Three times as many lamina I neurons project to the periaqueductal gray than to the thalamus: a retrograde tracing study in the cat

Social touch-like tactile stimulation activates a tachykinin 1-oxytocin pathway to promote social interactions

It is well known that affective and pleasant touch promotes individual well-being and facilitates affiliative social communication, although the neural circuit that mediates this process is largely unknown. Here, we show that social-touch-like tactile stimulation (ST) enhances firing of oxytocin neurons in the mouse paraventricular hypothalamus (PVH) and promotes social interactions and positively reinforcing place preference. These results link pleasant somatosensory stimulation to increased social interactions and positive affective valence. We further show that tachykinin 1 (Tac1⁺) neurons in the lateral and ventrolateral periaqueductal gray (l/vlPAG) send monosynaptic excitatory projections to PVH oxytocin neurons. Functionally, activation of PVH-projecting Tac1⁺ neurons increases firing of oxytocin neurons, promotes social interactions, and increases preference for the social touch context, whereas reducing activity of Tac1⁺ neurons abolishes ST-induced oxytocin neuronal firing. Together, these results identify a dipeptidergic pathway from l/vlPAG Tac1⁺ neurons to PVH oxytocin neurons, through which pleasant sensory experience promotes social behavior.

Eliminating the VGlut2-Dependent Glutamatergic Transmission of Parvalbumin-Expressing Neurons Leads to Deficits in Locomotion and Vocalization, Decreased Pain Sensitivity, and Increased Dominance

The calcium-binding protein parvalbumin (PV) is a recognized marker of short-axon GABA-ergic neurons in the cortex and the hippocampus. However in addition, PV is expressed by excitatory, glutamatergic neurons in various areas of the brain and spinal cord. Depending on the location of these neurons, loading of their synaptic vesicles with glutamate is mediated by either of three vesicular glutamate transporters (VGlut): VGlut1, VGlut2, or VGlut3. Driven by our interest in one of these glutamatergic/PV-expressing cell clusters-the lateral hypothalamic parvafox nucleus-we investigated the functions of this population of neurons by the selective deletion of VGlut2 expression in PV-expressing cells according to the Cre/Lox-approach. PV-Cre;VGlut2-Lox mutant mice are phenotypically characterized by deficits in locomotion and vocalization, by a decreased thermal nociception, and by an increased social dominance. We conducted a search of the Allen Brain Atlas for regions that might co-express the genes encoding PV and VGlut2, and that might thus contribute to the manifestation of the observed phenotypes. Our survey revealed several structures that could contribute to the deficits in locomotion and vocalization, such as the red, the subthalamic and the deep cerebellar nuclei. It also disclosed that a shift in the balance of afferental glutamatergic neurotransmission to the periaqueductal gray matter might be accountable for the decrease in sensitivity to pain and for the increase in social dominance. As a whole, this study broadens the state of knowledge about PV-expressing excitatory neurons.

A quantitative study of spinothalamic neurons in laminae I, III, and IV in lumbar and cervical segments of the rat spinal cord

The major ascending outputs from superficial spinal dorsal horn consist of projection neurons in lamina I, together with neurons in laminae III–IV that express the neurokinin 1 receptor (NK1r) and have dendrites that enter the superficial laminae. Some neurons in each of these populations belong to the spinothalamic tract, which conveys nociceptive information via the thalamus to cortical areas involved in pain. A projection from the cervical superficial dorsal horn to the posterior triangular nucleus (PoT) has recently been identified. PoT is at the caudal end of the thalamus and was not included in injection sites in many previous retrograde tracing studies. We have injected various tracers (cholera toxin B subunit, Fluoro-Gold, and fluorescent latex microspheres) into the thalamus to estimate the number of spinothalamic neurons in each of these two populations, and to investigate their projection targets. Most lamina I and lamina III/IV NK1r-immunoreactive spinothalamic neurons in cervical and lumbar segments could be labeled from injections centered on PoT. Our results suggest that there are 90 lamina I spinothalamic neurons per side in C7 and 15 in L4 and that some of those in C7 only project to PoT. We found that 85% of the lamina III/IV NK1r-immunoreactive neurons in C6 and 17% of those in L5 belong to the spinothalamic tract, and these apparently project exclusively to the caudal thalamus, including PoT. Because PoT projects to second somatosensory and insular cortices, our results suggest that these are major targets for information conveyed by both these populations of spinothalamic neurons.

