Firing of ‘possibly’ cholinergic neurons in the rat laterodorsal tegmental nucleus during sleep and wakefulness

To clarify functional roles of mesopontine cholinergic neurons as a component of an activating system, single neuronal activity in the laterodorsal tegmental nucleus (LDT) of undrugged rats, whose head was fixed painlessly, was recorded along with cortical EEG and neck EMG. Activity of some dorsal raphe (DR) neurons was also recorded for comparison. Most of the animals had been sleep-deprived for 24 h. Observation was made only on neurons generating broad spikes, presumed from previous studies to be cholinergic or monoaminergic. The position of recorded neurons was marked by Pontamine sky blue ejected from the glass pipette microelectrode, and was identified on sections processed for NADPH diaphorase histochemistry which specifically stained cholinergic neurons. According to their firing rates during wakefulness (AW), slow-wave sleep (SWS) and paradoxical sleep (PS), 46 broad-spike neurons in the LDT were classified into 4 groups: (1) neurons most active during AW and silent during PS (some of these neurons might be serotonergic rather than cholinergic, as all the 9 neurons in the DR); (2) neurons most active during PS and silent during AW; (3) neurons equally more active during AW and PS than SWS; and (4) others mainly characterized by transiently facilitated activity at awakening and/or onset of PS. Neurons of groups 2 and 3 were the major constituents of the LDT. In most neurons change in firing preceded EEG change, except at awakening from PS. These results suggest that: (1) the LDT is composed of cholinergic neurons with heterogenous characteristics in relation to sleep/wakefulness; and (2) some tegmental cholinergic neurons play a pivotal role in induction and maintenance of PS.     

Ultrastructure of cholinergic neurons in the laterodorsal tegmental nucleus of the rat: Interaction with catecholamine fibers

The ultrastructure of choline acetyltransferase (ChAT)-immunoreactive neurons in the laterodorsal tegmental nucleus (TLD) of the rat was investigated by immunohistochemical techniques. The immunoreactive neurons were medium to large in size, with a few elongated dendrites, contained well-developed cytoplasm, and a nucleus with deep infoldings. They received many nonimmunoreactive, mostly asymmetric synaptic inputs on their soma and dendrites. ChAT-immunoreactive, usually myelinated, axons were occasionally seen in TLD. Only one immunoreactive axon terminal was observed within TLD, and it made synaptic contact with a nonimmunoreactive neuronal perikaryon. The synaptic interactions between ChAT-immunoreactive neurons and tyrosine hydroxylase (TH)-immunoreactive fibers in the TLD were investigated with a double immunohistochemical staining method. ChAT-immunoreactivity detected with a beta-galactosidase method was light blue-green in the light microscope and formed dot-like electron dense particles at the electron microscopic level. TH-immunoreactivity, visualized with a nickel-enhanced immunoperoxidase method, was dark blue-black in the light microscope and diffusely opaque in the electron microscope. Therefore, the difference between these two kinds of immunoreactivity could be quite easily distinguished at both light and electron microscopic levels. In the light microscope, TH-positive fibers were often closely apposed to ChAT-immunoreactive cell bodies and dendrites in TLD. In the electron microscope, the cell soma and proximal dendrites of ChAT-immunoreactive neurons received synaptic contacts from TH-immunoreactive axon terminals. These results provide a morphological basis for catecholaminergic regulation of the cholinergic reticular system.     

Effects of chronic alcohol consumption and withdrawal on the cholinergic neurons of the pedunculopontine and laterodorsal tegmental nuclei of the rat: An unbiased stereological study

The brain cholinergic system comprises two main recognized subdivisions, the basal forebrain and the brainstem cholinergic systems. The effects of chronic alcohol consumption on the basal forebrain cholinergic nuclei have been investigated extensively, but there is only one study that has examined those effects on the brainstem cholinergic nuclei. The last one comprises the pedunculopontine tegmental (PPT) and the laterodorsal tegmental (LDT) nuclei, which are known to give origin to the main cholinergic projection to the ventral tegmental area, a key brain region of the neural circuit, the mesocorticolimbic system, that mediates several behavioral and physiological processes, including reward. In the present study, we have examined, using stereological methods, the effects of chronic alcohol consumption (6 months) and subsequent withdrawal (2 months) on the total number and size of PPT and LDT choline acetyltransferase (ChAT)-immunoreactive neurons. The total number of PPT and LDT ChAT-immunoreactive neurons was unchanged in ethanol-treated and withdrawn rats. However, ChAT-immunoreactive neurons were significantly hypertrophied in ethanol-treated rats, an alteration that did not revert 2 months after ethanol withdrawal. These results show that prolonged exposure to ethanol leads to long-lasting, and potentially irreversible, cytoarchitectonic and neurochemical alterations in the brainstem cholinergic nuclei. These alterations suggest that the alcohol-induced changes in the brainstem cholinergic nuclei might play a role in the mechanisms underlying the development of addictive behavior to alcohol.     

