Opsin evolution in the Ambulacraria.

Opsins--G-protein coupled receptors involved in photoreception--have been extensively studied in the animal kingdom. The present work provides new insights into opsin-based photoreception and photoreceptor cell evolution with a first analysis of opsin sequence data for a major deuterostome clade, the Ambulacraria. Systematic data analysis, including for the first time hemichordate opsin sequences and an expanded echinoderm dataset, led to a robust opsin phylogeny for this cornerstone superphylum. Multiple genomic and transcriptomic resources were surveyed to cover each class of Hemichordata and Echinodermata. In total, 119 ambulacrarian opsin sequences were found, 22 new sequences in hemichordates and 97 in echinoderms (including 67 new sequences). We framed the ambulacrarian opsin repertoire within eumetazoan diversity by including selected reference opsins from non-ambulacrarians. Our findings corroborate the presence of all major ancestral bilaterian opsin groups in Ambulacraria. Furthermore, we identified two opsin groups specific to echinoderms. In conclusion, a molecular phylogenetic framework for investigating light-perception and photobiological behaviors in marine deuterostomes has been obtained.

Stimulation of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception.

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.

Body region shocked need not critically define the neurochemical basis of stress analgesia.

Both opioid and non-opioid forms of stress-induced analgesia have been demonstrated in rats, although the conditions leading to their selective activation are still being investigated. We have shown that variations in shock intensity, duration or temporal pattern can determine whether opioid or non-opioid stress analgesia occurs. Others have suggested that body region shocked is the critical determinant, analgesia from front paw shock being opioid and that from hind paw shock non-opioid. We now report that either opioid or non-opioid stress analgesia can be evoked from either front or hind paws depending only on footshock intensity when duration and temporal pattern are held constant.

Intrinsic mechanisms of pain inhibition: activation by stress.

Portions of the brain stem seem normally to inhibit pain. In man and laboratory animals these brain areas and pathways from them to spinal sensory circuits can be activated by focal stimulation. Endogenous opioids appear to be implicated although separate nonopioid mechanisms are also evident. Stress seems to be a natural stimulus triggering pain suppression. Properties of electric footshock have been shown to determine the opioid or nonopioid basis of stress-induced analgesia. Two different opioid systems can be activated by different footshock paradigms. This dissection of stress analgesia has begun to integrate divergent findings concerning pain inhibition and also to account for some of the variance that has obscured the reliable measurement of the effects of stress on tumor growth and immune function.

Involvement of central muscarinic cholinergic mechanisms in opioid stress analgesia.

Stress has been shown capable of differentially activating opioid- and non-opioid-mediated endogenous analgesia systems. In this study, the muscarinic cholinergic antagonist, scopolamine, but not the centrally inactive methylscopolamine, blocks opioid, but not non-opioid stress analgesia. Additionally, naltrexone, an opiate antagonist, attenuates analgesia induced by oxotremorine, a cholinergic agonist. These findings support the existence of a muscarinic cholinergic synapse in a central nervous system opioid pain-inhibitory pathway.

Evidence for the independence of brainstem mechanisms mediating analgesia induced by morphine and two forms of stress.

Exposure to inescapable footshock stress causes potent analgesia in the rat. According to several criteria, prolonged, intermittent footshock elicits analgesia mediated by opioid peptides, whereas brief, continuous footshock produces non-opioid analgesia. We now report that these neurochemically discrete forms of stress analgesia also have different neuroanatomical bases. Electrolytic lesions damaging greater than 85% of the n. raphe magnus ('complete' NRM lesions), but not lesions of the same size causing less NRM damage (partial NRM lesions) significantly reduce only the non-opioid form of stress analgesia. In the same animals, complete and partial NRM lesions disrupt morphine analgesia; however, our analyses indicate that this effect is not mediated by the same substrate involved in either form of stress analgesia. These results support the existence of multiple endogenous analgesia mechanisms and indicate a complex role for the NRM in these systems.

N. raphe magnus lesions disrupt stimulation-produced analgesia from ventral but not dorsal midbrain areas in the rat.

We previously found that the opiate antagonist, naloxone, partially blocks stimulation-produced analgesia (SPA) elicited from ventral but not dorsal regions of the medial midbrain in rats. The present study compares the effects of n. raphe magnus (NRM) lesions on SPA from these same two midbrain areas. SPA thresholds were measured with the tail-flick method and compared before and for up to two weeks after NRM lesions. A high positive correlation was found between percent NRM destruction and percent increase in SPA threshold for rats with ventral but not dorsal electrode placements. Damage to brain areas other than NRM seemed not to contribute to these effects. We conclude that n. raphe magnus is a critical relay in the pain-suppressive path from that area of the rat midbrain mediating an opioid form of stimulation-produced analgesia.

Evidence for opioid and non-opioid forms of stimulation-produced analgesia in the rat.

This study compares stimulation-produced analgesia (SPA) elicited from two different midline regions of the midbrain of the rat. Dorsal electrode placements were in the caudal periaqueductal gray matter; ventral placements lay within or subjacent to the dorsal raphe n. SPA thresholds were measured by the tail-flick method both during and immediately after the period of brain stimulation. Thresholds were consistently higher in the post-stimulation test. SPA from dorsal and ventral regions differed in the following ways: (1) Post-stimulation analgesia was significantly more difficult to obtain in ventral than in dorsal regions, whereas during-stimulation analgesia did not vary as a function of electrode location; (2) Although a continuous distribution of thresholds was seen for ventral placements, thresholds for dorsal placements tended to be either high or low on both during- and post-stimulation tests; (3) Naloxone (0.01--10 mg/kg) reliably elevated SPA thresholds for ventral but not dorsal stimulation placements. We conclude that different substrates of SPA lie in close proximity to one another in the medial midbrain of the rat. This portion of the midbrain appears to mediate both opioid and non-opioid mechanisms of analgesia.

Effects of naloxone and hypophysectomy on electroconvulsive shock-induced analgesia.

Powerful analgesia follows electroconvulsive shock in both hypophysectomized and sham-operated rats. Antagonism of this analgesia by naloxone implicates opioid peptides in its mediation, its occurrence in hypophysectomized animals implicating opioids of central nervous system rather than pituitary origin. Because naloxone only partially reduces electroconvulsive shock analgesia in hypophysectomized rats, the participation of another, non-opioid analgesia substrate also seems indicated.

Opioid and nonopioid mechanisms of stress analgesia.

Inescapable foot shock in rats caused profound analgesia that was antagonized by naloxone or dexamethasone when shock was delivered intermittently for 30 minutes, but not when it was delivered continuously for 3 minutes. Thus, depending only on its temporal characteristics, foot-shock stress appears to activate opioid or nonopioid analgesia mechanisms. Certain forms of stress may act as natural inputs to an endogenous opiate analgesia system.

Long ascending projections from substantia gelatinosa Rolandi and the subjacent dorsal horn in the rat.

Small neurons of the substantia gelatinosa Rolandi and the subjacent dorsal horn of the spinal cord have been thought to exert a direct modulatory effect only on neurons located within a distance of a few spinal segemnts. By using the technique of retorograde transport of horseradish peroxidase, however, it has been found that in the rat a significant number of these cells, particularly those of the subjacent dorsal horn, ascend many spinal segments to the lateral cervical nucleus and to the lower brainstem. These data provide an anatomic basis for a role of substantia gelatinosa Rolandi and subjacent dorsal horn cells in madulating or contributing to sensory information transmission not only in nearby segments but in far distant structures.