The calcium binding proteins calbindin, parvalbumin, and calretinin have specific patterns of expression in the gray matter of cat spinal cord

Calcium binding proteins (CBPs) regulate intracellular levels of calcium (Ca(2+)) ions. CBPs are particularly interesting from a morphological standpoint, because they are differentially expressed in certain sub-populations of cells in the nervous system of various species of vertebrate animals. However, knowledge on the cellular regulation governing such cell-specific CBP expression is still incomplete. In this work on the L7 segment of the cat spinal cord, we analyzed the localization and morphology of neurons expressing the CBPs calbindin-28 KD (CB), parvalbumin (PV), and calretinin (CR), and co-expressing CB and PV, CB and CR, and PV and CR. Single CBP-positive ((+)) neurons showed specific distributions: (1) CB was present in small neurons localized in laminae I, II, III and X, in small to medium size neurons in laminae III-VI, and in medium to large neurons in laminae VI-VIII; (2) PV was present in small size neurons in laminae III and IV and in medial portions of laminae V and VI, medium neurons and in lamina X at the border with lamina VII, in medium to large neurons in laminae VII and VIII; (3) CR labeling was detected in small size neurons in laminae I, II, III and VIII, in medium to large size neurons in laminae I and III-VII, and in small to medium size neurons in lamina X. Double labeled neurons were a small minority of the CBP(+) cells. Co-expression of CB and PV was seen in 1 to 2% of the CBP(+) cells, and they were detected in the ventral and intermediate portions of lamina VII and in lamina X. Co-localization of CB and CR was present in 0.3% of the cells and these cells were localized in lamina II. Double labeling for PV and CR occurred in 6% of the cells, and the cells were localized in ventral part of lamina VII and in lamina VIII. Overall, these results revealed distinct and reproducible patterns of localization of the neurons expressing single CBPs and co-expressing two of them. Distinct differences of CBP expression between cat and other species are discussed. Possible relations between the cat L7 neurons expressing different CBPs with the neurons previously analyzed in cat and other animals are suggested.

In cat four times as many lamina I neurons project to the parabrachial nuclei and twice as many to the periaqueductal gray as to the thalamus

The spinothalamic tract, and especially its fibers originating in lamina I, is the best known pathway for transmission of nociceptive information. On the other hand, different studies have suggested that more lamina I cells project to the parabrachial nuclei (PBN) and periaqueductal gray (PAG) than to the thalamus. The exact ratio of the number of lamina I projections to PBN, PAG and thalamus is not known, because comprehensive studies examining these three projections from all spinal segments, using the same tracers and counting methods, do not exist. In the present study, the differences in number and distribution of retrogradely labeled lamina I cells in each segment of the cat spinal cord (C1-Coc2) were determined after large wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections in either PBN, PAG or thalamus. We estimate that approximately 6000 lamina I cells project to PBN, 3000 to PAG and less than 1500 to the thalamus. Of the lamina I cells projecting to thalamus or PAG more than 80%, and of the lamina I-PBN cells approximately 60%, were located on the contralateral side. In all cases, most labeled lamina I cells were found in the upper two cervical segments and in the cervical and lumbar enlargements.

Anatomy and neurochemistry of the pair bond

Studies in monogamous rodents have begun to elucidate the neural circuitry underlying the formation and maintenance of selective pair bonds between mates. This research suggests that at least three distinct, yet interconnected, neural pathways interact in the establishment of the pair bond. These include circuits involved in conveying somatosensory information from the genitalia to the brain during sexual activity, the mesolimbic dopamine circuits of reward and reinforcement, and neuropeptidergic circuits involved specifically in the processing of socially salient cues. Here we present an integrated description of the interaction of these circuits in a model of pair bond formation in rodents with a discussion of the implications of these findings for evolution, individual variation, and human bonding.

Alterations in dorsal horn neurones in a rat model of cancer-induced bone pain

Cancer-induced bone pain is a major clinical problem. A rat model based on intra-tibial injection of MRMT-1 mammary tumour cells was used to mimic progressive cancer-induced bone pain. At the time of stable behavioural changes (decreased thresholds to mechanical and cold stimuli) and bone destruction, in vivo electrophysiology was used to characterize natural (mechanical, thermal, and cold) and electrical-evoked responses of superficial and deep dorsal horn neurones in halothane-anaesthetized rats. Receptive field size was significantly enlarged for superficial neurones in the MRMT-1 animals. Superficial cells were characterised as either nociceptive specific (NS) or wide dynamic range (WDR). The ratio of WDR to NS cells was substantially different between sham operated (growth media alone) (26:74%) and MRMT-1 injected rats (47:53%). NS cells showed no significant difference in their neuronal responses in MRMT-1-injected compared to sham rats. However, superficial WDR neurones in MRMT-1-injected rats had significantly increased responses to mechanical, thermal and electrical (A beta-, C fibre-, and post-discharge evoked response) stimuli. Deep WDR neurones showed less pronounced changes to the superficial dorsal horn, however, the response to thermal and electrical stimuli, but not mechanical, were significantly increased in the MRMT-1-injected rats. In conclusion, the spinal cord is significantly hyperexcitable with previously superficial NS cells becoming responsive to wide-dynamic range stimuli possibly driving this plasticity via ascending and descending facilitatory pathways. The alterations in superficial dorsal horn neurones have not been reported in neuropathy or inflammation adding to the evidence for cancer-induced bone pain reflecting a unique pain state.