Human reticular formation: cholinergic neurons of the pedunculopontine and laterodorsal tegmental nuclei and some cytochemical comparisons to forebrain cholinergic neurons

Choline acetyltransferase immunohistochemistry showed that the human rostral brainstem contained cholinergic neurons in the oculomotor, trochlear, and parabigeminal nuclei as well as within the reticular formation. The cholinergic neurons of the reticular formation were the most numerous and formed two intersecting constellations. One of these, designated Ch5, reached its peak density within the compact pedunculopontine nucleus but also extended into the regions through which the superior cerebellar peduncle and central tegmental tract course. The second constellation, designated Ch6, was centered around the laterodorsal tegmental nucleus and spread into the central gray and medial longitudinal fasciculus. There was considerable transmitter-related heterogeneity within the regions containing Ch5 and Ch6. In particular, Ch6 neurons were intermingled with catecholaminergic neurons belonging to the locus coeruleus complex. The lack of confinement within specifiable cytoarchitectonic boundaries and the transmitter heterogeneity justified the transmitter-specific Ch5 and Ch6 nomenclature for these two groups of cholinergic neurons. The cholinergic neurons in the nucleus basalis (Ch4) and those of the Ch5-Ch6 complex were both characterized by perikaryal heteromorphism and isodendritic arborizations. In addition to choline acetyltransferase, the cell bodies in both complexes also had high levels of acetylcholinesterase activity and nonphosphorylated neurofilament protein. However, there were also marked differences in cytochemical signature. For example, the Ch5-Ch6 neurons had high levels of NADPHd activity, whereas Ch4 neurons did not. On the other hand, the Ch4 neurons had high levels of NGF receptor protein, whereas those of Ch5-Ch6 did not. On the basis of animal experiments, it can be assumed that the Ch5 and Ch6 neurons provide the major cholinergic innervation of the human thalamus and that they participate in the neural circuitry of the reticular activating, limbic, and perhaps also extrapyramidal systems.     

Neuroplasticity in cholinergic neurons of the laterodorsal tegmental nucleus contributes to the development of cocaine addiction

The laterodorsal tegmental nucleus (LDT) is a brainstem nucleus that sends cholinergic, glutamatergic, and gamma‐aminobutyric acid (GABA)‐ergic projections to the ventral tegmental area (VTA), a key brain region associated with reward information processing and reinforcement learning, and thus, with addiction induced by drugs of abuse, including cocaine. Recent studies have revealed that the LDT, in addition to the VTA, plays important roles in the development and expression of cocaine‐induced addiction and stress‐induced enhancement of addictive behaviors. Additionally, neuroplasticity induced in LDT cholinergic neurons by repeated cocaine administration critically contributes to these behaviors. Elucidation of the underlying mechanisms of cocaine‐induced neuroplasticity in the LDT that influences reward circuit activity may lead to the development of therapeutic strategies to treat cocaine addiction and stress‐induced reinstatement of cocaine use. This review summarizes recent progress in the study of the LDT, specifically neuroplasticity in LDT cholinergic neurons induced by cocaine and its functional roles in the development and modulation of addictive behaviors associated with cocaine.     

Activity of cholinergic neurons in the laterodorsal tegmental nucleus during emission of 22kHz vocalization in rats

