Ipsilateral Medial Forebrain Bundle Research Articles

Failure of caspase inhibition in the double-lesion rat model of striatonigral degeneration (multiple system atrophy)

In the present study we assessed the neuroprotective effects of the pan-caspase inhibitor z-VAD.fmk [N-benzyloxycarbony-valine-alanine-aspartate-(OMe)-fluoromethylketone], and the caspase-3 inhibitor Ac-DEVD.CHO (acetyl-aspartate-chloromethylketone) in the double-lesion rat model of striatonigral degeneration (SND), the core pathology underlying levodopa-unresponsive parkinsonism associated with multiple system atrophy (MSA). Male Wistar rats were divided into three groups, receiving either Ac-DEVD.CHO, z-VAD.fmk or normal saline before lesion surgery, comprising a sequential unilateral quinolinic acid (QA) lesion of the striatum followed by a 6-hydroxydopamine (6-OHDA) lesion of the ipsilateral medial forebrain bundle. At 2 weeks post lesion, all rats underwent testing of spontaneous nocturnal locomotor behavior in an automated Photobeam Activity System (FlexField). Immunohistochemistry was performed with tyrosine hydroxylase, dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein and glial fibrillary acidic protein antibodies. Morphometry was performed using computerized image analysis. Behavioral and morphological analysis failed to show striatal or nigral protection in caspase inhibitor-treated animals. Our findings suggest that anti-apoptotic strategies are unrewarding in the SND rat model and, therefore, alternative neuroprotective interventions such as anti-glutamatergic agents or inhibitors of microglial activation should be explored instead.

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague–Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.

Zinc transport from the striatum and substantia nigra

65Zn was unilaterally injected into the striatum or substantia nigra of rats to see the transport of intracerebral zinc (Zn). In the case of intrastriatal injection, 65Zn was densely distributed in the ipsilateral medial forebrain bundle (MFB) and the substantia nigra. On unilateral colchicine injection into the MFB, 65Zn distribution in the ipsilateral substantia nigra decreased significantly compared to that of the contralateral one after 65Zn injection into the bilateral striata. These results suggest the presence of axonal transport of 65Zn taken up by striatonigral γ-aminobutyric acid (GABA)-ergic and/or nigro-striatal dopaminergic neurons. On the other hand, in the case of intranigral injection, 65Zn was distributed in the ipsilateral MFB, striatum, globus pallidus, pontine reticular nuclei, and pontine nuclei. 65Zn distribution in the pons 1 day after intranigral injection was very similar to that 6 days after intrastriatal injection, suggesting that, in the case of intrastriatal injection of 65Zn, nigral 65Zn, which was transported anterogradely and/or retrogradely from the striatum, was transported to the postsynaptic neurons through the synapse and then transported to the pons.

Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of α-[ 3H]methyl-5-hydroxytryptamine (α-[ 3H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and α-[ 3H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated ( P ⪯ 0.001); the calculated TPH-specific activity was 0.76 nCi of α-[ 3H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and α-[ 3H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side, P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and α-[ 3H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in α-[ 3H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in α-[ 3H]M5-HT and TPH levels (136% and 93.8%, P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of α-[ 3H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.

Activation of single neurons in the rat nucleus accumbens during self- stimulation of the ventral tegmental area

Single neurons (n = 76) were recorded in the nucleus accumbens septi (NAS) of rats self-stimulating the ipsilateral medial forebrain bundle (MFB) at the level of the ventral tegmental area (VTA). Responses evoked by rewarding trains of stimulus pulses fell into five categories. The first category (40% of the sample) was characterized by a single discharge at invariant latency in response to individual pulses of the train, and hence was termed "tightly time locked" (TTL). Two TTL neurons were collision tested, and both showed collision, suggesting that self-stimulation of the VTA may involve antidromic, and thus direct, activation of a substantial number of NAS axons. The second category (26%) was characterized by discharges that varied in latency from pulse to pulse and hence was termed "loosely time locked" (LTL). Responses of the remainder of the sample showed no coupling to individual pulses but were categorized based on general firing patterns during the train: excited (7%), inhibited (4%), and no change (23%). Irrespective of category, immediately after the self-stimulation session, the likelihood of evoked discharge at monosynaptic latency by single pulse stimulation of the ipsilateral fimbria was reduced (relative to pre-session level), concurrent with elevations in mean firing rate and motor activity. NAS neurons thus exhibit vigorous activation, apparently both antidromically and orthodromically, in response to VTA self-stimulation. The responses of certain LTL and TTL neurons increased as a function of pulse number in the train, suggestive of integrative mechanisms important for brain stimulation reward. Conduction velocities of directly activated (TTL) axons (0.41-0.65 m/sec) were slower than those previously reported for first-stage, reward-relevant axons. Nonetheless, an implication of direct activation of NAS (and other MFB) axons is that rewarding stimulation triggers action potentials that could invade all axonal branches, including those between the stimulation site and the soma, and send synaptic signals to target neurons. Such signals from NAS neurons could contribute to the increased motor behavior accompanying MFB self-stimulation, and/or could interact with dopamine-mediated signals projected to the NAS from reward circuitry.