A quantitative and morphological study of projection neurons in lamina I of the rat lumbar spinal cord

In the rat lumbar spinal cord the major supraspinal targets for lamina I projection neurons are the caudal ventrolateral medulla (CVLM), lateral parabrachial area (LPb) and periaqueductal grey matter (PAG). In this study we have estimated the number of lamina I neurons retrogradely labelled from each of these sites in the L4 segment, as well as the proportion that can be labelled by injecting different tracers into two separate sites. Our results suggest that this segment contains approximately 400 lamina I projection neurons on each side, and that approximately 85% of these can be labelled from either the CVLM or the LPb on the contralateral side. Around 120 lamina I cells in L4 project to the PAG, and over 90% of these cells can also be labelled from the CVLM or LPb. Most lamina I neurons projecting to CVLM or LPb are located in the contralateral dorsal horn, but in each case some cells were found to have bilateral projections. We also examined horizontal sections to investigate morphology and the expression of the neurokinin 1 (NK1) receptor in cells labelled from CVLM, LPb or PAG. There were no consistent morphological differences between these groups, however, while cells with strong or moderate NK1 receptor-immunostaining were labelled from LPb or CVLM, they seldom projected to the PAG. These results suggest that many lamina I cells project to more than one site in the brain and that those projecting to PAG may represent a distinct subclass of lamina I projection neuron.

Differential distribution of calbindin D28k and parvalbumin among functionally distinctive sets of structures in the macaque brainstem

In a study of brainstem in the cynomolgus monkey, we found that the distribution of calbindin D28K (CB) and parvalbumin (PV) is nonoverlapping among functionally distinct sets of brainstem structures. Nuclei involved in representation and regulation of the organism's internal state contain CB, whereas those involved in the representation of the external environment and the representation or execution of externally directed actions contain only PV. Moreover, our findings indicate that different nuclei known as components of the ascending reticular activating system (ARAS) contain either CB or PV or both, suggesting that this system in primates operates with both CB and PV. In line with previously reported findings, we also found that unmyelinated pathways contain only CB, whereas myelinated pathways contain PV. Distribution of CB and PV in the macaque brainstem follows a pattern comparable to, but in some instances significantly different than, the pattern previously reported in the rat. We argue that the nonoverlapping distribution of CB and PV among different structures of the brainstem might reflect underlying differences in the physiological, anatomic, and perhaps phylogenetic properties of these structures. Considering our recent findings of selective vulnerability of brainstem structures to Alzheimer's disease, the present data suggest that the majority of macaque brainstem nuclei that contain CB are vulnerable to neurofibrillary tangles in humans. By contrast, only few nuclei that contain PV exhibit pathologic changes. Some of these nuclei are affected with a high number of neuritic plaques without ever developing neurofibrillary tangles.

Endocannabinoids in pain modulation

Five major approaches have been employed to determine the role of endocannabinoids in pain modulation: (1) studies of various markers of endocannabinoid action aimed at determining whether the necessary cannabinoid biochemical machinery is present in those brain areas that control pain sensitivity; (2) administration of exogenous cannabinoids to determine whether endocannabinoid action at appropriate sites would lead to a loss of pain sensitivity; (3) administration of compounds that would affect endocannabinoid action such as antagonists and transport inhibitors to determine whether drug-induced preterbation of cannabinoid action would alter pain sensitivity; (4) studies of genetically altered animals aimed at determining whether pain responses or responses to cannabinergic drugs are altered; and (5) studies that measure the release of endocannabinoids. Converging evidence from each of these research areas indicates that endocannabinods function to control pain in parallel with endogenous opioids but via different mechanisms.