It has been postulated that the ascending cholinergic tegmental system is responsible for the initiation of the aversive emotional state with a concomitant alarm vocalization in the rat. It is assumed that the activity of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) will cause release of acetylcholine in the target areas and will initiate the emission of 22 kHz vocalizations. The goal of the present study was to test the hypothesis that the cholinergic neurons of the LDT increase their activity during emission of 22 kHz alarm calls. Vocalizations were induced by an air puff or by intrahypothalamic-preoptic injection of carbachol. The activity of the LDT cholinergic neurons was studied by a double histochemical labelling for choline acetyltransferase, as a marker of cholinergic somata, and for c-Fos protein, as a marker of cells with heighten metabolic activity. Both air puff stimulation and intracerebral carbachol induced comparable 22 kHz alarm vocalizations. The activity of neurons in the LDT was significantly higher during prolonged emission of 22 kHz alarm calls induced by air puff or injection of carbachol than in the non-vocalizing or low-vocalizing controls. There were approximately two times more of all c-Fos-labelled cells in the LDT of vocalizing animals and 2.5 times more active cholinergic neurons during prolonged 22 kHz vocalization than in the control conditions without vocalization. However, the active cholinergic neurons constituted only a small proportion of all active LDT cells (2.3%). At the same time, there were no significant increases in the number of c-Fos-labelled cells in the neighbouring pedunculopontine nucleus (PPT). These findings lead to the conclusion that the neurons of the LDT, including cholinergic neurons, but not those of the PPT, significantly increased their activity during prolonged emission of alarm vocalizations, as evidenced by the c-Fos immunoreactivity.     

Discriminable excitotoxic effects of ibotenic acid, AMPA, NMDA and quinolinic acid in the rat laterodorsal tegmental nucleus

Excitotoxins are valuable tools in neuroscience research as they can help us to discover the extent to which certain neurones are necessary for different types of behaviour. They have distinctive neurotoxic effects depending on where they are infused, and this study was conducted to delineate the neurotoxic profiles of excitotoxins in the laterodorsal tegmental nucleus (LDTg). Two 0.1 ml infusions of 0.1 M ibotenate, 0.1 M quinolinate, 0.04–0.1 M NMDA, or 0.05–0.015 M AMPA, were made unilaterally into the LDTg under either pentobarbitone or Avertin anaesthesia. The injection needle was oriented at an angle of 24° from vertical in the mediolateral plane. After 23–27 days, sections through the mesopontine tegmentum were processed using standard histological procedures for NADPH-diaphorase histochemistry, tyrosine hydroxylase or 5-hydroxytryptamine immunohistochemistry, and Cresyl violet. Lesions were assessed in terms of the size of the damaged area (identified by reactive gliosis), the extent of cholinergic cell loss in the mesopontine tegmentum (by counting NADPH-diaphorase-positive neurones), and neuronal loss induced in the locus coeruleus and dorsal raphe nucleus. Ibotenate induced compact lesions in the LDTg (more than 80% cholinergic loss) and did little damage to the locus coeruleus and dorsal raphe nucleus. Quinolinate and low doses of AMPA and NMDA made very small lesions with less than 35% cholinergic loss, while at higher doses, AMPA and NMDA induced large areas of reactive gliosis but killed only a proportion of the cholinergic neurones. AMPA appeared to have a particular affinity for noradrenergic neurones in the locus coeruleus, with the 0.015 M dose injected into the LDTg typically destroying the majority of these neurones. The results are discussed in the context of what is known about the mechanisms of excitotoxins and the glutamate receptor profile of mesopontine neurones.     

Fos expression in rat pontine tegmental neurons following activation of the medial preoptic area

Fos immunohistochemistry was used to map the distribution of pontine neurons excited by activation of the medial preoptic area (MPO). Although we have previously shown that Barrington's nucleus receives a very dense focal input from the MPO, electrical stimulation of the preoptic area unexpectedly induced very little Fos expression in Barrington's neurons. These results suggest that the MPO→Barrington's projection utilizes a transmitter(s) that does not involve transduction of the Fos protein; alternatively, MPO afferents to Barrington's nucleus may be inhibitory in nature. As Barrington's nucleus plays a critical role in micturition, MPO projections to Barrington's nucleus may regulate voiding reflexes during sexual behavior. Interestingly, while the locus coeruleus (LC) proper receives only a sparse projection from the MPO, extensive Fos expression was present in LC. The finding of Fos immunoreactive LC neurons suggests that the excitatory influence of MPO may regulate LC neuronal activity and NE release during reproductive behaviors.     

