Postnatal development of cholinergic neurons in the mesopontine tegmentum revealed by histochemistry

Biological Functions of Rat Ultrasonic Vocalizations, Arousal Mechanisms, and Call Initiation

This review summarizes all reported and suspected functions of ultrasonic vocalizations in infant and adult rats. The review leads to the conclusion that all types of ultrasonic vocalizations subserving all functions are vocal expressions of emotional arousal initiated by the activity of the reticular core of the brainstem. The emotional arousal is dichotomic in nature and is initiated by two opposite-in-function ascending reticular systems that are separate from the cognitive reticular activating system. The mesolimbic cholinergic system initiates the aversive state of anxiety with concomitant emission of 22 kHz calls, while the mesolimbic dopaminergic system initiates the appetitive state of hedonia with concomitant emission of 50 kHz vocalizations. These two mutually exclusive arousal systems prepare the animal for two different behavioral outcomes. The transition from broadband infant isolation calls to the well-structured adult types of vocalizations is explained, and the social importance of adult rat vocal communication is emphasized. The association of 22 kHz and 50 kHz vocalizations with aversive and appetitive states, respectively, was utilized in numerous quantitatively measured preclinical models of physiological, psychological, neurological, neuropsychiatric, and neurodevelopmental investigations. The present review should help in understanding and the interpretation of these models in biomedical research.

Sex-specific alterations in GABA receptor-mediated responses in laterodorsal tegmentum are associated with prenatal exposure to nicotine

Smoking during pregnancy is associated with deleterious physiological and cognitive effects on the offspring, which are likely due to nicotine-induced alteration in the development of neurotransmitter systems. Prenatal nicotine exposure (PNE) in rodents is associated with changes in behaviors controlled in part by the pontine laterodorsal tegmentum (LDT), and LDT excitatory signaling is altered in a sex and age-dependent manner by PNE. As effects on GABAergic LDT signaling are unknown, we used calcium imaging to evaluate GABA_A receptor- (GABA_A R as well as GABA_A -_ρ R) and GABA_B receptor (GABA_B R)-mediated calcium responses in LDT brain slices from female and male PNE mice in two different age groups. Overall, in older PNE females, changes in calcium induced by stimulation of GABA_A R and GABA_B R, including GABA_A -_ρ R were shifted toward calcium rises. In both young and old males, PNE was associated with alterations in calcium mediated by all three receptors; however, the GABA_A R was the most affected. These results show for the first time that PNE is associated with alterations in GABAergic transmission in the LDT in a sex- and age-dependent manner, and these data are the first to show PNE-associated alterations in functionality of GABA receptors in any nucleus. PNE-associated alterations in LDT GABAergic transmission within the LDT would be expected to alter output to target regions and could play a role in LDT-implicated, negative behavioral outcomes following gestational exposure to smoking. Accordingly, our data provide further supportive evidence of the importance of eliminating the consumption of nicotine during pregnancy.

Plasticity in the Brainstem: Prenatal and Postnatal Experience Can Alter Laterodorsal Tegmental (LDT) Structure and Function