Parallel circuits mediating distinct emotional coping reactions to different types of stress

All animals, including humans, react with distinct emotional coping strategies to different types of stress. Active coping strategies (e.g. confrontation, fight, escape) are evoked if the stressor is controllable or escapable. Passive coping strategies (e.g. quiescence, immobility, decreased responsiveness to the environment) are usually elicited if the stressor is inescapable and help to facilitate recovery and healing. Neural substrates mediating active versus passive emotional coping have been identified within distinct, longitudinal neuronal columns of the midbrain periaqueductal gray (PAG) region. Active coping is evoked by activation of either the dorsolateral or lateral columns of the PAG; whereas passive coping is triggered by activation of the ventrolateral PAG. Recent anatomical studies indicate that each PAG column receives a distinctive set of ascending (spinal and medullary) and descending (prefrontal cortical and hypothalamic) afferents. Consistent with the anatomy, functional studies using immediate early gene expression (c-fos) as a marker of neuronal activation have revealed that the preferential activation of a specific PAG column reflects (i) the type of emotional coping reaction triggered, and (ii) whether a physical or psychological stressor was used.

Consciousness and the brainstem

In the first part of this article we summarize a theoretical framework and a set of hypotheses aimed at accounting for consciousness in neurobiological terms. The basic form of consciousness, core consciousness is placed in the context of life regulation; it is seen as yet another level of biological processing aimed at ensuring the homeostatic balance of a living organism; and the representation of the current organism state within somato-sensing structures is seen as critical to its development. Core consciousness is conceived as the imaged relationship of the interaction between an object and the changed organism state it causes. In the second part of the article we discuss the functional neuroanatomy of nuclei in the brainstem reticular formation because they constitute the basic set of somato-sensing structures necessary for core consciousness and its core self to emerge. The close relationship between the mechanisms underlying cortical activation and the bioregulatory mechanisms outlined here is entirely compatible with the classical idea that the reticular formation modulates the electrophysiological activity of the cerebral cortex. However, in the perspective presented here, that modulation is placed in the setting of the organism's homeostatic regulation.

Lamina I-periaqueductal gray (PAG) projections represent only a limited part of the total spinal and caudal medullary input to the PAG in the cat

The periaqueductal gray is well known for its involvement in nociception control, but it also plays an important role in the emotional motor system. To accomplish these functions the periaqueductal gray receives input from the limbic system and from the caudal brainstem and spinal cord. Earlier studies gave the impression that the majority of the periaqueductal gray projecting cells in caudal brainstem and spinal cord are located in the contralateral lamina I, which is involved in nociception. The present study in the cat, however, demonstrates that of all periaqueductal gray projecting neurons in the contralateral caudal medulla less than 7% was located in lamina I. Of the spinal periaqueductal gray projecting neurons less than 29% was located in lamina I. However, within the spinal cord large segmental differences exist: in few segments of the enlargements the lamina I-periaqueductal gray projecting neurons represent a majority. In conclusion, although the lamina I-periaqueductal gray projection is a very important nociceptive pathway, it constitutes only a limited part of the total projection from the caudal medulla and spinal cord to the periaqueductal gray. These results suggest that a large portion of the medullo- and spino-periaqueductal gray pathways conveys information other than nociception.

Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and midline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in discrete regions of the PAG, and axonal labeling was examined in the thalamus. PHA-L was also placed into the dorsal raphe nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nuclei-lateral parafascicular, medial parafascicular, central lateral (CL), paracentral (PC), or central medial nucleus-or one of the midline thalamic nuclei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The distribution of CTb labeled neurons in the PAG was then mapped. All PAG regions (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to most of the other intralaminar and midline thalamic nuclei. PAG inputs to the VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventroposterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nociceptive-, and behavior-related forebrain circuits associated with defense and emotional responses.

Pain modulation by release of the endogenous cannabinoid anandamide

Synthetic cannabinoids produce behavioral analgesia and suppress pain neurotransmission, raising the possibility that endogenous cannabinoids serve naturally to modulate pain. Here, the development of a sensitive method for measuring cannabinoids by atmospheric pressure-chemical ionization mass spectrometry permitted measurement of the release of the endogenous cannabinoid anandamide in the periaqueductal gray (PAG) by in vivo microdialysis in the rat. Electrical stimulation of the dorsal and lateral PAG produced CB1 cannabinoid receptor-mediated analgesia accompanied by a marked increase in the release of anandamide in the PAG, suggesting that endogenous anandamide mediates the behavioral analgesia. Furthermore, pain triggered by subcutaneous injections of the chemical irritant formalin substantially increased the release of anandamide in the PAG. These findings indicate that the endogenous cannabinoid anandamide plays an important role in a cannabinergic pain-suppression system existing within the dorsal and lateral PAG. The existence of a cannabinergic pain-modulatory system may have relevance for the treatment of pain, particularly in instances where opiates are ineffective.