Plasticity in the rat olfactory cortex

Lifelong olfactory deprivation-dependent cortical reorganization restricted to orbitofrontal cortex

Prolonged sensory deprivation has repeatedly been linked to cortical reorganization. We recently demonstrated that individuals with congenital anosmia (CA, complete olfactory deprivation since birth) have seemingly normal morphology in piriform (olfactory) cortex despite profound morphological deviations in the orbitofrontal cortex (OFC), a finding contradictory to both the known effects of blindness on visual cortex and to the sparse literature on brain morphology in anosmia. To establish whether these unexpected findings reflect the true brain morphology in CA, we first performed a direct replication of our previous study to determine if lack of results was due to a deviant control group, a confound in cross sectional studies. Individuals with CA (n = 30) were compared to age and sex matched controls (n = 30) using voxel- and surface-based morphometry. The replication results were near identical to the original study: bilateral clusters of group differences in the OFC, including CA atrophy around the olfactory sulci and volume increases in the medial orbital gyri. Importantly, no group differences in piriform cortex were detected. Subsequently, to assess any subtle patterns of group differences not detectable by our mass-univariate analysis, we explored the data from a multivariate perspective. Combining the newly collected data with data from the replicated study (CA = 49, control = 49), we performed support vector machine classification based on gray matter volume. In line with the mass-univariate analyses, the multivariate analysis could accurately differentiate between the groups in bilateral OFC, whereas the classification accuracy in piriform cortex was at chance level. Our results suggest that despite lifelong olfactory deprivation, piriform (olfactory) cortex is morphologically unaltered and the morphological deviations in CA are confined to the OFC.

Subtle Differences in Brain Architecture in Patients with Congenital Anosmia

People suffering from congenital anosmia show normal brain architecture although they do not have functional sense of smell. Some studies in this regard point to the changes in secondary olfactory cortex, orbitofrontal cortex (OFC), in terms of gray matter volume increase. However, diffusion tensor imaging has not been explored so far. We included 13 congenital anosmia subjects together with 15 controls and looked into various diffusion parameters like FA. Increased FA in bilateral OFC confirms the earlier studies reporting increased gray matter thickness. However, it is quite difficult to interpret FA in terms of gray matter volume. Increased FA has been seen with recovery after traumatic brain injury. Such changes in OFC point to the plastic nature of the brain.

Acquired olfactory loss alters functional connectivity and morphology

Removing function from a developed and functional sensory system is known to alter both cerebral morphology and functional connections. To date, a majority of studies assessing sensory-dependent plasticity have focused on effects from either early onset or long-term sensory loss and little is known how the recent sensory loss affects the human brain. With the aim of determining how recent sensory loss affects cerebral morphology and functional connectivity, we assessed differences between individuals with acquired olfactory loss (duration 7-36 months) and matched healthy controls in their grey matter volume, using multivariate pattern analyses, and functional connectivity, using dynamic connectivity analyses, within and from the olfactory cortex. Our results demonstrate that acquired olfactory loss is associated with altered grey matter volume in, among others, posterior piriform cortex, a core olfactory processing area, as well as the inferior frontal gyrus and angular gyrus. In addition, compared to controls, individuals with acquired anosmia displayed significantly stronger dynamic functional connectivity from the posterior piriform cortex to, among others, the angular gyrus, a known multisensory integration area. When assessing differences in dynamic functional connectivity from the angular gyrus, individuals with acquired anosmia had stronger connectivity from the angular gyrus to areas primary responsible for basic visual processing. These results demonstrate that recently acquired sensory loss is associated with both changed cerebral morphology within core olfactory areas and increase dynamic functional connectivity from olfactory cortex to cerebral areas processing multisensory integration.

Normal Olfactory Functional Connectivity Despite Lifelong Absence of Olfactory Experiences

Congenital blindness is associated with atypical morphology and functional connectivity within and from visual cortical regions; changes that are hypothesized to originate from a lifelong absence of visual input and could be regarded as a general (re) organization principle of sensory cortices. Challenging this is the fact that individuals with congenital anosmia (lifelong olfactory sensory loss) display little to no morphological changes in the primary olfactory cortex. To determine whether olfactory input from birth is essential to establish and maintain normal functional connectivity in olfactory processing regions, akin to the visual system, we assessed differences in functional connectivity within the olfactory cortex between individuals with congenital anosmia (n = 33) and matched controls (n = 33). Specifically, we assessed differences in connectivity between core olfactory processing regions as well as differences in regional homogeneity and homotopic connectivity within the primary olfactory cortex. In contrast to congenital blindness, none of the analyses indicated atypical connectivity in individuals with congenital anosmia. In fact, post-hoc Bayesian analysis provided support for an absence of group differences. These results suggest that a lifelong absence of olfactory experience has a limited impact on the functional connectivity in the olfactory cortex, a finding that indicates a clear difference between sensory modalities in how sensory cortical regions develop.

