Effect of superficial radial nerve stimulation on the activity of nigro-striatal dopaminergic neurons in the cat: role of cutaneous sensory input

Mediation of lateral hypothalamus orexin input to lateral habenula in the inhibitory effects of mechanical stimulation on psychomotor responses induced by cocaine

Introduction

The lateral hypothalamus (LH) plays an important physiological role in brain function and also plays an important role in substance abuse. The neuropeptides called orexin (or hypocretins) have been identified as being located exclusively in the cell bodies of the LH. Our previous studies have demonstrated that mechanical stimulation (MS) of the ulnar nerve produces strong inhibitory effects on cocaine addiction-like behaviors through activation of LH projection to the lateral habenula (LHb).

Methods

Therefore, the present study hypothesized that ulnar MS would suppress the psychomotor responses induced by cocaine through the orexinergic LH-to-LHb pathway.

Results

Ulnar MS attenuated cocaine enhancement of locomotor activity and 50-kHz ultrasonic vocalizations, which was prevented by antagonism of orexin-receptor type 2 (OX2R) in the LHb. Injection of orexin-A into the LHb reduced the cocaine-induced psychomotor responses. MS of the ulnar nerve excited LH orexinergic neurons. In addition, the excitation of LHb neurons by MS was blocked by the systemic administration of an OX2R antagonist.

Discussion

These findings suggest that MS applied to the ulnar nerve recruits an orexinergic LH-to-LHb pathway to suppress the psychomotor responses induced by cocaine.

Role of the central amygdala in acupuncture inhibition of methamphetamine-induced behaviors in rats

Methamphetamine (METH) enhances dopamine (DA) transmission in the mesolimbic system implicated in its reinforcing effects. Our previous studies have shown that acupuncture attenuates drug-seeking behaviors by modulating GABAergic transmission in the ventral tegmental area and DA release in the nucleus accumbens (NAc) of the striatum. The effects of acupuncture on METH-induced behaviors and its mediation by neural pathways remain a relatively understudied area of research. The central amygdala (CeA) plays a critical role in physiological and behavioral responses to somatosensory and drug stimuli and has been implicated in negative reinforcement. Thus, we evaluated the role of the CeA in acupuncture effects on locomotor activity, positive affective states, and DA release in the NAc following acute administration of METH. Acupuncture at acupoint HT7 reduced locomotor activity, 50-kHz ultrasonic vocalizations (USVs), and NAc DA release following systemic injection of METH, which was prevented by electrolytic lesions or optogenetic inhibition of the CeA. Acupuncture alone excited CeA neurons and reversed the suppression of CeA neurons induced by METH. These results suggest that acupuncture can relieve psychomotor responses and positive affective states following METH by inhibiting NAc DA release and this effect is mediated by activation of CeA neurons.

Spinal pathways involved in somatosensory inhibition of the psychomotor actions of cocaine

Previous studies have demonstrated that somatosensory stimuli influence dopamine transmission in the mesolimbic reward system and can reduce drug-induced motor behaviors, craving and dependence. Until now, the central links between somatosensory and brain reward systems are not known. Here, we show that the dorsal column (DC) somatosensory pathway contains projections that convey an inhibitory input from the periphery to mesolimbic reward circuits. Stimulation of the ulnar nerve under HT7 acupoint suppressed psychomotor response to cocaine, which was abolished by disruption of the DC pathway, but not the spinothalamic tract (STT). Low-threshold or wide-dynamic range neurons in the cuneate nucleus (CN) were excited by peripheral stimulation. Lesions of dorsal column or lateral habenula (LHb) prevented the inhibitory effects of peripheral stimulation on cocaine-induced neuronal activation in the nucleus accumbens (NAc). LHb neurons projecting to the ventral tegmental area (VTA)/rostromedial tegmental nucleus (RMTg) regions were activated by peripheral stimulation and LHb lesions reversed the inhibitory effects on cocaine locomotion produced by peripheral stimulation. These findings suggest that there exists a pathway in spinal cord that ascends from periphery to mesolimbic reward circuits (spino-mesolimbic pathway) and the activation of somatosensory input transmitted via the DC pathway can inhibit the psychomotor response to cocaine.

