage (e.g., following intraventricular hemorrhage [IVH] or hypoxic-ischemic encephalopathy [HIE]). The degree to which anticonvulsant drug treatment will alter outcomes in the presence of underlying injury remains to be examined, but avoiding drugs (when possible) that induce WMA may be beneficial.

Profile of retigabine-induced neuronal apoptosis in the developing rat brain

OBJECTIVE

Acute neonatal exposure to some, but not all, anticonvulsant drugs induces a profound increase in neuronal apoptosis in rats. Phenobarbital and phenytoin induce apoptosis at a therapeutically relevant dose range, lamotrigine and carbamazepine do so only at supratherapeutic doses or in polytherapy, and valproate does so even at subtherapeutic doses. Levetiracetam is devoid of pro-apoptotic effects. Retigabine, a new-generation drug, acts uniquely by enhancing the M-type potassium current. Because its safety profile in developing animals is unstudied, we sought to determine if retigabine would induce apoptosis.

METHODS

Postnatal day (P) 7 rat pups were treated with retigabine (5-30 mg/kg), vehicle (saline), or comparator drugs (phenobarbital, lamotrigine, levetiracetam, or carbamazepine). Cell death was assessed using amino-cupric-silver staining. A separate group of animals was treated repeatedly (three times over 24 h) with retigabine (15 mg/kg) or vehicle. To establish a pharmacokinetic profile for retigabine, we measured plasma and brain levels after drug treatment.

RESULTS

Consistent with prior studies from our group and others, we found phenobarbital-induced cell death throughout thalamus, nucleus accumbens, and several neocortical areas. By contrast, levetiracetam, lamotrigine, and carbamazepine were found to have no appreciable apoptotic effect on the aforementioned structures. Acute (single) exposure to retigabine, even at doses of 30 mg/kg, was also without effect on apoptosis. However, repeated (three times) exposure to retigabine triggered apoptosis in a subset of brain areas. The half-life of retigabine in plasma was 2.5 h, with appreciable concentrations reached in the brain within 1 h of administration.

SIGNIFICANCE

These data demonstrate that retigabine, like many other anticonvulsant drugs, is capable of triggering neuronal apoptosis in the developing rat brain. Unlike other drugs, repeated dosing of retigabine was necessary to induce this effect. This may be due to its shorter half-life as compared to other drugs, such as phenobarbital.

Prenatal protein level impacts homing behavior in Long-Evans rat pups

OBJECTIVE

This study assessed the effect of varying prenatal protein levels on the development of homing behavior in rat pups.

METHODS

Long-Evans rats were fed one of the four isocaloric diets containing 6% (n = 7 litters), 12% (n = 9), 18% (n = 9), or 25% (n = 10) casein prior to mating and throughout pregnancy. At birth, litters were fostered to well-nourished control mothers fed a 25% casein diet during pregnancy, and an adequate protein diet (25% casein) was provided to weaning. On postnatal days 5, 7, 9, 11, and 13, homing behaviors, including activity levels, rate of successful returns to the nest quadrant and latencies to reach the nest over a 3-minute test period were recorded from two starting positions in the home cage. Adult body and brain weights were obtained at sacrifice (postnatal day 130 or 200).

RESULTS

Growth was impaired in pups whose mothers were fed a 6% or, to a lesser extent, a 12% casein diet relative to pups whose mothers were fed the 18 and 25% casein diets. The 6 and 12% prenatal protein levels resulted in lower activity levels, with the greatest reduction on postnatal day 13. However, only the 6% pups had reduced success and higher latencies in reaching the nest quadrant when compared with pups from the three other nutrition groups. Latency in reaching the nest quadrant was significantly and negatively associated with adult brain weight.

DISCUSSION

Home orientation is a sensitive measure of developmental deficits associated with variations in prenatal protein levels, including levels of protein deficiency that do not lead to overt growth failure.

Consequences of early postnatal benzodiazepines exposure in rats. II. Social behavior

Social behavior represents an integral part of behavioral repertoire of rats particularly sensitive to pharmacological and environmental influences. The aim of the present study was to investigate whether early postnatal clonazepam (CZP) exposure can induce age-dependent changes related to expression of social behavior. The drug was administered from postnatal day (P) 7 until P11 at daily doses of 0.1, 0.5 and 1.0 mg/kg i.p. We designed three experiments to assess whether exposure to CZP affects social behavior in respect to the age of rats and the test circumstances, specifically their familiarity with test conditions during adolescence (P32), social behavior in juveniles and adolescents (P18–P42) and social behavior in a resident-intruder paradigm. The frequency and duration of a various patterns of social behavior related to play and social investigation not related to play were evaluated. The results showed that CZP postnatal exposure decreased social play behavior regardless of age and familiarity or unfamiliarity of experimental environment but did not affect the social investigation per se. When rats were confronted with an intruder in their home cages intense wrestling and inhibition of genital investigation were found. In conclusion, these findings show that short-term CZP postnatal exposure inhibits social play behavior and alters specific patterns of social behavior in an age and environment related manner.