Role of cochlear integrity in cochlear nucleus glucose metabolism and neuron number after cochlea removal in aging broiler chickens

In the chicken auditory system, cochlear nucleus (nucleus magnocellularis, NM) neurons receive their only excitatory input from the ipsilateral cochlea. Cochlea removal (CR) results in an immediate decrease in NM neuron electrical activity, followed by death of approximately 30% of NM neurons. Previous work showed a decrease in NM activity and subsequent loss of NM neurons in all chicks. Egg layer adults showed NM neuron loss after CR, while neuron number remained stable in broiler adults. This suggested that effects of CR on NM were age- and breed-dependent. We now know that most aging egg layer chickens maintain largely normal cochleae throughout adulthood. Some exhibit cochlear damage with age. The converse is true of broiler chickens. Most aging broiler chickens display cochlear degeneration, with some maintaining normal cochlear anatomy throughout adulthood. The presence of extensive cochlear damage may alter the effect of CR on NM, leading to the described differences. Here, we examine the effect of unilateral CR on NM glucose metabolism and neuron number in 2, 30, 39, and 52 week-old broiler chickens found to have normal cochleae. Chickens with damaged cochleae were excluded. Using 2-deoxyglucose uptake to evaluate bilateral NM glucose metabolism, we found significantly decreased uptake ipsilateral to CR at each age examined. Bilateral cell counts revealed significant neuron loss ipsilateral to CR at each age examined. This suggests that NM glucose metabolism decreases and subsequent neuron death occurs in aging broiler chickens when a normal cochlea is removed. The status of the cochlea must play a role in the effect of deafferentation on NM glucose metabolism and neuron survival. The effect of CR appears to be dependent upon neither age nor breed, but upon cochlear integrity instead.

Effect of cochlear integrity on cochlear nucleus neuron glucose metabolism in aged adult broiler chickens

Abrupt removal of excitatory input is devastating to post-synaptic neurons in normally functioning sensory systems. In both mammalian and avian auditory systems, abrupt temporary or permanent experimental deafferentation stimulates a cascade of changes in central auditory structures that can result in neuron death. Effects of naturally occurring progressive deafferentation on central auditory structure and function have not been fully described. Extensive naturally occurring cochlear damage is found in some aged chickens, despite their regenerative capacity, providing the opportunity to examine the effects of this type of deafferentation on the avian cochlear nucleus (nucleus magnocellularis, NM). Previous evaluation of NM oxidative metabolism using cytochrome oxidase histochemistry revealed that naturally occurring cochlear damage results in down-regulated metabolism in corresponding regions of NM. It is unknown how progressive hair cell damage and loss affects NM glucose uptake. Here, NM glucose metabolism is assessed using 2-deoxyglucose uptake as a marker for metabolic activity in the presence of normal, mildly damaged, severely damaged, and totally damaged cochlear hair cells. Results indicate that while severe and total cochlear damage significantly decrease NM oxidative metabolism, only total damage results in significantly decreased NM glucose metabolism. Results are discussed in the context of functional reorganization and trophic support.

Effects of age and cochlear damage on the metabolic activity of the avian cochlear nucleus

Most aging commercially raised broiler chickens display a progressive loss of cochlear hair cells in a pattern similar to the cochlear degeneration found in aging humans: basal (high frequency) hair cells are affected first, followed by apical (low frequency) hair cells [Durham et al., Hear. Res. 166 (2002) 82-95]. Here, cochlear anatomy was assessed from scanning electron micrographs. Then, the metabolic activity of cochlear nucleus (nucleus magnocellularis, NM) neurons in 15-19, 30, 39, 40, and 65-66 week old broiler chickens was examined using cytochrome oxidase histochemistry and compared to the degree of cochlear abnormality. Cochleae of 15-19 week old birds are largely normal; therefore the level of NM metabolic activity is considered the baseline. Cochleae of the 30 week old group display mild damage and hair cell regeneration in the base. Metabolic activity in rostral (high frequency) NM is increased relative to the baseline, while activity remains unchanged in caudal (low frequency) NM. The 39 and 65-66 week old groups display severe and total damage extending into the apex of the cochlea. Metabolic activity is decreased in rostral and caudal NM at these ages. These results suggest that auditory central nervous system metabolism (cytochrome oxidase activity) is affected by changes in the aging chicken cochlea.