Locomotor stepping elicited by electrical stimulation of the lateral hypothalamus requires an ipsilateral descending pathway

The effects of unilateral midbrain lesions on stepping produced by ipsilateral medial forebrain bundle (mfb) stimulation was determined in acute experiments. Rats were anesthetized with nembutal, mounted in a stereotaxic apparatus and suspended over a wheel which rotated when the rat stepped. Stimulation consisted of 10-sec trains of 50 Hz cathodal pulses (0.5 msec) at currents up to 200 μA delivered through monopolar electrodes. The stimulation sites were in the mfb at the level of the subthalamic nucleus. One side of the midbrain was randomly selected to receive a radio-frequency lesion. Certain midbrain lesions (N=11) abolished or severely reduced stepping elicited from ipsilateral mfb stimulation but produced no effect on contralaterally elicited stepping. Areas of common damage in these lesions included the medial half of the medial lemniscus and the ventral tegmental area (VTA). Lesions that were without effect (N=18) generally spared the dorsal VTA but damaged extensively either the ventrolateral VTA, medial lemniscus, substantia nigra, red nucleus, or central gray. A supplemental experiment further implicated a critical descending system since neither unilateral nor bilateral mfb lesions blocked stepping elicited by VTA stimulation. Together these results suggest that a descending path through the ipsilateral dorsal VTA mediates stepping elicited by mfb stimulation in the anesthetized rat.

Behavioural effects mediated by unilateral nigral dopamine receptor stimulation in the rat

Unilateral intranigral injections of dopamine in conscious rats pretreated with nialamide resulted in either ipsiversive or contraversive rotation depending upon the site of injection. Injection of dopamine (50 micrograms) into the zona compacta of the substantia nigra induced weak ipsiversive or mixed ipsiversive and contraversive rotation. Injection of dopamine (12.5-50.0 micrograms) into zona reticulata of substantia nigra induced only contraversive circling. Destruction of the ipsilateral medial forebrain bundle (MFB) using 6-hydroxydopamine (6-OHDA) abolished ipsiversive circling but enhanced contraversive circling produced by dopamine or apomorphine. The combination of a unilateral 6-OHDA lesion of MFB with a kainic acid or electrolesion of the ipsilateral strio-nigral and pallido-nigral pathways reduced contraversive circling to intranigral apomorphine (10 micrograms). Ipsiversive circling produced following intranigral injection of dopamine is dependent upon the integrity of ascending dopamine neurones. Contraversive rotation is independent of ascending dopamine pathways but is reliant upon afferent input to the substantia nigra from the striatum and/or globus pallidus.

Origins of substance P-containing fibers in the lateral septal area of young rats: immunohistochemical analysis of experimental manipulations.

The majority of substance P-like immunoreactive (SPLI) fibers in the lateral septal area (LS) are supplied by SPLI cells in the area (BAL) between the anterior hypothalamic nucleus and the lateral hypothalamus, and by those in the nucleus latero-dorsalis tegmenti (TLD). These conclusions are based on following: (1) Unilateral destruction of the BAL resulted in an ipsilateral decrease in the septal SPLI fibers similar to that seen after the destruction of the TLD, and (2) simultaneous destruction of the BAL and TLD caused a marked reduction of SPLI fibers in the LS on the operated side. The possibility that the destruction of the BAL affected the ascending SPLI system from the TLD seems to be excluded, because (1) the destruction of the TLD resulted in a decrease in SPLI fibers in the ipsilateral medial forebrain bundle (MFB), but failed to reduce the number of SPLI fibers in the BAL, and (2) the destruction of the BAL caused a decrease in SPLI fibers in the perifornical area rostral to the lesion, but failed to reduce the number of SPLI fibers in the MFB. These facts further suggest that ascending SPLI fibers from the BAL travel in the perifornical area and those from the TLD pass through the MFB. It should be noted that a few SPLI fibers remained intact following the simultaneous destruction of the BAL and TLD. The present study suggests that these remaining SPLI fibers might be innervated by intrinsic SPLI cells. In support of this, several SPLI cells were detected in the septal area after colchicine pretreatment.