"On-line" measurement of acetylcholinesterase release from the substantia nigra of the freely-moving guinea-pig

Acetylcholinesterase is released from dopaminergic cells within the substantia nigra. The functional significance of this phenomenon has been studied in the freely-moving animal by a novel system for measuring acetylcholinesterase release from the substantia nigra "on-line" and in vivo. In the unanaesthetized guinea-pig the amount of acetylcholinesterase released was significantly greater than during anaesthesia, and release occurred in a more pulsatile manner. In addition, release of acetylcholinesterase could be evoked by either pharmacological or physiological manipulations, i.e. (1) a depolarizing concentration of potassium ions administered locally; (2) metamphetamine, administered systematically, which also resulted in increased locomotor activity; (3) drinking behaviour, elicited by presentation of a water bottle. Although all three treatments were accompanied by an increase in acetylcholinesterase release within the substantia nigra, potassium-evoked release did not cause any detectable change in behaviour. It is therefore suggested that release of the protein acetylcholinesterase within the substantia nigra is not necessarily a direct cause of locomotor activity: rather, it reflects diverse sensorimotor events.

Apomorphine and haloperidol effects on striatal 3H-dopamine release in anesthetized, awake restrained and freely moving rats

The ability of apomorphine (APO) and haloperidol (HAL) to affect the spontaneous release of newly synthesized 3H-DA in the striatum was studied in halothane anesthetized, gallamine paralyzed, awake restrained and freely moving rats. The striatum was continuously superfused through a push-pull cannula with a physiological medium enriched in 3H-tyrosine. Basal levels of 3H-DA release were different in the four experimental models: highest in halothane anesthetized rats, intermediate in awake restrained and gallamine treated rats and lowest in freely moving rats. In all experimental models IV or SC injection of APO (1 mg/kg) inhibited the release of 3H-DA (30-50%) from 15 to 90 min following its administration. In awake restrained and freely moving rats, stereotyped behaviour was observed for one hour following the APO injection. In halothane anesthetized rats the inhibitory effect of APO on 3H-DA release was prevented by pretreatment with HAL (2 mg/kg IV). Injection of HAL (2 mg/kg IV or SC) failed to enhance the release of 3H-DA in anesthetized and awake restrained rats, whilst a long-lasting increase in 3H-DA release was observed in gallamine treated and freely moving animals (55% and 120% respectively). However, catalepsy was observed in both restrained and freely moving rats. It is concluded that the modifications of 3H-DA release produced by HAL but not those produced by APO are dependent on the experimental model used, a fact possibly related to the different sites of action of these two drugs.

Substantia nigra dopaminergic unit activity in behaving cats: effect of arousal on spontaneous discharge and sensory evoked activity

Single-unit activity of dopaminergic neurons in the substantia nigra was recorded in freely moving cats during a variety of conditions designed to shed light on the hypotheses that these neurons are involved in the regulation of arousal-stress and/or selective attention. Both aversive and non-aversive arousing experimental conditions were used, including tail pinch, immersion of feet in ice-water, white noise, inaccessible food, feeding, grooming, inaccessible rats, and somatosensory stimulation. None of these conditions had an effect on tonic neuronal discharge rate. However, these neurons did exhibit brief excitatory and inhibitory responses to phasic auditory or visual stimuli presented when the cat was sitting quietly. These responses were dramatically attenuated if these stimuli were presented during the aforementioned conditions of behavioral arousal. This sharply contrasts with the inability of these same conditions to influence spontaneous discharge rate. The sensitivity of this neuronal sensory response to the concurrent behavioral condition supports the hypothesis that these neurons are involved in attentional processes or selective responding. The lack of responsiveness of these neurons to a variety of arousal/stress manipulations supports the hypothesis that dopaminergic neurons play a permissive, rather than an active, role in these processes.

Prolonged changes in dopaminergic terminal excitability and short changes in dopaminergic neuron discharge rate after short peripheral stimulation in monkey

Time-courses of responses to peripheral somatosensory stimulation were studied in the nigrostriatal dopamine (DA) system by comparing rates of neuronal discharges with changes in nerve terminal excitability, an indicator of DA release. The excitability of DA nerve terminals in the putamen was assessed as probability for evoking an antidromic response in substantia nigra DA cells with electrical stimulation in an anesthetized monkey. At about 30-60% decrease of excitability was seen during and about 15 min beyond pain pinch stimulation (PPS) in 12 of 17 tested DA neurons, while 4 neurons showed a 40% increase. Discharge rates were decreased in 7 and increased in 5 of the 17 DA neurons during, but not after PPS. It is concluded that the release of DA in the striatum may be controlled in two ways: rapid reactions would be mediated by changes in discharge rate, while slower, prolonged responses could be due to presynaptic interactions with other striatal afferents.