Trans-neuronal modification of anterior piriform cortical circuitry in the rat

Long-Evans rats received unilateral naris closure on postnatal day 1 (PN1) or sham surgery. On PN30, brains were processed for anterograde horseradish peroxidase (HRP) labeling of lateral olfactory tract (LOT) fibers in anterior piriform cortex (aPCX) Layer Ia, Timm staining of association/commissural fibers in aPCX Layer Ib, or Golgi staining for reconstruction of aPCX semilunar cell dendrites. The results demonstrate that the width of aPCX Layer Ia was reduced ipsilateral to the sealed naris compared to undeprived controls. No significant change in Layer Ib was detected. Furthermore, semilunar cell dendrites were reduced by unilateral deprivation compared to undeprived controls. The reduction in dendritic tree size was localized to distal dendritic segments, roughly corresponding to Layer Ia. These results suggest an activity-dependent change in both the distribution of cortical afferents and in the dendritic field of their target cells. While these results are similar to those reported for other sensory systems, the relatively simple architecture and laminated organization of bilateral inputs to the aPCX make the olfactory system an ideal model system to examine experience-dependent synaptic reorganization and its functional consequences.

The organization of piriform cortex and the lateral olfactory tract following the loss of mitral cells in PCD mice

Homozygous Purkinje Cell Degeneration (PCD) mice exhibit a selective loss of olfactory bulb mitral cells (MCs) after 4 months of age. This selective degeneration leaves a subpopulation of denervated granule cells which establish new reciprocal dendro-dendritic synapses with unaffected tufted cells (TCs) (14). This suggests a capacity for plasticity in TCs and raises the question of whether a comparable degree of reorganization occurs in their axonal terminals in piriform cortex (PC) following the loss of MCs. Homozygous (experimental) and heterozygous (control) PCD mice were routinely perfused and processed for electron microscopy. A quantitative electron microscopic analysis was performed on radially oriented micrograph montages spanning from the pia into layer II of PC. After MC loss in the experimental animals there was a decrease in density of larger myelinated axons in the lateral olfactory tract (LOT). Myelinated axons in the LOT had a mean cross-sectional diameter of 1.26 +/- 0.04, and 0.81 +/- 0.025 microm in the control and experimental mice, respectively. In superficial layer I of PC, control mice had presynaptic axonal terminals from mitral and tufted cells with characteristic electron lucent (light) profiles establishing asymmetric synapses with pyramidal cell dendrites. In contrast, the experimental mice showed a decrease in electron lucent terminals and a robust increase in electron dense (dark) presynaptic associational terminals. Although the overall synaptic density did not differ between the control and experimental mice (16.40 +/- 0.94 and 18.10 +/- 0.96 synapses/100 microm2, respectively), an overall decrease in the thickness of Layer 1 suggests that the total number of synapses decreases following MC loss. In addition to the apparent increase of associational terminals, the diameter of terminal enlargements increased as well as the number of multiple synaptic contact per terminals in the experimental animal, suggesting further compensatory mechanisms for the loss of MC presynaptic terminals.

Recovery of olfactory function in thirteen-day-old rats after olfactory bulb transplantation but not after olfactory bulb ablation

Previous studies have shown recovery of olfactory ability along with reconnectivity of olfactory nerve (ON) following both olfactory bulb (OB) lesions and OB transplants (TX) when performed in newborn rats. The purpose of the present study is to correlate functional recovery with patterns of anatomical reconnectivity in older, postnatal (PN) 13-day-old rats (a possible critical period for plasticity in the system). Reinnervation of olfactory areas was seen in all OB TX animals regardless of the extent of functional recovery. Eight of nineteen animals with OB TXs demonstrated some degree of behavioral recovery. No reinnervation or behavioral recovery of OB lesion animals was observed. At this age, behavioral recovery is dependent upon reconnectivity within the system and transplantation may be required to facilitate this process.