Chronic cocaine exposure induces noradrenergic modulation of inhibitory synaptic transmission to cholinergic neurons of the laterodorsal tegmental nucleus

The laterodorsal tegmental nucleus (LDT), which sends cholinergic efferent connections to dopaminergic (DA) neurons in the ventral tegmental area (VTA), plays a critical role in the development of addictive behavior and the reinstatement of cocaine‐seeking behavior. Although repeated cocaine exposure elicits plastic changes in excitatory synaptic transmission and intrinsic membrane excitability in LDT cholinergic neurons, it remains unclear whether inhibitory synaptic transmission is modulated by cocaine exposure. The LDT receives fibers containing noradrenaline (NA), a neurotransmitter whose extracellular levels increase with cocaine exposure. Therefore, it is hypothesized that repeated cocaine exposure induces plastic changes in LDT cholinergic neurons via NA. Ex vivo electrophysiological recordings in LDT cholinergic neurons were obtained from rats repeatedly exposed to cocaine. Bath‐application of NA induced similar levels of hyperpolarization in both saline‐ and cocaine‐treated neurons. However, NA attenuated the amplitude of inhibitory postsynaptic currents (IPSCs) in cocaine‐ but not saline‐treated neurons through α2 adrenoceptors. This NA‐induced IPSC attenuation was observed in the presence of strychnine, but not gabazine, indicating that NA modulated GABAergic but not glycinergic neurotransmission. NA increased the paired‐pulse ratios of evoked IPSCs and decreased the frequencies of miniature IPSCs (mIPSCs) without affecting their amplitudes, suggesting a presynaptic mechanism. These findings suggest that repeated cocaine exposure induces neuroplasticity in GABAergic synaptic transmission onto LDT cholinergic neurons by probably modulating presynaptic α2 adrenoceptors. This potentially increases the activity of LDT cholinergic neurons, which might contribute to the development of addictive behavior by enhancing VTA DA neuronal activity.     

Feedback loop between locus coeruleus and spinal trigeminal nucleus neurons responding to tooth pulp stimulation in the rat

IGARASHI, S., M. SASA AND S. TAKAORI. Feedback loop between locus coeruleus and spinal trigeminal nucleusneurons responding to tooth pulp stimulation in the rat. BRAIN RES. BULL. 4(1) 75–83, 1979.—Studies were performed to elucidate reciprocal relationships between locus coeruleus (LC) and spinal trigeminal nucleus (STN) neurons responding to tooth pulp (TP) stimulation using rats anesthetized with α-chloralose. LC conditioning stimulation inhibited STN field potential as well as orthodromic spike generation of STN neurons produced by ipsilateral TP stimulation, confirming the previous findings in cats that LC neurons played an inhibitory role in the orthodromic transmission in STN neurons. Forty-one out of 56 LC neurons were activated by ipsilateral TP stimulation and 12 neurons by stimulation of both ipsi- and contralateral TP. STN stimulation usually excited LC neurons with a significantly shorter latency than did TP stimulation, including three LC neurons with a latency of less than 2.0 msec. These results indicate the existence of input from TP to LC neurons via multisynapses. In addition, neurons antidromically activated by STN stimulation were found in LC. It is highly probable, therefore, that there is a feedback loop between LC and STN, which might control input from TP to STN.     

Melanin-concentrating hormone is expressed in the laterodorsal tegmental nucleus only in female rats

Melanin-concentrating hormone (MCH) is a neuropeptide originating from prepro-MCH. In male rats, neurons expressing MCH are found in the lateral hypothalamic area and medial zona incerta, as well as, sparsely, in the olfactory tubercle and pontine reticular formation. The wide distribution of MCH fibers suggests the involvement of this neuropeptide in a variety of functions, including arousal, neuroendocrine control and energy homeostasis. In lactating females, MCH is expressed in the preoptic area, indicating sexual dimorphism in MCH gene activation according to the female reproductive state. We hypothesized that MCH is also expressed differentially in the brainstem of female rats. Adult male rats and female rats (in the afternoon of diestrus and proestrus days; ovariectomized; or on lactation days 5, 12 and 19) were perfused between 2 and 4 p.m., and the brainstems were processed for in situ hybridization using a 35S-labeled prepro-MCH riboprobe. As described in males, prepro-MCH was expressed in the pontine reticular formation of females. We also observed consistent prepro-MCH expression in the caudal laterodorsal tegmental nucleus (LDT) of females but no differential expression comparing the various female reproductive states. Using dual-label immunohistochemistry or dual-label in situ hybridization, we found that brainstem MCH neurons coexpress glutamic acid decarboxylase mRNA, the gamma aminobutyric acid (GABA) processing enzyme, but do not colocalize choline acetyl transferase (acetylcholine processing enzyme). Since changes in LDT GABAergic cell activity are associated with rapid eye movement (REM) sleep, our findings suggest that MCH interacts with LDT GABAergic neurons and plays a role in REM sleep regulation.     