The brainstem has traditionally been considered an area of the brain with autonomous control of mostly homeostatic functions such as heart rate, respiration, and the sleep and wakefulness state, which would preclude the necessity to exhibit the high degree of synaptic or cellular mechanisms of plasticity typical of regions of the brain responsible for flexible, executive control, such as the medial prefrontal cortex or the hippocampus. The perception that the brainstem does not share the same degree of flexibility to alter synaptic strength and/or wiring within local circuits makes intuitive sense, as it is not easy to understand how "soft wiring" would be an advantage when considering the importance of faithful and consistent performance of the homeostatic, autonomic functions that are controlled by the brainstem. However, many of the molecular and cellular requirements which underlie strengthening of synapses seen in brain regions involved in higher-level processing are present in brainstem nuclei, and recent research suggest that the view of the brainstem as "hard wired," with rigid and static connectivity and with unchanging synaptic strength, is outdated. In fact, information from studies within the last decades, including work conducted in our group, leads us to propose that the brainstem can dynamically alter synaptic proteins, and change synaptic connections in response to prenatal or postnatal stimuli, and this would likely alter functionality and output. This article reviews recent research that has provided information resulting in our revision of the view of the brainstem as static and non-changing by using as example recent information gleaned from a brainstem pontine nucleus, the laterodorsal tegmentum (LDT). The LDT has demonstrated mechanisms underlying synaptic plasticity, and plasticity has been exhibited in the postnatal LDT following exposure to drugs of abuse. Further, exposure of the brain during gestation to drugs of abuse results in alterations in development of signaling pathways in the LDT. As the LDT provides a high degree of innervation of mesoaccumbal and mesocortical circuits involved in salience, as well as thalamocortical circuits involved in control of arousal and orientation, changes in synaptic strength would be expected to alter output, which would significantly impact behavioral state, motivated behavior and directed attention. Further, alterations in developmental trajectory within the LDT following prenatal exposure to drugs of abuse would be expected to impact on later life expression of motivation and arousal.

Age-related changes in nicotine response of cholinergic and non-cholinergic laterodorsal tegmental neurons: implications for the heightened adolescent susceptibility to nicotine addiction

The younger an individual starts smoking, the greater the likelihood that addiction to nicotine will develop, suggesting that neurobiological responses vary across age to the addictive component of cigarettes. Cholinergic neurons of the laterodorsal tegmental nucleus (LDT) are importantly involved in the development of addiction, however, the effects of nicotine on LDT neuronal excitability across ontogeny are unknown. Nicotinic effects on LDT cells across different age groups were examined using calcium imaging and whole-cell patch clamping. Within the youngest age group (P7-P15), nicotine induced larger intracellular calcium transients and inward currents. Nicotine induced a greater number of excitatory synaptic currents in the youngest animals, whereas larger amplitude inhibitory synaptic events were induced in cells from the oldest animals (P15-P34). Nicotine increased neuronal firing of cholinergic cells to a greater degree in younger animals, possibly linked to development associated differences found in nicotinic effects on action potential shape and afterhyperpolarization. We conclude that in addition to age-associated alterations of several properties expected to affect resting cell excitability, parameters affecting cell excitability are altered by nicotine differentially across ontogeny. Taken together, our data suggest that nicotine induces a larger excitatory response in cholinergic LDT neurons from the youngest animals, which could result in a greater excitatory output from these cells to target regions involved in development of addiction. Such output would be expected to be promotive of addiction; therefore, ontogenetic differences in nicotine-mediated increases in the excitability of the LDT could contribute to the differential susceptibility to nicotine addiction seen across age.

Cocaine exposure enhances excitatory synaptic drive to cholinergic neurons in the laterodorsal tegmental nucleus

Accumulating evidence indicates that the laterodorsal tegmental nucleus (LDT) is associated with reward processing and addiction. The cholinergic projection from the LDT to the ventral tegmental area is essential for a large dopamine release in the nucleus accumbens, which is critically involved in the reinforcing effects of addictive drugs, including cocaine. In contrast to the large number of studies on plasticity induced after cocaine exposure in the mesocorticolimbic dopaminergic system, it remains unknown whether LDT cholinergic neurons exhibit plastic changes following cocaine administration. To address this issue, we performed ex vivo whole-cell recordings in LDT cholinergic neurons obtained from rats following cocaine administration. Neurons obtained from 1 day after 5-day cocaine-treated rats showed significantly smaller paired-pulse ratios of evoked EPSCs and higher miniature EPSC frequencies than those from saline-treated rats, indicating an induction of presynaptic plasticity of increased glutamate release. This plasticity seemed to recover after a 5-day withdrawal from repeated cocaine exposure, and required NMDA receptor stimulation and nitric oxide production. Additionally, pharmacological suppression of activity of the medial prefrontal cortex inhibited the presynaptic plasticity in the LDT. On the other hand, AMPA/NMDA ratios were not different between saline- and cocaine-treated groups, revealing an absence of postsynaptic plasticity. These findings provide the first direct evidence of cocaine-induced synaptic plasticity in LDT cholinergic neurons and suggest that the presynaptic plasticity enhances the activity of LDT cholinergic neurons, contributing to the expression of cocaine-induced addictive behaviors through the dysregulation of the mesocorticolimbic system.