Age-related development of olfactory bulb transplants in rats

We are using the rat olfactory system to study developmental aspects of neurotransplantation (TX). Age-related TX maturation and subsequent establishment of connections are of special concern. Previous studies of deafferentation by olfactory bulb (OB) removal suggested "critical" periods of plasticity in the system. We present here preliminary attempts at relating age of host receiving TX to maturation of the TX and its connections. This investigation used hosts of postnatal age (PN) 13-14 days with fetal donors at Embryonic Day 15; the former having one OB ablated and receiving a fetal donor OB TX immediately placed in the vacated space. The fetal tissue was labeled previously in utero with tritiated thymidine. After 2 months a small coagulation lesion was placed in the OB TX and 2 days later the tissue was taken, serially sectioned, and processed for [3H] autoradiography, degeneration, and olfactory marker protein (OMP). Extensively 3H-labeled OB TXs with localized small lesions were studied. The cellular architecture of the TX is less well organized than in normals but substantial OMP reactivity occurs throughout. Degeneration occurs mainly near the lesion and little if any degeneration is seen beyond the 3H-labeled TX tissue. The results show that OB TX survive and develop in the PN 13-14 age group as they do in the younger animals and that primary olfactory neurons likewise reinnervate the TX but that PN 13-14 TX efferent projections are far more limited than those of younger hosts.

Age-related remodeling of glutamic-acid decarboxylase-labeled elements in deafferented piriform cortex of rats

Olfactory bulb (OB) removal has been shown to result in plasticity in the piriform cortex (PC) that is age dependent. We are studying this phenomenon using immunoelectron microscopy of glutamic acid decarboxylase immunoreactivity (GAD, the enzymatic precursor for GABA) at selected postnatal ages and in adults with emphasis on short survival times of 4-7 days after OB ablation. Normally GAD-labeled synaptic terminals form type II symmetric contacts onto unlabeled dendrites and GAD-labeled dendrites receive type I, asymmetric contacts from unlabeled terminals (Westenbroek, et al., 1988a). The OB lesion results in degenerating terminals with type I contacts onto unlabeled and onto GAD-labeled dendrites. Type I postsynaptic sites may be seen partially contacted by or entirely devoid of degenerating terminals and occasionally may be apposed to variable degrees by normal unlabeled or by GAD-positive terminals. Subsequently, some GAD-labeled terminals may form asymmetric type I contacts usually with unlabeled dendrites and rarely with GAD-labeled dendrites. The findings are most common in the youngest subjects and essentially absent in the adult subjects. A sequence of reinnervation of deafferented type I sites by GAD-labeled terminals is suggested for the formation of this "atypical" synapse and the sequelae of this reorganization are discussed.

Fetal olfactory bulb transplants send projections to host olfactory cortex in the rat

We are using the rat olfactory system to study developmental details of neurotransplantation. Tritiated [3H]thymidine-labeled fetal olfactory bulbs (OBs), were transplanted immediately into sites from which the neonatal host OB was removed. Subsequently, a small lesion was placed in the region of the transplanted OB and the tissue studied, using degeneration methods and autoradiography. Only OB's with extensive [3H]-label and precise lesions confined to the labeled areas were used. Degeneration was found mainly in the ipsilateral piriform cortex with lesser amounts at other nearby sites. The results demonstrate successfully transplanted donor OBs that send axons to specific and appropriate target areas of the host brain.

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.

Electron microscopy of plasticity in rat olfactory cortex

Electron microscopy (EM) is being used to study the ultrastructural basis for the age-dependent reorganization of afferents in the olfactory cortex (OC) of rat after deafferentation of the area by removal of the ipsilateral olfactory bulb (OB). The double-lesion technique was used with a primary lesion of the OB at various postnatal (PN) ages between PN 0 and 30 and in the adult (PN 100). After appropriate survival times to remove initial lesion-degenerated terminals from the OB lesion, a second lesion was placed in the ipsilateral OC. One to 3 days later the tissue is prepared for EM with emphasis on a study of changes in the superficial and deep dendritic layer (Ia and Ib respectively) rostral to the lesion. In control litter mates with both OBs intact, but with a single OC lesion only, degenerating synaptic terminals occur onto dendritic spines and branches only in deeper Ib. However, in adults with OB lesions at PN 0-9, OC lesions produce degenerating terminals throughout Ia and Ib including immediately subjacent to the pia. In Ia degenerating terminals are greatly reduced in the PN 13 group and rare to absent in experiments with OB lesions at older ages (PN 30-100). Electron-dense debris within glia occurs throughout layer I in each double-lesion group but is greatest in experiments with OB lesions at older ages. Some transsynaptic alterations are seen throughout, especially in the PN 30-100 group even at a distance from the OC lesion. The results support earlier light microscopic (LM) findings, suggesting PN 9-13 as critical ages for developmental plasticity and prove that at least in the younger ages, synapses are involved in the phenomenon. This may be explained by either reinnervation of deafferented sites or persistence of synapses that would otherwise have been eliminated by afferents from the OB. In addition, some of the LM degeneration particles probably are engulfed masses of debris and not synaptic structures, especially in cases which were operated at older ages and survived for 3 days. The various afferent pathways involved in the events as well as factors that limit the phenomenon in older ages are discussed.