Direct septo-hypothalamic projections in the rat

Axonal projections from neurons located in the medial and lateral septal nuclei (MSN and LSN) were traced autoradiographically. The MSN projected bilaterally via a midline route to the diagonal band of Broca (DBB), the preoptic area (POA), the suprachiasmatic (SCN), paraventricular (PVN), ventromedial (VHM) and arcuate (ARC) nuclei including the median eminence (fiberous zone). Lateral coursing fibers traveled with the medial forebrain bundle (MFB) to the supraoptic (SON) and pre- and supra-mammillary nuclei. The majority of LSN axons projected to, and terminated in, the MSN. Fibers projected ipsilaterally through the POA, SON, SCN, ARC, ME and MMN with axonal projections observed throughout the MFB. In summary, the neurons of the MSN project to the medial hypothalamic nuclei via a midline route while LSN axons projected strongly to the MSN with the remaining ribers coursing along the ipsilateral MFB to terminate in several hypothalamic nuclei. These data indicate that a direct septo-hypothalamic pathway exists in the rat.

Neural inputs to the prefrontal agranular insular cortex in the rat: Horseradish peroxidase study

The prefrontal cortex, dorsal to the rhinal sulcus of the rat (hereinafter termed the agranular insular cortex) has been examined with the use of the retrograde transport of horseradish peroxidase. Labelled perikarya were seen in the region of the ipsilateral medial forebrain bundle, consistently rostral to the caudal edge of the posterior mammillary nucleus. These cells were interpreted as being the rostral members of the A10 dopaminergic cell group. Labelled cells were also seen in the dorsal raphe nucleus, the ipsilateral pyriform cortex, the contralateral agranular insular cortex, the ipsilateral basolateral amygdaloid nucleus, the ipsilateral locus coeruleus, and the medio-dorsal, ventromedial, and parafascicular thalamic nuclei. Detailed examination of the thalamic input confirmed a differential innervation of the dorsal and ventral regions of the agranular insular cortex. Possible functional correlates of these prefrontal afferents are discussed.

Evidence for an ascending inhibitory histaminergic pathway to the cerebral cortex.

Previous observations from our laboratory indicate that metiamide is a specific histamine antagonist in rat cerebral cortex. In view of the recent finding that histamine levels and L-histidine decarboxylase (EC 4.1.1.22) activity in cerebral cortex decrease following disruption of the ipsilateral medial forebrain bundle (MFB), the present investigation was undertaken to examine whether iontophoretically applied metiamide antagonizes the inhibition of deep cerebral cortical neurones produced by stimulation of the MFB. In rats anaesthetized with a mixture of methoxyflurane, nitrous oxide and oxygen, stimulation of the ipsilateral MFB or the cortical surface with iontophoretically applied histamine depressed the firing of cortical neurones. Metiamide antagonized the histamine-induced depression and reduced the duration of inhibition produced by MFB stimulation. However, it did not alter the inhibition induced by the cortical surface stimulation. These results indicate that a histaminergic pathway ascending through the MFB may inhibit rat cerebral cortical neurones.

Pathway mediating hypotension elicited by stimulation of the amygdala in the rat.

Cardiovascular responses were elicited from 372 histologically verified sites in the amygdala and closely adjacent structures in 82 rats under urethan or chloralose anesthesia. Arterial hypotension was elicited at low current intensities from the medial, central, lateral, and basal nuclei of rats under either anesthetic; the hypotension was still elicited in artificially respired paralyzed rats. Stimulation of the cortical nucleus produced variable changes in arterial pressure. The hypotension elicited under urethan was not found to be correlated with changes in the frequency of electrical activity of hippocampus, amygdala, or septum or to be associated with electrical seizures. Hypertension was usually elicited from the medial nucleus at high current intensities and was attributed to current spread to the cortex ventral to the medial nucleus from which hypertension was elicited at low current intensities. Stimulation of the central, basal, and lateral nuclei under chloralose elicited bradycardia. Lesions of the stria terminalis had no effect on the hypotension, whereas lesions of the ipsilateral medial forebrain bundle abolished this response.