Developmental aspects of the cholinergic system

Beyond its importance in sustaining or modulating different aspects of the activity of the central nervous system (CNS), the cholinergic system plays important roles during development. In the current review, we focus on the developmental aspects associated with major components of the cholinergic system: Acetylcholine, choline acetyltransferase, vesicular acetylcholine transporter, high-affinity choline transporter, acetylcholinesterase, nicotinic and muscarinic receptors. We describe when and where each one of these components is first identified in the CNS and the changes in their levels that occur during the course of prenatal and postnatal development. We also describe how these components are relevant to many events that occur during the development of the CNS, including progenitor cells proliferation and differentiation, neurogenesis, gliogenesis, neuronal maturation and plasticity, axonal pathfinding, regulation of gene expression and cell survival. It will be noticed that evidence regarding the developmental aspects of the cholinergic system comes mostly from studies that used agonists, such as nicotine, and antagonists, such as hemicholinium-3. Studies using immunohistochemistry and genetically altered mice also provided valuable information.

Electrophysiological effects of ghrelin on laterodorsal tegmental neurons in rats: an in vitro study

Ghrelin, a gut and brain peptide, is a potent stimulant for growth hormone (GH) secretion and feeding. Recent studies further show a critical role of ghrelin in the regulation of sleep-wakefulness. Laterodorsal tegmental nucleus (LDT), that regulates waking and rapid eye movement (REM) sleep, expresses GH secretagogue receptors (GHS-Rs). Thus, the present study was carried out to examine electrophysiological effects of ghrelin on LDT neurons using rat brainstem slices, and to determine the ionic mechanism involved. Whole cell recording revealed that ghrelin depolarizes LDT neurons dose-dependently in normal artificial cerebrospinal fluid (ACSF). The depolarization persisted in tetrodotoxin-containing ACSF (TTX ACSF), and is partially blocked by the application of [D-Lys3]-GHRP-6, a selective antagonist for GHS-Rs. Membrane resistance during the ghrelin-induced depolarization increased by about 18% than that before the depolarization. In addition, the ghrelin-induced depolarization was drastically reduced in high-K+ TTX ACSF with a K+ concentration of 13.25 mM. Reversal potentials obtained from I-V curves before and during the depolarization were about -83 mV, close to the equilibrium potential of the K+ channel. Most of the LDT neurons recorded were characterized by an A-current or both the A-current and a low threshold Ca2+ spike, and they were predominantly cholinergic. These results indicate that ghrelin depolarizes LDT neurons postsynaptically and dose-dependently via GHS-Rs, and that the ionic mechanisms underlying the ghrelin-induced depolarization include a decrease of K+ conductance. The results also suggest that LDT neurons are implicated in the cellular processes through which ghrelin participates in the regulation of sleep-wakefulness.

Electroencephalographic responses to tail clamping in anaesthetized rat pups

We investigated electroencephalographic (EEG) responses to tail clamping in lightly anaesthetized rat pups (5–22 days) in order to determine the ontogeny of EEG activity and at what age they may be capable of experiencing pain. Median frequency (F50) and spectral edge frequency (F95) of the power spectrum in the range of 1–30 Hz were determined before and after the application of a noxious stimulus and power spectra were compared by multivariate analysis. There was a postnatal increase in EEG power as, before clamping, pups aged 5–7 days exhibited isoelectric traces, whereas those aged 12–14 days and 21–22 days had intermittent EEG activity where the power in all frequencies was significantly lower at the former than at the latter age. Pups aged 5–7 days exhibited no EEG response to clamping in view of their isoelectric traces. Pups aged 12–14 days showed a significant decrease in F95 ( P = 0.002), whereas those aged 21–22 days showed highly significant reduction in F50 and F95 ( P = 0.028 and P < 0.001, respectively) as well as changes in EEG power of specific frequencies after clamping. The results and related literature suggest that rat pups aged 5–7 days and younger are not likely to perceive pain and that the ability to perceive pain develops gradually between postnatal ages 12–14 days and 21–22 days.