Localization of cholinergic neurons in the forebrain and brainstem that project to the suprachiasmatic nucleus of the hypothalamus in rat

In mammals, the suprachiasmatic nucleus is responsible for the generation of most circadian rhythms and their entrainment to environmental cues. Cholinergic agents can alter circadian rhythm phase, and fibres immunoreactive for choline acetyltransferase, the biosynthetic enzyme for acetylcholine, are present in the suprachiasmatic nucleus. Since there are no cholinergic somata in the suprachiasmatic nucleus, these fibres must represent the terminals of cholinergic neurons whose cell bodies are located elsewhere in the brain. This study was aimed at locating the cholinergic neurons that project to the suprachiasmatic nucleus by retrograde and anterograde tract-tracing and immunohistochemistry for choline acetyltransferase in the rat. After injection of fluorogold, a retrograde tracer, into the suprachiasmatic nucleus, retrogradely labelled neurons that were immunopositive for choline acetyltransferase were located throughout the rostrocaudal extent of the cholinergic basal nuclear complex, with highest densities in the substantia innominata and the nucleus basalis magnocellularis. A few cells were also located in the medial septum and in the vertical and horizontal limbs ofthe diagonal band of Broca. In the brainstem, double-labelled neurons were located in the laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus and the parabigeminal nucleus. Injections of the anterograde tracer biocytin in these three brainstem nuclei resulted in fibre labelling in the suprachiasmatic nucleus, consistent with the retrograde findings. No clearly double-labelled cells were located in the retina. These results suggest that the suprachiasmatic nucleus receives cholinergic afferents from both the basal forebrain and mesopontine tegmentum which may mediate cholinergic effects on circadian rhythms. © 1993 Wiley-Liss, Inc.     

Firing of 'possibly' cholinergic neurons in the rat laterodorsal tegmental nucleus during sleep and wakefulness.

To clarify functional roles of mesopontine cholinergic neurons as a component of an activating system, single neuronal activity in the laterodorsal tegmental nucleus (LDT) of undrugged rats, whose head was fixed painlessly, was recorded along with cortical EEG and neck EMG. Activity of some dorsal raphe (DR) neurons was also recorded for comparison. Most of the animals had been sleep-deprived for 24 h. Observation was made only on neurons generating broad spikes, presumed from previous studies to be cholinergic or monoaminergic. The position of recorded neurons was marked by Pontamine sky blue ejected from the glass pipette microelectrode, and was identified on sections processed for NADPH diaphorase histochemistry which specifically stained cholinergic neurons. According to their firing rates during wakefulness (AW), slow-wave sleep (SWS) and paradoxical sleep (PS), 46 broad-spike neurons in the LDT were classified into 4 groups: (1) neurons most active during AW and silent during PS (some of these neurons might be serotonergic rather than cholinergic, as all the 9 neurons in the DR); (2) neurons most active during PS and silent during AW; (3) neurons equally more active during AW and PS than SWS; and (4) others mainly characterized by transiently facilitated activity at awakening and/or onset of PS. Neurons of groups 2 and 3 were the major constituents of the LDT. In most neurons change in firing preceded EEG change, except at awakening from PS. These results suggest that: (1) the LDT is composed of cholinergic neurons with heterogenous characteristics in relation to sleep/wakefulness; and (2) some tegmental cholinergic neurons play a privotal role in induction and maintenance of PS.     

Noradrenaline-like terminals in the cat nucleus ventralis posterior of the thalamus

Noradrenaline-like immunoreactivity in the cat nucleus ventralis posterior of the thalamus was investigated using an indirect immunocytochemical technique. Specific antinoradrenaline antibodies, raised in rabbits, were used. It was first verified that these antibodies recognize noradrenaline cells bodies of the locus coeruleus and their ascending axons in the ascending noradrenergic tract. In the nucleus ventralis posterior itself, noradrenaline-like fibers were observed. They were either randomly distributed or grouped around nonlabeled cell bodies. These neurons were generally oblong and measured 60-80 microns. With electron microscopy, preliminary results showed immunoreactive fibers in close apposition to unlabeled cell bodies or dendrites. The precise nature of these profiles was sometimes difficult to ascertain, since experiments were done in presence of detergent. In some cases symmetric synapses might be observed between immunoreactive axon terminals and unlabeled dendrites. The specificity of the reaction is discussed in the light of several control experiments.     