Plasticity of intra-amygdaloid connections following the denervation of fibers from accessory olfactory bulb to medial amygdaloid nucleus in adult rat: immunohistochemical study of anterogradely transported lectin (Phaseolus vulgaris leucoagglutinin)

The rearrangement of the terminations of intra-amygdaloid fibers was examined in the medial amygdaloid nucleus (MAN) following denervation of the fibers from the accessory olfactory bulb (AOB) of adult rat using immunohistochemistry of anterogradely transported lectin (Phaseolus vulgaris leucoagglutinin, PHA-L). The AOB was removed unilaterally by suction at two months before the PHA-L injection. The PHA-L was injected bilaterally into the posteromedial region of the amygdala (posteromedial amygdaloid nucleus and amygdala-hippocampal transitional area). After the tissue was processed by the routine immunohistochemical method, the immunologically labeled axons and axon terminals in the MAN were observed with the light microscope. On the control side, labeled axons were observed in the cellular part of the MAN, but they were not found in the molecular layer in which the fibers from the accessory olfactory bulb terminate. On the side of the AOB removal, the labeled axons were found not only in the cellular part but also in the molecular layer. Electron microscopic observations showed that the labeled axon terminals made synaptic contacts in the molecular layer. These results indicate that terminals of intra-amygdaloid fibers expand from the cellular part to the molecular layer in the MAN and make synaptic contacts in this layer following the denervation of the AOB fibers.

Normal development and effects of early deafferentation on choline acetyltransferase, substance P and serotonin-like immunoreactivity in the interpeduncular nucleus

The normal postnatal development and response to neonatal fasciculus retroflexus (FR) lesions of serotonin, substance P (SP), and choline acetyltransferase (ChAT) distribution are described for the rat interpeduncular nucleus (IPN). Serotonin-, SP- and ChAT-containing axons differed in development, distribution, and response to deafferentation. Serotonergic axons and cell bodies were present at birth. SP was present in the FR and in the lateral subnuclei by 3 days of age but did not appear in the rostral or dorsal subnuclei until 7-14 days. Intrinsic SP perikarya were not seen until 17 days of age. The development of ChAT was late, appearing only during the second week of life and not reaching adult patterns and density until after 21 days of age. The pattern of development of cytochrome oxidase and Bodian silver staining are also described. Both cytochrome oxidase and Bodian staining paralleled the patterns of localization and development of ChAT staining. Bilateral neonatal FR lesions resulted in a permanent loss of ChAT and cytochrome oxidase staining throughout the IPN and of SP in the lateral and rostral subnuclei. No changes were seen in the serotonergic system. Following unilateral lesions, the pattern of SP loss and replacement paralleled that seen after adult lesions. The pattern of replacement of ChAT differed from that after adult lesions in that there was partial replacement in the ipsilateral intermediate subnucleus following neonatal lesions. This result suggests that late developing cholinergic axons can innervate the contralateral intermediate nucleus to a much greater extent following infant lesions than following adult lesions.

Immunocytochemical localization of cholecystokinin and glutamic acid decarboxylase during normal development in the prepyriform cortex of rats

Immunocytochemical localization of specific neurotransmitters in the brain is becoming increasingly important in studies of maturation. We have used the trilaminar prepyriform cortex (PC) of rats to study the distribution, patterns and relative number of cells, fibers and terminals during postnatal development using antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD). Both antisera show distinct patterns of immunoreactivity at birth and subsequent periods of distinct changes in these patterns. CCK immunoreactivity is rare but present at birth mostly in layer II. There is a dramatic increase of CCK-labeled structures between postnatal (PN) days 6 and 9 and between PN 13 and 21. The adult pattern is observed by PN 21 with large numbers of labeled cells in layer II, numerous terminals in layers II and deep I and large immunoreactive fibers in the lateral olfactory tract. At birth GAD-immunoreactive terminals are present mainly in layer I, forming a distinct pattern of superficial and deep bands. Subsequent major changes occur in this pattern between PN 9 and 13 and again between PN 13 and 21. By PN 21 there appears to be a loss in deeper laminae of GAD positive terminals which are possibly replaced by the increasing numbers of CCK terminals in the same sublaminae. The adult pattern of GAD immunoreactivity is established by PN 21 with terminals and a few cells in layer I. Therefore, throughout development of the rat PC, there is a distinct complementary and changing distribution of GAD and CCK. Factors that may influence these changes in immunoreactivity are discussed.