Recovery by push-pull perfusion of neurochemicals released within the cuneate nucleus of the cat by somatosensory stimulation

The present work describes a combination of techniques for the identification of neurochemicals released within the cuneate nucleus. During electrical stimulation of the superficial radial nerve, the extracellular fluid of the nucleus is continuously sampled by push-pull perfusion. In addition, the population electrical activity of peripheral nerve as well as the activity of cuneate neurons are recorded. Subsequently, the neurochemical content of the sampled fluid is assessed by HPLC analysis. The comparison of sampled fluid content during control (no stimulation) versus stimulation runs indicates that somatosensory stimulation elicits the release of specific neurochemicals within the cuneate nucleus. The possible sources of released neurochemicals are discussed.

Environmental stimuli but not homeostatic challenges produce apparent increases in dopaminergic activity in the striatum: an analysis by in vivo voltammetry

Using in vivo voltammetry in rats, we examined the relationship between the electrochemical signal measured in the striatum and the behavioral responses associated with various types of stimulation. Three patterns emerged. First, a series of homeostatic challenges, including abrupt decreases in glucose utilization, blood volume, or arterial blood pressure, were ineffective in altering the electrochemical signal despite the sympathoadrenal response produced by each. Second, intense exteroceptive stimuli, such as an electric shock applied to the tail or placing animals in a shallow ice-water bath, provoked large and abrupt rises in the signal which decayed rapidly. Third, rats eating after a 24-h fast, drinking after a period of dehydration, or presented with novel olfactory or visual stimuli, exhibited much smaller and more gradual rises in the electrochemical signal which were more long-lasting. In each case, the magnitude of the change in electrochemical signal was generally related to the level of behavioral activation, being most prominent when treatments produced a startle response. Those large increases in signal were markedly attenuated by pretreatment with alpha-methyl-p-tyrosine or gamma-butyrolactone, drugs known to decrease the release of dopamine, suggesting that the signal observed was associated with an increase in the activity of central dopaminergic neurons.

Effects of unilateral electrical stimulation of various thalamic nuclei on the release of dopamine from dendrites and nerve terminals of neurons of the two nigrostriatal dopaminergic pathways

The role of several motor and intralaminar thalamic nuclei in the regulation of dopamine release from terminals and dendrites of the nigrostriatal dopaminergic neurons was investigated in halothane-anaesthetized cats. For this purpose, the effects of the unilateral electrical stimulation of various thalamic nuclei on the release of newly synthesized [3H]dopamine were simultaneously determined in both substantiae nigrae and caudate nuclei using the push-pull cannula method. The electrical stimulation of the motor nuclei was the only one to induce asymmetric changes in the four structures since [3H]dopamine release was enhanced in the ipsilateral caudate nucleus and reduced in the contralateral structure while opposite responses were observed in the corresponding substantiae nigrae. A reduction of [3H]dopamine release occurred in the four structures or only in the contralateral substantia nigra and caudate nucleus following the stimulation of the parafascicularis nucleus and the adjacent posterior part of the nucleus centrum medianum or of the nucleus centralis lateralis and the adjacent paralaminar part of the nucleus medialis dorsalis, respectively. The stimulation of the anterior part of the nucleus centrum medianum, which in contrast to other thalamic nuclei examined, receives few nigral inputs, selectively enhanced [3H]dopamine release in the contralateral substantia nigra. No significant changes in [3H]dopamine release were seen either in the substantiae nigrae or in the caudate nuclei following the stimulation of midline thalamic nuclei. These results indicate that the motor and intralaminar thalamic nuclei exert multiple and selective influences on the release of dopamine from terminals and/or dendrites of the dopaminergic neurons. They also further support a role of thalamic nuclei in the transfer of information from one substantia nigra to the contralateral dopaminergic neurons. The possible involvement of connections between paired thalamic nuclei was underlined by the observations of evoked potentials in contralateral homologous nuclei following unilateral stimulation of motor, or some intralaminar, nuclei. The present report provides new insights on the mechanisms contributing to the reciprocal and/or bilateral regulations of nigrostriatal dopaminergic pathways.