Activity of cat locus coeruleus noradrenergic neurons during the defense reaction

The single-unit activity of locus coeruleus noradrenergic (LC-NE) neurons was recorded in freely moving cats during naturally induced defense reactions. Defense reactions, consisting of arched back, piloerection, flattened ears and mydriasis, were elicited by exposing the cat either to a dog, or to a cat displaying aggressive behavior induced by electrical stimulation of the hypothalamus. LC-NE neurons were identified using previously established criteria, including suppression of firing during rapid eye movement (REM) sleep and in response to clonidine administration. Exposure to a dog evoked defense reactions and increased the tonic firing rate of LC-NE neurons (n = 8) from a baseline of approximately 0.9 spikes/s to approximately 2.5 spikes/s. Exposure to an aggressive cat evoked defense reactions that were qualitatively very similar to those produced by dog exposure, and elevated the tonic firing rate of LC-NE neurons (n = 8) from a baseline of approximately 1.0 spikes/s to approximately 2.5 spikes/s. In addition to these tonic elevations of activity, LC-NE neurons discharged in phasic bursts (as high as 10 spikes in a 500 ms period) in close association with specific threatening acts made by the dog or hypothalamically stimulated cat. The mere presence of a dog was sufficient to evoke tonic activation of LC-NE neurons, even in the absence of threatening advances by the dog, whereas exposure to a hypothalamically stimulated cat produced LC-NE neuronal activation only when the stimulated cat showed aggressive behavior. These results extend our previous work, which examined the response of LC-NE neurons to environmental and physiological stressors, into a more ethologically relevant domain, and suggest that LC-NE neuronal activation may play a role in the response to threatening or challenging situations.     

Norepinephrine in the interpeduncular nucleus of the rat: normal distribution and the effects of deafferentation

We used correlative biochemical and histochemical methods to examine (1) the norepinephrine (NE) projection from the paired locus coeruleus (LC) to the midline interpeduncular nucleus (IPN) of the adult rat and (2) the ability of the LC to respond to denervation of their target following removal of noradrenergic afferents (6-hydroxydopamine lesions of the LC) or non-noradrenergic afferents (lesion of the paired fasciculi retroflexi(FR]. Histofluorescence revealed that the NE innervation from the two LC to the IPN is symmetric and overlapping. This projection is confined to rostral, central, and intermediate subnuclei and is absent from lateral and dorsal subnuclei. We found no evidence for homotypic collateral sprouting of undamaged LC neurons into the IPN following unilateral LC lesion. Bilateral LC lesions also did not induce sprouting by NE-containing neurons from other systems (e.g. the superior cervical ganglion or the lateral tegmental group) or from those LC neurons that survived the 6-hydroxydopamine lesion. Histofluorescence following bilateral FR lesions confirmed an earlier observation that apparent hyperinnervation of the IPN by LC afferents is elicited following removal of non-noradrenergic afferents. Measurements of the turnover rate of NE in the IPN of control animals and those that received bilateral FR lesions indicate an increased NE content and increased turnover rate of NE in the IPN of lesioned animals. Taken together these results suggest an increased number of NE terminals and an increase in the activity of tyrosine hydroxylase. No change in NE content or turnover rate was seen in the frontal cortex from these same animals. This is consistent with a target-dependent regulation of heterotypic collateral sprouting.     

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.     

Neurochemical evidence for inflammation-induced activation of the coeruleospinal modulation system in the rat

By using the microdialysis technique, the concentration of noradrenaline (NA) in the dorsal horn during unilateral hindpaw inflammation was compared between rats receiving bilateral lesions of the locus coeruleus (LC) and non-operated control rats. Bilateral lesions of the LC were made using an anodal current one week before testing. Unilateral hindpaw inflammation was produced by a subcutaneous injection of carrageenan (6 mg in 0.15 ml saline). Under conditions of sodium pentobarbital anesthesia, the microdialysis probe was inserted into the dorsal horn either ipsilateral or contralateral to the site of inflammation. The NA concentration in the dialysate was measured by high-performance liquid chromatography with electrochemical detection. Prior to carrageenan injection, the NA level (baseline level) did not differ between the LC-lesioned and the non-operated groups. After carrageenan injection, in the non-operated rats, the NA level increased significantly compared to the baseline level only in the dorsal horn ipsilateral to the site of inflammation, but not in the dorsal horn contralateral to the site of inflammation. An increase of the NA level was not observed in the LC-lesioned rats and in rats receiving an injection of saline. The result suggests that unilateral hindpaw inflammation produces excitation of descending NA-containing neurons from the LC, resulting in an increase of the NA level in the dorsal horn ipsilateral to the site of inflammation.