Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia

Region-specific effects of nicotine on brain activity: a pharmacological MRI study in the drug-naïve rat

We have applied pharmacological magnetic resonance imaging (phMRI) methods to map the functional response to nicotine in drug-naïve rats. Nicotine (0.35 mg/kg intravenous (i.v.)) increased relative cerebral blood volume (rCBV) in cortical (including medial prefrontal, cingulate orbitofrontal, insular) and subcortical (including amygdala and dorsomedial hippocampus) structures. The pharmacological specificity of the effect was demonstrated by acute pretreatment with the nicotinic acetylcholine receptor (nAChR) ion-channel-blocking agent mecamylamine, which suppressed the rCBV response to nicotine. Control experiments with norepinephrine, a potent non-brain-penetrant vasopressor, at a dose that mimics the cardiovascular response induced by nicotine were performed to assess the potential confounding effects of peripheral blood pressure changes induced by nicotine. In an attempt to highlight the relative contribution of different nAChR subtypes to the observed activation pattern of nicotine, we also investigated the central phMRI response to an acute challenge with (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (cpdA, at 5, 10, 20, and 30 mg/kg i.v.) and 5-iodo-A-85380 (5IA, 5 mg/kg i.v.). CpdA is a selective agonist at homomeric alpha7 nAChRs, while 5IA features high in vivo affinity for the alpha4beta2* and other less-abundant beta2-containing nicotinic receptors. CpdA did not produce significant rCBV changes at any of the doses tested, whereas 5IA induced a pattern of activation very similar to that induced by nicotine. The lack of phMRI response to cpdA together with the high spatial overlap between the activation profile of nicotine and 5IA, suggest that the acute functional response to nicotine in drug-naïve rats is mediated by beta2-containing nAChR isoforms, presumably belonging to the alpha4beta2* subtype.

Response of the brain to oligemia: gene expression, c-Fos, and Nrf2 localization

Oligemia is blood flow reduction without acute tissue damage that occurs in shock, migraine, and stroke penumbra. We developed a mouse model of oligemia by lowering mean arterial pressure to 30-40 mm Hg, resulting in a 50% reduction in cerebral blood flow as measured by laser Doppler, and reperfusing the blood after 30 min. Control experiments included anesthesia-only and surgery without blood withdrawal. Using immunohistochemistry, we localized the transcription factors Nrf2, which regulates expression of antioxidant and detoxification protein, and c-Fos, a marker of neuronal activation. Nrf2 was found only in oligemia mice and was localized in neurons of the cingulate cortex and cerebellar Purkinje cells. By contrast, c-Fos was found widely expressed in both groups and was localized in neurons in regions associated with response to stress, immunomodulation, and fluid homeostasis, including the periaqueductal gray and periventricular nucleus. These data indicate that c-Fos expression occurs as a result of surgical stress, but Nrf-2 upregulation is specific to oligemia. The CLONTECH Atlas 1.2 Mouse Array was used to assess genes that were up or down-regulated in oligemia versus surgery controls. Of 1176 genes, 29 differed between oligemia and surgery groups. Upregulation of oxidative stress induced (OSI) protein, heat shock protein (HSP) 84 and transthyretin (TTR) precursor in the oligemia group was confirmed with RT-PCR. The expression of HSP 84, transthyretin precursor, and OSI genes adds further evidence that oligemia induces an oxidative stress response in the brain.

Phasic activation of substantia nigra and the ventral tegmental area by chemical stimulation of the superior colliculus: an electrophysiological investigation in the rat

The source of short-latency visual input to midbrain dopaminergic (DA) neurons is not currently known; however, the superior colliculus (SC) is a subcortical visual structure which has response latencies consistently shorter than those recorded for DA neurons in substantia nigra and the ventral tegmental area. To test whether the SC represents a plausible route by which visual information may gain short latency access to the ventral midbrain, the present study examined whether experimental stimulation of the SC can influence the activity of midbrain DA neurons. In urethane-anaesthetized rats, 63 pairs of extracellular recordings were obtained from neurons in the SC and ipsilateral ventral midbrain, before and after local disinhibitory injections of the GABA antagonist bicuculline (20-40 ng/200-400 nL saline) into the SC. Neurons recorded from substantia nigra and the ventral tegmental area were classified as putative DA (25/63, 39.7%) or putative non-DA (38/63, 60.3%). In nearly half the cases (27/63, 42.8%), chemical stimulation of the SC evoked a corresponding increase in neural activity in the ventral midbrain. This excitatory effect did not distinguish between DA and non-DA neurons. In 6/63 cases (9.5%), SC activation elicited a reliable suppression of activity, while the remaining 30/63 cases (47.6%) were unaffected. In almost a third of cases (16/57, 28.1%) intense phasic activation of the SC was associated with correlated phasic activation of neurons in substantia nigra and the ventral tegmental area. These data suggest that the SC is in a position to play an important role in discriminating the appropriate stimulus qualities required to activate DA cells at short latency.

Covariant maturation of nocifensive oral behaviour and c-fos expression in rat superior colliculus

Injections of formalin into the rodent paw elicit a rapid orientation of the head and mouth to the source of discomfort, followed by licking and biting the injected area. Previous work has shown this response is dependent on the integrity of the midbrain superior colliculus. The present experiments were initiated to examine the ontogeny of this oral nocifensive reaction and to determine whether it is correlated with the functional maturation of collicular responses to noxious stimuli (as indicated by c-fos immunohistochemistry). Rat pups at various postnatal ages received formalin injections in either the hindpaw or perioral regions. Behaviour was videotaped, and after 120 min, animals were killed and the brain and spinal cord processed for Fos-like immunoreactivity. Uninjected controls were treated identically. Formalin-induced oral responses following injections into the hindpaw and the expression of Fos in the superior colliculus were virtually absent until 10 days postnatal, despite the presence of Fos-like immunoreactivity in many other structures (e.g. spinal cord, parabrachial area, periaqueductal grey). In contrast, animals from day 1 were able to use limbs to localise the perioral injection site. From day 10 onward, there was a progressive increase in oral nocifensive behaviours and Fos expression in the superior colliculus. Our observations are consistent with the hypothesis that the normal elaboration of pain-induced oral behaviour is initiated only after a functionally active superior colliculus has developed, and support previous observations that link the colliculus particularly with oral nocifensive behaviours.

Basal increase in c-Fos-like expression in superior colliculus of Royal College of Surgeons dystrophic rats can be abolished by intraocular injection of tetrodotoxin

In normal rats maintained in the dark, very few cells in the primary visual centers, including the superior colliculus, show Fos-like immunoreactivity. By contrast, in rats presented with flashing lights many Fos-like immunoreactivity cells are observed distributed throughout the visual centers. In the dystrophic Royal College of Surgeons rat, in which there is major loss of photoreceptors over the first 3 months of life, similar numbers of Fos-like immunoreactivity cells are seen on light presentation, but in marked contrast, cell densities in the rats maintained in the dark are many times higher than in non-dystrophic rats maintained under similar conditions. Here we show that this elevated dark response can be abolished by intravitreal injection of the sodium channel blocker tetrodotoxin, indicating that this effect results from changed retinal activity, rather than being centrally generated. We suggest that since Fos-like immunoreactivity is not usually elicited by steady state conditions, the elevated levels in the superior colliculus in these animals reflect the return of waves of activity, first seen in development coursing across the retina, but lost with photoreceptor maturation.