In utero methanesulfonyl fluoride differentially affects learning and maze performance in the absence of long-lasting cholinergic changes in the adult rat

There is increasing evidence that acetylcholinesterase (AChE) may have various specific developmental roles in brain development. Nevertheless, specific effects of AChE inhibition during early brain development have not been adequately described. Therefore, methanesulfonyl fluoride (MSF), an irreversible AChE inhibitor that shows high selectivity for the CNS was used to produce AChE inhibition in utero to study subsequent adult behaviors, sleep, and cholinergic markers. Rats exposed to MSF in utero showed a deficit in spatial learning tasks using appetitive motivation but, surprisingly, they performed equally well or better than controls when aversive motivation was used. One hypothesis was that MSF treatment in utero affected the response to stress. Tests of anxiety however showed no differences in basal levels of anxiety. Studies of sleep behavior, however, indicated a higher level of REM sleep which is only seen during the light phase of male rats exposed to MSF in utero as compared to controls. No differences in cholinergic markers in the brains of adults were found except that females exposed to MSF in utero had a higher level of ChAT activity in the synaptosomal fraction of the hippocampus. Even so, whether cholinergic alterations accompany the in utero MSF exposure remains to be determined. The failure to find widespread changes in cholinergic markers in the adult brains suggests changes in behaviors should be further investigated by testing the participation of postsynaptic mechanisms, measuring of cholinergic markers during earlier development periods and the possible participation of other neurotransmitter systems to clearly reveal the role of the cholinergic system following in utero MSF exposure.

Early life blockade of 5-hydroxytryptamine 1A receptors normalizes sleep and depression-like behavior in adult knock-out mice lacking the serotonin transporter

In serotonin transporter knock-out (5-HTT-/-) mice, extracellular serotonin (5-HT) levels are markedly elevated in the brain, and rapid eye movement sleep (REMS) is enhanced compared with wild-type mice. We hypothesized that such sleep impairment at adulthood results from excessive serotonergic tone during early life. Thus, we assessed whether neonatal treatment with drugs capable of limiting the impact of 5-HT on the brain could normalize sleep patterns in 5-HTT-/- mutants. We found that treatments initiated at postnatal day 5 and continued for 2 weeks with the 5-HT synthesis inhibitor para-chlorophenylalanine, or for 4 weeks with the 5-HT(1A) receptor (5-HT(1A)R) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), induced total or partial recovery of REMS, respectively, in 5-HTT-/- mutants. Early life treatment with WAY 100635 also reversed the depression-like behavior otherwise observed in these mutants. Possible adaptive changes in 5-HT(1A)R after neonatal treatment with WAY 100635 were investigated by measuring 5-HT(1A) binding sites and 5-HT(1A) mRNA in various REMS- and/or depression-related brain areas, as well as 5-HT(1A)R-mediated hypothermia and inhibition of neuronal firing in the dorsal raphe nucleus. None of these characteristics were modified in parallel with REMS recovery, suggesting that 5-HT(1A)Rs involved in wild-type phenotype rescue in 5-HTT-/- mutants are located in other brain areas or in 5-HT(1A)R-unrelated circuits where they could be transiently expressed during development. The reversal of sleep alterations and depression-like behavior after early life blockade of 5-HT(1A)R in 5-HTT-/- mutants might open new perspectives regarding preventive care of sleep and mood disorders resulting from serotonin transporter impairments during development.